bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of a Soliton Pulse Train in an Optical Fibre Using Two Cw, Single-Frequency, Diode Lasers
Authors: Chernikov S.V. , Taylor J.R. , Mamyshev P.V. , Dianov E.M.
Published: February 2023

The potential application in fibre optical communication systems has stimulated research into high repetition rate sources of solitons. Several schemes have been suggested, see for example the references quoted in [1]. One such method proposed the use of the cw beat signal derived from two frequencies for the generation of a cw train of solitons at repetition frequencies in the Gigahertz regime [1,2]. The transformation of the beat signal into a train of solitons results from the nonlinear propagation in an optical fibre exhibiting a slow amplification or decreasing dispersion along the length. The combined effect of the self-phase modulation and group velocity dispersion, influenced by weak amplification or the decreasing dispersion of the fibre leads to a reshaping of the sinusoidal beat signal into solitons and to pulse compression, with the repetition frequency of the generated soliton pulse train equal to the frequency difference between the two input modes. Theoretical analysis has shown that using this technique, a cw train of high quality, well separated solitons can be generated with negligible pedestal component.
Generation of ultra Intense pulses and applications
Authors: Mourou G.
Published: February 2023

We will review the development of ultra intense lasers, capable of producing 1018 W/cm2 now and 1020 W/cm2 in the near future. We will describe the potential applications, including the observation of relativistic effects, i.e. harmonic generation, self-focusing etc., the acceleration of electrons, the generation of collimated X-ray and X-ray beams by nonlinear compton scattering, observation of nonlinear quantum electrodynamic dynamic effects and ignition of thermonuclear fusion.
Evidence of slow hole spin relaxation in n-modulation doped GaAs/AlGaAs quantum well structures
Authors: Rolland P. , Ferreira R. , Roussignol Ph. , Delalande C. , Vinattieri A. , Martinez-Pastor J. , Carraresi L. , Colocci M. , Palmier J.F. , Etienne B.
Published: February 2023

Almost instantaneous hole spin relaxation has been often assumed in semiconductor structures [1]. On the other hand, theoretical models predict a slowing down of the hole spin relaxation time in GaAs/AlGaAs quantum wells due to the size quantization along the growth axis [2].
THz Pulses from the Creation of Polarized Electron-Hole Pairs in Biased Quantum Wells
Authors: Planken Paul C. M. , Nuss Martin C. , Knox Wayne H. , Miller David A. B.
Published: February 2023

The generation of THz pulses on the surface of semiconductors has long been believed to be caused by transient transport currents parallel to the surface depletion field of the material after excitation with an ultrashort laser pulse. [1] Recently, an additional explanation of those experiments was given, in which optical transitions in the depletion field create electrons and holes in states where they are already polarized. [2] This creates a polarization P that grows with the integrated pulse energy, and hence radiates an electrical transient according to E~∂2P/∂t2. The final proof of the importance of this process however, can only be given in a sample where transport current is inhibited!
Dynamics in Adsorbates by Picosecond Time Resolved Surface Second Harmonic Generation
Authors: Morgenthaler M. J. E. , Meech S. R.
Published: February 2023

One interesting application of surface specific non linear optical signals is as a probe of ultrafast processes at interfaces. Here we report a time resolved study, by surface second harmonic generation, of the coverage and pump intensity (Ip) dependence of the ground state recovery (GSR) of malachite green (MG) adsorbed on quartz.
Radiative Recombination of Free Excitons in GaAs Quantum Wells
Authors: Sermage B. , Satzke K. , Dumas C. , Roy N. , Deveaud B. , Clérot F. , Katzer D. S.
Published: February 2023

We have studied the free excitons life time in GaAs-AlAs quantum wells as a function of excitation wavelength around the resonnance. The samples were 4.8 nm thick MBE grown GaAs quantum wells between two 4.2 nm thick AlAs barriers with growth interruption at the inter faces. They showed nice monolayer splittings in low temperature luminescence1. The exciton lifetime was measured by time resolved luminescence after short pulse excitation. The laser was a synchronously pumped mode locked dye laser and the detection was obtained by a synchroscan streak camera. The decay time resolution was about 7 ps.
Investigation of the Primary Event in Vision using 10 fs Blue-Green Optical Pulses
Authors: Schoenlein R.W. , Peteanu L.A. , Mathies R.A. , Shank C.V.
Published: February 2023

Recent developments in the generation and compression of optical pulses in the bluegreen spectral regime [1] now enable the direct investigation of fundamental dynamics in a variety of interesting molecular systems. In this paper we describe a unique laser system which generates 10 fs pulses in the blue-green. This laser system is used to resolved the kinetics of the primary event in vision, the cis-trans isomerization of the retinal chromophore in the pigment rhodopsin [2].
Primary charge separation in Photosystem Two Reaction Centres
Authors: Klug David R. , Durrant James R. , Hastings Gary , Hong Qiang , Barber James , Porter George
Published: February 2023

Studies of primary charge separation in photosynthetic reaction centres have largely been confined to the reaction centres of purple bacteria such as Rhodobacter sphaeroides and Rhodopseudomonas Viridis . Although reaction centres (RC's) from purple bacteria were first isolated 23 years ago, and despite the solution of their crystal structures, the precise mechanism of primary radical pair formation remains the subject of much contention.
Femtosecond Broadband Spectroscopy of the Styryl Dye DASPI in Solution: Interpretation in Terms of Dielectric Relaxation, Vibrational Energy Redistribution, and Isomerization
Authors: Bingemann D. , Bultmann T. , Ernsting N.P. , Lochschmidt A.
Published: February 2023

