bluuebunny/crossref_metadata_2025 huggingface dataset:
Specifying the Surface Finish of X-Ray Mirrors
Authors: Church E. L. , Takacs P. Z.
Published: February 2023

Our measurements of synchrotron x-ray mirrors indicate that the power spectral densities of their finish errors have inverse power-law or fractal forms, rather than being flat at low frequencies as is usually assumed. This paper reviews these data and discusses how this apparent divergent behavior leads to finite but unconventional effects in imaging. Results are then used to develop more rational and realistic surface-finish specifications.
Monochromator Optics for Coherent Illumination of an Undulator Beamline
Authors: Koike Masato
Published: February 2023

An undulator having a long, periodic magnetic structure installed at a low emittance storage ring generates quasi-monochromatic, narrowly collimated, and partially coherent radiation. However, undulators currently operated and even those planed at third-generation synchrotron sources still do not provide a narrow bandwidth and a low emittance enough to meet spatial and temporal coherence requirements for experiments such as soft X-ray interferometry, holography, and scanning microscopy. This paper presents a beamline design which comprises a variable-spacing spherical grating monochromator with a Kirkpatrick-Baez (KB) prefocusing system to deliver a spatially and temporally coherent flux into an end station without sacrificing throughput for the coherent flux radiated from the undulator.
Printing a 500Å Period Grating Using X-Ray Interference
Authors: Wei Max , Anderson Erik
Published: February 2023

Recently a great deal of attention has been directed towards developing, analyzing, and fabricating a new generation of electronic devices which depend upon and attempt to exploit quantum effects [1-3]. Such devices, known as quantum effect devices, exhibit transport characteristics which are described by quantum mechanics and not by macroscopic averaged quantities. Quantum effect devices are required to be much smaller in size than typical microelectronic devices. A major issue in the fabrication of these devices is the ability to write fine patterns of less than 0.05 (上m pitch. Some candidate technologies which have been explored include electron beams, x-rays, and focused ion beams [4]. Electron beam lithography has been the most commonly used method thus far to produce features at this length scale, but is limited by the proximity effect. In this work, we choose to pursue a two grating x-ray interference technique as a method to produce finely spaced lines and spaces. X-rays are not limited by the proximity effect and thus a higher contrast resist pattern can be expected.
Laser Plasma Source Characterization for Soft X-ray Projection Lithography
Authors: Silfvast William , Richardson M.C. , Bender H. , Eigon A.M. , Hanzo A. , Miesak E. , Jin F.
Published: February 2023

A review of the status of laser plasma soft x-ray sources will be made. This will include discussions of radiation properties as well as particulate emission and interdiction techniques.
Picosecond Carrier Dynamics in Semiconductors
Authors: Göbel E.O , Kuhl J.
Published: February 2023

Picosecond and more recently femtosecond spectroscopy is established as a powerful tool for the direct investigation of the dynamics of nonequilibrium charge carriers and phonons in semiconductors [1]. In this paper we will discuss some recent results of the effect of localization of electronic states on carrier dynamics. Three different systems with different nature and degree of localization have been chosen, namely: GaAs/GaAlAs quantum wells, amorphous semiconductors, and the quaternary compound III-V semiconductor GalnAsP.
Observation of the 0π Pulse
Authors: Rothenberg Joshua E. , Grischkowsky D. , Balant A. C.
Published: February 2023

In coherent optics the pulse area θ is defined as the total angle the atomic state vector rotates around the electric field of the resonant driving pulse. The atomic excitation remaining after passage of the pulse is determined by this angle. For example, a π/2 pulse will put the atoms in a coherent superposition of the ground and excited states, a π pulse will put the atoms in the excited state, and a 2π pulse will take the atoms to the excited state and then back to the ground state. Another type of pulse which has generated a considerable amount of theoretical interest1 but for which there has been only a few experimental investigations2 is the 0π pulse. For this pulse, the initial part of the pulse excites the atoms and then due to a phase change of π in the electric field, the latter part of the pulse takes the atoms back to the ground state. Thus, just as in self-induced transparency, the atoms are completely in the ground state after passage of the pulse. There are two ways to produce such a 0π pulse. The first is to switch the phase of the second half of the pulse by electro-optic techniques. For this method it is difficult to get the 0π pulse area to the desired accuracy. The second method is to rely on the automatic reshaping to the 0π pulse which occurs when a weak (θ(t) ≪ 1) pulse is propagated through a resonant vapor. This method will produce the 0π pulse area to any desired degree of accuracy. This fact is due to an area theorem for these weak pulses which states that the pulse area decays exponentially with the on-resonance absorption coefficient.1
Femtosecond Time Resolved Multiphoton Ionization: Techniques and Applications
Authors: Greene B. I.
Published: February 2023

Ionization spectroscopies have long been known to be extremely sensitive methods for detecting small number densities of molecular or atomic species. In this paper we report the extension of a well known pulsed laser technique into the 10−13 sec time regime. The utility of this method is demonstrated by time resolved studies on cis-stilbene, benzene and toluene.
Ultrafast Intramolecular Redistribution and Energy Dissipation in Solutions. The Application of a Molecular Thermometer
Authors: Scherer P.O.J. , Seilmeier A. , Wondrazek F. , Kaiser W.
Published: February 2023

