© 2013 The Chemical Society of Japan

Bull. Chem. Soc. Jpn. Vol. 86, No. 7, 755­783 (2013)

755

Award Accounts

The Chemical Society of Japan Award for 2005

Time-Resolved Spectroscopic and Imaging Studies on Laser Ablation of
Molecular Systems: From Mechanistic Study to Bio/Nano Applications#
Hiroshi Masuhara
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University,
Ta Hsueh Rd. 1001, Hsinchu 30010, Taiwan
Received March 5, 2013; E-mail: masuhara@masuhara.jp

Studies on laser ablation of polymer ﬁlms, molecular crystals in solution, protein solution, and culture media
containing living cells are summarized and considered. Dynamics and mechanism of laser ablation were systematicallystudied by utilizing time-resolved spectroscopy and imaging; femtosecond­nanosecond transient absorption and emission
spectroscopy, nanosecond shadowgraphy, nanosecond­nanometer interferometry, femtosecond surface light scattering
imaging. It was conﬁrmed by integrating both data that primary processes of laser ablation can be well understoodin the framework of Jablonski diagram. For nanosecond laser ablation of doped polymers, it was demonstrated thatcyclic multiphoton absorption is an eﬃcient photothermal conversion process leading to photothermal ablation. For
femtosecond laser ablation of dye ﬁlms, transient pressure mechanism was proposed indicating photomechanical ablation.
As applications of laser ablation, nanoparticle preparation, protein crystallization, and manipulation of living cells arepresented. Laser ablation of molecular crystals in poor solvent gives small fragments whose size are in a few tens nm. The
fabricated nanocolloids are stable without adding detergents and their size was the smallest as nanoparticles producedby the top-down-method. Multiphoton laser ablation of water generates local impulsive force due to bubble formation,
shockwave propagation, and local convection ﬂow. The force triggers molecular and protein crystallization in theirsupersaturated solutions, whose mechanisms are described and considered. The impulsive force is also very useful for
manipulating living cells and its high potential was conﬁrmed by examining cell functions such as division, diﬀerentiation, death, and migration. Finally summary and future plan are presented.

Studies on molecular spectroscopy and photochemistry
introduced lasers very early in 1960’s and used them as lightsources for spectroscopic measurements as well as for inducing
photochemical reactions. When chemists started to use pulsed
lasers, they soon became aware that laser irradiation induced
vaporization, decomposition, and fragmentation of materials
and found that the irradiated surface was etched. The left
patterns on the colored papers were practically used to conﬁrm
optical alignment in experimental setups before infrared (IR)viewer became popular. Later this laser-induced phenomenon
was called laser ablation by Srinivasan,1 and its systematic
application was quickly developed to fabricate microstructuresof various materials by electronics engineers and to microsurgery by medical doctors. Laser ablation is made possible
by applying pulsed lasers and never brought out by weakcontinuous wave (CW) laser even when its irradiation
continues for 1 year. Laser ablation is one of representative
nonlinear photochemical phenomena and has a threshold withrespect with laser ﬂuence. Initially most of laser ablation
studies were carried out for metals, semiconductors, ceramics,
and glasses,2 while organic materials,3­6 cells, tissues, andorgans have recently been receiving more and more attentions.7

In general ablation mechanism of organic and bio materials was discussed from photochemical and photothermal
viewpoints. The former was simply expected, as multiphoton
excitation energy is sometimes higher than ionization potential
and chemical bond energy, leading to ion formation and bondcleavage, respectively. The molecules in the irradiated volume
may decompose typically to small gas phase molecules andthe volume should be etched. This interpretation is due tothermodynamic consideration and time scale at which thedecomposition takes place is not examined. When the relaxation from the higher excited states to the ﬁrst excited state is fast,
the excess energy is converted to heat before ionization anddecomposition takes place from the higher excited states. The
irradiated area is instantaneously heated to high temperature andshows explosive melting, leading to fragmentation and ejection.
This decomposition behavior is interpreted as photothermal
ablation. At the early stage of ablation studies of molecularmaterials, such mechanistic considerations seemed reasonable
and were presented in many papers.1­8 From theoretical viewpoints some photothermal models were presented for polymer
ablation, taking into account the movement of the interface
between initial solid and product gaseous phases.9,10 On the

Published on the web July 15, 2013; doi:10.1246/bcsj.20130066

756

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

other hand a simple sublimation model was proposed assuming
the similarity between polymers and inorganic materials,11,12
which was later developed to discriminate various models.13
Further systematic studies on cryogenic molecular solids andliquids were extended, leading to deep understanding on laser
ablation mechanism of molecular materials.14
It is not easy to conclude on ablation mechanism in general,
as ablation behavior is dependent on laser ﬂuence, wavelength,
pulse width, repetition rate, and total shots of laser pulses,
and of course on materials properties as well as their surface
and environmental conditions. Most of ablation studies had
been done for practical applications, so that exact mechanistic
approach was relatively limited. We considered that the timeresolved spectroscopic measurements are necessary and indispensable to reveal how electronic excitation energy is used formaterial ejection, what chemical species are involved, and inwhat time scale electronic excitation evolves to morphologicalchanges. Such spectroscopic measurements have been combined with newly developed time-resolved imaging, whose
results have given us better pictures of laser ablation dynamics
and mechanism at the molecular level. Furthermore the resultson time-resolved studies have led to various proposals of laserbased tools, methods, and technologies. Now laser ablation
studies are growing up to new research horizons where molecular implantation, molecular and protein crystallization, growth
control of crystals, patterning of protein cubes, or even manipulation of living cells in solution become contemporary topics.
In the present Account our systematic studies on laser
ablation utilizing time-resolved spectroscopy and imaging
are summarized and discussed in view of photophysics andphotochemistry. In Section 1, nanosecond laser ablation dynamics and mechanism are described, where experimental
results were obtained by nanosecond emission and absorption
spectroscopies, time-resolved shadowgraphy, and nanosecond­
nanometer interferometry. Photothermal expansion/contraction
dynamics and photochemical ablation are summarized forpolymer ﬁlms. In Section 2, the results on femtosecond laser
ablation dynamics and mechanism are presented, which are
all based on ablation phenomena induced by irradiation witha 100 fs Ti:sapphire laser pulse. In Section 3, nanoparticle
preparation in solution is introduced as practical outputs
from our laser ablation study and demonstrated to have highpotential in nanomaterials research. In Section 4, as anotherinteresting output, crystallization and crystal growth by femtosecond laser ablation is described and considered. In Sections
5 and 6, we summarize on femtosecond ablation of solution giving local impulsive force with bubble formation anddescribe on its application to crystal growth and crystallizationof molecules and proteins by the bubbling. In Section 7, it issummarized how living cells in solution can be manipulated by
the laser-induced bubbling, where the cells are not irradiateddirectly. It is worth to note that whole manipulation can bedone in solution without exposing cells to air. Summarizingthese results and our ideas, we will present our perspective on
laser ablation studies in molecular systems.
1. Nanosecond Laser Ablation Dynamics and Mechanism
For elucidating dynamics and mechanism of molecular
excited states under laser ablation condition, it should be useful

to correlate primary processes of laser ablation with wellknown photophysical and photochemical properties of molecules. From this viewpoint we chose aromatic molecules suchas biphenyl, p-terphenyl, N,N,N¤,N¤-tetramethyl-p-phenylenediamine, carbazole, anthracene, and pyrene, and doped themin polymer ﬁlms.15­41 Furthermore photoreactive groups werechemically bonded into polymers chains, and their ﬁlms wereformed.42­49 Ablation is initiated by absorption of excitation
photons by those dopants, and their dynamics was studied
by applying time-resolved absorption and ﬂuorescence spectroscopy, transmittance measurement at excitation wavelength,
time-of-ﬂight mass spectroscopy, nanosecond shadowing, andphotoacoustic measurement. In general no appreciable decomposition product indicating photochemical ablation mechanism
was detected and photothermal conversion was considered
to be responsible to the fragmentations. The excited states of
dopant molecules were detected spectroscopically, molecularradicals suggesting photodecomposition were not identiﬁed,
temperature elevation was conﬁrmed by analyzing ﬂuorescence
spectral broadening and time-of-ﬂight mass spectrometry, andsurface morphology suggesting melting of ablated area wasobserved after ablation. Furthermore time-resolved interferometry clariﬁed expansion and contraction dynamics before
fragmentation and ejection take place. These experimental
results indicate that the excited molecules convert electronic
excitation energy to intermolecular and lattice vibrations very
eﬃciently without appreciable chemical decomposition of
dopants. On the basis of these results, we proposed photothermal mechanism based on “cyclic multiphotonic absorption”
for aromatic molecule-doped polymers.
On the other hand, when photodecomposable groups are
introduced to main chains or side substituents of polymers,
ablation behavior is much changed. Upon densely electronic
excitation of the polymers with high intensity laser pulse, very
rapid dissociation is induced densely and polymer morphology cannot be maintained anymore, undergoing the etching in
the irradiated area, which can be assigned as photochemical
ablation. The specially designed polymers are necessary forconﬁrming photochemical ablation and we used nitrocellulose,
triazene polymer, polyurethane, and polyimide ﬁlms. Also wefound that their ablation sometimes involves combustion anddetonation, which is one of extreme cases of the photochemical
ablation. Here our experimental results demonstrating nanosecond photothermal and photochemical ablation are summarized and considered.
1.1 Spectroscopic Study of Laser Ablation of Doped
Polymer Films. In laser ablation sample ﬁlms are etchedeﬃciently, so that the decomposed materials, molecules, atoms,
and so on should be ejected to air. Under such condition it was
believed that transient absorption spectral measurement wasmuch disturbed. Also plasma emission is frequently observed,
which was considered to make transient absorption spectral
measurement diﬃcult. However, by conducting various timeresolved measurements including time-resolved shadowgraphy,
we found these experimental diﬃculty is not critical inobserving transient species at early stages of laser ablation,
namely, ejection occurs relatively later and the primaryprocesses of laser ablation can be probed not being so much
disturbed optically by the dispersed materials. As an exam-

H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

757

Figure 2. Schematic diagram of nanosecond photography.
This ﬁgure is reproduced from Figure 1 of Ref. 25.
Copyright 2013 American Chemical Society.

Time/ns
Figure 1. Comparisons between transient absorption at 360
nm and nanosecond photograph of poly(methyl methacrylate) ﬁlm doped with 2.0 wt % biphenyl upon laser
ablation. Laser ﬂuence for absorption measurement andphotograph is 670 and 600 mJ cm¹2, respectively. Thisﬁgure is reproduced from Figure 8 of Ref. 25. Copyright2013 American Chemical Society.

ple the result on poly(methyl methacrylate) ﬁlm doped withbiphenyl is described here.25 In Figure 1 absorbance at 360 nm
and shadowgraph near the ﬁlm surface are shown as a functionof the delay time after 248 nm excitation. Here the materials ejection behavior was followed directly by nanosecond
dynamic photography where an excimer laser (248 nm) and thesecond harmonic (532 nm) of nanosecond Nd3+:YAG laser
were used to induce ablation and to light a dye solution. The
ﬂuorescence of a methanol solution of rhodamine 101 worked
as a ﬂash lamp for nanosecond photography and its pulse width
was 17 ns. Since morphological change upon intense excitation was induced at the small area of the sample ﬁlm
edge, observation was done under a microscope as shownin Figure 2.
As clearly seen in Figure 1, no appreciable ejection offragmented debris was initially observed with the spatial
resolution of a few tens of ¯m. In the delay time range of a
few tens ns, the absorbance at 360 nm was detected even at
the excitation of 360 mJ cm¹2 and ascribed to the triplet state(T1) of doped biphenyl molecule. This band rose rapidly anddecayed fully before 50 ns, which is interpreted as T1­T1
annihilation due to its dense excitation condition. After that avery intense absorption increased, corresponding to ejection of
the fragmented materials debris which was observed in the
shadowgraphs. By introducing a streak camera into the setup in
Figure 2, transient absorption spectra upon laser ablation were
measured as one-shot event. The results are summarized in

Figure 3. The transient absorption spectrum at early stage canbe conﬁrmed to be T1, while the absorption band due to excited
singlet state (S1) was not observed in the examined temporaland wavelength regions. At later delay time an extremely
intense absorption tailing to the long wavelength emerged,
which was ascribed to scattering due to ejected particles withsize larger than the monitoring light wavelength.
Emission spectra were measured with a streak cameraand summarized in Figure 4. The spectrum at early stage isassigned to biphenyl ﬂuorescence, and its ﬂuorescence decay
dynamics is well correlated with the rise of the triplet absorption, namely, the singlet state is ﬁrst generated and followed byintersystem crossing, involving S1­S1 and T1­T1 annihilation
processes. Their decays under the present ablation condition
are faster than the conventional monomolecular decays due totheir mutual interactions. At later stages a very broad band
overlapped with sharp peaks were detected, which should
be ascribed to plasma emission and small radicals such as
CH2, C2, and so on, respectively. These emissions came to beprominent only at higher excitation intensity. Just above
the ablation threshold, etching was observed without plasma
emission. There results indicate that plasma and small radicals’emissions are ascribed to the subsequent processes and only
S1 and T1 states are responsible to ablation, although plasma
emission was quite frequently reported in the literatures onlaser ablation of organic materials.50 Therefore we consider thatprimary processes of laser ablation can be understood in theframework of Jablonski diagram.25 This experimental result
gave us an important remark that laser ablation of molecularmaterials can be studied as one of photochemistry researches.
In the following we summarize two reports of our spectroscopic studies on laser heating of polymer ﬁlms.
Laser heating dynamics of poly(methyl methacrylate) ﬁlms
doped with biphenyl or p-terphenyl was analyzed and photothermal laser ablation mechanism was considered to hold.38
Temperature-dependent T1­T1 annihilation was analyzed inview of the thermally activated diﬀusion of the dopant species.
The second-order rate constant for the T1 absorption was
obtained at several temperatures under weak excitation (2­4758

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

(a)

(b)

Wavelength/nm

Wavelength/nm

Figure 3. Time-resolved absorption spectra of poly(methyl methacrylate) ﬁlm doped with 2.0 wt % biphenyl upon laser ablation.
Laser ﬂuence is (a) 30 and (b) 360 mJ cm¹2. Gate time is given in the ﬁgure. This ﬁgure is reproduced from Figure 6 of Ref. 25.
Copyright 2013 American Chemical Society.

