bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonance Raman Studies of Electronic and Vibrational Relaxation Dynamics in Heme Proteins
Authors: Champion P. M. , Sage J. T. , Li P.
Published: February 2023

We discuss the technique of resonance Raman saturation spectroscopy and present experimental results that probe relaxation processes in heme proteins following electronic excitation in the Soret band. The observable relaxation time scales are limited by the laser excitation rate, kL, rather than by the laser pulse width (~10 ns). Analysis of the data using a model that allows for parallel relaxation channels leads to effective electronic relaxation times for deoxymyoglobin (2ps), ferrocytochrome c (3ps), and deoxyhemoglobin (1ps). The more rapid relaxation times found for hemoglobin and its isolated chains are not currently understood. The Raman depolarization ratio is predicted to increase at high laser flux, due to the preparation of a partially oriented sample by photoselective excitation. Such effects are observed in heme systems and the relaxation times extracted from the depolarization analysis are in agreement with the measurements of Raman intensity saturation. Studies of the asymmetric broadening of the
ν
4 mode of cytochrome c at high laser flux, reveal that the lineshapes in the Stokes and anti-Stokes region are inequivalent. Time-reversal symmetry dictates that this broadening is due to an underlying Raman band, associated with an excited electronic state that is populated at high laser flux. Similar linebroadening effects, observed in hemoglobin and myoglobin samples, are also shown to arise from Raman scattering of excited electronic states rather than Rabi broadening1 or anharmonic coupling to vibrationally hot low frequency modes2.
Soft-X-Ray Pulse Length Measurement by Pump-Probe Absorption Spectroscopy
Authors: Sher M.H. , Mohideen U. , Tom H.W.K. , Wood O.R. , Aumiller G.D. , McIlrath T.J. , Bokor J. , Freeman R.R.
Published: February 2023

Current developments in high power subpicosecond lasers are making possible a new class of experiments in which matter may be pumped or probed with picosecond or even subpicosecond x-ray pulses. These experiments require accurate knowledge of the x-ray pulse duration. We have demonstrated a picosecond time scale, soft-x-ray continuum absorption spectroscopy technique and are using it to measure the temporal profile of laser-generated soft-x-rays. The technique is based on the rapid but permanent switching of the transmission of an x-ray absorbing medium by multiphoton ionization with a 200 fsec laser pulse. We use Kr gas as the absorbing medium and monitor the transmission on two ~90 eV transitions from the neutral ground state. When the Kr is ionized these absorption features vanish. The change in transmitted energy as a function of the time delay between the ionizing and x-ray pulses is then the integral of the x-ray pulse shape. In principle, the resolution of this technique is limited only by the ionizing pulse duration which can be much better than the current state-of-the-art for x-ray streak cameras [1]. An advantage of this method over the transient Stark shift method reported previously [2] is that it is useful for soft-x-ray pulse lengths ranging from the resolution limit, to the recombination time of the absorbing medium. This type of measurement will have applications to transient species spectroscopy and other aspects of high-intensity laser and plasma physics.
An Overview: Femtosecond Spectroscopy at the Cutting Edge of Time Resolution
Authors: Shank C.V. , Schoenlein R.W. , Mittleman D.M. , Shiang J.J. , Alivisatos A.P.
Published: February 2023

We report the first direct measurements of electronic dynamics in CdSe nanocrystals on a femtosecond time scale. Two pulse photon echo measurements show a rapid echo decay, on the order of 8 fs. Complimentary three pulse echo measurements indicate that the two pulse results are dominated by quantum beats associated with the 210 cm–1 LO phonon mode. Using a novel technique to cancel the effects of the quantum beat, we observe in the three pulse echo results evidence of two echo decay times, on the order of 8 fs and 18 fs respectively. In addition, time resolved spectral measurements show evidence of hole burning and dynamics on a femtosecond time scale. These results suggest that the electronic properties of these materials are more complicated than previously believed.
Ultrafast energy transfer in the core light harvesting complex of photosynthetic bacterium Rhodobacter sphaeroides observed by fluorescence upconversion.
Authors: Bradforth S. , Jimenez R. , Fidler V. , Fleming G. , Nagarajan S. , Norris J. , van Mourik F. , van Grondelle R.
Published: February 2023

Energy transfer in the core light harvesting complex (LHI) of the purple photosynthetic bacterium Rb. sphaeroides has been measured by the fluorescence upconversion technique1. LHI is thought to be constructed from aggregates of small building blocks containing transmembrane polypeptides and associated carotenoid and bacteriochlorophyll a (BChla) pigment moieties.2 There is evidence for a high degree of ordering of the pigments in the basic "minimal unit". The mechanism of energy transfer from carotenoid to BChla and between BChla pigments is not well understood. Although the two energy transfer processes proceed via different mechanisms, they are both highly efficient, and are both expected to occur on sub-picosecond timescales. LHI makes an excellent system for study of ultrafast energy transfer in a biological system because of the existence of only a single BChl spectral pool and one type of carotenoid. We have investigated energy transfer between B870 BChla pigments in the complex as well as between carotenoid and BChla pigments. A sub-picosecond decay observed in the fluorescence depolarization of LHI excited at 870 nm is assigned to energy transfer between BChla's in the minimal unit of the complex. In preliminary measurements where the carotenoid band is excited at 505 nm, a rise in the isotropic emission from BChla is characterized by two time constants.
Ultrafast Solid State Lasers: Status and Prospects
Authors: Krausz F. , Spielmann Ch. , Curley P. F. , Brabec T. , Kelly S. M. J. , Stingl A. , Szipöcs R. , Wintner E. , Schmidt A. J.
Published: February 2023

