bluuebunny/crossref_metadata_2025 huggingface dataset:
Design Considerations for Femtosecond Titanium:Sapphire Lasers
Authors: Krausz F. , Spielmann Ch. , Brabec T. , Wintrier E.
Published: February 2023

Recently much emphasis has been placed on mode locking the Ti:sapphire laser owing to the unique characteristics of this new solid state laser medium. Novel passive amplitude modulation techniques allowed self-starting and/or self-sustaining passive mode locking of continuous wave Ti:sapphire lasers.1,2 However, the comparatively inefficient all-optical modulators have not been capable of generating femtosecond pulses in the presence of net positive dispersion in the cavity. A breaktrough was achieved by exploiting solitonlike shaping mechanisms.2,3 In a Ti:sapphire laser the high intracavity power and long optical path within the amplifier medium (compared with dye lasers) lead to strong self-phase modulation. With the introduction of negative group-velocity dispersion this nonlinearity can result in efficient pulse shortening down to the femtosecond regime. Femtosecond Ti:sapphire lasers are different from CPM dye lasers in three respects: i) steady-state pulse duration is determined primarily by phase-modulation rather than amplitude modulation, ii) steady-state pulse parameters may be significantly different at different positions in the cavity iii) the broad gain bandwidth does not limit pulse duration down to 10 fsec. This implies that special attention has to be paid to phase affecting cavity elements in the design of these lasers.
Hybrid fs Dye/Excimer Lasers for Pump-Probe and High-Intensity Experiments
Authors: Schäfer F.P. , Szatmári S. , Simon P.
Published: February 2023

For many experiments in physics, chemistry, and biology one needs a femtosecond pump-probe spectrometer with a strong pump pulse, preferably in the ultraviolet, and a jitter-free synchronized probe pulse, tunable over a wavelength range from the near ultraviolet to the near infrared. Such a spectrometer is usually very complex consisting of several lasers, synchronization and other electronics and electro-optical instrumentation. By contrast, we have designed a relatively simple, reliable spectrometer using only a single laser for the generation of the pump and the probe pulses absolutely jitter-free and without any electronic or electro-optic instrumentation. The spectrometer is schematically shown in Fig. 1. It contains a commercial excimer laser with two discharge channels operated by one thyratron for stable synchronization of the two discharges. One channel is operated as a XeCl oscillator whose output pumps a series of dye lasers giving finally an output pulse of .5 ps duration at 497 nm with more than 100 μJ pulse energy. Part of the output pulse is frequency-doubled and sent into the second discharge channel which is filled with KrF mixture. After a triple pass in an off-axis arrangement the output energy is 20 mJ in less than .5 ps. Since the output pulse has a strong positive chirp it can be compressed to less than 100 fs. The rest of the green pulse passes a two-grating compressor and is focussed into a 5 mm thick piece of quartz glass to produce a continuum. The collimated white light is sent through a dye amplifier cell pumped by the rest of the XeCl laser pulse energy through a delaying quartz fiber. After a second pass through the dye amplifier cell, the pulse is compressed in a pulse compressor consisting of two LiF prisms and a retroreflector to give a probe pulse output of less than 30 fs duration and 1 μJ energy. An aperture allows fine tuning the spectral position and width to give transform-limited pulses in a well collimated probe beam. With 7 different dyes one can cover the spectral region from 420 to 650 nm. In a similar scheme we have also used other excimers in order to reach different ultraviolet wavelengths.
Picosecond Transient Absorption and Time-resolved Fluorescence Spectroscopy of C60 and C70 in Solution
Authors: Kim Dongho , Lee Minyung , Suh Yung Doug , Kim Seong Keun
Published: February 2023

The recent developement of the production of C60 and other fullerenes in high yields made it possible to study the basic properties of the newly discovered molecules, employing various experimental techniques. The excited state photophysical properties of C60 and C70 have been investigated extensively, using time-resolved spectroscopy by many groups. In this study, we carried out transient absorption and fluorescence lifetime measurements on those molecules and present new experimental results which would provide useful information on understanding more fully the excited state properties of C60 and C70.
Electrical Soliton Devices as >100 GHz Signal Sources
Authors: Carman E. , Case M. , Kamegawa M. , Yu R. , Giboney K. , Rodwell M.
Published: February 2023

We report a GaAs device which with 39 GHz and 490 mW input, generates 33 mW at 117 GHz, and 2.8 mW at 156 GHz using electrical soliton propagation. This device will serve as a signal source for ultrafast electronics.
Plasmon-Phonon Coupling and Hot Carrier Relaxation in GaAs and Low Temperature Grown GaAs
Authors: Devlen R.I. , Kuhl J. , Ploog K.
Published: February 2023

Within the first picoseconds after photogeneration, hot carriers in bulk GaAs relax predominantly by emitting optical phonons. The emitted phonons exist as a non-thermal phonon population until they decay anharmonically. Previously, the dynamics of the nonthermal phonon population was studied using time resolved anti-Stokes Raman spectroscopy [1]. For a carrier density larger than 1017 cm−3 the plasma frequency is comparable to the phonon frequency and coupling of the two excitations becomes non-negligible. In GaAs plasmon-phonon coupled modes have been observed in both single component plasmas (via doping)[2] and two component plasmas (via optical excitation)[3]. Due to the interaction of the phonons with the carrier plasma there have been no measurements of the phonon population dynamics for carrier densities larger than about 1017 cm−3. However, these non-equilibrium phonons play an important role in theories of hot carrier relaxation at high densities. In particular the existence of a large non-thermal phonon population and its reabsorption by carriers has been used to explain a reduction in the carrier energy loss rate[4].
Temporal Characteristics of the Ytterbium-Erbium Figure-8 Laser
Authors: Khrushchev I.Yu. , Grudinin A.B. , Dianov E.M.
Published: February 2023

