bluuebunny/crossref_metadata_2025 huggingface dataset:
Fundamental performance limitations of surface-relief incoupling gratings for substrate-mode devices
Authors: Haggans Charles W. , Kostuk Raymond K.
Published: February 2023

In substrate-mode devices, the propagation angle of the substrate-mode beam must exceed the critical angle at all substrate-air interfaces.1 High spatial frequency asymmetrical profile (blazed) incoupling gratings are required to obtain this large propagation angle when the external beam is normally incident on the incoupling grating. However, the efficiency of blazed gratings is rapidly degraded as the wavelength/period ratio increases.2 The maximum achievable efficiency for normal incidence polarization unselective incoupling gratings is investigated by using rigorous coupled-wave theory. The ellipticity introduced during incoupling is also evaluated. High efficiency polarization selective and unselective incouplers for obliquely incident external beams are presented. Maximum achievable selective and unselective efficiencies for these elements are summarized. Additionally, the properties of surface-relief and volume grating incouplers are compared, and fabrication limitations for normal incidence surface-relief incouplers are discussed.
Temperature and excitation intensity dependent luminescence lifetime studies on GaAs/AlGaAs quantum well structure
Authors: Kim Dongho , Jeong Hong Sik , Park Seung Han
Published: February 2023

Time-resolved photoluminescence spectra of a GaAs/AlGaAs quantum well structure with uncoupled four single quantum wells having 2 nm, 4 nm, 6 nm, and 10 nm well widths, respectively, reveal that exciton and free carrier recombination processes in the quantum wells are dependent on excitation intensity. At a low excitation level, exciton lifetimes were measured as 100 ps, 110 ps, 150 ps, and 220 ps for 2 nm, 4 nm, 6 nm, and 10 nm wells, respectively, at 10 K, which confirms quantum confinement effect. Exciton lifetimes increase linearly with an increase in temperature in larger wells(6 nm and 10 nm) but do not show linearity in 2 nm and 4 nm wells, which shows that excitons in smaller wells are likely to be localized on interface defects.1 This result is in accordance with photoluminescence spectrum and rise-time measurement. As excitation intensity increases, free carrier recombination becomes dominant by exciton screening effect and the time evolution of luminescence has different features from those at low excitation levels. We will disscus how excitons collapse into free carriers by exciton screening and how free carriers in each quantum well recombine, from the picosecond luminescence lifetime measurement at various temperatures (10–100 K) and excitation intensities.
An energy–time Bell inequality in two-photon interferometry
Authors: Kwiat P. G. , Steinberg A. M. , Chiao R. Y.
Published: February 2023

We have performed an EPR-type experiment first proposed by Franson,1 resulting in two-particle interference that is both nonclassical and nonlocal. We used photon pairs produced in an entangled state of energy via spontaneous down-conversion. Each photon enters an unbalanced interferometer, and we detect singles and coincidence rates at the output ports. If the long- and short-arm path-length difference is much greater than the photons’ coherence lengths, no fringes are seen in singles as the arm lengths are varied. Yet when the difference between the path-length differences of the two interferometers is less than these coherence lengths, fringes are observed in coincidence, resulting from an interference of the (indistinguishable) “long–long” and “short–short" coincidence possibilities. If the electronic resolution is sufficient to eliminate the background of "short–long" and "long–short” coincidences, the visibility may be as high as 100%.2 For V > 71%, with certain reasonable auxiliary assumptions, Bell’s inequalities can be violated. The interpretation is that no local, realistic model can yield the observed results. The use of this type of entangled state is significant because it is much closer to the original EPR paradox than the spin-singlet case commonly considered.
Characteristics of planar waveguide grating couplers with double surface corrugations
Authors: Li Lifeng
Published: February 2023

Planar waveguide grating couplers are often fabricated by first etching a surface-relief grating into the substrate material and then depositing a thin film waveguide on top. Unless the film thickness is much greater than the corrugation depth of the grating, the top surface of the waveguide will inevitably have ripples following the periodic corrugation of the etched grating. The propagation and radiation characteristics of the grating coupler will be influenced by both surface corrugations by means of optical interference. The consideration of this interference effect is very important in the actual design and analysis of grating couplers. However, this issue has not received any attention in the literature until recently, when Avrutsky et al1,2 analyzed (using an approximate method) and experimentally demonstrated single-order couplers utilizing phase-shifted double corrugations. In this paper, planar waveguide grating couplers with double surface corrugations are analyzed by using a rigorous vector diffraction grating model. The effects of both the relative corrugation depth and the relative phase shift on the total radiation loss and the radiation branching ratio are investigated.
Directional, enhanced fluorescence from molecules near a corrugated metallic surface
Authors: Sullivan K. G. , King O. , Hall D. G.
Published: February 2023

