bluuebunny/crossref_metadata_2025 huggingface dataset:
Extremely large χ(2) by resonant Stark tuning in coupled quantum wells
Authors: Sirtori C. , Capasso F. , Sivco D. L. , Cho A. Y.
Published: February 2023

Recently large second-order nonlinearities associated with intersubband transitions have been reported in asymmetric AlGaAs/GaAs quantum well structures.1,2 In this paper we report what we believe to be the first observation of second harmonic generation (SHG) due to intersubband transitions in AlInAs/GaInAs coupled quantum wells. Previous structures were rectangular wells with applied electric field1 or step graded asymmetric wells.2 Our structure, grown lattice matched to a semi-insulating InP substrate, consists of forty coupled well periods separated from each other by 150-Å undoped AlInAs barriers and sandwiched between n+ InGaAs contact layers. Each period consists of a 64-Å GaInAs well, doped with Si (n ≈ 2 × 1017 cm-3) separated from a 28-Å undoped GaInAs well by a 16-Å undoped AlInAs barrier. To achieve resonant enhancement of SHG the asymmetric structure is designed in such a way that the energy separations ΔE12 and ΔE23 are comparable and within the tuning range of a CO2 laser. In addition, the composition profile is tailored to maximize the product of the dipole matrix elements of the three transitions since χ(2) is proportional to   2. We fabricated a multipass waveguide by polishing a 45° angle on both edges of the sample and illuminating one of them at normal incidence with CO2 laser; for a primary λ = 10.5 μm we have measured χ(2)(2ω) ≈ 2.8 × 10-8m/V at zero electric field. As a bias of the appropriate polarity is applied, the SHG increases and χ()2 reaches a maximum of 1.2 × 10-7m/V (~300 times the bulk values in InP, GaAs, and InAs). This maximum occurs when ΔE12 = ΔE23 as a result of the Stark shift of the energy levels.
Gabor/Bragg neural association in information storage systems
Authors: Jannson Tomasz , Aye Tin , Savant Gajendra , Hirsh Jay
Published: February 2023

Volume multiplex holographic storage has been investigated by many authors from the point of view of random angular addressing in optical disk uses, automatic pattern recognition, etc. However, little attention has been given to the specific features of associative memory introduced by Bragg angular selectivity.1 Here we discuss the fundamental tradeoff between the material2 and the geometrical limits of volume holographic information storage capacity as well as the specific types of neural architectures introduced by Bragg holography. It is shown that, within the same optical system geometry, we can obtain entirely different types (Gabor or Bragg) of neural association, depending on the value of the Bragg selectivity.
Photorefractivity in doped nonlinear polymers
Authors: Moerner W. E. , Scott J. C. , Ducharme S. , Walsh P. , Burland D. M. , Bjorklund G. C. , Lux M. , Twieg R. J.
Published: February 2023

Photorefractive polymer materials offer a number of potential advantages over inorganic materials, particularly low cost of material, low dc dielectric constant, ease of fabrication, and compatibility with integrated optics. Our first reports of photorefractivity in an organic polymer1 established the presence of the effect by a combination of hologram erasability, correlation with photoconductivity and electro-optic response, and enhancement by external fields in numerous samples (≃350 μm thick) of the epoxy polymers bisA-NPDA (bisphenol-A-diglycidylether-nitrophenylenediamine) or NA-APNA (N,N-(diglycidyl)-4-nitroaniline-N-(2-aminophenyl)-4-nitroaniline) mixed with three different hole transport agents similar in structure to p-diethylaminobenzaldehyde-diphenyl hydrazone (DEH). Diffraction efficiencies up to 0.1% (NA-APNA) and 0.06% (bisA-NPDA) were observed at bias fields of 84 kV/cm and 126 kV/cm, respectively. Using standard Kukhtarev theory to model the field dependence of the results, we find bestfit values for the space-charge field in the bisA-NPDA/DEH mixture of 26 kV/cm at an applied field of 126 kV/cm and a grating spacing of 1.6 μm; the estimated photorefractive trap density was 1.9 × 1015 cm-3. The steady-state diffraction efficiencies show a polarization anisotropy in agreement with the photorefractive interpretation. Recent experiments to fully characterize the photoconductivity and to extend this work to other materials systems are summarized.
Wavelength-tunable 2-D bioimaging and tomography based on a coherent detection imaging method with widely tunable lasers
Authors: Inaba Humio , Toida Masahiro , Osada Hiroyuki , Ichimura Tsutomu
Published: February 2023

We report further progress in our experimental demonstration of computer-assisted tunable laser absorption tomography (Laser CT) of biological organisms and samples. This was achieved by coherent detection imaging based on the laser heterodyne detection technique and the projection slice theorem. Its operating principle and simple demonstration have already been published.1,2 Although the idea of optical imaging inside the living body is appealing, strong light scattering by the tissues and other optical distortions create severe practical difficulties. To overcome such a diffuse optical image quality in highly turbid, scattering media, we have proposed and verified, we believe for the first time, the use of the optical heterodyne method utilizing tunable coherent laser beams. It should be stressed that this method possesses both the properties of a highly sensitive receiver and an extremely directional antenna, so that ballistic photons can be detected passing straight through multiple-forward scattering in the tissues and systems.
Hyperbolic tangent graded- index antireflection coatings
Authors: Biricik V. Warren
Published: February 2023

