bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Parametric Amplification of White-Light-Continuum Components at 250 kHz with a Ti:Sapphire Regenerative Amplifier
Authors: Reed M. K. , Steiner-Shepard M. , Negus D. K.
Published: February 2023

A ti:sapphire regenerative amplifier pumped by a cw-argon-ion laser was recently demonstrated by T. B. Norris [1]. We have developed a compact ti:sapphire mode-locked oscillator and regenerative amplifier system (Coherent RegA-9000) with a single argon-ion laser providing 8W to pump the oscillator and 14W to pump the amplifier. A 110fs duration, 820nm pulse from the Mira-900 oscillator is injected into the RegA-9000 amplifier using a TeO2 acousto-optic cavity dumper. Chirped pulse amplification is accomplished over 25 roundtrips by allowing the dispersion in a TeO2 Q-switch within the amplifier to expand these pulses to 30μs length. The cavity dumper then extracts a single pulse of more than 6μJ energy at a repetition rate of 250 kHz, corresponding to an average power of 1.5W.
Observation of Biexcitons in Time-Resolved Degenerate-Four-Wave-Mixing on GaAs Quantum Wells
Authors: Mayer E.J. , Smith G.O. , Kuhl J. , Bennhardt D. , Meier T. , Schulze A. , Thomas P. , Hey R. , Ploog K.
Published: February 2023

Although the importance of exciton/exciton interactions for the nonlinear optical response of GaAs quantum wells (QW) is well established by many experimental observations, the role of biexciton states in these processes is still an open question, even at low excitation densities. Evidence for the formation of biexcitons has been independently derived from pump-probe studies [1], from the density dependence of the photoluminescence (PL) line shape [2] and from signal modulations observed in degenerate-four-wave-mixing (DFWM) with incoherent light pulses [3]. Here, we report on time-resolved DFWM with parallel (PP) and cross-polarized (CP) pulses. Depending on the polarization geometry, the signals reveal strong quantum beats which correspond to the splitting between either the heavy hole (hh) and light hole (lh) exciton or the hh exciton and the biexciton.
Use of self-trapped filaments in air to trigger lightning
Authors: Zhao Xin Miao , Diels Jean-Claude , Braun Alan , Liu Xinbing , Du Detao , Korn Georg , Mourou Gerard , Elizondo Juan M.
Published: February 2023

A study of the possibility of triggering lightning with optical pulses has been investigated. A theoretical analysis shows that a continuous ionized path of several tens of meters should be created. In recent experiments, long focal length lens were used to confine a low energy (≈ 200 μJ) femtosecond ultra-violet laser pulse, resulting in a weak ionization (< 10−6 of the air molecules) in a column between two electrodes spaced by 25 cm. A discharge was induced with this technique at half the self breakdown field of air [1]. The mechanism of laser induced guided discharge is a local field enhancement resulting from the migration of charges in the applied electric field. This mechanism is different from the standard laser induced spark gap, where the laser energy is sufficient to induce a plasma between the electrodes. Self-trapped filaments can create a continuously ionized path over very long distances, which is the main requirement for “seeding” and guiding a discharge over long distances with minimal laser energy.
Molecular Pulse Shaping
Authors: Tull J. X. , Hillegas C. W. , Goswami D. , Warren W. S.
Published: February 2023

Interest in the uses of controlled radiation fields to alter molecular states or drive chemical reactions–originally called “laser selective chemistry,” but now commonly called “quantum molecular control”1–has resurged in the last few years, largely because the technology for controlling optical fields has improved dramatically. Programmable pulse shaping has been demonstrated with temporal resolution of ≈30 fs and peak powers limited only by the amplifying lasers, and a tremendous range of applications can be readily envisioned. Nonetheless, pulse shaping has not become widely applied in the ultrafast optical community, for two basic reasons. Robustness is a fundamental issue–it is relatively easy to calculate waveforms which can break strong bonds or alter reaction dynamics, but in many cases, laboratory inhomogeneities or lack of knowledge of the exact Hamiltonian make practical implementation impossible. In addition, none of the demonstrated techniques for controllable fs pulse shaping is truly “easy.” Arbitrary ps pulse shape generation was first demonstrated in 19852, but each new waveform required a new fabricated amplitude mask. Our electro-optic approach3 was the first method which gave programmable pulse shapes, but in 1986 we used traveling wave EO modulators and microwave waveform generators which are past the commercial state-of-the-art even today. More recently, liquid crystal modulators (LCMs)4. or holographic patterns5 have been used to tailor the optical waveform. Tremendous improvements have been demonstrated, particularly in the LCM-based approach; however, alignment, pixel isolation and calibration, and gaps between pixels become limiting factors when the desired waveform requires substantial frequency modulation.
Femtosecond Spectroscopic Studies of Barrierless Reaction of Triphenylmethane Dye Molecules in Alcohol Solutions
Authors: Du Mei , Jia Yiwei , Fleming Graham R.
Published: February 2023

