bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrated atomic force microscope and ultrafast sampling probe
Authors: Nees John A. , Wakana Shinichi , Chen Chun-Ying
Published: February 2023

The reduction of line dimensions in integrated circuits and the increase in the device speeds has led to the need for high-speed probes with sub-micron spatial resolution. This need is addressed by combining Atomic Force Microscopy (AFM), with its sub-nanometer spatial resolution, and photoconductive sampling, with single picosecond temporal resolution.[1]
Interferometric measurements of femtosecond laser cavity dispersion
Authors: Naganuma K.
Published: February 2023

Wavelength dispersion measurement method has been drawing much attention as an indispensable tool for femtosecond laser development. For individual optical element, these methods have already been established using interferometers.1–3
Tunable Femtosecond sources and Optical Parametric Generators
Authors: Salin François , Estable Frederick
Published: February 2023

Sources of tunable, energetic, femtosecond pulses are of substantial interest to a variety of scientific communities. Previously, femtosecond pulses in the microjoule range were only possible through complex dye amplifier schemes. With the introduction of self-modelocked Ti:sapphire oscillators, tunable sub-100 fs pulses became available although most of the experiments ask for spectral ranges not reachable by Ti:sapphire oscillators alone. In addition to generating short pulses, Ti:Al2O3 has also been shown to be an outstanding amplifier for producing ultra-high peak power pulses. Although energies up to 1 J have been produced, the maximum repetition rate of these amplifiers is typically on the order of 10 Hz. This low repetition rate prevents the use of powerful spectroscopy techniques such as heterodyne or phase sensitive detection. We previously demonstrated multi-kHz amplification of femtosecond pulses1 wich was a first step toward the production of tunable high power pulses tunable over the whole optical spectrum. We present here a femtosecond source producing microjoules pulses tunable from 250 nm to 2 μm.
Coherent Control? Panel Speakers
Authors: Wilson Kent , Warren Warren , Nelson Keith
Published: February 2023

No summary available
Ultra-wideband Free-Space Electro-Optic Sampling
Authors: Wu Q , Litz M. , Zhang X.-C.
Published: February 2023

In this presentation, we report the broadband capability of collinear electro-optic sampling for the characterization of freely-propagating pulsed electromagnetic radiation with gigahertz to terahertz bandwidth.
Multi-Dimensional Femtosecond Pulse Shaping and Spectroscopy
Authors: Wefers Marc M. , Nelson Keith A. , Weiner Andrew M.
Published: February 2023

There has been considerable attention recently regarding the shaping of ultrashort optical pulses into waveforms with arbitrary temporal profiles [1-3]. Applications to ultrafast optical switching [2] and coherent control of quantum mechanical systems have been demonstrated [3] and further results can be anticipated. Optical processing of shaped waveforms and their interconversion to and from spatial images have also been reported [4,5]. More recently, ultrafast time-varying two-dimensional spatial images have been generated using volume holographic techniques [6]. Here we demonstrate combined temporal and spatial shaping of ultrashort optical pulses based on linear spectral filtering of spatially separated frequency components [1-3]. This permits generation of coherent high-fidelity ultrashort optical waveforms with tailored profiles along both the propagation direction, i.e. the temporal coordinate, and one perpendicular direction, i.e. one spatial coordinate. Temporal and spatial pulse shaping offers novel possibilties for manipulation of propagating excitations and other non-local material responses, and permits a very large multiplex advantage in terahertz-bandwidth optical signal generation.
Reduction of Multiphoton Ionization in Dielectrics due to Collisions
Authors: Du D. , Liu X. , Mourou G.
Published: February 2023

The multiphoton ionization of atoms and molecules has been extensively studied, both experimentally and theoretically, ever since its discovery. The recently discovered phenomena such as ATI and transient resonance have enriched the understanding of laser-matter interaction on the atomic scale[1]. However, most of the studies on the MPI were done with low pressure gases, where inter-atomic distance is larger than the electron excursion amplitude, and hence the collision effect is negligible.
Charge-Transfer Dynamics in Blue Copper Proteins: Experiment and Simulation
Authors: Scherer N.F. , Ungar L.W. , Arnett D.C. , Book L.D. , Hu H. , Voth G.A.
Published: February 2023

Blue copper proteins function as mobile electron carriers in biological systems by transfering electrons to and from their "type I" copper active sites liganded to the protein matrix.1 In reduced form, these active sites have a strong ligand-to-metal charge transfer transition between the copper atom and a cysteine sulfur ligand in the region of 595-630 nm, which gives the proteins their characterisitic blue color.2 This strong absorption makes blue copper proteins suitable for ultrafast spectroscopic studies of protein electron transfer. Elucidation of electronic and nuclear dynamics of these systems requires classical and quantum simulations in conjunction with experiment. The resultant spectral density describing the optically induced charge transfer process may be useful in understanding the long range electron transfer of physiological function.
Ultrafast Photoisomerization of Retinal in Bacteriorhodopsin: A New Twist
Authors: Gai Feng , Hasson Kenton C. , Anfinrud Philip A.
Published: February 2023

