bluuebunny/crossref_metadata_2025 huggingface dataset:
Low Temperature Ultrafast Charge Separation; Rate vs Free Energy
Authors: O’Neil Michael P. , Gaines George L. , Svec Walter A. , Niemczek Mark P. , Wasielewski Michael R.
Published: February 2023

Photoinduced charge separation reactions in a large series of rigid porphyrin-triptycene-quinone (donor-spacer-acceptor) molecules were studied with picosecond transient absorbance and fluorescence techniques in polar and non-polar solvent at room temperature and at 77K in MTHF glass. At 77k the fluorescence yield as a function of free energy sharply inflects at 0.6eV. However at room temperature in toluene a less pronounced inflection occurs at 0.3eV. These inflection points represent the driving force where the rates of electron transfer approach the fluorescence decay rate of the porphyrin. Only when the energy of the ion pair state is lower than the lowest excited singlet state of the porphyrin may electron transfer occur.
Picosecond Transient Absorption and Dichroism Studies of Excitation Transfer in the Cyanobacterial Light-Harvesting Proteins C-Phycocyanin and Allophycocyanin
Authors: Beck Warren F. , Debreczeny Martin , Yan Xinwei , Sauer Kenneth
Published: February 2023

C-phycocyanin and allophycocyanin are light-harvesting proteins that occur in the phycobilisome in cyanobacteria.1 They represent relatively simple systems in which the structural factors that govern the direction and rate of excitation transfer can be addressed through the methods of picosec-ond spectroscopy. Several aspects of the structure of C-phycocyanin, as recently determined by X-ray crystallographic methods,2 present challenges to current theories for excitation transfer in light-harvesting proteins. We have begun a detailed characterization of the picosecond transient absorption and dichroism kinetics of C-phycocyanin in order to compare measured patterns of excitation transfer in C-phycocyanin with those indicated by direct calculation, using the known coordinates and orientations of the pigment molecules bound by the protein. We are also examining excitation transfer processes in allophycocyanin, a similar phycobiliprotein that directs excitation energy received from C-phycocyanin towards the photosystem II reaction center.
Ultrafast Processes in Hemoglobin-Co Photodissociation
Authors: Han C. H. , Lian T. , Anfinrud P. A. , Hochstrasser R. M.
Published: February 2023

Recent developments in ultrafast infrared spectroscopy have made it possible to study the photo-induced release of carbon monoxide from hemoglobin-CO (HbCO) in some detail (1). Not only was it possible to observe the bound metal carbonyl absorption transients but we have now observed the free carbon monoxide signals that were ca. 50 times weaker.
Soliton-like Pulse Shaping in cw Passively Mode-Locked Ti:Al2O3 Laser
Authors: Ishida Yuzo , Sarukura Nobuhiko , Nakano Hidetoshi
Published: February 2023

We have generated stable pulses less than 200 fs around 780 nm from a cw passively mode-locked Ti:Al2O3 laser with a prism-pair dispersion compensator inside the cavity. The pulse shaping mechanism mainly originates from the combined actions of self-phase modulation (SPM) and group velocity dispersion (GVD) or soliton-like pulse formation.
Femtosecond Pulses in the Medium Infrared by Down-Conversion of a Traveling Wave Dye Laser
Authors: Elsaesser Thomas , Nuss Martin C.
Published: February 2023

Parametric frequency mixing in nonlinear crystals represents a widely applied technique to generate ultrashort pulses in the wavelength range between 1 and 10 μm [1,2]. In this paper, we present a novel parametric device to produce femtosecond pulses around 5 μm with a repetition rate of 8 kHz and a higher efficiency and stability than previously reported.
Broadband Microwave Reflection Experiments with Picosecond Transient Radiation
Authors: Robertson W. M. , Arjavalingam G. , Kopcsay G. V. , Halbout J.-M.
Published: February 2023

Recently there has been much interest in broadband spectroscopy techniques based on optically generated ultrashort electromagnetic pulses.1-3 Auston et. al. used Cherenkov-like radiation produced by optical rectification in non-linear crystals to perform contact reflection measurements.1 We have demonstrated the Coherent Microwave Transient Spectroscopy (COMITS) technique to measure the complex dielectric properties of materials in the 15-130 GHz range.2 More recently this technique has been extended to higher frequencies by Grischkowsky et. al.3 Here we present results of broadband microwave reflection spectroscopy with optoelectronically generated, freely propagating, picosecond transient radiation. The technique is illustrated with the measured complex reflection function of Fabry-Perot interferometers, the dispersion from amplitude gratings, and reflection from materials. The experimental results are compared with theory and in all cases very good agreement is found.
Ultrafast Coplanar Air-Transmission Lines
Authors: Dykaar D.R. , Levi A.F.J. , Anzlowar M.
Published: February 2023

A limit to ultrafast electronics with single picosecond switching time is the ability to rapidly propagate undistorted electrical signals from one device to another. While subpicosecond electrical transients can be propagated for short distances (⪝100μm) without significant loss in fidelity (risetime) (Refs. 1,2), there has been little attempt to propagate these signals large distances (e.g. a few mm) because of dispersion mechanisms involving dielectric discontinuities and materials related losses (Refs. 2,3). We show experimentally that ultrafast Coplanar Air-Transmission (CAT) lines can be used to propagate subpicosecond electrical signals across typical chip dimensions of a few mm, at very high speeds and little distortion.
Subpicosecond Fluorescence Anisotropy Measurements of Tryptophanyl Residues in Proteins
Authors: Hansen John E. , Rosenthal Sandra J. , Fleming Graham R.
Published: February 2023

