bluuebunny/crossref_metadata_2025 huggingface dataset:
New Frontiers in Ultrafast: Panel Speakers
Authors: Shank C.V. , Campbell M. , Sprangle P.
Published: February 2023

No summary available
A Femtosecond Absorption Study of Skin’s Ultraviolet Chromophore: Urocanic Acid
Authors: Li Bulang , Hanson Kerry M. , Simon John D.
Published: February 2023

Urocanic Acid (trans) is a metabolic product of histidine and accumulates in the epidermis as a result of the absence of the enzyme urocanase in the upper layer of the skin1. Trans-urocanic acid absorbs in both the UVA and UVB regions (Fig. 1) and is proposed to be a major ultraviolet protection agent in the skin because its absorption spectrum covers a similar wavelength region as DNA2,3. The entire physiological implications of urocanic acid are, however, more complicated. Upon exposure to UVA and UVB radiation, trans-urocanic acid isomerizes into cis-urocanic acid (Scheme 1), and the latter has been reported to be a mediator for UV induced immunosuppression4 and is also a suspected carcinogen.
10- to 80-Gb/s, Highly Extinctive Electrooptic Pulse Pattern Generation
Authors: Otsuji Taiichi , Yaita Makoto , Nagatsuma Tadao , Sano Eiichi
Published: February 2023

Practical ultrafast signal sources are urgently required for testing ultrafast photonic and electronic devices [1]-[4]. Pulse-rate tunability and synchronization to electric clocks exciting the device under measurement are substantial requirements for characterization use. Sinusoidal electrooptic phase modulation of around π radian and linear chirp compensation makes possible pulse compression of 0.1 or less from a CW laser light [5], By quadruplexing the bit rates, we have achieved 10- to 72-Gb/s tunability using a 18-GHz bandwidth LiNbO3 phase modulator and a dispersion shifted fiber [2]. The problem is the nonuniform, insufficient extinction ratio varying from 13.5 to 20 dB with the bit rate [6], which seriously degrades the pulse eye opening. This variation is caused by the residual wings caused by nonlinear chirp components generated by sinusoidal phase modulation.
Measuring femtosecond pulses with linear optics and nonlinear electronics
Authors: Diddams Scott , Prein Steffen , Diels Jean-Claude
Published: February 2023

Current techniques for the measurement of femtosecond laser pulses require an optical nonlinearity with a femtosecond response time. Common examples are a second harmonic generating crystal or a material with a Kerr nonlinearity. However, a need exists for measurement techniques for applications involving weak pulses and/or pulses in spectral regions where appropriate nonlinear crystals are not available (i.e. ultraviolet). Semiconductor detectors holds the promise of good sensitivity over a very broad bandwidth, but unfortunately, the fastest response times are still in the picosecond domain. Nonetheless, creative combinations of linear optics and fast electronic detection can be used to characterize optical fields on a femtosecond time scale. Our measurements employ a GaAs Schottky photodiode monolithically integrated with a microwave detector [1] to perform cross-correlations between the pulse to be measured and spectrally filtered slices of the same pulse [2]. To our knowledge, this is the first direct measurement of the amplitude and phase of a femtosecond pulse employing only an integrated opto-electronic circuit. Although demonstrated here for the meausrment of pulses from a standard Ti:sapphire laser, we believe this technique should also enable the complete characterization of ultraviolet pulses. In addition, we are studying various “single-shot” implementations.
Two-photon excited fluorescence microscopy with time- and wavelength resolved detection
Authors: Andersson-Engels Stefan , Carlsson Jörgen , Rokahr Ingrid
Published: February 2023

Two-photon excited fluorescence microscopy [1] is emerging as a possible alternative of confocal microscopy. Using two-photon excitation a three-dimensional resolution is obtained as a direct result of the tight focusing of the ultrashort excitation pulses. Consequently, it can be performed in a conventional fluorescence microscope. In the present study, two-photon excited fluorescence microscopy is performed for different types of samples and combined with time- and wavelength resolved spectroscopy [2, 3].
Coherent Control In Ultrafast Photon Echo Spectroscopy: Liouville-Space Pathways Interference
Authors: Pshenichnikov Maxim S. , de Boeij Wim P. , Wiersma Douwe A.
Published: February 2023

Dephasing-induced resonances, arising from interference between different excitation pathways, are well-known in frequency-domain spectroscopy.[1] It is generally thought that the time-domain analog of this phenomena is difficult to achieve due to the explicit time ordering of the interaction fields.[2,3] In this contribution we report on the observation of time-domain interference in phase-locked stimulated photon echo. We also show how complete control over the interference effect can be exercised.
One pump beam impulsive excitation of nonsymmetric phonons.
Authors: Gautier C. A. , Albert O. , Loulergue J.C. , Etchepare J.
Published: February 2023

The trivial way to drive phonons with a specific symmetry consists in the use of a two-pump-beam geometry. The associated set-up is well known as transient grating or four wave mixing experiment. The phonon wave-vector value q is there a function of θ angle value between the pump beams. The lowest value of q one can reach is obtained when θ=0. In the case of nonsymmetric modes this value depends on the anisotropy of the crystal under study. Nevertheless, all experimentally published works using one-pump-beam apparatus, lead to the excitation of completely symmetric modes: the processes involved had an isotropic polarization dependence[1].
Ultrafast surface reaction and desorption probed by femtosecond second harmonic generation: Cl etching of Si(111) surface
Authors: Sasaki F. , Haraichi S. , Kobayashi S. , Tani T. , Komuro M.
Published: February 2023