Many chemical and biological reactions proceed on a potential surface with a low barrier or lack of barrier from reactant to product along the reaction coordinate1,2. In a barrierless case, the interaction between the surrounding medium and reactants (or products) provide the only frictional force in affecting the chemical reactions. In general, the frictional force includes two parts: frictional force from the solvent and from non-reactive vibrational modes of reactants or products. Thus, from the perspective of chemical reactions in solution, a barrierless reaction is a prototype in studying both the interactions between reactants(or products) with the surrounding medium and the role of vibrational relaxation during the course of chemical reaction. In this work, we choose triphenylmethane dye molecules: malachite green and crystal violet in alcohol solutions as two model systems to investigate the nature of the barrierless process at initial time and the effect of both solvent and intramolecular vibrational modes during the reaction.
Pulse-Doped GaAs Nonlinear Transmission Lines for 630 fs, 1 V Shockwaves
Authors: van der Weide Daniel , Brunemeier Paul
Published: February 2023

We have developed nonlinear transmission lines (NLTLs) having the fastest output transition times yet reported for electronic circuits: waveforms with 630 fs, 1 V edges, measured at T = 300 K. These circuits were realized on GaAs with a pulse-doped profile for maximum capacitance-voltage nonlinearity, and measured with on-chip integrated sampling circuits strobed by identical NLTLs. This epitaxial profile also allows high-cutoff-frequency sampling diodes by a simple etching process, enabling us to combine two essential requirements for an ultrafast electronic circuit onto one monolithic IC.
Optical breakdown in wide-band-gap material as function of pulse duration
Authors: Du D. , Liu X. , Korn G. , Squier J. , Mourou G.
Published: February 2023

The phenomena of optical breakdown of normally transparent optical materials such as fused silica has been studied for three decades[1]-[3]. Through the nonlinear photon-matter interaction (electron avalanche process or multiphoton ionization), a plasma is generated at the focal spot and laser energy is deposited to material, which heats the material that can lead to the irreversible damage to the material. The breakdown threshold is pulse-width dependent, and an empirical scaling law of breakdown threshold exists: F
th
, ~ τ1/2, for pulse duration greater than 10 ps.
Direct Observation of the Accessory Bacteriochlorophyll in the Primary Electron Transfer in Bacterial Reaction Centers
Authors: Zinth W. , Schmidt S. , Arlt T. , Huber H. , Nägele T. , Meyer M. , Scheer H.
Published: February 2023

Early femtosecond experiments on bacterial reaction centers (RC) performed nearly one decade ago did not exhibit transient absorption features related to the reduction of the accessory bacteriochlorophyll (B) located between the primary electron donor (the special pair P) and the electron accepting bacteriopheophytin H. The absence of such a component led to the conclusion that B is not a real electron carrying intermediate. However in later experiments |1–3| we found a subpicosecond component which had relatively weak amplitudes. The spectral dependence of this component and additional experiments on mutated and modified RC suggested that B is a real electron carrier. This interpretation was not generally accepted in the literature. One main objection came from the fact that the subpicosecond kinetic component was only observed in "congested" spectral region where other intermediates caused strong absorption changes. It is the purpose of this contribution to present new experimental results proving unambiguously that the accessory bacteriochlorophyll is a real electron carrier.
Picosecond Time-Resolved Vibrational Spectrum of Batho-Rhodopsin at Room Temperature: Concentration Effect
Authors: Popp A. , Ujj L. , Atkinson G.H.
Published: February 2023

The first vibrational spectrum of batho-rhodopsin (Batho), an intermediate formed during the initial 50 ps of the visual process of rhodopsin (Rh) at room temperature, has been measured by picosecond time-resolved coherent anti-Stokes scattering (PTR/CARS). The Batho spectrum is obtained through a third-order susceptibility (χ(3)) analysis of the measured PTR/CARS data in which the resonant and non-resonant contributions to the signal are separated. The PTR/CARS spectrum of Batho reveals structural changes during the Rh photo-reaction which have not previously been recognized.
Nonequilibrium Dynamics of Holes in p-Type Germanium Studied by Femtosecond Infrared Spectroscopy
Authors: Woerner M. , Elsaesser T. , Ludwig C. , Portella M. T. , Frey W. , Kaiser W.
Published: February 2023

The ultrafast thermalization of nonequilibrium carriers in semiconductors, i.e. the relaxation to a Fermi-like distribution, has mainly been investigated by femtosecond absorption and luminescence studies of photoexcited electron-hole plasma. In this paper, we present the first time resolved observation of thermalization in a single component plasma, providing specific insight into the subpicosecond dynamics of holes in p-type germanium.
Femtosecond Dynamics in Hydrogen-Bonding Solvents: Temperature-Dependence of the Inertial and Diffusive Relaxation of Liquid Acetic Acid
Authors: Chang Yong Joon , Castner Edward W.
Published: February 2023

Femtosecond nonlinear optical spectroscopy has proven to be a powerful tool with which to probe both the inertial and diffusive dynamics of molecular liquids.1 Only very recently have these techniques been applied to the study of dynamics in complex, strongly-interacting Hydrogen-bonding (H-bonding) liquids.2 We present here new results on the study of neat liquid acetic acid, CH3CO2H. The measurements of the inertial and diffusive dynamics were made using the optical-heterodyne-detected, optical-Kerr-effect (OHD-OKE) technique, which is an electronically non-resonant polarization spectroscopy providing a signal linear in the third-order nonlinear-optical response.1a Our measurements have been made as a function of sample temperature, with data taken at 19 and 108 °C. The melting and boiling points of acetic acid are ca. 16.2 and 117 °C, respectively, so our measurements have probed the extremes of the liquid phase. One of the most important uses for such ultrafast solvent relaxation data is to predict the rate of solvent-limited chemical reactions in solution, particularly adiabatic electron-transfer reactions.
Infrared Pulse Generation using a Subpicosecond OPO
Authors: Kafka J. D. , Watts M. L. , Pieterse J. W.
Published: February 2023