The fluorescence anisotropy of tryptophanyl residues in proteins is used as a probe of protein motion. We have recently shown(1) that the short life time (200 fs - 10 ps) region of the fluorescence anisotropy of tryptophan in solution is dominated by an electronic relaxation process, which has the potential to obscure molecular motion. The question arises as to the relevance of this result for the behavior of tryptophanyl residues in proteins. To address this question we have examined the subpicosecond fluorescence anisotropy of tryptophanyl residues in two very different enviroments - exposed to the solvent in melittin, a 26 amino acid peptide, and buried in a hydrophobic region in apoazurin purified from the bacteria Pseudomonas aeruginosa. Measurements were made by ultraviolet fluorescence upconversion described elsewhere(1). Samples were degassed and flowed in absence of air. Concentrations were adjusted so the optical density at the excitation wavelength was between 0.6 and 1.0 for a 1mm pathlength.
Picosecond Time-Resolved Measurements of Photorefractive Grating Dynamics in Zincblende Semiconductors
Authors: Smirl A. L. , Dawson M. D. , Schroeder W. A. , Stark T. S. , Boggess T. F. , Valley G. C.
Published: February 2023

We describe a novel nondegenerate, polarization-sensitive, transient-grating technique, which we use to isolate, to probe the symmetry of, and to time resolve (with resolution < 5-ps) the individual dynamics of several co-existing gratings written in GaAs:EL2 and CdTe:V. Specifically, we utilize optically-induced anisotropy to allow the first temporal resolution of the formation and decay of photorefractive gratings in these semiconductors on picosecond timescales. The measurements allow us to identify various contributions to the photorefractive effect, including the first direct evidence of a photorefractive grating arising from the Dember field associated with free electron hole pairs. The technique also allows the instantaneous electronic and the cumulative free-carrier gratings (which normally obscure the weaker photorefractive grating) to be simultaneously monitored. Measurements were conducted with a unique, amplified, Styryl 13 dye laser system that was developed for these studies.
Pulse Compression of a Mode-locked Ti:sapphire Laser
Authors: Kafka J. D. , Watts M. L. , Roach D. J. , Keirstead M. S. , Schaaf H. W. , Baer T.
Published: February 2023

A mode-locked Ti:sapphire laser has the potential to produce ultrashort pulses while preserving the high power and tunability inherent in this gain media. Several mode-locking techniques have produced pulses of less than 10 psec duration from a Ti:sapphire laser [1-3], however to date the output powers have been limited to 300 mW. We have previously reported on 6 psec pulses with 750 mW of average power generated by active mode-locking with an acousto-optic modulator [4]. In this paper, we use active mode-locking to generate picosecond pulses with 2.0 W of average power and pulse compression to generate tunable pulses shorter than 50 fsec with 750 mW of average power.
Dynamics of cis-Stilbene Isomerization in Solution
Authors: Jean J.M. , Todd D.C. , Rosenthal S.J. , Ruggiero A.J. , Fleming G.R.
Published: February 2023

The influence of the solvent on the dynamics of reactive processes continues to be of great interest.1,2 The past decade has seen a substantial increase in our understanding of the effect of solvent friction on activated barrier crossing processes. Less well understood is the role of the solvent in reactions for which there is no potential energy barrier separating reactant from products.3 An example of a barrierless process is the isomerization of stilbene. Isomerization from either the cis or trans side of this molecule involves a twisted intermediate which rapidly internally converts to the ground state. The lack of a barrier along the the torsional coordinate on the cis side leads to extremely rapid cis-trans isomerization rates.
Isomerization of cis-Stilbene in Ar and Kr Clusters. Direct Measurements of trans-Stilbene Formation Rates on Picosecond Time Scale
Authors: Petek Hrvoje , Yoshihara Keitaro , Fujiwara Yoshihisa , Frey Jeremy G.
Published: February 2023

Few problems have attracted so much interest of picosecond spectroscopists as the photoisomerization of cis- and trans-stilbene.1 Femtosecond transient absorption1,2 and multi-photon ionization3 experiments have shown that in solution cis-stilbene S1 state decays in ~1.4 ps independent of solvent temperature and friction. This decay process has been attributed to rapid torsion around the central C = C bond to the 90° twisted geometry which is thought to be the common intermediate for both cis- and trans-stilbene S1 state decay, and has historically been designated as the "phantom state.1-3. However, another isomerization channel, the dihydrophenathrene formation occurs with as high as 30 % quantum yield.4 In order to establish the mechanism for cis-stilbene non-radiative decay, and to investigate the role of the phantom state in cis-trans isomerization, this paper presents preliminary measurements of cis-stilbene dynamics in rare gas clusters that are formed by supersonic expansion.
Excitation of low frequency polaritons in PbTiO3 with femtosecond pulses.
Authors: Etchepare J. , Grillon G. , Antonetti A. , Loulergue J. C. , Fontana M. D. , Kugel G. E.
Published: February 2023

Perovskite PbTiO3 is tetragonal
(C4v1) in its ferroelectric phase. Its lattice dynamics[1,2] reveals the existence of two (A1. + E ) Raman active polar modes in the low frequency spectral range (lower than 200 cm-1). We show that the use of femtosecond pulses in a nonlinear spectroscopy technique[3,4] is well suited to drive coherently these phonon modes and measure their dispersion.
Femtosecond Dynamics of Photo-generated Carriers in Amorphous Hydrogenated Germanium
Authors: Wraback M. , Tauc J. , Pang D. , Paul W. , Lee J.-K. , Schiff E.A. , Vardeny Z.
Published: February 2023

