bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrafast Time-resolved Spectroscopy of Structural Phase Transitions, Vibrating Crystals and Molecules, and Chemically Reacting Molecules
Authors: Farrar Margaret R. , Williams Leah R. , Nelson Keith A.
Published: February 2023

An "impulsive" stimulated scattering (ISS) technique has been developed through which ultrashort laser pulses can be used to excite coherent vibrational motion and to probe the vibrating material at various stages of vibrational distortion. Recent ISS experiments and related theory are reported.
Propagation of Ultrashort Electrical Pulses on Superconducting Transmission Lines
Authors: Duling I. N. , Chi C.-C. , Gallagher W. J. , Grischkowsky D. , Halas N. J. , Ketchen M. B. , Kleinsasser A. W.
Published: February 2023

Electrical pulses 0.6 ps in duration have been generated on a transmission line structure by use of a fast-recombination-time photoconductive switch driven by a subpicosecond optical pulse. [1] These wide bandwidth electrical pulses are ideal for measuring the absorption and dispersion of a transmission-line-substrate combination by observing their propagation along the structure. [2] This technique has been applied here to the study of superconducting transmission lines. Briefly, by Fourier transform techniques the complex frequency content of the pulse can be found, and by comparing the pulse before and after propagation, the absorption and dispersion are calculated. The niobium lines become superconducting at 9 degees Kelvin. The transmission lines are He vapor cooled and temperature regulated in an optical dewar. The opto-electronic sampling technique used is well suited to operation inside a dewar since all fast electrical signals (0-1 THz) are confined to the sample and only slow signals (0-1 KHz) must leave the dewar.
The Stabilized Soliton Laser
Authors: Mitschke F. M. , Mollenauer L.F.
Published: February 2023

The soliton laser [1] consists of a synch-pumped, mode-locked color center laser, tunable in the 1.5 μm region, coupled to a second cavity, containing a single mode, polarization preserving optical fiber. (See Fig. 1.) Feedback from the fiber, where pulse compression and soliton formation take place, enables the color center laser to produce transform limited pulses of ~sech2 intensity profile and of any desired pulse width, down to about 100 fsec, through choice of the control fiber length.
Ultrafast Dynamics in GaA1As Diode Laser Amplifiers
Authors: Stix M. S. , Kesler M. P. , Ippen E. P.
Published: February 2023

Studies of the nonlinear and ultrafast properties of semiconductor diode lasers are necessary for an improved understanding of their high speed modulation, and ultrashort pulse generating, characteristics. Previous workers have used picosecond light pulses to probe the electrical modulation characteristics and time-average gain in these devices. In our work we have used optical pulses to both induce and probe ultrafast dynamics of diode laser gain. Our results, the first observations of subpicosecond behavior in diode lasers, provide direct information on the performance of active devices.
An experimental study on cavity-mirror dispersion dependence of pulse duration generated from a simple CPM laser
Authors: Yamashita Mikio , Ishikawa Mitsuru , Torizuka Kenji , Sato Takuzo
Published: February 2023

From recent studies on CPM dye lasers, it was verified that the most important thing to generate pulses shorter than 100 fsec is to compensate chirping leaded by dispersion and phase modulation due to optical elements composing of the laser cavity. Dietel et al. produced pulses shorter than 6 0 fsec by the adjustment of the optical path of a positive dispersion prism-glass in the cavity for compensation of down-chirping, while Valdmanis et al. recently have produced pulses as short as 27 fsec by the adjustment of the distance between prisms causing negative cavity dispersion for compensation of up-chirping. Consequently, the former conclusion that down-chirp is dominant in the CPM laser is contrary to the latter one that up-chirp is dominant. In addition, both the cavity configurations are complicated for optical alingment and experimental estimation of dispersion proper to the CPM laser because of the additional insertion of one or four prisms leading to negative cavity dispersion.
Collective Time Modes - An Analytical Model for Active Mode-Locking in the Transient Case
Authors: Aechtner P. , Heinz P. , Laubereau A.
Published: February 2023

Analytical investigations of active mode-locking have been restricted in the past to the stationary case or to a specific initial situation. The statistical character of the initial condition of multimode lasers starting from spontaneous emission was completely omitted. In this work a novel concept is presented which incorporates the complicated temporal and frequency structure of the initial condition, introducing collective temporal laser modes instead of conventional frequency modes.
A 4 PS Dye Laser Using the Sweeping Oscillator Method.
Authors: Meyer Y.H. , Martin M.M. , Breheret E. , D’azy O. Benoist
Published: February 2023

Production of ultrashort pulses at various wavelengths is generally based on mode locking techniques and, for practical applications, often requires the combined use of two expensive pump lasers, one CW, one pulsed. Other ways have been studied to produce short pulses using cavity transients, distributed feedback or cavity competition processes (1). In this work we report progress in a new technique, recently proposed (2), for obtaining high power ultrashort pulses from dyes using a single standard nanosecond pump laser.
Picosecond Photoluminescence and Energy-Loss Rates in GaAs Quantum Wells under High-Density Excitation
Authors: Kobayashi Takayoshi , Uchiki Hisao , Arakawa Yasuhiko , Sakaki Hiroyuki
Published: February 2023

Several features of the luminescence from the GaAs/AlGaAs semiconductors with quantum well structures have been extensively studied. Among the features studied the relaxation mechanism of hot carriers in quantum wells are still controversial among the studies at the present stage. The investigation of the relaxation process must be performed in order to clarify the essential mechanisms of excellent characteristics of laser oscillation such as the reduction of threshold current and the control of lasing over wide wavelength region.
Spin Dephasing Kinetics in Alloy Semimagnetic Semiconductors Cd1−xMnxSe
Authors: Junnarkar M. , Alfano R. R.
Published: February 2023