We describe a broadband and broad angle-of-incidence response antireflectance coating design for multispectral applications. This antireflectance coating has a graded-index profile consisting of a hyperbolic tangent function. We present spectral transmission and reflectance as functions of angle of incidence for both the TE and TM polarizations. We calculate the effect of random index fluctuations, such as are caused by monitoring feedback systems employed in coating fabrication, on the spectral transmission and reflection. This coating design is tolerant of manufacturing errors. We also present transmission calculations as functions of total coating thickness and the steepness parameter of the index profile. For infrared applications, a total physical coating thickness equaling one half the maximum operational wavelength provides nearly optimal transmission. The coating performance is not a strong function of the steepness of the index variation. We compare the performance of the hyperbolic tangent graded-index antireflectance coating to that of a coating having a linear index variation. We show that the hyperbolic tangent graded-index antireflectance coating transmission is significantly better than the transmission of a coating having a linear index variation through the thickness.
Ternary phase and amplitude synthetic discriminant function filters
Authors: Downie John D.
Published: February 2023

The use of synthetic discriminant function filters (SDFs) has long been proposed as a means of achieving distortion-invariant pattern recognition in optical correlators. Various design algorithms have been developed toward this end, including binary phase-only SDFs that can be readily implemented with current types of spatial light modulator. Recently, optical filters have been developed in which certain of the binary filter’s pixels are blocked to achieve a ternary state filter.1 These filters have been demonstrated to provide better correlation discrimination between the in-class object and an out-of-class object. In this work we design ternary phase and amplitude synthetic discriminant function filters in simulation with an algorithm that allows equal correlation peaks to be obtained for in-class objects while suppressing the responses from out-of-class objects, relative to the discrimination ratios achieved with binary phase-only SDFs. We investigate issues pertinent to practical systems’ use including variations in the design procedure, the degree of discrimination improvement afforded, response level of images between the training set images, maximum distortion range possible and signal-to-noise considerations.
Designs of 50-mm f/2 20° field apochromatic Gauss lenses
Authors: Mercado R. I.
Published: February 2023

Designs of apochromatic and superachromatic Gauss objectives utilizing only two optical materials have been described recently.1 These earlier designs were developed with moderate values of focal ratio (f/4) and fields of view (12–20°) and have reasonably excellent optical performance over wavelength bands covering the visible and near infrared regions of the electromagnetic spectrum. In this paper, design derivatives of the earlier work are described and design configuration of apochromatic Gauss lenses at f/2 are presented. The performance evaluation of these designs at 50-mm length, f/2, and 20° field of view without vignetting are shown.
Rotational level dependence of predissociation in OH A2Σ+, v′ = 3
Authors: Heard Dwayne E. , Crosley David R. , Jeffries Jay B. , Smith Gregory P. , Hirano Akira
Published: February 2023

Laser-induced fluorescence (LIF) is commonly used to detect the OH radical and other chemical intermediates in flames. Fluorescence from the strongly predissociating v′ = 3 level of the A2Σ+ state of OH can be excited via absorption of KrF laser radiation in the weak 3,0 band at 248 nm. This approach has been developed1 to potentially circumvent effects of collisional quenching that can complicate the interpretation of LIF signals. However, the dependence of the predissocation rate kpre (approximately 1011 s-1) on rotational quantum number N′ is not known. These relative kpre were measured using low pressure flames where the quantum yields are wholly determined by predissociation. Using a frequency doubled tunable dye laser, LIF excitation scans are made for the 0,0 and 3,0 bands, in the burnt gases of CH4/O2 and CH4/air flames at 27 and 30 Torr, respectively. The 0,0 band yields the true rotational populations (described by a temperature near 1700 K). Apparent populations from the 3,0 scan then furnish the relative quantum yield. These can be well described by an apparent temperature near 1000 K, showing that kpre(N′) increases as exp[cN′(N′ + 1)]. Additionally, kpre for the F1 levels appears slightly faster than that for F2 levels for the same N′.
Optical 3-D omega networks
Authors: Cheng L. , Sawchuk A. A.
Published: February 2023

With the increasing demand for solving computation-intensive tasks such as image processing and image understanding, a volume (3-D), dynamic multistage interconnection network that interconnects a 2-D array of channels to another is desirable.1 The understanding of the properties of these networks and their relationships is important for constructing useful; 3-D networks. Here, we study the class of 3-D Omega networks based on different shuffling principles and arrangements of the switch elements. The results show that the 3-D Omega networks using 2-D folded shuffles combined with 2×2 bypass/exchange switch element arrays2 in the 2-D plane are either topologically identical or equivalent to 2-D Omega networks using 1-D perfect shuffles combined with 2×2 bypass/exchange switch element arrays. Networks using 2-D separable shuffles combined with 2×2 bypass/exchange switch element arrays are full access only for a particular arrangement of the switch element arrays at the stages. A reduced-state 4×4 switch element, formed by four 2×2 switch elements with one pair lying horizontally, the other pair standing vertically and a direct connection between the two pairs, can be used at each stage. When reduced-state 4×4 switch element arrays are used, 3-D Omega networks with 2-D folded shuffles and 2-D separable shuffles are topologically identical.
Simple method to start and maintain self-mode-locking of a Ti:sapphire laser
Authors: Liu Yanming , Sun K. W. , Prucnal P. R. , Lyon S. A.
Published: February 2023

