bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsed Diffusing-Wave Spectroscopy: Pathlength Specific Observation of Speckle Fluctuation Spectra From Dense Colloids
Authors: Yodh A.G. , Kaplan P.D. , Pine D.J.
Published: February 2023

Nonlinear optical gating has enjoyed wide success over the years particularly in connection with time-resolved luminescence studies of semiconductors.1 In this paper we describe a new class of gating experiment which we have developed and applied to study particle diffusion in colloids at high concentration. In contrast to previous work, the new scheme exploits the phase fluctuations of gated photons and enables the experimenter to access properties of dense colloidal suspensions that multiply scatter light.
Pump-Probe Investigations of Femtosecond Desorption:NO/Pd(111)
Authors: Budde F. , Heinz T.F. , Loy M.M.T. , Misewich J.A. , Smith B.D.
Published: February 2023

It has recently been demonstrated that strongly nonequilibrium excitation can be induced in a metal irradiated by a femtosecond laser pulse.1 The laser energy is initially absorbed as electronic excitation, which rapidly thermalizes. Only on the scale of a hundreds of femtoseconds is energy transferred to lattice vibrations of the metal. Thus, one may describe the solid by separate electronic (T
e
) and lattice (T
l
) temperatures. Because of the lower heat capacity of the electrons, the peak electronic temperature rises to a much higher value than the lattice temperature. This observation suggests that nonequilibrium electronic excitation induced by femtosecond laser pulses may give rise to distinctive new physical processes at the surface of a metal. In this paper, we report results concerning molecular desorption of NO from a Pd(111) surface under irradiation by 616-nm laser pulses of 200-fs duration.
Femtosecond Thermomodulation Study of Conventional and High-Tc Superconductors
Authors: Brorson S. D. , Kazeroonian A. , Moodera J. S. , Face D. W. , Cheng T. K. , Ippen E. P. , Dresselhaus M. S. , Dresselhaus G. , Doll G. L. , Venkatesan T. , Wu X. D. , Inam A.
Published: February 2023

The Elishberg generalization of BCS theory includes a parameter λ describing the strength of the electron - phonon coupling constant.1 In 1987, P. B. Allen made the suggestion that λ might be measured in the metallic superconductors using femtosecond thermomodulation (pump - probe) spectroscopy.2 Such experiments3,4 have been performed on Cu and Au, although not in the context of measuring λ. Here we report the results of systematic femtosecond thermomodulation measurements of λ in metallic thin films of Cu, Au, Cr, Ti, W, Nb, V, Pb, NbN, and V3Ga. The agreement between our measured λ values and those obtained by other means is excellent, thus confirming Allen’s theory. This method to measure λ has several advantages over other techniques: it is the most direct measure of λ〈ω2〉, it works at room temperature, it can be applied to both superconducting and non-superconducting samples, and it is not affected by interactions competing with superconductivity such as the antiferromagnetism in Cr.
Rate of electron-phonon relaxation in Niobium
Authors: Yoo K. M. , Siddique M. , Alfano R. R. , Osterman D. P. , Radparvar M. , Cunniff J.
Published: February 2023

Niobium is an important superconducting metal for the fabrication of Josephson junctions and other superconducting devices. One of the parameters which governs the ultimate switching speed of a Josephson junction is the electron-phonon relaxation time. Using ultrafast laser excitation on the Josephson junction1,2, the transition from superconducting to normal and back to superconducting state is monitored by the transient change in voltage across the junction1,2. The rise time of the transient change of voltage is mainly governed by the electron-phonon relaxation time which is ultrafast and cannot be resolved by voltage measurement. In this talk, the electron-phonon relaxation time and electron-phonon coupling constant of niobium was measured by a time resolved thermal modulation3-5 optical transmission technique.
Terahertz Radiation from Coherent Electron Oscillations in a Double-Quantum-Well Structure
Authors: Roskos Hartmut G. , Nuss Martin C. , Shah Jagdeep , Leo Karl , Miller David A.B. , Schmitt-Rink Stefan , Köhler Klaus
Published: February 2023

We report the direct observation of coherent charge oscillations in an asymmetric semiconductor double-well potential with a thin tunneling barrier between the wells. Such a structure allows tuning of the energy levels of the two wells by an external electric bias field. At a certain resonance bias (here: -0.4 V), the conduction band levels are aligned and the electron wave functions extend over both wells. A 100 fs optical pulse resonant in energy to the lowest interband transition generates an exciton wave packet in a single well only.[1] From this nonstationary state the electrons begin to oscillate between the two wells at a frequency that is determined by the tunneling probability through the barrier between the two wells. The holes remain localized because the valence band energy levels are not aligned at this bias. Electrons and holes together form an oscillating dipole that emits electromagnetic radiation in the THz frequency range until the phase coherence of the oscillating wave packets is destroyed.
High-Order Harmonic Generation in Intense Laser Fields
Authors: L'Huillier Anne , Balcou Philippe , Lompré L-A , Mainfray G.
Published: February 2023