Bacteriorhodopsin (BR) is a membrane protein which converts light energy to chemical energy by pumping protons unidirectionally across the membrane. The driving force for pumping protons is derived from the photoisomerization of all-trans retinal to 13-cis retinal, the quantum yield of which is reported to be approximately 0.61. The quantum yield appears to be independent of temperature and is minimally affected by mutations to numerous neighboring residues in the protein. Previous studies have suggested that photoexcited BR relaxes in 0.5 ps to J which in turn relaxes in 3 ps to K2. The long-lived intermediate K has been identified as the 13-cis isomer of retinal3. The relative constancy of the quantum yield for the formation of K is not well understood, nor are the differences between J and K. To investigate the primary photoprocesses of BR as well as the role of the protein in mediating the photoisomerization of retinal, we measured time-resolved absorbance spectra over a broad spectral range with high sensitivity.
Frequency-resolved optical grating using third-harmonic generation
Authors: Tsang Thomas , Krumbügel Marco A. , Delong Kenneth W. , Fittinghoff David N. , Trebino Rick
Published: February 2023

Frequency-resolved optical grating (FROG) is a techique for measuring the full time-dependent intensity and phase of an arbitrary ultrashort optical pulse.1-2 Unlike the polarization-gated (PG) method, FROG using second-harmonic generation (SHG) has a time-reversal ambiguity. The envelope of the SHG FROG signal field is given by E
SHG
−
sig
(t, τ) ∝E(t)E(t − τ) and is always symmetric in the time delay τ, therefore the resulting spectrograms are somewhat unintuitive. Nevertheless, because of the simplicity of the SHG FROG, it becomes the most common technique for detail characterization of ultrashort optical pulses of laser oscillators. We extend this FROG technique with the use of third-harmonic generation (THG FROG), where the signal field is now given by E
THG
−
sig
(t, τ) ∝E2(t)E(t − τ) which has no time ambiguity. Therefore the resulting THG FROG trace is as appealing as the PG FROG. Although THG is dipole-allowed, the coherent length in the bulk of a material is short therefore THG in the bulk is often too weak to be measured. But THG is much enhanced at an interface3 giving adequate THG photons for pulse characterization of laser oscillators.
13fs Frequency-Resolved Optical Gating Measurements with Thin Poled Nonlinear Polymers
Authors: Yankelevich D.R. , Knoesen A. , Taft G. , Murnane M.M. , Kapteyn H.C. , Twieg R. J.
Published: February 2023

We use the technique of Frequency Resolved Optical Gating (FROG) [1] to measure the amplitude and phase of an ultrashort pulse.[2] Although there are several variations of the FROG technique which use different nonlinear processes, second harmonic generation (SHG) FROG is particularly well suited for measuring unamplified nJ pulses directly from a modelocked Ti:sapphire oscillator, since SHG produces a much larger signal than the higher order nonlinear processes. Experimentally, SHG FROG is similar to a second-order intensity autocorrelation, except that the second harmonic spectrum is measured as a function of time.
Femtosecond IR Studies of Alkane C-H Bond Activation by Organometallic Compounds
Authors: Lian T. , Bromberg S. E. , Yang H. , Asplund M. , Bergman R. G. , Harris C. B.
Published: February 2023

The mechanism of alkane C-H bond activation by transition metal complexes such as CpM(CO)2 (M=Rh, Ir) has been intensely studied because it represents a first step in a catalytic process using unreactive hydrocarbons.[1] The bond activation reaction starts with the formation of monocarbonyl intermediates such as CpRh(CO). These species have been detected in the gas phase[2] and in liquefied rare Kr and Xe[3] by µs time resolved IR spectroscopy. Unfortunately, the subsequent oxidative insertion of CpRh(CO) into the C-H bond is not well understood due to its rapid rate and low quantum yield (~1%) for formation of the C-H activated product. These properties have hindered previous femtosecond and picosecond time-resolved studies of activation reaction in room temperature alkane solution. [4]
Wave-packet dynamics in epitaxial polydiacetylene thin films
Authors: Pham T.-A. , Daunois A. , Halté V. , Le Moigne J. , Bigot J.-Y.
Published: February 2023

Polydiacetylenes (PDA) are model materials as quasi one-dimensional semiconductors. Their optical properties, which have been extensively investigated in the past years, display large nonlinearities associated to the π electron delocalization along the backbone [1]. The dynamics of the PDA excitons has been studied under various experimental configurations, and has allowed to determine the relaxation associated to the exciton self-trapping mechanism [2-4]. The dephasing properties of the excitons has also been measured using incoherent wave-mixing [5] and coherent photon echo techniques [6]. A determining mechanism for the coherent relaxation processes in PDA systems is the influence of the molecular motion on the electronic delocalization. The most important contribution comes from the vibrations of the backbone which are strongly coupled to the excitonic wavefunction. Recently it has been shown that quantum beats with 23 fs period, associated to the C=C vibration modes, appear during the polarization dephasing process [6]. The aim of the present paper is to study the wave-packet dynamics associated to this backbone coherent motion, using ultrashort optical pulses. The wave-packet motion is probed in the pump-probe configuration as reported in the case of molecular systems [7].
Enhancement of Degenerate Four Wave Mixing from a Conjugated Polymer by Femtosecond Optical Pumping
Authors: Sakaguchi Hiroshi , Nagamura Toshihiko
Published: February 2023