Wavelength/nm
Figure 4. Time-resolved emission spectra of poly(methyl
methacrylate) ﬁlm doped with 2.0 wt % biphenyl uponlaser ablation. Laser ﬂuence is 3.7 J cm¹2. This ﬁgure is
reproduced from Figure 10 of Ref. 25. Copyright 2013
American Chemical Society.

mJ cm¹2), and its Arrhenius plot gave a straight line that wasused to estimate the rate constant at higher temperatures forthe simulation of T1­T1 annihilation. Under intense excitation(100­190 mJ cm¹2), the T1 states of both ﬁlms decayed fasterwith an increase in laser ﬂuence. The comparison of theseobserved T1 decays with simulated ones revealed that most of
the observed photon energy, at least of 70%, is used to elevatetemperature of the ﬁlm. Also, the cooling of the polymer ﬁlm,

mainly due to heat transfer to a quartz substrate, slows down
the diﬀusion of the dopants and consequently the T1­T1
annihilation. The elevated temperature of the ﬁlm remains
almost constant at least for a few ¯s after excitation, during
which fragmentation leading to etching may be induced.
Another kind of example is an poly(N-vinylcarbazole) ﬁlm,
where we could not ﬁnd any appreciable transient absorption upon nanosecond laser ablation, but a tail of the groundstate absorption was observed to be temporarily enhanced.19
This behavior was interpreted that the excited singlet states
disappeared eﬃciently due to their mutual interactions giving
no appreciable transient species. By analyzing the absorption
proﬁles at diﬀerent laser ﬂuence, the eﬀective transient
temperature below, at, and above the ablation thresholds wasestimated. The temperature rise time was less than 50 ns, andthe ﬁlm was cooled down in a few ¯s. The temperature attainedat the excitation intensity just below the ablation threshold(60 mJ cm¹2) was 260 °C and between decomposition temperatures (230­300 °C) of the ﬁlm. Based on these results it was
concluded that the photothermal decomposition mechanism is
responsible to the ablation of this ﬁlm.
1.2 Cyclic Multiphoton Absorption as an Eﬃcient
Photothermal Conversion Process. It is well known thatsimultaneous quantum mechanical and successive multiphoton
absorption through some intermediate states are induced eﬃciently under intense laser excitation. The former takes place
directly from the ground to excited states, while the latter
consists of multistep absorptions via transient states which areusually excited singlet and triplet states. Under nanosecond
laser ablation of the doped polymer ﬁlms, the excitation poweris so high that the concentration of the excited states of dopedor chemically bonded molecules becomes often comparable tothat of the ground state soon at the early stage during excitation

H. Masuhara

Figure 5. Possible mechanisms for cyclic multiphotonic
absorption. ① Sn­S1, mechanism; ② Tm­T1 mechanism;③ S1­G mechanism. This ﬁgure is reproduced from
Figure 2 of Ref. 22. Copyright 2013 Elsevier.

pulse. Thus the photon absorption by the intermediate stateenough competes with that by the ground state. The key point
here is that the higher excited (Sn or Tm) state formed bysuccessive absorption of S1 or T1 states relaxes rapidly to theoriginal S1 or T1 state in the time scale of 10 ps and the latterstate again can absorb successive photons. The absorption toand the relaxation from Sn or Tm take place rapidly so that they
can repeat many times during nanosecond excitation, and as
a result emitted intramolecular and lattice vibrational energy
heats up to the surroundings enough to induce thermal decomposition. We named this process “cyclic multiphoton absorption” and concluded that this absorption results in photothermal
ablation of dopant molecular systems.22 The process is schematically given in Figure 5, along which the experimental
results were summarized and discussed.
Polystyrene ﬁlm doped with anthracene or antracene-d10 was
ablated by 351 nm laser irradiation in a high-vacuum apparatus,
and velocity distribution of ejected species were measured witha quadrupole mass spectrometer.20 When the etching of thesurface took place at irradiation energy of several hundredsmJ cm¹2, the detected species were mainly anthracene andstyrene monomers. The result was analyzed by assuming acomposite Maxwell­Boltzmann (MB) distribution with hightemperature (several hundreds K) and low-temperature (250­300 K) MB components. It was considered that photon energy
absorbed by anthracene is converted into thermal energy
through rapid internal conversion in a polystyrene matrix,
resulting in thermal decomposition of the polymer matrix.
Moreover it was pointed out that 30 photons should beabsorbed by individual molecules to prepare a melt polymer
at 995 K, which is estimated translational temperature ofanthracene obtained in the mass spectrometry measurement.
It is interesting to understand how one anthracene molecule
absorbs a few tens of photons during laser irradiation and why
the photon energy is converted to heat without decomposing
itself. This ﬁnding is well explained with “cyclic multiphoton
absorption” process which we described above. As the ﬁrst

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

759

excited singlet state of anthracene absorbs photon and theformed higher excited state quickly relaxes to the ﬁrst excitedstate, emitting vibrational energy, and the recovered excitedstate absorbs photons again. Thus anthracene is regarded to bea “molecular converter” of absorbed photon energy into thesurrounding heat energy.
To conﬁrm this cyclic multiphoton absorption process inmore detail, temporal proﬁle of the dopant ﬂuorescence wasmeasured and compared to the computational results basedon cyclic multiphoton absorption in the ground state, S1, or T1
state.22 It was revealed that simulation based on the cyclic
multiphoton absorption mechanism through Sn­S1 and Tm­T1
relaxation well reproduced the ﬂuorescence rise and decay
curves upon the laser ablation of the polystyrene ﬁlm dopedwith 0.1 M anthracene. Moreover calculated temperature basedon the mechanism through Tm­T1 transition agrees closely withthe mean temperature of ejected anthracene and styrene
obtained by mass spectroscopy. Thus we concluded that thecyclic multiphotonic absorption mechanism through Tm­T1
relaxation is most probable in the anthracene-doped polystyrene ﬁlm.
The 248 nm ablation of pyrene-doped poly(methyl methacrylate) ﬁlm was investigated with much attention to the
dynamics of pyrene transient species under intense excitation.27
It was shown that pyrene is stable even when it absorbs morethan several photons during the laser pulse, which is again well
explained in terms of cyclic multiphoton absorption process.
This means that the photodecomposition of a dopant is not
involved, while multiphoton absorption partly may lead toionization giving pyrene cation. This is reasonable, since only
ionization can compete with fast vibrational relaxation taking
place with 10 ps and decomposition reaction needs longer
time scale. Indeed the excited singlet, triplet, and cation statesof pyrene were detected within and after the laser pulse bynanosecond emission and absorption spectroscopy. The broadening and red shift of the ﬂuorescence of pyrene at 440
mJ cm¹2 mean that not only pyrene but also the polymer ﬁlm
are heated and become hot, which supports photothermal
ablation of pyrene-doped poly(methyl methacrylate) ﬁlm. The
triplet and cation states of pyrene are produced through theexcited singlet state of pyrene, contributing to cyclic multiphotonic absorption as intermediate states. It was also shownthat the temperature elevation due to photoabsorption facilitates the excited singlet­singlet and triplet­triplet annihilation
processes and also the recombination of the cation and electron
or anion.
Dynamics of N,N,N¤,N¤-tetramethyl-p-phenylenediamine inpoly(methyl methacrylate) was also studied spectroscopicallyunder 248 nm ablation condition.31 The spectral behavior waswell interpreted in terms of the dynamics of the excited singlet
and triplet states and a cation of the doped amine. The decay
processes of all the detected transient species became fast withan increase in the laser ﬂuence. It was directly shown that one
dopant molecule can absorb 8 photons during the nanosecond
laser pulse at the 248 nm ablation threshold. These results
are well explained again by the cyclic multiphoton absorption
mechanism where the transient species of the dopant work as
respective absorbers of the laser photons. The mechanism isconsistent with the fact that the excited state energy level of

760

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

ion radicals is usually low and small energy gap between the
excited and ground states of ions gives short lifetime to ion
radical. Namely the ion radical can repeat absorption andrelaxation eﬃciently.
The 248 nm laser ablation dynamics of poly(methyl methacrylate) (PMMA) ﬁlm doped with 5-diazo meldrum’s acidwas investigated as we expected that the dopan is easily
decomposed and induces photochemical ablation.26 We conﬁrmed that the photodecomposition of the dopant itself has
little or no eﬀect on laser ablation as it did not damage theﬁlms, while temperature elevation through the cyclic multiphotonic absorption by the generated ketene eventually led tolaser ablation. Namely the ketene which is formed throughphotodecomposition of 5-diazo meldrum’s acid undergoes
repetitive absorption and relaxation during the excitation pulse
as the excited state lifetime of ketene is enough short. Thus weconsider that cyclic multiphoton absorption process is also
applicable to 5-diazo meldrum’s acid dopant and its photochemical transient species is responsible to repetitive absorption. It is concluded that cyclic multiphotonic absorption
process holds generally for laser ablation of various doped
polymer ﬁlms.
1.3 Expansion and Contraction Dynamics of Polymer
Films by Nanosecond Time-Resolved Interferometry.
Ablation is a nonlinear photochemical phenomenon withrespect to laser ﬂuence and its resultant etching is observed
only when the ﬂuence is higher than the threshold value.
Even below the threshold, however, a number of photons areabsorbed by polymer ﬁlms, so that heating eﬀect is considered
to be appreciable. In other words the ﬁlm should be expandedby heating and then cooled down leading to shrinking. If thisexpansion and contraction are monitored by time-resolved
measurement, dynamic morphology change will be made clear.
In 1994 we ﬁrst developed nanosecond interferometry andsucceeded in observing directly expansion/contraction dynamics of polymer ﬁlms.24 This system is in principle based on aMichelson-type interferometer whose schematic diagram isshown in Figure 6. An excimer laser or a tunable OPO system
was used as an excitation light source, while the 2nd harmonic
pulse (532 nm, 14 fwhm, 10 ns) of the Q-switched Nd3+:YAG
laser was used as a probe light. The excitation pulse was
focused onto a sample surface at a slightly tilted angle, while
the probe beam was divided into two; one for the sample
surface and the other for a reference quartz glass withrespective normal angles. By combining two beams reﬂectedfrom the sample surface and the quartz glass, interference
patterns were formed, which was detected by a CCD camera.
The original of the delay time (¦t) was deﬁned as the time
when both peaks of excitation and probe pulses arrive at thepolymer surface simultaneously. In the experiment, a fringe
shift to the left or right corresponds to an expansion or etching depending on optical condition. One fringe spacing is ahalf wavelength of the probe laser, that is, 266 nm, and one
twentieth can be resolved by our CCD camera. Consequently
the surface displacement (expansion or contraction) can beobtained with resolution of 13 nm, while time resolution is
10 ns which is determined by the probe light pulse. We call thissystem nanometer­nanosecond interferometry and applied tovarious polymer ﬁlms.

Figure 6. A schematic diagram of a nanosecond timeresolved Michelson interferometer; PF, a polymer ﬁlm;
QS, a quartz substrate; M1­M2, mirrors; L1­L2, lenses;
BS, a beam splitter; PG, a pulse delay generator; OSC,a sampling oscilloscope; PD1­PD2, photodiodes; CCD,
a video camera; BPF, a band pass ﬁlter. This ﬁgure is
reproduced from Figure 1 of Ref. 24. Copyright 2013
American Institute of Physics.

Surface morphological change dynamics below and/or
above the ablation threshold of various polymer ﬁlms was
examined systematically, whose representative images of polyimide ﬁlm are shown in Figure 7.48 The fringe shift towardleft side corresponds to expansion under the experimental
condition. Below the ablation threshold polymer ﬁlm expanded
immediately after excitation and then slowly contracted to theoriginal thickness, just as etching started just after excitation
above the ablation threshold. Thus, transient expansion andetching dynamics can be elucidated obtained by analyzing thenanosecond time-resolved interferometric images.
A comparative interferometric study of poly(methyl metharcrylate) and poly(methyl acrylate) gave us new informationof glass-rubber transition dynamics, which was obtained fortheir ﬁlms doped with pyrene.41 At 150 mJ cm¹2 of the 248 nm
excitation, which is below the ablation threshold, polymer ﬁlm
of ca. 2 ¯m thickness underwent rapid expansion of about 250
nm and the following slow contraction, resulting in no permanent etching on the surface. Shrinking behavior of irradiated
PMMA ﬁlms on two diﬀerent kinds of substrates, quartz andundoped commercial PMMA plate, was compared with eachother. The expansion of the PMMA ﬁlm on the quartz plate
disappeared after 20 ms, while PMMA ﬁlm on the undoped
PMMA plate showed very slow contraction, that is, the expansion disappeared after a few seconds. This result indicatedthat contraction dynamics of the PMMA ﬁlm aﬀected by thesubstrate. Since the thermal diﬀusivity of a PMMA substrate islower than that of quartz one, which supports our proposal thatthe thermal process has a major role in the transient expansionof PMMA ﬁlms upon pulsed laser irradiation.
Based on photothermal mechanism, we extended systematic
study on pyrene-doped PMMA and related ﬁlms. Fluence
dependence of the maximum expansion amplitude of PMMA
and PMA ﬁlms doped with 2 wt % pyrene was investigated andH. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

(a)

761

(b)

Figure 8. Fluence dependence of the maximum expansionof PMMA ( ) and PMA ( ) ﬁlms below the ablationthreshold (180 and 330 mJ cm¹2, respectively). Both ﬁlms
are doped with 2 wt % pyrene. This ﬁgure is reproduced
from Figure 3 of Ref. 41. Copyright 2013 American
Chemical Society.