We use a four-mirror Ti:sapphire laser cavity for self-mode-locking1,2 with two prisms to control the group velocity dispersion and the pulse width. The Ti:sapphire crystal was pumped with an argon ion laser (all lines). However, this configuration is not self-starting, and one must perturb the cavity (tap a mirror, for example) to initiate mode-locking. We have taken a simple approach of mounting one of the mirrors (the total reflector in this case) on a shaker of the variety typically used in autocorrelators. With the mirror on the shaker oscillating at 5 Hz or faster and with an amplitude of ~0.4 mm, a stable mode-locked pulse train appears immediately. If the shaker is turned off, the laser continues to mode-lock but will eventually revert to cw operation. However, with the shaker on, mode-locking was never observed to cease. Using a spectrum analyzer we have measured the amplitude noise of the mode-locked laser, and find that the shaker has not introduced significant noises into the pulse train. The envelope modulation from the shaker is less than that introduced by the 60-Hz noise on the pump laser. We have extensively investigated and optimized the mirror oscillation frequency and amplitude for mode-locking as a function of different wavelengths (750–900 nm), pump levels and cavity lengths. We also present data on the phase noise (timing jitter) and a discussion of a possible mechanism of self-mode-locking.
Amplification of short pulses in tunable solid-state materials
Authors: Harter Donald J. , Rapoport William R. , Miller Edward J. , Mourou Gerard , Squier Jeff
Published: February 2023

There has been recent interest in amplifying femtosecond-range pulses to the terawatt level by the use of chirped pulse solid-state laser amplifiers.1 In this paper, we describe our progress in amplifying short pulses to the joule level using a variety of tunable solid-state materials.
Cutoff conditions for hybrid modes in integrated-optical anisotropic (biaxial) waveguides
Authors: Maldonado Theresa A. , Gaylord Thomas K.
Published: February 2023

Dielectric planar waveguides are important components in integrated-optical devices and circuits. The guiding layer of a waveguide may be naturally biaxial (anisotropic) or may become biaxial via the electrooptic effect,1 thereby affecting the characteristics of the propagating modes. However, the anisotropic nature of these waveguides is usually ignored, since the analysis required to include anisotropy is much more complex than for the isotropic case. The allowed modes are hybrid, in general, as the corresponding propagation constants are dependent on the orientation of the principal dielectric axes of the biaxial layer with respect to the waveguide/device coordinate system. These constants are calculated electromagnetically without approximations. The electromagnetic fields of these modes consist of the weighted sum or four extraordinarylike plane waves which are coupled. Three types of allowed hybrid mode are identified: homogeneous pure guided inhomogeneous pure guided, and leaky guided. Existing within these mode classes are unconditionally stable modes and critically stable modes. Mode cutoff can occur with a transition from one of the latter two mode types to either a leaky guided mode or leaky unguided wave. The cutoff conditions are quantified numerically.
Second-order algorithm for the complex propagation constant calculations of multilayer planar waveguides
Authors: Hulse Charles A. , Knoesen André
Published: February 2023

Eight unique mode configurations may occur in a multilayer planar waveguide.1 A correspondence exists between these modes and zero entries in the composite transfer matrix of the structure.2 The modes of such a structure are obtainable by equating transfer matrix entries to zero and solving for the complex propagation constants of individual mode types. Based on this theory to find the bound mode of waveguides with any number of layers, a numerical technique was developed that minimizes the magnitude of the bound entry in the transfer matrix.3 This downhill method can find minima of an arbitrary function, but it only converges with first-order accuracy and requires four evaluations of the transfer matrix at each iteration. A new method that takes advantage of the mathematical properties of the transfer matrix to achieve second-order convergence with a reduced computational stencil is presented. This method is capable of finding the propagation constants and field distributions of structures containing materials with complex permittivities. This new method offers improved accuracy and efficiency, compared to the first-order downhill method. Results accurate to within 0.01 dB/cm are typically obtained with one-seventh the computation of the downhill method.
Nonlinear spectroscopy of cold atoms
Authors: Hu Z. , Tabosa J. W. R. , Kimble H. J.
Published: February 2023

Nonlinear spectroscopy of cold atoms has recently been reported for a sample of cesium atoms confined in a magnetooptical trap.1,2 In particular, we report measurements of the spectral response of the cold sample to a weak probe beam for a range of detunings and power for the saturating trapping beams. Transmission spectra for the probe beam reveal the presence of broad absorption and gain features as well as narrow resonances. The power dependence of the broad part of the spectrum can be reasonably well described by earlier work for two-level atoms in a strong field.3 However, narrow dispersive shaped features around the frequency of the trapping beams are also observed and can have appreciable single pass gain (>20%). An exploration of the correlation between the narrow spectroscopic features and the mechanical properties of the atomic sample leads to the observation of both positive and negative radiation pressure associated with stimulated absorption from and emission to the probe beam.
Second-order neural network classifier with on-line learning capabilities
Authors: Horan Paul
Published: February 2023