Dipolar styryl dyes have been used as molecular probes for the non-equilibrium dynamics of the surrounding solvent after ultrafast S0 -> S1 excitation [1,2]. Typically, the dynamic Stokes shift of fluorescence is measured by fluorescence upconversion at different wavelengths. The kinetic traces which are thus obtained must then be used for spectral reconstruction of the fluorescence band as function of time. This method is capable of highest time resolution, but the finer spectral details are likely to be lost in the reconstruction step.
Femtosecond infrared spectroscopy of the photosynthetic reaction center
Authors: Walker G. C. , Maiti S. , Cowen B. R. , Moser C. C. , Pippenger R. S. , Dutton P. L. , Hochstrasser R. M.
Published: February 2023

The photosynthetic reaction center (RC) from the purple bacterium Rb. sphaeroides is a member of the closely related family of light harvesting membrane proteins that are responsible for carrying on photosynthesis in nature. The two C2 symmetry related subunits, L and M, consist of transmembrane alpha helixes and eight cofactors [4 Bacteriochlorophylls (Bchl), 2 Bacteriopheophytins (Bph) and 2 Ubiquinones (Q)]. The cofactors are arranged in a wishbone geometry, with a dimerized pair of bacteriochlorophylls at the center (also called the 'special pair', P), and one each of Bchl, Bph and Q (in that order) along each side of the wishbone. The primary events of photosynthesis consist of the formation of the excited singlet state P* of the dimer by the absorption of a near infrared (NIR) photon by P, and subsequent transfer of an electron to the Bph on the L side with a time constant of ca 3ps forming P+Bph−. The role of the L side Bchl in this process is uncertain.
Sub-100fs homogeneously and inhomogeneously broadened Nd:glass lasers
Authors: Kärtner F. X. , Kopf D. , Keller U.
Published: February 2023

Previously, we have introduced a spectral filtering technique and applied it to a passively modelocked Nd:glass laser using an antiresonant Fabry-Perot saturable absorber (A-FPSA) [1]. Pulses as short as 130 fs are achieved in a standard dispersion compensated delta - cavity design with a 4mm Nd:glass plate inserted at Brewster's angle. The Nd:glass laser system is interesting from the point of view of an all solid state laser technology because it can be diode pumped. To further investigate the modelocking dynamics in Nd:glass lasers, we performed the following experiments: First we demonstrated a regeneratively, actively mode-locked phosphate Nd:glass laser (LG760, Schott), resulting in stable pulses with 310 fs pulsewidth, secondly a passively modelocked phosphate Nd:glass (LG700) laser with 105 fs and thirdly a passively modelocked silicate Nd:glass (LG680) laser with 92 fs. The passive modelocking experiments used an A-FPSA to both start and sustain stable mode locking without self-Q-switching. The different Nd:glass hosts were chosen because the phosphate Nd:glass can be considered as a quasihomogeneous laser material whereas the silicate Nd:glass is strongly inhomogeneous. Despite the completely different cw-behaviour of the lasers when the homogeneous or the inhomogeneous gain material is used, the modelocking performance is comparable and leads to almost the same pulsewidth in contrast to previously obtained results based on additive pulse mode locking [2]. Calculations as well as the experimental results show that KLM does not contribute to the modelocking performance.
Femtosecond Studies of Photoinduced Electron Dynamics at the Liquid-Solid Interface of Semiconductor Nano-particles in Water
Authors: Zhang J. Z. , O'Neil R. H. , Roberti T. W. , McGowen J. L.
Published: February 2023

We report direct measurements of the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale. Surface electron trapping was found to occur in < 100 fs, followed by rapid decay of the trapped electrons through primarily electron-hole recombination.
Carrier transport in 1.5 μm quantum well laser structures
Authors: Marcinkevičius S. , Olin U. , Fröjdh K. , Wallin J. , Landgren G.
Published: February 2023

Carrier transport time is one of the main factors limiting the high-speed performance of the quantum well lasers [1]. Thus, investigations of the transport mechanisms are essential for the improvement of the laser characteristics. In this paper we examine the carrier transport in the 1.5 μm InGaAs/InP graded-index separate-confinement hetero structure (GRINSCH) laser by means of time-resolved photoluminescence (PL). Such measurements produce a direct picture of the carrier motion and allow to draw conclusions about the transport mechanisms.
Phase-matching optima for high-order multiwave mixing and harmonic generation beyond perturbation limit
Authors: Shkolnikov P. L. , Kaplan A. E. , Lago A.
Published: February 2023

High-order optical harmonics generation (HHG) has recently attracted much attention as a possible source of short-wavelength coherent radiation [1]; harmonics of the order up to the 135th and wavelengths as short as 7.6 nm have been observed [2,3]. Conversion efficiency of HHG, however, remains too low for applications, largely due to poor phase matching. Phase­matching optimization could substantially increase the output. The actual experimental conditions, however, are, most likely, very far from optimal. Indeed, reported HHG experiments are conducted in almost dispersionless (gases) or positively dispersive (plasma) media, whereas phase-matching optimization of HHG, at least according to perturbation theory, requires strong negative dispersion, if beam are focused. To avoid this obstacle, another multiphoton process, high-order difference frequency mixing (HDM)
Femtosecond Time–Resolved Thermomodulation of Small Copper Particles in Glass
Authors: Nakamura Arao , Tokizaki Takashi
Published: February 2023