(c)Figure 7. Nanosecond time-resolved interferometric imagesof polyimide ﬁlm on a wedged quartz plate at the ﬂuenceof 130 mJ cm¹2 below the ablation threshold, which was
obtained with the surface conﬁguration. The excitation
wavelength is 351 nm. The fringe shift to the left represents
an expansion. Delay time is (a) before excitation, (b) 24 ns,
and (c) enough after excitation. The black bar in panel(c) indicates 1 mm in the image. This ﬁgure is reproduced
from Figure 1 of Ref. 48. Copyright 2013 American
Chemical Society.

obtained result is plotted in Figure 8.41 Around 65 mJ cm¹2,
a knick point was observed for PMMA ﬁlm and the slope
increased by a factor of about 4.2, whereas the PMA ﬁlm did notalter the slope. It is worth noting that only PMMA ﬁlm shows
the discontinuous ﬂuence dependence. Surface temperature of
PMMA at the knick point was estimated to be about 410 K,
which is a little higher than glass­rubber transition temperature (Tg) of PMMA; 378 K. We estimated the temperature byassuming that the Lambert­Beer law holds and all absorbed
photons are converted to heat. The absorbance of ﬁlms obtainedby conventional absorption spectrometry, while the thicknessof the ﬁlms measured by a depth proﬁler were used for theestimation. Although this estimation was later deemed too
rough, it was enough to consider that the discontinuous ﬂuence
dependence of the expansion amplitude was due to the glass­rubber phase transition of PMMA. Thermal expansion coefﬁcient of the PMMA ﬁlm increased when temperature exceeds
Tg, which explains the slope change. On the other hand, PMA isin the rubbery state at room temperature even before excitation,
so that such a knick point was not observed.
In addition to expansion/contraction dynamics, etching
behavior of a neat PMMA ﬁlm was also investigated by 248
nm excitation under higher excitation condition and directly
measured by nanosecond interferometry.34 Above the ablationthreshold (1400 mJ cm¹2), the ﬁlm expands during excitation,
and after 25% expansion of the thickness, we could not get aclear fringe pattern of the interference images of the irradiatedarea. This is because interference images became dark, which is
eventually ascribed to ablated fragments. Below the threshold,the expansion rate is still high (a few nm ns¹1), and permanentswelling is observed after transient expansion and succeeding
contraction. Permanent swelling was not observed by decreasing the laser ﬂuence below 800 mJ cm¹2, which was called
swelling threshold. The expansion amplitude as a function oflaser ﬂuence indicates that the neat PMMA ﬁlm also undergoes
phase transition from glass to rubber, which takes place at a
ﬂuence higher than 450 mJ cm¹2.
In further low ﬂuence range where the transition is not
involved, neat poly(methyl methacrylate) showed a novel
expansion and contraction behavior as given in Figure 9.33 The
morphological change of several tens nm occurred repetitively
in a few hundreds ns time range, and then the ﬁlm shrinked tothe original thickness. This behavior was not observed for otherpolymers such as polystyrene, triazenopolymer, nitrocellulose,
and so on. In glassy region below the phase transition temperature, the ﬁlm behaves like a rigid solid and quick laser
heating results in a rapid expansion and its overshooting maylead to the oscillatory behavior. On the other hand, a large
slippage of polymer chains upon heating results in the glass­rubber transition.
These photothermal behaviors are probably ascribed to thediﬀerences in polymer chain entanglement in the given states.
The entanglement was also reﬂected in cooling dynamics
of laser-heated polymer ﬁlm. We studied molecular weight
dependent surface morphological changes of polystyrene ﬁlm
after 248 nm excimer laser irradiation.36 In the case of low
molecular weights (3.0 © 103 and 5.0 © 103), the surface contracts exponentially. In the case of high molecular weight(1.0 © 106), however, two diﬀerent contraction processes wereobserved, that is, a major contraction and a minor tail. The
thermal diﬀusion coeﬃcients acquired experimentally from thecontraction of polymers with both high and low molecularweights are almost the same as the value in the literature, whichindicates thermal diﬀusion is dominant for the major contraction. On the other hand the minor tail was observed only

762

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

(a)

(b)

Figure 9. Expansion and contraction dynamics of PMMAat ﬂuence of (a) 400, (b) 300, and (c) 250 mJ cm¹2. A solid
curve represents an excitation pulse shape. This ﬁgure is
reproduced from Figure 3 of Ref. 33. Copyright 2013
Wiley-VCH Verlag.

when the molecular weight is larger than that corresponding tocharacteristic length, so that it was be considered that the tailshows the eﬀect of the entanglements.
1.4 Nanosecond Laser Ablation of Photodecomposable
Polymer Films. Photothermal ablation is induced by local
and transient temperature elevation due to eﬃcient conversion of excitation energy to molecular and eventually lattice
vibrations. If bond cleavage of the excited states forming small
gaseous molecules is enough fast to compete with electronic
and vibrational relaxations, photochemical ablation is realized.
Some polymers having photodecomposable bonds or chromophores, which are specially designed, show this ablation, whose
representative examples are summarized below. It is clear thatthe time-resolved interferometry is very useful to analyze laserinduced reaction of polymer ﬁlms, particularly to discriminate
between photothermal and photochemical ablation.
1.4.1 Nitrocellulose Film: Laser-induced decomposition
and accompanying ablation dynamics of a reactive nitrocellulose ﬁlm was investigated by doping a copper­phthalocyanine derivative as a light absorber.51 Their chemical
structures are given in Figure 10. While nitrocellulose does
not absorb 351 nm XeF excimer laser pulse, the ﬁlm heatedinstantaneously via rapid photothermal conversion in the doped
copper­phthalocyanine derivative molecule. This rapid heatingprocess in nanosecond excitation is well elucidated spectroscopically for phthalocyanine solid ﬁlm.52 Below the ablationthreshold, thermal expansion and contraction processes ofnitrocellulose ﬁlm were directly followed by the nanosecond­
nanometer interferometry. The dynamic interferometric observation is summarized in Figure 11. Above the ablation threshold, the expansion of the ﬁlm was started during the excimer
laser irradiation, and then explosive decomposition was initiated, continuing for a few hundreds ns after the excitation.
Temperature elevation caused by the excimer laser irradiation results in an exothermic decomposition of nitrocellulose,

(c)

(d)

Figure 10. Chemical structures of (a) nitrocellulose (left)
and savinyl blue (right), (b) triazeno polymer, (c) polyurethane (left) and phenol resin (right), and (d) polyimide.

Figure 11. Nanosecond interferometric images of nitrocellulose/savinyl blue/polyurethane ﬁlm at 110 mJ cm¹2.
The bar in the ﬁgure represents 1 mm. Schematic illustration of expansion and etching at each stage is given above
or below the images. This ﬁgure is partly reproduced from
Figure 6 of Ref. 51. Copyright 2013 American Chemical
Society.

H. Masuhara

Figure 12. Illustration for combustion of materials: Solidsurface is decomposed and vaporized, and the surface
draws back. This ﬁgure is reproduced from Chart 2 ofRef. 51. Copyright 2013 American Chemical Society.

leading to a further heating of the ﬁlm. Consequently selfacceleration of the reaction is enhanced and an explosive selfsustaining decomposition is induced after reaching the explosive decomposition condition. This is indeed characteristic of
this ﬁlm which is brought about by intense laser irradiation andschematically illustrated in Figure 12.
1.4.2 Toriazene Polymer Film: The dynamic behaviorof photoetching process of a photosensitive traizenopolymer
ﬁlm was studied during and after 351 nm laser irradiation thenanosecond­nanometer interferometry.44 Its chemical structure
is given in Figure 10. At a ﬂuence of 250 mJ cm¹2, a slightswelling of the ﬁlm and darkening of the irradiated surface
was initially observed, and then the etching process of the ﬁlm
was brought about around the peak time of the excitation
laser pulse. The etching proceeded during laser irradiation andstopped almost at the end of the excimer laser pulse. On theother hand, at a ﬂuence of 60 mJ cm¹2, the etching started fromnearly the end of the excimer laser pulse and the etchingprocess continued until 140 ns. When the laser ﬂuence is highenough, photodecomposition occurs densely leading to sudden
ablation, so that thermal degradation cannot be observed.
On the other hand, at 60 mJ cm¹2 the thermal process was
successfully measured as photodecomposition is stopped. Since
the decomposition of the triazenopolymer is an exothermic
reaction, it is considered that photodecomposition of thepolymer results in the temperature elevation, causing thermal
degradation.
1.4.3 Polyurethane Film: A polyurethane sample ﬁlm was
prepared by adding phenol resin as a structural stabilizer anddynamics of laser-induced morphological change was investigated upon 248 nm excitation.35 Chemical structures areshown in Figure 10. The ablation threshold by 248 nm excitation was determined to be 40 mJ cm¹2. Below the ablationthreshold an expansion forms a bump and followed by rapid
contraction, that is, expansion disappeared at about 1 ¯s. Abovethe ablation threshold interferometric images were not disturbed by the decomposed products, which indicates that theﬁlm was decomposed rapidly into debris, fragments, aggregates, and so on. They should be smaller than visible light

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)763

wavelength, as images are clear and not disturbed. Indeed noappreciable debris was left. It is also noticeable that transient
expansion was not observed. At the ﬂuence of 310 mJ cm¹2,
etching proceeds fast during the initial half of the excitation
laser pulse, but stopped at the late stage of the pulse. The
morphological behavior is quite diﬀerent from that of photothermal expansion and contraction dynamics characteristic ofPMMA. Hence it is considered that laser ablation is induced
photochemically.
1.4.4 Polyimide Film: Intense excimer laser excitation ofpolyimide ﬁlm gives transient expansion and the following
contraction, permanent swelling, and ablation, depending on
excitation wavelength.48 The chemical structure of the polymer
is given in Figure 10. Ablation by 248 nm excitation takes
place only during the excitation laser pulse and no debris wasobserved, while ablation by 351 nm excitation is brought about
after expansion. Permanent swelling was observed only for
351 nm excitation. Expansion and contraction dynamics at low
ﬂuence of 351 nm excitation is consistent with the simulationof temperature rise. This means that photothermal mechanism
surely holds for the 351 nm excitation. At 248 nm excitation,on the other hand, a hump in expansion and contraction proﬁlebecomes clear. We explained this dynamics in terms of photochemical formation and recombination/leakage of decomposed
small molecules. On the basis of the morphological dynamics,
we considered that photochemical and photothermal mechanisms mostly explain the results obtained by 248 and 351 nm
excitation, respectively.
2. Femtosecond Laser Ablation Dynamics and Mechanism
Femtosecond laser ablation has been receiving much attention as a fabrication method since the etched surface is generally smoother compared to nanosecond ablation. This suggeststhat the ablation takes place involving less thermal eﬀect. The
advantages and disadvantages had been discussed mainly basedon observation of etched surface morphology, but information
on ablation dynamics by applying time-resolved spectroscopy
and imaging was and is still limited. Plasma emission upon
ablation was reported frequently, which is ascribed to thefact that emission spectral measurement is relatively simple andeasy compared to UV­visible absorption, IR absorption, andRaman spectroscopy. As a consequence it is roughly recognizedthat femtosecond ablation is due to plasma formation. By
studying dynamics and mechanism of nanosecond laser ablation
as summarized above, however, we had come to the consideration that molecular photophysical and photochemical primaryprocesses should hold for femtosecond ablation of organic
materials and be just modiﬁed because of very intense laser
excitation. We expected that similar spectroscopic and imaging
approaches which we had applied to nanosecond ablation
studies would be useful for elucidating femtosecond ablation
dynamics and mechanism. Furthermore it was reasonable toassume that optical measurements in early time range are easierthan in nanosecond one as materials ejection is always delayedcompared to photochemical and photophysical processes. Thus
our strategy was to apply various time-resolved measurement
methods for elucidating ablation the phenomena depending
on femtosecond, nanosecond, ¯s, and later time ranges, whichis schematically shown in Figure 13.

764

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Figure 13. Our strategy for elucidating laser ablation by direct measurements with time-resolved spectroscopy and imaging.

2.1 Surface Roughening Dynamics by Femtosecond
Surface Light Scattering Imaging. In the framework of
Jablonski diagram, photoexcitation energy generates excited
singlet and triplet states which are followed by energy, electron,and proton, or hydrogen atom transfers. These processes canbe directly probed by emission and absorption spectroscopic
methods. The plasma state which is sometimes considered to bea key step for ablation is also identiﬁed by its broad emission
in the visible region. On the other hand, ablation include
various phenomena such as melting, expansion, vaporization,gas bubbling, fragmentation, nm and ¯m particle formation,and so on are monitored by time-resolved imaging methods.
Time-resolved photography which is called shadowgraphy is
generally applied, but its spatial detection limit is of course
larger than the wavelength, namely, a few hundreds nm.
Expansion and contraction of polymer ﬁlms can be followedwith 10 ns and 10 nm resolutions by our interferometry as
described above, however, the moving polymer surface should
be enough ﬂat over the lateral direction of a few mm. It was
not easy to follow initial dynamics of fragmentation as far
as we knew. When we started femtosecond ablation study, it
was considered that nanometer surface morphological change
is one of the early events following electronic excitation andrelaxation in laser ablation. This surface dynamics becamemonitored by developing time-resolved surface light scattering
imaging, whose advantage was demonstrated ﬁrst for laser
ablation of liquid benzene53 and amorphous copper­phthalocyanine.54 Here we summarize experimental setup, example,
and how to get information on surface morphology change by
showing data on copper­phthalocyanine ﬁlms.
The schematic diagram of our surface light scattering imaging system is given Figure 14. The 800 nm light of a regenerated ampliﬁed femtosecond Ti:sapphire laser was divided
into two pulses; one was used as an excitation light andthe other for generating femtosecond white light continuum

extending from 400 to 1000 nm. The excitation and probe
pulses are almost normally incident on the ﬁlm, while thescattered light from the latter was detected at an angle of50 « 18°, and recorded with a CCD camera. The observation
wavelength region adopted was 535 « 17 nm. The origin of
the time axis was determined by measuring transient absorption spectra of amorphous copper­phthalocyanine ﬁlms, whichmeasurement is described later. All the measurements weredone for a fresh surface of the ﬁlm.
As typical examples, the scattering images of multicrystalline copper­phthalocyanine ﬁlm upon laser ablation are shownin Figure 15. At 62 mJ cm¹2, which is larger than the thresholdﬂuence by 1.5 times, the surface scattering from the central
part of the irradiated area was observed, which is consistent
with the Gaussian-like laser beam proﬁle. It is worth noting thatno appreciable change was detected in the picosecond time
region, while its intensity grew in a few ns. In general two
possible origins are examined; one is Rayleigh scattering due
to vaporized copper­phthalocyanine and/or its nanoparticles,
and the other is ascribed to nonspecular reﬂection from theroughened surface. As Rayleigh light scattering is usually very
weak, it is expected that the diﬀusely reﬂected light fromthe roughened surface gives the present scattering images. The
scattered intensity was practically correlated to the roughnessof the surface and we evaluated quantitatively the increase in
the roughness. Namely we prepared some copper­phthalocyanine ﬁlms with diﬀerent roughness by applying spincoating and casting, and their surface morphology was evaluated by AFM observation and the average surface roughness
was obtained. The same ﬁlms were probed by the femtosecond
surface light scattering imaging system without femtosecond
excitation. Then we could get a correlation curve between the
average surface roughness and the scattering intensity. This is
our new approach which has never been reported as far as weknow. The evolution of the surface roughness of microcrystal-H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

765

Figure 14. Femtosecond surface light scattering imaging system and sample images.