In recent years we have studied the relaxation of the vibrational energy in small molecules. Here we wish to discuss the flow of vibrational energy in large molecules of 30 to 60 atoms.
Picosecond Time-Resolved Fluorescence Spectra of Liquid Crystal: Cyanooctyloxybiphenyl
Authors: Tamai N. , Yamazaki I. , Masuhara H. , Mataga N.
Published: February 2023

Liquid crystals are highly anisotropic molecular systems which exhibit phase transitions in several stages on going from the crystal to the isotropic liquid phases. Mesophases such as smectic A and nematic phases are defined depending on orientational and spatial ordering of molecules. Effects of molecular anisotropy of the liquid crystal on chemical reactions and photo-physical processes now receive increasing attention [1-4]. It has been shown [5] that, in a luminescent liquid crystal, dodecylcyanobiphenyl (12CB), two kinds of fluorescence are emitted in various phases. The dual fluorescence emissions was interpreted in terms of singlet monomer and excimer formation.
The Origin of Chloroplast Fluorescence Decay Kinetics: Picosecond Fluorescence of Mutants and Sub-chloroplast Particles of Chlamydomonas reinhardii and Barley
Authors: Gulotty Robert J. , Mets Laurens , Alberte Randall S. , Fleming Graham R.
Published: February 2023

The form of the fluorescence decay function from the light harvesting system of photosynthetic organisms has now been determined quite accurately by low-intensity synchronously pumped dye laser excitation coupled with single photon counting detection. Several groups [1,2,3] have found it necessary to use a sum of three exponential decay components (eq. 1) to fit the decay curves of both green plants and algae.
An Analysis of Coherence Effects in Pump-Probe Measurements
Authors: Palfrey S.L. , Heinz T.F. , Eisenthal K.B.
Published: February 2023

When pump-probe measurements are performed using pulses originating from the same laser, it is necessary to consider the coherent interaction between the two pulses in determining any material relaxation rates on the time scale of the laser pulse.1-5 For pump and probe beams crossing at an angle, the source of the coherent artifact can be understood in the following manner.1 When the two beams are simultaneously present in the sample, they interfere to produce a spatial modulation of the dielectric constant. Light from the pump beam then scatters from this induced grating into the probe direction. This model has led to the suggestion that for the collinear, copropagating beams, where the spatial grating disappears, the coherent artifact should vanish.3 While the spatial modulation is eliminated by choosing this geometry, we demonstrate here that the coherent artifact, which results directly from the nonlinear polarizability of the material, still remains. We have, however, been able to develop a general method for isolating the incoherent response, thus permitting the material relaxation to be extracted without any knowledge of the coherence properties of the pulse.
Picosecond Laser Studies of Photoinduced Electron Transfer in Porphyrin-Quinone and Related Model Systems
Authors: Mataga Noboru , Karen Akiya , Okada Tadashi , Sakata Yoshiteru , Misumi Soichi
Published: February 2023

In relation to the mechanisms of photoinduced electron transfer (ET) and charge separation processes in biological photosynthetic systems, investigations on porphyrin-electron acceptor model systems have been made quite extensively.1 Contrary to the biological photosynthetic systems, where ET from excited singlet chlorophyll donors to near-by acceptors takes place on a ps time scale quite efficiently leading to a complete charge separation, the solvated radical ions or ET states produced from the S1 state were not detected even by means of pslaser spectroscopy in the case of intermolecular porphyrin-quinone exciplex systems in strongly polar solvents.
Picosecond Time-Resolved Spectroscopy of Electronically Excited tris (2,2'-bipyridine)Ruthenium(II) Dichloride
Authors: Philips Laura A. , Brown W. T. , Webb S. P. , Yeh Sheila W. , Clark J. H.
Published: February 2023

Transition metal complexes display a rich and varied photochemistry which has found broad application in such areas as solar energy conversion, electron transfer reactions, and lasers.1-5 Although tris(2,2'-bipyridine)-ruthenium(II) dichloride
(Ru(bpy)32+) has been extensively studied,5-14 many details of its excited-state dynamics are as yet unresolved.
Structural and Solvent Effects on the Excited State Dynamics of 3-Hydroxyflavones
Authors: Barbara Paul F. , Strandjord Andrew J.G.
Published: February 2023

The kinetic investigation of intramolecular excited state proton offers an exciting opportunity to probe the role of solvent configurational dynamics in a simple ultrafast chemical processes. One of the most interesting examples of excited state proton is observed for 3-hydroxyflavone (3HF) which is presently being studied by several research groups. In spite of the many recent advances made in this area, a specific, detailed and quantitative understanding of the role of solvent dynamics on the proton, transfer process of 3HF has remained elusive. In an attempt to remedy this situation we undertook a systematic investigation, by picosecond fluorescence spectroscopy, of solvent and structural effects on the photodynamics of 3-hydroxyflavones.
Isomerization Intermediates in the Photochemistry of Stilbenes
Authors: Doany F. E. , Hochstrasser R. M.
Published: February 2023