It is well known that the optical properties of molecules are modified by nearby surfaces or structures. We demonstrate here a technique by which the fluorescent signal detected from a near-monolayer of molecules can be enhanced by a factor that can exceed 1000. The technique adds to the multilayered substrate geometry of a previous work1 a periodic surface corrugation that produces directional, rather than nearly isotropic, fluorescent radiation. The directional nature of this emission also allows for polarization discrimination, since different polarization components are radiated preferentially into well-resolved angular directions. The technique we describe is of fundamental interest as an aspect of the dipole-surface problem; it is also potentially of practical importance to the biomedical/biochemical community as a sensitive and versatile diagnostic tool.
Photorefractivity in nonlinear organic polymers
Authors: Moerner W. E. , Silence S. , Scott J. C. , Walsh C. A. , Hache F. , Twieg R. J. , Matray T. , Lee V. Y. , Burland D. M. , Bjorklund G. C.
Published: February 2023

Photorefractive polymer materials offer a number of potential advantages over inorganic photorefractives, particularly low cost of material, low dc dielectric constant, ease of fabrication, and compatibility with integrated optics. Measurements of photoconductivity, nonlinearity, and grating formation are used to understand the newly observed photorefractive effect1 in several nonlinear polymers doped with charge transport agents. In particular, we find that the simple presence of photoconductivity and optical nonlinearity is not sufficient to guarantee the dominance of the photorefractive effect, as some materials show strong photochromic grating production. Direct measurements of the amplitudes and phase shifts of index and absorption gratings2 provide a powerful tool to assess the relative importance of photochromic and photorefractive processes. Similar tests should be used for all potential photorefractive polymers. The current and potential dynamical properties as well as the advantages and disadvantages of this new class of photorefractive materials will be summarized.
Density of states in finite-barrier quantum wells
Authors: Osinski Marek , Mojahedie Mohammad , Prairie Michael W.
Published: February 2023

Density of states in finite-barrier quantum wells is examined critically. In the infinite barrier limit, the two-dimensional (2D) density of states (DOS) has been shown to correspond to the bulk case.1 When finite wells are considered, this correspondence may no longer hold. In this paper, we propose a modification to the finite-well DOS, which retains the elegance of the infinite-well case while preserving the effects of the finite barrier. This is accomplished by either defining an effective infinite-well width that matches the parameters of the finite well or by defining a new effective mass. Both approaches are based on rigorous calculations of the quantized wave vectors. In the spirit of the effective mass concept, we concentrate on the latter case, particularly in the limits of very thin and very thick wells. We investigate the relationship between the DOS and the quantum-well effective mass, based on the allowed wave vectors. Examples will be given illustrating the conventional definition of 2D DOS, the modified DOS, and their comparison with the bulk case.
Effects of kernel profiles and dimensions and angular resolution on rotating kernel operations: experiments
Authors: Lee Yim K. , Son Jung Y. , Jeon Hyung W.
Published: February 2023

The rotating kernel min-max transformation (RKMT) has been proved useful for directional image processing and particularly well suited for optoelectronic implementation.1,2 The method compares extremely well with the best competing algorithms we have found. A related processing operation can be used to obtain object signatures that are independent of the angular orientation of the 2-D object and that are largely independent of the object scale.2 In this method, the input is convolved with a long, narrow kernel, and the maximum (MAX) and minimum (MIN) values of the convolution are stored. A typical output is obtained by some function of MAX and MIN. Basic RKMT operations are implemented by incorporating an optical convolver with a CCD camera as a detector on the output plane. The effects on performance of the kernel profile and relative dimension to directional features to be processed are discussed. The RKMT processing is also affected by the angular resolution. Thus the effect of the resolution is discussed for various kernel profiles, including rectangular and Gaussian profile. Experimental results will be discussed both qualitatively and quantitatively.
Hyperchaotic oscillations of Raman lasers with injected Stokes
Authors: Wright Robert V. , Englund John C.
Published: February 2023

The dynamical behavior of a Raman ring laser is investigated in the plane-wave limit. Assuming that the polarizability adiabatically follows the fields and that the molecular ground state remains undepleted, we obtain a discrete mapping for the pump and Stokes field amplitudes. We find that, when the two injected fields are sufficiently detuned from the Raman transition, the transmitted fields can become chaotic. Additionally, when the injected fields are of comparable magnitudes, hyperchaos—characterized by two positive Lyapunov exponents—is observed. This system thus represents an alternative for observing "bad-cavity" optical chaos indicated for nonlinear interferometers filled with two-level atoms1, 2 but not so far observed in purely optical systems. We compare the Lyapunov and correlation dimensions in the chaotic regime. Finally, we relax the assumption of adiabatic following to show the importance of the molecular dynamics in determining the characteristics of the chaotic field oscillations.
Spatial fidelity in photorefractive image amplifiers
Authors: Zhou Shaomin , He Q. Byron , Yeh Pochi , Liu H. K.
Published: February 2023