Significant improvement in the optoelectronic properties of amorphous hydrogenated germanium (a-Ge:H) has recently been attained, and this material may become of greater technological interest in the near future. It is also an attractive physical system for femtosecond pump and probe studies with 2eV photons because with an optical gap of 1.06eV it allows studies of dynamics of photoexcited carriers with large excess energy in the band states, in contrast to similar experiments1,2 on the extensively studied a-Si:H, where the optical gap is 1.8eV and the carriers have very little excess energy.
Continuous Wave Diode Laser Seeding of Tunable Femtosecond Soliton Pulses
Authors: Greer E J , Patrick D M , Vukusic J I , Wigley P G J , Taylor J R
Published: February 2023

In the near infra red, frequency tunable,femtosecond pulses can be readily obtained using the soliton Raman generation process (1). This continuum consists of several solitons the number of which is not particularly controllable although it is dependent on the pump power. In addition, the spectral extent is not finely controllable and as such the soliton Raman continuum is of limited use as a practical source. It is possible however, to spectrally filter a region of interest to generate a wavelength selectable femtosecond soliton and this method has been used to give a practical source for pump-probe experiments (2), where the time jitter between consecutive solitons in the pulse train is unimportant. One problem with this method is that it is inefficient, since most of the generated radiation from the continuum is discarded.
Microscopic Theory of the Transient Grating: The Role of Polaritons
Authors: Knoester Jasper
Published: February 2023

The traditional approach for calculating nonlinear optical signals is based on performing response theory of the material system under study with respect to the external laser (electric) fields. The optical response is then characterized completely by the eigenstates of the material system with instantaneous Coulomb interactions. For a molecular crystal, these eigenstates are the Coulomb excitons and the optical response may be expressed in terms of the exciton energies scattermg rates, and transition dipoles. This picture, however, neglects the transverse internal radiation field (retardation)[1], which is a quantum degree of freedom and cannot be switched off experimentally. Consequently, in low temperature pure crystals with sufficiently high density of oscillator strength, the proper eigenmodes are those of the coupled material-radiation system: polaritons [2-4]. The polaritons are coupled to the external fields through the boundary conditions on the crystal’s surface and in a proper description of the optical experiment, we should consider the response of the polariton system to these external fields [1]. The optical signal then contains characteristics of polaritons rather than excitons. Recently, such explicit signatures of retardation have received much experimental and theoretical attention [3-7].
Femtosecond Transient-Grating Experiments in Quantum Wells
Authors: Leo Karl , Göbel Ernst O. , Damen T.C. , Shah Jagdeep , Schmitt-Rink S. , Schäfer W. , Stafford C. , Müller J.F. , Köhler K. , Ganser P.
Published: February 2023

Time-resolved four-wave mixing allows to study the loss of coherence of optically excited states in molecules and solids. We have applied the transient grating technique with femtosecond time resolution to GaAs/AlGaAs Double Quantum Wells to show that the decay of the macroscopic polarization due to dephasing yields information about the nature and interaction of the excited states.
Anomalous Spectral Shifts of Femtosecond Pulses in the Collisional, Strong-Field Ionization Regime
Authors: Wood Wm M. , Banyai W. C. , Trisnadi J.I. , Downer M. C.
Published: February 2023

Laser-produced plasmas have recently received a great deal of interest because of their possible role as a high intensity source of soft x-rays. One method for achieving such a source involves the creation of an inverted population of ionized core states on an ultrashort time scale. Multiphoton ionization of gases with high intensity ultrafast light pulses can result in a cold plasma of highly stripped ions. The electrons recombining into the deeper core states emit soft x-rays.1 With 100 fs, mJ energy, 616 nm pulses, we are able to both create such a plasma on an ultrashort time scale, and study the rapid collisional and recombination dynamics of the plasma.
Femtosecond Studies of Electrons in Water
Authors: Long Frederick H. , Lu Hong , Eisenthal Kenneth B.
Published: February 2023

Although the equilibrium properties of the aqueous electron have been well-studied, the solvation dynamics and other non-equilibrium properties are still not well understood. A fundamental question concerning the electron solvation dynamics is the nature of the transition from the wet electron to the solvated or equilibrium electron. Theoretical studies have predicted dynamics involving a continous change in the electron absorption spectra.1 Early experimental work has claimed that only two-states were involved, i.e. the solvation dynamics were not continous.2 In order to resolve these issues we have done experiments with improved time resolution and have looked for an isosbestic point in the solvation dynamics.3
A Broadly Tunable CW Femtosecond Source for Ultrafast Spectroscopy
Authors: Wachman E.S. , Edelstein D.C. , Tang C.L.
Published: February 2023

We describe a cw mode-locked optical parametric oscillator (OPO) providing subpicosecond tunability throughout the near- to mid-infrared with transform-limited pulses and milliwatt outputs. This will be followed by a report of recent optical transmission-correlation measurements of hot carrier relaxation in In GaAs using the femtosecond OPO as a tunable excitation source.
Analysis of Femtosecond Pulse Systems Using Ray-Pulse Matrices
Authors: Kostenbauder A. G.
Published: February 2023