For many ultrafast experiments it is desirable to have synchronized subpicosecond pulses at multiple wavelengths. Recently, synchronously pumped parametric oscillators (SPPO) have been used to satisfy this requirement as well as produce ultrafast pulses at wavelengths beyond 1 µm. In this paper, we demonstrate the synchronism inherent in an SPPO by measuring a 100 fsec cross-correlation between the pump and signal pulses. We then use the synchronization between the signal and idler to generate pulses at longer wavelengths by difference frequency mixing in lithium niobate and silver gallium sulfide.
Interplay between Inhomogeneous Broadening and Coulomb Interaction in the Dynamics of Excitonic Wave Packets
Authors: Koch M. , Jahnke F. , Meier T. , Feldmann J. , Schäfer W. , Thomas P. , Koch S.W. , Göbel E.O. , Nickel H.
Published: February 2023

A proper description of subpicosecond four-wave-mixing (FWM) experiments for excitonic transitions in semiconductor heterostructures must consider the fact that the spectrally broad laser pulses excite not only the 1s bound exciton state but also energetically higher bound exciton states and the lower-lying part of the unbound continuum states. In this paper, we report results of transient and time-resolved FWM-experiments in which such ’excitonic wave packets’ play a crucial role. In addition, a theoretical treatment of the problem based on the solution of the semiconductor Bloch is presented. We show that the time-dependent nonlinear response is determined by an interplay between inhomogeneous broadening and Coulomb interaction.
Brave New Nanoworld: Probing the Realm of Ultrafast, Ultrasmall Electronics
Authors: Bloom D.M. , Hou A.S. , Ho F.
Published: February 2023

We have developed a new tool for measuring ultrafast voltages on integrated circuit devices. Our scanning force microscope probing system has 100-picosecond, submicron resolution and the ability to probe though passivating layers.
Time-Resolved Nonlinear Near Field Scanning Optical Microscopy of Semiconductor Microdisks
Authors: Stark J. B. , Mohideen U. , Betzig E. , Slusher R. E.
Published: February 2023

The experiments described here use nonlinear, time-resolved, near field scanning optical microscopy (NSOM1) techniques to study the local carrier dynamics and spatial distributions in GaAs microdisks. GaAs quantum wells in the plane of the microdisk provide a gain material for microlasers based on the whispering-gallery modes propagating around the edge of the microdisk . We are exploring the carrier density regime required for lasing. What are the energy relaxation times and spatial diffusion lengths near the microdisk edge? How do these properties vary with surface treatment and carrier densities? These questions can best be answered using the high spatial resolution provided by NSOM and the temporal resolution of short excitation and probe pulses tuned to frequencies near the GaAs bandedge.
Near-field Time Resolved Fluorescence Spectroscopy on Single Proteins: Application to Photosynthetic Membranes
Authors: Xie X. Sunney , Dunn Robert C. , Holtom Gary R. , Mets Laurens
Published: February 2023

The spatial resolution of conventional optical measurements is limited by far field diffraction effects. Recently a new optical technique has been demonstrated in which the spatial resolution can be much higher than the diffraction limit. Near-field optical microscopy involves scanning a spot of light, of dimension much smaller than the optical wavelength, in close proximity to the surface of a sample. To form the spot of light, a technique has been developed in which a single mode optical fiber is pulled to an ultrafine tip and coated with aluminum.1 The tapered fiber tip funnels light through a small aperture (tens of nanometer) whose size determines the near-field spatial resolution. Furthermore, because of the inherently low background signal in these microscopes, emission signals from a single molecule can be detected.2,3
Frequency Domain Study of Cross-coupling in a Two-wavelength Self-modelocked Ti:Sapphire Laser
Authors: Dykaar D. R. , Knox W. H. , Darack S.B.
Published: February 2023

The recent demonstration of self-modelocking in broadband solid state gain media [1] has caused an resurgence of interest in studies of bulk self-focusing interactions. In particular, two-color nonlinear interactions have been the subject of much interest in soliton communications [2], and operation of the Ti:Sapphire laser some soliton-like features [3]. Recently, the Ti:Sapphire laser has been modelocked at two wavelengths simultaneously using a single laser rod [4-6]. These lasers produce two independently tunable trains of pulses of <100 fs duration. Under certain conditions these pulse trains will precisely synchronize themselves, exhibiting a jitter of < 50 fs. In the present paper we discuss measurements of the frequency-pulling characteristics of a 2-color self-modelocked Ti.Sapphire laser based on the design of Dykaar [6]. These measurements demonstrate the strong nonlinearity of the cross-modelocking process, and may provide insight into two-wavelength soliton interactions.
Femtosecond Measurements of Ballistic and Interactive Transport of Nonequilibrium Electrons in Thin Gold Films
Authors: Juhasz T. , Suárez C. , Bron W. E. , Elsayed-Ali H. E.
Published: February 2023