Picosecond time resolved spin relaxation kinetics of high density free carriers is investigated from luminescence at low temperatures in CdSe (x=0) and in dilute in the semimagnetic semiconductor Cd1−xMnxSe for x=0.05 and 0.10. The fast spin relaxation observed in CdSe results from a mechanism associated with the non-centrosymmetric character of the band structure for this material. This process is similar to the one proposed by D'yakonov and Perel' (D-P) for the zinc blende crystal structures. The spin relaxation times in CdSe are of the order of 30 psec and the observed spin polarization factor for carriers in CdSe is in good agreement with theory. The spin relaxation times are < 20 psec in Cd1−xMnxSe and are consistent with spin flip Raman scattering measurements. The increase in spin relaxation rate relative to CdSe is explained in terms of the carrier spin exchange between the carriers and the magnetic spin sites. The carrier spin exchange with magnetic spin sites and the D-P process inherent in the host crystal are equally important in semimagnetic semiconductors. The spin polarization factor at t=0, ρ1(0), obtained in CdSe is ~ 50% using one photon excitation, while for Cd0.95Mn0.05Se and Cd0.9Mn0.1Se the values (ρ2(0)) are ≈ 18% and ≈ 28% using two photon excitation.
Photoisomerization Studies of Substituted Stilbenes: 4,4’-Dihydroxystilbene and 4,4’-Dimethoxystilbene
Authors: Zeglinski David M. , Waldeck David. H.
Published: February 2023

Recent studies of photoisomerization in liquid solution have revealed significant modification of isomerization rates by frictional and dielectric solute-solvent interactions. Reported here are initial studies of the isomerization dynamics of 4,4’-dihydroxystilbene and 4,4’-dimethoxystilbene in the normal alcohols. Comparisons are drawn between this work and previous studies of the unsubstituted stilbene [1].
Polarization Dependence of Picosecond CARS in Liquids
Authors: Kohles N. , Laubereau A.
Published: February 2023

We have performed a theoretical and experimental study of time resolved coherent Raman scattering under various polarization conditions. Three mechanisms are shown to contribute to the scattering signal in the general case: (i) scattering off the isotropic part of the resonant material excitation with vibrational dephasing time T2. (ii) scattering from a second, oriented component of the resonant material excitation with "anisotropic" time constant τan (iii) four wave mixing via the non-resonant part χNRof the third order susceptibility.
Picosecond Pulse Generation from a Continuous Wave Neodymiun: Phosphate Glass Laser
Authors: Yan L. , Ling J. D. , Ho P.-T. , Lee Chi H.
Published: February 2023

Up to now, very short (a few picoseconds) pulses from neodymium:glass laser can only be generated by Q-switching at repetition rates of a few hertz. Here, we report the CW operation of an actively modelocked Nd: phosphate glass laser which can generate 7 ps pulses. These pulses are about 14 times shorter than those from a CW modelocked Nd:YAG laser and 10 times shorter than those previously obtained by our group [1].
Time-Resolved Observation of Electron-Phonon Relaxation During Femtosecond Laser Heating of Copper
Authors: Elsayed-AIi H. , Pessot M. , Norris T. , Mourou G.
Published: February 2023

Thermal modulation of the optical properties of metals is a widely used technique in studying critical points in band structure. Recently the modulation of reflectivity of copper has been used to observe nonequilibrium electron-lattice temperatures during picosecond (~5 ps FWHM) laser heating of up to a few degrees1. Although nonequilibrium heating was demonstrated in these experiments, the time resolution was insufficient to resolve electron-phonon relaxation. In a subsequent report, the phenomenon of thermally enhanced multiphoton photoemission was used to time-resolve electron-phonon relaxation in tungsten2. Results indicated that such relaxation is accomplished in a few hundred femtoseconds. We report results obtained using amplified 150-300 fs laser pulses to time-resolve electron-phonon relaxation by monitoring the laser-heating-induced modulation of the transmissivity of 200 Å copper films.
Nickel(II) Octaethylporphyrin: Conformational Dynamics, Ligand Binding and Release
Authors: Courtney S.H. , Jedju T.M. , Friedman J.M. , Rothberg L. , Alden R.G. , Park M.S. , Ondrias M.R.
Published: February 2023

Metalloporphyrins are important in a variety of biological functions, such as oxygen transport, electron transfer, and energy conversion. To understand their ability to bind ligands and participate in electron transfer reactions, the excited state dynamics of these complex molecules must be elucidated. In non-coordinating solvents, photoexcitation of NiOEP leads to the formation of a relatively long lived excited state (*Blg) that decays to the 1Alg ground state in ≈300ps(1,2). To further investigate the excited state dynamics, we undertook a subpicosecond transient absorption study in chloroform and methylene chloride. We observed the appearance of transient absorption bands in <2ps and spectral evolution of the transient bands (see figure 1).
Vibrational Temperatures of Photoexcited Deoxy-Hemoglobin
Authors: Courtney S.H. , Friedman J.M. , Alden R.G. , Chavez M.D. , Ondrias M.R.
Published: February 2023