In order to establish a new maicrofabrication of semiconductors, we need to clarify the mechanisms of reaction between material surfaces and reactive species. Especially, the dry etching process on Si surface is the most spreading process in the electronics applications and has been extensively studied [1] The optical second harmonic generation is also a powerful technique to study surface science because of its surface sensitivity, non-detrimental and in-situ observation, and the ultrafast response [2]. Femtosecond time-resolved studies of the desorption and/or reaction on the metal or semiconductor surfaces have been earned out [3,4]. In this letter we report the results of the laser-induced surface reaction and/or desorption on the Si(111)/Cl2 system by using the femtosecond second harmonic generation.
A different view of the quantum control of matter
Authors: Walmsley Ian A. , Stroud Carlos R.
Published: February 2023

Quantum control theory has been developed in the past few years [1] by extending the control theory of classical systems into the quantum domain. It has lead to new insights into the way in which matter can be manipulated that may have important consequences for chemistry, biology, materials science and possibly optical communications and computing.
Selective Excitation of Wavepacket Motion in the Condensed Phase Using Chirped Pulses
Authors: Bardeen Christopher J. , Wang Qing , Shank Charles V.
Published: February 2023

Recent theory and experiments have shown that chirped ultrashort pulses may be used to control the dynamics of wavefunctions evolving on potential energy surfaces of isolated molecular systems [1]. In this work we show that the phase structure of the excitation pulse can substantially affect the coherent vibrational dynamics of molecules in room temperature liquids. The interaction of a short pulse with a two electronic state system can induce vibrational motion on both the excited and ground state potential energy surfaces, which in turn can lead to ambiguity in the interpretation of pump-probe experiments [2,3]. Figure 1 illustrates the impulsive stimulated Raman scattering process that gives rise to motion on the S0 state and shows how a negatively chirped (NC) pulse enhances this contribution through a single photon “pump-dump” sequence of field interactions. Conversely, a positively chirped (PC) pulse discriminates against this process. In accord with previous theoretical work [4], one can to some extent turn on or off the ground state contribution to the transient absorption signal by using an appropriately chirped excitation pulse.. In this way chirping the pump pulse can help determine the origin of experimentally observed coherent vibrational dynamics.
Harmonic Generation from a Sharp Plasma-Vacuum Boundary
Authors: von der Linde D. , Engers T. , Jenke G. , Agostini P. , Grillon G. , Chambaret J.-P. , Mysyrowicz A. , Antonetti A.
Published: February 2023

During the interaction of an intense, sharply rising femtosecond laser pulse with the surface of a solid target a thin layer of plasma is formed in which the density falls from approximately solid density to vacuum in a very short distance. It has been recognized more than a decade ago that the dynamics of electrons driven across such a sharp plasma-vacuum boundary is highly nonlinear, giving rise to the generation of optical harmonics of high order1-2. With the extremely high laser intensity available today the laser-induced excursion of the electrons may reach a considerable fraction of the wavelength, and significant contributions to the electronic anharmonicity from relativistic effects come into play. The relativistic interaction regime offers higher efficiency and an extension of harmonic generation to much shorter wavelengths. Thus the generation of harmonics using a solid target is a promising tool for the generation of coherent beams of ultrashort soft x-ray pulses.
Comparison of Spontaneous and Impulsive Stimulated Resonance Raman Excitation Profiles
Authors: Johnson Alan E. , Myers Anne B.
Published: February 2023

Spontaneous resonance Raman (RR)1 and resonant impulsive stimulated Raman scattering (RISRS)2 both contain vibrationally specific information about the structure and dynamics of a resonant excited electronic state, even when that state has a very short (sub-vibrational period) lifetime. Steady-state RR cross sections have a time-domain interpretation in which the laser field transfers part of the initial ground-state wavefunction to the excited state surface, and the spectrum of the spontaneously emitted light provides information on the evolution of that excited-state amplitude on the upper surface. In RISRS, a femtosecond pump pulse drives the initial thermal equilibrium state into a coherent superposition of ground-state vibrational levels, and a delayed probe pulse monitors the resulting perturbation of the chromophore's absorption. This time-domain signal can be Fourier transformed to give a spectrum usually dominated by Franck-Condon active vibrational frequencies of the ground state. However, the Fourier transform of the RISRS signal (FT-RISRS) is by no means equivalent to a RR spectrum, and important qualitative errors of interpretation can result if this assumption is made.3
Ultrafast dephasing dynamics and multi-level electronic structure of InP nanocrystals
Authors: Cerullo G. , Banin U. , Guzelian A. A. , Bardeen C. J. , Schoenlein R. W. , Alivisatos A. P. , Shank C. V.
Published: February 2023

Semiconductor nanocrystals are a novel class of materials intermediate between the molecular and solid state forms of matter. Their homogenous linewidths and electronic structure are of considerable interest for the understanding of the physics of quantum confinement. In this work we study the electronic and vibrational dephasing dynamics in InP nanocrystals using femtosecond three pulse photon echo (3PPE) experiments1 complemented by nanosecond transient hole-burning (HB) spectroscopy.2 The echo decay is explained by taking into account the multi-level electronic structure as observed in the hole-burning experiment.
Femtosecond Infrared Studies of Photosynthetic Reaction Centers: New Charge Transfer Bands and Ultrafast Energy Redistribution
Authors: Haran Gilad , Wynne Klaas , Moser Chris C. , Dutton P. Leslie , Hochstrasser Robin M.
Published: February 2023