Martinez1,2 has discussed the use of ABCDEF matrices with frequency dependant entries to describe wideband optical systems. This treatment works only for time invariant optical systems and requires the use of an auxiliary phase function. In this paper, we show how to model dispersive optical systems (including those with time varying elements) by the use of 4 by 4 matrices that are highly analogous to the ubiquitous ABCD matrices.
Anomalous Dispersion of the Electronic Kerr Effect n2
Authors: Hagan D.J. , Wu Y.Y. , Canto E. , Sheik-bahae M. , Said A.A. , Young J. , Wei T.H. , Junnarkar M. , Van Stryland E.W.
Published: February 2023

We have performed picosecond Degenerate Four-Wave Mixing (DFWM) experiments and beam distortion experiments, including using the newly developed Z-scan technique,[1] to accurately measure the bound electronic nonlinear refractive index n2 in a variety of materials at 1.06 and 0.53 μm. In semiconductors where two-photon absorption (2PA) is allowed, we find that n2 decreases from its maximum value near the 2PA (2ħω≃Eg) edge to zero as the wavelength is increased, changing to negative nearer the fundamental absorption edge. In order to measure n2, we experimentally separated the effects of 2PA excited carrier refraction (coefficient σ) from the electronic Kerr effect induced refraction. Using DFWM this is accomplished by observing that the carrier effect lasts for hundreds of picoseconds and varies as the fifth power of the incident irradiance I, while the χ(3) effect follows the temporal variation of the pulse and varies as I3. Such a signal is shown in Fig. 1.
Generation and amplification of femtosecond pulses at 800 nm
Authors: Georges P. , Salin F. , Estable F. , Brun A.
Published: February 2023

There is nowadays a great interest in the production of optical pulses with very high peak power (> 1 TW). With the recent development of new solid-state materials such as Ti:Al2O3, having a large amplification bandwidth, the generation of femtosecond pulses in the near infrared, is very attractive.
Femtosecond Laser Cavity Dispersion Measurement using Solitons Properties
Authors: Salin F. , Grangier P. , Georges P. , Roger G. , Brun A.
Published: February 2023

The soliton-like character of the pulses generated by colliding pulses modelocked (CPM) dye lasers has been widely evidenced in the recent years. The non linear Schrodinger equation (NLSE) which governs solitons behaviors seems to also explain fairly well most of the CPM laser behaviors even if the laser is a non linear media much more complicated than that assumed in the NLSE. We have tried to used the NLSE predictions in order to obtain some fundamental parameters of CPM lasers. Since the soliton properties arise from the balancing between group velocity dispersion (GVD) and self phase modulation (SPM), the soliton-like pulses behaviors depend widely on the laser GVD. Inversely measuring the pulse parameters can lead to an estimation of the cavity dispersion.
Electronically Programmable Femtosecond Pulse Shaping Using a Multielement Phase Modulator
Authors: Weiner A.M. , Leaird D.E. , Patel J.S. , Wullert J.R.
Published: February 2023

Considerable progress has been made in developing techniques for generating ultrashort pulses with arbitrarily controllable pulse shapes [1-3]. Nevertherless, it has not generally been possible to achieve both high resolution and precision [1] and electronic programmability [2] simultaneously. In this paper we describe a programmable, high resolution, fsec pulse shaping apparatus based on a multielement liquid crystal phase modulator. Applications include fsec pulse position modulation and programmable fsec pulse compression.
Design and Perfomance of Multiterawatt Subpicosecond Nd:Glass Lasers
Authors: Perry M.D. , Pattersoon F.G.
Published: February 2023

We have utilized chirped-pulse amplification1 to construct a compact, Nd:Phosphate glass based system capable of producing 10 Joules in a 700 fsec transform-limited pulse at 1053 nm. The system employs a novel regenerative amplifier2 to produce a net gain of 108 (to 15 mJ) followed by single-pass power amplifiers to boost the energy to 15 J. These pulses are then compressed using a conventional diffraction grating pulse compressor. The extremely low loss of the regenerative amplifier results in minimum gain narrowing. Use of spectral shaping3 before compression yields a subpicosecond compressed pulse having a contrast ratio of better than 105:1 (Figure 1). In the absence of shaping, the compressed pulses are typically 1.5 psec and exhibit a contrast of only 10:1 (Figure 2).
Determination of Photonic Band Gaps and Dispersion in Two-Dimensional Dielectric Arrays with Ultrafast Electromagnetic Transients
Authors: Robertson W.M. , Arjavalingam G. , Meade R.D. , Brommer K.D. , Rappe A.M. , Joannopoulos J.D.
Published: February 2023

Coherent scattering of radiation in periodic dielectric structures results in an electromagnetic dispersion relation which exhibits novel features such as new propagation modes and photonic band gaps. The latter topic has received much current attention for a variety of fundamental and practical reasons [1,2]. Furthermore, the theoretical calculation of the electromagnetic dispersion relation in such structures has become relatively sophisticated. However, experimental verification using traditional microwave techniques has been limited to determining the frequencies which define the gaps and to study localized defect modes. Here, we present the dispersion relation of a periodic dielectric array measured explicitly using the coherent microwave transient spectroscopy (COMITS) technique [3,4]. The experimental results are compared with theoretical predictions obtained using the plane-wave expansion technique [5].
Subpicosecond Study of the Dynamic Processes in Push-Pull Styrenes and the Role of Solvation
Authors: Hébert P. , Baldacchino G. , Gustavsson T. , Kabelka V. , Baldeck P. , Mialocq J.-C.
Published: February 2023

