bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond Studies of Electron Dynamics in Two-Dimensions
Authors: Harris C. B. , McNeill J. D. , Ge N.-H. , Jordan R. E. , Lingle R. L. , Wong C. M.
Published: February 2023

Angle resolved two-photon photoemission (TPPE) spectroscopy has shown that the energies and effective masses of image potential electrons outside metal surfaces are very sensitive to the morphology and electronic structure of atomically thin adlayers [1]. On a bare metal surface, an excess electron is bound by a Coulomb potential arising from the polarization it induces in the metal surface. As layers of a dielectric material are grown on the surface, the image state binding energies change because the Coulombic interaction is modified by dielectric effects and the electronic structure of the adlayer slab. The effective mass for motion parallel to the surface is influenced by the band structure of the metal and the adlayer potential. Here we show that femtosecond angle-resolved TPPE allows us to study in detail the factors which control the dynamics of electrons at interfaces over the transition from 2-D to 3-D behavior.
All-Optical Recognition of Terabit-per-Second Bit Patterns
Authors: Mahoney Keith M. , Nuss Martin C. , Morrison R. L.
Published: February 2023

An important operation in a communications system is packet header recognition. For future high-speed time-division-multiplexed (TDM) systems, a packet header recognizer needs to distinguish between the various packet headers on a ultrafast time scale to avoid placing a bottleneck on the system. Frequency-domain processing [1,2] of femtosecond optical pulses has been shown to be capable of performing correlations in the frequency-domain in parallel rather than serially in the time-domain. Unfortunately, the input optical pattern can only be compared to a single packet at a time, which forfeits some of the advantages gained by the frequency-domain processing.
Toward Coherent Control on a Sub-10fs Time Scale
Authors: Spielmann Christian , Xu Lin , Krausz Ferenc
Published: February 2023

Advances in ultrafast laser technology led to the generation of optical pulses that comprise less than 3 optical cycles within their FWHM directly from mode-locked lasers.1 Further progress in femtosecond oscillator technology2 are expected to yield shorter pulses in the near future. As the pulse duration approaches the optical oscillation period, a parameter that has not received attention previously, namely the phase of the electric field at pulse center, ψ gains rapidly increasing significance. This becomes apparent by comparing the electric field evolution in two pulses having identical envelopes but different carrier phases, as shown in Fig. 1.
Superradiant Terahertz Emission from Bloch Oscillations
Authors: Martini R. , Klose G. , Roskos H.G. , Kurz H. , Hey R. , Grahn H.T. , Ploog K.
Published: February 2023

Impulsively excited Bloch oscillations in semiconductor superlattices are attractive for the generation of coherent terahertz (THz) electromagnetic radiation tunable with a dc bias field over the wide frequency range from 0.2 to 10 THz. So far, only the emission characteristics at low density of the photo-excited charge carriers (1-3×109 cm-2 per well) have been investigated.1 For technical acceptable emission, the carrier density must be increased. According to the standard theory of emission from coherent ensembles (superradiant emission)2, the radiation power should scale quadratically with excitation density. For the signal decay rate, however a linear density dependence is expected.
Temporally One- and Two-Dimensional Spectroscopies on Overtone Vibrational Dephasing of Liquids
Authors: Tominaga Keisuke
Published: February 2023

Vibrational spectra in solution are very sensitive to environment around the oscillator, and vibrational line shapes have been analyzed to investigate microscopic details in liquids such as intermolecular interactions and dynamics [1]. Dependencies of vibrational dephasing on the vibrational quantum number,v, differs with relaxation mechanisms. Therefore, investigation of quantum number dependence of vibrational width has been crucial for studies of dephasing mechanism [2].
Femtosecond analysis of the spinning motion of liquid crystal molecule in Smectic A phase
Authors: Le Calvez A. , Montant S. , Freysz E. , Rassoul R. M. , Ducasse A. , Zhuang X.W. , Shen Y.R.
Published: February 2023

Molecular orientational dynamics of liquid crystalline (LC) materials is a subject of many researchers and has been studied by a large variety of techniques. More recently, with ultrashort laser pulses, time resolved measurements have been used to probe directly the fast dynamics for many different LC. Most of the works focus on the isotropic phase of nematogenic substances where the LC molecules are assumed to have cylindrical symmetry. Then orientational dynamics arise from molecular tumbling motion. This assumption is not valid if the molecules are biaxial and the molecular spinning motion about the molecular long axis is not negligible. For isotropic liquids, this happens on the picosecond time scale and has been demonstrated, for example, in liquid biphenyl1. The smectic A (Sm-A) phase is most suitable for investigation of the spinning orientational dynamics using for instance the transient optical Kerr effect. When homeotropically aligned, a Sm-A film has its molecules oriented with their long molecular axis parallel to the surface normal. A linearly polarized laser beam propagating along the surface normal will reorient the molecules around the surface normal via the spinning motion while the Fredericksz transition is forbiden thanks to the high elastic constants of the smectic layers. The molecular reorientation can be measured through the optical birefringence induced by the reorientation. This geometry was previously used to investigate the collective spinning dynamics at the Sm-A- Sm-C* transition2. So far, however, no such study in the femtosecond time scale pertaining to individual molecular reorientation has ever been reported. In this paper we present such a study in a number of cyanobiphenyl ( nCB with n denoting the number of carbons in the alkyl chain) Sm-A LC media.
Temperature-Dependent Electronic Dephasing of Molecules in Polymers Measured by Femtosecond Three Pulse Photon Echoes
Authors: Bardeen Christopher J. , Cerullo Giulio , Shank Charles.V.
Published: February 2023