As is well known, the interaction of a laser field with an atomic gas leads to generation of odd harmonics of the fundamental frequency. When the incident laser is intense and of short pulse duration, high order odd harmonics can be created. This has been extensively studied with excimer lasers (248 nm), with Nd-YAG or Nd-Glass lasers and recently with visible dye lasers [1-3]. The intensities of these harmonics in general are found to be approximately constant, up to a very high order. This "plateau" behavior is characteristic of a strong (non-perturbative) laser-atom interaction, as discussed in several theoretical papers [4,5]. The width of the plateau increases with the laser intensity, up to the intensity at which the medium becomes ionized. For example, we have observed up to the 53rd harmonic of a 1ps 1053 nm Nd-Glass laser at an intensity of 5×1014 W.cm-2 (The photon energy of 62 eV). The observation of harmonics of higher order than 100 of a 800 nm 150 fs Ti:S laser has been reported by a group at Stanford University (which means a photon energy of up to 200 eV) [6]. This radiation emission in the XUV domain has unique properties of coherence and brightness. Its pulse width follows that of the pump field and can therefore be very short. The investigation of these processes is worthwhile not only for its basic interest as a probe of the behavior of atoms in strong laser fields but also because it might lead to short pulse, bright, compact and possibly tunable sources of coherent radiation in the XUV range.
Femtosecond Pulse Propagation in Erbium-Doped Single-Mode Fibers
Authors: Hickmann J Miguel , Gomes A S L , de Araújo Cid B , Gouveia-Neto A S
Published: February 2023

Nonlinear pulse propagation and amplification in Erbium doped fibers (EDF) has arisen considerable attention [1-3]. Pulse propagation without amplification can be affected not only by nonlinear mechanisms such as self-phase modulation (SPM) and intrapulse stimulated Raman scattering (ISRS) [4], but also can be influenced by the dopant susceptibility [2,3]. Recently Zhu and Sibbett [3] described results of pulse propagation in EDF in the femtosecond regime, without amplification, but the observed spectral and temporal features have not been fully explained. In this communication, we report on a similar experiment which is further supported by numerical simulations. Pulses with 130fs centered at 1515nm, were generated with a time-bandwidth product of 0.45 through a soliton-Raman process in an optical fiber (minimum dispersion was at 1360nm) pumped at 1320nm with a Nd : YAG laser [5]. The pulses were launched through 3m of EDF (core diameter: 6.5µm, Er concentration of ~ 0.01M). The output spectrum and autocorrelation of the pulses are shown in fig. 1(a-c) for two different incident peak powers.
Dephasing of Excitonic Transition of J-Aggregates Measured by Time-resolved Resonance CARS
Authors: Kamaloyl V. F. , Inaba R. , Yoshihara K.
Published: February 2023

J-aggregates are the subject of great interest due to high values of optical susceptibilities of intensive excitonic resonance, originated from delocalization of electronic excitation in molecular aggregate. In [1,2] resonant enhancement of third-order susceptibiltity of J-aggregates of pseudoisocyanine (PIC) solutions was observed. The dephasing time of excitonic transition of PIC J-aggregate solutions was determined in [3] as 16 ps by low-temperature photon echo, and T2=0.17 ps at room temperature by measuring the dispersion of nonlinear susceptibility in the process of four-wave mixing [2].
Femtosecond Luminescence Spectroscopy of Indium Phosphide
Authors: Fazio E. , Gale G. M.
Published: February 2023

The investigation of weakly-excited, heavily-doped bulk semiconductors by femtosecond excitation and luminescence up-conversion techniques allows the dynamics of a single carrier species to be explored [1], as the doping provides a large reservoir of conjugate charge carriers at thermodynamic equilibrium, which is only slightly perturbed by weak excitation. Furthermore, in the case of heavy p-doping (hole reservoir), luminescence directly related to electron population can be observed well above the band-gap, because of the extensive hole distribution.
Femtosecond Pulse Amplification and Continuum Generation at 250 kHz with a Ti:sapphire Regenerative Amplifier
Authors: Norris T. B.
Published: February 2023

Amplification of femtosecond pulses to the microjoule level at high (>1 kHz) repetition rates has generally been accomplished using dye amplifiers pulse-pumped by copper-vapor lasers or by frequency-doubled Nd:YAG regenerative amplifiers [1,2]. The repetition rate of these systems is generally limited by the repetition rate of available pulsed pump sources. In this work, I demonstrate that the superior energy storage of Ti:sapphire allows one to amplify femtosecond pulses to the microjoule level in a cw-pumped regenerative amplifier. The gain storage time of Ti:sapphire is about 3 (μs, which makes it possible to efficiently extract the energy stored by the pump at repetition rates up to 250 kHz. Q-switching, injection, and cavity-dumping of the amplifier are done acoustooptically in order to achieve the very high repetition rate [3].
Ultrafast Studies on Intervalence Charge Transfera
Authors: Kliner D.A.V. , Tominaga K. , Barbara P.F.
Published: February 2023

Mixed-valence metal complexes have played a central role in the development of the theory of electron-transfer (ET) reactions.1 These compounds contain two metal atoms in different oxidation states, typically +2 and +3. An absorption band is often observed in the near IR or visible, corresponding to the transfer of an electron between the two metal centers. Much theoretical work has addressed the connection between this photoinduced charge transfer and the analogous thermal ET reaction.
Field-Enhanced GaAs/AlGaAs Waveguide Saturable Absorbers
Authors: Karin J.R. , Derickson DJ. , Helkey R.J. , Bowers J.E. , Thornton R.L.
Published: February 2023

