Selected Papers

Femtosecond Laser-driven Soft X-Ray
Sources for Time-resolved Spectroscopy
Katsuya Oguri†, Yasuaki Okano, Tadashi Nishikawa,and Hidetoshi Nakano
Abstract
We are studying time-resolved x-ray absorption fine structure (XAFS) techniques using a femtosecond-laser-produced plasma x-ray source to measure ultrafast dynamics by x-ray absorption spectroscopy. In this paper, we describe the enhancement of x-ray emission intensity achieved by using
nanostructured targets to improve the x-ray fluence. We also report a demonstration of the time-resolved
XAFS technique applied to the laser-induced dynamics of Si foil. Our results clearly show the potentialof the time-resolved XAFS technique for capturing ultrafast changes in the electronic state and atomic
structure of materials.

1. Introduction
X-rays were discovered and named by Röntgen in
1895. They were a new type of radiation and theirdiscovery had a huge effect on scientists throughout
the world. Since then, various x-ray techniques havebeen developed for investigating the structural andelectronic properties of matter using the unique feature of x-rays, namely that they mainly interact withlocalized core electrons. X-ray techniques using two
main x-ray sources—conventional x-ray tubes andsynchrotron sources—have helped drive rapid
advances in our understanding of the static or timeaveraged properties of matter. Such x-ray researchhas become well established as one of the mostimportant fields in physics, chemistry, and biology,
and it is now called x-ray science.
Recently, new x-ray sources that can generate x-ray
pulses with a duration of a few picoseconds (10–12 s)
down to the femtosecond (10–15 s) level have beendeveloped as a result of the great technologicalprogress made on high-power and ultrashort-pulse
lasers. The emergence of these laser-based ultrashortpulse x-ray sources (hereafter shortened to ultrashort† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: oguri@nttbrl.jp

36

x-ray sources) has opened up new horizons for x-ray
science, i.e., the study of the dynamic behavior ofmatter on a picosecond or femtosecond timescale bymeans of time-resolved measurements using ultrashort x-ray pulses. Since this time scale corresponds
to one vibrational period, typically of the order of
10–13 s (100 fs), these time-resolved x-ray techniques
will provide us with a deep and clear insight into theultrafast dynamics of matter on the atomic size andtimescale. Currently, the study of ultrafast dynamic
processes on this timescale, known as ultrafast science, is generally carried out with ultrafast opticaltechniques based on sophisticated femtosecond laser
technology. This research field now covers matter
ranging from molecules to solids [1], [2], and its scientific importance was recognized with the award of
the 1999 Nobel Prize for Chemistry to A. H. Zeweil.
However, optical photons probe only valence electronic states, which usually include the superpositionof multiple atoms, so access to information about theelectronic states of specific atoms or their atomic
arrangement is limited in methods that use opticalphotons. If we use x-ray photons instead of opticalphotons, we can expect to observe processes that
occur during the making and breaking of chemicalbonds and during atomic rearrangements in photoinduced chemical reactions or phase transformations.
This provides a “movie” of a chemical reaction,
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Selected Papers

Dynamic processes∆tChemical reaction, ionization,
breakdown, phase transition,etc.

Pump beam(laser pulse)
Diffraction
Probe beam(x-ray pulse)

Absorption
Detector
SampleFig. 1. Basic concept of time-resolved x-ray techniques.