Most work on the electronic dephasing of molecules in the condensed phase has concerned either liquids at room temperature [1]. or organic solids at very low temperatures (< 15 K) [2]. In this work we study the dephasing of the molecules LD690 in the polymers PMMA and PVA and LDS750 in PMMA in the intermediate temperature range 300-30 K. We find that the observed dephasing rate for LD690 is strongly temperature-dependent in both polymers, while that of LDS750 changes only slightly with temperature. LDS750 is known to undergo a large change i n charge distribution and fast Stokes shift upon excitation [3], unlike LD690, and the nature of its coupling to the bath is thus very different from that of LD690.
Confinement Effects on the Electron Thermalization Process in Tin Nanocrystals
Authors: Stella A. , Nisoli M. , De Silvestri S. , Svelto O. , Lanzani G. , Cheyssac P. , Kofman R.
Published: February 2023

Clusters and aggregates of atoms in the nanometer size range have been the object of intensive work particularly in the case of semiconductors quantum dots [1]. Less extensive investigations have been performed on metallic nanoparticles [2-3]. For particle size greater than 10 Å the behavior of the nanoparticles tends to approach that of the bulk metal, but with some significant differences concerning thermodynamic properties (like decrease of melting temperature with size) and non-linear effects in the optical spectra [4]. A careful investigation of the role of particle dimensions on the electron relaxation dynamics appears to be essential to improve our knowledge of the basic properties of metal nanoparticles as compared to bulk.
Direct Observation of Coherent Two-Photon Medium Response Induced by Femtosecond Deep UV-Laser Pulses in Krypton
Authors: Korn G. , Kittelmann O. , Ringling J. , Nazarkin A. , Hertel I. V.
Published: February 2023

Resonant and near-resonant two-photon excitations with ultrashort laser pulses of forbidden atomic transitions give access to a wide class of nonlinear-optical processes such as two-photon absorption, stimulated Raman scattering, four-wave sum- and difference-frequency mixing. The latter has proofed to be a powerful tool for efficient femtosecond pulse generation in the VUV-spectral range [1]. Nevertheless, up to now only few experimental results have been obtained which allow the direct observation of the coherent dynamics in two-photon induced radiation-matter interactions (see experimental papers [2-5] and monograph [6]).
Terahertz Waveform Synthesis and Enhanced Narrow-band Terahertz Generation Using Femtosecond Pulse Shaping
Authors: Liu Yongqian , Park Sang-Gyu , Weiner Andrew M.
Published: February 2023

Recently, generation of ultrafast terahertz (THz) radiation using femtosecond lasers has attracted much attention for a variety of potential applications [1-2]. In this contribution, we present a technique for manipulating ultrafast THz electromagnetic waveforms through optical pulse shaping. By using a programmable pulse shaper to design optical excitation pulses, we have demonstrated our ability for controlling the amplitude and phase of THz waveforms, or time-division multiplexing terabit signals at THz frequencies. Meanwhile, we have shown that we can increase the generation efficiency of narrowband THz radiation by overcoming saturation effects through multiple optical pulse excitation. This scheme also allows us to generate high power, tunable narrow-band THz radiation.
Fiber Laser Pumped Femtosecond Optical Parametric Generator
Authors: Galvanauskas A. , Arbore M. A. , Fejer M. M. , Fermann M. E. , Harter D.
Published: February 2023

We report here the first fiber-laser pumped optical parametric generator (OPG), which offers a compact, robust and cost-effective source of broadly tunable femtosecond pulses.
Measurement of Laser Wakefield Oscillations by Femtosecond Longitudinal Interferometry
Authors: Siders C. W. , Le Blanc S. P. , Fisher D. , Tajima T. , Downer M. C. , Babine A. , Stepanov A. , Sergeev A.
Published: February 2023

Because the electrostatic fields present in plasma waves can exceed those achievable in conventional accelerators and approach atomic scale values (E
a
~ 500 GV/m), plasma based accelerators have received considerable attention as compact sources of high-energy electron pulses [1]. Although stimulated Raman scattering [2] and terahertz radiation at the plasma frequency ω
p
[3] provide spatially averaged optical signatures of the plasma wave, new diagnostic techniques are required to map the the temporal and spatial structure of the plasma wave directly since such information is vital for addressing fundamental issues of wakefield generation and propagation. In this paper, we report femtosecond time resolved measurements of the longitudinal and radial structure of laser wakefield oscillations using an all optical technique known as ’photon acceleration’ [4].
Observation of Relativistic Self-Focusing, Self Channeling and Filamentation of Multiterawatt Ultra-Short Laser Pulses in Optical-Field Ionized Argon Gas Jets
Authors: Stenz C. , Blasco F. , Pellicer J.C , Antonetti A. , Chambaret J.P , Chériaux G. , Darpentigny G. , Grillon G. , Le Blanc C. , Mysyrowicz A. , Rousse A. , Salin F.
Published: February 2023