Semiconductors have recently been shown to be useful as saturable absorbers. Both bulk and quantum well materials have been used to generate picosecond and femtosecond pulses in color center[1], erbium-doped fiber[2], dye[3] and semiconductor[4] laser systems. These applications typically use damage sites or traps to decrease the absorption recovery time. In semiconductor diode lasers, monolithic structures incorporating waveguide saturable absorber sections for passive modelocking have produced pulses of 0.6 picoseconds, at repetition rates up to 350 GHz. However, the exact mechanism for such high frequency modelocking is not well understood. In order to produce a stable train of short pulses at these millimeter-wave repetition rates, the absorption recovery must be fast enough to recover on each round trip in the cavity, e.g., 10 ps for a 100 GHz repetition rate. The saturable absorber sections are typically p-i-n structures operated under reverse bias to remove photogenerated carriers and achieve this fast recovery, as in a p-i-n photodiode. Knowledge of the dynamics of the saturable absorbers is necessary for design optimization, as well as insight into the potential of these devices for producing short optical and electrical pulses. Absorption recovery is also thought to be responsible for elimination of multiple pulses in external cavity lasers[5]. In this paper we will present a study of the bleaching and recovery dynamics of these waveguide saturable absorbers for different bias conditions, and discuss the implications of the results for modelocked laser systems.
A Diode Pumped Picosecond Oscillator at 1053 nm
Authors: Mercer IP , Chang Z , Miller MRG , Danson CN , Edwards CB , Hutchinson MHR
Published: February 2023

We report on the development of a novel diode-pumped modelocked laser oscillator operating on the 1053 nm transition in Nd:YLF. The device has been used to generate 1.6 ps pulses for injection into a high power Nd:glass laser system producing multi-terawatt output using Chirped Pulse Amplification (CPA) techniques [1].
Features of the Dual Fluorescence of 4-N,N-dialkylaminoalkylbenzoates in Alkanes
Authors: de Lange M.C.C. , Leeson D. Thorn , Huizer A.H. , Varma C.A.G.O.
Published: February 2023

The appearance of a normal band FN and an anomalous band Fx, to the red of the former, in the fluorescence spectrum of solutions of 4-N,N-diethylaminoethylbenzoate in alkanes has been considered as evidence for the existence of a low energy twisted intramolecular charge transfer (TICT) excited state in electron acceptor substituted aniline derivatives [1]. Objections have been raised against this interpretation of the dual fluorescence [2]. Nevertheless, the TICT state seems to be widely accepted as a reasonable explanation of the phenomenon. Here we will present evidence that the long wavelength band in the spectrum arises from an adiabatic reaction of the molecule on the S2-surface, in which a conformational change takes place along a reaction coordinate Qxinvolving a deformation along the inversion coordinate of the amino group as well as an in-plane deformation of the benzene ring.
All solid femtosecond oscillator-amplifier laser chain with 100 mJ per pulse
Authors: Le Blanc C. , Chambaret J.P , Grillon G. , Boyer G. , Franco M. , Mysyrowicz A. , Antonetti A.
Published: February 2023

We describe a femtosecond laser system relying entirely on Solid State technology, capable of delivering femtosecond optical pulses with an energy of 100 mJ per pulse at a repetition rate of 10 Hz (see figure).
Fast Processes in Liquid Alkane Photolysis above the Ionisation Threshold
Authors: Sander M. , Brummund U. , Luther K. , Troe J.
Published: February 2023

Fast processes occurring in liquid alkanes upon radiolytical or optical excitation have been under investigation for many years. However, pulse radiolytical experiments have so far been limited to time scales larger than 10 ps and the only two-photon laser-experiments with subpicosecond resolution [1] have been performed at excitation energies below the ionisation threshold of alkanes. The attempted interpretation of the transient absorption signals in [1] as due to geminate electron-hole recombination therefore has to be rejected.
Transient Hole Burning Studies of Electronic State Solvation: Phonon and Structural Contributions
Authors: Yu Jongwan , Fourkas John T. , Berg Mark
Published: February 2023

Previously we showed that at room temperature the solvation of the nonpolar solute dimethyl-s-tetrazine (DMST) is subpicosecond, even in a variety of viscous solvents expected to show slow relaxation times.1 Measurements of solvation of DMST in glycerol have been extended from room temperature down to the low temperature glass. The solvation is shown to decompose into two major components: one describable in terms of phonon-modulated interactions, and one connected to the structural relaxation of the solvent.
Charge separation in photosynthetic reaction centers via Rydberg states
Authors: Fischer Sighart F. , Scherer P.O.J.
Published: February 2023

Quantum calculations of the MRINDO /1/ type for the prosthetic groups of the reaction center Rps. viridis including outer atomic orbitals are presented. In Fig.l the electron density distribution of the lowest Rydberg type orbital is depicted. It concentrates on the two pyrrol rings I their methyl groups and those around the dimer. It extends also into the monomers BL and bm and weakly into the pheophytines HL and HM. There are asymmetries which favour the L-branch. In particular the electron density on the tyrosine M208 is larger than the one on phenylalanine L181. The transition from the HOMO of the dimer into this Rydberg orbital becomes a low excitation denoted PR close to the well known P* transition. It carries no oscillator strength couples only weakly to P* but has a strong permanent dipole of 20D pointing along the symmetry axis.
Ultrafast dephasing of coherent phonons in GaAs
Authors: Kütt W. , Pfeifer T. , Dekorsy T. , Kurz H.
Published: February 2023