Optical coherent image amplification can be realized by means of two-beam coupling in photorefractive crystals. Image degradation due to the nonuniform pump beam depletion has been studied by Vachss and Yeh.1 In this work, we present a theoretical analysis of the image degradation due to spatial gain variation at the Fourier transform domain. Since the photorefractive crystal is placed at the rear focal plane of the Fourier transform lens, different spatial components of the input image correspond to different coupling coefficients in the amplification process. This leads to a degradation of the output image. The degradation increases with D/f, where D is the aperture of the input image and f is the focal length of the Fourier transform lens. Strong degradation exists in the plane formed by the pump beam and the axis of the image-bearing beam. Experimental results with a 45° cut, 1.2 mm thick barium titanate crystal will be presented.
Nonlinear optical properties of ocular components
Authors: Rockwell Benjamin A. , Roach W. P. , Rogers Mark , Mayo M. W. , Toth C. A. , Cain Clarence , Noojin Gary
Published: February 2023

We have measured the nonlinear optical refractive index for various ocular components with ultrashort laser pulses by using the z-scan technique1 to assess the extent of self-focusing in the eye. The sign and magnitude of the nonlinear optical refractive indices are measured for these materials along with water and physiological saline. Carbon disulfide is used as a standard for measurement since the spatial profile of our beam is non-Gaussian, as was required for previous z-scan analysis. We tabulate the nonlinear constants measured and discuss the implications for ultrashort-laser-pulse propagation and ensuing damage in ocular materials.
A new test system for the dynamic modulation transfer function of display devices
Authors: Gao M. L. , Alam M. S. , Karim M. A.
Published: February 2023

Dynamic performances are critical for most imaging system applications. Among the various components of an imaging system, it can be shown that the display devices have the largest dynamic effect. Therefore, it is important to calculate or test the dynamic performance of display devices. The dynamic modulation transfer function (DMTF)1 is the most important of all dynamic performance measures. Therefore, testing the DMTF of display devices should be the first task in evaluating an imaging system. However, the existing experimental setup for determining DMTF involves a flashing light source of infinitesimal time interval for generating the temporal delta function. In practice, such an ideal source, especially the thermal source required by the test system for infrared imaging systems, does not exist.2 To overcome the aforementioned limitation, we propose a practical test system that does not require a flashing light source for measuring the DMTF. Simulation results are presented to verify the effectiveness of the proposed scheme.
Vortex forces in magneto-optical traps for cesium and sodium: justification for spatial structures
Authors: Surdutovich G. , Vitlina R. , Oria M. , Marcassa L. , Zilio S. , Bagnato V. S.
Published: February 2023

In this communication we show that the existence of previously observed spatial structures—balls, rings, and rings with balls—in MOT experiments with Cs (Ref. 1) and Na (Ref. 2) can be explained from a single general viewpoint, through the use of vortex forces.1, 2 The theory uses the saturable aspect of the vortex forces that originates from shifted Gaussian laser beams in MOT experiments and the coordinate dependences of the vortex forces. Depending on the conditions of saturation, any of the above observed structures can be explained only with spontaneous radiative forces. High saturation regimes and moderate detunings provide conditions for the formation of rings with a central ball. Moderated saturation and larger detunings provide conditions for formation of rings without a central ball. We apply the present theory to the experiments of Ref. 1.
Threshold photoelectron spectroscopy of benzyl radical with cm−1 resolution
Authors: Eiden G. C. , Weisshaar J. C.
Published: February 2023

Pulsed field ionization (PFI)1 is used to record vibrational bands from 0 to 2100 cm−1 for ben-zyl+–-h7′, benzyl+–αd2, and benzy+–d7. Over 100 vibrational states are observed; assignments utilizing ab initio calculations identify 12 normal modes. Our data for benzyl+firmly establishes the accuracy of the calculations, which are typically 2–3% higher than experiment. Comparison of the ab initio frequencies for benzyl radical and cation show substantially greater double bond character in the exocyclic C-C bond in the cation than in that of the radical. PFI spectra recorded via each of the neutral resonances indicate the character of the vibronically mixed neutral excited states. Our data indicate that previous models of the vibronically mixed 12A2-22B2 system have included unimportant modes and have neglected important ones. We report adiabatic ionization potentials of 58464 ± 5 cm−1 for benzyl+–h7, 58414 ± 5 cm−1 for benzyl+ad2, and 58382 ± 5 cm−1 for benzyl+–d7.
Upconversion lasing in Ho3+-doped ZBLAN fiber
Authors: Funk D. S. , Stevens S. B. , Eden J. G.
Published: February 2023

Upconversion lasing has been reported by Allain et al.1 in single mode Ho3+-doped fiber at room temperature. We extend the work of Allain et al. and present here a detailed study of the upconversion lasing characteristics of a Ho3+-doped fluorozirconate fiber. Our fiber has an 11 pm core diameter (multi-mode for both pump and lasing wavelengths) and was doped with a concentration of 1000 ppm Ho3+. The fiber exhibits strong upconversion fluorescence in a band centered at around 550 nm. The peak of the upconversion fluorescence was found to depend on fiber length. With a pump wavelength of 647 nm, we have observed cw lasing in this fiber at 550 nm at room temperature with a threshold pump energy of 155 mW. We have observed lasing for pump wavelengths from 643 to 651 nm. The laser cavity consisted of two external dielectric mirrors butted against the uncoated fiber ends. Coupling into the fiber was accomplished by using a 10× microscope objective.
Diode-laser pumped Raman-shifted oscillation in cesium vapor
Authors: Poelker Matt , Kumar Prem , Hemmer Phil R.
Published: February 2023