A laser beam propagating in an underdense plasma undergoes relativistic self focusing and self-channeling, arising from the relativistic and charge-displacement mechanisms, as soon as its total power P exceeds the critical value Pcr= 16.2(ω0/ωp)2 GW. This as been established both theoretically [1] and experimentaly for laser pulses duration in the range of 400-500 fs [2]. Stability analysis against azimuthal perturbations of the propagation of an intense short pulse governed by relativistic and charge-displacement self-channeling non linearities have been recently examined[3]. The propagation, when P>Pcr, is considered as stable when it leads to a single main channel wich contains "30%-50% of the incident power P. Conversely the regime of strong filamentation (unstable) is caracterized by the formation of multiple channels each containing a power of approximately Per. The location of the domains of stability of the propagation of hyper-Gaussien beams are established in terms of two parameters, ρo=(ωpo.ro/c and η= Po/Pcr where ρo is the focal spot radius, ωpo is the unperturbed plasma frequency and c the speed of light. Far above threshold the formation of self generated magnetized light channels, due to a strong current of relativistic electrons and the coalescence of channels are also predicted [4].
Ultrafast oscillations in the optical emission from a semiconductor microcavity
Authors: Koch M. , Shah J. , Wang H. , Damen T.C. , Cunningham J.E. , Jan W.Y.
Published: February 2023

We report here the first investigation of femtosecond time-resolved emission from a semiconductor microcavity resonantly excited with intense femtosecond pulses. The emission intensity, which exhibits a strong threshold behavior, shows novel, well-pronounced oscillations that have a period of 600 fs and that last for several picoseconds. These THz oscillations can be associated with the appearance of a new mode in the spectrum and become more pronounced with pump intensify. The frequency of these oscillations is 20 times higher than those reported previously [1]. Our investigation probes novel aspects of semiconductor microcavities and vertical-cavity surface-emitting lasers that have attracted considerable interest recently [1-6]. Observations reported earlier include: coherent vacuum Rabi oscillations after weak resonant optical excitation [2-4], the emission of single pulses after nonresonant optical pumping [5], and relaxation oscillations after electrical pumping [1].
Ultrafast spin dynamics in ferromagnetic Nickel
Authors: Beaurepaire E. , Merle J.-C. , Daunois A. , Bigot J.-Y.
Published: February 2023

Fast magnetization changes can be achieved in a ferromagnet using optical pulses. Such changes which are due to an increase of the spin temperature above the Curie point are generally induced by the temperature elevation of the lattice induced by pico-nanosecond pulses [1,2]. In this case, the electron and phonon population are in equilibrium, and the delay between spin and lattice dynamics has been attributed to spin phonons interactions. As reported in this paper, the initial hot electron distribution created by femtosecond pulses can induce an ultrafast spin heating via electron-spin collisions. It is shown that in the first picosecond a new situation occurs where the electrons, spins and phonons are out of equilibrium and where an ultrafast demagnetization takes place [3].
Ultrafast gain in semiconductors
Authors: Likforman Jean-Pierre , Joffre Manuel , Hulin Danièle
Published: February 2023

Excitonic resonances in direct-gap semiconductors have proved to exhibit similar optical non-linearities as in atoms. One example is the optical Stark effect [1] which occurs when the sample is excited by a strong pump field whose frequency is detuned from the optical transition. The optical Stark effect has been described as a coupling between the electromagnetic field and the resonance with no real absorption. It can also be seen as the creation of a virtual population in the excited state. A transient gain, quadratic in pump intensity, is associated to this effect as already observed in atomic systems [2]. This hyper-Raman process involves two pump photons and occurs at frequency 2ω
p
– ω0, where ω
p
and ω0 are the pump and atomic transition frequencies, respectively (see inset of Fig. 1).
Molecular Bond Length Control by Phase Shaped Femtosecond Laser Pulses
Authors: Assion A. , Baumert T. , Seyfried V. , Gerber G.
Published: February 2023

Active laser control of chemical reactions has achieved again considerable attraction 1 due to progress in femtosecond laser technology. One basic idea is that different product channels are opened at different internuclear distances. This can be achieved in the so called Tannor-Kosloff-Rice scheme 2 by controlling the motion of a wave packet on an excited surface. Controlling the internuclear distance of the Na2 molecule in a femtosecond pump-probe MPI experiment confirmed this approach experimentally 3. Other ideas are based on tailored femtosecond laser pulses 4, 5.
Femtosecond Coherent Spin Control in Quantum Wells
Authors: Heberle Albert P. , Baumberg Jeremy J. , Köhler Klaus , Ploog Klaus
Published: February 2023