A series of experiments were carried out to spectroscopically identify intermediates in the trans-to-cis isomerization of stilbenes. The molecule used in these investigations is a fused ring analog of stilbene (A), trans-1,1’-biindanyldene (B). The photophysical properties of this "stiff" stilbene [1-3] are very similar to those of stilbene with the differences accounted for by a smaller barrier (1.5 kcal/mol) to isomerization [1] than in t-stilbene. This low barrier results in an extremely fast (3 ps in hexane) isomerization [2], thus providing an ideal system for investigation of twisted intermediates since it would cause an increased population of short-lived intermediate states compared with molecules with slower isomerization rates.
The Effects of Cavity Dispersion on Femtosecond Mode-Locked Dye Lasers
Authors: De Silvestri S. , Laporta P. , Svelto O.
Published: February 2023

Significant progress has recently been made in the generation of femtosecond dye laser pulses, in particular by means of the colliding pulse technique [1]. Some observed features of the mode-locked (ML) lasers still remain largely unexplained, however. In a few cases, for instance, a strong negative frequency chirp has been experimentally observed [2] and attributed to a phenomenon of self-phase-modulation (SPM) arising from the saturation of the saturable absorber [3]. In other cases, the choice of the cavity mirrors [4] and their layout in the laser cavity [5] have been shown to influence the pulse duration.
The Role of A and A′ States in the Geminate Recombination of Molecular Iodine
Authors: Kelley David F. , Abul-Haj N. Alan , Jang Du-Jeon
Published: February 2023

The geminate recombination dynamics of solution phase molecular iodine have been extensively studied both experimentally and theoretically. These dynamics were first studied directly by Eisenthal et.al.1 using picosecond absorption spectroscopy. In these studies the ground state was depleted by a 530nm, 6ps laser pulse, and its recovery monitored by a 530nm, 6ps probe pulse. Ground state recovery was found to take place on the ~100ps time scale. Several later studies have obtained similar results.2,3 These recovery times were interpreted as time required for the iodine atoms to recombine by a diffusive process. This interpretation assumes that the low lying A(3Π1u) and A′(3Π2u) states are not populated upon recombination, and that ground state vibrational relaxation is quite fast. Although the first of these assumptions has received little attention, the second has been a subject of extensive theoretical studies. These studies have suggested that the observed ~100ps recovery time is due to ground state vibrational relaxation. Both assumptions will be addressed in this paper.
Temporal Dependence of Non-Linear Optical Susceptibilities of Fused Quartz and Liquid CCl4
Authors: Etchepare J. , Grillon G. , Thomazeau I. , Chambaret J.P. , Orszag A.
Published: February 2023

Temporal discriminations between non linear effects involving mechanisms with different characteristic times have been, this last decade, the object of many works on organic liquids, supercooled liquids or plastics. Nevertheless, for materials with one or two orders of magnitude smaller non linearities, still subsists the problem of directly measuring the relative contributions of electronic, nuclear or molecular reorientational origin. We present, here, the kinetics of the optical Kerr effect (OKE), taken on a subpicosecond time scale, in two materials which are of clear interest on this point of view.
Temporal Development of Absorption Spectra in Alkali Halide Crystals Subsequent to Band-Gap Excitation
Authors: Faust W.L. , Williams R.T. , Craig B.B.
Published: February 2023

The electronic states of a semiconducting or of an insulating crystal often are treated as though the lattice were rigid, quite indifferent to the presence of carriers. However, there are well known departures from this picture, in varying degrees. In polar AB crystals, conduction-band electrons have enhanced mass, existing as polarons; and in many halide salts the hole is actually immobilized. Tields associated with an electron in the vicinity of such a hole, a self-trapped exciton (STE), can give rise to substantial displacements of nuclei and even to photochemistry. There are near-unity yields of transient lattice defects which can be recognized in optical absorption or in emission; similar transients are found in alkaline earth halides. Such absorption may cover much of the spectrum, from the near-infrared to the near-ultraviolet. For the alkali halides, there are minor yields of stable or long-lived defects; these populations have been studied for many years by conventional spectroscopic techniques.
Phase Coherent Laser Multiple Pulse Spectroscopy
Authors: Warren W. S. , Banash M. , Loiaza F. , Spano F.
Published: February 2023

Complex multiple pulse sequences have proven extremely valuable in NMR as probes of intermolecular and intramolecular interactions. Optical analogs of many of these sequences can be useful in elucidating the effects of collisional perturbations in gases, guest-guest and guest-host interactions in mixed molecular crystals, or any general perturbations which make the usual two-level approximations invalid.1 But all of these sequences require phase coherence (the phase of each pulse must be specified relative to the first) which was more difficult to achieve in optical spectroscopy than at radiofrequencies.
Modelocking a Ti:LiNbO3-InGaAsP/InP Composite Cavity Laser Using a High-Speed Directional Coupler Switch
Authors: Alferness R. C. , Eisenstein G. , Korotky S. K. , Tucker R. S. , Buhl L. L. , Kaminow I. P. , Veselka J. J.
Published: February 2023