The passive generation of femtosecond pulses in erbium-doped fibre lasers has recently become of considerable interest. The Figure-8 laser configuration whose operation is based on the reflection properties of the Nonlinear Amplifying Loop Mirror has been the object of considerable study [1,2]. It has been shown that the Figure-8 laser can operate in at least two distinctly different modes: in a 'soliton' regime producing relatively chirp-free, transform-limited, sub-picosecond pulses and in a 'square' pulse regime characterised by long (≈150ps) pulses, having a broad optical bandwidth (>35 nm)[3].
Energy transfer and primary charge separation in heliobacteria by picosecond transient absorption spectroscopy.
Authors: van Noort P.I. , Aartsma T.J. , Amesz J.
Published: February 2023

In photosynthetic bacteria the excitation energy of excited antenna pigments is transferred to the reaction center within tens of picoseconds. After trapping of the excitation energy by the reaction center, charge separation is induced as the first step in a series of electron transfer reactions.
Integrated Devices for Single Picosecond Pulse Measurements
Authors: Gerbe V. , Cuzin M. , Gentet M.C. , Lajzerowicz J.
Published: February 2023

An autocorrelator has been developped as an X-ray monitor to measure the bunch length of the CERN synchrotron in Geneva [1]. An extented application is the temporal measurement of single laser pulses. Using the same technology, we are now developping an optoelectronic sampler.
Femtosecond intermolecular electron transfer : dye in weakly polar electron-donating solvent
Authors: Yoshihara K. , Douhal A. , Nagasawa Y. , Yartzev A. , Kandori H. , Kenmitz K.
Published: February 2023

Ultrafast intermolecular electron transfer (0.1-0.2 ps) was observed for xantheme dyes in electron-donating solvents. Rates were limited by ultrafast nuclear relaxation not by solvent longitudinal relaxation, (470 mn) and AN (405 nm) and DMA (470 nm) cations in the subpicosecond timescale. From the recovery of ground-state depletion and decay of cation radicals, the reverse ET was also found to be fast, i.e., 4.0 ps in NB/DMA and 2.7 ps in NB/AN.
Femtosecond proton transfer in the electronic ground state of vibrationally hot molecules
Authors: Elsaesser Thomas , Frey Wilhelm , Portella Marcia T.
Published: February 2023

Photoinduced intramolecular proton transfer represents a chemical reaction that has mainly been studied in electronically excited molecules. For benzothiazole and -triazole compounds, recent femtosecond experiments gave evidence of excited state proton and deuterium transfer on a time scale of 100 to 200 fs [1,2]. In 2-(2’-hydroxy-5’-methylphenyl)- benzotriazole (trade name: TINUVIN P, TIN), the excited state enol→keto reaction (see structures (I) and (II)) is followed by the radiationless relaxation to the ground state of the keto-type molecules, occurring with a time constant of 150 fs. This species eventually undergoes a proton back-transfer to the ground state of the original enol structure. Until now, there was little information on the ground state reaction of TIN and of other compounds undergoing a closed reaction cycle. In particular, vibrational redistribution processes randomizing the large amount of energy from the initial photoexcitation (≃ 4 eV) have not been observed. In this paper, we discuss the first study of those processes with a time resolution of 100 fs, directly revealing the subpicosecond proton transfer in the ground state of TIN and the strong vibrational heating of the reaction product.
Soliton-like self-trapping of three-dimensional patterns
Authors: Froehly C. , Shalaby M. , Barthelemy A.
Published: February 2023

Description of experiments demonstrating the theoretically unexpected selftrapping stability of particular three-dimensional patterns ; experimental analysis of their connection to usual two-dimensional "soliton beam self-trapping".
Generation of pairs of solitons in an all-fibre, femtosecond soliton source
Authors: Richardson D.J , Afanasjev V.V. , Grudinin A.B. , Payne D.N.
Published: February 2023

In recent years much work has been devoted to the development of convenient solid-state laser sources of ultrashort pulses (1,2). Passive generation of femtosecond pulses in optical fibres is of particular interest. In this paper we report for the first time the generation of pairs of solitons in an all-fibre femtosecond fibre laser/amplifier combination.
Impulsive Stimulated Raman Scattering Study of Soft Mode Dynamics in Ferroelectric Crystals
Authors: Wiederrecht Gary P. , Dougherty Thomas P. , Nelson Keith A.
Published: February 2023