Figure 15. Sequential surface light scattering imaging of phthalocyanine ﬁlm as a function of delay time upon laser ablation. Brightspots outside the excited area are due to protrusions during ﬁlm casting.

line copper­phthalocyanine was estimated as in Figure 16. It
is extremely interesting to see that the surface roughening isbrought about in ns and even tens ns, although the excitation
was done by the 100 fs pulse.55,56 This induction time is very
meaningful for elucidating ablation mechanism as discussed
below.
2.2 Time-Resolved Electronic Absorption Spectroscopy.
As shown in nanosecond laser ablation of doped poly(methyl
methacrylate) ﬁlms, initially optical conditions of the irradiatedsample ﬁlms are not much disturbed and optically clear, which
is ﬁt to absorption spectroscopy. Indeed we succeeded inmeasuring femtosecond transient absorption spectra of copper­
phthalocyanine ﬁlm upon intense excitation.55­57 Typical
spectra below and above the threshold are given in Figure 17.
A broad positive band was observed around 520 nm andassigned to electronically excited state of copper­phthalocyanine, namely exciton in this case, while the negative peaks
at late delay times are due to spectral change of the groundstate absorption. The exciton decay depended upon excitation

intensity and was in the time domain of 10 ps, and replacedby the absorption having negative and positive peaks. It was
indeed a turning point of our research when we found thesimilarity between the spectral at the late stage and the temperature diﬀerence spectra. We set the microcrystalline ﬁlm
at the sample holder for the femtosecond absorption spectral
measurement and measured absorption spectra without excitation, but by changing the sample temperature. Thus we
obtained the temperature diﬀerence spectra of the ground stateby the femtosecond setup. The transient spectrum observed at a
few ns delay time and the temperature diﬀerence spectrum are
similar to each other, except in the wavelength region above
700 nm. In the latter region some dimer peak is located, and its
contribution to the exciton and ground state hot bands are
considered not to be identical. Neglecting this contribution, we
could estimate the local and transient temperature by comparing the transient spectra with temperature diﬀerence spectra. At
the ﬂuence of 100 mJ cm¹2, temperature elevation was estimated to be 250 K and at the ablation threshold the temperatureBull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTSRRMS /nm

766

Time/ps

Figure 16. Temporal evolution of surface roughening (Rrms) of phthalocyanine ﬁlm upon laser ablation analyzed by femtosecond
surface light scattering imaging.

Wavelength/nm

Wavelength/nm
Figure 17. Femtosecond pump-fs probe absorption spectra
at 0.5 ps by changing laser ﬂuence (A) and at the laser
ﬂuence of 64 (solid line) and 1.5 mJ cm¹2 (broken line)(B). Delay times are given in the ﬁgure. This ﬁgure is
reproduced from Figure 6 of Ref. 57. Copyright 2013
Elsevier.

jump was 100 K. These values of temperature are too low to
induce melting, decomposition, and rapid vaporization, but
it should be noticed that the temperature elevation speed isextremely fast; 1013 K s¹1.
2.3 Transient Pressure Mechanism for Femtosecond
Laser Ablation. On the basis of femtosecond imaging andtransient absorption spectral measurements, it was concluded
under the examined conditions that electronic excitation energy
is converted to molecular and lattice vibrational energies inabout 20 ps, and the attained temperature is not high at theablation threshold (room temperature + 100 K). It is known
that the sublimation starts at 380 °C and decomposition reactionis brought about at 550 °C. Thus, photothermal mechanism
cannot explain the present femtosecond laser ablation. We
noticed that we should pay our attention to the time scale ofphotothermal conversion of 20 ps which was clariﬁed directly
by transient absorption spectroscopy. On the other hand, it isdescribed above that nanometer surface morphological changes
were not observed until 1 ns after the 100 fs excitation. Namely
there is some time lag between the local and transient heating(ca. 20 ps) and the morphological change (ca. 1 ns) as illustrated in Figure 18, which is very important to understand thefemtosecond laser ablation mechanism.56 During this lag time,
vigorous molecular motions lead to pressure increase in every
direction so that a mechanical stress should be accumulated
in the ﬁlm.54 Thermal conduction is relatively slow in organic
materials, so that the time lag is kept to be appreciable. Since
the surrounding of the excited area is cold and the bottom isa quartz substrate, the stress can be released only by splitting
the irradiated ﬁlms into two layers. Here an example ismulticrystalline copper­phthalocyanine ﬁlm which is composed of nanocrystals where they associate with each other due
to van der Waals force. Consequently, the upper layer wasejected while the lower layer remained on the substrate. Thus
the mechanism is called “transient pressure mechanism,” whichis consistent with molecular dynamics simulation on laser
ablation by Zhigilei and Garrison.58
This mechanism is exclusively applied to femtosecond
ablation and does not hold for nanosecond one. During thenanosecond excitation mutual interactions between excitons
take place and enhance molecular and lattice vibrations, andH. Masuhara

Figure 18. The temporal evolution of photothermal processes and morphological change when (A) fs, (B) ps, and(C) ns pulses were applied as the excitation pulse. Thisﬁgure is reproduced from Figure 12 of Ref. 57. Copyright2013 Elsevier.

their motions are eﬃciently transferred to the surroundings inthis time range. Namely some gradient of molecular and lattice
vibrations from the irradiated area to the outside is formed andtransient pressure is not raised. Therefore photothermal heating
mechanism is applicable, which was conﬁrmed directly just by
replacing the 100 fs pulse of the Ti:sapphire laser to 100 ns
pulse of the same laser for the same copper­phthalocyanineﬁlms.56 Without any doubt the nanosecond ablation mechanism of the copper­phthalocyanine ﬁlm is consistent with themechanism of nanosecond mechanism discussed above forpolymer ﬁlms.
3. Laser Ablation in Solution for Nanoparticle Preparation
Laser ablation has been applied for fabrication of metals,
silicone, and related semiconductors materials, and theirfabricated structures still possess original properties.2 On theother hand organic materials, cells, tissues, and so on are
considered to involve chemical decomposition upon ablation,
resulting in large changes not only in physical and chemical
properties but also in morphology. Our mechanistic studies
described above, however, indicate that conventional photophysical and photochemical understanding of component
molecules can explain laser ablation dynamics. Nanosecondand femtosecond laser ablation is interpreted well in terms ofphotothermal heating and transient pressure eﬀects, respectively. Consequently, as a possible mechanism we can exclude
plasma state, where materials are ionized and many free electrons are produced, leading to complex dissociation processes

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)767

and giving small molecules, radical, and even atoms. We know
in some cases how we can avoid chemical decomposition
accompanying laser ablation, on which we have developed
some new methodologies for organic matters. Laser ablation oforganic materials should be a potential fabrication method toprepared nanometer-sized materials and structures, and here wesummarize nanoparticle preparation as a typical example.
Most of laser ablation studies are conducted for surface andsurface layer of materials in air and in vacuum, and resultant
fabricated surfaces and prepared structures have not beenstudied. On the other hand, the fragmented and ejectedmaterials were usually deemed to be dusts, and their morphology, structure, and property were not examined in detail.
Actually it is not easy to collect small ejected fragments
eﬃciently in air and vacuum. If we catch the ejected materials
and characterize them, a new approach toward small nanomaterials formation is made possible. Along this idea we proposedsolution laser ablation method to prepare organic nanoparticles
in inert solution.59 This method is based laser ablation of
microcrystals in solvent. High intensity laser pulse irradiationinduced fragmentation of the surface layers of the microcrystals
and small ejected particles are dispersed in solvent, giving
stable colloidal solution. Some results of quinacridone red
dye in water are given in Figure 19, where color change upon
ablation and the corresponding absorption spectral changes
are presented. The particles are usually charged to some extent,
so that they are dispersed stably without additives likedetergent, polymer, and so on. This method is simple and easy
as it does not need big facility and complex chemical processes.
Furthermore the method is enabling us to control nanoparticle
size, shape, crystal structure, and morphology by adjustinglaser parameters and chemical conditions.60­74
3.1 Nanoparticle Preparation of Dye Molecules. Here,
we describe our nanoparticles preparation of four dye compounds, whose chemical structure and abbreviated names are
depicted in Figure 20, using nanosecond YAG laser as an
excitation light source.73 Their micrometer-sized crystalline
powders were added into a poor solvent where no appreciabledissolution of those molecules was detected. Here the dyeconcentration was about 5 © 10¹3 wt % by applying sonication,and the solvent composition was water/ethanol (7:1) for BY,
water/ethanol (39:1) for BP and DR, and just distilled water for
QA and AlPc. For all examined dye crystals, most of the initial
microcrystalline powder sank dawn on the bottom of a cuvettebefore laser irradiation, and its supernatant was almost colorless. This means that dye molecules are hardly dissolved in thecorresponding poor solvent, since all compounds have strong
absorption in the visible wavelength region. The microcrystals
were dispersed in poor solvents by stirring the mixture
vigorously with a magnetic stirrer, and then exposed to thethird harmonic of nanosecond YAG laser (355 nm, 7 ns, 10 Hz).
The morphology of initial microcrystals and formed nanoparticles were examined with a scanning electron microscope(SEM), absorption spectra of the supernatants prior to irradiation and of the obtained colloidal solutions were measured
with a spectrophotometer. The latter measurement due to color
changes was even conducted from time to time during processing, which gives information of how nanoparticle preparation evolves. By irradiating the dispersed solution at the laser

768

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Wavelength/nm

Figure 19. Chemical structure of quinacridone, and its solution color and absorption spectral changes upon solution ablation.

Figure 20. Chemical structures and abbreviation of dye compounds used for nanoparticle preparation.

ﬂuence above the laser ablation threshold, the supernatantbecame colored and transparent by laser irradiation. Figure 21
shows the absorption spectra before and at 1 h after 5 min-laser
excitation for QA, BY, AlPc, BP, and DR. The color solutions exhibited the characteristic absorption spectra of parent
molecule, while they are more or less modiﬁed compared withthe spectra of molecules fully dissolved in organic solvents. It
is considered that dye molecules were dispersed in their poor
solvents as their nanoparticles, because the solutions werecolored and transparent with no tail in the visible wavelength
region due to light scattering. The color of the colloidal
solutions was observed for longer than a few weeks, although
it graduallys became pale due to their slow precipitation of
nanoparticles.

To conﬁrm nanoparticle formation more directly, SEM
observation of the nanoparticles was performed, whose representative examples of AlPc, BP, and DR are given in Figure 22.
It is interesting to see that small particles of the size of lessthan 100 nm are prepared from microcrystals just by laser pulse
irradiation. In Table 1, the mean size and its distribution foreach compound, and the thresholds of nanoparticle formation
are listed. The mean size is ranged from 40 to 60 nm andindependent of each dye, although their chemical structures andspectroscopic properties are quite diﬀerent from each other.
We also demonstrated similar solution ablation method byfemtosecond laser and reported that femtosecond ablation gives
smaller nanoparticle than nanosecond one. The size was 13 and17 nm for QA and vanadyl phthalocyanine (VOPc), respec-

H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

769

Figure 21. Solution color and absorption spectral changes of AlPc, BP, and DR.

size/nmsize/nmsize/nm

Figure 22. SEM observation of fabricated nanoparticles of AlPc, BP, and DR, and their size distribution.

tively, by using the femtosecond laser (780 nm, 200 fs FWHM)
as an irradiation light source.62,72 How nanoparticle size
depends on laser pulse was conﬁrmed for quinacridone in
Figure 23. These experimental results indicate that the formed
particle size is regulated by the pulse width of the excitation
laser, which is applicable to various dye molecules.
We consider that nanoparticle formation mechanism is thesame to ablation mechanism of polymer and dye ﬁlms which
we explained above. Laser-induced fragmentation of microcrystals should be due to photothermal laser ablation of organic
solids in the case of nanosecond excitation. The excited states

are density formed in the irradiated area and undergo eﬃcientmutual interactions leading to nonradiative relaxation. As thisannihilation process is so rapid and much faster than original
ﬂuorescence decay process, the diﬀerence in ﬂuorescence
lifetime between the dyes is not reﬂected in the nanoparticle
size. Temperature jump at local irradiated area of crystals leads
to fragmentation into small pieces, while temperature elevationof the area and ejected nanoparticles should be balanced byenergy dissipation to solvent. We assumed that laser ablation
is dependent on solvent cooling ability, which should becharacteristic of solution laser ablation. Actually we conﬁrmed

770

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Table 1. Mean Size and Dispersibility of Dye Nanoparticles Fabricated by Solution Ablation with
Its Ablation Threshold (See text)

Size/nm

Figure 23. Size distribution of fabricated QA nanoparticles.