We report observing Raman-shifted oscillation within a ring cavity containing cesium vapor and pumped with the output of a single-frequency diode laser tuned near the D2 line. The oscillation is shifted from the diode-laser pump frequency by ≃9 GHz, the hyperfine splitting of the 6S1/2 ground state of cesium. Many different transverse modes of oscillation are observed for slight variations in the cavity alignment. In addition, the oscillation is observed to turn on and off at 60 Hz as a result of the pump-frequency fluctuations associated with the line noise on the diode-laser drive current. The pump-frequency fluctuations (≃10 MHz) are small compared with the free spectral range (175 MHz) of the ring cavity. These characteristics indicate that the Raman oscillation in cesium vapor is narrow-band, similar to the behavior observed in sodium vapor.1–3 As little as 30 mW of pump power is required for oscillation when highly reflective cavity optics are used. We have obtained 0.5 mW of Raman-shifted output with an appropriate output coupler at 65 mW of pump power.
Joint transform correlator based arithmetic unit
Authors: Alam M. S. , Karim M. A.
Published: February 2023

Digital computing operations are generally performed either by using Boolean logic gates or by truth table lookup processing. The latter method is very fast since the output is realized in one step but requires a large content-addressable memory.1 Arithmetic processing may also be achieved by using optical pattern recognition systems. However, filter-based pattern recognition is not suitable for real-time operation since a complex filter is needed for each new input. A joint transform correlator (JTC) is particularly suitable for real-time optical pattern recognition since it does not involve filter fabrication. Herein, we investigate the application of a JTC for real-time optical arithmetic processing. As an illustration, we consider the implementation of a binary full adder with a JTC. But as the number of objects in the input joint image increases, the correlation signal is expected to suffer from reduced discrimination, large correlation sidelobes, large correlation bandwidth, and low optical efficiency, thus making the detection rather difficult. To alleviate these difficulties, therefore, we propose a multichannel JTC for implementing multi-input arithmetic operations. The performance of the proposed technique is finally verified by computer simulation.
Roberts operator based edge detection using polarization-encoded optical shadow-casting
Authors: Alam M. S. , Iftekharuddin K. M. , Karim M. A.
Published: February 2023

Polarization-encoded optical shadow-casting (POSC) is found to have remarkable promise for fully utilizing the parallelism of optics in performing arithmetic and logical operations with spatial coded patterns as inputs.1 However, so far little work has been done to exploit its role in realizing 2D image processing techniques in the spatial domain. For image processing applications, it is often necessary to extract the outline features of an image. Recently, POSC schemes have been used for edge detection by using a modified difference operator, i.e., an L-shaped operator.2 However, a difference operator is not necessarily suitable for detecting edges with equal edge strength since it may produce weak response for such cases. On the other hand, because of the symmetric nature of the Roberts mask, all the edges of an image will have equal strength. In this paper, accordingly, we implement the Roberts operator based edge detection by using the POSC technique. The performance of the proposed technique is verified by computer simulation.
Real-time motion-degraded image restoration
Authors: Awwal Abdul Ahad S. , Cherri Abdallah K.
Published: February 2023

Restoration of velocity-degraded images owing to phosphor persistence may be achieved by using a Fourier transform filter setup. However, such a system suffers from very stringent alignment and filter fabrication requirements and is therefore less amenable to a real-time restoration scheme. In this work a joint Fourier transform setup is used to perform real-time adaptive restoration on images degraded owing to relative motion and phosphor persistence.
Soliton transmission with control filters
Authors: Haus H. A. , Mecozzi A. , Moores J. D.
Published: February 2023

Transoceanic soliton transmission with erbium doped fiber amplifier stages and no repeaters has to overcome amplifier noise in order to achieve acceptable bit-error rates. The optical power must be high enough to overcome the amplifier background noise and low enough to avoid the Gordon-Haus limit. This limit is caused by random frequency changes of the solitons upon passage through the ampliers that translate into timing jitter after propagation over long distances. These two bounds limit the range of acceptable transmission powers and may not allow for gradual detrioration of the fiber link. We have shown1 that the insertion of passive filters can reduce the the timing jitter imposed by the Gordon-Haus effect. In addition, the linear noise contribution is filtered so that the available range of transmission powers is greatly increased. The bandwidth of the system linearly excited is greatly reduced. High bit-rate transmission with solitons is made possible only because the solitons recover their total spectral width after passage through each of the filters.
Spectral interference effects in long-distance soliton propagation
Authors: Chbat M. W. , Prucnal P. R. , Islam M.N. , Soccolich C.E. , Gordon J.P.
Published: February 2023