In recent years, optical nonlinearity of small metal particles in dielectric has attracted much attention, because of the possibility of the high polarizability and the fast response [1,2]. Enhancement of the local field inside the particle at the surface plasmon resonance causes enhancement of a third–order nonlinear susceptibility χ (3).
Upper satellite conduction band population dynamics in GaAs measured by femtosecond UV pump - IR probe absorption spectroscopy
Authors: Cavicchia Michael A. , Alfano R. R.
Published: February 2023

Over the past twenty years hot electron dynamics have been studied in the central (i.e., k=0), Γ, and satellite (i.e., k≠0), L and X6, lower conduction band valleys in bulk GaAs and quantum wells using ultrafast laser spectroscopy.1-4 A Monte Carlo calculation has shown that the satellite, X7, upper conduction band valley plays an important role in the high-field transport properties of GaAs.5 We will present for the first time an experimental study of the intervalley relaxation dynamics of a hot electron population in the satellite, X7, upper conduction band valley in GaAs using time-resolved UV pump-IR probe absorption spectroscopy.
The Femtosecond Dynamics of Aggregate Excitons in Liquids
Authors: Burgel M. V. , Wiersma D.A. , Duppen K.
Published: February 2023

Room temperature photon echo experiments show that optical dephasing in aggregates of dye molecules in solution is very slow (T2> 100 fs), due to motional narrowing over dynamic perturbations. By pump-probe experiments on multiple-exciton transitions the range of exciton delocalization was determined to be 18 molecules.
Femtosecond Coherent Generation of Hot Electrons Monitored via Band-to-Acceptor Luminescence in GaAs
Authors: Leitenstorfer A. , Elsaesser T. , Lohner A. , Kuhn T. , Haas S. , Rossi F.
Published: February 2023

Luminescence spectra observed after femtosecond excitation demonstrate initial electron distributions strongly affected by the coherent coupling of the incident light field and the semiconductor.
Deep Ultraviolet Femtosecond Multiphoton Ionization Studies of the Excited State Dynamics of Small Gas Phase Molecules
Authors: Owrutsky J.C. , Baronavski A.P.
Published: February 2023

Femtosecond studies using laser-induced fluorescence and multiphoton ionization (MPI)[1] have lead to considerable progress in understanding small molecule gas phase photodissociation dynamics. In this letter we describe experiments that extend these studies to shorter wavelengths. This makes it possible to investigate molecules that absorb weakly in the visible or near UV, which is often the case for many small molecules. We have generated femtosecond deep UV (near 205 nm) pulses, which have been used to directly measure the excited state lifetime for the predissociative S
3
state of CS2 and to study the two-photon state of NO at 102 nm using time-resolved MPI. The lifetime measured for CS2 is compared with several indirectly measured values, and the pressure dependence observed for the decay times for NO was measured and analyzed to obtain both the unimolecular lifetime and bimolecular rate constant.
Femtosecond Dynamics of Nan and Hgn Metal-Cluster
Authors: Baumert T. , Thalweiser R. , Weiss V. , Wiedenmann E. , Gerber G.
Published: February 2023

In recent years clusters and in particular metal clusters have been the fascinating subject of many experimental and theoretical studies. Cluster physics bridge the gap between molecular physics and solid state physics. Metal clusters exhibit distinct features ranging from molecular properties seen in small particles to solid state like behaviour of large aggregates. We report cluster size dependent studies of physical and chemical properties such as absorption resonances, lifetimes, decay channels and ionisation processes of the one- and two electron metal clusters Nan and Hgn. For these studies we introduced a new experimental technique consisting of the combination of a cluster beam, ion- and electron spectroscopy and tunable femtosecond laser pump-probe techniques. A major result of our femtosecond experiments is that the conventional view of the optical response of metal cluster, e.g. absorption, ionisation and decay processes as well as the relevant time scale, had to be changed. Our results clearly show that for Nan and cluster sizes n≤21 molecular structure, excitations and properties prevail over collective excitations and surface plasmon-like properties.
Coherence Properties of High Order Harmonic Radiation
Authors: Ditmire T. , Budil K. S. , Crane J. K. , Perry M. D. , Salieres P. , Huillier A. L'
Published: February 2023

The understanding of high order harmonic generation in gases exposed to strong laser fields has progressed significantly in recent years. Emphasis has shifted to the characterization and use of these harmonics as a coherent, high brightness source of XUV radiation. We have investigated some of the coherence properties of very high order harmonics in argon and neon by measuring the spectral distributions and the far-field spatial profiles of the harmonic radiation. We have conducted these studies at intensities both below and above the ionization saturation intensity.
Hybrid grating-prism compressor with cubic phase tunability and decreased alignment sensitivity
Authors: Kane S. , Rudd J.V. , Tien A.-C. , Squier J. , Korn G. , Mourou G. , Tournois P.
Published: February 2023