To address the vibrational population changes of deoxy-Hemoglobin following photoexcitation(1,2) we have measured the anti-Stokes/Stokes ratio of ν7, the strongest mode in the low frequency region of the spectrum and find an area ratio of 6.7% to 10.2%. Several observations suggest that this mode is not vibrationally hot on a 30ps timescale. 1) A two color pump/probe protocol using a strong 532nm pump pulse temporally and spatially overlapped with the 436 nm probe pulse resulted in no change in the relative intensities. 2) Experiments using 10ns pulses at the same wavelength yield an anti-Stokes/ Stokes ratio of 8.6% for the 674 cm−1 mode. 3) The anti-Stokes/Stokes ratio is consistent with the value expected at 300K (The Boltzmann ratio of ν7 corrected with a Kramers-Kronig transform technique is 8.2%). Figure 1 shows the anti-Stokes and Stokes Raman spectra of deoxy-Hb taken with 30ps pulses of three heme modes, ν7, ν16, and ν32 at 671, 756, and 790 cm−1 respectively.
Excited State Dynamics of Jet Cooled Substituted Indoles: The Role of Polar Interactions in 1La-1Lb State Coupling
Authors: Demmer David R. , Leach Gary W. , Wallace Stephen C.
Published: February 2023

One of the most compelling reasons for undertaking the study of indole and its derivatives under jet-cooled conditions is to elucidate the unusual excited state properties which make them useful as fluorescent probes in biological systems. The remarkable sensitivity of the fluorescence quantum yield and emission spectra of these compounds to the environment of the indole nucleus has been extensively used as a probe of conformational fluctuations in polypeptides and proteins. Efforts to understand this sensitivity are based on a description involving two electronic states which underlie the lowest energy absorption band systems in condensed phases. The highly structured absorption near the red edge has been attributed to transitions to the 1L
b
state, while a broad continuum peaking to higher energies has been assigned to the 1L
a
state, which appears to lie above the 1L
b
state for most indole derivatives studied. It is generally agreed that the 1L
a
state has a much larger excited state dipole moment than the 1L
b
state. It is therefore subject to a larger stabilization in polar solvents, which can lead to an inversion in the ordering of the excited states, thus providing an explanation for the extreme sensitivity to solvent polarity.
Molecular Alignment in the cis to trans Photoisomerization of Stilbene
Authors: Repinec Stephen T. , Abrash Samuel A. , Hochstrasser Robin M.
Published: February 2023

A series of experiments were carried out on the cis to trans isomerisation of stilbene. The lifetime of excited cis-stilbene was measured in a series of n-alkane solvents. The decay times ranged from 1 picosecond in hexane to 1.5 picoseconds in hexadecane. Absorption anisotropy measurements were performed at three probe wavelengths (335, 350, and 650 nm) and in two solvents (hexane and hexadecane). These measurements are used to study the alignment of the transition dipoles as the molecule isomerizes.
Femtosecond Laser Photolysis Studies on the Conformation Change of Benzil in Solutions
Authors: Miyasaka Hiroshi , Mataga Noboru
Published: February 2023

It is well-known that benzil (diphenylethanedione) has near-skewed form in the ground state, while the structure in the excited singlet state has been attributed to trans-planar geometry. (The terms "skewed" and "planar" used here refer to the dihedral angle between the planes containing the benzoil groups.) This structural change of benzil in solution has been considered to be very fast at room temperature. In this study, we have directly measured the structural change process by means of transient absorption spectroscopy with femtosecond laser. On the basis of the experimental result, we will discuss the factors controlling the change of molecular structure.
Ultrafast predissociation dynamics of isolated small molecules from uv resonance Raman depolarization ratios
Authors: Myers Anne B. , Li Bulang
Published: February 2023

Depolarization ratios of nonresonant Raman scattering depend on the molecular point group and the symmetry of the vibrational mode excited. When the excitation frequency approaches an electronic resonance, however, the "time" that may formally elapse between destruction of the incident photon and creation of the scattered photon may become long, and the polarization properties then depend on the rotational dynamics and relaxation pathways in the resonant electronic state as well [1]. For small isolated molecules, the rotational dynamics are often well defined, and the polarization of the emission observed with steady-state excitation can allow accurate determination of pico- and subpicosecond excited state lifetimes. This technique is essentially the light scattering analog of photofragment spectroscopy in cases where the lifetime is limited by dissociation.
Femtosecond Generation of Mid Infrared Pulses at 8.5 kHz via Parametric Mixing in AgGaS2
Authors: Becker P.O. , Gershoni D. , Prosser A.
Published: February 2023

There is currently a need for a stable and widely wavelength tunable femtosecond laser source for time resolved spectroscopic investigations. One approach is to amplify CPM pulses and generate a broad spectral continuum which can be either used directly or reamplified to reach wavelengths other than the fundamental CPM wavelength of 620 nm. Due to the high intensities available with the amplified CPM and continuum, there is potential for parametric mixing in a variety of nonlinear crystals so as to obtain femtosecond pulses in the 1-20 μm wavelength range. We report here preliminary results of the successful mixing of a femtosecond amplified CPM pulse and a femtosecond continuum so as to generate pulses in the mid infrared, at a repetition rate of 8.5 kHz. "Seeded" parametric amplification has been used previously by Jedju and Rothnerg to generate pulses in the 2-5 μm wavelength range using 350 fs pulses at a 10 Hz repetition rate.1
Vibrational Exciton Decay in Uranyl Crystals by Picosecond Coherent Raman: Energy Bottlenecks and High Order Relaxation Mechanisms
Authors: Chronister E. L. , Crowell R. A.
Published: February 2023