The dynamics of nonequilibrium electrons in solids has attracted considerable recent attention. Nonequilibrium electrons in metals can be excited by laser pulses with durations less than, or comparable to the excited-electron energy-loss lifetime (τ
e
), such that a transient inequality between the effective electron and lattice temperatures (T
e
and T
l
) occurs. The excited electron population can be probed through measurements of the transient differential reflectivity (or transmissivity) of the sample. Loss of electron energy in the excited volume is attributed to energy transfer to the lattice through electron-phonon interactions plus electron transport out of the excited region. It has been shown that the relaxation of the nonequilibrium electrons is influenced by the crystal structure of the films, and by the consequent enhanced electron-phonon scattering in the polycrystalline films [1]. It has been established on experimental and theoretical grounds that electron-grain boundary interactions impede electron transport in polycrystalline films [2]. Earlier experiments on this topic investigated electrons thermalized with the lattice. Unresolved, however, is the role of grain boundaries in the transport of nonequilibrium electrons in thin metal films. In the present experiment we investigate the transport of femtosecond laser generated nonequilibrium electrons in poly-and single-crystalline gold films, through measurements of the time-of-flight across the film thickness which is made possible through an application of a thermoreflectivity technique with femtosecond resolution [3].
Femtosecond, Cr4+:YAG laser
Authors: Nathel Howard , Sennaroglu Alphan , Pollock Clifford R.
Published: February 2023

Recent advances in mode-locked solid-state lasers have resulted in pulses as short as 8.5 fs in the 800 nm region from Ti:sapphire oscillators [1]. Using similar techniques, sub-50 fs pulses [2-4] have been generated in another solid-state laser system, Cr:forsterite, which operates in the 1.2 to 1.3 μm range. We report here results from both a regeneratively-initiated and self-initiated, mode-locked Cr:YAG laser [5] which is tunable from 1.51 to 1.53 μm. One hundred and twenty femtsosecond, nearly transform-limited pulses have been generated with peak output powers of 45 kW. The stable, high peak power pulses and room temperature operation of this laser make it a very suitable alternative to the cumbersome, cryogenic mode-locked NaCl laser [6] commonly used in both narrow bandgap semiconductor and optical communications research.
Femtosecond Pulse Generation At 1.3 μm From A Praseodymium Fibre Laser
Authors: Guy M. J. , Boskovic A , Noske D. U. , Taylor J R
Published: February 2023

Femtosecond pulses have been generated in erbium doped fibre lasers using a number of different cavity designs and mode-locking techniques. These have proven capable of directly generating pulses as short as 98 fs at 1.55 μm1, the lowest loss "window" in optical fibre used for telecommunications. A second low loss window exists at 1.3 μm and recently considerable interest has been directed towards Praseodymium (Pr3+) doped fluoride fibre as an amplifier providing gain at this wavelength. Pr3+ doped into a fluoride glass host high gain over a range of ~50 nm and this broad bandwidth has the potential to support femtosecond pulses.
Time-resolved thermionic electron emission and nonthermal ion emission from Au excited by intense fs pulses
Authors: Riffe D. M. , Wang X. Y. , Lee Y.-S. , Downer M. C.
Published: February 2023

Electron and ion emission have long been used as probes of the interaction of laser pulses with solid-state materials. [1] For excitation by femtosecond pulses most of the focus has been on either the low excitation regime where single or multiphoton electron emission dominates and surface states play an important role or in the very intense regime where significant x-ray generation occurs. Notably absent are emission studies in the transition region across the damage threshold where electron temperatures are ~ 1 eV, a temperature region where the relaxation dynamics of electrons are important in nanoscale microelectronic devices, ultrafast desorption of adsorbates, and fs-laser plasma physics. [2]
Noncontact Wafer Characterization with T-Rays
Authors: Mittleman D. M. , Nuss M. C. , Cunningham J. , Geva M.
Published: February 2023

Recently, Hu and Nuss [1] demonstrated an imaging system using terahertz transients generated by standard optoelectronic methods. We have used this system to measure spatial inhomogeneities in the doping density of epitaxial GaAs films. The absorption of THz radiation by doped semiconductors is a strong function of the conductivity, and as a result we are able to detect small variations in the level of doping with 250 µm spatial resolution. Such information is not easily obtainable by any other method, and this demonstration could lead to improved quality control in the growth of doped semiconductor wafers.
Laser-Induced Softening of Coherent Phonons: Pathway to Nonthermal Melting
Authors: Hunsche S. , Wienecke K. , Dekorsy T. , Kurz H.
Published: February 2023

We investigate the dynamics of coherent optical phonons in single-crystal Tellurium after high-density excitation of free electrons by intense femtosecond laser pulses. The experiments are performed in a standard time-resolved pump-probe setup using pulses of a Ti-Sapphire oscillator that are amplified in a 6.8 kHz dye amplifier. The pulse duration is <100 fs, much shorter than the oscillation period of the A1 optical phonon mode of Tellurium, leading to an impulsive and selective excitation of this phonon via the “displacive excitation” mechanism.1 The coherent lattice vibrations are detected via transient reflectivity changes that show clearly-resolved oscillations as a function of pump-probe delay, as shown in the inset of Fig. 1. The main plot shows Fourier spectra of the oscillating signal contribution, obtained after subtraction of the slowly-varying background, at various excitation densities.
Tunable photoconducting emitters and detectors of free space terahertz radiation
Authors: Weling Aniruddha S. , Auston David H. , Heinz Tony F.
Published: February 2023