Until recently the passively mode-locked dye laser has been the only reliable, widely used source of femtosecond optical pulses. Dramatic advances in all-optical modulation techniques and the appearance of novel broadband solid state gain media over the last few years opened up a new era in femtosecond pulse technology [1]. More than two decades after the first demonstration of a picosecond mode-locked laser, we have recently witnessed the emergence and evolution of a new generation of ultrashort-pulse lasers based exclusively on solid state components [2]. Intensive research efforts resulted in a good understanding of the basic physical processes dominating pulse formation in these novel sources [3], giving rise to a rapid progress of femtosecond solid state laser technology. This research culminated in the generation of sub-20 fs optical pulses directly from Ti:sapphire laser oscillators [4].
Ultrafast Dynamics as reflected in MPI-photoelectron spectra: Aniline, Azulene, Phenanthrene and Carbon disulfide
Authors: Thantu N. , Kim B. , Weber P. M.
Published: February 2023

Very short lived resonances can cause unobservably small yields of resonant multiphoton ionization (MPI) processes, and severely alter the photofragmentation pattern of the resulting ions.1 These effects, which are frequently observed when using high lying electronic states of medium size or large molecules, are usually interpreted as a consequence of fast intramolecular processes such as internal conversion (IC), intersystem crossing (ISC), Intramolecular Vibrational redistribution (IVR) or Photodissociation. In the present paper we investigate the interplay of such dynamical phenomena with the laser pulse characteristics. To illustrate the range of possible effects we investigated the photoionization of aniline (C6H5NH2), phenanthrene (C14H10), azulene(C10H8) and carbon disulfide (CS2).
Solvent Influence on Barrier Crossing in the S1-State of Cis- and Trans-"Stiff”-Stilbene
Authors: Mohrschladt R. , Schroeder J. , Troe J. , Vöhringer P. , Votsmeier M.
Published: February 2023

Diffusion-controlled chemical rearrangements in the liquid phase are in general well described by the Kramers-Smoluchowski model for barrier crossing processes under the influence of frictional forces.
Impulsive Stimulated Raman Scattering and Coherent Soft Phonons in LaAlO3
Authors: Liu Y. , Frenkel A. , Whitaker J. F. , Merlin R. , Uher C.
Published: February 2023

The generation of coherent phonons and molecular vibrations using pulsed lasers has recently attracted much attention [1-8]. Systems for which such coherent transient modes have been studied include molecular crystals and dyes [1], semiconductors [2], semimetals [3], high-Tc superconductors [4-5] and ferroelectrics [6-7]. As with other stimulated processes, phonon coherences result from nonlinear coupling between photons and the vibrational field. While in most cases the relevant mechanism has been ascribed to stimulated Raman scattering, this has not always been supported by a symmetry analysis of its properties. In addition, there is some controversy surrounding experiments performed under resonant excitation [5,8].
Coherence Transfer in Photoinitiated Solution Phase Reactions
Authors: Pugliano Nick , Szarka Arpad Z. , Palit Dipak K. , Hochstrasser Robin M.
Published: February 2023

The use of ultrashort laser pulses to photoinitiate chemical reactions in solution has lead to the observance of vibrational coherence transfer from reactant states into the product manifold. By resolving the wavepacket contributions in the transient response, the solvent influence on the reaction pathway may be probed. As a result, information regarding product state distributions and the existing offdiagonal density matrix elements becomes accessible. For many photochemical processes the initiation pulse must be in the ultraviolet spectral region. Therefore, a femtosecond pump/probe spectrometer, capable of delivering 70 fs; 5 μj 310 nm pulses, has been constructed for this purpose. It has enabled the study of the solvent sensitivity of wavepacket dynamics in two photochemical reactions; the photodissociation of HgI21 and the photoisomerization of cis-stilbene. These experimental results, and an improvement in the time resolution of the femtosecond spectrometer will be summarized below.
Determination of 2-Pulse Photon Echos from Solvent Spectral Densities
Authors: Arnett D.C. , Vöhringer P. , Westervelt R.A. , Feldstein M.J. , Scherer N. F.
Published: February 2023

The dynamic nature of a liquid or fluid medium causes structural changes to occur on timescales corresponding to the Fourier transform of the far-infrared or Raleigh-wing spectrum of the material.1,2 Experiments that are performed on such short timescales are capable of directly capturing the solvent effect on or response to chemical processes.3 For example, successful comparison of the solvent spectral density obtained from optical Kerr effect measurements and the time correlation function for solvation, obtained from fluorescence Stokes-shift measurements, has already been made.3
Ultrafast observation of an electron gas in SiO2
Authors: Audebert P. , Daguzan P. , Santos A. Dos , Gauthier J.C. , Geindre J.P. , Guizard S. , Hamoniaux G. , Krastev K. , Martin. P. , Petite G. , Antonetti A.
Published: February 2023