experimentally that the size of produced nanoparticle becomes
smaller with the decrease in thermal conduction of thesolvent.61 Also the cooling rate for small particles should befaster than for large one because of their large surface-tovolume ratio. We consider that fragmentation proceeds until
the temperature of the irradiated and fragmented materials iscooled down to the temperature at which thermal ablation
does not occur anymore. The minimum particle size is thus
determined by absorbed laser energy and dissipation to solvent.
It is worth noting that femtosecond laser ablation in solution
gives very small nanoparticles compared to the nanosecond
excitation. This means that our method of laser ablation insolution enable us to form the smallest nanoparticles as aso-called bottom-up method.72 It is reasonable to say that the
femtosecond mechanism in solution is quite diﬀerent from thatof nanosecond ablation, so we consider that transient pressure
mechanism is responsible to give the smallest organic nanoparticle of phthalocyanine dyes.
3.2 Applications of Organic Nanoparticles Prepared by
Laser Ablation in Solution. Many applications are considered for those organic nanoparticles and here we describe someexamples demonstrated in our laser laboratory. SEM picturesof fullerene microcrystals and its nanoparticles prepared uponsolution ablation are shown in Figure 24 with their size
distribution. By changing irradiation condition of nanosecond
laser, the concentration of the nanoparticle in solution can becontrolled and the accompanying color change was clearlyobserved. The prepared solution is referred as a standard
nanoparticle solution, as it is highly concentrated and very
stable over for a few years without any additives.70
More direct and eﬃcient application of the nanoparticles
fabrication by solution ablation should be drug preparation.
Usually the compounds designed and synthesized for medical
use are complex and unstable, and sometimes they are notdissolved in aqueous solution. By adding detergents or bymodifying chemical structures, the target compounds are
dispersed under physiological aqueous conditions, but thoseresults in some toxic eﬀect and/or reduction in their originalfunctionality. Our ablation method is just ﬁt to such drug
application, as we can disperse the target molecules as nanoparticles in water without any additives. Our preliminary trial
was done for irinotecan hydrochloride for large intestine andlung cancers, drug for osteroarthritis,74 myocardial infraction
drug, and so on. The nanoparticles of these drugs are alsostably dispersed in water, which is very useful for the next step
biological examination for developing medicines.
The other application is to fabricate thin ﬁlms from nanoparticles prepared by laser ablation. The fabricated nanoparticles can be regarded as an important building block toconstruct organic nanostructures for photonic and electronic
devises.67 Usually organic molecules and their self assembling
structures, vacuum deposited thin ﬁlms, and LB ﬁlms were
examined as building blocks. These are now quite conventional
materials, but the prepared ﬁlms are always suﬀering fromthe intrinsic defects, distribution of grain size, and so on.
We prepared QA nanoparticles by the ablation method andthen formed their ﬁlms by utilizing electrophoretic method.66
Fortunately QA nanoparticles are charged to be minus, which is
the reason why the nanoparticles are dissolved in water without
detergents, so they are attracted on to a plus-charged electrodegiving ﬂat ﬁlm. These processes are schematically shown in
Figure 25. It is interesting to see that size-, shape-, and phasecontrolled nanoparticles prepared are used as a new buildingH. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

771

Figure 24. SEM observation of fullerene nanocrystals before and after the laser ablation, and their size distribution.

Figure 25. Experimental setup and illustration of electrophoretic deposition of QA nanoparticles.

block and transferred to very homogeneous ﬁlms with new
functionality.
4. Crystal Growth in Solution by
Femtosecond Laser Ablation
Laser ablation of molecular crystal in solution leaves its
etched surface and gives nanoparticles, and both can be nuclei
for crystallization if solution of the molecules is supersaturated.
This possibility was demonstrated for the ﬁrst time for aqueous
solution of urea crystal.75 When its single crystal prepared
spontaneously by conventional way was irradiated by focusing

a single 800 nm femtosecond pulse, a quite novel phenomenonof crystal growth was observed. As shown in Figure 26, aneedle-like crystal grew almost normally against to the mothercrystal and its growth rate was estimated to be 100 ¯m s¹1. Its
needle shape is similar to that of the mother crystal which
is characteristic of urea crystal. The subsequently prepared
crystals tended to grow perpendicularly to the mother crystal,and the orientation of needle axis to the subsequent daughter
crystal did not depend on polarization of the laser pulse. At
the higher energy irradiation, several crystals grew near theirradiation area. The number of the daughters increased with

772

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Figure 26. Subsequent crystal growth of urea from the irradiated area marked with an open circle by a single shot of (a) an
800 nm femtosecond laser pulse at 0.12 ¯J/pulse through a 10© objective lens (NA = 0.4), (b) an 800 nm pulse at 0.030 ¯J/pulse
through a 20© objective lens (NA = 0.46). This ﬁgure is partly reproduced from Figure 1 of Ref. 75. Copyright 2013 American
Chemical Society.

both the laser energy and the number of the pulse. These
results are consistent with laser ablation behavior of molecular
crystals. Upon every shot of laser pulse, ablation gives fragments and their number increases with laser ﬂuence.
Etching and fragmentation of urea crystal are the originof the crystal growth, which is strongly supported by AFM
observation. The ablation of a urea crystal was carried out in air
on the glass substrate and the obtained pictures are shownin Figure 27, where a hole etched in the crystal and fragments
ejected on to the glass substrate are clearly identiﬁed. At higher
laser ﬂuence, many fragments with the size ranging from a
few tens nm to a few ¯m were observed, and they spatiallydispersed on the substrate. These fragmentations are considered
to be caused by the transient pressure eﬀect as explained in
Sections 2 and 3, so that destructive thermal eﬀect should be
suppressed in this femtosecond ablation.
The present laser-induced crystal growth is unconventionaland considered useful to form complex structures of crystals insupersaturated solution. The sequential irradiation gives something a set of blocks as shown in Figure 28. Another idea is
to use this phenomenon to generate a new single crystal frommulticrystalline or amorphous precipitates which are unfortunately formed. Sometimes proteins and newly synthesizedcompound are very expensive and/or diﬃcult to get themagain, so that repetitive trials to get single crystals should bevery important, indeed, we succeeded in forming a new single
crystal of lysozyme by ablation the mother crystal in saturated
solution by a single shot femtosecond pulse.76
5. Multiphoton Laser Ablation of
Solution Giving Local Impulsive ForceMultiphoton laser ablation is easily induced by focusing
femtosecond laser pulse into water, leading to bubbling, which
phenomenon should be started at the focal point, pushing
the surrounding water toward its outside. This can cause local
and transient pressure, which is similar to that in femtosecond

Figure 27. AFM images of the ablated fragments on a glass
substrate (right) and the corresponding etched area of aurea single crystal (left) by single shot irradiation of an
800 nm laser pulse with (a) 0.031 and (b) 0.056 ¯J/pulse
through a 20© objective lens. Each image shows a 10 ©
10 ¯m2 region. This ﬁgure is reproduced from Figure 4
of Ref. 75. Copyright 2013 American Chemical Society.

laser ablation mechanism as discussed. Consequently shockwave is generated and propagates with high speed and local
convection ﬂow surrounding the focal point is induced. As
a result mechanical force and water ﬂow are loaded on small
objects located surrounding the laser focus. This force isgenerated locally and temporarily in the three-dimensional
space in solution, so that it is called local impulsive force. Then

H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

the bubbles shrink, which is accompanied with local counterconvection ﬂow. These multiphoton laser excitation behaviors
are well known and have received much attention particularly for the biomedical subjects.13 Most of the femtosecond
laser applications, however, were carried out by focusing
the femtosecond pulse to target materials, living cell, and so on.

Figure 28. (a) Crystal patterning procedure for urea. The
ﬁrst pulse was irradiated to the open circle area numbered
1, the second pulse was to the circle 2, and so on. A single
shot of the 800 nm femtosecond laser pulse with 0.12 ¯J/pulse was irradiated through a 10© objective lens. (b) The
ladder-like spatial pattern of urea crystals obtained bysuccessive single shot laser irradiation of the ﬁve open
circle areas each under the same irradiation conditions.
This ﬁgure is reproduced from Figure 5 of Ref. 75.
Copyright 2013 American Chemical Society.

(a)

773

In our case we do not irradiate the objects, but focus thepulse near the objects, and then the generated local impulsive
force is used for their manipulation and processing.77 Thisnew method has an advantage that the target is not excited,no photophysical and no photochemical process is induced,
and no damage is given. This has opened new ways in application of femtosecond laser ablation to manipulation andprocessing.
One application of the local impulsive force is to transfer
a microparticle with the diameter of 90 ¯m on a substrate insolution.78 The femtosecond laser was scanned around themicroparticle by using the scanning mirror to form a trapping
circle, as shown in Figure 29. The trapping circle radius was
set to 150 ¯m and the scanning frequency was tuned to 10
cycles/s. The laser pulse energy was adjusted to 5 ¯J and its
repetition rate was set to 100 Hz, so that one cycle would
consist of ten laser shots. A representative experimental resultis shown in Figure 30, where we shifted the microscopestage. Small stains on a substrate moved clearly, but thetarget bead still remained in the center area of the trapping
circle. Thus, it is obvious that the particle was trapped in thecircle by the local impulsive forces from the surroundings. It
is worth noting that a microparticle adhered to the substrate
could be detached and moved easily. This method is applicableeven to large microparticles which cannot be manipulated byconventional optical tweezers because of their limited trapping
force.
We remember our former work on laser manipulation of ¯msized metal particles by a focused CW laser beam.79 This beamexerts repulsive force upon optically reﬂective metal particles.
As we are the ﬁrst group to combine laser tweezers and beamscanning technique using Galvano mirror, we came to the
proposal that a single metal particle should be surrounded bya circle locus formed by a scanning laser beam. As the force
generates repulsive force, the metal particle was conﬁned by
the repulsive potential. This former idea is reproduced as a
new manipulation method due to repulsive force, although its
physical origin is diﬀerent.

(b)Figure 29. (a) Experimental setup for trapping and manipulation. The femtosecond laser beam was focused by a 10© objective.
Laser focal point was controlled by scanning mirror and objective or L2 lens. (b) A trapping circle was formed by scanning the
femtosecond laser beam around the target. Sample was moved by controlling the motor-driven stage. This ﬁgure is reproduced from
Figure 1 of Ref. 78. Copyright 2013 American Institute of Physics.

774

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

6. Crystallization of Molecules and Proteins by
Femtosecond Laser-Induced Local Impulsive Force

(a)

(b)

(c)

(d)

(e)

(f)

Figure 30. Trapping and manipulation of a single beadin water by the intense femtosecond laser irradiation. (a)Trapping is started by scanning the laser along the dotted
circle; (b) the bead moves to the center of the trapping
circle; (c)­(e) the bead is transferred laterally when thestage is driven, which is conﬁrmed by the movement of
the stains in the background; (f ) the bead escapes fromthe trapping circle when the femtosecond laser irradiationis stopped. This ﬁgure is reproduced from Figure 2 ofRef. 78. Copyright 2013 American Institute of Physics.

The mechanical force induced by the impulsive force wasestimated by analyzing the movement of the model bead, as
shown in Figure 31. It is worth noting that the force exerted on
the bead was estimated to be 1 ¯N order and is extremely larger
than that by the conventional laser trapping force (usually form
pN to fN range). Quite recently Hosokawa et al. reports theprecise determination of this mechanical force by utilizing
AFM tip as a force sensor.80,81 This femtosecond laser-induced
force is transiently loaded and applicable only when the
femtosecond laser was introduced, while the conventional laser
trapping force by CW laser irradiation is constantly exerted.
Another advantage of the laser-induced local impulsive force isthat the target is not irradiated directly as mentioned above. The
focal point is usually set to the position which is about 10 ¯m
far from the target. We can say that we have now two laser
manipulation methods of conventional laser tweezers and thepresent manipulation by local impulsive force and can choose
one of them or combine them depending upon a target and its
condition.

Laser-induced impulsive force is considered to be an eﬀective stimulation to form nucleus in the supersaturated solutionsof molecules and proteins. The propagating shockwave may
increase local concentration temporarily and the local convection assists mass transfer giving higher local concentration.
As a result, the system initially set in the meta-stable state
may be shifted to crystallization area in the phase diagram(Figure 32) by the impulsive force and the convection. Thisidea explains the femtosecond laser-induced crystallization
which we succeeded for the ﬁrst time in 2002.82 The molecule
was lysozyme, a standard protein, and this femtosecond laserinduced crystallization is now extended to various kinds ofmolecules and proteins.83­90 One typical example is on the
800 nm femtosecond laser irradiation of aqueous solution of
egg-white lysozyme.83,84 A similar experimental result wasalso reported by Watanabe et al.91 Lysozyme crystallization
is not diﬃcult, but more eﬃcient crystallization giving better
quality compared to the conventional method was demonstrated. We considered that the bubbling induced by multiphoton
laser ablation is responsible to lysozyme crystallization. In
Figure 33, it is shown how crystallization proceeds on the
bubble surface generated by the femtosecond laser irradiation.
Yoshikawa et al. examined eﬀects of laser pulse energy andwidth, and reported that the threshold values of crystallizationand the bubbling are similar to each other.92 This indicates thatthe crystallization is started by the bubbling. As both laser
bubbling accompanied with the impulsive force and crystallization are complex phenomena involving various processes,
such an approach clarifying responsible parameters (in thiscase, the threshold) is very promising and is useful as the initialstage of research.
The questions we have received most frequently are onwhether multiphoton excitation of the protein is involved ornot. Namely photochemical reaction leading to less soluble
species possibly triggers nucleation. Indeed Okutsu alreadyreported interesting results, showing that photochemicallyformed tryptophan dimer is responsible to protein crystallization.93 To make clear this issue we conducted an irradiation
experiment shown in Figure 34. A single droplet of lysozyme
aqueous solution was covered by paraﬃn oil and set on a
microscope stage. The femtosecond laser pulse irradiatedparaﬃn oil or aqueous solution with high intensity. In bothcases bubble formation was observed and then some precipitants were identiﬁed. Namely laser-induced local impulsive
force results in aggregation of proteins. In this experiment we
set the condition to see visible ¯m-sized aggregates immediately after the irradiation, although crystals come at least after
leaving the irradiated solution for a few hrs. Intense femtosecond laser pulse was introduced to the oil phase, while we
conﬁrmed formation of several bubbles not only in oil but alsoin protein solution. In several seconds the precipitant appearedin the aqueous solution. As the aqueous solution is not irradiated, this experiment clearly indicates that nondirect irradiation
mechanism contributes to the crystallization. We consider thatthe shockwave propagates over the interface between both
solutions in a droplet, forms bubbles, and triggers nucleation.

H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

775

Figure 31. Optical setup, schematic illustration, and microphotographs of a single polystyrene nanosphere which was transferred by
the local impulsive force.

Crystallization

By the local impulsive force
Initial stage

Figure 32. Schematic illustration of femtosecond laserinduced crystallization in phase diagram of crystallization.