Optical excitation of coherent LO-phonons in surface space charge fields of GaAs was recently demonstrated using 50fs light pulses from a CPM-ring dye laser[1]. The measurement of both amplitude and phase of the coherent phonons via reflective-electro-optic sampling allows the study of coherent phonon dephasing in the presence of a dense carrier plasma in opaque semiconductors.
Aspects of Nonclassical Dynamics of Vibrational Wave Packets in Sodium Dimers
Authors: Dunn Tom , Sweetser John , Radzewicz Czeslaw , Walmsley Ian
Published: February 2023

The dynamics of vibrational wave packets in molecules that are generated by an ultrashort light pulse have revealed effects that are clearly attributable to the quantum mechanical nature of the system. Several nonclassical aspects of the dynamics have been reported, notably the decay and revival of a single wave packet1, and quantum interference between two wave packets2.
Wave Packet Interferometry and Other Applications of Phase Locked Femtosecond Pulses
Authors: Fleming G.R. , Scherer N.F. , Cho M. , Matro A. , Cina J.
Published: February 2023

The ability to control the relative optical phase of ultrashort light pulses [1] as well as their duration and time delay [2] offers new capabilities for the study and manipulation of molecular responses. We have recently reported a new experimental method for setting and maintaining the relative optical phases within a sequence of ultrashort light pulses [1]. The initial studies involved fluorescence detected interfermotry measurements on iodine vapor. These measurements were shown to be extremely sensitive to the transition energies and potential energy surface of the excited state. The experimental interferograms have been analyzed in detail and the effect of rotational motion elucidated. By extending the technique to in-quadurate phase-locked pulses a relationship between the interferograms and the linear susceptibility to be obtained directly.
Femtosecond Dynamics of Molecules in Liquids
Authors: Wiersma Douwe A. , Nibbering Erik T.J. , Duppen Koos
Published: February 2023

The theoretical basis for a study of condensed phase dynamics was generally provided by the famous Bloch equations. There is mounting evidence, however, for glasses and liquids that a division of time scales, as implied by the Bloch equations, is not warranted. In glasses and liquids the fluctuations dominating the optical dynamics do not necessarily have a memory time much shorter than the time scale of molecular coherence decay. A more general non-Markovian approach must then be used.
Femtosecond Photodissociation of Aromatic Disulfides Followed by Solvent Relaxation
Authors: Ernsting N. P.
Published: February 2023

The dynamical properties of liquids may be probed in the time domain in two ways: either in the neat liquid by nonlinear scattering experiments, or else indirectly by studying the solvation dynamics of molecular probes [1,2]. In the latter case, optical excitation of a suitable dye molecule leads to a change in molecular dipole moment. As the surrounding solvent adjusts to a new equlibrium, solvent-solute interaction stabilizes the excited molecule, and this results in a red-shift of the fluorescence spectrum. Typical compounds for this purpose are coumarin or styryl dyes. However their use for ultrafast solvation dynamics is limited by a relatively large spectral width of fluorescence, or by the possibility of geometrical isomerisation. Furthermore, measurement requires fluorescence upconversion and spectral reconstruction which may be quite tedious.
The Homogeneity of Liquid Phase Vibrational Line Broadening From Raman Echo Experiments
Authors: Muller Laura J. , Vanden Bout David , Berg Mark
Published: February 2023

The dynamic interaction of a vibration with a solvent is reflected in the vibrational dephasing time. However, essential questions about the correlation time and physical origin of the interaction can only be answered if the homogeneity of the dephasing is known. Numerous indirect arguments, both experimental and theoretical, have been advanced for both homogeneous and for inhomogeneous dephasing without reaching an overall consensus.
Phase Dependent Ionization Using An Intense Two-Color Light Field
Authors: Schumacher D.
Published: February 2023

We have studied Multi-Photon Ionization (MPI) in Krypton using a field made up of two sub-picosecond laser pulses, one the second harmonic of the other, as a function of phase shift between the two pulses. Recent previous experiments have used either two pulses of incommensurate frequencies, or pulses that were in the "long-pulse" regime1 where ponderomotive effects dominate the final kinetic energies of ejected photoelectrons. The sub-picosecond pulses of this experiment allow us to identify laser induced resonances during the ionization process2 and hence specific ionization pathways. The variation of the total electric field in time and the amplitude of the peak electric field is phase dependent.
Investigation of Femtosecond Pulse Induced Phase Transitions in GaAs
Authors: Schröder T. , Heist P. , Govorkov S. , Shumay I. , Rudolph W.
Published: February 2023