Higher-order neural networks allow a priori implementation of geometric invariances with the concomitant reduction in training complexity.1 The symmetries inherent in a second-order translation-invariant classifier can be exploited in an elegant optical implementation.2 An optical system based on a commercially available liquid crystal display, operating in transmission under external computer control, and with on-line learning capabilities has been constructed. The binary inputs are displayed as a series of black/white bars. A second pass through the same pattern arranged at right angles produces all the terms of the vector cross-product with itself. A third pass institutes the weight multiplication. The summed light totals are monitored with photodiodes. During the training phase the weight updates are determined by the measured light output at each iteration. Because of device limitations the learning rule must be adapted to produce a finite range of weighing values.
Optical implementation of the Hamming net
Authors: Yang Xiang Y. , Yu Francis T. S. , Gregory Don A.
Published: February 2023

The Hamming net is a neural network implementation of the optimum image classifier.1 It is essentially a two-layer neural network. The first layer calculates the Hamming distance between the input pattern and each exemplar, i.e., classes. The second layer, known as MAXNET, selects the node with the maximum output. We have developed a modified Hamming net model in which the effect of the Hamming distance is enlarged. The modified model alleviates the dynamic range requirement to the spatial light modulators and reduces the iteration cycles in the MAXNET. The optical Hamming net is constructed based on an architecture using a lenslet array and inexpensive liquid crystal televisions (LCTVs).2 The input patterns as well as the interconnection weight matrices (IWMs) in both layers are bipolar signals in the Hamming net. To accomplish the bipolar multiplication optically, the IWMs and the input patterns are area modulated before being displayed on the LCTVs. The experimental results have shown that the optical Hamming net has a larger information capacity than other previously reported optical neural networks.2
Ray matrices and single surface imaging in optical systems with reduced symmetry
Authors: Zabel Mark , Collins Stuart A.
Published: February 2023

We are considering an extension of ray matrices to cover situations of reduced symmetry. Examples of the cases considered include paraxial rays with rotated cylindrical and toroidal lenses and rays that make a small angle with any general skew ray. Here 4×4 matrices are necessarily required. In addition to the usual translation and refraction matrices we also need a rotation matrix to provide a proper system description. Even when point sources are initially used the images are often astigmatic images. In this paper the basic matrices are reviewed and single surface imaging equations for astigmatic line derivations outlined. These equations are compared to the wave trace equations for describing optical fields propagating through an optical system.
Coupled-cavity resonant passive mode-locked Nd:YLF laser
Authors: Keller Ursula , Chiu T. H.
Published: February 2023

Resonant passive mode-locking (RPM) is a new technique for picosecond to femtosecond pulse generation in solid-state lasers using a quantum well reflector in a coupled cavity. This approach has several key features for generating short pulses with a compact and reliable laser, such as one mode-locker works for any repetition rate; the RPM does not require active cavity length control, and the QW reflector is a bulk device requiring no critical alignment. The RPM was first demonstrated with a Ti:sapphire laser.1,2 Femtosecond pulses were achieved in a dispersion-compensated Ti:sapphire laser where the quantum well reflector continuously started Kerr lens mode-locking.3 An RPM Nd:YLF laser was first demonstrated with a 1.06-µm InGaAs/GaAs light modulator.4 We present results using improved low-temperature grown material, which produced as short as 3.7 psec with good stability. Average output power was 370–450 mW at 250-MHz repetition rate with a pump power of 1.3 W from a cw Ti:sapphire laser. Spontaneous self-Q-switching is suppressed by using an aperture to insure TEM00 spatial mode. The low temperature grown InGaAs/GaAs MQW device has two advantages: no surface striation which reduces its fixed loss and a short carrier lifetime which is necessary for high repetition rates.
Polarization-encoded optical shadow-casting: efficient combinational logic circuit design
Authors: Alam M. S. , Karim M. A.
Published: February 2023

For multioutput logic systems, the polarization-encoded optical shadow-casting (POSC) algorithm1 requires switching the source pattern2 or switching of the decoding mask for a fixed input pattern. Recently, Rizvi et al.2,3 proposed an extension of this algorithm to avoid this limitation. A careful observation of this algorithm2 reveals that it may not always lead to a minimized design. However, a minimized design is often essential for logic circuits having large number of inputs and outputs, which is particularly important for POSC based logic circuits since the size of the input overlap pattern, source LED pattern, and the output overlap pattern are entirely dependent on the number of minterms and their proper spatial allocation. To achieve this objective, we propose an extremely efficient design algorithm that yields a minimized design and, at the same time, avoids the problems encountered with the algorithm of Refs. 1 and 2. To illustrate, we design an 8-to-3 line encoder using the proposed algorithm and compare it with that designed using the algorithm presented in Ref. 2. The proposed design is more efficient in terms of both the number of LEDs and spatial encoding size. Since the proposed algorithm requires less space for encoding the inputs, it is cost-effective as well as suitable for logic circuits having a large number of inputs and outputs.
Biochemical representation of the ERG a-wave rising phase
Authors: Breton M. E. , Pugh E. N. , Schueller A. W. , Montzka D. P.
Published: February 2023