We have observed experimentally, and modelled both analytically and numerically, the appearance of oscillations in the output spectrum after a reshaped optical soliton has propagated over a few tens of soliton periods in an optical fiber. An optical pulse with sufficient intensity will reshape into a fundamental soliton after propagating a few soliton periods by stripping off the non-soliton part of its energy, which will propagate radiatively in the fiber. This radiative field has a spectrum that is overlapped with the output soliton spectrum, and the phase difference between the two spectra is both frequency and distance dependent. Thus the interference between the two fields will cause a modulation of the output field spectrum, with peaks at discrete frequencies that depend on the distance of propagation. An analytical study, based on the analysis in Ref. 1, gives results in good agreement with the observed spectra. This study, carried for hyperbolic secant-shaped input pulses for which the soliton part of the asymptotic solution of the nonlinear Schrödinger equation is exactly known,2 allows the calculation of the non-soliton field after several soliton periods of propagation, and therefore the spectrum of the total output field. In addition, numerical results obtained by a split-step Fourier transform technique show good agreement with the analytical results.
Numerical investigation of the modulation properties of an AM soliton pulse upon reflection from a nonlinear/dispersive interface
Authors: Chatterjee M. R. , Banerjee P. P. , Ziv O.
Published: February 2023

Recently, in an analytical approach based on incommensurate spectral decomposition, Banerjee et al.1 showed that an AM pulse, upon transmission across a linear, nondispersive/ nonlinear, dispersive interface, appeared to indicate the formation of collinear AM and narrowband FM "channels," each with distinctive phase velocities. This indicated that these modulations would spatially separate after some characteristic distance. The above work was based on a nonlinear Klein-Gordon (NKG) system in which the interface was linear/ nondispersive on one side and nonlinear/ dispersive on the other. In this paper, we assume a nonlinear Schrödinger (NLS) system in which the core is assumed to have a Sellmeier-type material dispersion and an n2-type quadratic refractive index nonlinearity. The cladding is made similarly nonlinear but nondispersive for simplicity. By numerical simulations that accommodate exact soliton pulses, the nature of the reflected soliton pulses is examined for variable angles of incidence and for dispersion and nonlinearity parameters. We report here the formation of self-phase modulations on the low-amplitude regions of the propagating pulse, as well as possible phase modulation (resembling the narrowband FM channel analytically derived in Ref. 1) in portions of the reflected pulse. The propagation velocities and amplitudes of the modulations and the carrier are also numerically estimated and are compared with available theory. The case of discrete, incommensurate sidebands is also tested.
Spectrally narrow, high-power F2 laser
Authors: Herman Peter R. , Hill Peter C. , Sia Robert
Published: February 2023

Recent development of high-pressure electric discharge technology1,2 for the F2 laser has produced a 10-fold increase in output energy to >100 mJ. Such energy at a vacuum ultraviolet wavelength of 157 nm is desirable for photolithographic and photochemical applications. This paper summarizes the further optimization of this technology and the performance of a low inductance discharge laser that produces 12 ns pulses, a peak power of 8.3 MW and an extraction energy density of 4.1 J/L.
Sub-Poissonian photon statistics in second-harmonic generation
Authors: Ispasoiu Radu Gh. , Cristescu Constantin P. , Feru Valentin M.
Published: February 2023

The quantum statistical equations for the field produced in the process of second-harmonic generation (SHG) are expressed, based on a method proposed for four-wave mixing.1 We start from the following assumptions: 1) the field absorption in the nonlinear medium may be neglected; 2) the amplitude of the fundamental field is much greater than that of the SH field; 3) the Wigner function associated with the density operator of the field depends significantly only on the variables of the SH field. The coefficients of the Fokker–Planck equation thus obtained depend on the Laplace transforms of the correlation functions of the atomic polarization operator. The moments of the Wigner function2 are related to the moments of the SH field photon number; consequently, we impose the condition for sub-Poissonian photon-statistics and derive the corresponding relationships satisfied by the atomic correlation functions. A numerical interpretation is given.
Barium autoionizing states in dc electric fields near the second ionization threshold
Authors: Armstrong D. J. , Wood R. P. , Smith S. J.
Published: February 2023

Experimental measurements of the autoionizing spectrum of barium in dc electric fields are presented. The experiments are performed with two pulsed dye lasers, which are used to populate and photoionize the doubly excited 5d6p3D1 state. The photo ion signal is monitored while the ionizing laser frequency is scanned across a field ionization threshold lying below the second (zero-field) ionization threshold. Of particular interest is the investigation of doubly excited states that lie near the second ionization threshold in a dc electric field when the zero field autoionization rate and the field ionization rate are comparable. These experimental results will be compared with theoretical calculations now in progress. The theoretical results are obtained from R-matrix eigenchannel calculations combined with a multichannel quantum defect theory (MQDT) treatment. A frame transformation theory1 is used to calculated amplitudes and phases of hydrogen-Stark wave functions that provide the asymptotic boundary conditions used in the MQDT treatment.
Instability of dark soliton stripes
Authors: Law C. T. , Kaplan A. E. , Swartzlander G. A.
Published: February 2023