Traditional pulse stretcher/compressor systems [1,2] have relied on high-groove-density gratings (~2000 l/mm) for systems operating in the 800-nm wavelength regime. These highly dispersive gratings provide significant stretching and compressing of >100-fs pulses in a relatively compact system. In addition, high-groove-density gratings are very efficient, typically yielding a 70% net throughput for a double-passed compressor. However, while these gratings work well for stretching and compressing >100-fs pulses, the alignment sensitivity of traditional high-groove systems makes them almost unusable for pulses shorter than 100 fs. Here we present a novel high-groove-density grating compressor which has relaxed alignment tolerances and can be used with ultrashort pulses.
Ultrafast Vibrational Energy Relaxation in [(NC)5MIICNMIII(NH3)5]1-(M = Ru, Os) Studied by Picosecond Infrared Spectroscopy
Authors: Stoutland Page O. , Doom Stephen K. , Dyer R. Brian , Woodruff William H.
Published: February 2023

Mixed-valence transition metal complexes have long been used to study electron transfer reactions due to the availability of well-defined complexes with favorable spectroscopic properties.1 These studies have resulted in a plethora of information concerning the fundamental aspects of electron transfer reactions. We have recently used complexes of this type to investigate ultrafast electron transfer and vibrational energy relaxation dynamics in cyanide-bridged mixed-valence transition metal dimers. These studies have allowed us to observe, for the first time in the solution phase, the vibrational excitation which accompanies electron transfer reactions. In previous publications2 we have shown that (1) optical excitation into the metal to metal charge transfer (MMCT) transition of a series of mixed-valence transition metal complexes leads to formation of the excited state redox isomer, and that (2) subsequent ultrafast back electron transfer regenerates the original species, with much of the electronic excitation energy being converted into vibrational energy in the product. To date we have discussed the electron transfer and the coupled vibrational excitation. Here, we concentrate on the relaxation of the vibrationally excited molecules produced following the back electron transfer.
Femtosecond Spectral Hole Burning in InGaAs/AlGaAs Strained-Layer Single-Quantum-Well Diode Lasers
Authors: Sun C.-K. , Golubovic B. , Choi H. K. , Wang C. A. , Sanders G. D. , Stanton C. J. , Fujimoto J. G.
Published: February 2023

Studies of the high speed modulation response of semiconductor lasers, exhibited a strong damping of the relaxation oscillation resonance and suggested the presence of a nonlinear gain process with a characteristic time much shorter than the carrier lifetime[1]. Several possible mechanisms were proposed which might produce this nonlinear gain, including spectral hole burning, carrier heating, and four wave mixing related effects[2]. CW studies have been performed in an effort to indentify these nonlinear gain mechanisms[2,3]. However, time domain femtosecond measurements have the advantage that they can directly characterize the ultrafast gain dynamics. Several single-wavelength femtosecond pump-probe investigations were performed on various diode laser systems[4,5]. Carrier heating effects and spectral hole burning were reported, but to date, no direct femtosecond measurements of the wavelength dependence of spectral hole buring have been possible.
Ultrafast Magnetoexciton Dynamics in GaAs
Authors: Mycek Mary-Ann , Siegner Uwe , Chemla Daniel S.
Published: February 2023

A bulk semiconductor provides an excellent system for optically studying the ultrafast dynamics of Coulomb correlated magnetoexcitons. The application of a magnetic field to a 3-D system quantizes the energy levels of states with momentum perpendicular to B and, in contradistinction to experiments performed on 2-D quantum well systems, leaves a 1-D continuum of states with momentum parallel to B. The presence of the continuum states has a dramatic influence on both the linear and nonlinear optical properties. Thus, the experiments described below track the evolution of the semiconductor from a 3-D to a 1-D system and reveal surprising optical properties not observed in the previously reported studies in quantum wells.
Adsorbate Infrared Spectral Response Following Femtosecond Metal Substrate Heating
Authors: Yodh A. G. , Culver J. P. , Li M. , Jahn L. G. , Hochstrasser. R. M.
Published: February 2023

Ultrafast laser heating pulses create unique environments for surface chemistry on metal surfaces. Recent femtosecond experiments have shown, for example, that nonequilibrium substrate conditions can play a central role in laser induced desorption[1]. In these experiments extremely high electronic temperatures (4000 °K) are produced which modify adsorbates in a manner which is impossible to achieve via a conventional thermal process. Here we report the results of probing the internal vibrations of CO adsorbate molecules as a function of time following the impulsive excitation of electrons in the underlying Cu(111) substrate by 300 fs visible light pulses[2]. We find that a low frequency CO mode couples to substrate electrons and phonons, and that the representative coupling rates can be separately determined[2,3]. Importantly, with increased excitation of the substrate, our measurements provide indication of stronger, temperature dependent couplings between the adsorbate vibrations and the substrate reservoirs[4].
Distinguishing between Energy and Electron Transfer Processes in Photosystem Two Reaction Centres.
Authors: Klug David R. , Rech Thomas , Joseph D. Melissa , Durrant James R. , Knox Robert S. , Barber James , Porter George
Published: February 2023

The photosystem two (PS2) reaction centre (RC) undertakes both electron and energy transfer processes as part of its function. Distinguishing between these two processes is an essential pre-requisite for meaningful studies of the primary photochemistry of this complex. In the reaction centre (RC) of purple bacteria, the energy transfer processes are relatively simple, and result in the unidirectional transfer of excitation energy to the special pair in approx. 100fs. In contrast, in the PS2 RC of higher plants, extensive forward and backward energy transfer is expected due to the relatively small energy separation between the S1 states of the RC chlorins.
Collinear Transient Four-Wave-Mixing and Rabi Flopping in Semiconductors
Authors: Cundiff S.T. , Knorr A. , Feldmann J. , Spill B. , Stolz W. , Koch S.W. , Göbel E.O.
Published: February 2023

