bluuebunny/crossref_metadata_2025 huggingface dataset:
Large excitonic optical nonlinearity in optimized asymmetric coupled quantum wells
Authors: Ding Y. J. , Guo C. L. , Li S. , Khurgin J. B. , Law K. K. , Stcllato J. , Law C. T. , Kaplan A. E.
Published: February 2023

Asymmetric coupled quantum wells (ACQWs) were used to observe red-Stark shift1 and optical nonlinearity.2 On the other hand, the blue-Stark shift3 for the optimized ACQWs and large optical nonlinearity4 were proposed. Here, we report our photocurrent measurements performed at 78 K for the optimized ACQW structure and confirm the prediction in Ref. 2. The optimized ACQW sample was grown using molecular beam epitaxy and consisted of 25 × (a 18-Å GaAs well, a I5-Å Al0.4 Ga0.6As tunnel barrier, a 32-Å GaAs well, and a 100-Å Al0.4 Ga0.6As separating layer). The wafer has been processed into mesa diodes with ACQWs in the intrinsic region of p-i-n diode. Argon-pumped cw dye laser was used for photo-current study. At the lower laser intensity ~9.2 mW/cm2, the unusually large blue-Stark shift of the heavy-hole excitons up to 6.1 meV (27 Å) was observed. As the laser intensity increased, we also observed the intensity-enhanced blue-Stark shift up to ~10 meV (44 Å) at laser intensity ~270 mW/cm2. At the constant reverse bias of ≃3 V the red-Stark shift of the excitons up to 4.9 meV (22 Å) was observed, as the intensity increased from 9.2 mW/cm2 to 850 mW/cm2. The external reverse bias at which the resonance occurs increase as the laser intensity increases. The large optical nonlinearity in this structure may be attributed to the excitonic many-body effects.
Second-order optical susceptibilities in asymmetric quantum wells
Authors: Yoo S. J. B. , Fejer M. M. , Byer R. L. , Harris J. S.
Published: February 2023

Recently, there have been extensive studies of intersubband transitions. The large oscillator strengths of transitions between subbands of quantum wells1 have led to investigations of their application to lasers, detectors and nonlinear optical devices. In this paper, we discuss the extremely large second order optical susceptibility [χ(2)] predicted and observed2 in structures with inversion symmetry broken through external electric fields or compositional asymmetry. Second order susceptibility of the quantum well system is obtained by solving effective mass Hamiltonian in the conduction band. The large oscillator strengths and relatively narrow linewidths lead one to expect large . Experimental measurements of χ(2) have been obtained for second harmonic generation over the tuning range of CO2 laser in both electric field biased and compositionally asymmetric modulation doped structures, resulting in 28 nm/V and 100 nm/V near resonance, respectively. These are extremely large compared to χ(2)of bulk GaAs, for example, which is 0.38 nm/V at similar wavelengths. In actual frequency conversion devices, the strong absorption at resonance adversely affects efficiency, which ultimately depends on the ratio of χ(2) to absorption. We discuss the design of quantum well devices for large conversion efficiency.
Multiple-pupil synthesis using a detuned Fabry–Perot cavity
Authors: Raj K. , Poon T-C. , Indebetouw G.
Published: February 2023

Scanning optical systems are widely used in a variety of imaging schemes (such as in remote sensing, microscopy, etc.). On-line processing and incoherent imaging are some of the advantages of a scanning imaging system. However, the operations achievable using such a system are restricted due to its corresponding real and positive point spread functions (PSFs). Two-pupil synthesis1 of PSFs removes this restriction as the PSF synthesized is bipolar in general. The capabilities of a scanning optical system can be enhanced if the input is processed by several different operations in parallel.2 The purpose of this talk is to demonstrate that multiple parallel processing can be achieved by scanning the input with a PSF resulting from the interaction of more than two pupils. Multiple-pupil interaction is achieved in a system employing a detuned Fabry–Perot driven by a piezoelectric transducer. The spatially separated pupils are shifted in frequency by Doppler effect and interact with each other on the input. Since each pupil is offset by a different temporal frequency, the output contains terms oscillating at the sum and difference frequencies, each of which can perform a different bipolar filtering operation and can be sorted electronically. A four-band spatial filtering operation is demonstrated.
VUV electronic absorption and magnetic circular dichroism study of HI molecules
Authors: Rupnik K. , Felps W. S. , McGlynn S. P.
Published: February 2023