Short, high peak power optical pulses in the wavelength region λ=1.3-1.6μm are potentially attractive for very-high bit-rate lightwave systems and signal processing. InGaAsP/InP lasers, the most widely used semiconductor lasers for this wavelength range, have previously been actively modelocked by current modulating the diode in a passive, extended cavity.3 The achievement of low loss2, high speed titanium-diffused lithium niobate waveguide optical switches has made possible a new approach to active modelocking of semiconductor lasers3. We have fabricated a low-loss, broadband Ti:LiNbO3 waveguide directional coupler switch that is coupled to the AR coated facet of a DC biased InGaAsP/InP diode. By driving the switch with a sinusoidol signal, we have modelocked the composite cavity to produce a train of 22 psec wide (FWHM) pulses at repetition rates as high as 7.2 GHz. This new method of modelocking semiconductors lasers offers important advantages including the potential for combined modelocking/cavity dumping, ring laser geometry and electrically tunable modelocked pulses. In addition, the generated pulses could be multiplexed or encoded with another Ti:LiNbO3 switch/modulator integrated on the same chip.
Cerenkov Radiation from Femtosecond Optical Pulses in Electro-Optic Media
Authors: Auston D. H. , Cheung K. P. , Valdmanis J. A. , Kleinman D. A.
Published: February 2023

It was recently suggested that under suitable conditions, the propagation of femtosecond optical pulses in electro-optic materials should be accompanied by the radiation of an extremely fast electromagnetic transient1,2. This phenomenon, which arises from the inverse electro-optic effect3, produces a Cerenkov cone of pulsed radiation having a duration of approximately one cycle and a frequency in the THz range. Although closely resembling the classical Cerenkov radiation from relativistic charged particles in dielectric media, the physical basis for this effect has some unique distinguishing features. Most important, it is a nonlinear optical effect, arising from a second order nonlinear polarization, and consequently requires a lack of inversion symmetry in the host material. Also, the charge state of the effective source is neutral, being a dipole moment, rather than a point charge. The velocity of the source, however, does move at a velocity which exceeds the radiation velocity due to the additional contribution to the low frequency dielectric permittivity from the infrared lattice vibrations. This is expected to produce a characteristic cone of radiation in the form of a shock wave. Unlike classical Cerenkov radiation, however, the radiation source is spatially extended, being proportional to the intensity envelope of the optical pulse. Consequently the details of the radiation field are expected to depend sensitively on both the duration and beam waist of the optical pulse. A discussion of these effects, based on an analysis of this phenomenon, is given in references 1 and 2.
Ultraviolet Photoemission Studies of Surfaces Using Picosecond Pulses of Coherent XUV Radiation
Authors: Haight R. , Bokor J. , Storz R. H. , Stark J. , Freeman R. R. , Bucksbaum P. H
Published: February 2023

Ultraviolet photoemission spectroscopy provides a powerful tool for studying both the bulk and surface electronic structure of solids. While previous photoemission studies of solids have concentrated on the electronic structure of filled electronic levels, i.e. the valence bands of semiconductors and the filled conduction bands of metals, the extension of this technique to study the dynamic transient processes experienced by electrons excited into otherwise unoccupied states has only begun to be exploited [1]. In this paper we describe an experimental system which utilizes the generation of short pulsewidth coherent vacuum ultraviolet light to perform photoemission studies at solid surfaces and in the bulk. Furthermore, the nature of the short time duration of the light pulses, 10 picoseconds, provides the opportunity to study the transient dynamics of electrons which are excited into unoccupied levels in the solid.
Picosecond Photoemission Study of Laser-Induced Phase Transitions in Silicon
Authors: Malvezzi A. M. , Kurz H. , Bloembergen N.
Published: February 2023

Pulsed laser-induced phase transitions in semiconductors are characterized by an ultrafast energy transfer from the generated electron-hole plasma to the phonons. Time-resolved optical measurements provide ample evidence that the thermal equilibrium between carriers and phonons is established within a few picoseconds [1]. However, optical techniques mainly explore the evolution of plasma density and lattice temperature. Direct information about the energy content of the electron-hole plasma and the energy loss due to phonon interaction cannot be obtained. Picosecond photoemission provides a direct approach to this problem.
Synchronous Mode-Locking of a GaAs/GaAlAs Laser Diode by a Picosecond Optoelectronic Switch
Authors: Kuhl J. , Göbel E. O.
Published: February 2023

The generation of coherent ultrafast light pulses with semiconductor lasers is of considerable interest for scientific as well as practical applications. Active and passive mode-locking has been applied so far to produce optical pulses in the picosecond or even subpicosecond time regime1,2. We report a new technique for synchronous mode-locking of a diode laser and an acousto-optically mode-locked Ar+ ion laser by using a picosecond optoelectronic switch for synchronized excitation of the diode laser. Synchronously mode-locked pulses with 30ps duration (FWHM) are generated at a repetition rate of 80.32 MHz with a buried heterostructure GaAs/GaAlAs double heterostructure laser.
Picosecond Gain Measurements of a GaAlAs Diode Laser*
Authors: Lenth W.
Published: February 2023

The success of high-data-rate optical communication systems relies on the modulation and switching capabilities of semiconductor laser oscillators and amplifiers. The fundamental physical processes which govern the dynamic device performance depend in a complex fashion on the material and design characteristics of diode lasers and are yet not well understood. In this context it is important to study the transient response of the laser gain to rapid changes of the injection current. We have used a new direct gain measurement technique to investigate the time evolution of the diode laser gain after the injection of an 80 ps current pulse.
Picosecond Optoelectronic Studies of Microstrip Dispersion and Solidstate Device Response
Authors: Cooper Donald E.
Published: February 2023