The ready availability of sub-picosecond pulses from mode locked lasers has led to a large number of experiments being performed on the coherent interaction between light and extended electronic states in semiconductors and semiconductor heterostructures [1]. In tandem with these experimental efforts the theoretical understanding of these phenomena has also made significant strides [2]. However, one theoretical prediction which has not been directly experimentally observed is the phenomenon of Rabi-flopping in semiconductors [3]. Rabi-flopping is the oscillation of the excited state population induced by a strong light field. Here we present experimental results based on the nonlinear interaction of two copropagating pulses and show that in the theoretical treatment the observed behavior arises from the occurrence of a Rabi flop.
Resonance Transient Population Grating Spectroscopy of Ultrafast Solvation Dynamics
Authors: Fainberg B.D , Goldberg S.Y. , Huppert D.
Published: February 2023

Solvation dynamics has been extensively studied in the past decade using time resolved luminescence (TRL) techniques [1]. Large polar molecules embedded in polar liquids exhibit a broad and featureless electronic optical spectra. For such molecules, there is a hierarchy of relaxation times of the vibronic transition under consideration [2]: σ2-1/2<T′<τC. Here σ2-1/2plays the role of the reversible dephasing time of a vibronic transition, σ2is the second central moment of a spectrum corresponding to the vibronic transition, τCis the vibrational relaxation time, and T′=(τCσ2-1)1/3. The experimental conditions of our transient degenerate four-wave mixing experiment have been chosen to provide similar information to the ones given by time-resolved luminescence studies. TRL experiments investigate the hot luminescence processes occurring after the completion of the electronic transition phase relaxation (with a characteristic decay time T′) and during the vibrational and solute-solvent relaxation in the excited electronic state.
Direct Determination of the S2 Lifetime of β-carotene by Femtosecond Time-Resolved Fluorescence Spectroscopy
Authors: Kandori Hideki , Sasabe Hiroyuki , Mimuro Mamoru
Published: February 2023

Femtosecond time-resolved fluorescence spectroscopy (up-conversion) was applied to directly determine the S2 lifetime of β-carotene at room temperature. Upon excitation of β-carotene in n-hexane with a 425 nm femtosecond pulse, the fluorescence lifetime was estimated to be 195 ± 10 fs throughout the whole emission wavelength. Absence of dynamic Stokes shift suggests that the intramolecular relaxation processes occur within 50 fs, followed by the internal conversion from S2- to S1-state in 195 fs.
Tunable Femtosecond Pulses in the Visible By Optical Parametric Amplification
Authors: Watson J. , Lépine T. , Georges P. , Brun A.
Published: February 2023

For femtosecond time resolved spectroscopy experiments, it is often interesting to change the wavelength of the pump and the probe. Generally, the production of tunable femtosecond pulses is obtained through spectral continuum generation in order to produce a white light. However, there are some limitations in energy and the spatial profile is altered by self focusing. Optical parametric generation is a non linear process which plays an important role in the development of easily and widely tunable coherent sources [1-3].
Femtosecond Optical Spectroscopy of Partially Deoxygenated Y-Ba-Cu-O Thin Films
Authors: Shi L. , Gong T. , Xiong W. , Weng X. , Sobolewski Roman , Fauchet P. M.
Published: February 2023

Very recent spectroscopic data of YBa2Cu3O7-δ (YBCO) films suggest that oxygen doping dramatically changes the electronic structure of the CuO2 plane.1,2 The femtosecond 2-eV-pump/white-light-continuum-probe experiments performed at room temperature on YBCO thin films with various oxygen contents showed that in fully oxygenated YBCO samples (Tc ~90 K) the transient optical reflectivity (AR/R) signal remains positive for all probe energies from 1.91 eV to 2.25 eV, whereas the response of oxygen-poor films (Tc ~0 K) is negative down to 1.46 eV.3 The lack of sign reversal for AR/R in either type of YBCO suggests that the results for the oxygen-rich samples could be interpreted as intraband free-carrier absorption, while the response in the oxygen-poor YBCO resulted from the saturation of interband absorption across the charge-transfer gap.
Trends in Ultrafast Physics of Quantum Confined Systems
Authors: Chemla Daniel S.
Published: February 2023

In the last decade, quantum confined semiconductor structures (QCSS) have attracted much attention because of their novel properties and their potential for electronic and photonic applications [1]. Quantum size effects appear in a semiconductor structure when one of its dimensions is in the range 10Å <L<500 Å. In these materials the typical values of group and phase velocities of elementary excitations are such that the corresponding times are in the range 10fs<t<500fs. Therefore reduced dimensionality also implies ultrafast dynamics. In this talk we review the most significant results obtained in the investigation of ultrafast processes in QCSS. We also describe recent experiments that reveal some open questions in ultrafast physics of QCSS, which we discuss. The present summary gives a flavor of the topics covered by the lecture.
Time-Resolved Studies of Bloch Oscillations in Semiconductor Superlattices
Authors: Leisching P. , Dekorsy T. , Waschke C. , Beck W. , Roskos H. , Leo K. , Kurz H. , Köhler K.
Published: February 2023