There has been a great deal of interest in the investigation of the primary steps of phase transitions at semiconductor surfaces excited by an ultrashort laser pulse. Raman experiments [1] revealed that energy from the electronic system is transferred to the phonon system (lattice) on a time scale of about 1-2ps. Direct observation of the sample surface after excitation showed the onset of melting to be within a few or less than 1ps [2]. Second harmonic generation in reflection turned out to be a powerful tool to detect symmetry changes of the sample surface [3-5]. The general observation is that the drop of the second harmonic efficiency occurs within the first one hundred fs after excitation. In contrast the reflectivity of the fundamental changes on a time scale of several hundred fs up to one ps depending on the excitation density. While the SHG probes only about a layer of lOnm the reflectivity of the fundamental is determined by a layer thickness of several hundred nm. Therefore one possible explanation for the different time constants rests on propagation effects of the melt front, another assumes an intermediate state reflecting a changed lattice symmetry which is reached before a considerable part of energy is transferred to the phonon system (cold melting) [3]. We supplemented pump-probe and time resolved SHG in reflection by transient grating measurements. The latter are extremly sensitive against propagation effects because they change the grating structure and thus the diffraction efficiency. Corresponding measurements are shown in Fig.l. It is intresting to note that observable first-order diffraction occured only at excitation densities above the melting threshold. From Fig.1 it is evident that the diffraction dynamics is similar to that of the fundamental reflection. A theoretical model explains this as a consequence of the fact that propagation effects can be neglected within the first few ps. An upper limit for the velocity of the melt front velocity can be estimated and is in the order of 500 m/s. Since the diffraction rise time of the UV-probe (310nm) is considerably longer than the decay of the second harmonic efficiency the existence of an intermediate state or phase prior the melting becomes even more likely.
Recombination XUV Lasers Driven by Low Energy Picosecond Pulses from a High Brightness KrF Raman Laser
Authors: Dwivedi L , Jacoby J , Kato Y , Key M H , Kodama R , Lewis C L S , Lightbody M , Murai H , Norreys P , Pert G J , Ramsden S , Shiraga H , Smith C , Tallents G J , Uhomoibhi J , Yuan G , Zhang Jie
Published: February 2023

There is great interest in the development of XUV lasers with driver energies of a few joules rather than a few kilojoules. One of the most promising ways to accomplish this is based upon the use of fast adiabatic cooling to produce population inversion in recombination to hydrogenic ions(l). Theoretical modelling(2) indicates that the optimum driver for this scheme should produce pulses of duration < 10 ps, ultraviolet wavelength and focusability to sub 10 µm line foci with length of the order 1 cm. Under these conditions saturated laser action at wavelengths below 20 nm is predicted in plasmas created from fibre targets of < 10 µm radius with driver energy < 10 J (see eg Fig 1). Preliminary experimental studies have been carried out using a high brightness 5 J, 10 ps KrF pumped Raman laser beam(3) with observations of fluorescence and possible amplification on the Balmer-α transitions of C VI, O VIII(4). In a current experiment these phenomena are being studied in detail. Our paper will review the theoretical basis for the X-ray laser scheme, describe the experimental and diagnostic system, review previous preliminary results and report on the results of the current more detailed experiment.
Ultrafast Fluorescence Spectroscopy of Reaction Centers of Photosynthetic Bacteria
Authors: Hamm P. , Zinth W.
Published: February 2023

In the primary reaction of photosynthesis an electron is transferred through a pigment-protein complex called reaction center (RC). Starting from the primary donor, a dimer of bacteriochlorophyll molecules called special pair P, the electron reaches a quinone (Q) via two intermediate acceptors, a bacteriochlorophyll-monomer (B) and a bacteriopheophytin (H). In a number of publications this electron transfer process was investigated by transient absorption spectroscopy /1/. At least four kinetic constants of 0.9 ps, 3.5 ps, 200 ps and infinity are required to explain the absorption data /2,3/. The interpretation of these measurements is difficult since absorbance changes from different intermediates interfere. At present the most straight-forward interpretation of the data is the stepwise electron transfer model via the radical pair state P+B–: Complementary information from fluorescence up-conversion experiments is given in this contribution. By this way we follow directly the decay of the excited electronic state of the special pair. The experimental system was optimized to high time resolution and high sensitivity at low repetition rates in order to meet the requirements of the biological samples.
Distortion of a 6 fs pulse in the focus of a BK7 lens
Authors: Bor Zs. , Horváth Z. L.
Published: February 2023

It was shown in [1-3] that the chromatic aberration of a lens causes a distortion of a femtosecond pulse front. In this paper the behaviour of the pulse front in the vicinity of a lens is described by using wave optics. For the calculation the input pulse (6 fs, 620 nm) is decomposed into monochromatic plane waves, which are converted by the lens into Airy-type of diffraction patterns. Due to chromatic aberration the positions of the diffraction patterns are slightly shifted with respect to each other along the optical axis. The pulse front is obtained as a result of the superposition of diffraction patterns of the components [4]. The calculation were made for BK7 lenses having the same focal length (f=30 mm) but different diameters (d=1,2,4,7 and 10 mm). The total input energy of the pulse was regarded to be constant. Spatially homogeneous illumination was assumed.
Vibrational Relaxation of I 2 ¯ in Polar and Non-Polar Solvents
Authors: Walhout P.K. , Alfano Joseph C. , Thakur Khalid A.M. , Barbara Paul F.
Published: February 2023

In this paper we present new results on the photodissociation, geminate recombination, and vibrational relaxation of
I2¯ in various solvents. The photodissociation of
I2¯ involves localization of the excess charge on a recoiling I• photofragment. This charge again delocalizes as the I• and I− geminately recombine. The "charge flow" associated with recombination and subsequent large-amplitude vibrational motion is believed to greatly accelerate the vibrational relaxation of the nascent
I2¯. Coupling of the charge flow with accepting solvent modes leads to relaxation rates which are extremely fast and non-exponential, even near the bottom of the potential well. These results are in stark contrast to similar experiments performed on neutral I2 which show nearly exponential vibrational relaxation rates that are an order of magnitude slower and are well-described by isolated binary interaction (IBC) theory.
Ultrafast Mid-IR Spectroscopy of Carbonmonoxymyoglobin: The Dynamics of Ligand Motion
Authors: Lim Manho , Jackson Timothy A. , Anfinrud Philip A.
Published: February 2023