In recent years, various optoelectronic techniques have been developed to generate and coherently detect broadband pulses of terahertz (THz) radiation in free space [1]. While the power of these methods for far-infrared spectroscopy has been amply demonstrated, certain measurements such as of nonlinear interactions in the far-infrared would clearly benefit from a scheme of spectral control of both the generation and detection of THz radiation. Recently, we reported a new scheme of producing quasi-sinusoidal modulation of optical radiation at tunable THz frequencies [2]. This was accomplished by mixing two linearly chirped broadband optical pulses with a variable delay τ between one another and led to the production of tunable narrowband THz radiation. In this summary, we demonstrate the applicability of the chirped pulse beating technique to incorporate spectral selectivity in the phase-coherent detector, as well as the emitter, of THz radiation. We have developed and carefully characterized a THz system composed of photoconducting dipole emitters and detectors independently tunable from ~60 GHz to ~0.8 THz [3]. In addition, we report our results on the possibility of circumventing the saturation effects normally encountered in generating high-intensity THz radiation from photoconducting antennas by the use of the chirped pulse mixing scheme.
Experimental Demonstration of a Sub-Picosecond X-Ray Streak Camera
Authors: Chang Z. , Rundquist A. , Zhou J. , Kapteyn H.C. , Murnane M.M. , Liu X. , Shan B. , Liu J.
Published: February 2023

Although autocorrelation techniques have demonstrated femtosecond time resolution from the infrared to the ultraviolet wavelength region, the streak camera is still the fastest broadband x-ray/xuv detector. This is due to the difficulty of using x-ray nonlinear optical effects for correlation techniques over a broad wavelength range. [1] The fastest x-ray streak camera measurement to date was demonstrated to be ≈ 2ps, and was limited both by the streak camera itself, and by the x-ray source.[2,3] Up to now, X-ray pulses from laser-produced plasmas have been use for streak camera calibration. However, such pulses are far from ideal because their duration can be comparable to the resolution of the camera being testted. For this work, we use high-harmonics from a 25fs laser[4] to successfully calibrate a newly-developed x-ray streak camera.
Ultrafast Sampling with an Amplified, Polyimide-Based Photoconductive Probe
Authors: Al-Hemyari K. , Cheng H.-J. , Williamson S.L. , Hwang J.-R. , Whitaker J.F.
Published: February 2023

We have developed and characterized a novel, amplified, high-impedance circuit probe which utilizes a polyimide film as the substrate for a probe tip and photoconductive sampling gate. While the utility of freely-positionable photoconductive probes has been substantiated for both high- and low-impedance testing of electronics1-3, the invasiveness (in terms of both field disturbance and charge extracted), mechanical flexibility, and sensitivity at low light levels limit the usefulness of instruments based on these devices. The probe described in this work has many advantages over conventional, unamplified probes using semiconductor substrates. This is due to the low permittivity of the strong, but flexible, polyimide film employed, the optical transparency of the polyimide, and the low capacitance and high resistance of the JFET source follower connected to the output of the sampling gate. For instance, while still maintaining a single-picosecond temporal resolution, the amplified polyimide probe disturbs the fields surrounding a circuit less than a typical probe, while providing two orders of magnitude improvement in sensitivity and the ability to convert current to voltage (and thus measure absolute voltages). In addition, problems associated with extracting excessive amounts of charge from a circuit, as well as with optical heating and scattering of the probe light, are reduced because only very small laser intensities are required to gate the signal being measured.
Observation of Laser Assisted X-ray Photoionization and Measurement of Ultrashort Soft X-ray Pulses
Authors: Glover T.E. , Schoenlein R.W. , Chin A.H. , Shank C.V.
Published: February 2023

Ultrashort x-ray pulses are powerful diagnostic tools, combining the temporal resolution typically reserved for optical (laser) pulses with atomic scale spatial resolution which, in principle, can be achieved using high energy electromagnetic radiation. Progress in the development of femtosecond x-ray sources has been hindered by an inability to measure x-ray pulses of femtosecond duration. Streak cameras are typically used to measure ultrashort x-ray pulses, however the ~ 1 ps resolution limit of these instruments abates their usefulness in the femtosecond regime. We report demonstration of a visible/x-ray cross correlation technique based on laser assisted x-ray photoionization which has been used to measure 50 fs soft x-ray pulses. The technique permits the first direct measurements on the duration of femtosecond high order harmonic radiation. Additionally, the free-free scattering inherent to the technique provides an important test for models of high power laser interaction with matter.
The photochemical dynamics of trans-1,3,5-hexatriene in solution investigated by the ultrafast transient absorption spectroscopy
Authors: Ohta Kaoru , Naitoh Yukito , Tominaga Keisuke , Hirota Noboru , Yoshihara Keitaro
Published: February 2023

The photophysical and photochemical dynamics of small linear polyenes is a subject of current interest since these molecules play important roles in the biological systems. However, the excited state dynamics of small polyenes have not been well understood because these molecules exhibit no fluorescence and give only broad absorption spectra even in the low temperature matrices and supersonic expansions. It has been considered that after the photoexcitation, the lifetimes of the initial excited states of small linear polyenes are extremely short and the immediate transfer to the other dark electronic state occurs. Thus, direct ultrafast time-resolved studies are needed in order to understand the fate of the photoexcited state of small linear polyenes in the condensed phase. In this study, we carried out the ultrafast transient absorption measurement of trans-1,3,5-hexatriene in cyclohexane and acetonitrile, respectively.
Electron Transfer in the Hexamethylbenzene/Tetracyanoethylene Charge-Transfer Complex in the Supercritical Fluids
Authors: Kimura Y. , Takebayashi Y. , Hirota N.
Published: February 2023