Organic compounds have been attracted much attention as nonlinear optical materials because these provide the large nonlinear susceptibility and the ultrafast response by the use of the electronic transition rather than inorganics. Although organic compounds possess a large susceptibility compared to the inorganics, nonlinear susceptibility from these reported so far was two or three orders of magnitude less than the criterion for a practical use. Thus the enhancement of optical nonlinearity for organic compounds is seriously expected to fulfill the future phtonic applications. One of the most interesting techniques to control and enhance the nonlinear optical properties of organic compounds is to exploit the optically pumped excited states.1-4 In the present paper, one-dimensional conjugated polymer which exhibits a large optical nonlinearity in the ground state was used to investigate the optical pumping effect on the degenerate four wave mixing, third order nonlinear optical process. Femtosecond optical pumping of conjugated polymer could produce a significant enhancement of the third order nonlinearity.
UV pulse assisted ultrashort laser pulse induced breakdown in fused silica
Authors: Tien A.-C. , Liu X. , Mourou G.
Published: February 2023

Laser induced breakdown by ultrashort pulses in optically transparent materials has become increasingly interesting. As has been shown[1], avalanche ionization is the dominant mechanism for laser induced breakdown in such materials for pulsewidth down to ~ 100 fs, and multiphoton ionization is responsible for the initial electron generation. The breakdown threshold for the avalanche process is a function of the initial free electron density n0. By controlling the initial seed electron density, one may vary the breakdown threshold value. We have performed a pump-probe experiment on fused silica (SiO2) to test this hypothesis. A UV “pump” pulse, illuminating the same location, generates seed electrons for the later-arrived IR pulse by two-photon absorption. By varying the delay between the two pulses so that the UV pulse arrives before and after the IR pulse, we can observe the difference in the breakdown thresholds.
Time-Resolved Two-Colour Photoelectron Spectroscopy of Clean and Adsorbate Covered Metal Surfaces
Authors: Hertel T. , Knoesel E. , Wolf M. , Ertl G.
Published: February 2023

The dynamics of photoexcited electrons and their interaction with adsorbates is of fundamental importance for many photostimulated reactions at metal surfaces. Such reactions are frequently mediated by 'hot' substrate electrons and the corresponding reaction cross sections will, therefore, depend critically on the charge carrier dynamics in the near surface region. On the other hand the perturbation of the local electronic structure by an adsorbate may allow to alter lifetimes of electronically excited states at surfaces.
Hot-electron dynamics at Cu surfaces
Authors: Ogawa S. , Petek H.
Published: February 2023

Dynamics of electrons in solids is important to many phenomena such as optical, electrical, magnetical, and chemical properties of matter. Direct measurements of electron-electron (e-e) scattering rates provide critical tests for many body theories. Recent developments in ultrafast laser technology make it possible to probe directly femtosecond phenomena such as e-e scattering in metals. As a consequence of a large cross section for electron scattering hot-electon lifetimes in metals are in femtosecond regime, and until recently the scattering rate could only be evaluated by indirect measurements of heat and electrical transport properties. Two-photon time-resolved photoemission (TPTRP) measures the direct change of hot-electron population as a function of time. So far, this technique has been applied to direct measurement of electron relaxation by electron-phonon scattering in polycrystalline Au[1], hot electron thermalization by e-e scattering in Cu(100)[2], and image potential state decay on Ag(100) and Ag(111) surfaces[3]. With higher time resolution(<10fs) we have been able to resolve coherent component in hot-electron decay due to optical dephasing from the slower population decay rates of hot-electrons at Cu(100) and Cu(110) surfaces[4]. Here we report the results on Cu(100), Cu(110) and Cu(111) surfaces and compare them with Fermi liquid theory.
Strong Instantaneous Contribution in Femtosecond Degenerate Four–Wave Mixing from 350 μm InP due to Virtual Excitation
Authors: Yu Sungkyu , Chu Jang Hee , Lee Joo In , Kim Dongho , Yee Yonghwan , Kim Dai-Sik , Lee Jong Hyun
Published: February 2023

Femtosecond degenerate four-wave mixing (FWM) is a powerful technique for studying the ultrafast nonlinearities and the fast relaxation processes of the optical phase coherence in semiconductors. [1] Up to now, most experiments have been performed at the excitonic resonance for multiple quantum well structures or thin epilayers, [2]
A Comparative Study of the Initial Photoinduced Event in Bacteriorhodopsin: Can it be Isomerization?
Authors: Zhong Q. , Ruhman S. , Ottolenghi M. , Sheves M. , Friedman N. , Atkinson G.H. , Delaney J.K.
Published: February 2023

Rhodopsins are a family of photoactivated 7 α-helix membrane proteins which play central roles in visual-transduction and bacterial photosynthesis. They share the same chromophore: A retinal polyene bound to the protein via a protonated Schiff base linkage. A comprehensive description of the molecular mechanism of light energy conversion in rhodopsins requires a detailed understanding of the primary photophysical events.
Electro-Optic Imaging of THz Beams
Authors: Zhang X.-C. , Wu Q. , Hewitt T.D.
Published: February 2023

We demonstrate electro-optic imaging of THz beams.
Photophysics and Photochemistry of the Solvated Electron in Higher Alcohols and Saturation Experiments on the Hydrated Electron
Authors: Silva C. , Barbara P. F.
Published: February 2023