Myoglobin (Mb) is an oxygen-binding protein found in muscle. The active binding site is an iron-II-containing porphyrin (heme) that is located within the hydrophobic interior of the protein. The X-ray structure of Mb reveals a distal cavity large enough to accommodate a ligand the size of O2, but no channel large enough for the ligand to diffuse between the heme pocket and the surrounding solvent. Clearly, structural fluctuations of the protein are required to open channels for ligand transport to and from the binding site. What is the functional role of the heme pocket and the residues that circumscribe it? What pathways exist for ligand transport between the heme pocket and the solvent? On what time scale does the ligand escape into the surrounding solvent? We have sought to answer these questions by probing the time-resolved vibrational spectrum of CO, a ligand similar in size to O2, following photodissociation of MbCO at physiological temperatures. We have developed a femtosecond time-resolved mid-IR spectrometer with the sensitivity required to measure the “free” CO absorbance. Ultrafast diffusion to a “docking” site has been observed, the orientation of the docked CO relative to the plane of the heme has been measured, and the rate of escape from the heme pocket has been determined. Additional studies with mutants of Mb are being conducted to probe the location of the docking site within the heme pocket and the pathways for ligand escape from the heme pocket.
A self-modelocked Ti:Al2O3 laser pumped by a frequency-doubled Nd:YLF minilaser.
Authors: Lamb K. , Spence D.E , Hong J. , Yelland C. , Sibbett W.
Published: February 2023

Recent advances in high power laser diode technology has led to vigorous research activity in the development of diode-laser-pumped, solid-state lasers. These so-called minilasers are especially useful because of their compactness and excellent beam quality, and in particular, a frequency-doubled Nd:YLF system is well-suited to the pumping of a Ti:A12O3 crystal. Argon-ion lasers having powers ranging from 1-20W, have generally been used as pump sources for Ti:A12O3 lasers. However these large-frame systems tend to have relatively noisy outputs, and are prone to having 50Hz and 300Hz intensity ripple. An efficient diode-laser-based pump source for self-modelocked Ti:A12O3 lasers is therefore more desirable. It is expected that such a system will exhibit less phase noise due to the superior characteristics of the power supply units that are used. This is especially relevant to applications in optical sampling and to synchronous streak camera measurements which both require sources of low noise, ultrashort laser pulses.
Transient Resonance Impulsive Stimulated Raman Scattering study of nascent diiodide ions; experiment and simulation.
Authors: Banin Uri , Kosloff Ronnie , Ruhman Sanford
Published: February 2023

A novel coherent spectroscopy is applied to measure ultrafast vibrational relaxation. Its performance is quantitatively evaluated through computer simulations. Transient Resonant Impulsive Stimulated Raman Scattering (TRISRS), is used to record vibrational relaxation in rapidly evolving nascent
I2−, following photodissociation of
I3− in solution. The TRISRS experiment involves a sequence of three pulses. A primary UV photolyzing pulse at 308 nm initiates triiodide dissociation. At a certain delay with respect to the photolysis a secondary intense 'push' pulse at 616 nm sets in motion ground state coherent vibration. Its decay is followed via periodic transmission modulations of a continuously delayed tunable probing pulse1. In essence these modulations record a Raman Free Induction Decay (FID) whose duration defines the minimum time that a dynamic variable must be followed in order to reconstruct its power spectrum. Master simulations of vibrational relaxation, coupled to quantum wave-packet representation of the light-matter interaction2, are combined to provide a quantitative analysis of the experimental results.
Applications of the Chronocyclic Representation of Ultrashort Pulses
Authors: Paye J.
Published: February 2023

The chronocylic representation1 is a way to display an ultrashort light pulse in a time-frequency space. It uses the Wigner distribution function applied to the complex electric field of the pulse. It thus contains both the amplitude and phase information but is a two-dimensional real function, although it can take negative values. Its mathematical properties make it as close as possible to a representation of the energy distribution of the pulse as a function of both time and frequency.
Coherent Optical Polarization of Large Coherence Length Frenkel Excitons in Pseudoisocyanine J Aggregates
Authors: Kobayashi Shunsuke , Sasaki Fumio
Published: February 2023

Recently the collective optical response in mesoscopic structures has attracted much attention because such structures are expected to show large nonlinear optical (NLO) properties and ultrafast optical response. These features are essential for the materials of the photonics application.
Highly efficient second harmonic generation of ultra-intense Nd:glass laser pulses
Authors: Korn G. , Chien C.Y. , Coe J.S. , Squier J. , Mourou G. , Craxton R. S. , Kieffer J.C.
Published: February 2023

The technique of chirped pulse amplification (CPA) [1] has been successfully used to generate peak laser powers well beyond the terawatt region [2-4], These subpicosecond laser pulses when focused can produce intensities exceeding 1018 W/cm2, thus giving access to new regimes of laser-matter interaction. Second harmonic generation (SHG) in nonlinear crystals is important because it extends the available wavelength range and significantly improves the contrast ratio. This is an important issue for high density laser-plasma experiments in which the fs-laser radiation should interact with the solid rather than with a pre-formed plasma.
Femtosecond Switching of the Solid-State Phase Transition in VO2
Authors: Becker Michael F. , Buckman A. Bruce , Walser Rodger M. , Lépine Thierry , Georges Patrick , Brun Alain
Published: February 2023