Soft Modes, ISRS, and Perovskite Ferroelectrics The dynamics of cooperative ordering in crystals near structural phase transitions have long been elusive. Most picosecond and femtosecond time-resolved measurements have involved rapid laser heating of a sample whose phase transition (e.g. melting) dynamics are then monitored1. In this case many lattice degrees of freedom are excited, and little information is provided about the roles or dynamics of particular lattice motions. In structural phase transitions, the order parameter is described by one or a very few specific ("soft") lattice modes whose motions bring crystalline constituents from their initial positions into their positions in a new crystalline phase. Experimental characterization of the dynamics is possible in some cases through Raman spectroscopy, but often the highly damped character of the soft mode leads to a central peak in the Raman spectrum which cannot be analyzed accurately. In many cases it is not even possible to tell whether the ordering motion is a heavily damped vibration, Debye relaxation, or some combination of the two. This information is essential for understanding the microscopic mechanism of a transition since vibrational or relaxational character is an indication of ionic motion within a single potential energy minimum (i.e. a displacive transition) or hopping between different sites (an order-disorder transition) respectively.
The Role of Covalency in Femtosecond Time-resolved Reflectivity of Hydrodynamically Expanding Solid Surfaces
Authors: Wang X. Y. , Ahn H. Y. , Downer M. C.
Published: February 2023

Intense femtosecond excitation of solid targets has been used to produce and measure the properties of solid density plasmas1 and pressurized liquids2. In several recent studies1 the ultrafast reflectivity response was observed with a single intense pulse serving both as excitation and optical probe. Consequently, the competing processes of electron heating and hydrodynamic surface expansion, which influence the reflectivity to different degrees and at different times, could be resolved only by a numerical deconvolution of single pulse data. In addition free electron metals were strongly emphasized1.
Hot hole capture by shallow acceptors in p-type GaAs studied by picosecond infrared spectroscopy
Authors: Lohner A. , Woerner M. , Elsaesser T. , Kaiser W.
Published: February 2023

The capture of free carriers by shallow impurities in doped crystalline semiconductors represents an important relaxation process which is of particular relevance for hot carrier transport. Until now, those phenomena have been investigated by stationary transport or noise measurements, giving the approximate cross sections of electron or hole trapping for a number of shallow donors or acceptors, respectively [1]. However, very limited information exists on the underlying scattering dynamics by which carriers are transferred from continuum states in the respective bands to localized states of the impurity atoms. In this paper, we discuss the first direct time-resolved observation of the ultrafast recombination of free holes with ionized shallow acceptors in a III-V semiconductor. Our data give evidence of hole capture on a time scale of 50 ps where emission of single longitudinal-optical phonons leads to a direct population of the acceptor ground state.
Ultrafast Recovery of Hetero n-i-p-i's by Enhanced In-plane Diffusive Transport
Authors: Smirl Arthur L. , McCallum D. S. , Cartwright A. N. , Huang X. R. , Boggess Thomas F. , Hasenberg T. C.
Published: February 2023

Hetero n-i-p-i's are one of several semiconductor quantum-well devices that use second-order electro-optic effects and that continue to attract interest for possible applications to low-power two-dimensional switching arrays and all-optical spatial light modulators. The turn-on time for a hetero n-i-p-i device is usually determined by transport perpendicular to the quantum wells. More specifically, it is determined by the time required for carriers generated in the quantum wells to escape the wells and to move to screen the built-in electric field, thus shifting the exciton. Consequently, typical turn-on times are of the order of a few ps. By contrast, when used in the conventional single-beam geometry, the recovery (or turn-off) time of hetero n-i-p-i's is determined by the slow recombination of the spatially-separated charges in the doped regions and is typically in the range of µs-ms. If instead, however, we use a two-beam mixing geometry for the device (where gratings are written in the material by the interference of the two beams), then the decay or turn off of the signal is determined by the decay of the gratings by in-plane transport over micron dimensions. Here, we use transient grating techniques to measure the recovery of such photorefractive and photoabsorptive gratings written in all-binary hetero n-i-p-i's. In this geometry, we show that the separation of photo-generated charge actually speeds the recovery by enhancing the effective in-plane ambipolar diffusion coefficient by roughly an order of magnitude (in contrast to the single beam geometry where charge separation elongates the recombination and recovery time).
Dynamics of Molecular Rotation at the Air/Water Interface by Time-Resolved Second Harmonic Generation
Authors: Castro Alonso , Zhang Dan , Eisenthal K. B.
Published: February 2023

To investigate orientational relaxation of molecules at the air/liquid interfaces we have used the technique of second harmonic generation as the probe. The idea of the experiment is to use a polarized picosecond pump pulse to preferentially excite molecules of a given orientation at the interface and thereby disturb the equilibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the second harmonic signal generated by a time-delayed second laser pulse at the interface. Such surface second harmonic signal continues to change as the ground and excited molecules undergo rotation towards their equilibrium distributions and as the excited molecules relax back to the ground state.
Ultrafast Energy Transfer within the B800-850 Light-Harvesting Antenna of Photosynthetic Purple Bacteria
Authors: Visscher K.J. , Oskarsson M. , van Grondelle R. , Sundstrom V.
Published: February 2023

Using ~500 fs tunable infrared laser pulses of low intensity, the energy transfer dynamics within several similar B800-850 antenna complexes from photosynthetic purple bacteria (Rhodobacter sphaeroides type) was investigated. Energy transfer from B800 to B850 was generally found to be very fast, -0.7 ps, at room temperature, and considerably slower at 77 K, ~2 ps. Transfer among identical B800 molecules is slow, >5 ps. The energy transfer was also studied in a different B800-850 complex and found to be very different, revealing very fast transfer between differently polarized states.
QED at 1020 W/cm2
Authors: Melissinos A. C.
Published: February 2023