We report the first direct femtosecond infrared pump-probe experiments in higher alcohols on the s→p transition of the equilibrated solvated electron e
S
(itself, a transient species). We employ a three laser pulse sequence (previously used for the hydrated electron),1,2 where a UV synthesis pulse generates excess electrons and, after a few nanosecond delay, an NIR pump pulse promotes the equilibrated ground-state electron to the excited p-state, with a tunable probe pulse monitoring subsequent spectral dynamics. Representative pump-probe absorption transients of solvated electrons in water and various alcohols are shown (Fig. 1-2). Transients obtained probing on the blue edge of the ground state absorption band (600 – 740 nm) show a fast, instrument-limited reduction in optical density (bleach) followed by a bleach recovery that occurs on two timescales, having both a fast, sub-picosecond component and a longer, > 1 ps component. Transients probing on the red edge of the ground state absorption band (950 nm) show a transient increase in optical density. Finally, transients probing in the intermediate spectral region (860 nm) show complicated non-monotonic behavior. The recovery of the initial bleach “overshoots” to yield a transient increased absorption, which decays on a picosecond timescale. The non-monotonic behavior of these transients indicates that other processes besides electronic relaxation, namely solvation dynamics, play a role in the observed spectral dynamics. These data contain information on the p-state to s-state internal conversion mechanism as well as the energy disposal and relaxation dynamics of the solvent molecules that are coupled to the electron. It is important to distinguish the three laser pulse experiment from the complimentary two-pulse approach of previous femtosecond experimental work on the solvated electron which involved an ultraviolet (UV) pump pulse to produce photoelectrons in the conduction band and a probe pulse to monitor the spectral dynamics due to trapping and subsequent relaxation.
Femtosecond Studies of Exciton Dynamics in a Novel Optically Active Conjugated Polymer
Authors: Zhang J. Z. , Kreger M. A. , Goka E. H. , Pub L. , Hub Q.-S. , Vitharana D. , Rothberg L. J.
Published: February 2023

Conjugated polymers have unique electronic and optical properties and potential applications in electro-optics and LEDs. The study of the exciton dynamics in conjugated polymers is essential to their applications [1]. This paper reports the study of the exciton dynamics in an optically active poly(arylene vinylene) (PVA) in solutions using femtosecond laser spectroscopy. This polymer has main chain chirality and is the first of its kind. It has potential applications in areas such as asymmetric synthesis [2]. Its strong and relatively blue absorption and emission spectra (Fig. 1) make it attractive for blue LEDs and diode lasers. The molecular structure is shown in Fig. 1.
New Advances in Measuring Hydrogen Bonding Dynamics
Authors: Arrivo S. M. , Dougherty T. P. , Grubbs W. T. , Heilweil E. J.
Published: February 2023

We report the only known comprehensive study of conserved vibrational energy transfer during association and dissociation of biologically relevant hydrogen-bonded complexes in dilute (0.1 M acid) room temperature solution. Newly applied picosecond infrared techniques which vibrationally tag and probe interacting proton donating (-OH, -NH) and accepting (e.g., -C = O, ≡ON) constituents will be presented. From these measurements, details of steric interactions, equilibrium reaction rates and unexpected vibrational excitation transfer during hydrogen-bond formation are revealed for the first time.
Time-Domain Observation of Nuclear Vibrational Motion in Glasses
Authors: Kang I. , Smolorz S. , Krauss T. D. , Wise F. W. , Aitken B. G. , Borrelli N. F.
Published: February 2023

Optical glasses can be fabricated with large refractive nonlinearities and reasonably small two-photon absorption, and thus have become strong candidate materials for optical switching applications. In particular, sulfide glasses and heavy-metal oxide glasses are found to have among the largest nonresonant nonlinearities reported to date.1
Coherence Transfer in Exciton-Exciton Scattering in a Semiconductor Microcavity
Authors: Rhee J.-K. , Norris T.B. , Citrin D.S. , Arakawa Y. , Nishioka M. , Hanamura E.
Published: February 2023

Semiconductor quantum microcavities are interesting physical systems in which fundamental interactions between light and excitons can be explored. For example, in the weak - coupling limit, modified excitonic spontaneous emission can be observed. In the strong-coupling limit, the cavity field modes and the quantum well exciton modes become coupled; the normal modes of the system are "cavity-polaritons" [1,2]. For in-plane wavevectors k// near zero, the normal modes are split by the dipole coupling between the excitons and the cavity vacuum field. This splitting is often referred to as a vacuum-Rabi splitting (VRS), as it has the same physical origin as the VRS observed in the interactions of atoms with single-mode cavities. The physical system in this work consists of a λ planar microcavity formed by two distributed-Bragg reflectors, with five GaAs quantum wells situated in the center of the cavity. In all the experiments described here, the exciton and cavity modes are on resonance (the low-field VRS is 6 meV for this sample).
Inter-subband scattering and thermalization of electrons in quantum wells studied with mid-infrared femtosecond pulses
Authors: Woerner M. , Lutgen S. , Kaindl R. , Elsaesser T. , Hase A. , Künzel H.
Published: February 2023

The dynamics of carriers in two-dimensional semiconductor structures is governed by both intra- and inter-subband scattering processes dominantly due to carrier-carrier and carrier-phonon interactions. In the case of subband separations larger than the LO-phonon energy a broad range of lifetimes of n=2 carriers between 150 fs [1], 1 ps [2], and 15 ps [3] was reported. In addition, there is only poor information about thermalization of two-dimensional carrier gases. Using femtosecond light pulses in the mid-infrared spectral range allows the direct selective observation of electron or hole dynamics in quantum wells [4], In this paper, we study the inter- and intra-subband relaxation of electrons in n-type modulation-doped GalnAs/AlInAs multiple quantum wells (MQWs). Our experiments provide new information on the n=2 lifetime and give the first direct insight into electron therm- alization after inter-subband excitation.
Sub-picosecond far-infrared transient-grating measurements of electron cooling in InAs and GaSb
Authors: Pellemans H.P.M. , Wenckebach W. Th. , Planken P. C. M.
Published: February 2023