Dark soliton stripes (DSS's) have been experimentally observed in self-defocusing nonlinear refractive media.1 Based on our numerical simulations, we have demonstrated that when the wavelength of intense modulations is at least a few times the soliton width, DSS's become unstable. Only the fundamental dark soliton stripes are immune from the instability due to transverse modulations. Our results are consistent with recent analytical results.2 The onset of instability and evolution of DSS's, which are generated by a long wire or a wire tapered at both ends, are investigated. Intense sinusoidal modulations are artificially introduced in the long wire case, whereas in the tapered wire case transverse modulations are generated naturally by the tapered ends. In both cases, vortex pairs appear after a long propagation length near the locations of maximum modulations; otherwise DSS's are stable or decay adiabatically. Numerical results were obtained by using the Cray YMP computer at the Pittsburgh Supercomputing Center.
Single photon avalanche diode for single molecule detection
Authors: Li L.-Q. , Davis L. M. , Soltesz S. I. , Trottier C. J.
Published: February 2023

Single dye molecules may be detected as they pass through a 0.4 pL sample volume by using repetitively pulsed laser excitation and time-gated single photon counting to detect the weak single-molecule fluorescence and simultaneously discriminate promptly scattered Raman photons arising from the >1013 solvent molecules.1 Previous experiments have employed a microchannel plate photomultiplier (MCP), which achieves good time response but mediocre quantum efficiency. For comparison, we have characterized the performance of a single photon avalanche diode (EG&G SPCM-200 series) custom modified to allow the avalanche pulses to be directly coupled to fast timing modules. For a reverse bias of 20 V above breakdown a considerably better overall photon detection efficiency of ~50% is achieved, with a dark noise of 60 s−1 when the sample is thermoelectrically cooled to about −45°C. Since the sample volume in the single molecule detection experiment is very small, the collected light may be tightly focused onto the detector. When care is taken to focus to a 10 μm spot correctly positioned on the diode, the time response for time-gated photon counting is comparable with or better than that obtained by the MCP. Although the prompt full-width-at-half-maximum (FWHM) is longer (168 ps), the absence of optical reflections results in a prompt with a shorter tail [FW(1/100)M = 790 ps], which allows the time gate to begin at earlier times.
Near dipole-dipole effects in dense media of two-level atoms
Authors: Dowling Jonathan P. , Bowden Charles M.
Published: February 2023

In condensed matter of densities such that there are many atoms within a cubic atomic resonance wavelength, induced near dipole-dipole interactions (NDD) effects can cause dynamic frequency chirp in near resonant electromagnetic field-matter interactions.1 Using semiclassical theory for two-level atomic systems and a Green's function solution to the wave equation, we derive a self-consistent set of renormalized Bloch–Maxwell equations, useful for discussion of nonlinear optical processes in condensed matter systems, and discuss some of their implications. The strength of the induced near dipole-dipole coupling is found to be ε = (4π/3) u2n, and it causes inversion-dependent frequency chirp in the system. Here, u is the matrix element of the transition dipole moment and n is the density of atoms.
Large infrared nonlinear optical response of C60 not observed
Authors: Tang N. , Partanen J. P. , Knize R. J. , Hellwarth R. W. , McLean M. , Dalton L.
Published: February 2023

Buckyball C60 molecules are reported to have large picosecond nonlinear optical response at 1.06 μm wavelength.1 We used 1.06 μm, 30 ps pulses to measure a degenerate four-wave mixing signal from C60/benzene solutions as a function of C60 concentration. We will present the results of our measurements normalized to the signal observed from pure benzene. The magnitude of the complex hyperpolarizability of a C60 molecule is at most 2000 times that of a benzene molecule. If the phases of the hyperpolarizabilities of the two molecules are the same, the magnitude ratio must be smaller than 100.
Anomalous nonlinear optical response of coated nanoparticle composites
Authors: McDonald J. K. , LaiHing K. , Inguva Ramarao , Bowden C. M.
Published: February 2023

A survey of the degenerate four-wave mixing measurements and their interpretation done on composite materials composed of nanometer coated particles, such as gold-on-aluminum, metal-on-metal, gold-on-silica, and silver-on-silica, will be presented. These data show an anomalous change in slope of the phase conjugate signal versus laser intensity from 3 to 4, depending on the thickness of the coating. To understand this feature, we study the effects of the surface properties coming from the inner surface of the particle. The effects of the second harmonic generation from such an interface, as studied, for example by Shen,1 are investigated. The metallic part of the grain is modeled via a Kerr type nonlinearity in the dielectric constant. We obtain an expression for the dielectric constant in the effective medium theory. Using a Maxwell-Garnett and self-consistent field approximation, we then calculate the phase conjugate signal versus laser intensity to study the effect observed above.
Spatial mapping of the dc electric field induced in optical fiber preforms by intense light
Authors: Dominic Vince , Feinberg Jack
Published: February 2023