Recent developments in picosecond optoelectronic techniques make it possible to measure the frequency response of solidstate electronic devices at frequencies beyond 60 GHz.1 In this presentation we report on the fabrication of optoelectronic switches for the generation of picosecond electrical pulses, including the proper implantation parameters for various pulse widths and amplitudes. Picosecond pulse propagation on microstrip lines, and the use of the pulses to measure the frequency response of GaAs FETs is also discussed.
Rotational Relaxation in Free and Solvated Rotors
Authors: Bain A. G. , Holt D. , McCarthy P. , Meyers A. B. , Periera M. , Hochstrasser R. M.
Published: February 2023

New results are presented on the rotational relaxation of moleclues using both fluorescence and nonlinear methods. Of special interest are nondiffusive effects and the passage from isolated molecule to condensed matter behavior.
Picosecond and Subpicosecond Laser Chemistry in Molecular Beams
Authors: Zewail Ahmed H.
Published: February 2023

Recent advances in the use of picosecond and subpicosecond laser pulses to study the dynamics of recoil processes of fragmentation in chemical reactions are reviewed. Particular emphasis is on elementary chemical reactions studied in collisionless conditions in molecular beams.
New Optical Design for A Jet Amplifier
Authors: Seddiki O. , Goddi A. , Mounet R. , Morhange J. F. , Hirlimann C.
Published: February 2023

The possibility to amplify light pulses in the femtosecond regime, using a gain jet, in a multipass geometry has recently been demonstrated (1, 2). We propose a very simple, compact, all reflective optical design for a multipass amplifier which minimizes group velocity dispersion.
Femtosecond Carrier Relaxation In Semiconductor-Doped Glasses
Authors: Nuss M.C. , Zinth W. , Kaiser W.
Published: February 2023

Semiconductor-doped glasses, incorporating small CdSxSe1–x or CdSexTe1–x crystallites in a glass matrix, show strong nonlinear absorption1 and large values of the nonlinear optical susceptibility χ8 have been reported in recent degenerate-four-wave-mixing experiments2. This nonlinearity has been attributed to the generation of a dense electron-hole plasma in the semiconductor crystallites.
Detection of Higher Order Fourier Components of Index Gratings in Picosecond Transient Grating Experiments
Authors: Saito H. , Göbel E.O.
Published: February 2023

In transient induced grating experiments the decay of an intensity or phase grating produced by interference of two coherent light pulses in e.g. solids or liquids is monitored by diffraction of a third probe pulse. Generally, the intensity of the first order diffracted beam is monitored as a function of time delay between the excitation pulses and the probe pulse to provide information on the dynamics of diffusion and recombination of the photoexcited species /1/. If the two interfering excitation pulses are additionally delayed with respect to each other phase relaxation processes can also be studied /2/. We report a further extension of this technique, which allows the study of the temporal variation of the spatial shape of an originally sinusoidal index grating by simultaneous detection of the decay of different diffraction orders.
Measured Nonlinear Optical Response of a Polydiacetylene on the Picosecond and Femtosecond Time Scale
Authors: Carter G.M. , Thakur M.K. , Chen Y.J. , Hryniewicz J.V. , Meyler S.E.
Published: February 2023

The large measured nonresonant third order nonlinear optical susceptibility, χ(3), of the organic polymeric system the polydiacetylenes has been attributed to their delocalized one-dimensional electronic states[1]. However, the lack of dynamical data has prevented the determination of the effect of the strong electron-phonon coupling in these materials on excited state life-times [1]. Further such coupling could slow the off-resonant nonlinear response (i.e. the change of index of refraction due to χ(3)) from its "instantaneous" (due to adiabatic following) value [1], which in principle could limit the suggested use of these materials in ultra-fast optical signal processing schemes based on χ(3) [2]. In the following we describe the first experimental determination of some of these dynamical effects.
Ultrafast Carrier Dynamics in GaAs
Authors: Lin W. Z. , Fujimoto J. G. , Ippen E. P. , Logan R. A.
Published: February 2023

The dynamics of excited carriers in GaAs are relevant to both electronic and optoelectronic devices. Recent advances in the generation of ultrashort optical pulses permit the investigation of these processes on a femtosecond time scale[1-3]. In this paper we describe investigations of femtosecond absorption recovery dynamics in GaAs using pulses as short as 35 fs generated by a colliding pulse modelocked (CPM) dye laser[4], Pump probe measurements reveal the presence of an initial carrier relaxation process which occurs on a time scale of less than 30 fs.
Detergent effects upon the picosecond dynamics of higher plant light harvesting chlorophyll complex (LHC).
Authors: Ide J. P. , Klug D. R. , Kuhlbrandt W. , Porter. G.
Published: February 2023

The light-harvesting chlorophyll a/b protein complex (LHC) has been shown to form crystalline arrays in vitro. These arrays have P321 symmetry. The unit cell is composed of trimeric protein oligomers arranged with a 3-fold symmetry axis running centrally through each complex[1]. Each polypeptide has an apparent molecular weight of 25–27kd within which are bound 6-11 chlorophyll molecules.
Picosecond Studies of Barrierless Torsional Diffusion
Authors: Ben-Amotz Dor , Harris C. B.
Published: February 2023

The large amplitude motion of a solute molecule can be viewed as one of the simplest liquid phase chemical reactions. The dynamics of such a process is dictated by the shape of the solute's potential energy surface and its coupling to the stochastic thermal forces of the solvent bath. Numerous systems which undergo large amplitude motion involving barrier crossing have been studied over the past several years. Few systems whose reaction coordinate is barrierless have so far been investigated. The goal of the present experiments is to test the validity of current theories for barrierless liquid phase chemical processes.
Picosecond Addition and β-Scission Reactions of Thiyl Radicals with Unsaturated Hydrocarbons
Authors: Scott T. W.
Published: February 2023