An experiment is in preparation(1) at the 2-mile Stanford Linear Accelerator (SLAC) to study the scattering of 50 GeV electrons from a focussed short pulse of IR (λ = 1054) with peak intensity 1019 to 1020 W/cm2. The electric field seen by the electron in its own rest frame is E
*
= 2γELAB where γ is the ratio of the electron energy to its rest mass. For this experiment, where γ = 105 and if I = 1019 W/cm2, the ratio of E
*
to the critical field E
c
=m2c3/eħ = 1.32 × 1016 V/cm, reaches unity. At such field strengths multiphoton absorption is dominant and calculations must be carried out beyond simple perturbative QED.
Stimulated Emission Pumping and Selective Excitation by Adiabatic Passage with Frequency Modulated Picosecond Laser Pulses
Authors: Melinger Joseph S. , Hariharan Anand , Gandhi Suketu R. , Warren Warren S.
Published: February 2023

Optical excitation of material transitions by adiabatic passage with frequency modulated (chirped) laser pulses is a useful spectroscopic tool.1,2 Here we demonstrate that picosecond frequency modulated laser pulses produce more efficient population transfer, and more selective excitation than equivalent (same spectral bandwidth) nearly transform limited laser pulses. We apply frequency modulated pulses to gas-phase and condensed phase systems. We also point out the "robustness", or insensitivity of the properties of the material excitation to the parameters of the laser field, that is inherent to the adiabatic process. The robustness obtainable by adiabatic passage will ultimately prove useful to laboratory applications of theoretical schemes which require frequency modulated ultrashort laser pulses to control chemical reaction dynamics.3
Femtosecond time-resolved ionization spectroscopy of polyatomic molecules
Authors: Seel Matthias , Domcke Wolfgang
Published: February 2023

It is generally appreciated that femtosecond pump-probe techniques have the potential to provide novel and important information on elementary photophysical and photochemical processes. It has been demonstrated by several applications that time-resolved two-pulse ionization, that is, optical excitation and subsequent ionization of a molecular system with two short laser pulses, is a promising tool to observe excited state dynamics in real time.1 In comparison to alternative real-time probing techniques such as stimulated emission or excited-state absorption with fluorescence detection, two-pulse ionization spectroscopy possesses the advantage of extreme sensitivity of signal detection. The possibility of recording, for each pulse configuration, a complete photoelectron spectrum suggests that time-resolved ionization spectroscopy may be able to yield unprecedentedly detailed information on ultrafast intramolecular processes.
Squeezing of the Molecular Vibrations by Femtosecond Laser Pulses
Authors: Vinogradov An. V. , Janszky J.
Published: February 2023

In the last few years considerable interest was manifested to the stimulation of Franck-Condon transitions in molecules by ultrashort (femtosecond-range) laser pulses [1]. Since the spectrum of such a pulse definitely overlaps several vibrational levels, the molecule is put into a nonstationary vibrational state by the electron transition. This state represents a wave packet: a coherent superposition of vibrational eigenstates. In a number of papers [2-5] we studied how the properties of this wave packet relate to the characteristics of the laser pulse, i.e. to its duration and phase modulation. It was demonstrated that when the spectral width of the pulse is smaller than the width of the absorption band, but much larger than the frequency of vibrations, the vibrational ground state of the molecule transforms during excitation into a squeezed state with reduced value of the quantum uncertainty of the coordinate. All our previous results were obtained for Franck-Condon transition resulting exlusively from a spatial shift of harmonic nuclear potentials.
Transient Anisotropic Luminescence and Long-Living Polarization of an Optically Excited Dense Electron-Hole Plasma
Authors: Ivanov A.L. , Haug H.
Published: February 2023

Currently, transient optical phenomena in semiconductors are attracting considerable interest both theoretically [1-2] and experimentally [3-4] in connection with recent advances in ultra short-pulse spectroscopy. The relaxation into quasi-equilibrium of a dense electron-hole plasma in a semiconductor previously excited by a strong femtosecond optical pulse is determined by the carrier-scattering processes and lies in femtosecond time scale (τs typically 20 fs - 500 fs). In fact the carrier-scattering processes are determined both by the Coulomb mechanism which conserves the total momentum of the electron-hole system and dominates at high plasma densities and by carrier interaction with any crystal imperfections, i.e. phonons or impurities.
Plasma Physics with Ultra Short and Ultra Intense Laser Pulses
Authors: Pépin H.
Published: February 2023

The development of subpicosecond multi-terawatt lasers has been extremely rapid and laser intensities close to 1020 Wcm–2 will soon be available. These lasers have already brought new results in plasma physics and open the way to a broad range of new research avenues.
Ultrafast Dynamics of Femtosecond Laser-Produced Plasmas
Authors: Engers T. , Schüler H. , Schulz H. , von der Linde D.
Published: February 2023

We study the formation and subsequent evolution of microplasmas during and after the interaction of intense femtosecond laser pulses (peak intensities up to 1017 W/cm2) with solid materials. Pump-probe techniques are used to perform femtosecond time-resolved measurements of the optical reflectivity and of the second harmonic generation in the plasma. We have characterized the angular dependence, the polarization properties, and the frequency spectra of both the fundamental and the second harmonic (SH) of the reflected probe pulse.
X-Ray generated by femtosecond laser-produced plasmas
Authors: Geindre J.P. , Audebert P. , Gauthier J.C. , Mysyrowicz A. , Grillon G. , Chambaret J.P. , Antonetti A. , Mens A. , Verrecchia R. , Sauneuf R. , Schirman P.
Published: February 2023