Cohernt phenomena in semiconductors play a crucial role on time scales faster than the dephasing time. The relative phase in a sequence of laser pulses then strongly influences the dynamics. For example, experiments have demonstrated that excitons in quantum wells created by a first laser pulse can be destroyed again with a second phase-locked pulse when it interferes destructively with the coherent excitons from the first pulse. 1 This interference requires the same polarization orientation of both pulses. Here we consider the case that quantum wells are excited with a phase-locked Pair of perpendicularly polarized pulses as shown in Fig. 1. Such pulses do not interfere directly, even if they overlap in time. Their relative phase therefore does not influence the total exciton density. However, the pulses can produce phase dependent circularly polarized light, inducing interference effects in the angular momentum lZ and in the spin polarization of the excitons. This net angular momentum results in different reflectivities of positive and negative circularly polarized light, causing Faraday rotation of a linearly polarized reflected probe pulse.2 Coherent control of the excitonic angular momentum is also possible with non overlapping pulses when the second pulse interacts with the coherent excitons from the first pulse.
Dual pulse pumping of efficient, short pulse table top X-ray lasers
Authors: Nickles P.V. , Shlyaptsev V.N. , Kalachnikov M.P. , Schnuerer M. , Schlegel T. , Sandner W.
Published: February 2023

We report on the first realization of efficient x-ray lasers using transient gain in collisionally excited neon-like ions. These lasers are working on a double pulse excitation scheme proposed by us in 1993 [1] for solid and foil targets.In a first step, a long ns- pump pulse produces a plasma column of the desired neon-like ions with both a uniform electron density and temperature.The second but short ps- pump pulse arrives at the preformed plasma after a certain delay and heats it up very rapidly, producing very efficiently a transient gain. Lasing was experimentally realized on the 3p-3s transitions in Titanium XII at 32.6 nm, as well as in Vanadium XIV at 30.4 nm. Additionally, in Ti a bright lasing line at 29.8 nm was recorded, which is supposed to result from a photo pumped 3d-3p transition.
Ultrafast spectroscopy of wild type and genetically engineered photosystem II reaction centres
Authors: Merry Stephen , Kumazaki Shigeichi , Giorgi Linda , Nixon Peter , Barber James , Porter George , Durrant James , Klug David
Published: February 2023

Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centres. As reported previously [1], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Qy band bleach / stimulated emission is dominated by energy transfer processes with a 100 ± 50 fs time constant. In contrast, depolarisation of this signal occurs with a time constant of 400 ± 30 fs, from an initial anisotropy of ~0.4 to a value of ~0.15 at 1.5 ps. This is illustrated in figure 1 which shows the isotropic absorption and anisotropy transients at a probe wavelength of 683 nm. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction centre absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 ps and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction centre excited states occurs on a sub-picosecond timescale. These excited state equilibration processes preceed charge separation, which occurs primarily with a 21 ps time constant [3]. Transient data collected for pheophytin Qx absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process.
Delocalized states in isolated B800-850 complexes of the photosynthetic purple bacterium Chromatium tepidum
Authors: Kennis John T.M. , Streltsov Alexandre M. , Aartsma Thijs J. , Amesz Jan , Nozawa Tsunenori
Published: February 2023

This work concerns a study by time-resolved femtosecond spectroscopy on isolated B800-850 complexes of the purple bacterium Chromatium tepidum. The results provide direct evidence that the B850 absorption band is associated with a transition to a delocalized exciton state.
Chirped-pulse amplification with a fiber stretcher and a grism compressor
Authors: Kane S. , Squier J. , Yakovlev V. V. , Barty C. P. J.
Published: February 2023

In chirped-pulse amplification (CPA) lasers, pulses are temporally stretched by a factor of ~103, and to achieve good pulse compression it is necessary that the dispersion of the compressor match that of the stretcher. The original CPA lasers used a single-mode fiber to stretch and a grating pair to compress; however, the mismatch in third-order dispersion between the fiber and the grating made sub-picosecond compression impossible. The fiber stretcher was abandoned in favor of the more complicated grating-pair stretcher, which more closely matched the dispersion of the grating compressor.
Optical coherences in light-harvesting chlorosomes from green photosynthetic bacteria
Authors: Savikhin S. , Struve W. S.
Published: February 2023

Chlorosomes are light-harvesting assemblies that are found in green photosynthetic bacteria [1]. The principal antenna in a chlorosome consists of ~104 bacteriochlorophyll (BChl) c, d, or e pigments, which give rise to a broad Qy electronic absorption band centered at ~ 740 nm. Excitations in the BChl c/d/e antenna are transferred to a lower- energy BChl a antenna absorbing at ~795 nm, prior to trapping at the reaction centers [2]. BChl c/d/e antennae in chlorosomes appear to be pigment oligomers whose structure is determined by pigment-pigment rather than pigment-protein interactions, because spectroscopically similar BChl aggregates self-assemble spontaneously from the pigment monomers in solution [3]. The internal energy transfer kinetics of reconstituted BChl c aggregates from the green bacteria Chloroflexus aurantiacus and Chlorobium tepidum closely resemble those found in the BChl c antennae of the intact chlorosomes [4].
Femtosecond VIS and mid-IR Spectroscopy of the Photoisomerisation of Retinal
Authors: Hamm P. , Zurek M. , Röschinger T.
Published: February 2023