An accurate description of the mechanism of the primary electron transfer process in Photosynthetic Reaction Centers requires a full elucidation of the electronic structure of the cofactors involved in the process. In order to characterize low-lying excited states of reaction centers we performed ultrafast visible pump/ IR probe experiments. The mid- to near-IR difference spectra of optically pumped and unpumped Reaction Centers were recorded over a time range circa 0.1 to 20 ps (Wynne et al., 1996), allowing us to isolate kinetically the transient electronic spectra of the excited special pair P* and its oxidized form P+H-. The spectrum of P* between 1 and 5.2 μm (Figure 1A) exhibits two main bands, one at 5,300 cm-1 and another at 2,710 cm-1 while P+H- (Figure 1B) exhibits two bands, one at 2,600 cm-1 and another at 8,000 cm-1. These new states do not appear as transitions of unexcited Reaction Centers at E(P*)+5,300 cm-1 or E(P*)+2,710 cm-1 (although the former is close to the Qx transition region of the special pair) suggesting they derive almost exclusively from charge transfer (PL+PM- or PL-PM+) states. This conclusion is strengthened by the anisotropies obtained by pumping P* and probing these transitions with polarized light. The transition of P+H- at 2,600 cm-1 corresponds to the known hole transfer transition (Bréton et al., 1992) and this assignment is confirmed by kinetic and anisotropy measurements: The transition dipole is along the direction of charge transfer between PL and PM. The anisotropy for the transition at 8,000 cm-1 is consistent with a transition dipole along the y-axis of PM, in agreement with theoretical predictions for a trip-doublet transition. The results constitute the first direct measurement of the charge transfer bands of the Special Pair.
Regenerative pulse shaping: a new technique for ultrabroadband amplification
Authors: Squier J.A. , Guo T. , LeBlanc C. , Korn G. , Rose-Petruck C. , Raksi F. , Yakovlev V.V. , Yamakawa K. , Barty C.P.J.
Published: February 2023

Traditionally, pulse shaping of femtosecond pulses is accomplished by first dispersing the pulse using a grating, and modulating the spatially chirped beam at the transform plane of an imaging system. In this paper we introduce regenerative, intra-cavity pulse shaping of chirped pulses. This method has inherent advantages over traditional pulse shaping techniques for producing certain types of pulse shapes, and overcoming limiting processes such as gain narrowing. Using regenerative pulse shaping we demonstrate for the first time 1) amplification beyond the gain narrowing limit, 2) simultaneous amplification of two ~100-fs pulses separated in wavelength by as much as 100 nm, and finally, 3) difference frequency mixing of these two amplified pulses to produce tunable mid-IR (6-12 μm) ultrashort pulses.
Femtosecond Dynamics of the Bacteriochlorophyll-a Dimer in the B820 Subunit from the Core (LH-1) Antenna Complex of Rs. rubrum
Authors: Kumble Ranjit , Palese Steven , Visschers R. W. , Dutton P. Leslie , Hochstrasser Robin M.
Published: February 2023

The core (LH-1) antenna complexes of purple non-sulfur bacteria are membrane-bound light-harvesting complexes which enclose the reaction centers.1 They can be described as circular aggregates of a basic subunit (typically 12 per LH-1 complex1) consisting of two helical polypeptides (α and β) and two bacteriochlorophyll-a (BChla) chromophores. Under special conditions, the LH-1 complex can be reversibly dissociated into such subunits, which display spectral properties consistent with a those of strongly coupled BChla dimer.1 A crucial yet unresolved issue concerns the extent of interaction between neighbouring subunits in the intact complex, i.e. the degree to which dimeric properties are retained in the functional aggregate. It remains debated as to whether the electronic states are delocalized over several pigments,2 or may be viewed as being localized on individual dimers or tetramers.3 Knowledge of the exciton level structure and dynamics within the subunit, and the effects of aggregation on these properties is clearly required to establish an understanding of the mechanism of energy migration. We report here the preliminary results from studies of the femtosecond transient kinetics of the B820 subunit (which incorporates one dimer of BChla) from the LH-1 complex of Rhodospirillum rubrum G9.
New Features in the Ultrafast Ligand Dynamics and Energy Dissipation in Myoglobin
Authors: Kholodenko Yuriy , Volk Martin , Gooding Ed , Hochstrasser Robin M.
Published: February 2023

Numerous transformations in heme proteins can be triggered upon photoexcitation, providing perfect model systems for the study of protein dynamics. In particular, the recombination of CO after photodissociation has been investigated in great detail. It was found to occur on the ns time scale. In contrast, the geminate recombination of NO proceeds much faster, within several 100 ps, i.e. on the same time scale as the relaxation of the protein to its unligated structure [1]. Furthermore, the recombination of NO was found to be nonexponential. Several mechanisms have been suggested to explain this behavior: (i) distribution of the barrier to rebinding due to different protein substates (inhomogeneous model), (ii) time dependence of the barrier due to protein/heme relaxation to the unligated structure or (iii) dissociation of the ligand to different intermediate sites in the protein. We are investigating these possibilities by time resolved IR measurements of the NO recombination. Furthermore, the comparison of native heme proteins with specially designed mutants will discriminate between the different mechanisms. We have also investigated the effect of the amount of excess energy in the precursor state on the resulting dynamics. The question is whether a higher excitation energy leads to the release of a ligand with higher kinetic energy, and thereby results in different intermediate protein sites, or whether the excess energy simply leads to hotter heme product which then undergoes vibrational cooling.
Optimal Sequences for Chirped-Pulse Stimulated Raman Preparation of Large Amplitude Vibrational Motion
Authors: Hiller Eric M. , Cina Jeffrey A.
Published: February 2023