A great deal of attention has recently been devoted to the role of solvation in chemical reactions involving intramolecular charge transfer, LE (locally excited)-TICT (twisted intramolecular charge transfer) singlet excited state relaxation and trans-cis photoisomerization. In this paper we present a subpicosecond study of the solvation dynamics of the styryl 8 laser dye (2-(4-(4-dimethylaminophenyl)-1,3-butadienyl)-3-ethylbenzothiazolium perchlorate) in various solvents. Our results are discussed in the light of recent studies of styryl 7 [1,2] and of our previous studies of the solvatochromism of DCM (4-(dicyanomethylene)-2-methyl-6-[p-(dimethylaminostyryl)-4H-pyran]) [3,4]. Both styryls are remarkable as regards the weak overlap between their absorption and emission spectra. The large Stokes shift observed in DCM is related to a strong intramolecular charge transfer between the electron donor dimethylamino group and the electron acceptor dicyanomethylene group. However, the behavior of the two styryls with respect to solvent polarity is quite different. On the one hand, the neutral DCM presents a red shift of its absorption transition energy with increasing solvent polarity which is due to the feeble solvation of the electronic ground state and the strong response of the electronic polarization of the solvent molecules to the solute Franck-Condon electronic excitation. Its fluorescence spectrum shows a large dependency with respect to the solvent polarity indicating a strong increase of the dipole moment upon electronic excitation [3,4]. On the other hand, in the cationic styryl 8 molecule, an unsymmetrical polymethine-cyanine, the large blue shift of the absorption maxima in solvents of increasing polarity indicates the enormous stabilization of the electronic ground state with respect to the Franck-Condon excited singlet state. The subsequent relaxation of the fluorescent excited state is only minor as shown by the small variation of the wavelengths of the styryl 8 fluorescence maxima with solvent polarity. We thus infer that the dipole moment of the fluorescent state and its solvent cage are weakly affected upon electronic excitation.
“Nonthermal distribution of electrons in GaAs”
Authors: Snoke D.W. , Rühle W.W.
Published: February 2023

We have reported1 observation of a nonthermal energy distribution of electrons at low densities on picosecond timescales, using a streak camera to record the band-to-acceptor luminescence of Ge-doped GaAs. At densities less than 1014 /cm3, the electrons deviate substantially from a Maxwell-Boltzmann distribution for up to 50 ps following a 5 ps laser pulse. We discuss these results in terms of the expected rates of electron-electron and electron- hole scattering, and we present a Boltzmann-equation model for carrier-carrier scattering which gives good agreement with our data (see Fig. 1). We compare our experiments to other recent work.2,3,4
Investigation of Fast Relaxation Processes in Non-fluorescent Rhodamine Dyes
Authors: Plaza P. , Hung N. Dai. , Martin M.M. , Meyer Y.H. , Rettig W.
Published: February 2023

Non radiative relaxation of various N-substituted ethyl-ester-Rhodamine dyes (scheme A) was explained by assuming an intramolecular rotational motion towards a low lying non emissive TICT-like state with charge localization [1]. Stationary and time-resolved fluorescence experiments evidenced that fluorescence quenching in those Rhodamines is linked with the rotation of the amino group. On the other hand, if the o-carboxyphenyl group is exchanged for a p-dimethylamino electron donor group (scheme B), the fluorescence quantum yield is very low in neutral ethanol but increases by more than two orders of magnitude in acidic ethanol solutions, where the electron donating property of the anilino group can be blocked by protonation [1, 2].
Dynamics of excitons probed by accumulated photon echo
Authors: Bouma T. , Vledder P. , Dijkhuis J.I.
Published: February 2023

Accumulated photon echo1 (AccPE) is an ultra-sensitive tool for studying optical dephasing of excitations in solids. The essential prerequisite for the succesful application of this technique is the presence of metastable states in the optical pumping cycle, preventing the system from a quick return to the ground state. A frequency grating in the ground state may than be formed and serve to create a stimulated echo. The technique has been applied to study long-lived optical impurity centers and bound excitons in solids. Here we present the first exploratory measurements on short-lived excitons in semi-insulating GaAs, and we will discuss the accumulation mechanisms. These mechanisms hold promise for general application of AccPE to study excitons in semiconductors.
High-Sensitivity Picosecond-Response InxGa1-xAs Photodetectors
Authors: Gupta S. , Whitaker J. F. , Williamson S. L. , Ho Pin , Mazurowski J. S. , Ballingall J. M.
Published: February 2023

For ultrafast optoelectronic applications, photoconductors with sub-picosecond carrier lifetimes and low dark current are desired. Previous work has demonstrated the dramatic reduction in the carrier lifetime by lowering the growth temperature to ~200°C in molecular-beam-epitaxial (MBE) GaAs layers [1]. Optical transmission and reflection measurements, and photoconductive switching measurements have confirmed the sub-picosecond carrier lifetime in this material. In addition, the semi-insulating nature and high breakdown voltage of these layers has enabled the fabrication of submicron interdigitated photodetectors and optical correlators having an unprecedented bandwidth of 375 GHz and responsivity of ~ 0.1 A/W [2]. The excess-As incorporation during the low-temeprature MBE growth is believed to lead to the above properties [3], hence this approach of low-temperature growth should be applicable to other As-based materials. In particular, the ternary compound InxGa1-xAs is an important direct band-gap material for the long wavelength region, and with this in mind we investigate the ultrafast carrier dynamics and applications of low-temperature MBE grown InxGa1-xAs epilayers.
Transient Dichroism and Birefringence Studies of I2 Predissociation and Wave Packet Dynamics in Solution
Authors: Scherer Norbert F. , Ziegler Lawrence D. , Fleming Graham R.
Published: February 2023