which has long been a dream of scientists. From thispoint of view, the interdisciplinary research field
between x-ray science and ultrafast science has
emerged as “ultrafast x-ray science”.
The concept of the time-resolved x-ray techniques
is very simple (Fig. 1). A laser pulse (pump pulse)
excites the sample, and then an x-ray pulse (probe
pulse) with a variable time delay ∆t with respect tothe laser pulse is either scattered or absorbed by the
sample. We can obtain information about the sample
by detecting the scattered or absorbed x-rays, whichcorrespond to x-ray diffraction and x-ray absorption,
respectively. By tuning the time delay, we can measure the time evolution of the excited states. In particular, the fine structure that appears near the x-ray
absorption edge in the x-ray absorption spectrum,
known as the x-ray absorption fine structure (XAFS),
is sensitive to local order such as the atomic arrangement, chemical bonding, and electronic states of theabsorbing atom. XAFS is classified into two types: xray absorption near-edge structure (XANES) andextended x-ray absorption fine structure (EXAFS).
These provide information about the electronic states
and bond length of the absorbing atom, respectively.
In addition, XAFS can provide local structural information about various types of material, while x-ray
diffraction mainly provides information about thelong-range structure of crystalline materials. Therefore, time-resolved XAFS measurement with a temporal resolution of a picosecond or better is thought
to be an important tool for capturing ultrafast chemical reactions. Studies using ultrafast time-resolved
XAFS are still in their early stages, and considerable
effort is being made to establish it as a standard technique [3].
This paper describes our recent research on timeresolved x-ray absorption spectroscopy in which weused ultrashort x-ray pulses emitted from femtosecond-laser-produced plasma. Section 2 introduces theVol. 3 No. 12 Dec. 2005

main laser-based ultrashort x-ray sources, in particular, the laser-produced-plasma x-ray source that wehave been developing. Section 3 describes theenhancement of the x-ray conversion efficiency
achieved by using nanostructured targets. Section 4reports our recent results for time-resolved XAFS
measurements of Si using a laser-produced-plasma xray source.
2. Laser-based ultrashort x-ray sources
The key technology in a time-resolved XAFS measurement system is the laser-based ultrashort x-ray
source. Sources can be classified into two types
according to the physical mechanism of x-ray production. One is laser high-order harmonics producedby the nonperturbative nonlinear response of gaseousatoms to a strong laser field with an intensity of about
1013–1014 W/cm2 [4]. Its spectrum exhibits wellresolved discrete odd–order harmonics ranging typically from 10 to 100 eV. The second is a laser-produced-plasma x-ray source that can cover the x-ray
region, especially from a few tens of electron volts toover 100 keV [5]. This source utilizes the thermal xray emission from high-density high-temperatureplasma created on a target surface by laser irradiation
with an intensity of about 1015–1018 W/cm2. In general, the characteristics of the emission spectrum
depend strongly on the target material, i.e., strongline emissions are dominant in the spectrum forlighter elements (e.g., C, Al, and Si), while broadband emission with less pronounced line emission isdominant in the spectrum for heavier elements (e.g.,
Au, Ta, and W). When the laser intensity is about
1016–1018 W/cm2, the plasma emits continuous andcharacteristic x-rays in the keV region, so it is often
called a laser-driven electron x-ray source. This keVx-ray emission is based on the bremsstrahlung and Kα
radiation of fast electrons accelerated by the steep
37Intensity (a.u.)

Selected Papers

200
~5 ps
100

0

Wavelength (nm)

14
12

250

10

200
150

8
100
6

50
0

4

200

100

0

2
–20

–10

Intensity (a.u.)

0

10

20

30

40

Time (ps)Fig. 2. Temporally and spectrally resolved soft x-ray emission from Ta target at a laser intensity of ~1016 W/cm2.
The spectral profile at the maximum intensity and the temporal profiles at 5.5 nm are also plotted.

intensity gradient of the high-peak-power laser pulse.
In particular, we have been intensively studying thegeneration of x-rays from femtosecond-laser-produced-plasma because these x-rays possess an ultrashort pulse duration of the picosecond or subpicosecond order as well as a high x-ray fluence. This
makes them good x-ray sources for time-resolved xray techniques. The other important reason is that thesmooth broadband spectrum of laser-produced-plasma x-rays generated from heavy elements is suitablefor XAFS measurements. Figure 2 shows a spectrally resolved x-ray streak image of a laser-producedplasma x-ray pulse from a Ta target, its spectrum at
the maximum intensity, and its pulse shape at 5.5 nm.
Although the x-ray intensity in the longer wavelength
region is relatively low due to the photon-energy
dependence of the quantum yield of the CsI photocathode of the x-ray streak camera, the spectrum issmooth without any line emission. The pulse shapeexhibits a steep rise and a slow decay caused by thethermal emission from high-temperature plasma. The
pulse duration is about 5 ps, which determines thelimit on the temporal resolution of our time-resolved
XAFS system.