In this work we present new results on electron transfer (ET) rates in the hexamethylbenzene/tetracyanoethylene charge-transfer (CT) complex in supercritical carbon dioxide. The electron transfer process from the excited state of this CT complex is considered to correspond to the highly exoergic "Marcus inverted region", and extensive studies have been performed on the role of the intramolecular vibrational modes and the solvent relaxation[1-4]. The recent theoretical study on the absorption, fluorescence, and Raman spectra of this CT complex in CCl4 with a common model based on Marcus' and Jortner's theory[4] requires a large (2450 cm-1) solvent reorganization energy λs, and overestimates the ET rate. The large reorganization energy has been ascribed to the inseparability of the low frequency intermolecular complex vibrations and the solvent relaxation. The study using a supercritical fluid as solvent is a suitable approach to answer the problem of the mode separation:i.e., in dilute gases, there should be little contribution of the solvent reorganization energy, and the effect of the solvent on the ET process can be extracted from its density dependence.
Comparison of the photochemical ring opening of cyclohexadiene and dehydrocholesterol in solution, studied by ultrafast absorption spectroscopy
Authors: Fuß W. , Lochbrunner S. , Schmid W. E. , Kompa K. L.
Published: February 2023

The photochemical ring opening of 1,3-cyclohexadiene to 1,3Z,5-hexatriene is a prototype of the orbital-symmetry controlled electrocyclic reactions. In previous work [1], we studied by transient absorption spectroscopy (frequency-tripled Ti:sapphire laser around 267 nm, pulse length 250 fs, identical pump and probe wavelength) 7-dehydrocholesterol, which contains the cyclohexadiene moiety. The excited state (1B2) relaxes very rapidly (10 fs, deduced from the fluorescence quantum yield) to the 2A1 surface, which is a dark excited state and has a much lower minimum. From there, the product previtamin D is formed with a temperature independent rate within 5.2 ps. Subsequently, this primary product relaxes by rotation around a formally single bond to its stable conformer within 125 ps at room temperature (activation energy 15 kJ/mol).
Femtosecond IR Studies of Solvation by Directly Probing the Solvent
Authors: Lian T. , Yang H. , Asplund M. , Bromberg S. E. , Harris C. B.
Published: February 2023

The response of solvent to the change of charge or dipole of solute molecules has been intensely studied in recent years1. In previous solvation experiments, time dependent fluorescence Stokes shift of dye molecules in different solvents were measured, from which the solvation time for the solvents were determined1,2. Various theories, from the simple dielectric continuum model to instantaneous solvent normal mode analysis, have been used to relate solvent motions to solvation time3,4. MD simulations have also been carried out to understand the nature of these solvent motions in the solvation process5. However, these time dependent Stokes shift experiments, which measure the solute fluorescence, can only provide an indirect microscopic picture of the relevant solvent motions during the solvation process.
Transient grating with asymmetric arms for electro-optic generation of THz radiation
Authors: Kawashima Hitoshi
Published: February 2023

Recent progress in femtosecond pulse-shaping technology1,2 drives a strong impetus to try to find novel operations that a tailored optical waveform can achieve and that would be impossible with transform-limit one. In this paper, we propose a scheme to map an amplitude profile of an optical waveform on THz radiation that is generated via optical rectification in nonlinear crystals3. The present study was motivated by an anticipation that if chirp can be optically coded on the THz radiation, it could compensate for spreading of the THz wavepacket that propagates under group-velocity-dispersion (GVD). The idea is close to recently demonstrated "wavepacket focusing"4. In an arrangement that we envision, two independent optical waveforms, E0 and ES, are directed into an E-O medium, and they overlap both in time and space to form a fringe pattern in the medium as shown in Fig. 1. Source polarization PNL ∝ EO ES emits THz radiation along the direction of difference wavevector, by which we define z-axis. We denote amplitude of wavepacket propagating in (± z) direction by U±. If E0 is a monochromatic light and ES has some tailored waveform, PNL is expected to replicate the amplitude profile of ES instead of its intensity profile. Detailed analysis5 reveals features proper to the present configuration; i) spectral bandwidth of U± is limited by spatial monochromaticity, of the fringe pattern, ii) when a chirped pulse is used as ES, U± carries an opposite chirp to each other, and cancellation of material GVD with the coded chirp occurs in either one direction. Also, arrival time of the wavepackets are shifted from that for ES with no chirp. The time shift is in advance in one direction and in retardence in the other. Objective of the experiment is to see such asymmetry.
Solvent Effects on the Ultrafast Dynamics of Chlorine Dioxide Photochemistry
Authors: Chang Yong Joon , Simon John D.
Published: February 2023

The photochemical events following the photodissociation reaction of chlorine dioxide (OC1O) are strongly medium-dependent.1 The photoreactivity of OC1O is complicated by the existence of competing reaction pathways (i.e., photoproducts C1O + O vs. C1 + O2), the partitioning of which is significantly altered by the medium. We have investigated the ultrafast dynamics of OC1O photochemistry in the liquid phase to further our understanding of the effects of solute-solvent dynamical coupling on the reaction coordinate.2,3 The ultrafast dynamics of the photophysical and photochemical processes following the photodissociation of OC1O at 395 nm have been measured using femtosecond transient absorption spectroscopy. The transient absorption dynamics were probed using a wide range of spectral coverage, from 267 nm to 700 nm. Figure 1 shows a select set of the data obtained for OC1O in aqueous solution.
Diffracting Fourier-transform spectroscopy using mid-infrared femtosecond pulses
Authors: Bonvalet A. , Joffre M. , Migus A. , Martin J.-L.
Published: February 2023