It is well known that free radical reactions exhibit extreme product diversity through the action of a limited number of elementary steps. The most facile elementary reaction between sulfur centered thiyl radicals and hydrocarbon liquids is free radical addition to carbon-carbon double bonds (1). This is a rapid exothermic reaction which is generally reversible.
Fiber Raman Amplification Soliton Laser (FRASL)
Authors: Islam M.N. , Mollenauer L. F.
Published: February 2023

We have demonstrated the first Fiber Raman Amplification Soliton Laser (FRASL) by synchronously pumping a loop of fiber with ≈10 psec pulses from a modelocked color center laser. The FRASL output pulses are determined by pulse compression and soliton pulse shaping in the fiber, and have to date been as short as 240 fsec. The lasing wavelength can be time dispersion tuned[1]. The single cavity FRASL is a much simpler configuration than the color center soliton laser[2], which consists of two coupled cavities and requires feedback stabilization to overcome interferometric instabilities.
The Observation Of Chirped Stimulated Raman Scattered Light In Fibers
Authors: Johnson A. M. , Stolen R. H. , Simpson W. M.
Published: February 2023

Stimulated Raman scattering (SRS) limits the maximum achievable power in the fiber-grating compression of optical pulses [1]. We have recently experimentally investigated SRS in single-mode fibers in the regime of large Raman Stokes pulse walk off from the pump pulse by the effect of group-velocity dispersion (GVD) [2]. We found that the Raman Stokes pulse was produced within the first 3-4 walkoff lengths, where a walkoff length is defined as the distance in which the Stokes signal passes through one pump pulse width via the action of GVD. For a 21 m length of fiber (3.4 walkoff lengths) we were surprised to find that the Raman pulse was generated with a strong frequency chirp. This is not the first observation of chirped SRS in a fiber [3], but it is the first observation of chirped SRS in an experimental configuration simple enough that there is hope for a relatively straightforward understanding of the physical mechanisms involved. The distinctive features of the present results are that over this short length of fiber, subsequent modifications of the Raman pulse by self­phase modulation (SPM) or GVD are minimal and the pump pulse is only minimally chirped. The wings of a spectrally broadened pump pulse are thus not the source of the chirping of the Raman pulse.
Kinetics of Energy Relaxation and Localization in II-VI Compound Semiconductor Quantum Well Structures
Authors: Hefetz Y. , Goltsos W.C. , Lee D. , Nurmikko A.V.
Published: February 2023

With emerging new semiconductor superlattice structures it is important to evaluate their characteristics with emphasis on interfacial quality, particularly in the limit of very thin constituent layers (< 100 A). We report here on transient luminescence studies of new II-VI compound semiconductors quantum well structures, with two examples based on the ZnSe/(Zn,Mn)Se and CdTe/ZnTe heterostructures (1). In this work we have employed a tunable high repetition rate picosecond source in the blue and red regions of the spectrum. An optimum compromise for temporal broadening in the spectrosopic measurements was reached by using a spectrometer with a pair of gratings in the subtractive mode, enjoined with a Hamamatsu synchroscan streak camera. In this way a temporal/spectral resolution of approximately of 10 psec/5 A was achieved. As a consequence, time-resolved spectra from the quantum well structures has yielded direct information of key energy relaxation and localization processes of excitons.
Propagation of Coherent Phonon Polaritons in LiTaO3 Measured By Fir-Cerenkov Pulses
Authors: Nuss M. C. , Auston D. H.
Published: February 2023

We have previously shown that optical rectification of femtosecond laser pulses in electro-optic materials produces a Cerenkov cone of an extremely short electromagnetic pulse with a duration of ≃ 300 fsec and a frequency spectrum extending up to 6 THz1. In ionic crystals the far infrared pulse is strongly coupled to the transverse optical lattice modes, thus propagating as a polariton, i.e. a coupled phonon-photon excitation. The coherent polariton can be traced in time and space as it travels away from the generating optical femtosecond pulse.
Superheating During Ultrafast Laser Heating of Semiconductors
Authors: von der Linde D. , Fabricius N. , Danielzik B. , Hermes P.
Published: February 2023

When a solid is heated up and melted the rate of evolution of the solidliquid phase transition may be limited by the kinetics of the atomic rearrangement connected with the transition if the heating is sufficiently fast. In such a situation the solid may be driven into a highly superheated metastable state characterized by a temperature greater than the equilibrium melt temperature. Strong superheating with excess temperatures comparable with the melting temperature may be expected to occur during heating and melting on a picosecond or femtosecond time scale.
Observation of 7.2 THz Beats between the D-Lines of Atomic Rb
Authors: Golub J. E. , Mossberg T. W.
Published: February 2023

Over the past few years, dramatic advances in short pulse technology have pushed studies of dynamical processes to ever shorter time scales. Unfortunately, short pulse technology remains beyond the means of many investigators. A simple and clever alternative approach to achieving high temporal resolution was recently proposed by several groups1−4. In this approach, time resolution is limited only by the coherence time of the excitation light, rather than by its duration. Since laser light of short coherence time (i.e. large bandwidth) is much more easily generated than the corresponding short pulse, this result is of great importance.
Dispersion Compensated Fiber Raman Oscillator
Authors: Kafka J. D. , Head D.F. , Baer T.
Published: February 2023