Anharmonicity in the crystal potential leads to phonon interactions which result in vibrational relaxation and dephasing. For many crystal systems a satisfactory description of vibrational relaxation is obtained by considering the first anharmonic correction to the crystal potential, namely a cubic interaction. However, recent results on vibrationally sparse systems have shown that the next higher order anharmonic relaxation term (i.e quartic) is necessary to account for the observed thermally induced dephasing [1-3]. In this work we focus on the importance of anharmonic relaxation mechanisms by creating a large vibrational energy gap. One expects the mode immediately above the gap to have an increased lifetime relative to other vibrational modes of the molecule due to the absence of any cubic decay channels.
Second-harmonic Efficiency and Reflectivity of GaAs During Femtosecond Melting
Authors: Wang Juen-Kai , Saeta Peter , Siegal Yakir , Mazur Eric , Bloembergen Nicolaas
Published: February 2023

Studies of the ultrafast melting of semiconductor surfaces under intense laser irradiation at frequencies above the bandgap reveal changes in the dielectric properties on the sub-picosecond timescale. Reflectivity measurements of silicon with femtosecond pulses demonstrate significant changes within 300 fs.1 Second-harmonic measurements on silicon further show that the top 70-130 Å of the silicon surface loses its cubic symmetry within 150 fs.1 In this paper we report on simultaneous reflectivity and second-harmonic generation (SHG) measurements during the melting of GaAs with 160-fs laser pulses. The results show bulk melting to a depth of at least 90 atomic layers within the pulsewidth.
Studies of Diffusion-Influenced Fluorescence Quenching
Authors: Eads Daniel D. , Dismer Berit G. , Fleming Graham R.
Published: February 2023

Picosecond and nanosecond time resolve experiments are combined with steady-state fluorescence measurements to examine Smoluchowski and Collins and Kimball type descriptions of a fluorescence quenching reaction.
Femtosecond Relaxation of the Excited Carriers in CdSexSl-x Doped Glass Produced by High-Intensity Laser Pulse
Authors: Chekalin S.V. , Farztdinov V.M. , Lozovik Yu.E. , Matveetz Yu.A. , Stepanov A.G. , Yartsev A.P.
Published: February 2023

Carrier relaxation in semiconductor-doped glasses incorporating small (R~100 Å) CdSexSl-x crystallites in a glass matrix was investigated previously at carrier density up to 1018cm-3 [1,2]. It was found that the relaxation time depends strongly on the level of the excitation above the band gap [1].
The Impact Approximation in Transition-State Absorption Spectroscopy: On the Possibility of Obtaining Difference Potentials Directly from the Transient Spectra
Authors: Walkup R. E. , Misewich J. A. , Glownia J. H. , Sorokin P. P.
Published: February 2023

The impact approximation in transition-state absorption spectroscopy is introduced. Its usefulness in recovering difference potentials from transient spectra is evaluated.
Subpicosecond non Linearities of Semiconductor near Infrared Bragg Reflectors
Authors: Le Saux G. , Pocholle J.P. , Georges P. , Schnell J.P. , Salin F. , Papuchon M. , Brun A.
Published: February 2023

Bragg reflector devices have been built in semiconductor materials. Many applications covering the realization of surface emitting laser diodes and mirrors activated under electrical field have been demonstrated [1]. We describe in this paper experimental results showing the optical non linearities induced by light carrier interactions in Ga1-xAlxAs / Ga1-y Aly As Bragg reflectors, in the subpicosecond regime.
Measurement of Transient, High-Temperature Electrons from Solid-Density Plasmas
Authors: Banyai W. C. , Anacker D. C. , Wang X. Y. , Reitze D. H. , Focht G. B. , Downer M. C. , Erskine J. L.
Published: February 2023

Metal surfaces have been excited with high-intensity, ultrafast optical pulses in order to understand the relaxation dynamics of the transient nonequilibrium between the electron and lattice temperatures, the temperature-dependent collisionality of the material1 and the radiation of soft x-rays from a high-temperature, solid-density plasma2. Pulsewidths on the order of 100 fs offer the bonus of depositing large amounts of energy within a skin depth of the surface, creating transient, high electron temperatures. The electrons thermalize rapidly via electron-electron scattering while electron-phonon scattering is expected to take more than 100 fs. Experimental techniques used to study the electron cooling dynamics include thermally assisted photoemission3, thermal modulation of the optical constants4 and multishot optical damage5. The critical issues in the measurement of the electron-lattice coupling constant include the trade-off between diffusion of the electron gas and electron-lattice energy exchange, the energy distribution of the hot electrons and the thickness of the sample6. These same dynamics are important to the realization of a short-pulse, soft x-ray source since the tail of the x-ray pulse will follow the cooling of the electron energy distribution. We report the direct measurement of the time-resolved electron energy distribution from a Ag(111) surface under ultrafast optical excitation.
Hot Phonon Spectra of Polar Semiconductors Studied Via Picosecond Free Carrier Absorption Between 5µm and 10 µm
Authors: Elsaesser T. , Bäuerle R.J. , Klann R. , Kaiser W.
Published: February 2023

Excess LO phonons created by carrier cooling play an important role for the relaxation of hot electrons and holes in polar semiconductors. Recently, we have introduced a novel method for the direct study of hot phonons via transient changes of the intraband absorption of electrons /1/. Intraband absorption represents a three particle interaction of electron, photon and phonon, where the coupling to the lattice provides the conservation of the k-vector /2/. In our experiments, the electron gas in an n-doped sample is heated to a temperature of approximately 500 K by an intense infrared pump pulse. The resulting change of free carrier absorption was monitored by a weak probe pulse at the same wavelength. In InAs, a significant enhancement of intraband absorption was found, which relaxes on a time scale of several tens of picoseconds. The theoretical analysis of the data demonstrates, that this effect is mainly due to the occurrence of excess LO phonons, which couple in the absorption process and increase the absorption coefficient. In this paper, we discuss new frequency resolved measurements of the transient free carrier absorption in InAs and GaAs. Data taken at different wavelengths between 5 and 10 µm give detailed information on the spectral distribution of the excess phonons in k-space.
Spontaneous Emission and Ultrafast Dephasing in front of a Mirror.
Authors: Drabe Karel E. , Cnossen Gerard , Wiersma Douwe A.
Published: February 2023