A more direct conﬁrmation of dynamics of this femtosecond
laser-induced crystallization was demonstrated for simple solution system; anthracene in cyclohexane.89 Its crystallizationis very easy and crystal growth is fast, so that it is possible tomonitor the growing within 1 s. A single shot femtosecond
laser pulse with a pulse energy higher than above 3.1 ¯J/pulse
was irradiated into a suﬃciently supersaturated solution, andthen we found that anthracene crystallization was induced
at the vicinity of the laser focal point immediately after theirradiation. The threshold of crystallization was in agreementwith that of the laser-induced bubbling. The evolution of thecrystal is pictured as in Figure 35, where a bending ﬁlm-likecrystal grew and changed to a normal shape. The bending form

strongly suggests that the nucleation and initial growth arestarted at the curved bubble surface, although the bubble
disappeared already before 100 ms after the irradiation.
To conﬁrm visually the role of bubbling in femtosecond
laser crystallization, we show the spatial relation between
bubbling and crystallization in viscous solution of hen eggwhite lysozyme.90 By adding poly(ethylene glycol) the viscosity is increased, so that we expect that the formed bubbles
would not disappear soon and not diﬀuse out from theirradiated areas during the period necessary for crystal growth
to visible size. Actually 1 day after the irradiation the lysozyme crystals were associated with bubbles and not so many
crystals were found far from the bubbles. For an example, we
could observe a long-lasting bubble with an initial diameterof 120 ¯m. It shrunk to 80 ¯m in 1 h and interestingly lysozyme
crystals appeared at the surface of the bubble. This may also
indicate that the surface is a preferential ﬁeld for the crystallization. The long-lasting bubbles might be composed ofgaseous molecules produced by photodissociation and photodegradation of solute and solvent due to multiphoton absorption at the focal point. In the present supersaturated solution,
lysozyme and its clusters should be adsorbed at the surface andtheir ordering would evolve to nucleation, and furthermore thenuclei underwent to growth processes at the surface.
Similar approaches were adopted for clarifying crystallization processes of thaumatin protein. Yoshikawa et al. studied its
femtosecond laser-induced crystallization by adding agarosegel. The gel enables us to monitor the crystal growth processfrom the focal point in the time range from ¯s to days. Crystals
and bubbles were observed in the same area, which is ascribedto the suppressed diﬀusion of nuclei and bubbles. They also
examined supersaturated lysozyme solution containing ﬂuo-

776

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Figure 33. Time evolution of the HEWL crystallization at the surface of the long-lasting bubble created by a single scan irradiation
with pulse energy of 30.0 mJ/pulse. These photographs were taken at (a) 0, (b) 1, (c) 3, and (d) 12 h, respectively. This ﬁgure is
reproduced from Figure 3 of Ref. 90. Copyright 2013 Elsevier.

Figure 34. Lysozyme aggregation in water upon femtosecond laser-induced babbling in paraﬃn oil.

rescent tetramethylrhodamine-5-isothiocyanate-labeled lysozyme by time-resolved ﬂuorescence imaging. Initially a brightspot due to f5 laser-induced break-down was observed and the
bubble underwent shrinking. Again a bright domain appeared
around the focal point at 20 ¯s after irradiation, which wasinterpreted as highly concentrated lysozyme. Namely, theshrinking of the bubble causes a local increase of the protein
concentration, which is surely one possible reason of eﬃcient
nucleation.94
This femtosecond laser irradiation is useful to induce
crystallization even under low saturation degree. Murai et al.
demonstrated that the use of a gel solution with agaroseenhanced nucleation and produced egg white lysozyme crystals
at 3 to 5 times lower saturation compared to the experiments
without agarose and of spontaneous crystallization (withoutlaser irradiation) with agarose.95 Also their ﬂuorescence imag-

ing of the labeled lysozyme revealed the cavitation bubbles
formed the high concentration region at the focal point, which
may trigger the nucleation similarly discussed above. The
generated high concentration region remains longer in the
agarose system, so the probability of nucleation should be
increased. The present femtosecond laser crystallization method is improved by combining it with the solution-stirring
technique,96 and indeed this result has received much attention
as a successful crystallization of membrane protein.97
7. Manipulation and Processing of Living Cells byFemtosecond Laser-Induced Local Impulsive Force
The femtosecond laser-induced impulsive force accompanying bubbling has high potential as a tool for managing single
living cells in solution. We have devoted our eﬀorts to explore
new bubbling-induced behavior of living cells, on which weH. Masuhara

Figure 35. Microscopic images of generated anthracene
crystals upon single shot femtosecond irradiation withenergy of 6.7 ¯J/pulse (a) and its growth process of abending ﬁlm-like crystal generated at the surface of a large
laser-induced bubble formed at a pulse energy of 16.5 ¯J/pulse (b­d). Right side illustrations represent generated
crystals in left side photographs. This ﬁgure is reproduced
from Figure 3 of Ref. 89. Copyright 2013 American
Chemical Society.

have developed new manipulation methods.98­111 As the laser
is irradiated near the target, the living cells receive impulsive
force but are not irradiated directly by the laser pulse. Also
the intense force is loaded on the cells only transiently. These
characteristics are complementary to laser tweezers where weakand constant force is exerted.
Here we start this text by introducing nondestructive
isolation of single cultured animal cells.98 Mouse NIH3T3
ﬁbroblasts were cultured on a collagen matrix and its glass
substrate with the matrix was placed upside down on an
inverted microscope. Therefore a sample cell consists of theglass substrate with the matrix set upside down, solution, and abottom glass substrate as illustrated in the right-upper illustration in Figure 36. The femtosecond laser was introduced
through the bottom glass. Their ﬁlopodia were ﬁrst cut by

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

777

convention femtosecond direct irradiation, namely by laser
ablation, because cells are attached so strong that they cannot
be removed by the impulsive force. Then the femtosecond
laser was focused at a position about 10 ¯m from the matrix.
The cells were detached individually from the matrix by theimpulsive force and the laser bubbling. After that the cells fell
down to the bottom through the solution and regenerated theirﬁlopodia on the bottom substrate, coming back to the original
morphologies. It was estimated that the necessary force torealize this detachment was a few N ¯m¹2.
One important requirement for developing this femtosecond
manipulation method as a general tool is to examine whether
cell functions such as cell division, cell diﬀerentiation, cell
death, ad cell migration are damaged by the laser bubblingor not. The above detachment and regeneration is concernedwith cell death, and actually we showed that, when NIH3T3
cells were detached from a collagen coated substrate, 80% of
the cells adhered again on another substrate.99 This percentage
is comparable or higher than that of cells after reseedingby trypsin treatment. Another trial which we introduce here
is to elucidate eﬀects of the impulsive force on animal cell
diﬀerentiation using PC12 cells derived from rat pheochromocytoma. A schematic illustration of our experimental procedure100 is given in Figure 36. When nerve growth factor isinteracted with PC12 cells, they are partially diﬀerentiated into
neuron like-cells with neuritis. The diﬀerentiated and undiﬀerentiated PC12 cells were selectively isolated by the bubbling
and their morphological changes after falling onto the bottom
substrate were compared. Upon isolation their neuritis wereonce contracted and some of them gradually regenerated day byday. When diﬀerentiated cells were isolated, the percentage ofthe diﬀerentiated cells with neurites at 6 h after isolation was
about 3.3 times higher than that when undiﬀerentiated ones
were isolated. This result was comparable again with a control
experiment where the isolation was carried out with trypsin
treatment.
In view of cell division we studied dynamics of yeast ﬁssion
cells and conﬁrmed that the laser-induced local impulsive
force does not damage their liability. Furthermore we applied
this force to assist their ﬁssion. A similar enhancement of cell
activity was demonstrated for cultured mammalian cells suchas HeLa, PC12, P19CL6, and C2C12. They are stimulated by
the impulsive force due to laser bubbling and their cell growth
was examined. In comparison with the control condition, cell
growth was enhanced for the cells of C2C12 and P19CL6,
undergoing diﬀerentiation to myocytes. On the other handa suppression eﬀect was observed for the cell lines of PC12
and HeLa. These results suggest that cell growth is possibly
enhanced for myogenic cells.101
Furthermore we have examined an eﬀect of the impulsive
force on cell migration, using melanocyte and keratinocyte inhuman skin tissue on a fabricated surface. The directionalmigration of melanocyte toward keratinocyte was directly
observed, although the cells were arranged by the femtosecond
impulsive force. This means that these cells were not negatively aﬀected by loading the force.102 Thus we consider that
our laser manipulation method utilizing the local impulsive
force is recognized to be useful not only from the viewpointof laser fabrication but also as keeping cell functions during

778

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

Figure 36. This ﬁgure is reproduced from Figure 1 of Ref. 100. Copyright 2013 Springer Publishing Co.

manipulation. To examine why the functions are well maintained during the removal, we cultured animal cells on asubstrate which was coated a cell scaﬀold such as collagen.
The cells were connected to the scaﬀold through cell adhesion
molecules such as integrins and grown. As a conventional
detaching method, trypsin was used and the detachment eﬀect
was compared with the result by the present femtosecond laser
method. It is well known that the cell adhesion molecules
are chemically digested where trypsin works as an enzyme.
We studied both detachment methods in the single cell level byvisualizing the scaﬀold with bright emission of quantum dots.
When the cell was removed from the substrate by the laser
bubbling, the emission simultaneously disappeared from thesubstrate. This means that the cell is detached together withthe scaﬀold. On the other hand, only cells fell down and the
scaﬀold showing the emission remained in the initial substrate
upon the trypsin treatment. Thus we considered in the case oflaser irradiation that the cell functions are well kept as they are
surrounded by the scaﬀold.
The detachment technique was applied for various cells in
diﬀerent experiments. For example, the laser impulsive force
was loaded to a substrate where PC12, HeLa, or normal humanastrocyte (NHA) cells and the removed cells were translocatedonto microfabricated cell-adhesive domains. The domains wereprepared to be surrounded by cell-repellent surface, so that thepatterning is well performed. By combining thus prepared cellpatterns and additional femtosecond in situ fabrication, migration of HeLa or NHA cells along the fabricated channels was

examined. This approach will be useful not only to study thedeveloping networks of neuronal, glial, and capillary cells, butalso to the quantitative analysis of nerve function.103
The detachment by the impulsive force and accompanying
bubbling was soon developed to a micropatterning methodof living cells in solution. The mouse NIH3T3 ﬁbroblast cells
were detached from an upper substrate by the laser impulsive
force and transferred to an underlying bottom substrate, which
was fully conducted in water. By scanning the laser beam,consequently, shifting the bubbling, we can remove the cells
along laser-scanned lines and curves, and transfer them to thetarget substrate, giving the similar lines and curves. The spatial
resolution is not in the nanometer range, as laser-induced
impulsive force is caused about 10 ¯m far from the cell and thediﬀusion from the source to target substrates are involved. The
spatial resolution was estimated less than 80 ¯m in a full width
at the half maximum. About 80% of the patterned cells wereconﬁrmed to be alive at 3 h after the process. It is consideredthat this technique is soft for biological materials as no
exposure to the air is necessary. As a similar patterning method,
inkjet printing, micro-contact printing, and laser-induced
forward transfer in which a liquid droplet containing cells isejected from a source substrate to target one by UV laser
irradiation, are well known, but cells are more or less aﬀectedby air.99 Our in situ patterning method is of course applied toother soft micrometer-sized materials in solution.104,105
The mechanical impulsive force is now applied to injectpolystyrene nanoparticles in single living cells.106 The nano-H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

779

Figure 37. Transmission (left) and confocal ﬂuorescence (right) images of a NIH3T3 cell at 10 min (a) before and (b) after adding
polystyrene nanoparticles into the culture medium. The irradiation of 20 femtosecond laser pulses with focal spot size of 1 ¯m werecarried out at the laser focal point. Nanoparticles were absorbed on/in the cell membrane and their number increased with time,
which may be due to electrostatic attraction between membrane (minus charge) and particles (plus charge). This ﬁgure is
reproduced from Figure 3 of Ref. 106. Copyright 2013 Springer Publishing Co.

particles which are not soon absorbed are distributed on aliving cell in solution and the femtosecond laser is focused near
the cell, but not directly irradiated. A mouse ﬁbroblast NIH3T3
cell was used, and the size of the focal point was 1 ¯m and the
distance between the focal point and the edge of the cell was
20 ¯m. When the femtosecond laser was hit, the cell shrunk alittle, suggesting that its membrane structure should be changed
at least transiently. The force exerted on the cell was estimatedto be a few tens fN ¯m¹2, under which power the cell was notdetached. The results are summarized in Figure 37.
The focal point was located in the top-right side of the cell.
The bright spots observed at the right-upper and left-lower
sides can be ascribed to the ﬂuorescent polymer nanoparticles
which we tried to introduce into a single cell. The confocal
ﬂuorescence image clearly shows, in addition to the particles
on the stained membrane, that some particles are transferred
into the inside of the cell and located close to the bottom but
not on the bottom. Some bright spots look long shaped, suggesting the aggregation of the nanoparticles. The particle on thewhite lines (cursors) seems single, and its cross view along thez-axis is compared with those of the membrane stained withﬂuorescent dyes in Figure 37. Fluorescence intensity distribution of the stained dye has two peaks in the vertical proﬁle,corresponding to the bottom and top surface membranes of
the cell. On the other hand the ﬂuorescence from the bead islocated just between them, clearly indicating that the nanoparticles are injected into the cell.
8. Summary and Perspective
Initially laser ablation study of molecular systems wasstarted from polymer ﬁlms, as it received much attention as a
key technology for future electronics. One of the mechanistic
issues was concerned with plasma formation, as broad emission
overlapped with sharp ﬂuorescence peaks of diatomic radicals

and so on were usually observed. However, we considered thatprimary processes of laser ablation of molecular systems could
be well explained in the framework of Jablonski diagram,developed various time-resolved imaging methods, and appliedthem together with time-resolved spectroscopy to polymer anddye ﬁlms. It was proved that morphological changes evolving
to fragmentation and ejection do not take place immediately
after excitation and spectroscopic measurements of primaryprocesses can be carried out as in conventional spectroscopy.
Even under ablation condition we could identify excited
singlet, triplet, and ionic states of chromophores which absorb
excitation laser and conﬁrmed that their dynamics are modiﬁed
to large extent by high intensity excitation eﬀects. Further
plasma emission was demonstrated as the succeeding processes
characteristic of laser ﬂuence far higher than the ablationthreshold.
Laser ablation is typical of molecular solids excited with highintensity laser pulse. Not only photophysical and photochemical
dynamics but also the resultant morphological changes such as
expansion and contraction, surface roughening, fragmentation
to nanoparticles, and their ejection are sequentially induced, thatis indeed solid-state photochemistry. Therefore our approaches
integrating all the results obtained by time-resolved spectroscopic and imaging methods are very eﬀective and we could
clarify that photothermal, photochemical, or photomechanicaldynamics is responsible to laser ablation depending on molecules and excitation conditions. Photothermal mechanism due tocyclic multiphotonic absorption for nanosecond laser ablationand transient pressure mechanism for femtosecond ablation
was proposed and conﬁrmed by us. Here we did not mention
our systematic studies on laser ablation of neat liquid benzene
and its derivatives, but it is worth noting that photophysical,
photochemical, and photothermal processes induced by laser
irradiation are directly correlated to morphological dynam-