Compared to GaAs, little is known about hot-electron dynamics in narrow-gap semiconductors such as InAs and GaSb. These materials have a smaller bandgap, a smaller effective electron mass and a smaller electron-phonon coupling constant in comparison with GaAs. To investigate the dynamics of hot electrons in these materials, it is necessary to use mid- to far-infrared optical pulses with a photon energy low enough to avoid the generation of holes by (multi-photon) interband electron-hole pair generation. [1,2] Here, we show results of transient-grating experiments in n-type InAs and GaSb in which we time-resolve the ultrafast electron cooling of an electron distribution after optical excitation with intense sub-picosecond far-infrared pulses.
Ultrafast Excitation and Deexcitation of Local Vibrational Modes in a Solid Matrix: The Si-H Bond in Amorphous Silicon
Authors: Xu Zhiwei , Vandyshev Jury V. , Fauchet Philippe M. , Rella Chris W. , Schwettman H. Alan , Tsai Chuang Chuang
Published: February 2023

Recent advances in femtosecond laser technology have made it possible to answer experimentally the question of how long a vibrational excitation can remain localized on a single bond in a solid matrix. From these measurements, we can infer quantitative information on the local atomic arrangement and the coupling of a specific bond to the matrix. If energy can remain localized for long periods of time, several new applications may emerge, such as laser-assisted growth. Although related work is already being performed on isolated molecules, when the bonds are imbedded in a solid matrix, many new channels for energy decay open up. As a result, there is to our knowledge no reliable theoretical work on the deexcitation of localized vibrational modes in a solid matrix.
Physics and scaling of Saturable Bragg Reflector-modelocked diode-pumped solid state ultrafast lasers
Authors: Tsuda S. , Knox W. H. , Cundiff S. T. , Zyskind J. L. , Jan W. Y. , Pathak R. , Cunningham J. E.
Published: February 2023

We have recently demonstrated that a saturable Bragg reflector (SBR) structure can modelock a diode-pumped Cr:LiSAF (Fig. 1a) laser with very low loss and produce 100 fs pulses with average power of 42 mW using a single 0.5 W broad area 670 nm pump diode [1]. In the present paper, we discuss results obtained with a 0.5 W diffraction-limited pump diode, and scaling of SBR structures to other wavelengths. Further, we discuss the modelocking processes in such passively modelocked lasers.
Generation and applications of mid-infrared femtosecond pulses obtained by optical rectification of 15-fs near-infrared pulses
Authors: Joffre M. , Bonvalet A. , Martin J.-L. , Migus A.
Published: February 2023

Using optical pulses as short as 15 fs, we were able to extend the techniques of THz generation from the far-infrared to the mid-infrared spectral domain. Our method produces highly stable mid-infrared pulses at a 100 MHz repetition rate, simply by focusing the pulses delivered by a Ti:Sapphire oscillator on an electro-optic material. Since the bandwidth of the incident pulses is about 60 THz, this allows the generation of ultra-broad band femtosecond pulses extending from a wavelength of 6 μm to above 15 μm [1].
Ultrafast Ellipsometry of Coherent Processes and Exciton-Exciton Interactions in Semiconductors
Authors: Smirl Arthur L. , Bolger J. A. , Paul A. E.
Published: February 2023

Ultrafast degenerate four-wave-mixing (FWM) techniques have been enormously successful in providing new information about coherent processes and excitonic effects in semiconductors. One of the techniques that has proven to be extremely powerful is to study the polarization dependence of the FWM process. Typically, these studies have addressed the dependence of the magnitude of the FWM signal (or its spectrum) on the incident polarizations, and there have been very few attempts to monitor the polarization state of the FWM signal itself. Moreover, the attempts that have been made have been restricted to measurements of the direction of polarization (ignoring the ellipticity and degree of polarization) and, therefore, they have not completely specified the polarization state of the emitted light. In addition, these measurements have been restricted to positive delays.
Optical Coherence and Anisotropy Studies of the First Events in Photosynthesis
Authors: Arnett David C. , Moser C. C. , Dutton L. , Scherer Norbert F.
Published: February 2023

The bacterial reaction center of Rhodobacter Sphaeroides contains six pigments, arranged within a protein environment. These pigments act in a very concerted way to generate a remarkably efficient charge transfer which initiates the process of photosynthesis.1 The structure of the reaction center is well known, consisting of two strongly interacting bacteriochlorophyll (BChl) molecules known as the special pair (P), two accessory BChls, and two bacteriopheophytins (H) arranged in approximately C2 symmetry.2,3,4 Upon excitation of P, either directly (light absorption) or through energy transfer from the light harvesting antennae, an electron is transferred to H within about 3 ps. Although much work has been done on these types of systems, many issues remain unresolved: (1) what is the role of the accessory bacteriochlorophyll (B) in the electron transfer, (2) what is the influence of the upper excitonic state of P (Py+), (3) what is the spectrum of fluctuations (frequencies, couplings…) that describes the interaction of the reaction center chromophores with the protein environment?
Femtosecond Electron Transfer and Real Time Discrimination of a Disulfide Bond Formation
Authors: Gelabert H. , Belloni J. , Marignier J.L. , Gauduel Y.
Published: February 2023