We report a new technique to probe femtosecond dephasing processes in solids, requiring only low excitation intensities. The method is based on the measurement of the spontaneous emission spectrum of a thin sample (about 2µm) in front of a mirror, excited by 40 fs pulses from a colliding pulse modelocked (CPM) laser. Fig. 1A shows the simplified level scheme of a prototype molecule, and fig.1B gives the schematics of the experimental arrangement.
Optically Induced Electromagnetic Pulses from Semiconductor Surfaces
Authors: Zhang X.-C. , Hu B.B. , Darrow J.T. , Xin X.H. , Auston D.H.
Published: February 2023

Teraheratz bandwidth electromagnetic radiation can be optically induced from semiconductor surfaces.1 This diffraction-limited electromagnetic beam propagates in free space and can be optically steerable by varying the incident angle of the laser beam. The waveform of the radiated field depends on the impurity concentration, carrier mobility, and built-in field near the surface.
Picosecond Pulse Generation Circuits using a GaAs Nonlinear Transmission Line
Authors: Madden C.J. , Marsland R.A. , Bloom D.M. , Rodwell M.J.W.
Published: February 2023

Circuits for generation picosecond step and pulse signals have many applications in wide bandwidth time-domain and microwave instrumentation. The silicon step-recovery diode is widely used in sampling oscilloscopes and network analyzers for gating sampling diodes but has limited utility in optoelectronic and ultrafast applications due to its relatively slow speed (30-40 ps). Conversely, photodiodes and photoconductors produce much narrower electrical pulses [1,2] but the real pulse generator is a femtosecond or picosecond mode-locked laser. Using only RF drive, we have measured a 6 V, 1.8 ps falltime step output as well as a 2.4 V, 4.4 ps full width at half maximum (FWHM) pulse output from monolithic GaAs nonlinear transmission line (NLTL) circuits.
The Non-Linear Interaction of Visible Light and X-Rays: The Measurement of Femtosecond Duration Soft X-Ray Pulses
Authors: Freeman R.R.
Published: February 2023

We are developing a high resolution technique for the direct measurement of pulse durations of soft x-ray pulses that arise from laser-induced plasmas driven by sub-picosecond lasers. This technique relies on the ponderomotive effect (high intensity Stark Effect) of a high intensity (~1014W/cm2), 100 fsec visible laser on the absorption properties of core transitions in noble gases at soft x-ray wavelengths. We employ cross correlation in high density krypton of a 100 fsec 616nm laser pulse with an x-ray pulse produced from a laser plasma driven by a high intensity, 300 fsec 308nm laser. The 616nm laser effectively "switches" the absorption of the x-rays in the krypton off and on with a resolution exactly equal to its pulse duration. We have demonstrated the technique by making a preliminary measurement of the pulse duration of 100 eV soft x-ray pulses derived from the interaction of a 300 fsec, 3mj 308nm laser pulse on a gold rod. We observe a cross correlation signal on the order of 1 psec. This value is in rough agreement with computer modeling codes (LASNEX) for our experimental conditions. To our knowledge this is the only demonstration of a technique for the measurement of the duration of soft x-ray pulses with a resolution of better than a few picoseconds. In addition, it is an example of a rather rare experimental occurrence: the non-linear interaction of visible light and x-rays. This measurement technology is crucial to the development of a soft x-ray "framing" camera capable of recording changes in atomic positions in solids with a resolution sufficient to resolve electron-phonon interactions.
Ultrafast studies on intramolecular charge transfer and solvation dynamics
Authors: Barbara Paul F.
Published: February 2023

Much of the recent activity on the study of fast charge (electron) transfer ET reactions stems from the growing appreciation that the rates of ET can be affected by solvent dynamics in contrast to traditional theories of ET which are based on a quasi—equilibrium assumption. Our work has emphasized femtosecond studies of the excited state, S1 small barrier, ultrafast ET reactions of aromatics, including BA, ADMA, derivatives of ADMA, and several other molecules. We have shown theoretically that emission dynamics at the short wavelength edge of the fluorescence band accurately measures the decay of the prepared (reactant’s) concentration. Results on the solvent dependence of <τET> demonstrate that the ET kinetics of the various molecules are controlled by solvation dynamics, see Table 1. However, we have shown that simple continuum models for solvation dynamics (when combined with theories of ET) do not correctly predict ET rates. Rips and Jortner have analyzed our data and shown that the MSA model (combined with ET theory) more accurately agrees with experiments. This emphasizes the importance of molecular interactions in dynamic solvent effects on chemical reactions.
Femtosecond Intermolecular Dynamics of Liquid Carbon Disulfide at Elevated Pressures
Authors: Kohler Bern , Nelson Keith A.
Published: February 2023