Recent calculations [1, see also 2-5] indicate that suitably timed sequences of electronically preresonant femtosecond laser pulses can induce large amplitude coherent molecular vibrations of dilute chromophores in low temperature hosts, without undue electronic absorption or other loss processes. Here we demonstrate that frequency modulation, in the forms of variable pulse center frequency and variable chirp rate, can significantly further enhance the efficiency of impulsive stimulated Raman preparation of coherent vibrations in the electronic ground state.
Mid-Infrared Femtosecond Spectroscopy of Intersubband Hot Hole Relaxation in Quantum Wells
Authors: Fauchet P. M. , Vandyshev Ju. V. , Xu Z. , Rella C. W. , Schwettman H. A. , Wicks G. W.
Published: February 2023

We report the first direct measurement of the relaxation time of holes in p-type quantum wells using femtosecond mid-infrared laser pulses tuned on resonance with the intersubband transition energy. In the early experiments, we used the Stanford free electron laser (FEL) which generated submicrojoule, subpicosecond pulses tunable from 4 to 6 μm [1]. Recently, we have used microjoule femtosecond pulses tunable from 4 to 9 μm generated by frequency mixing the signal and idler outputs of a parametric amplifier pumped by an amplified Ti:S laser. Most of the measurements have been performed in the degenerate pump-probe configuration as a function of hole density, sample temperature, pump wavelength and pump intensity. The multiple quantum well (MQW) samples investigated to date consist of strained In0.5Ga0.5As wells and Al0.5Ga0.5As barriers. Fig. 1 shows the infrared absorption of a typical sample containing a hole sheet density of 1013 cm-2. The absorption at 5.25 μm corresponds to the n=1 heavy hole (hh) to n=1 light hole (lh) transition.
Determination of the electron mobility in multiple quantum wells by time-resolved optical measurements
Authors: Cameron A.R. , Riblet P. , Miller A.
Published: February 2023

Optical induced concentration gratings has been extensively used to determine the ambipolar diffusion coefficient in bulk1 and quantum well2 semiconductors. These gratings are based on the spatial modulation of equal numbers of excess electrons and holes across the excitation region. They are optically induced in the sample by the interference of two laser pulses with parallel polarisation. Monitoring the grating decay for different grating periods allows the determination of the ambipolar diffusion coefficient. This coefficient is dominated by the motion of the slower carriers, i.e. holes, and gives no accurate information about the faster electron motion.
Quasi-Zero-Dimensional Media Formed by Thin-Film Technique: Microstructure, Subpicosecond Optical Nonlinearities, Applications
Authors: Goncharova Olga V.
Published: February 2023

Three-dimensional (3D) confinement of excitons or electrons motion in nanometer-sized microcrystallites (nanocrystals — NCs) is an important phenomenon in physics, and is expected to bring a significant improvement to ultrafast functional devices. In the past ten years, several types of quasi-zero-dimensional (QZD) media such as dielectric matrices containing dispersed semiconductor- as well as metal NCs have been realized by using various techniques. Recently, we have succeeded in the preparation of thin-film QZD media. A reliable process has also been developed for the fabrication of thin-film Fabry-Perot interferometers (TFIs) with intermediate layers in form of QZD media.
Femtosecond Kinetics of Nonthermal Electron and Hole Distributions in GaAs
Authors: Leitenstorfer A. , Fürst C. , Laubereau A.
Published: February 2023

In intrinsic semiconductors, the ultrafast intraband relaxation of nonequilibrium carriers is governed by two important processes: carrier-carrier (cc) and carrier-phonon (cp) scattering. Valuable information on these basic interactions is extracted from experiments if the different contributions to the kinetics can be clearly separated. However, time and energy resolved data under conditions where cp scattering is the dominating effect for the first relaxation steps, are difficult to obtain because of the limited sensitivity of ultrafast measurements at low carrier concentrations.
Femtosecond interfacial electron transfer in dye sensitised nanocrystalline TiO2 photovoltaic solar cells
Authors: Durrant James R. , Tachibana Y. , Moser J. , Grätzel M. , Klug D.R.
Published: February 2023

The sensitisation of wide band gap semiconductors to visible irradiation by the adsorption of dyes to their surface is of widespread technological importance. It is the basis of silver photography and electrophotography, and has for many years been studied as a means of solar energy conversion. Electrochemical PV solar cells employing dye sensitised nanocrystalline TiO2 thin films have already achieved >10 % energy conversion efficiencies [1] and are now in commercial production. However, the mechanism of the dye sensitisation remains poorly understood, and this missing information is impeding technological applications.
Superlattice Exciton Dynamics Probed by a Novel Interband-Pump Intersubband-Probe Technique
Authors: Duer R. , Gershoni D. , Shtrichman I. , Ehrenfreund E.
Published: February 2023