The light induced isomerisation of the retinal molecule (more exactly protonated Schiff base retinal, PSBR) plays an essential role in a number of important functional biomolecules like rhodopsin (responsible for vision), bacteriorhodopsin (BR, photosynthetic light induced proton pump) and halorhodopsin (HR, light induced chloride pump). These proteins were investigated in detail by femtosecond spectroscopy. In all cases, a fast S1 reaction occurs prior to the transition to the ground state. However, the details of the photoreaction critically depend on the specific surrounding of the chromophore in the protein: The 11-cis→all-trans isomerisation in rhodopsin and the all-trans→13-cis isomerisation in BR occur extremely fast with 200 fs [1] and 500 fs [2], respectively. A slower and biexponential process is observed for the all-trans→13-cis isomerisation of HR with 1.5 ps and 8.5 ps [3]. In order to separate the influence of the protein from the intrinsic properties of the retinal molecule, isolated all-trans-PSBR (in ethanol) was investigated in the present paper. In addition to standard VIS measurements, femtosecond IR experiments will be presented in order to reveal structural aspects of the photoreaction.
Ultrafast Study of the Photodissociation and Recombination of Aqueous O3-
Authors: Walkout P. K. , Barbara P. F.
Published: February 2023

The photodissociation of the strongly solvated radical O3- (generating O2 and O-) has been studied in aqueous solution by femtosecond pump-probe spectroscopy for the first time. This study of O3- represents the first femtosecond pump-probe study of the photodissociation and geminate recombination of a triatomic radical anion in solution. I3-, a non-radical, is the only other charged triatomic in solution whose photodissociation has been studied with comparable time- resolution.1 From a solution dynamics standpoint there are many features of O3- which make it an interesting radical to examine. O3- has highly concentrated charges (small atomic radii) leading to extraordinarily large electrostatic solute/solvent interactions. O3- has two high-frequency vibrational fundamentals (1058 cm-1, 880 cm-1), thus requiring a quantum mechanical description of the relaxation process. Finally, O3- has a much lower dissociation energy in water (De = 0.34 eV) than the gas phase (De =1.7 eV), further emphasizing the especially strong solute/solvent coupling in this system.
Femtosecond Studies of Isomerization and Energy Relaxation in Small Polyenes
Authors: Pullen Stuart , Walker Larry A. , Anderson Neil , Sension Roseanne J.
Published: February 2023

Recent advances in laser technology have permitted the extension of femtosecond transient absorption studies into the deep ultraviolet. In the present investigation we have used tunable pump and probe pulses to study the photochemically initiated ring-opening reaction of 1,3-cyclohexadiene (CHD). A proper interpretation of the transient absorption signals obtained in the investigation of this ring-opening reaction requires an understanding of the vibrational and conformational relaxation processes which occur following formation of the hot cis-1,3,5-hexatriene photoproduct. To this end, we have also investigated the conformational and vibrational relaxation of hot cis-1,3,5-hexatriene and trans-1,3,5-hexatriene (HT) produced via ultrafast internal conversion following direct excitation of these two isomers.
Transient Product Vibrational Population Distribution in the Femtosecond-Photodissociation of Triiodide
Authors: Kühne Thomas , Vöhringer Peter
Published: February 2023

In liquid solution the dynamics of vibrational relaxation of diiodide generated through dissociative bond breakage of triiodide [1] and diiodide itself [2] has recently been studied in detail. In particular, the application of femtosecond optical pulses has proven to yield valuable information about time scales for thermalization and mean vibrational energies of the products directly after dissociation through the preparation and detection of vibrational superposition states [1]. We describe femtosecond pump-probe experiments on the dissociation reaction of triiodide in ethanol using 30 fs, 400 nm optical excitation pulses which are tuned to the red edge of the low energy absorption band of triiodide. Optical 50 fs pulses tunable from 450 to 950 nm cover the entire visible absorption spectrum of the diiodide product and are used for probing. The experimental data have been translated into vibrational product state distributions, thus giving additional unique information about the mechanism for photodissociation.
Molecular ATI and ATD with Femtosecond Laser Pulses
Authors: Assion A. , Baumert T. , Helbing J. , Gerber G.
Published: February 2023

In a typical ATI experiment the excited states are populated due to ac Stark shifts. Here we report ATI experiments on the sodium dimer molecule where all in the ionization process involved electronic states are in resonance with the light field. Additionally, the vibrational wave packet dynamics on the electronic potential curves introduce a time scale for ionization out of these states1. Using intense femtosecond pump-probe lasers we were able to show that the vibrational wave packet motion in different excited electronic states of the neutral molecule is found in the time resolved ATI photoelectron peak. In addition to that we found that the the coherent coupling of the involved neutral electronic states 2 3 is beautifully reflected in the ATI spectra. A further unexpected result is that even in the high intensity laser fields - leading to the ATI structure in the photoelectron spectra - we do not observe additional ionic fragmentation channels. Our results indicate that no matter how many photons are additionally absorbed in the continuum, the final electronic and vibrational ionic states are always the same. This finding has far reaching consequences for femtosecond experiments in cluster physics.
Compensation of SPM-induced pulse distortions in chirped-pulse amplification systems
Authors: Braun A. , Kane S. , Norris T.
Published: February 2023