The dynamics of small, highly polarizable molecules have been studied extensively by light scattering. We have shown that the time-resolved technique of Impulsive Stimulated Scattering (ISS) can offer distinct advantages over conventional frequency-domain light scattering.1 In particular, ISS is well-suited to the measurement and characterization of low-frequency heavily-damped modes of molecular motion. Various theories of the liquid state predict such modes at high densities as a result of molecular caging effects. As the density of a liquid is increased, nearest neighbor distances decrease on average and the short-time dynamics of molecules are expected to be dominated by repulsive intermolecular interactions. A temperature-dependent study of the transient response of CS2 showed a weakly oscillatory signal suggestive of intermolecular vibration.2 To further study intermolecular vibration in dense liquids we have combined femtosecond time resolution with diamond anvil cell technology to study the ultrafast dynamics of CS2 molecules at constant temperature as a function of pressure.
Femtosecond Transition-State Absorption Spectroscopy of Bi Atoms Produced by Photodissociation of Gaseous Bi2 Molecules
Authors: Glownia J.H. , Misewich J.A. , Sorokin P.P.
Published: February 2023

Spectroscopic observation in real time of the "transition states" of a chemical reaction has recently become possible with advances in ultrafast laser technology. A.H. Zewail et al. have con ducted femtosecond transition-state spectroscopy (FTS) experiments’ on several gas-phase photodissociation reactions. In these FTS experiments, a femtosecond UV pump pulse first excites a molecule to a repulsive electronic state. A tunable femtosecond probe pulse, delayed by a variable time from the pump pulse, then detects one of the photofragments via laser induced fluorescence (LIF) or multiphoton ionization. Spectral changes that arc observed in the (LIF) excitation spectrum at small pump-probe separations thus provide information about variations in the separation between the potential curves involved in the probe transition, occurring as the interatomic spacing is varied, with the photofragment still in the force field of the remaining parts of the molecule, i.e., with the whole photodissociating complex still in the process of passing through a continuous sequence of "transition states".
Dynamics of Excitons in Conjugated Polymers
Authors: Yoshizawa Masayuki , Taiji Makoto , Kobayashi Takayoshi
Published: February 2023

Optical and electrical properties of conjugated polymers have been extensively investigated because 'of their large and fast optical nonlinearities and their role as model compounds for quasi-one dimensional semiconductors. In the present study, time evolution in polydiacetylenes (PDA-3BCMU and PDA-4BCMU) and poly(3-methy1thiophene) (P3MT) was investigated by femtosecond absorption spectroscopy and picosecond luminescence spectroscopy [1]-[3]. The samples of PDA-3BCMU and PDA-4BCMU (butoxycarbonylmethylurethane) are cast films of about 100 nm thickness on glass substrates. About 1 μm-thick films of P3MT was prepared by electrochemical polymerization on glass substrates.
Femtosecond Dynamics of Single Electron Transfer in Aqueous Media
Authors: Gauduel Y. , Pommeret S. , Yamada N. , Migus A. , Antonetti A.
Published: February 2023

Recent advances in the femtosecond investigations of single electron transfer in aqueous media are reviewed. Primary events involved in radical reactions (geminate recombination, encounter pair formation) are discussed.
Chirped Atomic Electron Wave Packets
Authors: Noordam L.D. , Wolde A. ten , Muller H.G. , Lagendijk A. , van Linden van den Heuvell H.B.
Published: February 2023

A coherent superposition of atomic electron eigenstates gives rise to localization of the wave function. Creation and detection of these so-called wave packets is possible by picosecond dye lasers. In this contribution we describe two kinds of wave packets, and one application: radial wave packets (1-3) created by a superposition of n-states; parabolic wave packets (4,5) created by a superposition of k-states; finally we will show that the chirped character of these matter waves can be used to shorten optical pulses (6).
Dramatic Reduction Of The Amplitude Noise Of Colliding-Pulse-Modelocking Dye Lasers
Authors: Nuss M. C. , Keller U. , Harvey G. T. , Heutmaker M. S. , Smith P. R.
Published: February 2023

The colliding pulse modelocking (CPM) dye laser has established itself as the work horse of femtosecond optics and spectroscopy owing to the ease of generating stable optical pulses of less than 100 fsec duration. The probably most important improvement of the CPM laser has been the addition of a four-prism sequence to adjust the intracavity group velocity dispersion1. However, the characterization of its amplitude noise has not received sufficient attention. We show in this manuscript that the strong amplitude noise of the CPM laser at frequencies of several MHz can be significantly reduced by simply inserting a single frequency etalon in the cavity of the (argon-ion) pump laser.
Simultaneous Two-Photon Absorption and Second Harmonic Generation in CdSe
Authors: Penzkofer A. , Schaffner M. , Bao X.
Published: February 2023

CdSe is a hexagonal crystal of 6mm point symmetry. Two-photon absorption (TPA) and second harmonic generation (SHG) occur simultaneously when picosecond light pulses of a Nd:glass laser (wavelength 1054 nm) are passed through the crystal. Two-photon absorption coefficients determined previously are collected in ref.1. The contribution of the resonant non-phasematched second harmonic generation to the pump pulse transmission through CdSe [2] has not been studied in detail.
Physical Properties of Coupled Cavity Lasers Used for Ultrashort Pulse Generation
Authors: Morin Michel , Champagne Yves , Martin Francois , Vallée Réal , McCarthy Nathalie , Piché Michel
Published: February 2023