38

3. Nanostructured target to enhance x-ray
intensity
When femtosecond-laser-produced-plasma x-rays
are used as an ultrashort-pulse source for timeresolved XAFS, a sufficiently large x-ray fluence isessential to obtain a good-quality absorption spectrum. Methods for improving the conversion efficiency of laser-produced-plasma-x-ray generation can be
categorized into two approaches. One is to introduce
a weak prepulse before the main pulse to generate
preformed plasma [7], [8]. In general, x-ray generation from a laser pulse does not have very high energy conversion efficiency because the energy of thelaser pulse is mostly reflected by the surface of thetarget, especially in the case of metal targets. The
low-density expanding preplasma generated by the
prepulse can absorb the energy of the main strong
laser pulse efficiently, so the conversion efficiency ishigher. This method can control the x-ray fluence, xray pulse duration, and spectrum distribution by controlling the prepulse and main-pulse conditions, i.e.,
the pulse intensity ratio and separation time. However, this method inevitably leads to a broadening of theresultant x-ray pulse as a result of the low heat conductivity of the preplasma between the target and themain plasma [8]. The other approach involves using a
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target with a structured surface having a low averagedensity and a high local density [9], [10]. Here, we
introduce two novel types of target with artificiallydesigned nanostructured surfaces. Since the large surface area of the nanostructured targets increases theregion of interaction with the laser pulse, the conversion efficiency increases as a result of the efficientabsorption of the laser pulse energy. In contrast to the
prepulse method, the pulse broadening effect is suppressed because the region with high local density
retains high heat conductivity.
One type is an alumina nanohole-array target
formed using the self-organization process of theanodic oxidation of an aluminum plate [11]. We
obtained the x-ray emission spectrum from this target
irradiated by a Ti:sapphire laser pulse with an intensity of 1.5 × 1016 W/cm2 as shown in Fig. 3, which
also shows the spectrum from an ordinary alumina
plate target with a flat surface for comparison. Notethat the intensity scale on the flat alumina plate isexpanded by a factor of ten. The inset in Fig. 3 shows
a top-view scanning electron microscope (SEM)
image of the alumina nanohole-array target with ahole pitch of 100 nm, hole diameter of about 90 nm,
and hole depth of about 40 µm. This figure clearlyshows that an approximately 30-fold enhancement ofsoft x-ray intensity was achieved in the wavelength
region from 7 to 20 nm. The appearance of line emission from the Al5+ and Al8+ ions in the short-wavelength region implies that ionization to the higher
charged state was promoted by the increase in elec-tron temperature caused by the efficient energy
absorption of this target. In addition, we confirmedthat there was a 20-fold x-ray intensity enhancementin the same wavelength region when we used a gold
nanocylinder-array target, which was made by fillingthe nanoholes in the anodic alumina with gold byelectrodeposition and removing the alumina part[12].
The other nanostructured target is a carbon nanotube (CNT), which is made by microwave plasmaenhanced chemical vapor deposition [14]. Carbon
exhibits strong emission lines around 4 nm, whichcorrespond to the “water window” region (2.3–4.4
nm). The x-ray pulse in this region is suitable for the
x-ray microscopic imaging of biological specimens
because the natural contrast between protein andwater can be obtained due to the large absorption differences between carbon and oxygen. For this purpose, x-ray sources that have sufficient x-ray fluence
have been required. The inset in Fig. 4 shows a sideview SEM image of a CNT target. Vertically aligned
multiwalled nanotubes were formed with a spacing ofabout 150 nm on a silicon substrate. They are 30 nm
in diameter and 12 µm high. Figure 4 also shows theemission spectrum from an ordinary carbon plate andseveral types of CNT targets when the intensity of theincident laser pulse on the target was about 3.7 × 1016
W/cm2. Line emissions from the helium-like carbon
ions, Heα and Heβ, and from the hydrogen-like carbon ions, Lyα, Lyβ, and Lyγ, were observed in thewater-window region. This figure also clearly shows
12