The utility of nonlinear spectroscopic experiments in elucidating “sub-linewidth” vibrational dynamics of molecules in solution has been clearly demonstrated in recent years.1, 2 Still, the electronic state origin of the observed beats of the detected third-order polarization is generally not directly established.1 Clearly, detailed study of reaction dynamics in solution requires greater precision in the measurement of the real and imaginary contributions to the time dependent response function of the reactive moiety. We report here our results of the optically-induced dichroism and birefringence of 5mM I2 - alkane solutions on the femtosecond timescale. The solvent dependence of the medium-induced predissociation of the B-state is described.
50 Femtosecond Pulse Generation from a Self-Starting c.w. Passively Mode-Locked Cr:LiSrAlF6 Laser
Authors: Rizvi N. H. , French P. M. W. , Taylor J. R.
Published: February 2023

Femtosecond laser physics has been transformed by the advent of the titanium-doped sapphire laser which has been been mode-locked by conventional and novel means [e.g. 1-6], yielding pulses as short as 30 fs [5]. The observation of self-mode-locking [2] has led to the realisation that the nonlinear refractive index of the laser medium can be used to provide both amplitude modulation, through self-focussing or "Kerr Lens Mode-locking" [3], and phase modulation which, together with group velocity dispersion, can lead to further pulse compression. These pulse compression mechanisms are not restricted to Ti:sapphire but will work with any nonlinear medium in the laser cavity [4], We present here the observation of sub-100 fs pulse generation for the first time from a Cr:LiSrAlF6 laser.
Multi-kHz high power femtosecond pulses amplification in Ti:Al2O3
Authors: Salin François , Squier Jeff , Mourou Gérard , Vaillancourt Gary
Published: February 2023

We built a Ti:Al2O3 based oscillator / regenerative amplifier system capable of high power femtosecond pulse generation at high repetition rate. The oscillator used in this experiment was a self-modelocked Ti:Al2O3 laser1, pumped by an Argon-ion laser. It produces 100 mW, 90 fs pulses at 82 MHz. It is tunable from 730 to 800 nm and was used at 764 nm in the present experiment. It was followed by a regenerative amplifier pumped by a Q- switched Nd:YLF laser. In order to fully exploit the storage capability of Ti:Al2O3 we used chirped pulse amplification1. The experimental set-up is shown in Fig.1. The Ti:Al2O3 amplifier cavity is a folded, astigmatically compensated resonator. The pump laser is a newly designed, cw-pumped, acoustooptically Q-switched, Nd:YLF laser from Laser Photonics (model KYD-101). This laser runs on the 1.047 μm Nd:YLF line and is intracavity frequency doubled. The frequency doubled light contains up to 10 mJ per pulse at 1 kHz, and 1 mJ at 10 kHz in a uniformly distributed multimode output beam. The pulse duration is 250-400 ns, ideally suited for pumping Ti:Al2O3.
Ultrafast Dynamics in Solution: Wavepacket Motion and the Cage Effect in Iodine
Authors: Yan Yi Jing , Whitnell Robert M. , Wilson Kent R. , Zewail Ahmed H.
Published: February 2023

In a recent series of experiments at Caltech,1 65-90 fs tunable laser pulses are used to pump I2, both in the gas phase and in Ar solution, from the ground state to the electronically excited B state. The dynamics of I2 in the B state is then monitored by a time-delayed probe pulse which excites the I2 molecule further, to the f or E state. Absorption or fluorescence resulting from the probe pulse is measured as a function of delay time between the two pulses. The dynamics of the wavepacket on the B state is governed by the shape of the potential and the nature of the solvent environment. The initial pumped packet is influenced by the coherent nature of the pump pulse and the Franck-Condon overlaps between the initial thermal distribution of the ground state and the B state vibrational manifold.
Femtosecond Intracavity Dispersion Measurements
Authors: Knox W.H.
Published: February 2023

In the new class of CW-pumped widely tunable femtosecond solid-state self-modelocked lasers [1], dispersive phenomena represent an important limit. We present the first complete measurements of intracavity dispersion in a "live" femtosecond modelocked laser. These measurements illustrate the importance of higher order dispersion and its effects on the modelocking physics in the operating laser, and show for the first time the dispersive behavior of a complete operating system.
Time-resolved Measurement of Hot Carrier Cooling Rates in a-Si:H and a-Ge:H
Authors: Wraback M. , Tauc J.
Published: February 2023

The study of nonequilibrium carriers is essential for an understanding of the electronic properties of semiconductors and the physics of semiconductor devices. While great progress has been made in the measurement and understanding of hot carrier relaxation processes in crystalline semiconductors, little is known about the hot carrier cooling rates necessary for an understanding of carrier-phonon and carrier-carrier interactions in the extended states of amorphous semiconductors. Most experiments have been performed on the technologically important amorphous hydrogenated silicon (a-Si:H), a material which, because of its large optical gap (1.7eV), does not lend itself easily to the study of hot carrier dynamics with conventional femtosecond laser systems operating at or below photon energies in the spectral region surrounding 2eV. As a result, previous experiments [1] employing the pump-probe technique with 2eV excitation pulses as short as 100fsec have not revealed any evidence of hot carrier relaxation in a-Si:H, although indirect measurements [2] suggest that the carrier cooling rate is greater than leV/psec.
Multiple Excitation Pulse, Multiple Probe Pulse Femtosecond Spectroscopy
Authors: Wiederrecht Gary P. , Wang Weining , Nelson Keith A. , Wiener A.M. , Leaird D.E.
Published: February 2023