We have generated subpicosecond pulses from a synchronously pumped fiber Raman ring laser with a dispersion compensating delay line. The ring oscillator consists of 100 M of single mode fiber, a grating pair dispersive delay line, and an interference filter as a tuning element.(Fig. 1) The pump source is a CW modelocked Nd:YAG laser emitting 80 psec pulses at 1064 nm. We observe pulses at the the first Raman stokes band (1100 nm) with pulsewidths as short as 0.8 psec. (Fig. 2)
Fast Sampling of Atomic Velocities by Incoherent Light Stimulated Photon Echoes
Authors: Defour M. , Keller J-C. , Le Gouët J-L.
Published: February 2023

It has been recently pointed out that incoherent light can be used to generate coherent optical transients and that there are decided advantages in using such broad-band non-laser source to achieve very high time resolution {1,2}. When a light pulse and its time-delayed replica interact with a sample to produce the non-linear optical response, the ultimate time resolution should identify with the correlation time τC of the light field rather than with the pulse duration τL. The situation where τC ≪ τL is easily obtained with broad-band incoherent light sources. Time resolution in the femtosecond range is then attainable using nanosecond light pulses. This has been recently demonstrated through the observation of the relaxation of photon echoes in Nd3+-doped silicate glass {1,2}. In these cases, attention is focused on the measurement of fast homogeneous decay rates and the inhomogeneous broadening ΩD is assumed to be much larger than the spectral width of the excitation light.
“Third-Order Non-Linear Optical Interactions in Thin Films of Organic Polymers Investigated by Picosecond and Subpicosecond Degenerative Four Wave Mixing"
Authors: Prasad P. N. , Rao D. Narayana , Swiatkiewicz J. , Chopra P. , Ghoshal S.K.
Published: February 2023

This paper presents experimental measurements and theoretical analysis of the third-order optical susceptibility measurements by degenerate four wave mixing in several classes of polymeric films which have conjugated π-electron structure. A high χ(3) value derived from the π-electron conjugation effect is observed and a subpicosecond response is confirmed. The first group is a conjugated aromatic hetrocyclic polymer, poly-p-phenylenebenzobisthiozole (PBT) which has a very high mechanical strength due to its rigid rod conformation as well as environmental stability and a high laser damage threshold. The χ(3) measurement in a 33 μM thick biaxial film of PBT yields a value about an order of magnitude larger than that for CS2. The measurements at two wave lenghts suggests that they are non­resonant χ(3) values. The measured anisotropy of χ(3) as a function of angular orientation at two different sets of laser polarization is explained by using the tensor properties of χ(3) in on anisotropic medium.
Ultrafast electron-transfer processes in photosynthetic reaction centers. Photon-echo and holeburning studies of optical excitations in Rhodopseudomonas viridis and sphaeroides
Authors: Meech S.R. , Hoff A.J. , Wiersma D.A.
Published: February 2023

One of the most intriguing and fundamental processes on our planet is photosynthesis, whereby light is converted into chemical energy. The first step in the chain of events is known to involve a charge-separation reaction in which a specialized chlorophyll (Chl) or bacteriochlorophyll (BChl) pigment-complex reduces a nearby pheophytin (Ph) molecule. In subsequent reactions the electron moves along a chain of acceptors and finally becomes stabilized such that the remaining potential energy can be used in metabolic reactions. It is obvious that these fundamental electron-transfer reactions have been an ongoing challenge to spectroscopists. Since the functioning pigment-protein complex was isolated in pure form (1) much progress has been made towards elucidating the architecture of the pigment complex and the optical dynamics of the system. Very recently these investigations were greatly stimulated by an X-ray structural determination of the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis (2). One of the most interesting findings was that the pigments spatial arrangement was such that it contained a quasi C2-axis with a dimer at one end. It was known from previous time-resolved experiments that this dimer played a crucial role in the initial events of the charge separation. It is apparent that with the structure at hand one can ask very specific questions about the functioning of the reaction centre, such as: why is there a dimer; what is the role of the intermediate chlorophyll and why does the reaction only proceed along one branch of the pseudo-symmetric pigment complex?
Parametric Amplification Sampling Spectroscopy (Pass): A New Technique For Resolving Near Infrared Luminescence at Subpicosecond Time Scale.
Authors: Hulin D. , Migus A. , Antonetti A. , Ledoux I. , Zyss J.
Published: February 2023

In these last years optical fiber communication requirements have arised considerable interest in materials for optical devices operating in the 1.3 - 1.6μm spectral region. Time resolved spectroscopy is a fundamental tool for studying such material responses. We report here a new method, Parametric Amplification Sampling Spectroscopy (PASS), for resolving weak near infrared emission with a subpicosecond accuracy. We have applied this scheme to measure the ultrashort luminescence lifetime of an infrared dye and electronic transport properties of multiple quantum well structures (MQWS).
Time-resolved Photoluminescence Measurements in AlxGa1−xAs Under Intense Picosecond Excitation
Authors: Bohnert K. , Kalt H. , Boggess Thomas F. , Smirl Arthur L. , Loo R. Y.
Published: February 2023