25

Heα

Al3+
15

500 nmO5+

Al4+
Al5+ Al

10

O4+

5,4+

Al6+
5
0

Al

5

×10

8,7+

10

15

8
5 µm
6
Heβ
4
2

20

Wavelength (nm)Fig. 3. Time-integrated soft x-ray spectrum from analumina nanohole-array target (red). The
spectrum from an ordinary flat alumina plate(green) is shown for comparison (note the scale is×10). The inset shows a top-view SEM image ofan anodic alumina nanohole-array target.

Vol. 3 No. 12 Dec. 2005

Intensity (a.u.)

Intensity (a.u.)

10

O5+

20

Lyα

Lyβ
(O-Heα)

0
2.0

Lyγ

2.5

3.0

3.5

4.0

4.5

Wavelength (nm)Fig. 4. Time-integrated soft x-ray spectra in the waterwindow region from vertically aligned multiwalled
CNT (red), unaligned multiwalled CNT (blue),
unaligned single-walled CNT (green), and an
ordinary carbon plate targets (black). The inset
shows a side-view SEM image of the verticallyaligned multiwalled CNT target.

39

Selected Papers

that an approximately 7-fold soft x-ray intensity
enhancement was achieved with the aligned multiwalled CNT. The number of photons obtained in thewater-window region was estimated to be 2 × 1012
photons/steradian per pulse. A general comparison oflaser-produced-plasma x-ray sources with other conventional x-ray sources, such as rotating anode tubes,
is difficult because the former are pulse sources while
the latter are continuous wave (cw) sources. However, it is generally true that laser-produced-plasma xray sources produce x-rays with much higher peak
power but lower average power than rotating anodetubes do. When we convert the number of photons
obtained from the CNT targets into the peak spectral
brightness, which represents the peak photon flux
emitted per unit time (s), unit source size (mm2), unit
solid angle (mrad2), and 0.1% bandwidth, it corresponds to approximately 1 × 1018 photons/s/mm2/
mrad2/0.1%BW. Since the peak spectral brightness ofa rotating anode tube is typically ~108 photons/s/
mm2/mrad2/0.1%BW, it is easy to see that the laserproduced-plasma x-ray source generates a huge number of photons in an extremely short time.
4. Time-resolved XAFS measurement on laserexcited Si
Here, we describe an experimental demonstrationof time-resolved XAFS measurement using a femtosecond laser-produced-plasma soft x-ray source.
We used this system to measure the time evolution of
the L-edge XAFS in Si foil under two different incident laser conditions: low intensity irradiation (~1010

W/cm2) [14] and high intensity irradiation (~1012
W/cm2) [15].
The experimental setup is schematically illustrated
in Fig. 5. A laser pulse from a 100-fs Ti:sapphire laser
with a central wavelength of 790 nm was fed to abeam splitter, where 80% of the energy was reflected.
The reflected pulse was focused onto a Ta target atnormal incidence to generate broadband soft x-ray
pulses, as shown in Fig. 2. The emitted x-ray pulses
were focused onto an amorphous Si sample with athickness of about 100 nm by a condenser mirror. The
transmitted x-ray pulses were again focused onto theentrance slit of a flat-field grazing incidence spectrograph and detected with a microchannel plate combined with a cooled charge-coupled device. The
remaining laser pulses that passed through the beam
splitter were fed into an optical variable delay line
and softly focused onto the sample. This is our basic
design for a time-resolved XAFS system.
An example of the absorption spectrum of Si foil isshown in Fig. 6. We can clearly see the LII,III edge at
99 eV and the LI edge at 150 eV, which correspond to
the spectrum components representing transitions
from 2p to 3s and from 2s to 3p in the conduction
band, respectively. Therefore, the XANES of the Si
LII,III edge almost corresponds to the total density ofstates (DOS) in the unoccupied conduction band. In
the energy region from 160 to 270 eV, the absorption
spectrum decreases with a slight oscillation, whichcorresponds to L-edge EXAFS. From an analysis of
the EXAFS, we can obtain information about the SiSi bond length in the Si sample.
First, we observed dynamic changes in the XANES