We present the results of the first VUV (250–146 nm) magnetic circular dichroism (MCD) study of HI molecule in the gaseous state at various pressures below 1 Torr. MCD spectroscopy1 measures the difference in the absorption of right and left circularly polarized light, and refers to the situation in which a static magnetic field is applied parallel to the direction of the polarized beam. This molecule is of particular interest since it is a simple diatomic that exhibits resolved rotational, vibrational and electronic structure below the CaF2 cutoff of 140 nm. Progress in MCD measurements beyond this limit requires optics not presently available. We are investigating pressure effects and the dependence of spectra on magnetic field strength (1–5 T). The analysis of the absorption spectrum of HI molecule is also important to the proposed models of electron-nuclear momentum coupling. We are also investigating the possible role of spin-orbit mechanisms in the VUV electronic absorption and MCD of HI molecules.
Subband structure for narrow, coupled quantum wells
Authors: Biermann Mark L. , Stroud C. R.
Published: February 2023

The effect on the subband structure of the coupling between two coupled quantum wells is studied. An enhanced theoretical formalism based on the theory of Smith and Mailhiot1 is used to model two GaAs quantum wells coupled through narrow Al0.3Ga0.7As barriers of widths between 11 and 17 angstroms. The wells are symmetric and vary in width between 11 and 100Å. Conduction, heavy and light hole subbands are calculated. It is found that the zone-center energy position of the subbands can be given by a simple function whose coefficients depend on the subband being considered. Also, the splitting between the symmetric and antisymmetric band pairs can be described using a simple function. The symmetric and antisymmetric subbands do not split evenly about the single well subband position as is sometimes assumed. The light hole 1 subband crossed the heavy hole 2 subband for narrow barrier and well widths. Additionally of interest, is the clear transition from a region where the subband positions are mainly given by quantum confinement effects to a region where material properties are more important. Comparison is made to experimental results and device applications are discussed.
Spectral shifts produced by scattering on some model random media
Authors: James Daniel F. V. , Wolf Emil
Published: February 2023

The effect of the statistical properties of fluctuating light sources on the spectrum of the emitted radiation has been the subject of intense research over the last few years. It was first shown that due to spatial correlations, the spectrum of light was not, in general, invariant on propagation, even in free space.1 It was also shown that these effects can lead to shifts of spectral lines, a result which is of interest particularly for astrophysics.2 In view of the intimate connection between radiation from localized sources and scattering from media of finite extent, similar spectral changes may also be produced by scattering. In this paper a model scatterer is introduced, whose dielectric response function is a random function of space and time and whose space-time correlation function is a Gaussian function in all its variables. Using the first-order Born approximation, such scattering media are shown to generate frequency shifts of spectral lines that imitate the Doppler effect in its main features. The shifts may be arbitrarily large. Some numerical simulations of such shifts are presented and compared with Doppler shifts.
The McCollough effect is a by-product of an internal error-correcting mechanism
Authors: Roorda Jonathan E. , Wolfe Jeremy M.
Published: February 2023

The visual system requires error-correcting mechanisms that can cope with internal changes (e.g., cell death, lens yellowing). An error correcting mechanism might examine the statistical properties of the visual input and recalibrate if the input is inconsistent with normal visual stimulation.1 For example, on average, color and orientation are uncorrelated in natural stimuli. A correlation might indicate a change internal to the visual system (e.g., damage to one input to an orientationally-tuned, color opponent mechanism). In a standard McCollough effect (McE) paradigm, Ss adapt to red vertical and green horizontal stimuli, making white verticals appear greenish and white horizontals appear reddish. Adaptation to red vertical alone also produces a McE; however, adaptation to red vertical and red horizontal produces little or no McE. In all three cases, orientationally-selective color mechanisms are strongly stimulated and one might expect similar effects of adaptation. However, in the last there is no correlation between color and orientation and no stimulus for error-correction. These restrictions on the McE act to prevent erroneous recalibration under normal viewing conditions.2
Broadband excitation of the Jaynes–Cummings molecule
Authors: Tian Liguang , Carmichael H. J.
Published: February 2023