780

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

ics.53,112­121 The successful progresses in dynamics and mechanistic studies on laser ablation of molecular systems enabled
us to develop unique methodologies which are seminal andapplicable to material- and bio-sciences.
Molecular crystals in water are fragmented and ejected upon
their laser ablation, and produced nanoparticles are dispersed
well giving stable colloids. Their size usually ranges around a
few tens nm, but it can be controlled by laser ﬂuence, pulse
width, repetition rate, wavelength, solvent, and temperature.
The particle size of phthalocyanine dye fabricated by femtosecond laser ablation was 10 nm which is smallest as nanoparticles prepared by the top-down-method. The solution ablation method for nanoparticle preparation is contributing toemerging ﬁelds of nanoparticle science and technology,122 as
it is convenient in laboratory scale experiments.
When femtosecond multiphoton ablation of water, solution,and culture media for living cells is induced, impulsive force isgenerated due to bubble formation, shockwave propagation, andconvection ﬂow. The force is a very eﬃcient perturbation for
nucleation in supersaturated solution of molecules and proteins.123­125 Our approach on femtosecond laser crystallization
is now successfully developed and applied to more complex
proteins.126­128 The impulsive force can be loaded on individual
cells by irradiating femtosecond laser at the position of about
10 ¯m from the edge of cells. This means that the cells are
not photoexcited and never photochemically damaged. Laser
manipulation with the local impulsive force is very useful andapplied to determine quantitatively cell­cell interaction force.81
As a related method laser tweezers are well known, where thelater trapping force is constantly loaded and its amplitude isin the fN­pN order. The impulsive force is transiently loaded
and it reaches ¯N. Thus we consider that our manipulation due
to femtosecond laser is complementary to the manipulation bylaser trapping. In other words both manipulation methods are
selectively utilized or integrated together to achieve complex
manipulation depending on the biological targets.
In this Accounts we summarize our time-resolved spectroscopic and imaging studies of laser ablation, which convinces
us that laser ablation is not a peculiar phenomenon but oneof molecular photoprocesses under high intensity excitation.
For further developments in the studies on laser ablation ofmolecules we plan to conduct the following subjects. 1) Simultaneous spectroscopic measurement and imaging to reveal how
multiphoton excitation evolves to generate the local impulsive force. 2) Femtosecond electronic absorption spectroscopy
of protein and amino acid solutions under femtosecond laser
crystallization condition. 3) Separately we have been studying
laser trapping crystallization by using CW laser,129 where
nucleation and the following crystal growth cannot be observed
separately. By combining the present femtosecond crystallization with laser trapping crystallization, we will be able to control
necessary concentration for nucleation and crystal growth and toadjust timing of nucleation independently. 4) The manipulationof living cells by the local impulsive force can be combinedwith laser ablation by direct irradiation, for example, protein
patterning is achieved by the latter and the cells are transferredto the ablated area by the former. This approach in physiologicalaqueous solutions is believed promising for constructing futurecell-based devise. As conclusion molecular photophysical andphotochemical nature of laser ablation is being elucidated,which is a base for further developing new useful methodologies for material- and bio-sciences.
This study was partly supported by Japan Society for the
Promotion of Science, Japan Science and Technology Agency,
and Ministry of Education, Culture, Sports, Science andTechnology Japan, and is now by MOE-ATU (Ministry of
Education Taiwan - Aiming for Top University) Project of
National Chiao Tung University, National Science Council
Taiwan (No. NSC100-2113-M-009-001), and Foundation of
the Achievement for Outstanding Scholarship Taiwan. The
author thanks many colleagues and students whose names areincluded in the cited papers in the references, particularly toProf. Hiroshi Fukumura (Tohoku University), Prof. Tsuyoshi
Asahi (Ehime University), Prof. Yoichiroh Hosokawa (NaraInstitute of Science and Technology), Dr. Teruki Sugiyama(Instrument Technology Research Center, Taiwan), and Dr.
Kazunori Okano (National Chiao Tung University, Taiwan).
Thanks are also due to Prof. Masahiro Irie (Rikkyo University)
for his continued encouragement since 1969. The preparationof this manuscript is supported by Dr. Teruki Sugiyama.
References# Dedicated to the late Prof. Noboru Mataga for his
encouraging and stimulating the author by his research style.
1 R. Srinivasan, W. J. Leigh, J. Am. Chem. Soc. 1982, 104,6784.
2 D. Bäuerle, Laser Processing and Chemistry, Springer,
Berlin, 2000. doi:10.1007/978-3-642-17613-5.
3 T. Lippert, J. T. Dickinson, Chem. Rev. 2003, 103, 453.
4 T. Lippert, M. Hauer, C. R. Phipps, A. Wokaun, Appl. Phys.
A: Mater. Sci. Process. 2003, 77, 259.
5 R. Srinivasan, B. Braren, Chem. Rev. 1989, 89, 1303.
6 G. Paltauf, P. E. Dyer, Chem. Rev. 2003, 103, 487.
7 A. Vogel, V. Venugopalan, Chem. Rev. 2003, 103, 577.
8 D. D. Dlott, S. Hambir, J. Franken, J. Phys. Chem. B 1998,
102, 2121.
9 N. Bityurin, N. Arnold, B. Luk’yanchuk, D. Bäuerle, Appl.
Surf. Sci. 1998, 127­129, 164.
10 N. Arnold, N. Bityurin, Appl. Phys. A: Mater. Sci. Process.
1999, 68, 615.
11 B. Luk’yanchuk, N. Bityurin, M. Himmelbauer, N. Arnold,
Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 122, 347.
12 N. Arnold, B. Luk’yanchuk, N. Bityurin, Appl. Surf. Sci.
1998, 127­129, 184.
13 N. Bityurin, A. Malyshev, J. Appl. Phys. 2002, 92, 605.
14 S. Georgiou, A. Koubenakis, Chem. Rev. 2003, 103, 349.
15 H. Masuhara, H. Hiraoka, E. E. Martinero, Chem. Phys.
Lett. 1987, 135, 103.
16 H. Masuhara, H. Hiraoka, K. Domen, Macromolecules
1987, 20, 450.
17 H. Hiraoka, T. J. Chuang, H. Masuhara, J. Vac. Sci.
Technol., B 1988, 6, 463.
18 H. Fukumura, K. Hamano, H. Masuhara, Chem. Lett. 1993,
245.
19 H. Fukumura, K. Hamano, H. Masuhara, J. Phys. Chem.
1993, 97, 12110.
20 H. Fukumura, N. Mibuka, S. Eura, H. Masuhara, N. Nishi,
J. Phys. Chem. 1993, 97, 13761.

H. Masuhara
21 H. Masuhara, H. Hiraoka, K. Domen, IBM Res. J. 1986,
5385, 55174_1.
22 H. Fukumura, H. Masuhara, Chem. Phys. Lett. 1994, 221,373.
23 H. Fujiwara, T. Hayashi, H. Fukumura, H. Masuhara, Appl.
Phys. Lett. 1994, 64, 2451.
24 H. Furutani, H. Fukumura, H. Masuhara, Appl. Phys. Lett.
1994, 65, 3413.
25 H. Fukumura, E.-i. Takahashi, H. Masuhara, J. Phys. Chem.
1995, 99, 750.
26 H. Fujiwara, Y. Nakajima, H. Fukumura, H. Masuhara,
J. Phys. Chem. 1995, 99, 11481.
27 H. Fujiwara, H. Fukumura, H. Masuhara, J. Phys. Chem.
1995, 99, 11844.
28 H. Furutani, H. Fukumura, H. Masuhara, Rev. Laser Eng.
1997, 25, 288.
29 M. Hosoda, H. Furutani, H. Fukumura, H. Masuhara,
M. Nishii, N. Ichinose, S. Kawanishi, Rev. Laser Eng. 1997, 25,306.
30 H. Masuhara, K. Sasaki, H. Fukumura, H. Furutani, Analyst
1998, 123, 531.
31 H. Fujiwara, H. Fukumoto, H. Fukumura, H. Masuhara,Res. Chem. Intermed. 1998, 24, 879.
32 T. Mito, T. Masubuchi, T. Tada, H. Fukumura, H.
Masuhara, J. Photosci. 1999, 6, 109.
33 T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara,
ChemPhysChem 2000, 1, 137.
34 T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara,
J. Phys. Chem. B 2001, 105, 2518.
35 T. Masubuchi, H. Fukumura, H. Masuhara, K. Suzuki, N.
Hayashi, J. Photochem. Photobiol., A 2001, 145, 215.
36 T. Mito, H. Masuhara, Appl. Surf. Sci. 2002, 197­198, 796.
37 T. Tada, T. Asahi, H. Masuhara, M. Tsuchimori, O.
Watanabe, J. Photosci. 2003, 10, 97.
38 H. Fujiwara, H. Ishii, T. Ishiwata, T. Hayashi, H.
Fukumura, H. Masuhara, Bull. Chem. Soc. Jpn. 2003, 76, 1075.
39 T. Tada, T. Asahi, M. Tsuchimori, O. Watanabe, H.
Masuhara, Jpn. J. Appl. Phys. 2004, 43, 5337.
40 T. Tada, T. Asahi, M. Tsuchimori, O. Watanabe, H.
Masuhara, J. Nonlinear Opt. Phys. Mater. 2004, 13, 373.
41 H. Furutani, H. Fukumura, H. Masuhara, J. Phys. Chem.
1996, 100, 6871.
42 L. Bennett, T. Lippert, H. Furutani, H. Fukumura, H.
Masuhara, Appl. Phys. A: Mater. Sci. Process. 1996, 63, 327.
43 T. K. Lippert, L. S. Bennett, T. Kunz, C. Hahn, A. J.
Wokaun, H. Furutani, H. Fukumura, H. M. Masuhara, T.
Nakamura, A. Yabe, Proc. SPIE 1997, 2992, 135.
44 H. Furutani, H. Fukumura, H. Masuhara, T. Lippert, A.
Yabe, J. Phys. Chem. A 1997, 101, 5742.
45 T. Lippert, J. T. Dickinson, S. C. Langford, H. Furutani, H.
Fukumura, H. Masuhara, T. Kunz, A. Wokaun, Appl. Surf. Sci.
1998, 127­129, 117.
46 T. Mito, T. Tsujita, H. Masuhara, N. Hayashi, K. Suzuki,
Jpn. J. Appl. Phys. 2001, 40, L805.
47 T. Lippert, C. David, M. Hauer, T. Masubuchi, H.
Masuhara, K. Nomura, O. Nuyken, C. Phipps, J. Robert, T.
Tada, K. Tomita, A. Wokaun, Appl. Surf. Sci. 2002, 186, 14.
48 T. Masubuchi, T. Tada, E. Nomura, K. Hatanaka, H.
Fukumura, H. Masuhara, J. Phys. Chem. A 2002, 106, 2180.
49 T. Lippert, J. T. Dickinson, M. Hauer, G. Kopitkovas, S. C.
Langford, H. Masuhara, O. Nuyken, J. Robert, H. Salmio, T. Tada,
K. Tomita, A. Wokaun, Appl. Surf. Sci. 2002, 197­198, 746.

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

781

50 P. E. Dyer, J. Sidhu, J. Appl. Phys. 1988, 64, 4657.
51 H. Furutani, H. Fukumura, H. Masuhara, S. Kambara, T.
Kitaguchi, H. Tsukada, T. Ozawa, J. Phys. Chem. B 1998, 102,3395.
52 H. Van Mingroot, L. Viaene, M. Van der Auweraer, F. C.
De Schryver, M. Ichikawa, H. Fukumura, H. Masuhara, J. Phys.
Chem. 1995, 99, 17174.
53 K. Hatanaka, T. Itoh, T. Asahi, N. Ichinose, S. Kawanishi,
T. Sasuga, H. Fukumura, H. Masuhara, Appl. Phys. Lett. 1998, 73,3498.
54 Y. Hosokawa, T. Asahi, H. Masuhara, Rev. Laser Eng.
2001, 29, 710.
55 Y. Hosokawa, M. Yashiro, T. Asahi, H. Fukumura, H.
Masuhara, Appl. Surf. Sci. 2000, 154­155, 192.
56 Y. Hosokawa, M. Yashiro, T. Asahi, H. M. Masuhara, Proc.
SPIE 2001, 4274, 78.
57 Y. Hosokawa, M. Yashiro, T. Asahi, H. Masuhara,
J. Photochem. Photobiol., A 2001, 142, 197.
58 L. V. Zhigilei, B. J. Garrison, Appl. Phys. A: Mater. Sci.
Process. 1999, 69 (Suppl.), S75.
59 Y. Tamaki, T. Asahi, H. Masuhara, Appl. Surf. Sci. 2000,
168, 85.
60 Y. Tamaki, T. Asahi, H. Masuhara, J. Phys. Chem. A 2002,
106, 2135.
61 Y. Tamaki, T. Asahi, H. Masuhara, Jpn. J. Appl. Phys.
2003, 42, 2725.
62 T. Sugiyama, T. Asahi, H. Masuhara, Chem. Lett. 2004, 33,724.
63 T. Asahi, K. Yuyama, T. Sugiyama, H. Masuhara, Rev.
Laser Eng. 2005, 33, 41.
64 T. Sugiyama, T. Asahi, H. Takeuchi, H. Masuhara, Jpn. J.
Appl. Phys. 2006, 45, 384.
65 H.-G. Jeon, T. Sugiyama, H. Masuhara, T. Asahi, Jpn. J.
Appl. Phys. 2007, 46, L733.
66 H.-G. Jeon, S.-i. Ryo, T. Sugiyama, I. Oh, H. Masuhara, T.
Asahi, Chem. Lett. 2007, 36, 1160.
67 H.-G. Jeon, T. Sugiyama, H. Masuhara, T. Asahi, J. Phys.
Chem. C 2007, 111, 14658.
68 R. Yasukuni, T. Asahi, T. Sugiyama, H. Masuhara, M.
Sliwa, J. Hofkens, F. C. De Schryver, M. Van der Auweraer, A.
Herrmann, K. Müllen, Appl. Phys. A: Mater. Sci. Process. 2008,
93, 5.
69 T. Asahi, T. Sugiyama, H. Masuhara, Acc. Chem. Res.
2008, 41, 1790.
70 T. Sugiyama, S.-i. Ryo, I. Oh, T. Asahi, H. Masuhara,
J. Photochem. Photobiol., A 2009, 207, 7.
71 K. Yuyama, T. Sugiyama, T. Asahi, S. Ryo, I. Oh, H.
Masuhara, Appl. Phys. A: Mater. Sci. Process. 2010, 101, 591.
72 T. Sugiyama, T. Asahi, Chem. Rec. 2011, 11, 54.
73 T. Sugiyama, H. Takeuchi, T. Asahi, H. Y. Yoshikawa, Y.
Hosokawa, H. Masuhara, Laser Fabrication of Nanoparticles andCrystals in Solution, Proc. PICALO (Paciﬁc Int’l Conf. Appl.
Laser & Optics), 2008, pp. 807­812.
74 D. Ikegami, T. Iwai, S. Ryo, N. Gu, T. Sugiyama, I. Oh,
H. Yoshikawa, N. Tsumaki, Osteoarthritis Cartilage 2011, 19,233.
75 H. Y. Yoshikawa, Y. Hosokawa, H. Masuhara, Cryst.
Growth Des. 2006, 6, 302.
76 H. Y. Yoshikawa, Y. Hosokawa, R. Murai, G. Sazaki, T.
Kitatani, H. Adachi, T. Inoue, H. Matsumura, K. Takano, S.
Murakami, S. Nakabayashi, Y. Mori, H. Masuhara, Cryst. GrowthDes. 2012, 12, 4334.