Ti:sapphire laser system
Beam splitter

Variable delay

MgF2 lens
MgF2 lens

Ta target

ToroidalmirrorCondenser
mirror

Si sample

MCP + CCD

Flat-field grazing incident
spectrograph

MCP: microchannel plate
CCD: charge coupled device

Fig. 5. Schematic illustration of the experimental setup for time-resolved XAFS.

40

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Wavelength (nm)

1.0
EXAFS

0.0
0.5

(a)
Tp
Tu
(Tp – Tu)/Tu

0.5
100

150

0.1

200

250

–0.1

0.0
95

Energy (eV)(Tp – Tu)/Tu

1.5

LII,III

12
0.15 eVTransmittanceµd (E )

1.0

LI

2.0

13

–0.2
105

100

of the LII,III edge when the sample was irradiated by alaser pulse with an intensity of 3 × 1010 W/cm2, which
is well below the damage threshold. Typical transmission spectra with and without laser irradiation are
shown in Fig. 7(a). Also shown is the differential
transmittance between these two spectra. We
observed a significant dip in the differential transmittance at 99.5 eV, which corresponds to the energy difference between the LII,III level and the top of thevalence band [16]. In Fig. 7(b), the depth of the dip inthe differential transmittance is plotted as a functionof the time delay of the soft x-ray pulse with respect
to the laser pulse. Zero delay represents the x-ray andthe laser pulse peaks overlapping. This figure showsthat the dip at 99.5 eV appeared only when the timedelay was from –20 ps to 40 ps. In this figure, theincrease in the dip depth corresponds to the shape ofthe soft x-ray-pulse when the time delay was negative, i.e., when the soft x-ray pulse passed through thesample before the laser pulse arrived. This indicatesthat the change in transmission of the sample
occurred immediately after the laser irradiation. On
the other hand, the recovery time from the change inthe transmission can be estimated to be about 16 psfrom the decrease in the dip depth in the positivedelay region. The simplest model for explaining thelaser induced absorption change shown in Fig. 7
involves the addition of a new absorption line. In
semiconductors, transitions from the core level to thevalence band are usually prohibited because thevalence band is occupied. However, in photoexcited
semiconductors, holes (i.e., vacancies) are created in
the valence band. These vacancies allow the transiVol. 3 No. 12 Dec. 2005

Ip = 3 × 1010 W/cm2

0.2

1Intensity (norm.)

Fig. 6. Example of the absorption spectrum ofamorphous Si foil with a thickness of 100 nm.
Here, µd (E ) is an absorption coefficient defined byµd (E ) = ln[I0(E )/I (E )], where E, I0(E ), and I (E ) are
the photon energy, the emission, and thetransmission spectra, respectively.

–(Tp – Tu)/Tu (at 99.5 eV)

Photon energy (eV)0.1(b)
Probe pulse
0.0
–40

–20

0

20

40

0
60

Delay (ps)Fig. 7. (a) Transmission spectra of Si foil near the LII,III
edge. The red and blue curves represent
transmission spectra with and without laser
irradiation, respectively. The dashed green curveis the differential transmission spectrum between
these two spectra. We defined the differential
transmittance as (TP – TU)/TU, where TP and TU
represent the Si transmittance with and without
the pump beam, respectively. (b) Differential
transmittance at 99.5 eV as a function of the timedelay when the intensity of the laser pulse was 3
× 1010 W/cm2. The pulse shape of the soft x-ray
pulse (probe pulse) is also plotted in this figure(gray).