When a single atom interacts with one mode of an optical cavity and the coupling to the cavity mode is sufficiently large, the spectroscopic response of the composite system to a weak probe is a doublet, with peaks located at the eigenfrequencies of the one-quantum excited states of the Jaynes–Cummings Hamiltonian. So far as these one-quantum states are concerned the spectroscopy of the Jaynes–Cummings model is the same as that of coupled, damped harmonic oscillators. This equivalence has been used to observe a doublet spectrum (the so-called "vacuum" Rabi splitting) by exciting a collective dipole oscillator inside an optical cavity.1,2 At higher levels of excitation the spectroscopy of the Jaynes–Cummings model differs markedly from that of coupled harmonic oscillators. Coupled harmonic oscillators give a doublet at all excitation levels; because of its noncommensurate excited state energies the Jaynes–Cummings model gives a multitude of additional peaks. In this paper we show how the excited states of the Jaynes–Cummings Hamiltonian can be probed by driving a cavity containing a single atom with broadband chaotic light. For a sufficiently large dipole coupling strength the spectrum of the light scattered out of the cavity mode shows peaks at the many resonance frequencies of the Jaynes–Cummings Hamiltonian. A similar multipeaked spectrum is seen in transmission. The spectrum of the light reflected from the cavity shows corresponding absorption dips superimposed on the broadband input.
Electronically controllable shaping of femtosecond pulses using a 128-element liquid-crystal phase modulator
Authors: Weiner A. M. , Leaird D. E. , Patel J. S. , Wullert J. R.
Published: February 2023

Femtosecond pulses with nearly arbitrarily controllable pulse shapes can be generated by spatial masking of individual optical frequency components within a generalized grating compressor.1 Although our original pulse shaping work was performed using fixed, microlithographically patterned masks, we have recently demonstrated programmable femtosecond pulse shaping by using a 32-element liquid crystal phase modulator.2 With this apparatus we demonstrated switching of the optical pulse shape on a 10 ms time scale and achieved gray-level (continuous) control of the individual spectral phases. In the current paper we describe pulse shaping with a new, 128-element modulator. The device is a nematic-liquid-crystal phase modulator controlled by a linear array of individually addressed electrodes on 40-µm centers (with a gap of 1.5-µm between electrodes). Although this 128-element modulator offers improved spectral resolution comparable to the best we have obtained with fixed masks defined by microlithography, full utilization of this spectral resolution will also impose more stringent requirements on the uniformity and accuracy of the phase response. In a preliminary test using binary (0 or π) phase control, we have already demonstrated the ability to perform programmable pulse shaping over a temporal window in excess of 10 ps. We intend in our talk to present the results of a more complete evaluation of pulse shaping with the 128-element modulator, including an evaluation of the ability to utilize full gray- level phase control.
Computation speeds of different optical thin-film synthesis methods
Authors: Li Li , Dobrowolski J. A.
Published: February 2023

There is a need to compare the computation times of, and the results obtained by commonly used thin film design methods. Clearly it is desirable, for a given problem, to employ a method which yields good solutions in a reasonable time. In particular, the solution should be based on the use of only a few materials because such systems are easier to produce in practice. An intercomparison of ten different refinement methods has been published recently.1 In this paper a similar study is performed for the following numerical thin film synthesis techniques: comprehensive search, gradual evolution, flip-flop, minus filter and inverse Fourier transform methods. For this purpose a common master program was written that provides for timing and for the input and output of data, and which can invoke each of the above methods in turn. Calculations were performed on several examples. The results are presented for each method in diagrams that show the spectral performance of the resulting multilayer, its refractive index profile, and the variation of the merit function with computation time.
Radiation at Higher Free Electron Laser Frequencies
Authors: Bali I.M. , Pandya T. P. , Jha Pallavi , Srivastava Anchal , Shukla R.K.
Published: February 2023