Intense light can cause germanium-doped glass to act like a doubling crystal and convert light at frequency co into light at frequency 2ω. Recent theories of this effect invoke the presence of a dc electric field inside the glass. We present a method for mapping out this dc electric field inside a glass fiber preform. We first prepare the preform by injecting beams of both 1064 nm and 532 nm into the preform core. Both injected beams have the same polarization. We then map the resulting dc electric field by using the polarization properties of the induced nonlinearity, which we assume arise from the internal dc electric field and the non-resonant χ(3) of fused silica. By selecting vertically polarized reading light and a vertically oriented analyzer we probe only the vertical component of the induced field, and similarly for the horizontal component. We also measure the relative phase between these two components. We confirm the result of Mizrahi et al.1 that the polarization properties of the second-harmonic process are completely predicted by assuming a dc electric field and the χ(3) of the isotropic preform. This mapped field shows that charge is transported along the direction of polarization of the beams that prepared the glass preform.
Modeling ultrafast optical probes of ballistic transport
Authors: Grondin R. O. , El-Ghazaly S. , Joshi R. P.
Published: February 2023

The ballistic acceleration of electrons inside a semiconductor in response to an applied field has recently been optically observed along with a sub-picosecond burst of terahertz electromagnetic radiation related to the acceleration of the carriers.1 This experiment provides unique challenges to a modeler as it combines broadband electrodynamics with complex transport, e.g., the acceleration of carriers by the field during the finite duration of electron-phonon processes. In our paper, we discuss techniques by which all of these features may be modeled.2 We additionally consider the prospect that may arise in other situations of interest in ultrafast studies of semiconductor devices and circuits.
Ferroelectric liquid crystal optoelectronic ART1 neural processor
Authors: Caudell Thomas P. , Sharpe John , Johnson Kristina M.
Published: February 2023

Adaptive resonance theory (ART) neural networks are becoming an important component of neural network industrial applications. Electronic implementation of these network models has proven difficult to scale to practical input dimensionality. In this paper, a new implementation of ART1 is proposed that efficiently combines optical and electronic devices. Global computations are performed by the optics, while local operations are performed in electronics. A physical implementation of this architecture that uses ferroelectric liquid crystal modulators integrated with VLSI circuitry is presented.1 The system has the capacity to learn in real time and to store the neural weights in a two dimensional optically addressed smart spatial light modulator (SSLM). The implementation can be packaged in a multi-chip module of small physical dimensions. Macro-circuits can be constructed from these modules to perform complex logical functions. This system can also be modified to allow read-out and readin of the learned weights stored in the SSLM, for replication of trained systems. The sensitivity of the ART1 network algorithm to variations in modulator and detector device characteristics is discussed.
Photon-counting, quantum-phase, and phase-space distributions
Authors: Freyberger M. , Schleich W.
Published: February 2023

We analyze a recent proposal for a new phase operator along with an eight-port homodyne detection experiment1 using phase-space functions. We show that the moments of these sine and cosine operators and, in particular, of the measured dispersion result from a phase distribution obtained from an appropriate Wigner function by integration over radius. We also connect this experiment with a different phase operator2 and the integrated Q-function.
Computer modeling of lasing without inversion in atomic samarium
Authors: Wilkinson Steven R. , Smith Arlee V.
Published: February 2023

Remarkable effects are predicted in a coherently prepared atomic medium. These include gain without inversion and high index of refraction with zero absorption.1 Atomic samarium in a magnetic field is an attractive system for the experimental realization of these effects. We have modeled the behavior of this system for a strong, coherence-inducing pump and a weak probe. We calculate probe eigenpolarizations and the associated susceptibilities as a function of probe frequency using realistic values for oscillator strengths, damping rates, and number density but not including Doppler broadening. This eigenpolarization approach is an advance over previous treatments of these effects. We demonstrate that under certain conditions gain without inversion and high index of refraction still occur when this model is used. Furthermore, under certain conditions the probe exhibits double refraction. The current model assumes that the pump propagates without modification. The next step will be to model the pump propagation more realistically.
Mutual coherence between fluorescent and elastically scattered light from an atom
Authors: Narducci F. , Bali S. , Mandel L.
Published: February 2023

It has been predicted that when fluorescent photons emitted from an atom in the process of resonance fluorescence interfere with a portion of the coherent driving field, the visibility of the resulting interference exhibits nonclassical features and is zero at the moment when the atom is fully excited and when the fluorescent intensity is greatest.1 We show theoretically that a similar conclusion applies to interference between the fluorescent photons and the elastically scattered part of the driving field, which can be separated out with narrow band filters in practice. This involves the calculation of certain fourth order, three-time correlation functions that have not previously been studied. A possible application of the theory to an interference experiment will be discussed.
Frequency shift and wavelength scanning of a BaTiO3 self-pumped phase conjugator coupled to a ring dye laser
Authors: Liao Jianhong , King T. A.
Published: February 2023

Based on the literature,1-4 it has been assumed that the wavelength scanning of ring dye lasers coupled to SPPCMs can be controlled. We have tested this experimentally and have found that the wavelength scan can be controlled by adding an external field when it scans initially toward the blue; this is consistent with Fischer's theory. It is a dynamic process to build up the grating inside the crystal during the scan, so that the behavior of the SPPCMs cannot be analyzed according to the static theory described in literature. We believe that in wavelength scanning initially toward the red the frequency shift cannot be used to explain the scan.
Dynamics of large χ(2) induced in SiO2 by temperature and electric field stress
Authors: Mukherjee N. , Myers R. A. , Brueck S. R. J.
Published: February 2023