When an ultra-short and intense laser field propagates inside a dielectric medium, it induces a strong polarization field and, if the intensity is high enough, it produces a high density of electrons and holes. In this paper, we report on an experiment based on a new phase-sensitive technique [1,2] which allows the direct visualization of the polarization and the excitation density with a resolution of 120 femtosecond in time and 5 μm in space. To our knowledge, the very first relaxation events, concerning the photogenerated carriers in the conduction band, remain an unexplored domain in wide band-gap insulators.
THz study of field screening: 10 fs - 100 fs regime
Authors: Hu B. B. , DeSouza E.A. , Knox W. H. , Nuss M. C. , Cunningham J. E.
Published: February 2023

We report the first THz study of field screening dynamics in the 10- 100 fs regime. The instantaneous polarization contribution and the transport contribution are clearly resolved in the time domain.
Vibrational Dynamics at the Active Site of Myoglobin: Picosecond Infrared Free-electron Laser Experiments.
Authors: Peterson Kristen A. , Hill Jeffrey R. , Tokmakoff A. , Sauter B. , Zimdars D. , Dlott Dana D. , Fayer M. D.
Published: February 2023

Vibrational lifetimes of CO bound to myoglobin and to a water-soluble Fe:porphyrin were measured by ps infrared experiments. Vibrational relaxation rates are found to depend on protein conformational substate and porphyrin substitution. We have made direct measurements of molecular energy transfer at the active site of a protein using intense tunable mid-IR pulses from a free-electron laser to measure the loss of vibrational energy from CO bound to a bare water soluble heme and CO bound to the active sites of different conformational substates of myoglobin. Vibrational relaxation rates are found to depend on protein conformational substate and porphyrin substitution.
Observation of Fifth Order Optical Response of Liquid CS2 by Non-Resonant Six-Wave Mixing
Authors: Tominaga Keisuke , Naitoh Yukito , Kang Tai Jong , Yoshihara Keitaro
Published: February 2023

We repotrt the first observation of a fifth order optical response of liquid CS2 by non-resonant six-wave mixing and discuss the vibrational dephasing of intermolecular modes.
Ultrafast Dynamics of the Excited Br Atom - Mesitylene Eda Complex
Authors: Jarzeba W. , Hörmann A. , Thakur K.A.M. , Barbara P.F.
Published: February 2023

We recently made the first measurements of the electron transfer (et) kinetics of a bromide/aromatic cation contact ion pair (Br-/Ar+). This type of contact ion pair (CIP) is prepared by ultrafast optical excitation of the charge transfer band of a Br atom/aromatic molecule charge transfer complex.1
Frequency Doubling of Ultrashort Pulses from a Ti:sapphire Laser
Authors: Asaki M.T. , Backus S. , Shi C. , Murnane M.M. , Kapteyn H.C.
Published: February 2023

Recent advances in mode-locked Ti:sapphire laser technology make possible the routine generation of pulses as short as 11 fs directly from a laser.1 Frequency doubling the 800-850 nm fundamental of these lasers may make possible pulses of unprecedented short duration in the blue. Both intracavity and extracavity doubling of femtosecond pulses from Ti:Sapphire and CPM lasers have been demonstrated.2-4 Intracavity doubling is simpler, and also has the advantage of much higher power in the doubling crystal, and therefore, much higher efficiency. Previously, the shortest pulses generated using intracavity doubling were 54 fs at 430 nm.
Direct Observation of Infrared Free Induction Decay of Liquid Water Molecules with a Femtosecond OPO at 3 Micron
Authors: Crowell Robert A. , Holtom Gary R. , Xie Sunney X.
Published: February 2023

The importance of water as a universal and unique solvent for chemical and biological reactions has continued to stimulate theoretical, computational, and experimental efforts on understanding the structure and dynamics of liquid water. Liquid water has a broad absorption band in the 3μm region due to its fundamental stretching vibrations which are particularly sensitive to its hydrogen binding structure. Graener et al have originated picosecond hole burning experiments on liquid water in this spectral region and revealed the inhomogeneous nature of the hydrogen bonding structures (Ref. 1) Femtosecond coherent experiments would be particularly informative about the vibrational dephasing and other dynamical properties of the system. A 50fs pulse at 3μm is needed in order to coherently excite the entire inhomogeneously broadened band. However, this has been experimentally difficult due to the lack of femtosecond source in this spectral region.
Picosecond Dynamics of Carrier Cooling in CdSe : Reduced Energy Loss Rates Caused by Hot Phonon Effects
Authors: Vengurlekar Arvind S. , Prabhu S.S. , Roy S.K. , Shaht Jagdeep
Published: February 2023

We report the first picosecond luminescence measurements of hot carrier cooling rates in the II-VI semiconductor CdSe. We find that the carrier energy loss rates reduce with the excitation density n
o
. The rates are much smaller than those expected in a simple theory of carrier energy transfer to the lattice via various phonon emission processes. At n
o
= 8 × 1017 cm−3 for example, the rates are reduced by as much as factor of 75. We present a detailed comparison with the theory including the effects of hot phonons and all relevant energy loss mechanisms. We find that a large, density dependent value of 6 to 9 ps for the optical phonon lifetime (at 8 K) is required to explain the results.
Coherent Pulse Shaping in a Self-Modelocked Ti:Sapphire Laser
Authors: Harvey J. D. , Dudley J. M. , Curley P. F. , Spielmann C. , Krausz F.
Published: February 2023