tion from the core level to the top of the valence band,so the soft x-ray absorption at 99.5 eV increases. The
good agreement between the recovery time estimatedfrom Fig. 7(b) and the recombination time of an electron-hole pair supports the validity of our physicalmodel.
Second, we observed the time evolution of the Ledge EXAFS when laser pulses with an intensity of 5
× 1012 W/cm2 excited the sample. In this case, since
the excitation intensity was well above the damage
threshold, the sample was ablated by each laser shot.
Therefore, we scanned the sample to refresh the laser
irradiation region for each laser shot. Figure 8 shows
41

Selected Papers

(a)
0.1

Solid SiR = 2.32 Å

0.0
–0.1

–330 ps(b)

χ (k )

0.1
Liquid Si
R = 2.43 Å

0.0
–0.1
~0 ps
(c)
0.1

Ablation

0.0–0.1+1670 ps
4.0

4.5

5.0

5.5

–1

k (Å )

Fig. 8. Transient EXAFS spectra for various time delays (red line) when the intensity of the laser pulse was 5 × 1012
W/cm2. The EXAFS function χ(k) is defined as χ(k) = [µ(k) – µ 0(k)]/µ 0(k), where k is the wave number ofphotoelectrons and µ(k) and µ 0(k) represent the absorption coefficient of the sample and that of an isolated atom,
respectively. Data for the unpumped condition is also shown for comparison (blue line).

the time evolution of the EXAFS spectra obtained
from the absorption spectra in Fig. 6. When the x-ray
pulse passed through the Si sample 330 ps before thearrival of the laser pulse, no large difference wasobserved between the EXAFS spectra of the pumped(red curve) and unpumped (blue curve) samples (Fig.
8(a)). We obtained a Si-Si atomic distance of 2.32 Å
by Fourier transformation of the EXAFS spectra. The
good agreement between the obtained bond distanceand previously reported values (2.35 Å) [17] confirmed the validity of the measurement. Next, asshown in Fig. 8(b), we can clearly see that the
EXAFS oscillation amplitude decreased with a small
peak shift toward a lower wave number when the xray pulse almost overlapped the laser pulse. The
decrease in the oscillation amplitude is probably
caused by a mixture of thermal damping and structural disordering. Since the small peak shift indicatesthat the oscillation period became shorter than that of
the unpumped Si, this strongly suggests that the Si-Si
atomic distance broadened slightly due to the laser
excitation. By the Fourier transformation of this
EXAFS spectrum, we obtained a Si-Si atomic distance of 2.43 Å, which agrees well with the reported
42value of 2.45–2.50 Å for liquid Si at 1700–1800 K[18], thus the increase in Si-Si atomic distance can be
explained by the ultrafast production of liquid Si. At
a time delay of +1670 ps, the EXAFS became too
weak for us to analyze its oscillation structure (Fig. 8(c)). The disappearance of the oscillation structure
indicates further structural disordering of Si due tothe onset of evaporation from the liquid phase to a
gas-like phase. As discussed above, our study clearlyshows that ultrafast structural changes during laser
melting can be captured directly by using the timeresolved XAFS technique.
5. Summary
Time-resolved XAFS spectroscopy is expected to
become a useful tool for detecting the processes that
occur during the making and breaking of chemicalbonds and during atomic rearrangements of various
types of material, such as crystals, amorphous materials, and liquids. Laser-produced-plasma x-rays thathave both a high x-ray fluence and a short pulse duration constitute a promising x-ray source for such
spectroscopy. We conducted two experiments to
NTT Technical Review