The rising phase of the rod photocurrent response to a brief flash is approximated by the ERG a-wave leading edge at intensities high enough to approach rate-of-rise saturation and to minimize interference from the inner retinal (b-wave) component.1 Representation of the stages of the cGMP rising phase by a series of differential equations, as proposed by Cobbs and Pugh,2 provides a means for interpreting the photocurrent response of rods and the leading edge of the a-wave in biochemical terms at high intensities [ARVO 32(4): 1276, 1991]. At low flash intensities the activation time course of PDE is well approximated by a delayed ramp. When the PDE ramp is combined with the equations for cGMP hydrolysis and cGMP channel activation, the rising phase is predicted to be a Gaussian. Lamb and Pugh [ARVO 32(4):1170, 1991] confirmed a number of predictions derived from the Gaussian rise in salamander rods for flashes producing up to 105 isomerizations (out of 4 × 109 rhodopsins). We apply the Gaussian representation of the photocurrent rising phase to human a-wave leading edges and evaluate the limitations of the fit against the salamander results and the more general Cobbs and Pugh representation as a function of intensity.
Observation of modulated Hanle resonances in stochastic laser fields
Authors: Vemuri Gautam , Anderson M. H. , Cooper J. , Smith S. J.
Published: February 2023

We report an experiment on the observation of Hanle resonances in sinusoidally amplitude modulated, weak, phase diffusing optical fields. The J = 0 to J = 1 transition in an atomic beam of barium was probed with a frequency stabilized dye laser, whose bandwidth and bandshape were precisely controlled with an extra-cavity noise modulation system. Phase sensitive detection of the Hanle fluorescence signal at the amplitude modulation frequency revealed resonances in the in-phase and in-quadrature parts, whose position, shape, and strength were sensitive to the laser bandwidth. These resonances, which were Lorentzian or dispersion shaped or a combination of the two, occurred when the Zeeman splitting of the exited sublevels equaled the modulation frequency for monochromatic light and at splitting of half the modulation frequency for broadband light. The results are in good agreement with the theoretical predictions of Saxena and Agarwal.1
Picosecond backside optical detection of currents and voltages at an internal line-driver in a flip-chip mounted silicon circuit
Authors: Heinrich H. K. , Pakdaman N. , Kent D. S.
Published: February 2023

High performance silicon VLSI circuits are often covered with four or more layers of metal, which makes many internal nodes inaccessible to conventional front-surface probing approaches. Previously, Heinrich et al.1,2 demonstrated a backside optical probe for measuring signals in silicon circuits. This work uses a two-beam Nomarski interferometer to sense electronically modulated charge at a device or pn junction in the circuit. By focusing one beam on the emitter of a transistor and the other beam on the base of the device, we can sense the diffusion charge in the device (αIc). By focusing on a pn junction, we can sense the capacitively stored charge (αVj). This paper presents measurements of the current and voltage at the output of an internal line driver in a silicon IC, and compares these measurements to prior circuit simulations. The voltage measurement agreed well with the circuit simulations. However, it showed an additional overshoot not seen in the simulations, caused by current transients on the power supply. The measured rising edge of the current waveform agreed well with the simulation. However, the falling edge showed much deeper undershoot, caused by removal of majority carriers under strong reversed biased operation.
Laser scanning micromanipulation of fine particles
Authors: Sasaki Keiji , Koshioka Masanori , Misawa Hiroaki , Kitamura Noboru , Masuhara Hiroshi
Published: February 2023

A laser trapping technique is a powerful tool for manipulating single or plural particle(s). Bums et al. recently demonstrated that lattice structures of latex particles could be created on interference patterns of several laser beams.1 Also, we showed that particles were aligned in concentric circles or a line image of a single trapping beam, and demonstrated size-selective trapping of particles.2 The spatial patterning by these methods is, however, essentially based on the interference, so that there are severe limitations in forming arbitrary patterns. In this paper, we propose a laser scanning micromanipulation technique to produce any spatial patterns of fine particles. The principal key to the technique is repetitive scanning of a focused laser beam by computer-controlled galvano mirrors. If the repetition rate is faster than the time-scale of the Brownian motion, plural particles are trapped along a locus of the scanning spot. In addition to the pattern formation, the trapped particles can be moved continuously along the formed patterns, and the flow velocity is controlled by scanning speed and laser power.
Scattering of light by crystals: a full Kirchhoff approximation
Authors: Muinonen Karri
Published: February 2023