The use of multiple femtosecond excitation pulses for increased control over material behavior has been demonstrated experimentally1−3 and also discussed at some length theoretically4. Applications demonstrated to date include vibrational amplification in crystal lattices and manipulation of molecular electronic and vibrational coherences. One of the primary goals is increased control over chemical or structural rearrangements. Some of the experimental challenges involved include electronic and vibrational dephasing, which limit the extent to which the effects of the first pulse in a sequence are "remembered" by a sample as additional pulses arrive, and unwanted optical effects which lead to unwanted material responses or even sample damage and which limit the light intensity that can be used.
Polarization-Dependent Femtosecond Dynamics of MBE-grown Phthalocyanine Organic Thin Films
Authors: Sandalphon  , Williams V.S. , Meissner K. , Armstrong N.R. , Peyghambarian N.
Published: February 2023

We have observed ultrafast bleaching and induced absorption in MBE-grown chloro-indium-phthalocyanine (InPcCl) thin films. A 200-Å thick film was deposited using van der Waals molecular beam epitaxy onto a transparent tin disulfide substrate. RHEED measurements indicated epitaxy was maintained throughout sample growth. A comparison of linear absorption spectra for epitaxial versus polycrystalline InPcCl is shown in Figure 1. The Q-band absorption is significantly narrower for the epitaxial sample of Figure 1(a) with peaks at 760 nm and 680 nm compared to the extremely broad Q-band absorption for the 400-Å thick polycrystalline sample of Figure 1(b). The ultrafast measurements were performed using a CPM femtosecond laser system with a 620-nm pump and pulse widths of 80fs. The differential transmission (-Δαl) spectra were measured for various time delays between the probe and pump pulses.
Vibrationally unrelaxed cis-stilbene photoproducts examined through two-color uv pump-probe antistokes Raman spectroscopy
Authors: Phillips David L. , Rodier Jon-Marc , Myers Anne B.
Published: February 2023

The photoisomerization of cis-stilbene is of particular interest as a test for theories of solvent effects on reactions occurring on barrierless or nearly barrierless potential surfaces. Recently the dynamics of photoexcited cis-stilbene in solution have been studied by several groups through subpicosecond absorption and fluorescence methods. [1-4] While these techniques allow detailed kinetic studies to be performed, they provide relatively little information about structure. We are carrying out complementary studies which utilize pump-probe antistokes resonance Raman spectroscopy to probe the structural and vibrational relaxation of the ground-state photoproducts.
Femtosecond Waveform Processing via Spectral Holography
Authors: Weiner A.M. , Leaird D.E. , Reitze D.H. , Paek E.G.
Published: February 2023

Compared to the advanced technology now available for generating femtosecond optical pulses, the ability to manipulate and process ultrashort pulses is still rather primitive. Recently, a number of techniques for shaping and synthesizing femtosecond pulse waveforms have been developed.1,2 These techniques rely on linear filtering of the individual optical frequency components contained within an ultrashort pulse. Although these techniques can be very effective, certain basic signal processing operations, such as time reversal, correlation, and convolution, cannot be achieved by linear filtering. In this paper we discuss the use of spectral holography to perform storage, recall, and nonlinear filtering of shaped femtosecond pulses. In particular, we demonstrate time-reversal, correlation, and convolution of femtosecond optical waveforms.
Observation of the thermalization of electrons in a metal excited by femtosecond optical pulses.
Authors: Bokor J. , Fann W. S. , Storz R. H. , Tom H. W. K.
Published: February 2023

We have measured the evolution of the electron energy distribution excited in a 300 Å Au film by a 180 fsec optical pulse. The distribution is measured directly by UV photoemission with a time-resolution of 270 fsec and an electron energy resolution of ~0.04eV. We are able to observe: a) evidence of the nascent (as photoexcited) electron distribution, b) the time-evolution from the nascent to a Fermi-Dirac distribution, and c) that thermalization is more rapid for higher excitation levels.
Investigation of the Reaction Coordinate for Ligand Rebinding in Photoexcited Hemeproteins Using Transient Raman Spectroscopy
Authors: Zhu Huiping , Lingle Robert , Xu Xiaobing , Hopkins J.B.
Published: February 2023

Picosecond two-color difference Raman spectroscopy is used to study the ultrafast photodeligation and the subsequent geminate recombination of oxyhemoglobin and carbonylhemoglobin. Complimentary Stokes and anti-Stokes spectra are used to distinguish vibrational cooling from structural changes in the heme. These structural changes are related to ligand dissociation and rebinding. Transient Raman spectra unambiguously show vibrational cooling and slow ligand rebinding in the hemeproteins. The deconvolved time constant of vibrational cooling is about 2-5 picoseconds.
The C850X Ultrafast Streak Camera : An Instrument to Study Spatially and Temporally Subpicosecond Laser-Matter Interaction
Authors: Mens A. , Sauneuf R. , Schirmann D. , Verrecchia R. , Audebert P , Gauthier J.C. , Geindre J.P. , Antonetti A. , Chambaret J.P. , Hamoniaux G. , Mysyrowicz A.
Published: February 2023