The atomic-layer precision achieved in the MBE growth of GaAs/AlGaAs semiconductor superlattices (SLS) has allowed to observe a variety of interesting novel coherent phenomena on a pico- and subpicosecond time scale upon resonant excitation with femtosecond laser pulses.
Real time 2-photon confocal microscopy using a femtosecond, amplified Ti: sapphire system
Authors: Squier Jeff , Norris Ted , Bliton Christyne , Brakenhoff G. J. , Athey Brian
Published: February 2023

Confocal microscopes improve resolution by creating optical sections with spatial filtering. These optical sections can be reconstructed to produce complete three-dimensional images suitable for studying a variety of important biological problems. However, over the time course of an image scan, confocal microscopes illuminate the full specimen volume with the excitation wavelength (as in standard fluorescence microscopy) which can limit the image quality. The use of a short optical pulse (from an ultra-short pulse laser) allows one to utilize two-photon excitation, which occurs only at the focus of the (high NA) objective, with several advantages following. Background scattered light is lower, and fluorophore bleaching only occurs at the objective focus instead of throughout the sample volume. Consequently, three-dimensional imaging experiments may be performed at higher excitation powers, greatly improving signal-to-noise, or lower excitation power could be used, and the number of image sets collected over time could be increased. Finally, since the excitation volume is lower with two-photon excitation, caged compounds may be released in a small (diffraction-limited) volume in a specimen. Thus, a new tool may be developed for the researcher a miniature (micron-cubed) opto-chemical probe that can be used in a very non-invasive way.
THz Emission by Coherent Lattice Vibrations in Tellurium
Authors: Dekorsy T. , Auer H. , Waschke C. , Bakker H. J. , Roskos H. G. , Kurz H.
Published: February 2023

We report on the time-resolved observation of coherent infrared-active phonons excited by fs-laser pulses in single crystal Tellurium. The coherent lattice vibrations are detected in an optical pump-probe experiment in amplitude and phase. The emission of electromagnetic waves at THz frequencies associated with the coherent lattice vibration is observed. This is the first demonstration of THz emission by coherent phonons for any semiconductor.
Ultraviolet Picosecond Pulse Amplification in New Solid-state Laser Medium: Ce3+:LuLiF4
Authors: SARUKURA Nobuhiko , LIU Zhenlin , SEGAWA Yusaburo , EDAMATSU Keiichi , SUZUKI Yoshiro , ITOH Tadashi , SEMASHKO Vadim V. , NAUMOV Alexander K. , KORABLEVA Stella L. , ABDULSABIROV Ravil Yu. , DUBINSKII Mark A.
Published: February 2023

Recent progress in tunable solid-state-laser materials in the infrared region has enabled development of completely new ultrashort-pulse laser sources. Such innovations will also be expected in the ultraviolet region. Several Ce3+-doped tunable ultraviolet laser crystals have been developed including YLiF4,1 LaF3,2 LiCaAlF6,3 LiSrAlF6,4 and LuLiF4.5 In spite of their broad gain bandwidth, no short-pulse application of these materials has been reported. In this presentation, we will describe the properties of Ce3+:LuLiF4 and its application to picosecond-pulse amplification.
Wave packet interference in the free induction decay from molecules
Authors: Sweetser John N. , Dunn Thomas J. , Walmsley Ian A.
Published: February 2023

It is well known that the application of a pulse to a molecular electronic transition leaves the molecules vibrational modes in different electronic states in well-localized states, or wave packets[1]. One method of detecting the wave packets is via the time-dependent spectra of spontaneous emission from the excited molecule[2]. The emission consists in general of two parts, an incoherent part that depends only on the wave packet in the excited state, and a coherent part, that depends on the wave packets in both excited and ground electronic states (of a two-state molecule). This can be seen from the following simple calculation.
Picosecond Laser Induced Electric Field Modulation of Carotenoid Absorption Bands
Authors: Gosztola David , Yamada Hiroko , Wasielewski Michael R.
Published: February 2023

Observation of band shifts in the absorption spectra of in vivo carotenoids has been attributed to electric field effects generated by membrane potentials. This electrochromism is related to what is known as the Stark effect, a term which is used to describe the effect of an applied electric field on many physical processes. Previous in vitro studies of how electric fields effect the absorption spectra of molecules all have relied on applying the perturbing electric field by using external electrodes around a dielectric spacer containing the molecule to be studied. The maximum applied field in these examples has been between 0.5 MV/cm and 3 MV/cm. These electric field intensities are of the same magnitude as those thought to be present across the membranes studied in the in vivo experiments.
Dominant Effects of Two-Photon Coherence on Four Wave Mixing in GaAs Quantum Wells
Authors: Wang Hailin , Shah Jagdeep , Damen T. C. , Pfeiffer L.N.
Published: February 2023

We present in this paper polarization dependent four-wave-mixing (FWM) measurements in homogeneously broadened GaAs quantum wells (QW). We have unambiguously demonstrated the important contribution of the two-photon coherence (TPC) between the ground and biexciton states to the nonlinear response. We show for the first time the dominant effects of the TPC on the polarization dependence of FWM. In particular, by including the TPC-induced contribution, we are able to account for the anomalous polarization dependence, a subject of considerable current interest [1].
Enhancement of Coherent Charge Oscillations in Coupled Quantum Wells by Femtosecond Pulse Shaping
Authors: Weiner A.M.
Published: February 2023