We present a conclusive study of the dynamics of excitons in photoexcited GaAs/GaAlAs quantum wells, using a novel interband-pump intersubband-probe technique. This technique probes the mid IR absorption due to optical transitions of carriers between their subbands, following photogeneration by a picosecond interband pulse.
Dynamical Studies of Non-linear Refractive Index by Cross Phase Modulation
Authors: Ripoche J.-F. , Franco M. A. , Grillon G. , Nibbering E. T. J. , Prade B. S. , Mysyrowicz A.
Published: February 2023

The precise measurement of the dynamics of the Kerr-like non-linearity is an important issue since it affects the propagation of intense laser beams in transparent media. Besides the instantaneous electronic response, many materials exhibit a delayed response due to the nuclear motion. This may lead to very complex situations which often cannot be described by a single exponential decay law for the induced non-linearity.
Mid-infrared femtosecond pulse generation with a 250 kHz Ti:sapphire pumped β-BaB2O4 optical parametric amplifier and DFG in AgGaS2
Authors: Reed Murray K. , Steiner-Shepard Michael K.
Published: February 2023

A high power source of sub-picosecond light pulses tunable in the mid-infrared would have applications in the ultrafast spectroscopy of molecular vibrations and intersubband absorption in semiconductors. Demonstrations of such sources include a Nd:YAG based OPG/OPA with 4-mW output at 3-μm and 1-mW output at 5-μm [1] and Ti:sapphire laser based OPO/DFGs [2, 3] with 0.5-mW output at 3-μm and 0.2-mW output at 5-μm. Seifert et al [4] used a commercial 1-kHz 800-nm Ti:sapphire regenerative amplifier system and demonstrated mid-IR tuning by conversion of the output from a LBO OPG/BBO Type-I OPA through difference frequency generation (DFG) in AgGaS2. They report an output power of 0.05-mW at 5-μm.We have demonstrated that it is possible to operate an OPA system with pulse energies of only a few microjoules with a 400-nm pumped, β-barium-borate (BBO) Type-I OPA using a 250-kHz Ti:sapphire regen [5]. In this paper we describe the conversion of a low energy 800-nm pumped, BBO Type-II OPA through DFG in AgGaS2 to the midinfrared from 2.4-μm to beyond 12-μm with more than 10-mW of power from 3-μm to 6-μm.
Excitation of Surface Electromagnetic Wave in High-Temperature Near-Surface Femtosecond Plasmas; Resonance Second Harmonic Generation
Authors: Tarasevitch A.P. , Gordienko V.M. , Savel'ev A.B. , Timoshin A.O. , Volkov R.V.
Published: February 2023

The interest towards plasma generated by high-power femtosecond laser pulses on a solid target surface (femtosecond near-surface high-temperature plasma - FNHP), is related with its unique properties (including high optical nonlinearity) as well as with the possibility of high-power x-ray generation. In this paper we consider the conditions of surface electromagnetic waves (SEW) excitation in FNHP. There are several reasons for this research: i) It is rather promising to use resonance SEW excitation in order to modify FNHP parameters: to increase plasma absorption, x-ray yield and harmonic generation efficiency. Up to now it is well known that the resonance SEW excitation on a metal surface can cause a decrease in the specular reflectivity and a large increase in the surface effective optical nonlinearity; ii) Little is known about plasma waves in FNHP. The study of SEW can provide information about collective excitations in FNHP in general. iii) SEW are known to be a sensitive tool for investigation of surface characteristics.
Optical Coherence Tomographic Imaging Using a Mode Locked Cr4+:Forsterite Laser Source
Authors: Bouma B. E. , Tearney G. J. , Golubovic B. , Fujimoto J. G.
Published: February 2023

Optical coherence tomography (OCT) is a new technique for micron scale, cross-sectional, tomographic imaging.1 In biology and medicine, OCT can be a powerful technique for optical biopsy because it permits the direct imaging of cross-sectional tissue architectural morphology without the need to excisionally remove specimens. OCT imaging is somewhat analogous to radar or ultrasound, except that OCT uses a low coherence optical source and interferometry to perform time delay measurement of light which is backreflected or backscattered from optically turbid specimens. Superluminescent diodes can provide 15-20 micron resolution imaging at several fixed wavelengths, including 800 and 1300 nm, but output powers are limited to the order of a milliwatt. Studies at these wavelengths have demonstrated that 1300 nm wavelengths can achieve greater imaging penetration depths in biological tissues because of reduced scattering. Ultrahigh resolution imaging has been demonstrated using the fluorescent emission from solid state laser media2 and femtosecond pulses from a mode locked Ti:Al2O3 laser.3 OCT using mode locked solid state lasers3 is especially promising because the high output powers available enable higher signal to noise levels as well as faster data acquisition. Using a mode locked source, OCT imaging rates approaching real time can be achieved. In addition, the spectral bandwidth of femtosecond pulses can be broadened by using self phase modulation in an optical fiber thus yielding significant increases in imaging resolution. In this paper we present a Kerr lens mode locked Forsterite laser (at 1.3 micron wavelength) which is spectrally broadened through self-phase modulation in a dispersion shifted fiber. With this source 5.7 micron resolution OCT images with 116 dB signal to noise are demonstrated. Applications of mode locked solid state technology for OCT biomedical imaging is discussed.
Enhanced Radiative Decay and Optical Dephasing of Excitons in GaAs Quantum Well Bragg Structures
Authors: Kuhl J. , Hübner M. , Stroucken T. , Knorr A. , Koch S.W. , Hey R. , Ploog K.
Published: February 2023