The aim of this experimental work is to discriminate, in real time, the first steps of a S-S bond formation in solution (figure 1). One electron reduction or oxidation of a simple thioether, the dimethylsulfide (CH3SCH3), has been investigated by femtosecond UV-IR spectroscopy. Indeed, when an electron of excess energy is injected in a liquid organic sulfide characterized by a high electron affinity, an irreversible electron attachment competes with the solvation process. This electron transfer yields a primary anion (RSR-, R = CH3) which exhibits a high reactivity through an ultrafast ion-molecule reaction. This contribution will focus mainly on S-S bridge formation triggered by a primary anion/cation-molecule reaction.
Femtosecond Real-Time Spectroscopy of Small Molecules
Authors: Schreiber Elmar , Rutz Soeren , Ruppe Harald , Wöste Ludger
Published: February 2023

For a selected number of examples the application of ultrafast spectroscopy on small molecules will be presented. It allows to study the dynamics of these spezies after excitation with laser pulses of less than 80 fs duration with high temporal and —via Fourier analysis— spectral resolution. Information, never before received by stationary spectroscopy, can be revealed and, hence, the complementarity of these two approaches is clearly observable.
Parametric Trapping and Self-Ordering of Solitons
Authors: Grigoryan V.S. , Hasegawa A. , Maruta A.
Published: February 2023

One basic problem in high bit rate soliton communication and storage systems is the soliton’s time position jittering. This jittering is caused by the soliton frequency diffusion which originates either from amplified spontaneous emission in repeaters [1] or from soliton-soliton interactions. Use of filtering along with amplification in repeaters permits only partially suppress the time position jittering [2,3]. Without filtering mean-square time position increases as distance cubed, whereas with filtering it increases linearly in distance. Recently big progress was achieved in experiment of M.Nakazawa with the help of active modulation of losses along with filtering and amplification of solitons [4]. Modulator provides a clocking mechanism by which an ordering of solitons occurs, whereby each of solitons tends to occupy a time interval when the modulator has a maximum transmission. However, in practical use of this method for a communication line, when a number of modulators is needed, a serious problem of mutual synchronization of all those modulators arises. In addition, the bit rate is limited by the modulator electronics. The way-out is a passive mechanism of ordering or a mechanism of self-ordering of solitons which does not require electronics.
A Potassium Niobate OPA Pumped by an Amplified Ti:sapphire Laser
Authors: Kafka J. D. , Watts M. L.
Published: February 2023

Optical parametric amplifiers (OPAs) have been used to extend the wavelength range available from amplified Ti:sapphire lasers [1,2]. These systems typically produce 100 fsec pulses throughout the visible and near infrared. Recently, there has been much interest in generating wavelengths beyond 3 μm, often by means of difference frequency mixing [3]. Direct generation of pulses from an OPA however, offers the potential for higher energy. We report, for the first time, generation of 3 to 5 μm pulses directly from a potassium niobate (KNbO3) OPA pumped by an amplified Ti:sapphire laser [4].
Femtosecond X-rays Generated via Thomson Scattering of Terawatt Laser Pulses with Relativistic Electrons
Authors: Schoenlein R.W. , Chin A.H. , Glover T.E. , Shank C.V. , Leemans W.P. , Volfbeyn P. , Kim K.-J. , Chattopadhyay S.
Published: February 2023

We have developed a unique source of femtosecond x-rays operating in the ~300 fs, ~0.5 Å regime, which will allow us to apply x-ray techniques with ultrafast time resolution to directly probe the structural dynamics of materials. The study of atomic motion on the fundamental time scale of a vibrational period, h/kT~100 fs, is important for understanding processes such as chemical reactions, vibrational energy transfer, phase transitions, and surface dynamics. Our understanding of these processes has been limited by lack of appropriate tools for probing the ultrafast motion of atoms. X-ray techniques such as diffraction and extended x-ray absorption fine structure (EXAFS) are powerful probes of atomic structure, and are widely used with synchrotron x-ray sources. However, the time resolution provided by high-brightness synchrotrons is nearly three orders of magnitude too slow to directly observe fundamental atomic motion. Conversely, femtosecond lasers can measure the optical properties of materials on a 10 fs time scale but optical properties are poor indicators of atomic motion. Femtosecond x-ray pulses will allow us to directly probe atomic motion in condensed matter on an ultrafast time scale.
Modelling of the propagation of light in turbid media from femtosecond pulse transmission measurements
Authors: Andersson-Engels S. , Berg R. , Björn L.O. , Carlsson J. , Hellentin P. , Jarlman O. , Johansson J. , af Klinteberg C. , Lindquist C. , Malmqvist L. , Persson W. , Pifferi A. , Svanberg S.
Published: February 2023

Measurements of time-resolved transmission can be used to study the structure and scattering mechanisms of turbid media. Compared with conventional measurements of reflectivity and transmittivity, time-resolved transmission gives more detailed information on the propagation of light inside the sample. For performing such measurements on thin samples, such as paper or leaves, a picosecond or sub-picosecond resolution is needed.
Polarization Temporal Gating of Light Propagating in Highly Scattering Medium
Authors: Demos S.G. , Ullman J. , Celmer Edward J. , Zurawska-Szczepaniak Margaret , Alfano R.R.
Published: February 2023