The recent development of nonresonant passive modelocking techniques has stimulated considerable interest in the generation of ultrashort optical pulses from broadband solid state laser media. Self-modelocking [1] is now routinely exploited to generate sub-100 fs pulses from solid state lasers in the near infrared. Optimization of a solitonlike interplay between negative group delay dispersion (GDD) and self-phase modulation (SPM), and minimization of high-order dispersive perturbations [2] have resulted in sub-20 fs pulse generation from Ti:sapphire lasers [3 - 6]. In particular, the minimisation of the cubic phase distortion arising from intracavity prism pairs has resulted in the generation of exceptionally broad modelocked spectra with FWHM of up to 150 nm and pulse durations ≈ 10 fs [7].
Primary Electron Transfer Steps in Photosystem I Reaction Center with Reduced Number of Antenna Chlorophylls (12 - 30)
Authors: Kumazaki Shigeichi , Iwaki Masayo , Ikegami Isamu , Kandori Hideki , Itoh Shigeru , Yoshihara Keitaro
Published: February 2023

The ultrafast dynamics of electron relays in the Photosystem I (PS I) reaction center (RC) is of particular interest because the amino acid sequences of the polypeptides show almost no homology to those of PS II-purple-bacterial type RC’s. Electron carriers and energy levels of PS I RC are also different from those of the latter type of RC’s. However, studies in Photosystem I have been scarce due to its large number of antenna chlorophylls (more than 100) per RC and to resultant spectral congestion. Even though the presence of the primary electron acceptor chlorophyll (A0) has been suggested by a recent X-ray crystallographic work [1], spectroscopic detection of sequential reduction and reoxidation of A0 has never been successful.
Polarized Ultrafast Transient Spectroscopy of the Hydrated Electron: Quantum Non-Adiabatic Molecular Dynamics Simulation
Authors: Schwartz Benjamin J. , Rossky Peter J.
Published: February 2023

The transient spectroscopy of the aqueous solvated electron has been the subject of intense experimental1-3 and theoretical4-8 interest recently. Hydrated electrons play an important role as intermediaries in the radiation chemistry of water, as well as in solution photochemistry and electron transfer reactions. Furthermore, the coupling of solvent fluctuations to the electronic absorption spectrum makes the hydrated electron an outstanding probe of electronic solvation dynamics in the aqueous environment. The optical absorption spectrum of the hydrated electron is comprised of three s->p like transitions which are highly broadened and split by coupling to solvent fluctuations.6
Femtosecond electronic dynamics by time-delayed four-wave mixing with incoherent light: iodine and azulene
Authors: Yang Tzyy-Schiuan , Zhang Ruihua , Myers Anne B.
Published: February 2023

Spectrally broadband, temporally ’’incoherent” light has been employed to obtain femtosecond time resolution in various nonlinear optical experiments by exploiting the fact that in many such processes it is the coherence time of the light source, not the duration of its pulse intensity envelope, that dictates the temporal resolution of the experiment.1-4 One such process is the photon echo configuration of time-resolved four-wave mixing,5-9 which can distinguish the contributions to molecular electronic spectral linewidths from "homogeneous” and ’’inhomogeneous” broadening if the latter is static on the time scale of the entire matter-radiation interaction sequence. In liquids, spectral difiusion--the interconversion of spectroscopically different local environments for different chromophores--is usually fast relative to the actual pulse durations of most incoherent light sources, complicating efforts to study electronic dephasing with such sources. When femtosecond pulses are used, the overall time scale of the experiment can be made subpicosecond if desired, or it can be made arbitrarily long by employing the three-pulse technique. No such tunability is possible when the light source has a temporally long intensity envelope, however short its correlation time.
Single-shot, pulse-length-limited ultrafast transient-absorption spectroscopy
Authors: Fourkas John T. , Dhar Lisa , Nelson Keith A.
Published: February 2023

In solids and viscous liquids, irreversible ultrafast dynamics, such as high-intensity processes and photo-induced chemical reactions, have to date proven difficult to study spectroscopically. In low-viscosity liquids, such processes can be monitored with conventional ultrafast techniques, so long as the liquid is flowed fast enough that each set of laser pulses interrogates a new portion of the sample. Such an approach is not practical in solids or viscous liquids, however.
Direct observation of ultrafast ionic screening
Authors: Cheng T. K. , Dresselhaus M. S. , Ippen E. P.
Published: February 2023

Recently, it has been shown that coherent optical phonons can be excited in semimetals[1,2], semiconductors[2-4] and superconductors[5]. In all cases, pulses of laser light, which are short compared to the optical phonon period, are used to impulsively generate the coherent lattice vibrations. While the experimental methods for phonon excitation are similar, the physical mechanisms responsible for phonon generation appear to differ among the various materials types. For semimetals and narrow-gap semiconductors, the reflectivity modulation observed with the pump-probe technique have been particularly large (on the order of 10% in some materials) and have suggested significant modulation of the material's electronic structure. In these cases, we have determined that the coherent phonon excitation is caused by a mechanism we call Displacive Excitation of Coherent Phonons (DECP) [2,6].
Ultrafast Time-Resolved Dynamics of Hot Electron Distributions in Silicon
Authors: Prybyla J. A. , Goldman J. R.
Published: February 2023