Rapid superradiant decay of coherently excited excitons in thin quantum wells (QWs) on a time scale of approximately 10 ps has been theoretically predicted by several groups [1], however, the experimental observation of this ultrafast radiative recombination is still the subject of controversial discussions. A recent theoretical paper [2] concludes, that this radiative decay rate should be strongly enhanced in multiple QW (MQW) Bragg structures due to radiative coupling of excitons excited in different wells via the exchange of transverse photons.
Spin quantum beats in bulk and low dimensional semiconductors
Authors: Oestreich M. , Hallstein S. , Nötzel R. , Ploog K. , Bauser E. , Rühle W. W. , Köhler K.
Published: February 2023

The electron Landé g factor is one of the basic parameters in semiconductors which de scribes the magnitude of the Zeeman splitting of electronic states in magnetic fields. Since various theoretical models predict the value of g, accurate measurements of g provide a sensitive test of different band structure calculations. A recently introduced experimental technique enables the measurement of the electron g factor g* with high accuracy by spin quantum beats. [1] The technique proves to be feasible to measure various effects as the anisotropy of g* in quantum wires,[2] the dependence of g* on quantum well thickness,[3] and the temperature dependence of g* in bulk GaAs up to room temperature. [4, 5] The temperature dependent spin quantum beat experiments show interesting discrepancies between experiment and a well accepted five-band
k→⋅p→ theory model.[4]
Refractive Surgery — Success and the Public Health
Authors: Maguire Leo
Published: February 2023

Summary not available.
High and Low Contrast Acuity Following Excimer Photorefractive Keratectomy
Authors: Casson E. J. , Jackson W. B. , Mintsioulis G.
Published: February 2023

Studies of photorefractive keratecomy (PRK), have generally concentrated on determining the refractive outcome of the surgery. These studies determined that PRK produces predictable and stable results, particularly for mild to moderate myopes.1-4 Visual function estimates in these studies are usually limited to high contrast acuity and show that the vast majority of individuals remain within one line of their pre-operative, best-corrected acuity, while greatly improving their uncorrected visual acuity.5
Is Magnification a Cue in Defocus Induced Ametropia in Chicks?
Authors: Sivak J.G. , Curry T.A. , Callender M.G. , Irving E.L.
Published: February 2023

A number of experiments have demonstrated that it is possible to induce ametropias in the eyes of young animals by distorting early visual experience. Much of the work has involved the use of chicks (Gallus gallus domesticus) because of their precocial nature and because they grow and develop rapidly. The earlier studies generally involved depriving the retina of a clear image by either suturing the eyelids together or applying some kind of translucent or light scattering device over the eye during development (e.g., Pickett-Seltner, et al., 1987). This kind of treatment invariably leads to varying levels of myopia, often referred to as form deprivation or experimental myopia, through the axial elongation of the eye.
Scanning laser ophthalmoscope imaging of the papillomacular bundle of the nerve fiber layer
Authors: Grosof David H. , Lakshminarayanan Vasudevan
Published: February 2023

The nerve fiber layer (NFL), consisting of the axons of ganglion cells sweeping across the inner surface of the retina to the disc, transmits all the visual information from the eye to the brain. The normal, topographically regular pattern of nerve fibers is illustrated in Figure 1. Ophthalmoscopically, the nerve fiber layer is recognized as a pattern of subtle striations emanating from the disc a short distance; the striations are evidently bundles of axons contained within glial tunnels formed by Muller's cell processes6. Glaucoma and other optic neuropathies damage the nerve fibers and cause defects within the pattern. The bundles are fairly easy to see near the disc, and the circumpapillary nerve fibers have proved useful to image, because fiber damage and defects in the striation pattern sometimes precede detectable visual field defects. It has previously proved impossible, however, to image non-invasively the details of the perifoveally originating papillomacular bundles. Because the visual information from the perifoveal area is crucial to visual performance and because some neuropathies affect central vision preferentially (e.g., Leber's optic neuropathy), it may be useful to visualize the papillomacular bundle near the fovea, where a focal pattern of loss might be noticeable and well-correlated to focal visual deficits.
Methodological and Statistical Issues in the Evaluation of Amblyopia Therapy
Authors: Moseley Merrick J. , Irwin Mary , Auld Rosemary , Jones Helen S. , Cocker Kenneth D. , Fielder Alistair R.
Published: February 2023

Our concern is with the evaluation of treatments for amblyopia, specifically with that of occlusion therapy. Some of the issues considered here are also relevant to the new pharmacological treatments under development (e.g. see Leguire, 1993).
Early Detection of Macular Dysfunction in the Topography of the Electroretinogram
Authors: Bearse Marcus A. , Sutter Erich E. , Smith David N. , Rose Steven J.
Published: February 2023