We have measured the transient change in the optical parameters of VO2 thin films for laser excitation through the first-order phase transition near 68° C. Femtosecond laser excitation at 780 nm from an amplified, passively modelocked Ti:sapphire laser was used in a pulse-probe configuration to excite the VO2 film through the semiconducting to metallic phase transition. The transient reflection and transmission were measured on a femtosecond and picosecond time range by a delayed probe pulse also at 780 nm. We have transformed the raw measurements of dynamic reflection and transmission to complex refractive index using a thin film model.
Contributions of Bound and Unbound Two-Exciton States to the Nonlinear Optical Response of GaAs Quantum Wells
Authors: Kuhl J. , Mayer E.J. , Smith G.O. , Bennhardt D. , Meier T. , Schulze A. , Thomas P. , Hey R. , Ploog K.
Published: February 2023

The strong influence of exciton/exciton interaction on the nonlinear optical response of the 2D exciton in GaAs quantum wells has been demonstrated in several recent experimental studies. The theoretical model for the microscopic coupling mechanism is still the subject of controversy, however. In ref. [1], we have presented a model including a density dependent coupling between opposite spin excitons which assumes that the interaction results in a renormalization of the matrix elements, dephasing rates and energies of the transition from the single-exciton state to the two-exciton state with respect to the corresponding quantities for the single exciton transitions. In contrast, Wang et al. claim that all experimental observations are explained by a density induced dephasing rate and that biexciton states play no role [2]. Here we present a new 3-pulse degenerate-four-wave-mixing (DFWM) configuration which is able to differentiate between pure local field effects and biexcitonic contributions to the time-integrated signal. Experiments were performed on an almost homogeneously broadened GaAs/Al0.3Ga0.7As single QW (well width 20 nm, photoluminescence line 0.3 meV, homogenous linewidth 0.15 meV) in the backward reflection geometry. The sample was cooled to 10 K and excited by a sequence of three pulses (1.1 ps duration) with equal intensity, wave vectors
k→1,k→2,k→3 and delays τ12 and τ13 between the second and first and the third and first pulse, respectively. The signal was monitored in the direction
k→
s
=k→1+k→2−k→3 which provides no signal for the chosen pulse length if local field and renormalization effects are negligible. The peak intensity of the time-integrated DFWM signal has been calculated by solving the optical Bloch for a system of two non-interacting two-level systems (2LS) with opposite circular polarization selection rules (local field effect) and for the renormalized four-level system (4LS) depicted in Fig. 1 which assumes the formation of biexcitons between excitons with opposite spins. The signal strength is proportional to the strength of the local field in the case of the 2LS and to the renormalization for the 4LS. The peak signal intensities expected for pure local field and pure biexction contributions are summarized in the second and third column of Table 1 for eight experimental configurations applying different linear and circularly polarized pulses. The values are normalized to the signal strength predicted for three parallel linearly polarized pulses. Close inspection of the data reveals remarkable differences of the peak amplitude with polarization geometry for the two coupling models.
Operation Parameters of Antiresonant Fabry-Perot Saturable Absorbers for Passively Mode-Locked Solid-State Lasers
Authors: Brovelli L. R. , Keller U. , Chiu T. H.
Published: February 2023

The semiconductor A-FPSA (antiresonant Fabry-Perot saturable absorber) (Fig. 1) has passively mode-locked successfully many different solid-state lasers such as Nd-YLF [1, 2], Nd:YAG [2], Nd:fiber [3] and Nd:glass [4, 5] lasers with ps- to sub-100-fs pulses. So far, the A-FPSA is the only intracavity saturable absorber in an all-solid-state ultrafast laser technology for which the modelocking is always self-starting and stable against Q-switching. The main reason is that the operation parameters, such as saturation intensity, losses, and impulse response of the A-FPSA can be custom designed and adapted to the requirements of solid-state lasers. Knowledge of these parameters therefore is of central importance for the optimization of the mode-locking performance.
Microscopic study of ferroelectrics with ultrashort phonon polaritons
Authors: Bakker H.J. , Hunsche S. , Kurz H.
Published: February 2023

We investigate the low-frequency dielectric response of the ferroelectrics LiTaO3 and LiNbO3 via the impulsive excitation and phase-sensitive detection of THz phonon polaritons. The low-frequency dielectric response of these crystals is dominated by a ferroelectric mode of Ai symmetry. This mode leads to a strong absorption at 6 THz (200 cm−1) in LiTaO3 and to a strong absorption at 7.5 THz (250 cm−1) in LiNbO3 at room temperature The strength and frequency of this mode strongly change when the temperature is increased towards the ferroelectric phase-transition temperature (890 K for LiTaO3 and 1480 K for LiNbO3).
Femtosecond Photon Echo Study of Solution Dynamics Using a Cavity-Dumped Ti:sapphire Laser
Authors: de Boeij Wim P. , Pshenichnikov Maxim S. , Duppen Koos , Wiersma Douwe A.
Published: February 2023

It has long been recognized that a better grasp of solution dynamics is essential to our understanding of chemical reactions in the liquid phase. A prime example of reaction dynamics is the so-called "cage" effect, where products of a fragmentation reaction cannot escape from one another because of collisions with solvent molecules. A more profound understanding of this phenomenon and other solvent dynamics, like solvation, remains a formidable challenge and is at the heart of physical chemistry.
Programmable Wavefunction Engineering in Cs Rydberg States
Authors: Bucksbaum P. H. , Schumacher D. W. , Pinkos Dan , Hoogenraad J. H. , Krause Jeffrey L. , Wilson Kent R.
Published: February 2023