While ultrafast optical techniques have been applied extensively to the study of hot electron dynamics in GaAs, there have been very few such studies of Si because conventional optical pump-probe schemes are difficult to interpret for indirect gap materials. This is unfortunate since understanding hot electron behavior in Si is key to understanding and predicting the properties of next-generation Si devices. Here we report the first direct subpi-cosecond (150 fsec) time-resolved measurement of the energy relaxation of optically excited electrons in Si. This is accomplished in a laser pump-probe format by analyzing the photoelectron spectrum generated by an ultrashort uv probe pulse. By this method, we obtain directly the time-evolution of the photo-excited electron distribution function (bulk and interface) in Si with 150 fsec resolution. These measurements would not be possible by either reflectivity/transmission or luminescence schemes.
Femtosecond ellipsometric study of nonequilibrium carrier dynamics in Si1-xGex
Authors: Hu X.F. , Downer M.C.
Published: February 2023

In III-V semiconductors, comparison of femtosecond time scale optical measurements1 with microscopic carrier dynamic theory2 has become direct and quantitative. In Column IV semiconductors, on the other hand, quantitative experiment-theory comparison has been achieved only on a picosecond time scale.3 This paper presents a systematic study of the fs.-time-resolved ellipsometric response of epitaxial Si1.xGex (0≤x≤l) alloys as a first step in measuring and modelling subpicosecond bulk carrier dynamics. Pump and probe beams were derived from either an unamplified CPM laser4 (620 nm wavelength) or an unamplified KLM laser(800nm + 400nm second harmonic). The time-dependent optical response is related to free carrier density Neh(t), reduced effective mass μ*(t) via a time-varying Drude-Lorentz dielectric function:
Femtosecond energy transfer and coherent oscillations in BChl c light-harvesting antennae of chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus
Authors: Savikhir S. , Zhu Y. , Lin S. , Blankenship R. E. , Struve W. S.
Published: February 2023

Chlorosomes are the principal light-harvesting bodies in green photosynthetic bacteria. These 100×30×12 nm ellipsoidal bodies contain ~104 bacteriochlorophyll (BChl) c chromophores, as well as a BChl a pigment-protein complex that forms an interfacial baseplate between the chlorosome and the cytoplasmic membrane. The BChl c pigments in chlorosomes are organized into large oligomers, whose electronic and vibrational spectroscopy is remarkably similar to that of BChl c aggregates that form spontaneously from BChl c monomers in solution. This unique self-aggregating property has attracted wide attention because of its potential applications in artificial photosynthesis. The BChl c and BChl a antennae of chlorosomes from the green bacterium Chloroflexus aurantiacus exhibit broad Qy (S1←S0) electronic absorption bands centered at ~740 and ~790 nm, respectively. Downhill BChl c → BChl a energy transfer occurs with ~10 ps kinetics in isolated chlorosomes [1,2]. In this work, we have focussed on the femtosecond internal energy transfer events within the BChl c antenna. It is currently believed [3] that this 740 nm antenna comprises several distinct BChl c spectral forms (c727, c744, c766 etc.) Equilibration among chlorophyll and bacteriochlorophyll spectral forms requires several hundred fs in most pigment-protein antenna complexes that have been studied to date [4].
Femtosecond Photoemission Studies of Electron Relaxation at Cu Surfaces
Authors: Schmuttenmaer C. A. , Aeschlimann M. , Miller R. J. D. , Mantell D. , Cao J. , Gao Y.
Published: February 2023

A number of recent studies have illustrated the importance of photoexcited electrons in surface reactions at metal surfaces. This work focuses on directly determining the relaxation dynamics for photoexcited electrons at Cu(100) surfaces. This particular surface is important as recent observations for photoinduced desorption of CO have found quantum yields as high as 15% for the hot electron reaction channel at high laser fluences.1 Equal pulse correlation studies employing 3 eV pulses (50-60 fs) with both parallel and crossed polarization and low energy (<100 nJ/cm2) have been used to characterize the intrinsic electron relaxation dynamics in the absence of excess lattice heating or transient collective effects. This low energy limit corresponds to photoinduced surface chemistry employing conventional low flux CW light sources such as arc lamps. These results are shown in Figure 1. The inset shows the observed photoemission spectrum. The highest energy electrons give a pulse width limited response function which can be used to deduce the slower relaxation dynamics of the lower energy electrons. The slower relaxation dynamics are clearly resolved as a broadening in the FWHM of the two pulse correlation of the photoemitted electron at specific energies. The deconvolved relaxation times for the electron energy distribution are shown in which the solid line through the data is a fit to a calculation based on Fermi liquid theory for electron relaxation scaled by a factor of 2.45. The extent of this agreement for the electron relaxation dynamics at a single crystal surface is similar to the relaxation dynamics observed previously for polycrystalline gold surfaces at higher fluences.2 At lower energies above EF, hot electron transport effects become noticeable and can be understood in terms of a ballistic transport model. These results fully characterize the non-radiative relaxation of electrons at low fluence and peak power.
All Solid State Laser System for the Generation of Tunable Femtosecond Pulses down to 175 nm
Authors: Seifert F. , Ringling J. , Noack F. , Petrov V. , Kittelmann O.
Published: February 2023