With high intensity femtosecond lasers it is possible to create plasmas with near solid densities. Such plasmas can bring valuable informations on the dynamics of dense hot solids. They also appear very promising as sources of sub-picosecond X-Ray pulses. The availability of such X-Ray bursts can have a significant impact in the field of Plasma and Solid State Physics. We will review some recent experiments performed at L.O.A. in order to obtain a better understanding of femtosecond laser-matter interaction. Experiments have been performed with time-resolved optical pump and probe techniques and with X-Ray diagnostics.
Ionization and Dissociation of HCl by Infrared Radiation
Authors: Dietrich P. , Corkum P. B.
Published: February 2023

There have been many experimental and theoretical investigations of the interaction of high intensity infrared light with strongly bound diatomic molecules. All indicate that infrared multiphoton dissociation will only occur at very high intensities. We report the first observation of infrared multiphoton dissociation of a strongly bound diatomic molecule using HCl+. Furthermore, dissociation is observed at intensities of less that 1014W/cm2. We show that the dissociation threshold is determined by coupling of highly nonresonant electronic states, a process not included in previous theories.
Molecular Processes in the Primary Reaction of Photosynthetic Reaction Centers
Authors: Zinth W. , Finkele U. , Lauterwasser C. , Dressier K. , Umlauf E. , Schmidt S. , Kaiser W.
Published: February 2023

The primary electron transfer (ET) is the key process in photosynthesis. It occurs in a membrane bound pigment protein complex called reaction center (RC). Fortunately x-ray structure analysis has revealed the arrangement of aminoacids and chromophores of the RC giving the basic information for an understanding of the primary ET on a molecular basis: The important actors in the primary ET are four bacteriochlorphyll (BChl) and two bacteriopheophytin (BPh) molecules. After optical excitation of the primary donor P (which is a "special pair" of BChl molecules) the electron travels to or passes by the accessory BChl B and arrives within less than 4 ps at the BPh H before it finally (on the picosecond time scale) reaches the quinone Q after 200 ps. While the role of quinone Q and BPh H as real electron carriers is well established there is no general agreement about the function of the BChl B. Recently femtosecond absorption data have revealed a weak additional 0.9 ps kinetic component compatible with the assignment that the radical pair state P+B– is a real intermediate in the reaction (see model A in Fig. 1). However, alternative reaction models (B and C) - where the electron does not reside on the BChl B - could not be excluded by experimental data on native RC at room temperature.
Quantum Beats Spectroscopy of Exciton Spin Dynamics in GaAs Heterostructures
Authors: Bar-Joseph I. , Bar-Ad S.
Published: February 2023

The study of the spin dynamics in semiconductor quantum wells (QW) is of growing interest in recent years.1-5. In this paper we use a powerful novel technique, absorption quantum beats, to investigate this process. Controlling the initial spin orientation by the polarization of the pump pulse we excite the system in a superposition of two eigen-states with different energies. We observe two kinds of absorption quantum beats: Beats between Zeeman split levels and between excitons and bi-excitons. The frequency of these beats corresponds to the small energy splitting between the states involved, and the damping rate - to their dephasing. Experiments are performed on several QW samples at 4K and magnetic fields up to 5T, in a standard pump-probe configuration with a time resolution of ~ 1 ps.
Femtosecond Spectroscopy of Magneto-Excitons
Authors: Chemla D.S.
Published: February 2023

Recently, quasi-2D excitonic (X) effects have been investigated extensively in semiconductor quantum well (QW), resulting in fundamental advances in our understanding of the effects of dimensionality reduction from 3D to 2D on the optical response of semiconductors, and have lead to numerous new applications in optoelectronic [1]. Efforts to make new semiconductor structures where electronic states are further confined in quasi-0 or quasi-1 dimensions, have had only very limited success because of the difficulty of obtaining narrow size-distribution of defect free samples. However, electronic states strongly confined in all dimensions, in material of excellent uniformity and optical quality, can be obtained by immersing a high quality QW well in a perpendicular magnetic field [2,3]. In such a situation, the crystal band discontinuities confine the electronic states in the direction perpendicular to the QW. In the QW plane, in addition to the Coulomb potential, -e2/εor, the e-h pairs experience the confinement of the quadratic-potential, (eHr)2/8mc2, imposed by the magnetic field, H. The relative strength of two confining potentials is measured by the dimensionless parameter, λ = (ao/lc)2, where ao is the Bohr radius and lc = (c/eH)1/2 is the cyclotron radius. The e-h pairs then form magneto-excitons (MX), which extrapolate continuously between quasi-2D excitons (λ = 0) and quasi-0D Landau levels (λ → ∞).
Broadband transient IR spectroscopy of metal carbonyl photochemistry
Authors: Dougherty Thomas P. , Heilweil Edwin J.
Published: February 2023

A novel method for obtaining transient infrared molecular spectra with femtosecond time and high spectral resolution is described. New studies of metal-carbonyl photodissociation and solvation dynamics using this technique are described.
Electronic Relaxation Dynamics in Solution
Authors: Rossky Peter J. , Schwartz Benjamin J. , Sheu Wen Shyan
Published: February 2023