The theoretical foundation of the Kirchhoff approximation for light scattering by optically isotropic and homogeneous crystals was presented by Muinonen.1 In that approximation, the reflected and transmitted electromagnetic near fields were calculated from geometric optics ray-tracing. The corresponding far fields were then obtained from the vector Kirchhoff integral relation. The earlier computer simulations for randomly oriented crystals were, however, based on a modified Kirchhoff approximation that included further simplifications.
Simulation of lightning by laser-induced plasma
Authors: Lakkaraju H. S. , Jebens D. S. , Borucki W. J. , McKay C. P.
Published: February 2023

We report results of time resolved spectra of lightning simulation in air by laser-induced plasma (LIP).1 The measured electron temperatures varied from (1.7 ± 0.1)104 K at 600 ns to (1.2 ± 0.1)104 K at 4 μs after the ignition of the plasma. Electron densities in the plasma varied from (7.0 ± 1.4)1017/cm3 to (9.0 ± 1.8)1016/cm3 for the above times. We also report, we believe for the first time, a comparison of state variables for LIP and natural lightning. We find that immediately following the ignition both natural lightning2 and LIP reach temperatures >30,000 K and relax to ambient pressure with different rates. They appear to approach the 2000-4000 K temperature range, which is important in chemical synthesis, in an asymptotically similar fashion.
Absolute testing of flats using a shearing technique
Authors: Ai Chiayu , Albrecht Herve , Wyant James C.
Published: February 2023

This paper describes a method for measuring the absolute flatness of flats.1 The flat was measured at three positions: nominal, displaced in x, and displaced in y. From these three measurement positions, three phase maps were obtained. To avoid the tilt error during the translation or shearing operation, the tilts of the three phase maps were removed individually. The three phase maps were then used to obtain the shearing interferograms in the x and y directions. Using this technique the error of the reference surface error was removed. From the two shearing interferograms, the profile of the flat was reconstructed.2 The theoretical derivation and comparison of different methods, and the experimental results are presented.
Synchronization of a passively mode-locked laser and an actively mode-locked laser to better than 2 ps
Authors: Walmsley I. , Radzewicz C. , Palese S. , Sweetser J. , Dunn T. J. , Miller R. J. D.
Published: February 2023

Pulses from two mode-locked lasers have been synchronized to better than 2 ps. The technique, similar to that used by Halbout and Grischkowsky1 uses the output of a passively mode-locked CPM dye laser to generate a radiofrequency (rf) signal which can be used to drive a mode-locker for a second laser. In our case, the second laser is a diode pumped Nd:YLF (DPY) FM mode-locked with a lithium-niobate phase modulator.2 FM mode-locking has two advantages over AM mode-locking: it does not require a rf divide by two element, and the phase modulator tank circuit can be easily tuned without reducing its Q. The tank circuit has a relatively low Q that allows the DPY laser to be adjusted to the CPM laser frequency without tuning the circuit. The CPM can be set optimally to produce the shortest pulses, and the DPY adjusted to operate in synchrony. The DPY generates pulses as short as 15 ps at 100 Mhz; it operates stably in two modes corresponding to pulse durations of 15 and 30 ps. The shortest pulses, measured by a cross correlation with pulses from the CPM, are neither Gaussian nor hyperbolic secant in shape. They have an asymmetric shape with one or two satellite pulses preceding or following the main pulse depending on the cavity length detuning.
Uniqueness of higher-order autocorrelation functions
Authors: Yellott John I. , Iverson Geoffrey J.
Published: February 2023

The kth order autocorrelation function (ACF) of an image is formed by integrating the product of the image and k independently shifted copies of itself: the case k = 1 is the ordinary autocorrelation; k = 2 is the triple correlation. Bartelt et al.1 have shown that every image of finite size is uniquely determined up to translation by its triple correlation function. We point out that this is not true in general for images of infinite size, e.g., frequency band-limited images. Examples are given of pairs of simple band-limited periodic images, and pairs of band-limited aperiodic images, that are not translations of one another but that have identical triple correlations. Further examples show that for every k there are distinct band-limited images that have identical kth order ACFs. However certain natural subclasses of infinite images are uniquely determined up to translation by their triple correlations. We develop two general types of criterion for the triple correlation to have an inverse image that is unique up to translation, one based on the zeros of the image spectrum, the other on image moments.
Computational efficiency of the Gabor transform
Authors: Folsom Tyler C. , Pinter Robert B.
Published: February 2023

The two dimensional Gabor transform has been proposed as a model of the visual receptive field of the simple cells of mammalian striate cortex. Such a representation can make image features such as localized spatial frequencies and edge orientations explicit while achieving data compression. This paper analyzes the computational steps needed to achieve this transformation and presents typical execution times for several computers. The use of an adaptive iteration step size is shown to improve performance significantly. Computational requirements are examined for both sequential (SISD) and parallel (SIMD) computers. For an n × n image a SISD machine requires O(n2 log n) operations; SIMD computations are bounded by O(n log n), O(log n
2
), or O(log n), depending on the machine architecture. A parallel computer with global addition capability and with a processor for each pixel could perform a Gabor transform in near real-time. On existing computers near real time performance is not possible for moderate sized images. Sequential computers show better cost/performance ratios than parallel machines.
Optical beam steering using a liquid crystal TV
Authors: Tanone Aris , Yang Zhaohui , Zhang Zheng , Gregory Don A. , Yu Francis T. S.
Published: February 2023