The phenomenon of harmonic generation can lead to extension of the tunable range of FEL to higher frequencies without using ever higher electron beam energies.1 Linearly polarized wigglers have been utilised for this purpose. An analysis of the electron beam dynamics in such a configuration shows that the harmonics are generated due to the resulting axial electron oscillations of small amplitude. Following single particle approach, we show here that similar electron dynamics can be generated by using circularly polarized wigglers alongwith an appropriately directed axial magnetic field of suitable strength.
Volume holographic correlator
Authors: Gu Claire , Hong John , Campbell Scott
Published: February 2023

An optical correlator with a volume hologram as a matched filter is usually not shift-invariant in both directions. If the input image is shifted along the degeneracy direction, correlation peak can still be obtained.1,2 But if the input image is shifted perpendicular to the direction of degeneracy, there will be no correlation peak from the Bragg mismatch. We propose a 2-D shift-invariant optical correlator with a volume hologram as a matched filter. The shift-invariance along the degeneracy direction is obtained directly through Fourier transform. In the direction perpendicular to that of degeneracy, the image is shifted to obtain the correlation peak. Image shifting is achieved by deflecting the plane waves (Fourier components) at the Fourier plane. Either a rotating mirror or a Bragg cell can be employed as the deflector. Other methods to achieve image shifting that may be potentially applicable are also discussed. In addition, the amount of shift-invariance along the degeneracy direction is evaluated. The result shows that the amount of shift-invariance depends on the size of the image, focal length and also the angle between the reference and object beams. The combination of degeneracy and image shifting provides the 2-D shift-invariant optical correlation.
GaAs-based photorefractive time-integrating correlator
Authors: Liu Duncan T. H. , Luke Keung L. , Cheng Li-Jen
Published: February 2023

Photorefractive time-integrating correlator using BSO1 and BaTiO32 have been demonstrated previously. They have potential uses in radar pulse compression and adaptive processors for separating a narrowband noise such as a jamming signal from a broad-band signal such as a chirp radar signal or a phase-coded communication signal. However, in practice, a faster photorefractive medium is needed. In this paper, we investigate the feasibility of such an implementation using undoped photorefractive GaAs. In particular, the dynamic range, the bandwidth, and the response time of the system are investigated in detail. Both the old1 and the new2 configurations are considered. In addition, we observed in the output a fringe pattern with a spacing only a function of the bandwidth (chirp frequency range). This rules out the possibility that it is part of the correlation pattern. This fringe pattern exists in both the old and the new configurations. However, their fringe spacings are differed by a geometrical factor. The exact expressions for the fringe spacings in the old and the new configurations, respectively, are derived. They agree quite well with the experimental data.
Joint-Fourier transform correlation by optical heterodyning
Authors: Poon T.-C. , Pieper R. J. , Hirakawa S.
Published: February 2023

Current methods for real-time joint transform correlation (JTC) use a spatial light modulator (SLM) in the focal plane to store the interference intensity which is subsequently readout by coherent light. Because the resolution required for accurate representation of the image transform is extremely high, the spatial light modulators that have met the specifications are expensive. Recently, a novel approach to real-time JTC using an optical heterodyning technique had been proposed.1 The proposed technique obviates the use of a SLM in the focal plane. The patterns to be correlated are coherently illuminated at two different optical frequencies in the front focal plane of a lens. By scanning the interference intensity in the back focal plane of the lens with a chirp grating and subsequent optical heterodyning with a photodetector, spatial correlation between the two patterns carried by the heterodyne frequency can be extracted at the output of the photodetector.
Polarization-encoded optical shadow-casting: design of a 2-D register
Authors: Alam M. S. , Karim M. A. , Awwal A. A. S.
Published: February 2023

The use of polarization-encoded optical shadow-casting (POSC)1 for designing sequential logic circuits has been minimal. To date, only one POSC based sequential logical unit, a J-K flip-flop (FF)2 has been reported in the literature. Since the POSC technique possesses some important advantages when compared with other optical implementation techniques, it is important to investigate the use of this technique for the design of sequential logic circuits such as a multiple-bit optical register. In the current work, we design a toggle FF, and then a 2-D parallel-in parallel-out n × n-bit optical register is designed using the Toggle FFs. The coded input patterns are placed in perfect contact at the input overlap plane and illuminated by an LED. The subsequent outputs are detected at the output overlap plane by a n × n detector array (e.g., CCD detector array). Since the next state of the register is a function of its present state, the present state inputs (i.e., the feedback inputs) are fed back to the encoder from the detector array by an optical fiber cable containing n2 fibers.
Mode partition noise in two-mode laser models
Authors: Olsen T. , Adkison D. , Raymer M. G.
Published: February 2023