782

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

AWARD ACCOUNTS

77 Y. Hosokawa, J.-i. Takabayashi, C. Shukunami, Y. Hiraki,
H. Masuhara, Rev. Laser Eng. 2004, 32, 94.
78 Y. Jiang, Y. Matsumoto, Y. Hosokawa, H. Masuhara, I. Oh,
Appl. Phys. Lett. 2007, 90, 061107.
79 K. Sasaki, M. Koshioka, H. Misawa, N. Kitamura, H.
Masuhara, Appl. Phys. Lett. 1992, 60, 807.
80 T. Iino, Y. Hosokawa, Appl. Phys. Express 2010, 3, 107002.
81 Y. Hosokawa, M. Hagiyama, T. Iino, Y. Murakami, A. Ito,
Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 1777.
82 H. Adachi, Y. Hosokawa, K. Takano, F. Tsunesada, H.
Masuhara, M. Yoshimura, Y. Mori, T. Sasaki, J. Jpn. Assoc. Cryst.
Growth 2002, 29, 445.
83 H. Adachi, K. Takano, Y. Hosokawa, T. Inoue, Y. Mori, H.
Matsumura, M. Yoshimura, Y. Tsunaka, M. Morikawa, S. Kanaya,
H. Masuhara, Y. Kai, T. Sasaki, Jpn. J. Appl. Phys. 2003, 42, L798.
84 H. Adachi, Y. Hosokawa, H. Masuhara, M. Yoshimura, Y.
Mori, T. Sasaki, Rev. Laser Eng. 2004, 32, 84.
85 Y. Hosokawa, H. Adachi, M. Yoshimura, Y. Mori, T.
Sasaki, H. Masuhara, Cryst. Growth Des. 2005, 5, 861.
86 M. Kashii, H. Kitano, Y. Hosokawa, H. Adachi, Y. Mori,
T. Sasaki, H. Masuhara, K. Takano, H. Matsumura, T. Inoue, S.
Murakami, K. Sugamoto, H. Yoshikawa, Jpn. J. Appl. Phys. 2005,
44, L873.
87 H. Y. Yoshikawa, Y. Hosokawa, H. Masuhara, Jpn. J. Appl.
Phys. 2006, 45, L23.
88 Y. Takahashi, H. Adachi, T. Taniuchi, M. Takagi, Y.
Hosokawa, S. Onzuka, S. Brahadeeswaran, M. Yoshimura, Y.
Mori, H. Masuhara, T. Sasaki, H. Nakanishi, J. Photochem.
Photobiol., A 2006, 183, 247.
89 K. Nakamura, Y. Hosokawa, H. Masuhara, Cryst. GrowthDes. 2007, 7, 885.
90 K. Nakamura, Y. Sora, H. Y. Yoshikawa, Y. Hosokawa, R.
Murai, H. Adachi, Y. Mori, T. Sasaki, H. Masuhara, Appl. Surf.
Sci. 2007, 253, 6425.
91 S. Watanabe, S. Nagasaka, K. Noda, H. Tashiro, Jpn. J.
Appl. Phys. 2004, 43, L941.
92 H. Y. Yoshikawa, R. Murai, S. Maki, T. Kitatani, S.
Sugiyama, G. Sazaki, H. Adachi, T. Inoue, H. Matsumura, K.
Takano, S. Murakami, T. Sasaki, Y. Mori, Appl. Phys. A: Mater.
Sci. Process. 2008, 93, 911.
93 T. Okutsu, K. Furuta, M. Terao, H. Hiratsuka, A. Yamano,
N. Ferté, S. Veesler, Cryst. Growth Des. 2005, 5, 1393.
94 H. Y. Yoshikawa, R. Murai, S. Sugiyama, G. Sazaki, T.
Kitatani, Y. Takahashi, H. Adachi, H. Matsumura, S. Murakami, T.
Inoue, K. Takano, Y. Mori, J. Cryst. Growth 2009, 311, 956.
95 R. Murai, H. Y. Yoshikawa, Y. Takahashi, M. Maruyama,
S. Sugiyama, G. Sazaki, H. Adachi, K. Takano, H. Matsumura,
S. Murakami, T. Inoue, Y. Mori, Appl. Phys. Lett. 2010, 96,043702.
96 H. Adachi, A. Niino, S. Murakami, K. Takano, H.
Matsumura, T. Kinoshita, M. Warizaya, T. Inoue, Y. Mori, T.
Sasaki, Jpn. J. Appl. Phys. 2005, 44, 1365.
97 H. Adachi, S. Murakami, A. Niino, H. Matsumura, K.
Takano, T. Inoue, Y. Mori, A. Yamaguchi, T. Sasaki, Jpn. J. Appl.
Phys. 2004, 43, L1376.
98 Y. Hosokawa, H. Takabayashi, S. Miura, C. Shukunami, Y.
Hiraki, H. Masuhara, Appl. Phys. A: Mater. Sci. Process. 2004, 79,795.
99 T. Kaji, S. Ito, H. Miyasaka, Y. Hosokawa, H. Masuhara,
C. Shukunami, Y. Hiraki, Appl. Phys. Lett. 2007, 91, 023904.
100 Y. Maezawa, K. Okano, M. Matsubara, H. Masuhara, Y.
Hosokawa, Biomed. Microdevices 2011, 13, 117.

101 Y.-E. Kuo, C.-C. Wu, Y. Hosokawa, Y. Maezawa, K.
Okano, H. Masuhara, F.-J. Kao, Appl. Phys. A: Mater. Sci. Process.
2010, 101, 597.
102 Y. Maezawa, Ph.D. Thesis, Nara Institute of Science andTechnology, 2011.
103 K. Okano, D. Yu, A. Matsui, Y. Maezawa, Y. Hosokawa,
A. Kira, M. Matsubara, I. Liau, H. Tsubokawa, H. Masuhara,
ChemBioChem 2011, 12, 795.
104 Y. Hosokawa, T. Kaji, Y. Hiraki, H. Mori, H. Masuhara,
Proc. SPIE 2006, 6108, 610805.
105 Y. Hosokawa, T. Kaji, C. Shukunami, Y. Hiraki, E. Kotani,
H. Mori, H. Masuhara, Biomed. Microdevices 2007, 9, 105.
106 A. Yamaguchi, Y. Hosokawa, G. Louit, T. Asahi, C.
Shukunami, Y. Hiraki, H. Masuhara, Appl. Phys. A: Mater. Sci.
Process. 2008, 93, 39.
107 Y. Hosokawa, R. Yasukuni, T. Kaji, A. Yamaguchi, S.
Iguchi, T. Asahi, H. Masuhara, Rev. Laser Eng. 2007, 35, 430.
108 N. Takizawa, K. Okano, T. Uwada, Y. Hosokawa, H.
Masuhara, Proc. SPIE 2008, 6854, 685411.
109 Y. Hosokawa, Y. Jiang, I. Oh, N. Takizawa, T. Uwada, K.
Okano, H. Masuhara, Proc. SPIE 2008, 6854, 68541K.
110 K. Okano, N. Takizawa, T. Uwada, Y. Hosokawa, H.
Masuhara, Proc. SPIE 2008, 6854, 685414.
111 Y. Maezawa, Y. Hosokawa, K. Okano, M. Matsubara, H.
Masuhara, Appl. Phys. A: Mater. Sci. Process. 2010, 101, 127.
112 Y. Tsuboi, H. Fukumura, H. Masuhara, Appl. Phys. Lett.
1994, 64, 2745.
113 Y. Tsuboi, K. Hatanaka, H. Fukumura, H. Masuhara,
J. Phys. Chem. 1994, 98, 11237.
114 Y. Tsuboi, H. Masuhara, Rev. Laser Eng. 1995, 23, 2.
115 Y. Tsuboi, H. Masuhara, Rev. Laser Eng. 1995, 23, 9.
116 Y. Tsuboi, H. Fukumura, H. Masuhara, J. Phys. Chem.
1995, 99, 10305.
117 K. Hatanaka, M. Kawao, Y. Tsuboi, H. Fukumura, H.
Masuhara, J. Appl. Phys. 1997, 82, 5799.
118 Y. Tsuboi, K. Hatanaka, H. Fukumura, H. Masuhara,
J. Phys. Chem. A 1998, 102, 1661.
119 K. Hatanaka, T. Itoh, T. Asahi, N. Ichinose, S. Kawanishi,
T. Sasuga, H. Fukumura, H. Masuhara, Chem. Phys. Lett. 1999,
300, 727.
120 K. Hatanaka, T. Itoh, T. Asahi, N. Ichinose, S. Kawanishi,
T. Sasuga, H. Fukumura, H. Masuhara, J. Phys. Chem. A 1999,
103, 11257.
121 K. Hatanaka, Y. Tsuboi, H. Fukumura, H. Masuhara,
J. Phys. Chem. B 2002, 106, 3049.
122 Single Organic Nanoparticles, ed. by H. Masuhara,
H. Nakanishi, K. Sasaki, Springer, Berlin, 2002. doi:10.1007/
978-3-642-55545-9.
123 Nano Biophotonics, Science and Technology, ed. by
H. Masuhara, S. Kawata, F. Tokunaga, Elsevier, Amsterdam,
2007.
124 Nanophotonics: Integrating Photochemistry, Optics andNano/Bio Materials Studies, ed. by H. Masuhara, S. Kawata,
Elsevier, Amsterdam, 2004.
125 Molecular Nano Dynamics, ed. by H. Fukumura, M. Irie, Y.
Iwasawa, H. Masuhara, K. Uosaki, Wiely-VCH Verlag GmbH &Co., Weinheim, 2009, Vols. 1 and 2.
126 T. Tsukazaki, H. Mori, S. Fukai, T. Numata, A. Perederina,
H. Adachi, H. Matsumura, K. Takano, S. Murakami, T. Inoue,
Y. Mori, T. Sasaki, D. G. Vassylyev, O. Nureki, K. Ito, Acta
Crystallogr., Sect. F: Struct. Biol. Cryst. Commun. 2006, 62, 376.
127 T. Kinoshita, I. Nakanishi, T. Terasaka, M. Kuno, N. Seki,

H. Masuhara

Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)

M. Warizaya, H. Matsumura, T. Inoue, K. Takano, H. Adachi, Y.
Mori, T. Fujii, Biochemistry 2005, 44, 10562.
128 T. Numata, Y. Ikeuchi, S. Fukai, H. Adachi, H. Matsumura,
K. Takano, S. Murakami, T. Inoue, Y. Mori, T. Sasaki, T. Suzuki,

783

O. Nureki, Acta Crystallogr., Sect. F: Struct. Biol. Cryst. Commun.
2006, 62, 368.
129 T. Sugiyama, K.-i. Yuyama, H. Masuhara, Acc. Chem. Res.
2012, 45, 1946.

Hiroshi Masuhara graduated from Tohoku University (1966) in Sendai and obtained Ph.D. degreefrom Osaka University (1971). He is a physical chemist working in multidisciplinary areas indepartments of chemistry (Tohoku University), synthetic chemistry (Osaka University), polymer
science and engineering (Kyoto Institute of Technology), applied physics (Osaka University),
frontier bioscience (Osaka University), life science (Hamano Foundation), and materials science(Nara Institute of Science and Technology). In 2008 he joined Department of Applied Chemistry
and Institute of Molecular Science of National Chiao Tung University in Taiwan as a Chair
Professor. He was awarded Asian Photochemistry Association Award, Mukai Prize (2010), The
Purple Ribbon Medal (2008), Porter Medal (2006), the Chemical Society of Japan Award (2005),
Osaka Science Prize (1994), Chemical Society of Japan Award for Creative Work (1994), MoetHennessy-Louis-Vuitton International Prize “Science for Art” Da Vinci of Excellence (1993), andreceived Doctor Honoris Causa de Ecole Nomale Superier de Cachan (2006). Now he is foreign
fellows of Royal Flemish Academy of Belgium and the National Academy of Sciences India.
He has published more than 500 papers and wrote/edited 19 books. In Taiwan he is extending
seminal researches on (1) laser trapping crystallization of molecules and proteins and (2)molecular trapping phenomena by femtosecond laser pulses.