The phase modulation properties of a twisted-nematic liquid crystal television have been widely reported. However, the phase modulation depth depends on various factors, such as the birefringence of the liquid crystal material, the thickness of the cell and the wavelength of the illuminating light.1 When the modulation depth is equal to 2π, a phase ramp with maximum phase that is much larger than 2π can be displayed on a liquid crystal panel as a sawtooth phase variation with a maximum height of 2π. These sawtooth phase fronts then act as a series of prisms that deflect the incoming beam. Thus, in principle, we may create a programmable prism for beam steering purposes, which could be used in optical imaging or multiplex hologram applications. However, most of the commercially available LCTVs are unable to attain the 2π phase modulation depth. In this paper we discuss the effect of this limited modulation depth and provide some experimental verification of beam steering by using an LCTV having a modulation depth of it.
Two-dimensional photonic band structure
Authors: Leung K. M. , Carin L. , Agi K. , Garetz B. A. , Abers J.
Published: February 2023

The photonic band structure of a 2D photonic band-gap crystal is investigated by using the coherent microwave transient spectroscopy technique.1 A mode-locked, pulse-compressed, frequency-doubled Nd-YLF laser is used to switch planar antennas photoconductively. The freely propagating pulsed radiation generated by the coplanar strip horn antennas has a usable bandwidth extending from 5 to nearly 100 GHz. The crystal is made by drilling a parallel set of holes on a triangular lattice in a dielectric whose refractive index is 3.6. The theoretically determined optimal hole diameter is 0.96 times the lattice spacing. The photonic band structure for both the TE and TM modes and the attenuation length within the gap are mapped out directly by using the phase information obtained experimentally. These results are compared with theoretical predictions calculated by using the plane-wave expansion method.
Optical studies of GaInP/AIGaInP quantum wells: device-related materials parameters and substrate orientation effects
Authors: Dawson Martin D. , Duggan Geoffrey
Published: February 2023

Ga0.5In0.5P/ AI0.35Ga0.15In0.5P, lattice-matched to GaAs, has been used to fabricate laser diodes with room temperature emission in the 630-670 nm range. Growth, by MOCVD, on misoriented substrates has been shown1 to minimize band-gap variations and other anomalous behavior associated with spontaneous ordering in these alloys. Here, we use a variety of optical techniques to compare bulk and quantum well epilayers of this material grown on normal and misoriented substrates and to extract fundamental materials information. The normal substrates are (100) 2° off towards (110), and misoriented substrates are (100) 10° off towards (110); importantly, equivalent structures were grown on half wafers of these orientations in the same reactor at the same time for direct comparison. At 5 K the 10°-off bulk sample shows single-peaked photoluminescence (PL) at 1.995 eV, with a linewidth of 6.8meV. No anomalous behavior with temperature or fluence was observed, consistent with minimal effects of ordering. The PL linewidths of the 10°-off quantum wells (7.0 meV at 2.021 eV for 120 Å, 9.6 meV at 2.045 eV for 75 Å, and 11.4 meV at 2.072 eV for 50 Å wells) are, to our knowledge, the narrowest yet reported in this material system. The 2°-off quantum wells show linewidths ~1.5 to 2 times these values in each case, shifted to longer wavelengths by ~20 meV. The 2° bulk PL is double-peaked, with a tail to lower energy, and is strongly fluence and temperature dependent, consistent with known effects of ordering. Longitudinal optical phonon energies for Ga0.5In0.5P have been determined by resonant Raman spectroscopy. Photoluminescence excitation measurements are also reported. The results have been incorporated into envelope function calculations in an attempt to determine a consistent set of materials parameters (band gaps, excitonic binding energies, band offsets, effective masses, etc.) for device design purposes.
Folded Fabry–Perot cavity surface-emitting quantum well lasers
Authors: Cheung S. K. , Jain F.
Published: February 2023

A novel folded Fabry–Perot cavity surface-emitting laser (SEL) structure, employing two oppositively flipped (45° downward) etched facets and one λ/4 dielectric mirror (AlAs/GaAs), is proposed. Unlike the recently reported structures1 that use two parallel 45° etched facets (one downward and one upward) and two dielectric mirrors to form the cavity, the lasing cavity in the proposed device is formed by using only one dielectric mirror. The quantum well (QW) active region, which is confined by two cladding layers, is defined by two etched 45° downward facets. The output is determined by the reflectance of the dielectric mirror and is received from the substrate side of the device. The reflectance of the dielectric mirror is designed to optimize the operation of the device to achieve lower threshold current density. The structure is also shown to be compatible with high power operation.
Separable spatiotemporal components in displacement and disparity processing
Authors: Tyler Christopher W. , Kontsevich Leonid
Published: February 2023