Ultrashort optical pulses tunable in the vacuum ultraviolet (VUV) spectral region are of great interest for spectroscopic investigations of various ultrafast processes, e.g. photodissociation of molecules and clusters. We report on the generation of femtosecond pulses in the wavelength range from 175 to 182 nm by phase matched sum frequency mixing of femtosecond ultraviolet and near infrared optical pulses in lithium triborate (LBO). Due to its superior transparency range down to 160 nm and its high nonlinear coefficients only this crystal can be used for nonlinear conversion processes in the wavelength range below 189 nm. Unfortunately phase matching is possible only for two widely separated wavelength, e.g. λUV < 205 nm and λIR >1.7µm to obtain a wavelength at 185 nm or below (Ref.1). To ensure the synchronism both the ultraviolet (UV) and infrared (IR) pulses have to be derived from a single ultrashort pulse source. Up to now the shortest wavelength generated by sum frequency mixing in nonlinear crystals was 187.7 nm using nanosecond pulses (Ref.2).
Coherent Evolution Following Photolysis of Mn2(CO)10
Authors: Waldman Amir , Ruhman Sanford
Published: February 2023

In this letter we report a study of Mn2(CO)10 photodissociation using photoselective transient transmission on a 100 fsec timescale. Absorption in the UV is known to induce both metal-metal bond fission, as well as decarbonylation, leading to Mn(CO)5 and Mn2(CO)9 as primary photoproducts1. The overall quantum efficiency for decomposition is ~0.4, three quarters of which involves the metallic bond breaking. Flash photolysis studies have associated the Mn(CO)5 and Mn2(CO)9 with transient absorption bands centered at 800 and 500 nm respectively2. The former has been studied extensively through ultrafast spectroscopy. Rapid initial features in the near IR spectral region have been associated either with metallic bond cleavage through a metastable excited state, or to recombination following an ultrafast initial dissociation3,4.
Chirped pulse amplification of ultrashort pulses using neodymium-and erbium-doped fiber amplifiers
Authors: Yang L.-M. , Stock M. L. , Mourou G. , Galvanauskas A. , Fermann M. E. , Harter D. J.
Published: February 2023

As an important step toward the realization of compact, optical fiber-based, femtosecond pulse systems of the highest possible energies, we demonstrate chirped pulse amplification (CPA) of ultrashort pulses using rare-earth doped fiber amplifiers for the first time. The technique of CPA [1] is well established to produce the highest energy ultrashort pulses in bulk systems. Rare-earth doped fibers are attractive as amplifiers due to the broad bandwidth available in glass hosts. This property has already been exploited in rare-earth doped fiber lasers which have produced pulses as short as 42 fs with Nd-doped fiber [2] and 77 fs with Er-doped fiber [3]. Thus, these fibers can support sub-100 fs pulses. Here, Nd-doped fiber is used to amplify 1.06 µm seed pulses and Er-doped fiber is used to amplify 1.55µm seed pulses in initial experiments toward reaching the saturation energy of several micro-Joules [4] attainable in rare-earth doped fiber amplifiers.
Excitation-Induced Phase Shifts of Heavy and Light Hole Quantum Beats in Multiple Quantum Wells
Authors: Sha W. , Paul A. E. , Patkar S. , Smirl Arthur L.
Published: February 2023

In recent years, transient four-wave mixing (FWM) techniques have been used to investigate the dephasing times of non-equilibrium carriers in semiconductor heterostructures by monitoring the decay of the macroscopic polarization. Initially, this decay was measured by temporally-integrating the scattered FWM signal as a function of time delay between two (or more) pump pulses. More recently, the macroscopic polarization decay has been measured by time-resolving the FWM signal by cross correlating it with an ultrashort laser pulse via frequency up-conversion in a nonlinear crystal. When excitons with slightly different energies are excited, oscillations (or beats) have been observed both in the time-integrated FWM signal and in the time-resolved FWM signal. Such beats have been observed, for example, between light- and heavy-hole excitons and between excitons in wells of different widths. Both the spectral behavior1 of the time-integrated FWM signal and the temporal behavior2 of the time-resolved signal have been shown to allow the distinction between the polarization interference associated with two independent oscillators and the quantum beats associated with two coupled oscillators which share a common level. In addition, it has been demonstrated3 that the quantum beats produced by incident pulses with parallel polarizations are exactly out of phase (by π) with the beats produced by orthogonally polarized pulses. In each case1-3, the period and phase of the beats and the polarization of FWM signal were explained by using a six-level model, which excluded many-body effects, to describe the J=1/2 conduction and J=3/2 valence states. Within the last few months, however, it has been shown that a complete description of the polarization selection rules for the FWM signal requires excitation-induced dephasing4 or disorder-induced coupling of the σ
+
and σ
–
excitonic transitions5.
Ultrashort High-energy pulses from long-stripe actively-mode-locked laser diodes in external cavity
Authors: Stelmakh N. , Azouz A. , Lourtioz J-M. , Gavrilovic P.
Published: February 2023

Short-pulse mode-locked laser diodes could become an interesting alternative to more conventional sub-picosecond lasers for time-resolved spectroscopy and optical testing of materials. Pulsewidth limits have been substantially lowered down to ~ 200 fs in recent experiments[1,2] and still better performances are expected when considering the very large amplification bandwidth of laser diodes. However, practical applications also require high output energies (or peak-powers) and much effort has to be made in this direction. Up to now, pulse energy delivered by sub-picosecond laser diodes has been typically in the range of a few picojoules. In this paper, we report on a one-order-of-magnitude improvement of pulse energy with the use of long-stripe (1-2 mm) quantum well AlGaAs lasers in an active mode-locking scheme. The overall pulse performances attained with 2 mm long lasers (Δt ≈ 460 fs, E ≈ 50 pJ) are believed to be the best reported to date without using additional amplifier sections.
Intensity and phase measurements of femtosecond pulses from a chirped pulse amplifier
Authors: Kohler Bern , Yakovlev Vladislav V. , Wilson Kent R. , Squier Jeff , DeLong Kenneth W. , Trebino Rick
Published: February 2023