Recently Perry et al [1] showed that self-phase modulation (SPM) in CPA lasers can degrade the quality of the compressed pulse. Even in the absence of spectral broadening, a B-integral of ~1 can produce uncompressible wings; this result could be very significant for experiments which require clean, high-contrast pulses.
Size Dependent Absorption Anisotropy Measurements of CdSe Nanocrystals: Symmetry Assignments for the Lowest Exciton State
Authors: Rosenthal S. J. , Yeh A. T. , Alivisatos A. P. , Shank. C. V.
Published: February 2023

Semiconducting nanocrystals smaller in size than the bulk Bohr exciton display quantum confinement, leading to a size dependent energy gap and concentration of oscillator strength into a few transitions. Given these properties these materials are being investigated for potential device applications. CdSe nanocrystals have already been implemented in tunable light emitting diodes.1 We have performed absorption anisotropy experiments on CdSe crystals ranging in diameter from 30-50A with the goal of elucidating the symmetry of the lowest energy optical transition and the earliest events of carrier evolution. The optical anisotropy is defined as R(t)=I‖ (t)-I⊥ (t)/K(t), where K(t) is the total population decay (I‖ + 2I⊥). The initial value of the anisotropy, R(0), is indicative of the nature of the optical transition. For example, if the transition is spherically degenerate, as for C60, R(0) is expected to be 0.0. A linear dipole transition has R(0)=0.4 and for a degenerate planar transition R(0)=0.15.2 The decay in the anisotropy may contain contributions from rotational diffusion, rotation of the transition dipole, as in an isomerization reaction, or population transfer between two excited state levels with transition dipoles of different directions. High time resolution measurements performed on nanocrystals thus give us insight into the shape of the optical transition and the earliest dynamics of the excited carriers. We’ve discovered that a competition exists between the intrinsic wurtzite crystal field and the nonspherical confinement in the crystal3 which dictates the symmetry of the first exciton state. The crystals are thus intrinsically dichroic, a phenomenon which may be exploited in device applications.
Observation of Coherent Local Surface Phonons on Clean GaAs Surfaces
Authors: Chang Yu-Ming , Xu Li , Tom Harry W.K.
Published: February 2023

Surface phonons play a crucial role in the dynamics of surface chemical reactions because they mediate the flow of vibrational energy between the bulk and surface adsorbates. We have used ultrashort laser pulses to excite and detect coherent surface phonon waves on GaAs surfaces. This macroscopically large coherent excitation is significant because it not only allows us a new surface spectroscopic tool for measuring the phonon frequency and dephasing time (equivalent to linewidth) but also gives us the potential of exciting a single surface phonon and observing which degrees of freedom “receive” the energy.
Photon echo measurements in liquids using pulses longer than the electronic dephasing time
Authors: Passino Sean A. , Nagasawa Yutaka , Joo Taiha , Fleming Graham R.
Published: February 2023

Liquid dynamics has been studied by various time-domain techniques such as photon echoes [1] and dynamic Stokes shift measurement [2], which utilize electronic absorption as a probe. Attempts to measure electronic dephasing times in solution using the standard photon echo technique have been hampered by the rapidity as well as the non-Markovian nature of the dynamics. Much emphasis has been directed toward employing shorter and shorter pulses. However, photon echoes using short pulses often simply measure the ultrafast break up of the intra-molecular vibrational wavepacket created by the large spectral bandwidth of the pulses. Recently, it has been shown that three pulse stimulated photon echo peak shift (3PEPS) measurements give accurate dynamical information on solute-solvent interaction [1,3]. In this technique, peak shifts are determined precisely by simultaneously measuring signals in the phase matching directions –k1+k2+k3 and k1−k2+k3. The peak shift reflects the ability of the system to rephase after evolving in a population state for time, T. That is, the decrease of 3PEPS mirrors the electronic transition frequency correlation function, M(t). Here we report 3PEPS studies on various polar protic and aprotic solvents using 22 fs and 90 fs pulses. It is shown the 3PEPS with pulses much longer than a typical electronic dephasing time still gives accurate information on ultrafast as well as slow dynamics in liquids. The experimental results are consistent with the numerical simulations including finite pulse duration.
Measurement of Progressive Addition Lenses
Authors: Fowler Colin W
Published: February 2023

Progressive addition spectacle lenses (PALs) have one surface which is a non-rotationally symmetric aspheric, the second surface being spherical or toroidal as demanded by the prescription. Although such lenses are checked using conventional focimeters for distance prescription, reading addition and prism, this only gives the minimum information about a lens. For a complete picture, the power distribution over the whole lens is required.
Aspherization Does Not Significantly Make Single Vision Lenses Thinner
Authors: Jurkiewiez Fanny , Ahsbahs Françoise , Le Saux Gilles
Published: February 2023