Streak cameras used in laser-matter interaction experiments by CEA at CEL-V have been the subject of years of tube development with the help of Philips Components (now Philips Photonics). In particular the concept of bilamellar electron optics was introduced to simultaneously improve the spatial and temporal performances of streak tubes (1).
A femtosecond lightning rod
Authors: Zhao Kin Miao , Yeh Chao Yung , Diels Jean-Claude , Wang Cad Yi
Published: February 2023

Can laser light be used to trigger lightning? Previous attempts [1] have been unsuccessfull, because they created a dense plasma opaque to the beam by avalanche ionization with an intense infrared laser. The method under investigation here uses a fs UV pulse at 248 nm to create a moderate (1013/cm3) electron density in a narrow (a few cm in diameter) vertical column of air. The charges are injected in the high field region between ground and cloud (at least 100 m altitude). The motion of the charges in the “external” atmospheric field result in a field enhancement (up to tenfold) at the “tip” of the cylinder of charges. A long (up to 1 µs) visible laser pulse has to be sent simultaneously through the same path to prevent the creation of negative ions O− and
O2−
Displacive Excitation of Coherent Phonons
Authors: Vidal J. , Cheng T. K. , Zeiger H. J. , Ippen E. P. , Dresselhaus G. , Dresselhaus M. S.
Published: February 2023

We explain the mechanism for coherent phonon generation in Bi, Sb, Te and Ti2O3. Furthermore, this framework is used to analyze the temperature dependent transient reflectivity response of Ti2O3.
Increase in Exciton Radiative Lifetime in GaAs Quantum Wells at High Exciton Density due to Exciton-Exciton Scattering
Authors: Eccleston R. , Feuerbacher B.F. , Kuhl J. , Rühle W.W. , Ploog K.
Published: February 2023

We report time-resolved photoluminescence (TRPL) measurements of the radiative lifetime (τr) at 1.8K of resonantly excited heavy-hole excitons in a 27nm GaAs single quantum well as a function of exciton density. By the use of AlAs interface smoothing layers [1], the sample exhibits very little inhomogeneous broadening (≈0.15meV). The exciton centre of mass motion is therefore not significantly restricted by localisation in well width fluctuation islands. However, measurements of the exciton homogeneous linewidth (Γ
h
) using time-resolved degenerate four wave mixing in the self-diffraction configuration (Fig.1) [2], additionally show that the sample imperfection scattering contribution to the Γ
h
of these delocalised excitons is also small (0.2meV). Thus, as indicated by the dependence of Γ
h
on exciton density (N
x
), even at moderate exciton densities, exciton-exciton scattering is the dominant dephasing interaction within the photocreated exciton ensemble.
Picosecond Single Shot Pulse Shape Measurement by Stochastic Sampling of Detected Photon Times
Authors: Adams N. , Bovet C. , Rossa E. , Simonin A.
Published: February 2023

Sampling techniques are frequently used in order to measure the shape of repetitive pulses. They imply a synchronous trigger plus a variable delay and a peak sensing device which often sets the limit to the time resolution thus achievable. The best sampling oscilloscopes have a rise time in the range of 10 ps but they cannot be used for single shot measurement. A streak camera with a display of the pulse on a memory tube makes it possible to reach a resolution of 1 ps. However, the use of a streak camera is cumbersome and cannot be done on line because of the time needed for digital image evaluation. In some cases, transient recorders may be used, but the passband of such apparatuses is at present only of 6 GHz, which is insufficient for studying very brief light pulses. An alternative procedure consists in measuring the times of occurrence of events whose probability is proportional to the pulse height. The histogramme of these occurrences reproduces the pulse shape, provided the event probability is small enough for multiple event rate to be negligible. This method, called here stochastic sampling, is attractive because it consists in measuring the time of flight between a detected photon and a base time clock; and time measurements can easily be performed with TDCs or TACs to a resolution better than 10 ps. It is proposed to store the light pulse in a system of optical fibres which will deliver single photons as a periodic function. This will speed up the sampling by orders of magnitude, and eventually allow for single shot measurements.
100-Gbps Response of Microcavity Lasers
Authors: Yokoyama H. , Nambu Y. , Shimizu T.
Published: February 2023

A lot of research interest is presently focused on microcavity lasers. These structures are resonators having at least one dimension of the order of a wavelength. So far, attention has been paid mainly to controlling spontaneous emission and the threshold-less feature of the microcavity laser [1], i.e. the advantage of low power consumption. However, the microcavity laser offers another attractive feature: the ultrafast response of possibly over 100 gigabit-per-second (Gbps). One reason is due to the extremely short photon lifetime in an appropriately designed microcavity accompanied by the extremely short cavity length. The other reason for the response speed increase is the cavity-enhanced spontaneous emission rate. Assuming a linear gain, the resonant relaxation oscillation frequency is inversely proportional to the square-root of both lifetimes. A three-dimensionally or two-dimensionally confined microcavity could induce a marked reduction in the spontaneous emission lifetime. In a planar microcavity, although the enhancement in the spontaneous emission rate is at most a factor of ~2 [2], ultrafast response is still expected. For example, a symmetric Fabry-Perot cavity having a 0.99 reflectivity and λ/2 length gives a 0.2 ps photon lifetime. This value is ~1/10 of a conventional diode laser. Assuming a 1 ns spontaneous emission lifetime and an excitation level of 20 times higher than the threshold, the above photon lifetime gives a relaxation oscillation frequency of ~100 GHz. Under these conditions, large signal analysis has shown that the light output follows a series of 100 Gbps excitation pulses without serious distortions [3].