Chirped pulse amplification (CPA) with a Ti:sapphire regenerative amplifier has produced femtosecond pulses with millijoule energies at kHz repetition rates [1]. The performance of these laser systems has been limited primarily by the inability to compensate the spectral pulse phase to all orders in the compressor after the pulse has been stretched and amplified by making typically twenty roundtrips in a regenerative amplifier. Several authors have used dispersive ray tracing calculations to modify the design of the pulse stretcher and / or compressor in order to reduce the residual phase distortions which cause pulse broadening in a CPA system [2]. In order to further shorten the pulses from CPA systems, it would be extremely useful to measure directly the actual intensity and phase of the amplified pulses under different conditions. This information would then provide a means for checking the dispersive ray tracing models of the stretcher / compressor as well as providing information on precisely what sort of phase compensation is needed.
Femtosecond Transient Grating Studies of Electron Transfer in Porphyrin and Chlorophyll Donor-Acceptor Molecules
Authors: Wiederrecht Gary P. , Svec Walter A. , Wasielewski Michael R.
Published: February 2023

Transient grating experiments have been utilized to study a variety of condensed phase phenomena. Most of these studies are performed on transparent media, resulting in non-resonant probing of the material response of the system. Such experiments include the study of orientational dynamics in liquids, acoustic and optical phonon dynamics as they relate to phase transitions in solid state media, optical Kerr effects, intramolecular vibrations, etc. In its simplest form, the technique requires two beams to be temporally and spatially overlapped in the sample. An interference pattern is created, which modulates the real part of the index of refraction through the third order susceptibility (χ(3)). A phase grating results, through which a variably delayed probe beam incident on the sample may diffract and provide temporal information about the material response. Transient grating experiments have been utilized to study a variety of condensed phase phenomena. Most of these studies are performed on transparent media, resulting in non-resonant probing of the material response of the system. Such experiments include the study of orientational dynamics in liquids, acoustic and optical phonon dynamics as they relate to phase transitions in solid state media, optical Kerr effects, intramolecular vibrations, etc. In its simplest form, the technique requires two beams to be temporally and spatially overlapped in the sample. An interference pattern is created, which modulates the real part of the index of refraction through the third order susceptibility (χ(3)). A phase grating results, through which a variably delayed probe beam incident on the sample may diffract and provide temporal information about the material response.
High-repetition-rate blue-pumped βBaB2O4 femtosecond optical parametric oscillator tunable in the visible
Authors: Gale G. M. , Hache F. , Pavlov L. I.
Published: February 2023

High-repetition-rate femtosecond optical parametric oscillators (OPO) are potentially very useful devices in many areas of science due to their wide tunability, relatively short pulse duration and automatic synchronisation with the pump laser, which can allow, for example, sum and difference frequency generation, further extending the range of tunability of the OPO. To date continuous wave femtosecond OPO's have employed KTP [1-3] or KTA [4], mainly pumped by the fundamental wavelengths of a Ti: sapphire laser. These sources operate in the near- to mid- infrared, although intracavity frequency doubling of a KTP OPO has been described recently [5].
Ultrafast Electronic Processes in Porous Silicon
Authors: Fauchet P.M. , Kostoulas Y. , Vandyshev Ju. V. , Tsybeskov L. , Ucer K.B. , Waxer L. , Walmsley I.A. , Petrova-Koch V.
Published: February 2023

The discovery in 1990 [1] that porous silicon is a strong light emitter in the visible at room temperature has changed our assessment of the usefulness of silicon as an optoelectronic material and raises the possibility of practical silicon light emitters [2]. The luminescence from light emitting porous silicon (LEPSi) usually occurs in the deep red to orange region with a quantum efficiency of a few percents at 300 K and >10% at low temperature. The luminescence decays with a lifetime that ranges from ~1-10 μsec at 300 K to >1 msec at low temperature. More than 15 different models have been proposed to explain the strong luminescence from LEPSi [3] but only two models have received strong experimental support. In the first model, light emission results from the recombination of a free electron-hole pair or exciton across the bandgap of silicon nanoparticles [1,4]. The emission is in the visible because the effective bandgap is increased by quantum confinement and the quantum efficiency is high because the bandgap of these silicon nanoparticles has become quasi-direct and because nonradiative recombination is suppressed thanks to the good passivation of the nanoparticle surface. In the second model, light emission results from radiative recombination involving carriers trapped in states localized on the nanoparticle surface [5]. These surface states are in the forbidden gap of the silicon nanocrystallites which is increased by quantumconfinement. The long lifetime of the luminescence is explained by the small overlap between the wavefunctions of carriers localized at physically separated states. To date, no experiment has been able to decide which model is correct.
Relaxation of Highly Vibrationally Excited Cycloheptatriene in Liquids Studied by Picosecond UV Absorption Spectroscopy
Authors: Linkersdörfer S. , Benzler J. , Luther K. , Troe J.
Published: February 2023