Visible or near-infrared (hereafter called simply visible) femtosecond pulses make possible the generation of far-infrared (THz) [1] and even mid-infrared [2] femtosecond pulses. A powerful technique used to characterize these pulses is Fourier-transform spectroscopy [3], yielding the power spectrum through a Fourier-transform of the recorded linear autocorrelation. Although such a measurement can be performed using an infrared Michelson interferometer, a sequence of two collinear infrared pulses separated by a variable time delay can more simply be generated using other techniques [4]. Here, we propose and demonstrate a new Fourier-transform spectrometer in which two time-delayed visible pulses are focused on a single infrared generator, as shown in fig. 1, thus generating a sequence of two infrared pulses. The signal recorded as a function of time delay yields the linear autocorrelation of the infrared pulse. However, when the two incident visible beams are collinear, their interaction in the generator results in an additional emission. As a consequence, the signal then recorded as a function of time delay, shown in fig. 2(a), resembles the interferometric second-order autocorrelation of the visible pulse, with a contrast larger than 2 and high-frequency fringes. Fortunately, a non-collinear geometry eliminates this crossed emission, while preserving a nearly collinear geometry in the infrared. Indeed, since the natural angle of diffraction is larger in the infrared than in the visible, we can choose an angle θ between the two incident beams such that two non-collinear incident visible beams yield two strongly overlapping infrared beams, as shown in the inset of fig. 1. Therefore, the two infrared beams can interfere on the detector, nearly as much as if they were truly collinear. In this case, the detected signal is the linear autocorrelation of the infrared pulse, with a constrast close to 2, as expected (fig. 2(b)). A Fourier transform provides the power spectrum of the infrared radiation (inset). The broad spectral width results from the use of 15-fs optical pulses on GaAs as the infrared generator [2], and the sharp cut-off above 14 µm is due to our detector spectral response.
Ultrafast Vibrational Relaxation and Ligand Photodissociation/Photoassociation Processes of Nickel(II) Porphyrins
Authors: Eom Hyo Soon , Kim Cheon Min , Jeoung Sae Chae , Kim Dongho
Published: February 2023

The nickel(II) porphyrins have been well suited for an ideal system for investigating electronic decay, axial ligand photodissociation and photoassociation dynamics.1Of great significance in governing photophysics of the four- and six-coordinate nickel(II) complexes is the presence of a low-lying metal excited
(dz2,dx2−y2) state having ~250 ps lifetme below porphyrin ring (π,π*) states.1 The (d,d) excited state shows characteristic sharply featured absorption difference spectra, compared to the broader featured more diffuse spectra of the ring (π,π*) and metal⇔ring charge transfer excited states. This favorable properties of the nickel(II) complexes provide a good opportunity for examining the deactivation dynamics in the porphyrin and its interaction with environment that may accompany a transition from an electronic excited state of the macrocycle to an electronic excited state of the metal.
Vibrational Mode Suppression in the Non-Markovian Limit: Diagonal Time-Gating of the Stimulated Photon Echo
Authors: de Boeij Wim P. , Pshenichnikov Maxim S. , Wiersma Douwe A.
Published: February 2023

Numerous studies to date are aimed at a better understanding of the optical dynamics of dissolved dye molecules. Most dyes exhibit prominent vibronic features in their absorption spectra. For impulsive excitation the initially excited state is a vibronic wave packet, that strongly influences the observed dynamics. Shank and co-workers were the first to recognize the importance of vibrational mode suppression towards optical dephasing measurements.[1,2] However, mode suppression based on time-integrated stimulated photon echo works only in systems that exhibit Bloch-like dynamics.
Time-Space Conversion using Chirped Pulses for Three-Dimensional Shape Measurements
Authors: Minoshima Kaoru , Matsumoto Hirokazu
Published: February 2023

Three-dimensional (3-D) shape measurements have been scientifically and practically attractive and several methods have been proposed. Most of the conventional methods require either rather complex image processing or some mechanism to scan the laser beam and/or delay time.[1,2] We have proposed a new method in which simultaneous 3-D imaging is possible without the need for scanning by means of conversion between the time and color axes mediated by the chirped pulse.[3] The applicability of this method was demonstrated by measuring highly reflecting gauge blocks. In this paper, we apply the method to measure the internal structure of the transmission objects and the surface structure of the diffusing objects.
High-resolution ultrafast laser pulse shaping for quantum control and terabit per second communications
Authors: Dugan M. , Tull J.X. , Rhee J-K. , Warren W.S.
Published: February 2023

Advances in ultrafast laser pulse shaping technology will expand capabilities for quantum molecular control and high speed optical communications networks. Here we report a dramatic increase in pulse shaping resolution using a large aperture acoustooptic modulator[1]. We also present recent results with shaped pulse amplification and two applications–quantum control by steering wavepackets with frequency swept pulses, and an architecture for terabit/sec optical communications.
“Bubble” dynamics following Rydberg state excitation in rare gas matrices
Authors: Portella-Oberli Marcia T. , Jeannin Catherine , Chergui Majed
Published: February 2023