Coupling a mode-locked laser to a nonlinear external cavity has led1-6 the emission of ultrashort pulses whose duration can be reduced by two orders of magnitudes. The method has allowed to generate pulses in the 100 fs range from colour center lasers. With Ti: sapphire lasers, the emission of ultrashort pulses was observed6 to be self-starting, e.g. the laser did not require the use of a mode-locking device. Although the nonlinear element in the external cavity is generally an optical fibre, semiconductor amplifiers can also be used1,4. The physical process leading to pulse narrowing has been identified2,7: chirping of the pulse circulating in the coupled cavity leads to a time-dependent modulation after interferometric recombination with the pulse circulating in the main cavity.
Far-Infrared Spectroscopy of High Temperature Superconductors with Terahertz Electrical Pulses
Authors: Nuss Martin C. , Goossen K. W. , Mankiewich P. M. , Howard R. E. , O’Malley M. L. , Marshall J. L. , Kisker D. W.
Published: February 2023

In recent years, a variety of techniques have been developed to generate and detect ultrafast electromagnetic transients with bandwidths up to 1 THz1. The spectrum of these transients spans part of the microwave, submillimeter wave and far-infrared range of the electro-magnetic spectrum. The spectroscopic benefits include the enormous spectral bandwidth covered, the coherent detection that enables to extract both real and imaginary part of the dielectric response function, and a sensitivity that far exceeds the sensitivity of conventional far-infrared spectrometers in the same spectral range.
Femtosecond Absorption Studies of Nonequilibrium Electronic Processes in High-Tc Superconductors
Authors: Chwalek J. , Uher C. , Gupta S. , Whitaker J. , Agostinelli J. , Lelental M.
Published: February 2023

The time resolution of nonequilibrium electron processes using short laser pulses has produced important information regarding the thermal properties of the carriers and phonons in metals1-2. Recently the use of short laser pulses (~ 8 ps) has been used to study the electron-phonon relaxation rate in single crystal, non-superconducting lanthanum cuprate3. The authors found nonequilibrium heating to occur on a picosecond time scale. In this paper we present for the first time, evidence of nonequilibrium heating on a single picosecond time scale in epitaxial thin-film samples of the compounds YBa2Cu3O7-x (0 < x < 0.6) and Bi2Sr2Ca2Cu3O10.
Femtosecond Carrier-Carrier Interactions in GaAs
Authors: Gong T. , Ucer K. B. , Zheng L. X. , Wicks G. W. , Young J. F. , Kelly P. J. , Fauchet P. M.
Published: February 2023

Despite recent femtosecond measurements using pump-probe [1], photon echo [2], and photoluminescence [3], and theoretical studies [3,4] of screening, carrier-carrier scattering, such as electron-hole and electron-plasmon interactions, is still not well understood. We have made a series of femtosecond spectroscopic measurements on intrinsic, n-type, and p-type GaAs using the usual pump-probe arrangement and a novel three-pulse-pump-probe technique. By investigating the initial scattering and the cooling rate of hot carriers injected by 2-eV photons for densities (N) varied from the mid-1015 to 1019 cm-3, we have acquired quantitative knowledge on electron-hole, electron-electron and electron-plasmon interactions.
Experimental Investigation of Dark Solitons Interaction
Authors: Emplit Ph. , Hamaide J.-P. , Haelterman M.
Published: February 2023

Temporal dark optical solitons are stable odd solutions of the Nonlinear Schroedinger Equation describing the propagation, of a spatially single-mode plane wave in presence of normal (or positive) Group Velocity Dispersion (GVD) and Self-induced Phase Modulation (SPM). Such propagation of temporal dark signal in single-mode optical fibers has, after some pioneering experimental demonstrations in the visible region of the spectrum, received much attentions1. Simulations predicting the possibility of optically amplifying dark solitons2, on one hand, and establishing their stability with respect to the Gordon-Haus jitter effect in ultralong amplified systems3, on the other hand, opened the telecommunication domain as a potential field of applications for negative pulses. Recently, single dark pulse propagation experiments, with telecommunication compatible parameters (picosecond duration, infrared wavelength and 1km fiber), were reported4. In this paper, as a first step to a study of soliton sequences, we present experimental observations of the propagation of truncated dark pulse pairs.
Experimental analysis of gain modulation in Subpicosecond (0.45 Ps) Mode-Locked Laser Diodes
Authors: Stelmakh N. , Lourtioz J-M. , Pascal D.
Published: February 2023

Mode-locked laser diodes present important applications in the field of optical communications and are an interesting alternative to more conventional (sub)picosecond lasers for time-resolved spectroscopy and optical testing of various materials. During last years, many papers have been addressed to the experimental demonstration of laser diode mode-locking using different optical schemes and different types of diodes [1-4]. However, the mechanisms at the origin of coherent subpicosecond mode-locked pulses in laser diodes still remain unknown. In view of a better understanding of this phenomena, we propose a non-invasive method for probing the gain medium during the mode-locking process. Results are presented for a passively mode-locked AlGaAs laser emitting pulses as short as ~450 fs.
Femtosecond Solvent Dynamics Studied by Time Resolved Fluorescence and Transient Birefringence.
Authors: Rosenthal S. J. , Scherer N. F. , Cho M. , Xie X. , Schmidt M. E. , Fleming G. R.
Published: February 2023

Ultrafast dynamic solvent properties are recognized to play an important role in controlling the rate of charge transfer reactions. 1 Dynamic solvation has been studied by time dependent fluorescence Stokes shift measurements of dipolar chromophores. However, only one measurement with sufficient time resolution to reveal a large contribution arising from underdamped solvent motion has been reported.2 The solvent motions giving rise to the solvent response should also contribute to the collective nuclear response in transient birefringence measurements of the neat liquid.3 To explore the connection between the two experiments and further characterize ultrafast dynamic solvation we have performed both time resolved fluorescence and transient birefringence experiments in acetonitrile, methanol, and dimethyl sulfoxide.
Probing the Microscopic Molecular Environment in Liquids with Femtosecond Fourier-Transform Raman Spectroscopy
Authors: McMorrow Dale , Kim Seong K. , Melinger Joseph S. , Lotshaw William T.
Published: February 2023