To understand in detail the relaxation of highly vibrationally excited polyatomics in dense fluid media experimental answers on fundamental mechanistic aspects are still urgently needed. It has been demonstrated on the relaxation of azulene that too simple applications of isolated binary collision models do not account for the observed relaxation dynamics in liquid solutions [1] and that e.g. cluster type effects may play a role in media like rare gas liquids [2]. However, much more systematic experimental evidence is necessary to establish how far a gas phase type description of energy transfer rates <ΔE>·Z (with effective collision numbers Z and average amounts of transferred energy <ΔE>) is valid and scales e.g. with D-1 (the inverse self diffusion coefficient of the fluid [3]) as opposed to the onset of basically different energy transfer mechanisms in higher density fluids.
Sub-40 fs Pulse Generation in a KTP-based Optical Parametric Oscillator
Authors: Reid D. T. , Dudley J. M. , Ebrahimzadeh M. , Sibbett W.
Published: February 2023

The demand for femtosecond sources at new wavelengths in the near to mid-infrared together with the emergence of the self-mode-locked Ti:sapphire laser as a convenient pump source has led recently to the development of femtosecond optical parametric oscillators. We report here the operation of a synchronously pumped femtosecond optical parametric oscillator (OPO) based on a 1.5 mm thick crystal of KTP cut for noncritical phase-matching. Pumped by 85 fs pulses from a self-modelocked Ti:sapphire laser the OPO is tunable over the bandwidth of the mirrors from 1.1 -1.3 μm and pump tuning over 100 nm is demonstrated. Our intracavity dispersion-compensated OPO has been operated in both three-mirror and four-mirror standing-wave geometries using single-pass pumping and exploiting a variety of mode-matching configurations. In addition, the observation of soliton generation in the femtosecond OPO is described. A typical spectrum and interferometric autocorrelation of the signal pulses produced in the three-mirror configuration are presented in Figure 1 and correspond to bandwidth-limited 39 fs pulses with intracavity peak powers approaching 1 MW [1]. The shortest pulses are observed when operating at near-zero intracavity GVD.
An Efficient Kα X-ray Source from Femtosecond Laser-Produced Plasmas
Authors: Rousse A. , Audebert P. , Geindre J.P. , Falliès F. , Gauthier J.C. , Mysyrowicz A. , Grillon G. , Antonetti A.
Published: February 2023

The generation of intense and ultrafast x-ray pulses is a challenge in the study of unexplored physics of high density and high temperature plasmas [1]. Such plasmas are produced when an intense ultra-short laser pulse (100 fs, 1016-17 W/cm2) is focused on solid targets [2-6]. Laser energy absorbed within the laser skin depth gives rise to a thermal plasma of several hundred electron-volts temperature, close to solid density. Behind the thermal plasma, fast electrons resulting from the specific interaction of the incident laser light through a very steep density gradient [7] eject inner shell electrons of the target plasma. This produces fluorescence line radiation as the inner shell vacancy is filled from outer shells [4, 8].
Einstein causality in time - resolved intra - atom transverse quantum correlations
Authors: Aiello A. , De Martini F. , Maialoni P. , Murra D.
Published: February 2023

We report the first experimental investigation on the long standing, fundamental problem of QED intra-atom quantum correlations1,2,3. It is well known that the quantum analysis of the problem of the interacting atoms in free-space, based on a Dicke Hamiltonian, shows an unexpected non causal intra-atom correlation transfer. We show theoretically that the problem is fully resolved by the use of the complete hamiltonian, i.e., including the anti-resonant contribution4. However, in spite of the existence of this satisfactory quantum-theoretical approach, the transfer of QED correlations within a single-mode, plane, Casimir microcavity still shows a somewhat puzzling noncausal behavior when the atoms are correlated by transverse interactions, i.e., taking place in direction parallel to the plane of the cavity and then orthogonal to the K - vector of the allowed mode. The "spatial" conterpart of this problem, the establishment of the transverse "size" of the mode, was already investigated by us in an early paper5. In that work it was demonstrated that the transverse correlation between two atoms inside a microcavity takes place within the coherence length lc = 2λ(fm)1/2, where f and m represent the finesse and the order of the microcavity respectively. By the present work, the investigation is carried out to the theoretical, as said, and the experimental analysis of the complete correlation problem in space-time, then fully implying the Einstein causality within the process. The experimental layout is shown in Fig.1. Two equal, mutually delayed, 50 fs, 10 Mw laser pulses generated by an amplified CPM laser are injected and focussed in a region of 15 μm of diameter in an active microcavity with a molecular solution of oxazine - 725 as active medium: the two pulses are then the "pump" of two identical, virtually "thresholdless" microlasers placed at a transverse distance d in the cavity plane, with d⩽lc.
Pulsed Terahertz Spectroscopy of Solutions: Experiment and Memory Function Analysis
Authors: Flanders B. , Cheville R. , Grischkowsky D. , Scherer N. F.
Published: February 2023

Understanding the magnitudes and time-scales of the intermolecular interactions that occur during a chemical reaction in the liquid phase is a central problem in condensed phase studies. Assuming linear response and validity of the Fluctuation-Dissipation theorem, knowledge of the processes and related time scales by which the neat solvent molecules fluctuate and relax in an equilibrium system (that is, in the absence of the strong perturbation produced by a reacting chromophore) is directly related to knowing the solvent modes available for acceptance of excess energy during the course of a reaction.1 Such equilibrium spectra may be obtained from optical Kerr effect studies2 or far-infrared (i. e. terahertz) dipolar absorption measurements.3