In its earliest stages, macular disease is often difficult to confirm by objective means. A patient may report vision problems while little or no macular abnormality is evident upon examination. In a case where tests are inconclusive, the patient may become increasingly concerned with his or her vision and also frustrated by the lack of a certain diagnosis. For these reasons, and for a more complete characterization of the retinal abnormality, a sensitive and objective means is needed to detect retinal dysfunction in the early stages of macular disease.
Foveal Ganglion Cell Function in Patients with Stabilized Glaucoma and a Clinically Normal Fovea
Authors: Swanson William H. , Fellman Ronald L. , Lynn John. R. , Starita Richard J.
Published: February 2023

Clinical automated static perimetry for glaucoma typically measures white-on-white increment thresholds for a grid of locations throughout the central visual field.1,2 Relative scotomas (regions of the visual field for which threshold is abnormally high) are early signs of glaucomatous damage. Increase in number, size or depth of relative scotomas is an indication of progression of glaucomatous damage. Surgical reduction of intraocular pressure (IOP) to subnormal levels can halt progression of perimetric defect for several years even in cases of extreme damage.3
Accommodation, phorias, and AC/A ratios in school-age children
Authors: Gwiazda Jane , Bauer Joseph , Thorn Frank
Published: February 2023

Changes in the interactions between accommodation and vergence have been linked to the development of myopia in adults. Jiang (1995) reported that AC/A (accommodative convergence/ accommodation) ratios of young adults who became myopic over a 2 to 3 year period were higher at the outset than those of subjects who remained emmetropic. Previously we reported that newly myopic children show insufficient accommodative response to blur (Gwiazda et al, 1993). We have speculated that this accommodative lag might create a blurred image on the retina when an individual is engaged in near work, which could induce myopia, as occurs with animal models. Reduced accommodation could also account for elevated AC/A ratios in myopes.
Individual Variations in the Spatial Profile of Macular Pigment
Authors: Hammond Billy R. , Wooten Billy R. , Snodderly D. Max
Published: February 2023

Past psychophysical studies have shown that macular pigment optical density (MP) varies dramatically among individuals.1-5 Such studies have examined MP density differences using limited stimuli; typically a one-degree test field is used. Consequently, little is known regarding individual variations in other aspects of MP such as spatial dimensions. Psychophysical studies that have mapped MP spatial profiles5-7 have been conducted on very small samples providing no information on the range of individual differences. Such differences may be important for both basic and applied research. On a basic level, variability in MP distributions is an important consideration in quantitative models of color vision. On an applied level, since MP preferentially absorbs damaging short-wavelength light and peaks-in a region of the retina (i.e., the foveola) that appears to be most resistant to degenerative change, differences in the spatial profile of MP may have clinical consequences. This hypothesis is consistent with the findings of Weiter, Delori and Dorey (1988)8, who examined patients with annular maculopathy. They found that the spatial profile of MP was highly related to the "bulls eye" region of the retina that was spared in these degenerative conditions. Given the implications for both basic and applied research, normative data on individual variations in MP spatial profiles is needed.
Orientation Discrimination in the Periphery: Evidence for Aliasing by Receptoral and Neural Sampling Arrays
Authors: Coletta Nancy J. , Sharma Vineeta , Carter John
Published: February 2023

When a high frequency grating is imaged on the parafoveal retina, it can appear to be oriented 90° from its true orientation. The orientation reversal occurs when the grating spatial frequency is twice the Nyquist frequency of the parafoveal cone mosaic, because the frequency spectrum of the sampled grating has aliased spatial energy near its origin that is oriented perpendicular to the actual grating orientation (Yellott, 1983; Coletta and Williams, 1987).
Predicting Variations in Visual Performance Caused by Optical Defects
Authors: Atchison David , Woods Russell , Bradley Arthur
Published: February 2023

Uncorrected spherical and astigmatic defocus are the most significant sources of retinal image degradation in human eyes. When sufficient defocus is added to an otherwise diffraction limited optical system, the modulation transfer function (MTF) drops quickly from 1.0 at zero spatial frequency and becomes zero. Beyond this first zero, the MTF oscillates and additional zeros occur (Figure 1a). Each time the MTF passes through zero there is a phase shift of half a cycle. When spherical aberration is added to defocus, the shape of the MTF changes (Figure 1a). If the spherical aberration has the same sign as defocus (in wave aberration terms), the zeros in the MTF will-occur at lower spatial frequencies. The reverse is true if the signs are opposite. The dips and rises in the MTF are more marked when defocus and spherical aberration have the opposite sign than when have the same sign. Other aberrations or combinations of aberrations can cause the MTF to rise and fall without necessarily passing through zero, with shape and magnitude of the MTF often dependent on the object orientation (Figure 1b).
Reading Rates for RSVP and Scrolled Text are the Same Regardless of Acuity Reserve
Authors: Fine Elisabeth M. , Peli Eli , Labianca Angela T.
Published: February 2023

Whittaker and Lovie-Kitchin (1993) refer to the ratio of text character size to acuity threshold character size as acuity reserve (AR). Most studies of reading with normally sighted subjects have used character sizes about 4× the acuity threshold of the participants (0.33 deg). This is slightly larger than newspaper print (an 'x' measures approximately 1.8 mm) read from 40 cm, and Rubin and Turano (1992) have shown that varying the character size from 2 to 30× acuity threshold has almost no effect on reading rate from a computerized page display.
Contour Plots and the Limits of ‘Clear and Comfortable Vision’ in the Near Zone of Progressive Addition Lenses
Authors: Fisher Scott
Published: February 2023