Considerable interest has arisen in the prospect of using specially crafted ultrashort laser pulses for coherent control of quantum mechanical motions in atomic and molecular systems. In this paper we analyze the possibility of applying coherent control concepts and femtosecond pulse shaping to a quantum well system, in particular to enhance coherent quantum mechanical charge oscillations in GaAs/GaAlAs coupled quantum wells.
Excited-State Processes and Photoinduced Antiviral Activity in Hypericin
Authors: Gai F. , Fehr M. J. , Petrich J. W.
Published: February 2023

Hypericin (Figure 1) is a naturally occurring polycyclic quinone that has received recent notoriety for its antiviral capacity-in particular its ability to deactivate the human immunodeficiency virus (HIV) [1]. The antiviral activity of hypericin requires light [2]. Hypericin is also very closely related, both structurally and spectrally, to the chromophore of the photoreceptor complexes of the protozoan ciliates [3]. Optical excitation of hypericin produces both singlet oxygen [4] and a pH decrease [5] and gives rise to transient species of picosecond duration. The following observations indicate that excited-state, intramolecular proton transfer occurs in hypericin.
Use of Monolithic Tunable Laser Diodes for Chirped Pulse Amplification in Fiber Amplifiers
Authors: Galvanauskas A. , Fermann M. E. , Harter D.
Published: February 2023

Here we study the use of tunable three-section distributed-Bragg-reflector (DBR) diode lasers as pulse sources for chirped pulse amplification (CPA) in a fiber amplifier. The broad continuous-tuning range of the laser allows the generation of nanosecond linearly chirped pulses, which are amplified in a fiber amplifier up to microjoule energies and then compressed to transform-limited subpicosecond duration. The energy of these ultrashort pulses is more than two orders of magnitude higher than any previously reported energies from a compact diode laser and fiber source.
Femtosecond Blue Continuum Generation and its Application to the Time-Resolved Study of Mn2+ Emission in Mn-Doped ZnS Nanocrystals
Authors: Sosnowski T. , Klein P.B. , Norris T.B. , Bhargava R.N. , Gallagher D.
Published: February 2023

Recently, the achievement of high quantum efficiency light emission from semiconductor nanocrystals using transition metal doping was reported1 in the ZnS:Mn system. While this system is of considerable interest both scientifically (physics of low dimensional structures) and technologically (fast, high speed phosphors), the excitation dynamics in doped nanocrystals are still not well understood. In order to probe such a system on the picosecond or sub-picosecond time scale, a tunable ultrafast UV source is required. We have developed such a source by using the frequency-doubled output of a 250kHz Ti:sapphire regenerative amplifier2 to produce the blue continuum.
THz Radiation from delta-doped GaAs
Authors: Birkedal D. , Hansen O. , Sprensen C. B. , Jarasiunas K. , Brorson S. D. , Keiding S. R.
Published: February 2023

The emission of ultrafast THz pulses after femtosecond laser pumping was recently introduced by Zhang and Auston [1] as a sensitive probe of the semiconductor surface region. Photoexcited carriers created near the surface are accelerated by the electric field present due to band-bending. The resulting photo current radiates an electro-magnetic pulse posessing frequency components in the THz-regime. The detailed structure of the THz pulse carries information about the built-in surface fields in the sample.
Importance of Femtosecond Solvent Motions on Electron-Transfer Reactions: A Study of Electrolyte Solutions
Authors: Deuel Hans P. , Cong Peijun , Simon John D.
Published: February 2023

Electrolytes can influence chemical reaction dynamics in vastly different ways. In this paper we focus our attention on fast solvent motions in electrolyte solutions and their effect on the rate constants of electron transfer reactions. The experimental methodologies employed in this paper are femtosecond optically-heterodyne-detected Raman-induced-Kerr-effect spectroscopy (OHD RIKES) and transient absoption spectroscopy.
Previously Impossible Measurements made possible by THz-Time-Domain Spectroscopy
Authors: Grischkowsky D. , Cheville R.A.
Published: February 2023

Using a high-performance, optoelectronic, THz beam system, which can generate and detect fsec pulses of freely propagating THz radiation, we have made the first comprehensive THz (far-infrared) absorption measurement of flames and have observed the late time response from targets in THz impulse ranging studies. Other applications will also be discussed.
Exciton Coherence in Photosynthetic Light-Harvesting
Authors: Chachisvilis M. , Pullerits T. , Westerhuis W. , Hunter C.N. , Sundström V.
Published: February 2023

Excitation transfer in photosynthetic pigments is generally described as incoherent Förster hopping, but the exciton concept has also been applied for photosynthetic antenna systems (1,2). In that case excitation energy is delocalized over a number of pigment molecules and the dynamics occurs through the relaxation between different exciton states. However, often it is not so straightforward to unambiguously distinguish in experiment the two qualitatively different kinetic processes, incoherent hopping and exciton relaxation. The actual dynamics can be a combination of these limiting cases. For example, smaller sections of the full system might behave as a small exciton whereas the dynamics on a larger scale may correspond to the hopping-like transfer of this small exciton. An argument against the pure incoherent Förster mechanism is the recent observation of coherent nuclear motions in the antenna complexes of photosynthetic bacteria (3). The vibrational coherence is preserved for at least the same time as the estimated single step transfer time in these systems. This implies that one of the main assumptions of the Förster theory that the vibronic relaxation occurs much faster than the excitation transfer is not fulfilled. Furthermore, the structural data of several light-harvesting proteins (4,5) show densely packed pigment systems, where exciton interactions are quite strong.
The Primary Processes in Heme Protein Relaxation: The Coupled Reaction Coordinate Problem in Molecular Cooperativity
Authors: Chiu Hui-Ling , Deak John , Dwayne Miller R. J.
Published: February 2023