Due to the significance of local electric fields and rapid solvent response for the ultrafast processes involved in chemical reactions, explicit molecular descriptions of the solvent and solute have an essential role in understanding the underlying microscopic dynamics. Recent progress in laser technology is enabling direct experimental observation of the fundamental events. At the same time, significant advances have been made in theoretical methods. The present contribution focusses on the direct connection now accessible between the two at the spectroscopic level, and the molecularly detailed pictures that are emerging. Here, we outline selected theoretical results for two experimentally investigated systems, namely aqueous halide ions and the hydrated electron.
Ultrafast Optical and Vibrational Studies of some Simple Chemical Reactions in Solutions
Authors: Hochstrasser Robin M.
Published: February 2023

The dynamics of a few known gas reactions are different in solvents; evidenced by rates, solvent sensitivity, state distributions, phase propagation and transition state properties.
Influence of Inhomogeneous Broadening on Quantum Beats
Authors: Erland J. , Pantke K.-H. , Mizeikis V. , Lyssenko V.G. , Hvam J.M.
Published: February 2023

Current issues in the study of light-matter interactions are coherence and quantum interference. With ultrafast nonlinear spectroscopy such as transient four-wave mixing (TFWM) the imposed coherence on the optical excitations in e.g. GaAs multiple quantum wells has been studied. In this contribution, we study the influence of inhomogeneous broadening on quantum beats observed by spectrally resolved TFWM. In artificially grown semiconductor structures as e.g. quantum wells, inhomogeneities due to interface-disorder has, up till now, been inevitable. In the experiments, the light-hole exciton (lhx) and the heavy-hole exciton (hhx) in a 100 Å GaAs multiple quantum well are excited simultaneously with 150 fs pulses from a self mode-locked Ti:sapphire laser. The spectral profile is shown in the inset of Fig. 1.
Time-Resolved Optoelectronic Measurements of Nitrogen-Implanted GaAs Crystals
Authors: Hong S. , Jin Y. , Wagoner G. , Zhang X.-C. , Kingsley L.
Published: February 2023

Understanding the effects of electrochemical passivation on the electronic properties of semiconductor surface states is crucial to semiconductor device fabrication. Previous results indicated that the static electric field across the surface of a semiconductor is substantially modified by hydrogen plasma passivation. As an example, after 30 minutes of passivation, the surface field from n(p)-type GaAs samples has been shown to increase by up to a factor of 2(10). Herein, we report the results of our recent measurements of the transient optoelectronic properties of nitrogen-ion implanted GaAs, where the nitrogen ions served as isoelectronic centers and surface implantation densities ranged from 1x108 cm-2 to 5x1015 cm-2. Optically induced THz electromagnetic radiation and time-resolved photoreflectance (ΔR/R) measurements were made and the results of these two different optoelectronic techniques was compared. Both measurements show strong dependence on implantation density and either technique is a good diagnostic of surface implantation density.
Group–delay dispersion measurement of laser mirrors by spectrally resolved white light interferometry
Authors: Kovács A. P. , Szipőcs R. , Osvay K. , Bor Z.
Published: February 2023

It is known, that the temporal shape of a femtosecond pulse is altered while propagating through dispersive optical components and mirrors. Specially designed chirped dielectric high reflectors [1–3] have been proven to be efficient tools for controlling the intracavity group velocity dispersion in femtosecond lasers. In designing high performance femtosecond laser systems, the direct measurement of the frequency dependent group delay of mirrors is essential. Recently, a number of works have been devoted to the problem of measurement of the group–delay dispersion [4–9]. In [4,5] white light time–of–flight interferometer has been used. In [6,7] a phase locked Michelson interferometer was used to measure the phase response. Knox [8] measured the total group delay in an operating Ti: sapphire femtosecond laser by analyzing the pulse train with a digital frequency counter. Schwider [9] used superposition–fringes interferometry to determine the phase changes upon reflection from a dielectric multilayer. Spectrally resolved white light interferometry has been used for refractive index measurements of dye solutions [10]. In this paper a spectrally resolved white light Michelson interferometer is used to measure the group delay of mirrors.
Picosecond Optical Time Domain Reflectometer for Optoelectronic IC Device Characterisation
Authors: Beaulieu Y. , Normandin R. , van der Meer P. , Chatenoud F. , Delage A. , Garside B.K.
Published: February 2023

Several years ago Stegeman and Normandin predicted, and subsequently demonstrated [1], the nonlinear mixing of oppositely propagating guided waves. The resultant second harmonic field was coupled to radiation modes and propagated in a direction perpendicular to the waveguide surface, in the case of equal fundamental frequencies. Its application to picosecond signal processing [2], the creation of all optical transient digitizers [3] and spectrometers [4] demonstrated the potential usefulness of this approach. Finally, the nonlinear cross section for uniform GaAs optical waveguides was calculated and published [5], and recently this GaAs prediction was verified [6] by Vakhsoori et al. In their geometry, the oppositely propagating beam was supplied by the reflection from the exit surface. Their use of picosecond pulses also demonstrated the potential for optical correlation in a geometry similar to references 2 and 3. However, the use of simple GaAs films results in low conversion efficiencies, due to difficult input coupling and absorption and phase cancellation effects at the harmonic wavelength.
Femtosecond Studies of Charge Separation in the Fluorescent State of DCM
Authors: Zhang H. , Jonkman A.M. , van der Meulen P. , Glasbeek M.
Published: February 2023