Surface astigmatism contour plots are a commonly used tool in the evaluation of progressive addition lens (PAL) designs. Astigmatism contour plots are topographical representations of the lens surface as defined by isoastigmatism contours, lines joining points of equal astigmatism, Figure 1. These contour plots are often used to represent the lens optics. However, the relationship between this representation of the optics and human performance or subjective acceptance has not been well studied.
Variation in Retinal Pigment Epithelial Function with Blood Glucose as Measured with the Fast Oscillation of the Electro-Oculogram
Authors: Fortune Brad , Schneck Marilyn E. , Hong Happy , Adams Anthony J.
Published: February 2023

We have previously demonstrated that functional changes in the visual system accompany fluctuations in blood glucose occurring over minutes or hours. These changes in vision function occur in both diabetic and non-diabetic observers and can be demonstrated psychophysically1 as well as with visually-evoked cortical potentials (VEP's)2.
Effect of 10 Hz Flicker on Retinal Hemodynamics Measured by the Heidelberg Retina Flowmeter
Authors: Harrison J.M. , Elliott W.R. , Kiel J.W. , Sponsel W.E.
Published: February 2023

Localized flicker-induced increase in retinal and optic nerve head blood flow appears to be a replicable and robust phenomenon. Several investigators have shown that there are localized increases in retinal and optic nerve head (ONH) blood flow and velocity when the retina is stimulated by light flickering at around 10 per second.
A Comparison of Visual Field Loss to Multi-focal ERG Changes in Patients with RP
Authors: Hood D. C. , Holopigian K. , Greenstein V. , Seiple W. , Li J. , Sutter E. E. , Carr R. E.
Published: February 2023

Many patients with retinitis pigmentosa (RP) show cone ERG b-waves that are depressed in amplitude and delayed in timing even during the early stages of RPe.g.1-3. Further, patients' photopic, visual fields measured with static perimetry show a range of losses in sensitivity both within and across patients4-7. Because of the difficulty in obtaining focal ERGs with conventional methods, little is known about the relationship of local sensitivity to local retinal activity outside the central retina. The multi-focal technique as described by Sutter and colleagues8,9 allows for simultaneous measurement of 103 focal retinal responses. Although a general agreement has been observed between visual field losses and multi-focal ERGs in patients with RP10-12, the relationship between local sensitivity changes and multi--focal ERG changes has yet to be explored. Here patients' multi-focal ERGs are compared to local visual field sensitivity measured with static perimetry.
Aging Changes in the Human Accommodative Process
Authors: Koretz Jane F. , Cook Christopher A. , Kaufman Paul L.
Published: February 2023

Accommodation, the process by which the human eye focuses on near objects, takes place through carefully controlled changes of the crystalline lens in shape, thickness, and anterior surface location relative to the cornea. These changes, which involve an elastic recovery of the lens upon accommodation from the maximally stressed and flattened non-accommodated lens shape, are coupled with the contraction of the ciliary muscle, which itself changes in shape and location relative to fixed points within the globe. The refractive range over which accommodation can occur is age-dependent. In youthful eyes, the range can be as great as 15 diopters, but the near point recedes steadily with increasing age in the adult eye until, by about 50 yr, it closely approaches the far point. This diminution in accommodative range is accompanied by other aging changes in the lens, ciliary muscle, and anterior segment geometry.
Scattered Light Tomography: Instrumentation and Ocular Fundus Data with A Vertical Cavity Surface Emitting Laser
Authors: Eisner Ann E. , Dreher Andreas W. , Reiter Klaus , Zhou Qienyuan , Reed Geradus C. , Rogomentich Fran J. , Beausencourt Eva , Webb Robert H.
Published: February 2023

We present a new technique for multiply scattered light tomography, as well as first results, designed to probe the relative thickness and depth of ocular structures. Previous studies have shown that the relative thickness and depth of retinal and subretinal structures provides crucial information in the pathophysiology, progression, and treatment of many eye diseases. Recent noninvasive imaging techniques using scanning laser ophthalmoscopy and near infrared light have demonstrated: a) differences in thickness of the nerve fiber layer in glaucoma as opposed to normal eyes1,b) development of drusen in eyes of adults prior to clinical visualization2, c) widespread and otherwise undetected drusen in eyes with neovascularization3, and d) thickening as an early sign of neovascular membranes in age-related macular degeneration3,4.
The Influence of the AC/A Ratio and Phoria on Accommodative Esotropia
Authors: Semmlow John , Putteman Annie , Vercher Jean-Louis , Gauthier Gabriel , Bérard P.-V.
Published: February 2023

Modification of the focusing power of the lens, oculomotor vergence, and pupil movements are all evoked in response to distance-related stimulation. These three oculomotor responses are termed the "near triad" and are driven primarily by a synergistic combination of two different, but geometrically related, stimuli: retina image disparity and retinal image blur. Failure of near triad components to interact appropriately is a common cause of clinical disorders. Accommodative esotropia is an esodeviation that occurs in patients when they use their accommodative system (1) and is thought to be the result of excessive levels of accommodative convergence (1,2,3). These high levels of accommodative vergence could be the result of increased accommodative effort due to an uncorrected hyperopia, and/or a high effective accommodative vergence to accommodative stimulus ratio (AC/AS ratio). If the excessive accommodative drive cannot be compensated by the fusional vergence system, then accommodative esotropia occurs (2,4).