Data were analyzed from a previous study1 of psychophysical thresholds to spatiotemporal wiggle of lines in lateral displacement (monocular viewing) and in stereopsis (dichoptic viewing of the same lines with counterphase disparity wiggle). The threshold surfaces were not separable into the product of a spatial and a temporal frequency function. The aim of the present analysis was to determine whether, like spatiotemporal contrast thresholds, the data could be well described as the sum (or difference) of two spatiotemporally separable components. Singular value decomposition showed that two components accounted for both lateral and disparity surfaces to an accuracy of ~95%. We optimized separable components with the constraint that each component was simple in that it had only positive sensitivity and that it minimized the second component magnitude. These constraints revealed a similar excitatory component for both displacement and disparity domains. The second component was inhibitory with reduced sensitivity for low frequencies in both space and time, as for contrast-domain inhibition. The inhibitory component for disparity, however, had a purely spatial tuning, independent of temporal frequency. This behavior is reminiscent of that for stereomovement suppression2 and suggests that lateral and disparity thresholds are processed by independent neural mechanisms.
Against a “covariance metric” for motion perception: chromatic and luminance information do not combine
Authors: Gorea Andrei , Kovacs Ilona , Papathomas Thomas V.
Published: February 2023

Is perceived motion based on a similarity metric (subtending the classical motion detector whose spatially sampled inputs are processed by identical sensors), or is it based, as recently suggested,1 on a covariance metric implying a unique, undifferentiated motion detector allowing for spatiotemporal matching between qualitatively distinct elements to override spatiotemporal matching between identical elements? We will display the stimulus used to answer this question as it relates to the combination of chromatic contrast (CC) and luminance contrast (LC) information. Its spatiotemporal structure is such that CC-defined elements (red or green on a yellow background) will carry rightward motion, while the potential matching between CC- and LC- (positive or negative luminance increments) defined elements will carry leftward motion. According to the covariance model, leftward motion should be perceived any time that CC×LC>CC×CC, i.e., when LC is larger than the equivalent luminance contrast of the CC-defined element (~5%). For LC ranging from 0 to ±80%, leftward motion was never perceived, clearly reinforcing (together with additional experiments to be reported) the original view of a similarity metric for motion perception.
Power dissipation and speed tradeoffs associated with optical fan-in
Authors: Athale R. A. , Euliss G. W.
Published: February 2023

Optical techniques are projected to possess significant advantage over electronic techniques in implementing global, long distance, high bandwidth, high spatial density interconnects with large fan-in and fan-out. This paper focuses on the impact of large fan-in on the power dissipation of optical switches. Goodman1 has analyzed the inherent power loss in optical fan-in networks due to the constant brightness theorem. We extend the analysis to include the impact of the fan-in on the power dissipation and speed of an optical link. Both passive (non-regenerative) as well as active (optical detection-regeneration) cases will be analyzed. The connection between these limitations and the performance limits of optical processing architectures based on orthogonally oriented linear modulator arrays (e.g., outer product processors) will also be explored.
Polarization-encoded optical shadow-casting: design of a trinary logic-based multiprocessor
Authors: Alam M. S. , Karim M. A.
Published: February 2023

Among the multiple-valued ogic systems, trinary logic appears to be the most promising in terms of the number of logic levels and storage complexity. To implement multiple-valued logic, various optical computing techniques, such as threshold logic, spatial filtering, truth table look-up processing, and polarization-encoded optical shadow-casting (POSO),1 have been suggested. Among these techniques, the lensless POSC technique has been found to exploit completely the parallelism of optics in performing logic operations. The POSC scheme has already been used to design a binary logic-based multiprocessor.2 To exploit the potential of multiple-valued logic in optical computing, in our current paper we extend the scope of POSC systems and design a multiple-valued logic-based optical multiprocessor. As an illustration, we furnish the design of a trinary logic-based multiprocessor that can perform parallel trinary full addition and full subtraction. The POSC algorithm presented in this paper uses fixed source and detector patterns and yields a minimized design.
Spectral density and energy flux vector in radiometry with partially coherent fields
Authors: Foley J. , Wolf E.
Published: February 2023

In previous publications1,2 we derived an expression for the spectral radiance of fields generated by any planar, secondary, quasi-homogeneous source. In the present paper we carry out the analysis further and derive expressions, valid in the short wavelength limit, for the spectral density and for the energy flux vector at an arbitrary point in such fields. These formulas show clearly the contributions to these quantities made by the spectral density and by the degree of coherence of the source. We illustrate the results by examples.
Superresolution tomography using a moving detection system and convex projections
Authors: Wernick Miles N. , Ordonez Caesar E. , Chen Chin-Tu
Published: February 2023

Tomographic imaging systems such as those used in PET and SPECT are typically limited in their performance by the size of the detectors used in acquiring the data. By analogy to findings in the context of optical image-plane applications,1-2 we show that, by moving a tomographic detection system to expand the measurement set, image detail can be recovered by methods such as convex projections. It is demonstrated that, by underrelaxing the data constraints in a convex projection algorithm, excellent results can be obtained in the presence of the high levels of quantum noise typically encountered in the motivating applications. In addition, the method presented is shown to be robust to reasonable misspecifications of the spatial response functions of the imaging system. A “wobbling” PET system is used for purposes of demonstration; however, the concept can be applied in a variety of ways to various tomographic imaging configurations to improve achievable spatial resolution, either in image space or in projection space.