The photoluminescence of the ternary semiconductor AlxGa1−xAs under intense picosecond excitation is investigated spectrally and temporal1y resolved. The experiments are performed at room temperature and 15 K in samples with direct and indirect gaps. From an analysis of the spectra, the dynamics and nature of the nonequilibrium carrier recombination processes are determined as well as band gap renormalization due to exchange and correlation effects.
Time-Resolved Measurement of Laser-Induced Desorption of a Molecular Monolayer
Authors: Arjavalingam G. , Heinz T.F. , Glownia J.H.
Published: February 2023

Recently considerable interest has developed in the use of intense laser radiation to desorb molecular monolayers at surfaces. In addition to the importance of this effect for fundamental investigations of energy flow in absorbed molecules, laser-induced desorption has been applied to the study of surface reactions and surface diffusion.1 To date, however, no direct measurements of the dynamics of the process of laser-induced desorption have been performed. In this paper, we present experimental results on the desorption of a monolayer of dye molecules monitored on a picosecond time scale by means of surface second-harmonic generation (SHG).2
Broadband Ultrafast Absorption Spectroscopy in the Hard Ultraviolet: Evolution of the CF2 Radical Upon Photodissociation of CF2Br2
Authors: Gosnell T. R. , Taylor A. J. , Lyman J. L.
Published: February 2023

Time-resolved absorption spectroscopy is often rejected in favor of laser-induced fluorescence spectroscopy as a viable technique for the investigation of ultrafast photochemical phenomena primarily because the method does not allow zero-background detection. Photoproduct concentrations produced by the low-energy pump pulses normally available from ultrafast laser oscillators and amplifiers are simply too small to show strong absorption signals against a large background. Nevertheless, the possibility that absorption techniques offer advantages complementary to those of other methods has motivated us to reexamine the absorption technique. These advantages include the ability to more easily determine absolute photoproduct yields, the ability to obtain broadband spectra using a probing supercontinuum, and the ability to detect intermediate species whose lifetimes are short compared with their fluorescence lifetimes. These advantages can be purchased at the cost of a laser system able to deliver high-energy ultrashort pulses (> 1 mJ), and by requiring the photochemical reaction under investigation to produce photoproducts with large absorption cross sections (> 10-18 cm2) at high quantum yield (~1). Misewich et al.,1 exactly so, have satisfied these demanding requirements and obtained femtosecond near-uv spectra of Tl atoms produced by photolysis of thallium halides at 308 nm.
Femtosecond Transient Absorption Spectroscopy of Photochemical Reactions in Solution
Authors: Joly Alan G. , Nelson Keith A.
Published: February 2023

The availability of sub-100 femtosecond optical pulses has spawned much research directed at understanding chemical reaction dynamics in condensed phases. While for a small molecule in the gas phase the potential energy surface determines uniquely the dynamics of a chemical reaction, this is not true in condensed phases. Energy activation and dissipation through solvent-solute collisions as well as solvent induced barrier crossings and recrossings destroy the one-to-one correspondence between potential energy surface and reaction dynamics.
Subpicosecond Infrared Spectroscopy: the Condensed Phase Photochemistry of Iron Carbonyls
Authors: Anfinrud Philip A. , Han Chui-Hee , Lian Tianquan , Hochstrasser Robin M.
Published: February 2023

The condensed phase photochemistry of transition metal carbonyls has been investigated by various techniques in recent years. It is anticipated that such studies will contribute to our understanding of coordinatively unsaturated organometallic complexes and their role in homogeneous catalysis. IR characterization of photogenerated intermediates in isolated matrices has helped to identify photochemical reaction pathways, however, the frozen matrix imposes steric restrictions on pathways which may be important in the liquid phase at room or elevated temperatures. Transient IR methods offering microsecond time resolution have made an important leap in characterizing those reactive intermediates whose lifetimes are determined by diffusion limited bimolecular collisions. Characterization of the primary photochemical events and their products, however, requires significantly improved time resolution. Picosecond transient absorption methods which probe the uv/visible absorption spectrum offer the potential to temporally resolve the ultrafast events leading to the primary photoproducts, however the electronic spectrum is quite complex and generally does not contain features which are structure specific. Clearly, ultrafast IR methods can shed much light on the primary events in condensed phase photochemical processes.
Picosecond Polarization Studies of Protein Relaxation
Authors: Xie Xiaoliang , Dunn Robert , Simon John D.
Published: February 2023

Picosecond polarization spectroscopies have been used to probe relaxation processes in heme proteins induced by ligand photodissociation. In particular, probing the N-band of myoglobin at 355 nm reveals a relaxation of several hundreds of picoseconds, over two orders of magnitude longer than the dynamics of photo-induced bond cleavage. Similar dynamics are also observed for studies in the Soret region. Time resolved linear dichroism studies confirm that the evolution of the CD signal does not arise from a change in the direction or degeneracies of the two degenerate, perpendicular transition moments of the heme. In addition, the time dependent spectral properties of the charge transfer absorption between the iron and the proximal histidine which results from photodissociation does not reveal any noticeable spectral shift on the time scale of the CD dynamics, strongly suggesting that the tilting of the proximal histidine is not causing the time dependent changes in the CD signal. These results suggest that the transient CD data is measuring a relaxation in the surrounding protein structure which is triggered by the photodissociation process.