We have recently reported1 on the formation of a large, stable second-order nonlinearity χ(2) ~ 1 μm/V) in the near surface region (~10 μm) of commercial fused silica materials by a modest temperature and electric field poling process. This nonlinearity is approximately 103 larger than that found in fibers and approaches the nonlinearity of LiNbO3. We have proposed a mechanism involving mobile-ion charge transport and trapping to create a large internal field that interacts with the material χ(3) to produce this large χ(2) nonlinearity. In this paper, we report time-resolved measurements of the formation and decay of the nonlinearity for a sample maintained at elevated temperatures (~300°C) while the applied electric field is switched on and off. The nonlinearity is probed by monitoring the second harmonic generation with a cw mode-locked YAG laser excitation source. The observed time dependence of the nonlinearity is consistent with an ionic charge transport process.
Cone adaptation and cortical silent surrounds cooperate to produce color constancy and color induction
Authors: Courtney Susan M. , Buchsbaum Gershon , Finkel Leif H.
Published: February 2023

The contributions of Von Kries adaptation and cortical surround inhibition to color constancy and color induction are studied by using a neural network model of visual processing from the retina to area V4. We propose that the large silent surrounds in V41 carry out a push-pull operation that enhances or diminishes cone-specific contrast. The network does not explicitly estimate the illuminant spectrum nor make assumptions about the mathematical properties of illuminant-reflectance products. Using center-surround and Mondrian stimuli, we demonstrate that adaptation and contrast calculations cooperate to produce a broad range of color constancy and color induction behavior. Cone-specific contrast calculations account for spatial context effects, while adaptation accounts for luminance effects.2 The relative contributions of adaptation and contrast to color perception depend on the color and luminance proper ties of the stimulus.
Application of photorefractive fibers to acousto-optic sensors
Authors: Yu F. T. S. , Rajan S. , Yin S. , Uang C. , Bhalla A. S.
Published: February 2023

Recently Wu et al.1 have proposed an optical sensing technique using multimode fibers, called a fiber specklegram. In this paper we propose a system comprising an optical fiber coupled into a photorefractive crystal fiber to construct a compact sensor, which can be integrated into a large scale composite system. To physically simulate a photorefractive fiber, a LiNbO3 waveguide with a cross-section of 1 mm2 and a length of 10 mm is used. The specklegram is made by recording the interference pattern formed by the output speckle field and a plane wave both coupled into the waveguide. Since the readout from the specklegram, or speckle hologram, is sensitive to modal phase changes of the multimode fiber sensor, an application to detect acoustic disturbances on the fiber is feasible. We have shown that the proposed technique yields an efficient and compact sensing and measurement system that can be embedded into composite structures. We have also shown that by using a photorefractive fiber array, multiple sensors for different sensing applications can be synthesized, with little cross-talk among the sensors. Experimental demonstrations also verify its wavelength and angular multiplexing capabilities.
Differential interferometric readout optical disk spatial light modulators
Authors: Rilum John H. , Tanguay Armand R.
Published: February 2023

In combination with image-based recording and parallel readout, optical disks are promising spatial light modulator candidates for optical information processing and computing applications.1,2 Highly parallel readout performance of such optical disk spatial light modulators can be achieved by utilizing a differential interferometric readout technique.1 Gray scale capability can be incorporated into these optical disk spatial light modulators (ODSLMs) by binary area encoding at the expense of the pixel resolution. It has previously been shown experimentally1 that gray level encoding on ODSLMs read out by the differential interferometric technique produces extremely linear transfer function characteristics with relatively high total throughput efficiencies. We discuss a computer simulation of the ODSLM that verifies the transfer function linearities and quantitatively explains the experimentally obtained throughput efficiencies. The simulation is based on scalar diffraction theory using an angular spectrum approach in combination with a full three-dimensional ray-trace description of the (optically birefringent) components in the ODSLM. The technological limitations of this spatial light modulator approach are discussed in terms of the obtainable pixel resolution for a given requirement of throughput efficiency and linearity. Comparisons with other direct imaging readout techniques, such as Schlieren imaging, are also given in terms of total throughput efficiency, linearity, and usable dynamic range.
Characterization of a He 2 + ( C 2 Π u → A 2 Σ g + ). laser
Authors: Hill Peter C. , Herman Peter R.
Published: February 2023

The electronic assignment of a pure helium flashlamp continuum1 (1500–8000 Å) proves to be an essential element in the characterization of plasma reactions. This enables approximation of laser gain and resolves the paradox of extremely strong flashlamp emission, which has been unobserved in e- beam devices. The high temperature formation of the

He

2
+

(

C
2

Π
u

)

is discussed. Electron impact transitions are found to dominate radiative processes and enhance ion-molecule formation through Coulombic collisions. The excited dimer ions are formed extremely rapidly through two- body reactions with trimer ions. Numerous potential loss mechanisms are examined, the most significant being identified as

He

2
+

(

C
2

Π
u

→

B
2

Σ
g
+

)

absorption commencing at wavelengths shorter than 3000 Å. A conservative estimate of spontaneous emission for a capillary discharge is deduced to estimate gain.