For reasons of comfort and appearance, eyeglasses that are increasingly lightweight, flat and thin are more and more in demand. However, the techniques able to reach significant results for the wearer, while maintaining maximum visual quality (acuity and distortion), are relatively limited. Basically, we can increase the index of the material [1], and flatten the base curve of the lens. In any case, it is necessary to maintain visual acuity in most of the gaze direction. It is the aim of aspherization [2]. But distortion and thickness reduction are only natural consequences of this compensation.
Rod Phototransduction is Altered in Proliferative Central Retinal Vein Occlusion
Authors: Johnson Mary A. , Hood Donald C.
Published: February 2023

Central retinal vein occlusion (CRVO) is a common retinal vascular disorder. In about 20% of the cases of CRVO1, perfusion to the eye is severely reduced, and the eye becomes ischemic and at risk for the development of new blood vessels on the iris. Neovascularization of the iris (NVI) is a serious problem because the new vessels can quickly close off the anterior chamber of the eye, producing the painful and blinding condition of angle closure glaucoma. Treatment to prevent or reverse NVI is aggressive panretinal photocoagulation administered prior to complete angle closure. Identification of CRVO eyes at risk for NVI can be problematic using clinical and angiographic methods2,3.
Retinitis pigmentosa affects cone phototransduction as well as post-synaptic cone activity
Authors: Hood Donald C. , Birch David G.
Published: February 2023

Numerous reports have documented delays in the implicit times of the b-wave of the cone ERG in patients with retinitis pigmentosa (RP) (see ref 1 for review). Abnormalities of the cone a-wave combined with anatomical evidence have lead to the proposal that the cone ERG abnormalities are due, at least in part, to cone photoreceptor abnormalities, specifically to shortened outer segmentse.g.1-3. Other ERG evidencee.g.4-7 is more difficult to explain by a cone receptor defect.
On The True Shape of the Optical Point Spread Function in the Human Eye
Authors: Navarro Rafael , Losada M. Angeles
Published: February 2023

There are basically three different approaches to assess in vivo the optical quality of the retinal image of the eye [1], but none of them is able to provide a reliable estimate of the true shape of the Point Spread Function, PSF. Interferometric methods can provide accurate estimates of the ocular MTF, but to obtain the PSF the Phase Transfer Function, PTF is also needed. The PSF can be computed from wave aberration data. However, the reliability of these computations is unclear, since in practice, it is difficult to measure higher order aberrations, which may have an important contribution to the optical quality of the retinal image. Finally, the double pass method directly provides the aerial PSF, which, however, is affected twice by ocular aberrations. Furthermore, it has been recently shown [2] that the double-pass aerial PSF is actually the autocorrelation of the retinal PSF, which means that the single pass MTF can be accurately estimated, but the PTF is lost. Therefore, the true PSF can not be estimated, and odd aberrations such as coma are symmetrized in the double passage so that they can not be determined.
Effect of Higher Order Aberrations on Image Quality in the Human Eye
Authors: Liang Junzhong , Williams David R.
Published: February 2023

Though measurements of the wave aberrations of the human eye have been made with several techniques1-3, there remains considerable uncertainty about the contribution of higher order aberrations to the optical quality of the human eye4. Knowledge of very high order aberrations is important, for example, because they may limit resolution in fundus imaging. We have constructed a wavefront sensor based on the Hartmann-Shack principle3 to measure the contribution of higher order aberrations in a number of eyes. A point source from a 632.8 nm laser is imaged on the retina and the light reflected from the retina returns through the pupil of the eye. A two-dimensional array of lenslets is conjugate with the eye's pupil, dividing the wavefront exiting the eye into a large number of individual beams. The light from each lenslet is focused on a CCD array, produced an array of spots. Fig. 1a shows the regular array of spots that would have been obtained from a perfect eye with no aberrations; Fig. 1b shows the distorted array of spots obtained with a real human eye. Aberrations in the eye's optics cause displacements in the positions of the spots from which the wavefront error of the eye can be reconstructed.
Line Spread Function and Glare Disability Measurements of Cataractous Eyes
Authors: Magnante Peter , Noonan Carmel , Wolfe Jeremy , Chylack Leo
Published: February 2023

In clinical practice today, the means of relating the severity of cataract to visual function are subjective, requiring patients to respond to test stimuli. The principal objective of our work, however, is to develop an optical method for the objective quantification of image degradation due to cataract that does not require patient response.1 We have constructed a prototype optical instrument that projects and focuses a fine illuminated line on the retina, re-images the resulting retinal line image (the double-pass line spread function or LSF) onto a CCD camera, and quantifies the LSF by measuring its width (specifically the full width at half height or FWHH). In a study of sixty-three patients, we have compared measurements obtained with the LSF instrument to subjective vision test measurements.
Reflectometric Measurement of Human Cone Photoreceptor Directionality
Authors: Burns Stephen A. , Wu Shuang , Eisner Ann E. , Delori Francois
Published: February 2023