Intermolecular dynamics in liquids that occur on the sub-500 fs timescale axe determined primarily by the details of the intermolecular potentials. Such dynamics traditionally have been investigated with frequency-domain techniques, such as spontaneous light scattering (LS) spectroscopy. Frequency-domain techniques, however, have provided only limited information on the intermolecular vibrational dynamics of liquid systems.
Sequential Laser Emission in Multiple Quantum Well Vertical-Cavity Structures
Authors: Tanguy Christian
Published: February 2023

Optical pumping has been performed on vertical structures, concentrating on the very first picoseconds after the excitation. Above threshold, laser emission is observed, which can take place at two or more different wavelengths, corresponding to longitudinal modes of the Fabry-Perot cavity. Lasing occurs sequentially, the high-energy line appearing before the low-energy one. This shows that transient gain may be large enough to induce lasing over a wide spectral range, up to 46 nm in the present case.
Generation of 1.7ps Solitons by Amplification of Pulses from a Laser Diode with Saturable Absorber in an Erbium Doped Fibre
Authors: Khrushchev I. Yu. , Grudinin A.B. , Dianov E.M. , Kuksenkov D.V. , Portnoy E.L.
Published: February 2023

The combination of a picosecond laser diode, operating within the 1550nm wavelength region and an erbium-doped fibre amplifier, is an attractive proposition since it allows one to realise a miniature source of ultra-short pulses.
Creation of an anti wavepacket in a Rydberg atom
Authors: Noordam L. D. , Stapelfeldt H. , Duncan D. I. , Gallagher T. F.
Published: February 2023

At the 1990 UFP conference the observation of several kinds of wave packets was reported [1]. All these wavepackets have in common that the wavefunction is localised due to a coherent superposition of eigenstates. The localised ‘particle’ evolves in course of time. The oscillation time of the wavepacket is determined by the inverse spacing of the states out of which the wavepacket is formed. We reported on the localisation of an atomic electron wavefunction by coherent superposition of Rydberg states. Also the observation of exciton wavepackets was reported, as well as the observations of optic phonons in the time domain. At an earlier conference the CalTech group reported already the observation of the oscillation of vibrational wavepackets in diatomic molecules [2].
Observation of the Laser-Induced Desorption of CO from Cu(111) with 100 Femtosecond Time-Resolution
Authors: Tom Harry W. K. , Prybyla Judith A. , Aumiller Gary D.
Published: February 2023

The most promising aspects of ultrafast investigations of chemical systems are the possibility of resolving reactions on the time-scale of bond-formation and bond-breaking and of understanding the microscopic dynamical processes that drive reactions. For molecules adsorbed on solid surfaces, the relevant time-scale is 100 fsec: this time is short enough to resolve molecular bond changes as well as the transfer of energy between the electronic and vibrational degrees of freedom in the substrate and substrate-adsorbate complex. We have used 100 fsec time-resolved second-harmonic generation to probe the departure of adsorbed CO molecules from a Cu(111) surface after the surface is irradiated with a 100 fsec, 2 eV laser pump pulse. The desorption event is >90% completed within the first 325 fsec after the pump pulse. In contrast to techniques that measure the properties of desorbed molecules,[1] SHG allows us to probe the adsorbed molecules and in principle to directly measure the reaction time. Here, the reaction time is so short that we can identify the dynamical processes that drive this reaction. All conventional mechanisms (thermal and direct photochemical) are ruled out and a novel mechanism is proposed. We believe hot electrons, produced by 100 fsec excitation of the substrate, excite the adsorbate electronic state several times during the CO-metal stretch period and cooperatively pump the CO-metal stretch into high enough states to desorb the CO.
Generation of 50 TW subpicosecond pulses in a Nd-Glass Chain and focusability study
Authors: Rouyer C. , Mazataud E. , Allais I. , Pierre A. , Seznec S.
Published: February 2023

There has been recently a great deal of works on the development of ultrabright sources in order to study the interaction of matter with such intense pulses. A very interesting goal is to obtain intensities above 1019 W/cm2 because new effects are expected in this regime such as relativistic electrons, coherent X-Ray short pulses, huge shock pressures in dense and hot plasmas,... Solid state broadband amplifying media coupled to the Chirped Pulse Amplification technique [1] is the most direct way to get intense and ultrashort pulses. Ti-Sapphire, Cr3+:LiCAF, Alexandrite are such good media. Nd-Glass has the potential to amplify pulses only longer than 250 fs but because of the available amplifiers size allows very energetic pulses up to kilojoules. In a first experiment we started from a CW-Mode-Locked-Nd-YAG oscillator coupled to a fiber, injected into a Nd-Glass regenerative amplifier and in a Nd-Glass power chain. The 60 J chirped pulses were then compressed using double pass compressing scheme. 20 TW-24J and 1.2 ps pulses are then delivered [2]. However the intensity contrast in this system is very poor. We have developed a new source based on a double fiber scheme [3] and a Ti:S amplifier allowing to get clean 450 fs pulses. The chirped pulse is sent in a regenerative amplifier able to increase the pulse energy from a few nJ to 1 mJ without gain narrowing for a 8 nm spectral bandwidth [Fig. 1, Fig. 2.]. These pulse is then sent into the Nd-Glass power chain and further compressed.