Selected Papers

investigate this time-resolved XAFS technique basedon a femtosecond-laser-produced-plasma x-ray
source. One involved enhancing the x-ray fluence byfabricating an array of nanoholes on an Al surface or
carbon nanotubes on a Si substrate. We achieved alarge enhancement due to the enlargement of theinteraction region between the laser pulse and the target surface. The other experiment involved measuring picosecond time-resolved XAFS for laser-excited
Si foil. We found an increase in x-ray absorption near
the LII,III-edge structure as a result of the creation ofvacancies in the valence band by laser irradiation
with an intensity of ~1010 W/cm2. With a laser intensity of ~1012 W/cm2, we observed a slight shorteningof the EXAFS oscillation period and a decrease in its
oscillation amplitude. This can be explained by anincrease in the Si-Si atomic distance and the structural disordering as a result of the production of liquid
Si. This study clearly demonstrates the potential of
the time-resolved XAFS technique for capturing
ultrafast changes in the electronic state and atomic
structure of materials.
Acknowledgments
We thank Professor Hideki Masuda of Tokyo Metropolitan University and Dr. Otto Zhou of the University of North Carolina for providing us with thenanostructured targets, and Professor Naoshi Uesugiof Tohoku Institute of Technology for initiating thisproject and providing helpful suggestions andencouragement throughout this study. Financial support was provided by grant aid from the Ministry of
Education, Culture, Sports, Science and Technologyof Japan (Grant No. 16032219).
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[17] K. Laaziri, S. Kycia, S. Roorda, M. Chicoine, J. L. Robertson, J.
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[18] Y. Waseda, K. Shinoda, K. Sugiyama, S. Takeda, K. Terashima, andJ. M. Toguri, “High temperature x-ray diffraction study of melt structure of silicon,” Jpn. J. Appl. Phys., Vol. 34, pp. 4124-4128, 1995.

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Selected Papers

44

Katsuya Oguri

Tadashi Nishikawa

Quantum Optical Physics Research Group,
Optical Science Laboratory, NTT Basic
Research Laboratories.
He received the B.S. and M.S. degrees from the
University of Tokyo, Tokyo in 1996 and 1998,
respectively. He joined NTT Basic ResearchLaboratories in 1998. Since then, he has been
engaged in the study of high-power femtosecond-laser-based x-ray sources and their application. His current interest is ultrafast x-ray spectroscopy with laser-produced-plasma x-rays andhigh-order harmonics. He is a member of theJapan Society of Applied Physics (JSAP) and the
Optical Society of America (OSA).

Senior Research Scientist, Quantum Optical
Physics Research Group, NTT Basic ResearchLaboratories.
He received the B.S., M.S., and Ph.D. degrees
from Tokyo Institute of Technology, Tokyo in
1986, 1988, and 1998, respectively. He joined
NTT Basic Research Laboratories, Tokyo in
1988. In 2004, he received the 26th Japan Society of Applied Physics JJAP (Japanese Journal ofApplied Physics) Best Original Paper Award. He
is a member of JSAP.

Yasuaki Okano

Hidetoshi Nakano

Research Associate, Quantum Optical Physics
Research Group, Optical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.E. degree in engineeringfrom Saitama University, Saitama and the M.S.
and Ph.D. degrees in science and engineeringfrom Tokyo Institute of Technology, Kanagawa
in 1999, 2001, and 2004, respectively. He joined
NTT Basic Research Laboratories in 2004. He isa member of JSAP and the Japan Intense Light
Field Science Society (JILS).

Senior Research Scientist, Group Leader,
Quantum Optical Physics Research Group, Optical Science Laboratory, NTT Basic ResearchLaboratories.
He received the B.S. and M.S. degrees in electronic engineering, and the Ph.D. degree in electrical engineering from the University of Tokyo,
Tokyo in 1982, 1984, and 1987, respectively. He
joined NTT Basic Research Laboratories in
1987. Since then, he has been engaged in ultrafast optical physics research including applications of intense laser-field and ultrashort wavelength sources. He is a member of JSAP, theInstitute of Electronics, Information and Communication Engineers of Japan, the PhysicalSociety of Japan, the Laser Society of Japan,
JILS, IEEE, OSA, and the International Society
for Optical Engineering.

NTT Technical Review