Solvation dynamics has long been recognized to be of great importance for the understanding of the rate of intramolecular electron transfer (ET) reactions1,2. Especially when the ET reaction takes place in the so-called strong adiabatic coupling limit, solvation dynamics may predominantly determine the ET rate constant. Under such conditions one has κ ≫ 1, where the adiabaticity parameter3, κ, is given by κ = π V2 〈τ〉 /ħλm, in which V is the electronic coupling between the locally excited (LE) and charge transfer (CT) states involved in the electron transfer process, 〈τ〉 is the solvent relaxation time and λm is the solvent reorganization energy. Recently, for a number of molecules solvent controlled ET in the photo-excited state has been considered in the strong adiabatic coupling regime4. In this report, we present results of a femtosecond pump-probe study of the charge separation reaction in the photo-excited state of a large laser dye molecule, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4-H-pyran (DCM). We have performed a subpicosecond study of the solvatochromism of the DCM molecule. A major finding is that following the pulsed excitation of the molecule, the integrated intensity of the spontaneous fluorescence exhibits a transient decay to approximately 50% of its initial value in a few picoseconds. The results are consistent with strongly mixed LE and the CT excited states for DCM (i.e., the adiabatic coupling limit applies) so that the solvation dynamics constitutes the rate determining step in the ET process.
Mechanical relaxation and solvation dynamics in nonpolar solutions
Authors: Fourkas John T. , Benigno Andrea , Ma Jangseok , Bout David Vanden , Berg Mark
Published: February 2023

We have used transient hole burning measurements to make the first measurements of solvation dynamics in a completely nonpolar system: dimethyl-s-tetrazine (DMST) in n-butylbenzene.1,2 A new model for solvation based on the mechanical response of a viscoelastic solvent explains the main experimental features and predicts new features on the femtosecond timescale. Although the mechanical solvation mechanism is most prominent in nonpolar systems, it should play a role in all solvation3,4 and electronic dephasing5–8 processes.
Real World Ultrafast Sources
Authors: Panel Speakers  , Keller Ursula , Fermann Martin , Sibbett Wilson , French Paul M.W. , Delfyett Peter
Published: February 2023

No summary available
Real World Ultrafast Applications
Authors: Panel Speakers  , Weber Willes H. , Smith Kevin , Wiesenfeld Jay , Williamson Steve , Weiner Andy , Rodwell Mark , Alfano Robert
Published: February 2023

No summary available
Dephasing Dynamics of Epitaxial Polydiacetylene Films
Authors: Pham T.A. , Daunois A. , Le Moigne J. , Bigot J.-Y.
Published: February 2023

The dephasing dynamics of epitaxial polydiacetylene films resonantly excited with 10fs optical pulses is investigated. A 52fs relaxation time and quantum beats with 23fs period are reported.
Mode Properties of a High Intensity Plasma Waveguide
Authors: Durfee C.G. , Milchberg H.M.
Published: February 2023

New results are presented from the first experimental demonstration of the channeling of intense laser pulses over distances much greater than a Rayleigh length1. In this scheme, two laser pulses are used: The first pulse creates a column of hot plasma that forms a channel behind a radially expanding shock wave, and the second pulse is injected into the channel after an appropriate delay. Single-mode and multi-mode propagation of the channeled beam is observed, with the mode structure depending on the two-pulse delay, gas density, and input beam coupling. Pulses have been channeled for distances as long as 70 Rayleigh lengths at intensities of at least 1014 W/cm2. As much as 75% of the energy in the focal spot of the injection lens is coupled into the guide.
Time Resolved Dynamics in Large Molecular Cluster Ions
Authors: Nadal M. , Vorsa V. , Papanikolas J. M. , Campagnola P. J. , Parson R. , Lineberger W. Carl
Published: February 2023

Photoabsorption in large mass-selected cluster ions is detected via evaporation of molecules from the cluster. The shape and size evolution of the absorption spectrum with cluster size provides a mechanism to identify the chromophore and to detect the extent of charge delocalization in the cluster ion. Photofragment mass spectra have been obtained for
Br2−(CO2)n,I2−(CO2)n and ICl−(CO2)n cluster ions following photoexcitation of the
12g (2Πg,1/2)←12u(2Σu+) transition in
I2− and Br2− and the analogous transition in ICl-. Photodissociation of
X2− within the cluster results in the production of both X-- and
X2−-based ionic photofragments:
Uses of Optical Phase-Controlled Pulse Sequences
Authors: Cina Jeffrey A.
Published: February 2023

There has been rapid experimental progress in the shaping of single ultrashort light pulses into sequences of pulses of specified separation, duration, and relative optical phase 1,2a. Phase-controlled pulse sequences have found a number of applications to molecular systems 2b,c, and several more have been the subject of theoretical calculations. The salient feature of electronically resonant phase-controlled pulse trains is their ability to generate superpositions of multiple time-dependent contributions to the nuclear wave function in a given electronic state having well-defined relative quantum phases. Here we describe two uses to which that feature might fruitfully be put in preparing and probing time-dependent nuclear wave functions in the ground electronic state possessing chemically or spectroscopically useful properties.
Ultrafast Spectroscopy of Alkali Clusters
Authors: Schreiber E. , Kobe K. , Kühling H. , Rohland S. , Ruff A. , Rutz S. , Sommerer G. , Wöste L.
Published: February 2023