A light pulse propagating in a highly scattering medium experiences multiple scattering lossing coherence and polarization and leading to a much broader temporal profile.1-4 In this work we measure the degree of polarization of linearly polarized laser pulses emerging after propagation in polystyrene microsphere solutions or human breast tissues. The temporal profiles of the parallel and perpendicular polarization components of the emerging light pulses differ for over 100 ps following the ballistic component. These results demonstrate that polarization can be used for time-gating the early-arriving component of the output pulse with gate time duration of ≈100 ps.
Coherent THz plasmons in GaAs Schottky diodes
Authors: Fischler W. , Höpfel R. A. , Zandler G.
Published: February 2023

The research on coherent collective excitations in solids has been pioneered by the observation of coherent phonons [1]. Femtosecond studies of plasma excitations in solids were reported by us on one of the previous meetings in this series [2]. Recently, excitation and detection of coherent plasma oscillations in solids became a new and exciting topic [3].
First demonstration of quasi phase matched generation of EUV high-harmonic radiation
Authors: Bartels Randy A. , Paul Ariel , Backus Sterling , Tobey Raanan , Christov Ivan P. , Kapteyn Henry C. , Murnane Margaret M.
Published: February 2023

High harmonic generation (HHG) is an excellent source of extreme ultraviolet (EUV) radiation that upconverts an intense laser pulse to much shorter wavelengths. Over the past decade, many experiments have studied this process, providing detailed insights into the physics of HHG. Phase matching the HHG process, either using a hollow waveguide,(1) or using a near-plane-wave geometry, (2, 3) increases the brightness of the source by orders of magnitude and generates and EUV beam with full spatial coherence.(4) In this work, we experimentally demonstrate that phase-matching of HHG radiation can be extended to shorter wavelengths and higher pulse energies using a structured hollow waveguide. Moreover, this photonic structure results in the generation of a broad continuum that may support an isolated 250 attosecond EUV pulse, as has been previously predicted theoretically.(5) This work is a compelling demonstration of the applicability of photonics technology concepts to a new region of the spectrum.
A New Method for Characterizing Progressive Addition Lenses
Authors: Campbell Charles E.
Published: February 2023

A new method for characterizing the design of progressive addition lens is presented. The power gradient of the lens is measured in a small area using an automatic lensmeter. This measurement, designated PV, is repeated on selected, separated areas of the lens to create a profile of the lens gradient change. Results of measurements taken on lens of different designs are presented.
Representing corneal shape
Authors: Klein S. A. , Mandell R. B. , Barsky B. A.
Published: February 2023

The power and radius expressions that are commonly used for corneal topography measurements are frequently misinterpreted. There is no consensus for the most appropriate expression of corneal topography, perhaps due to bias in the measurement process and misunderstanding of what is being measured. Agreement is needed as to the most appropriate methods for expressing corneal shape independent of the available measurement methods. This is a worthwhile enterprise since corneal shape measurements are useful in such applications as diagnosing corneal abnormalities, refractive corneal surgery and contact lens design.
Aging and the eye's optical performance
Authors: Liang Junzhong , Westheimer Gerald
Published: February 2023

In the literature there have been a number of reports of tests of disability glare and straylight in the aged eye[1,2]. To address some fundamental questions about age-dependent loss in optical performances we have conducted a number of experimental studies [3-6]. Here we report some of the new developments in this study.
Photokinetic Analysis of Primate Cone Responses Implies Qualitative Differences from Rod Transduction
Authors: Tyler Christopher W. , Hamer Russell D.
Published: February 2023

Diagnosis of functional disorders of the human retina is based on indirect measures of retinal function, such as psychophysical response functions and electroretinography. Interpretation of the data obtained by such measures requires an accurate model of the processes underlying the retinal responses. Recent progress in this regard has been made by Hood and Birch (1993) and others for rod function, but a comprehensive model for the nonlinear behavior of human cone responses has not been attempted.
Retinoscopic Patterns: Explained by the Optics of an Eccentric Source
Authors: Roorda A. , Bobier W.R. , Teske C.
Published: February 2023

The optics of retinoscopy can be understood from a consideration of an important fact. That fact is that the retinoscopist views the eye through a sighthole which is not coincident with the source. In this context the optics of retinoscopy can be analyzed using a format similar to that previously conducted for eccentric photorefraction1,2 where patterns of light observed in the plane of the pupil are found to be a function of the eccentricity of the source. The fundamental difference in the techniques is in the variability of the source position and the small sighthole aperture employed in retinoscopy.
The Relationship between Defocused MTF and Spatial Frequencies Needed for Letter Recognition
Authors: Lang Alan , Lakshminarayanan Vasudevan
Published: February 2023