The phenomena of molecular cooperativity involves an interaction between two or more different protein moieties in which the protein's function is controlled synergistically by its neighbor. Changes in state of the adjacent protein, such as changes in ligation of a receptor molecule, affect the reaction rate of its neighbor, often several nanometers away. The reaction rate can be controlled by either changes in the reaction free energy or activation barriers. Our current understanding of this process is based on structural changes at one site affecting the adjacent activation barrier. In order to affect a reaction coordinate at a distance, these structural changes must involve highly correlated atomic displacements coupling thousands of degrees of freedom. The key question then is what is the mechanism by which the reaction forces at one site propagate to adjacent sites, i.e., what is the communication pathway? How do the reaction forces which develop on an atomic length scale couple to mesoscopic dimensions of motion?
Recent Developments in Frequency-Resolved Optical Gating: Measurement of Ultraweak Ultrashort Pulses
Authors: Fittinghoff David N. , Bowie Jason L. , Sweetser John N. , Jennings Richard T. , Krumbügel Marco A. , DeLong Kenneth W. , Trebino Rick , Walmsley Ian A.
Published: February 2023

Frequency-Resolved Optical Gating (FROG) [1] has been used to fully characterize ultrashort laser pulses over a wide range of wavelengths, pulse lengths, pulse energies, time-bandwidth products, and pulse repetition rates. There are, however, many situations in which the sensitivity of FROG is inadequate. For example, ultrafast spectroscopy experiments that generate signal pulses of sub-picojoule energies offer much additional information about the medium if the weak signal pulse could be better characterized. [2] In this work, we combine FROG with Spectral Interferometry (SI) [3] to yield a simple method for measuring nearly arbitrarily weak pulses. Here, FROG is used to characterize a reference pulse, and SI sensitively measures the phase difference between the reference pulse and an unknown pulse, thus yielding the full intensity and phase of the unknown, potentially ultraweak, pulse. We demonstrate this method on a pulse train containing 40 zeptojoules (zJ) of energy, or about 1/5 of a photon per pulse (See Figs. 1-3). This method is quite general because ultraweak ultrashort pulses do not exist by themselves; an optical nonlinearity is required to make ultrashort pulses, which are then strong enough to be measured with FROG and hence act as reference pulses. Because this method uses FROG, and because ST involves simply measuring the spectrum of a pair of pulses, we call it Temporal Analysis by Dispersing a Pair Of Light E-fields (TADPOLE).
Primary Ultrafast Events in the Proton Transfer Reaction from Excited Pyranine to Water
Authors: Tran-Thi T-H. , Prayer C. , Gustavsson T. , Pommeret S.
Published: February 2023

Pyranine or tsPyOH (8-hydroxy-1,3,6-trisulfonated pyrene) belongs to the family of aromatic alcohols which have been widely studied since Förster1 and Weller2. These molecules are known to undergo proton transfer (PT) both in the ground (GSPT) and singlet excited (ESPT) states, with Ka jump of seven orders of magnitude between the two processes. The dynamics of pyranine ESPT has been studied in various polar media 3-5. In all cases, ESPT was probed by observing the disappearance of the protonated species and the formation of the deprotonated form, using time resolved fluorescence techniques with picosecond or nanosecond time resolution. A long decay time (~100 ps) of tsPyOH*, correlated with the formation time of tsPyO-*, was found in aqueous solutions and ascribed to the ESPT time constant6. In contrast to these findings, Mataga's group, who probed ESPT of the related compound hydroxypyrene (PyOH) hydrogen-bonded to a strong base (triethylamine) in various non-polar and polar solvents, found a first, fast time constant (850 fs) followed by a slower one7. The first time constant, which is solvent independent, was ascribed to a fast ESPT, while the second one was assigned to the ion pair separation.
Electronic and Vibrational Dephasing in Solution by Dynamic Symmetry Breaking
Authors: Myers Anne B. , Johnson Alan E.
Published: February 2023

All spectroscopies, whether explicitly time-resolved or steady-state, are sensitive to fluctuations in the spectroscopic environment of the chromophore on the time scale of the radiation-matter interaction(s). Simple "two-state jump" models that assume random hopping between just two spectroscopically distinct environments have been well studied. Such models can provide some qualitative insight into the influence of fluctuations on a particular spectroscopy even if the actual system accesses a continuous distribution of states, as is usually the case for chromophores in liquids. The usual two-state models assume that the states differ in their transition frequencies to one or more accessible excited states. In linear spectroscopies, such models predict the well-known coalescence from two discrete resonances to a single broad one which then motionally narrows as the fluctuation rate increases. For multiphoton spectroscopies the effects are more complicated; in particular, for monochromatically excited spontaneous emission, increasing the fluctuation rate causes evolution from a sharp, "resonance Raman-like" spectrum to one having increasing contributions from broad emission.1 The transition frequency fluctuations constitute a source of electronic pure dephasing at the level of the chromophore's density matrix, generating a "fluorescence" component to the emission.