The fragmentation and wavepacket propagation dynamics of small neutral potassium and sodium clusters in a supersonic cluster beam were investigated using pico- and femtosecond two photon ionization spectroscopy followed by mass-selective detection.
Time gated x-ray imaging using an ultrashort pulse, laser produced plasma x-ray source
Authors: Gordon C. L. , Barty C. P. J. , Harris S. E.
Published: February 2023

The development of high peak power, ultrashort pulse duration Ti:sapphire laser systems1-2 has led to the generation of hard, incoherent x-rays from laser-produced-plasmas (LPP).3 In these experiments x-rays with energies as high as 1.5 MeV were produced. Also generated were copious amounts of diagnostic x-rays (20 keV to 150 keV). This fact suggests that ultrashort-pulse-pumped, LPP x-ray sources may have uses in medical imaging applications. In particular, there are two distinct differences between the LPP x-ray source and conventional x-ray sources, namely source duration and source size. The duration of the LPP source is believed to be <1 ps or nearly 6 orders of magnitude shorter than conventional x-ray sources. Because of this, it becomes possible to consider time gated imaging arrangements which require greatly reduced x-ray exposures to create the same quality image of a patient. The source size of the LPP x-ray source may be as much as 100 times smaller than conventional devices and thus may allow imaging of much smaller features than previously possible.
A dual beam sub-picosecond spectrometer in the 5-11μm spectral region.
Authors: Hamm P. , Lauterwasser C. , Zurek M. , Zinth W.
Published: February 2023

Time resolved pump and probe measurements with probing pulses in the mid-IR spectral region has proven to yield important information on fast reactional and relaxational processes in solid state physics, semiconductor physics, chemical physics, and biophysics. In order to make new experiments in those physical fields possible, one requires tunable sub-picosecond mid-IR-pulses and a experimental setup, which allows the sensetive detection of absorption changes as small as 0.1 mOD. Our laser system is based on a femtosecond Ti-sapphire laser and regenerative amplifier which is the master light source used for a subsequent difference frequency mixing to obtain sub-picosecond pulses in the mid IR-region. The amplifier operates at a repetition rate of 1 kHz, a wavelength of 815 nm, a pulse duration of 120 fs, and a pulse energy of 500 μJ [1].
Ultrafast Protein Relaxation: Time-Resolved Infrared Studies of Protein Dynamics Triggered by CO Photodissociation from CO Myoglobin
Authors: Dyer R. Brian , Causgrove Timothy P.
Published: February 2023

A critical feature of the biological function of heme proteins is the direct coupling of protein motion to the process of binding exogenous ligands to the heme. In carbonmonoxymyoglobin (MbCO), a substantial, specific conformational relaxation is associated with the transition from the ligated to the unligated form of the protein. The analogous tertiary structural changes of the monomer heme subunits of hemoglobin ultimately lead to the R→T quaternary structural transition, the allosteric control mechanism of O2 binding efficiency [1]. We have studied these processes on the earliest timescales, using picosecond, time-resolved infrared (TRIR) spectroscopy. It has long been known that infrared spectra in the amide region are sensitive to protein secondary conformation [2]. Recent advances in equipment and techniques have permitted researchers to quantitatively predict secondary structures from infrared spectra [3,4], particularly in the amide I region [4]. Therefore, it is now possible to study protein motion in time-resolved experiments on dynamics and function. The ligation reactions of small molecules such as CO with the heme site of Mb exemplify the mechanisms available to O2. CO is an ideal candidate for initial time-resolved IR experiments in the amide I region because it is easily photolyzed, little geminate recombination [5], and the structure of both MbCO and unligated Mb have been studied by crystallographic methods [6]. TRIR has already been applied to the stretching vibrations of the bound and free CO ligand [7,8]; dynamics of the protein, however, have yet to be probed by TRIR spectroscopy of the protein vibrations. Here we report results on the motions of the protein in response to ligation reactions, probed in the amide I region centered about 1650 cm-1.
Coherent Nuclear Motions and Exciton State Dynamics in Photosynthetic Light-Harvesting Pigments.
Authors: Chachisvilis M. , Pullerits T. , Jones M.R. , Hunter C.N. , Sundstrom V.
Published: February 2023

Introduction The light-harvesting (LH) pigment-proteins of photosynthetic purple bacteria have been extensively studied with picosecond [1] and now recently with femtosecond [2] pulses. These investigations have helped to make these pigments to the probably best studied and characterized LH systems. Several bacteria, including the presently studied species Rhodobacter sphaeroides, contain two types of LH pigments, a core LH1 antenna surrounding the photochemical reaction centers and a more peripheral LH2 antenna. Both these LH pigments are believed to be organized in small minimum units of 2-4 polypeptides holding about the same number of chromophores, which aggregate to form the network of the intact antenna. Energy transfer between minimum units is believed to occur in a non-coherent hopping fashion. Interaction within a unit is probably quite strong and spectral evidence suggest that it should be characterized as strong exciton interaction leading to the formation of delocalized exciton states. In this report we will mainly be concerned with the ultrafast dynamics within such a minimum unit.
Nonlinear Vibrational Spectroscopy of Liquids and Liquid: Air Interfaces with Ultrashort Laser Pulses
Authors: Laubereau A. , Wolfrum K.
Published: February 2023

Novel aspects of the vibrational dynamics of bulk and surface molecules are investigated by "magic" 3-colour femtosecond CARS and picosecond sum-frequency spectroscopy in the infrared.