In 1990, Frank Thom and Faye Schwartz published an interesting study of the effects of defocus on letter and sine-wave gratings. By comparing with theoretical estimates of the Modulation Transfer Function (MTF) of the observer's eye, they concluded that "letters are recognizable only if the spatial components used for recognition are free of spurious resolution or phase reversals". Because recognition requires both sufficient spatial frequencies and above-threshold contrast, this suggests that the highest spatial frequencies required for letter recognition are limited by the threshold contrast and do not exceed the first "zero crossing" of the MTF of the observer's retinal image. Since this conclusion is crucial to the development of any model predictive of clinical measures of human visual performance [Lang, 1993], we tested it using a method which allows concurrent assessment of letter images and measurement of the MTF of an optical model of the human eye which generates the images. As seen below, this procedure allows correlation of letter recognition with the location of spurious modulation. It does not measure contrast levels for threshold recognition.
Eye-Movement Tracking Using Compressed Video Images
Authors: Mulligan Jeffrey B. , Beutter Brent R.
Published: February 2023

Infrared video cameras offer a simple noninvasive way to measure the position of the eyes using relatively inexpensive equipment. Several commercial systems are available which use special hardware to localize features in the image in real time; typically the pupil and the first Purkinje image (corneal reflex) are tracked, the difference between these two signals giving a measure of eye rotation which is relatively independent of head position. While these systems have the advantage of providing a result in real time, this speed is obtained at the cost of reduced resolution and accuracy. The limitation is not imposed by the information content of the video image, however, but rather by the complexity of the processing algorithms which can be implemented in current hardware. More accurate results can be obtained when the imagery is analyzed off-line using more complex algorithms implemented in software.
The Critical Period for Surgical Treatment of Dense Congenital Unilateral Cataract
Authors: Birch Eileen E. , Stager David R.
Published: February 2023

The surgical treatment of adults with cataracts is one of the most common and most successful of procedures, with 93% of adult patients achieving 20/40 or better acuity (Stark et al., 1987). In contrast, outcome following surgical treatment of infants with dense congenital unilateral cataracts historically has been dismal, with few patients achieving 20/200 acuity and most achieving only light perception (Hiles and Waller, 1977; Shapiro et al, 1978). Despite routinely excellent surgical outcomes and the availability of infant aphakic contact lenses, the unilateral visual deprivation experienced by these infants during the first months of life results in a severe abnormality in visual development which is often resistant to any late efforts toward visual rehabilitation. However, data from animal models of unilateral visual deprivation suggest that there may exist an early critical period during which the effects of visual deprivation are, at least to some extent, reversible (Hubei, 1982; von Noorden and Crawford, 1979; Weisel, 1982).
Extraction of a Ganglion Cell Component from the Corneal Response
Authors: Sutter Erich E. , Bearse Marcus A.
Published: February 2023

Great efforts have been made over the past decades to find a direct objective measure of retinal damage in glaucoma and optic atrophy. The electroretinogram (ERG) was considered a prime candidate for such a measure, but so far, reliable ERG tests for inner retinal and specifically ganglion cell function are still not available in the clinic. Numerous scientific studies have reported glaucomatous changes in the pattern ERG, but even now many findings remain controversial. Often, the changes reported were significant within a group but not large enough to permit a diagnosis on individual patients. The problem with such applications of traditional ERG techniques is two-fold: 1) They test retinal performance with large stimuli and are, therefore, relatively insensitive to local damage encountered in the early stages of disease. 2) Most of the response originates from the distal retina. Local changes of the small inner retinal contribution are too small to detect in the presence of a large outer retinal component with its own inter-subject variabilities.
Telecentric Lens Minimizes Perspective Errors in Measurement of Purkinje Images
Authors: Barry J.C. , Dunne M.C.M. , Hartmann A. , Reim M.
Published: February 2023

Purkinje image measurements are used for a wide range of clinical and scientific applications, for instance in strabology (Purkinje I and IV Reflection Pattern Evaluation [1,2,3] Hirschberg Test) and in biometry or phakometry. Purkinje images I and IV are typically separated from Purkinje image III axially by 10mm to 15mm. This leads a) to image blurring due to limited depth of field and b) to perspective errors with simultaneous recording when using a conventional lens.
Receptor and Middle Retina ERG Components in Achromatopsia
Authors: Verdon Wayne A. , Haegerstrom-Portnoy Gunilla , Schneck Marilyn E.
Published: February 2023

The ganzfeld ERG of autosomal recessive achromats is characterized by the lack of cone responses, and frequently borderline or mildly subnormal dark adapted (rod) responses. The purpose of this research is to determine why rod responses of achromats often appear subnormal. One possibility is that the achromats' rod responses are compared to norms that have significant cone components, at least at some flash intensities. If so, we might expect that achromats' dark adapted responses to appear progressively more subnormal at higher flash intensities where the normals' cones make a larger contribution. In fact this has been documented (Krill, 1977). However, it is also possible that this accounts for only part of the discrepancy, and that the rods themselves or the inner retinal responses of achromats are in some way abnormal. To help understand the cause of the achromats' borderline dark adapted ERG we isolated components that correspond to rod receptor activity (Granit's P3, derived from the leading edge of the a-wave) and middle/inner retina activity (Granit's P2, which sums with P3 to give the b-wave). Fig 1 shows the relationship between P2, P3 and the ERG recorded at the cornea (data show the response to a standard Grass 18, for a dark adapted achromat). P3 components were fit using a computational model based on Lamb and Pugh's (1992) description of individual rod responses (Hood and Birch, 1993; Breton et al. 1994). By subtracting the P3 element from the corneal ERG, the electro-positive P2 component remains. We fit P2 responses with the Naka-Rushton equation, over approximately a 6 log unit range of flash intensities.