We used a computer controlled LCD to shape the spectrum of a short laser pulse which was then used to excite a shaped atomic wavepacket. We use amplified programmable shaped optical pulses to engineer Rydberg wavefunctions. The method employs a computer-controlled liquid crystal pulse shaper[l]. The wavefunctions, which are nonstationary states, are monitored by observing temporal interference via the optical Ramsey method[2,3].
Physics of solid density plasmas at 1018-1019 W/cm2
Authors: Kieffer J.C. , Jiang Z. , Chaker M. , Côté C.Y. , Pelletier J.F. , Beaudoin Y. , Pépin H. , Korn G. , Chien C.Y. , Coe S. , Peyrusse O.
Published: February 2023

Experiments were carried out with a table top terawatt laser system which employs the chirped pulse amplification technique [1]. The laser was delivering up to 2 J on targets in a 400 fs pulse at the wavelength of 1.053 μm. High contrast pulses have been obtained by frequency doubling (0.53 μm) the laser pulse using KDP crystals. Very high conversion efficiencies have been obtained [2] and the pulses have been focused with off axis parabola to intensities up to 4 x 1018 W/cm2 (Iλ2 = 1018 W μm2/cm2 for green irradiation). At these intensities relativistic effects and ponderomotive pressure [3] are important issues for the interaction.
Vibrationally Resolved Reaction Dynamics in the Condensed Phase: Excited Product States of DCN
Authors: Raftery D. , Gooding E. , Hochstrasser R. M.
Published: February 2023

One of the goals of condensed phase reaction dynamics is to contrast results from experiments in the gas phase with observations of similar reactions in liquids so as to elucidate the effects of solvation on the kinetics and energy partitioning of simple reactions. To date, there have been few experimental studies having the requisite spectral and temporal resolution to test current theories. Transient infrared spectroscopy has been useful in the study of a variety of unimolecular reactions [1] and recently in a bimolecular reaction that is not diffusion controlled [2]. The present work shows that for products with large reaction exoergicity and sufficient anharmonic shifts in the vibrational coordinate of interest, it is possible to observe highly excited modes in a variety of different reaction schemes, and to measure the nascent vibrational distributions. Such dynamical detail should allow tests of model potential energy surfaces for the condensed phase analogous to Monte Carlo studies of gas phase potential energy surfaces.
AC- Stark effect of the Fermi Edge Singularity: Observation of "excitonic polarons"?
Authors: Perakis I. E. , Brener I. , Knox W.H. , Chemla D.S.
Published: February 2023

Ultrafast technology has created new opportunities for investigating the properties of many-body systems. For example, an intense pump optical pulse tuned in the transparency regime induces virtual charge fluctuations between the conduction and valence bands that renormalize all the semiconductor parameters. Augmented by correlation effects, such processes result in a "dressed" interacting system whose excitation spectrum can be measured by a weak probe pulse. The exciton absorption in undoped semiconductors was investigated with this technique in [1]. As the pump intensity increases, the bound state resonance shifts to the blue; this is called the AC (optical) Stark effect. Unlike atomic systems, in semiconductors the optically excited electrons and holes interact with each other; and many-body effects need to be addressed [2]. A narrow-band pump pulse blue-shifts the resonance and slightly enhances its oscillator strength [3,4]. The oscillator strength is significantly reduced if the pulse is ultrashort (100 fs or less) [5].
Solvent Dependence of the Femtosecond Three Pulse Photon Echo for LD690 in n-Alcohols.
Authors: Bardeen C.J. , Shank C.V.
Published: February 2023

The characteristic timescales of the solvent-solute interaction are of great interest to those studying the spectroscopy and reaction dynamics of molecules in condensed phases. These timescales influence a variety of linear and non-linear spectroscopic measurements, and together these different types of measurements should constrain the quantitative values of the interaction parameters. In this work we use both linear (absorption and fluorescence) and non-linear (resonance raman and femtosecond time-resolved 4-wave-mixing) spectroscopies to systematically investigate the spectral broadening of the dye molecule LD690 in a series of n-alcohols.
Femtosecond Carrier Dynamics at InP/Liquid Interfaces in the Presence of Electric Fields
Authors: Rosenwaks Y. , Thacker B.R. , Nozik A.J. , Ellingson R.J. , Burr K.C. , Tang C.L.
Published: February 2023

Recently there has been considerable interest in studying photogenerated carrier dynamics in semiconductors under the influence of electric fields1. This is a process of critical importance in recombination kinetics and electron transfer processes at semiconductor interfaces. At semiconductor-liquid interfaces, very large electric fields (typically 105 to 106 volts/cm) exist in the semiconductor close to its surface because of the formation of a space charge layer (SCL)2. Externally-applied potentials will drop across the SCL to increase or decrease the space charge fields (SCFs) and the surface band bending-VB. Under bandgap illumination the SCFs are reduced because of screening by the photogenerated majority carriers, but strong residual SCFs are ubiquitous across any semiconductor liquid interface. It is expected that these SCFs may have a crucial role in electron transfer (ET) processes at semiconductor electrodes.