The photoreceptors of the human retina are normally aligned such that their long axis points toward the center of the pupil (Stiles; Enoch and Laties). This alignment probably serves several functions. It optimizes sensitivity to light entering the center of the pupil, it decreases sensitivity to intraocular stray light, and may improve the resolution of the eye by apodizing the pupil. Cone photoreceptor alignment is affected by a number of retinal diseases (Enoch, Smith and Pokorny), and has been shown to be related to exudative maculopathy (Smith et al). However, clinical investigation of the relation between photoreceptor alignment and retinal disease has been impeded both by the difficulty of the measurement techniques available and the large investment of patient time required to make measurements.
Theoretical MTF Performance of Intraocular Lenses
Authors: Sun Rong , Portney Valdemar
Published: February 2023

Lens aberrations are highly dependent on the lens shape and the material of the lens (or more exactly, its index of refraction). The common shapes of modem posterior intraocular lenses are plano-convex, biconvex, and equal bi-convex.
Ray-Tracing Computation of Spectacle Magnification and Eye Magnification for Single-Vision Lenses
Authors: Bourdoncle Bernard
Published: February 2023

The use of single-vision lenses to correct ametropias modifies the perception of the sizes of objects viewed. This modification, known as magnification, mainly takes two different forms: 1. The magnification of surrounding objects viewed by the wearer, which is referred to as Spectacle Magnification (SM). 2. The magnification of the wearer's eyes from the viewpoint of an observer, which is referred to as Eye Magnification (EM).
Useful Variations of the Badal Optometer
Authors: Atchison David A. , Bradley Arthur , Thibos Larry , Smith George
Published: February 2023

The Badal principle1 is that, if an eye is placed at the back focal point of a (Badal) lens, the angular size of the perceived object is independent of target position and the target vergence perceived by the eye is proportional to the distance of the target from the front focal point of the lens. The constancy of angular size can be appreciated from looking at Fig. 1. In this diagram, a ray is traced from the top of the target of height
h at
T and parallel to the optical axis through the lens. It is refracted at the point
P at the Badal lens of power F, and in the image space of the lens passes through the back focal point of the lens at
F. Regardless of target distance from the lens, the angle a subtended by the image is always
hF. If the eye is placed here, the apparent angular size of the object must also be invariant.
Influence of the Cardiac Cycle on Topographic Measurements of the Optic Nerve Head and Peripapillary Retina
Authors: Chauhan Balwantray C. , McCormick Terry A.
Published: February 2023

Confocal scanning laser tomography is an imaging technique used for determining the topography of the optic nerve head and retina. Since images can be acquired through undilated pupils and analysed rapidly, the technique is particularly suited for clinical use. One of the largest potentials of scanning laser tomography is the detection of structural alterations in the optic nerve head and peripapillary retina caused by glaucoma. The ability to detect change depends on obtaining an accurate estimate of test-retest variability of regional measurements due to instrumental, computational and a host of physiological factors.
Effect of Retinal Image Motion on Visual Acuity at Low Luminances in Normal Observers and Congenital Nystagmus
Authors: Chung Susana T.L. , Bedell Harold E.
Published: February 2023

Our ability to see fine detail decreases with reduced luminance. This worsening of visual acuity at low luminance can be attributed to at least three factors: increase in photon noise, change in the optical properties of the eye (increased peripheral optical aberrations and night myopia: Campbell & Green, 1965; Coletta & Clark, 1993) and the shift from cone- to rod-mediated vision, in which resolution capability is poorer (Hecht, 1928; Shlaer, 1937). Another characteristic of vision under low luminance is an increase in the integration period, defined as the duration within which energy from the stimulus can be summed to elicit a visual response. For visual acuity, the integration period has been reported to increase from about 100 ms to about 200 ms as luminance decreases from photopic to scotopic levels (Brown & Black, 1976; Kahneman, 1964).
An Economic Analysis of Screening for Amblyopia: Influences of Prevalence, Test Cost, Test Quality, Value of Detection, Sensitivity and Specificity
Authors: Howland Howard C.
Published: February 2023

When is it profitable to screen for amblyopia? In order to answer this question, we must place some values on the various outcomes of screening. What is the value of finding an amblyope at an early age? What is the cost of a false positive, a false negative, or a true negative? Because it is not easy to provide answers to these questions, the method of this paper is to investigate a range of assumptions, the possible values in this pay-off matrix. These are coupled with screening tests of various quality in order to find the minimum prevalence of the disease for which it would be profitable to screen. Indeed, the prevalence of amblyopia is itself unknown, variously estimated at between 0.05 and 0.005 (Ciuffreda et al. 1991; Oliver & Nawratzki, 1971) in normal populations.
The Reliability of Visual Performance Measures in Low Vision
Authors: Woods Russell , Lovie-Kitchin Jan
Published: February 2023

The reliability of measurements of visual performance of subjects with low vision may be expected to be worse than the reliability of subjects with 'normal' vision. Whilst this is commonly accepted in clinical practice, there have been no studies which have demonstrated this effect. This is of practical significance as it is important to know the expected variance in performance to be able to determine what is a 'real' change from the usual measurement noise. In low vision there is often the requirement to monitor on-going ocular disease, and often this involves assessment of vision.