The dynamics of Rydberg states in condensed matter is still a field to be unravelled. The radius of low-n Rydberg states is comparable to the nearest neighbour distance in rare gas solids. Consequently, following photoexcitation of such states in a solid matrix, there will be a short-range repulsive interaction between the Rydberg electron and the electronic cloud of the neighbouring matrix atoms. This repulsion leads to a local expansion around the electronically excited center and results in the formation of a "microscopic cavity", or "bubble", surrounding the molecule as the solvent atoms are pushed asunder to a new equilibrium position /1-4/. In conventional spectroscopy, this is inferred from the large blue gas-to-matrix shifts of absorption bands and the strong absorption-emission Stokes shifts.
Enhanced High-Harmonic Generation with Ultrashort 25fs Pulses
Authors: Zhou J. , Rundquist A. , Chang Z. , Peatross J. , Murnane M.M. , Kapteyn H.C. , Christov I.P.
Published: February 2023

In this talk, we present experimental and theoretical results on ultrashort high-harmonic generation in gases, using a high peak-power Ti:sapphire laser with 25fs pulsewidth. In these experiments, we focus the laser pulses into a gas cell to generate the high harmonics, and use a grazing-incidence soft-x-ray spectrometer to observe the soft x-ray emission. The bandwidth of the laser pulses is 32nm, centered at a wavelength of 805nm, and the laser system can provide up to 70mJ of energy per pulse at a 10Hz repetition rate.[1] These experiments are approaching a new regime in laser-matter interactions, since for our excitation pulses (10 optical cycles FWHM), at the half-maximum intensity of the temporal pulse envelope the laser intensity changes by more than 25% during a single cycle. This challenges the adiabatic assumption, which presupposes that the atomic dipole moment undergoes quasi-periodic motion from cycle to cycle, with no dependence on the history of the pulse.
Femtosecond nonlinear investigations of molecular motion in liquids
Authors: Steffen Thomas , Duppen Koos
Published: February 2023

Nuclear dynamics in liquids can be studied in real time by impulsive, Rayleigh/Raman-type ultrafast nonlinear spectroscopy. Nonresonant ultrashort laser pulses can induce and probe molecular motion in liquids, that shows complicated inertial behaviour on a subpicosecond time scale. The initially coherent motion dephases rapidly and after a few picoseconds only diffusive behaviour is observed. This dynamics gives rise to the broad, low-frequency spectrum of liquids in the Rayleigh-wing, that was studied in the past decade by third-order techniques like the optical Kerr effect [1], and transient grating scattering [2].
Wavepacket Dynamics via Femtosecond Time-resolved Photoelectron and Photoionization Spectroscopy
Authors: Fischer I. , Vrakking M.J.J. , Villeneuve D.M. , Stolow Albert
Published: February 2023

An emerging goal of fs pump-probe spectroscopy is the study of chemical reaction dynamics of polyatomic molecules and it is important to develop methods for their study. We describe such an experiment by S(t) = ||2 where S(t) is the measurement, |f> is the final state, used as a 'template' onto which we project the dynamics, μ(Q) is the coordinate dependent dipole coupling, and |ex(t)> is the wavepacket that we wish to study. In general, the choice of the final state |f> must take into consideration several effects: (1) In order to decompose the evolving wavepacket in the basis of the final states, |f> must be well known (unfortunately, higher excited states of polyatomic molecules are poorly known); (2) Variation of the dipole coupling between electronic states during the wavepacket evolution (i.e. due to non-adiabatic effects) will have a strong effect on the measured signals. It is important to choose a technique which 'follows' the reaction dynamics, even to 'dark' states. (3) Polyatomic reaction dynamics implies multidimensional wavepackets with many frequency components.
Enhanced Ionization of Molecular Ions in Intense Laser Fields: Experiments on the Iodine Molecule
Authors: Constant E. , Stapelfeldt H. , Corkum P. B.
Published: February 2023

Using nuclear wave packet techniques we show that the ionization rate in an intense laser field is very sensitive to the separation of nuclei in a molecular ion and therefore on the distribution in a plasma. The maximum we observe in the ionization rate occurs around a critical internuclear distance and is unrelated to resonances and Franck Condon overlaps. Our results are consistent with recent theoretical predictions [1-2].
The Primary Electron Transfer in Bacterial Reaction Centers with Altered Energetics of the Primary Acceptor
Authors: Wachtveitl J. , Arlt T. , Huber H. , Penzkofer H. , Zinth W.
Published: February 2023

Recent femtosecond experiments on bacterial reaction centers (RCs) could show that a bacteriochlorophyll anion is formed in the first step of the photosynthetic charge separation process [1]. This reaction was interpreted as the reduction of the monomeric bacteriochlorophyll molecule BA, located on one of the two pigment branches, between the primary donor (P, a bacteriochlorophyll dimer) and the secondary acceptor (HA, a bacteriopheophytin molecule).
Frequency-Dependent Mode-Size in Kerr-lens Modelocked Lasers
Authors: Cundiff S.T. , Knox W.H. , Ippen E.P. , Khatri F.I. , Haus H.A.
Published: February 2023

The basic mechanisms of self-focusing modelocking (also called Kerr-lens modelocking, KLM) in broadband solid state gain media are now well established [1]. Recent advances in dispersion management and cavity design have led to the generation of pulses in the sub 10 fs regime [2]. Here we show that, because self-phase modulation and self-focusing have the same underlying origin, there is an intimate connection between wavelength and spatial mode in KLM lasers.