Mode partition noise, the sharing of a total constant intensity among a number of laser modes, is a major source of noise in nearly single-mode lasers. Following Agrawal,1 we apply the equations of Lax2 to a semiconductor laser. We allow for two lasing modes, one suppressed in intensity relative to the primary mode. We solve these stochastic differential equations numerically and perform ensemble averages over many simulations. By solving fully nonlinear equations, we may examine the regions of applicability of common quasilinear approximations. We have employed a dynamic description of the quantum noise sources, valid away from the operating point. This allows us to study the buildup of the laser field. Auto- and cross-correlations are calculated between the modes.
Coherence collapse in semiconductor lasers with phase-conjugate feedback
Authors: Liby Bruce W. , Li Hua , McInerney John G.
Published: February 2023

Coherence collapse is a dramatic spectral broadening that occurs in single-mode semiconductor lasers subject to weak (10-3–10-4) optical feedback well above threshold.1 It is a particularly vexing and elusive phenomenon because it mimics the properties of deterministic chaos and noise-driven instabilities. Here we describe what we believe to be the first experimental observation of coherence collapse using phase-conjugate external feedback. A GaAs/AlGaAs index guided laser diode (Hitachi HLP-1400) is coupled by a variable attenuator—consisting of two polarizers and a half-wave plate—to a passive phase conjugate mirror (PPCM) that uses a barium titanate crystal.2 An optical isolator is placed in the ring to make the PPCM undirectional. This avoids deleterious effects caused by counterpropagating beams. The line broadening was observed with a 16.5-GHz Fabry-Perot interferometer. External cavity modes were seen in the rf intensity noise spectra. This PPCM could produce reflectivites up to 15% at the laser facet. For these experiments, the reflectivity was varied from zero up to 2 × 10-3. At the lower reflectivites (10-4) the initially well-behaved laser experiences undamped relaxation oscillations which beat with the external cavity modes in the nonlinear laser medium to produce the broadband (~10 GHz) single-mode spectra characteristic of coherence collapse.
Characteristics of dispersive acoustooptic tunable filters based on CdS
Authors: Feichtner J. D. , Chang I. C. , Katzka P. J. D.
Published: February 2023

Acoustooptic (AO) tunable filters operating in the dispersive region of the birefringent crystal exhibit improved acoustic drive power efficiency because the refractive indices and the relevant photoelastic coefficients, hence the AO figure of merit M2, increase as the wavelength approaches the bandedge. Also, the filter bandwidth is decreased because the dispersive constant increases in that spectral region. These advantages must be traded against increased optical absorption near the bandedge. A preliminary version of such a filter was reported previously; its interaction length was short, the bandwidth was wide, and the filter was not oriented for maximum angular aperture.1 The present filter is oriented with θi = 54.25°; acoustic shear waves propagate at 109.45° to the CdS c-axis. The measured filter bandwidth at 5320 Å was 1.2 Å FWHM. The angular aperture at 5320 Å was ±20° FWHM. Tuning curves and bandwidth are given for the 5260–5400 Å region. The bandedge of CdS shifts with temperature; measured filter characteristics vs temperature are presented.
Higher-order photon statistics of optical parametric oscillators
Authors: Vyas Reeta , DeBrito A. L.
Published: February 2023

We discuss two- and three-fold photon statistics for the squeezed light generated by subthreshold degenerate (DPO) and nondegenerate (NDPO) parametric oscillators. In higher-order statistics details of coherence properties are more strongly reflected than in the single-fold statistics. Many experiments use two-fold statistics to extract spectral information about the incident beam. We use phase-space representation to express the operator field variables in terms of independent Gaussian random processes.1 Using the Gaussian property of these field variables and making a Karhunen-Loeve expansion in terms of orthogonal functions, analytical expressions for the two- and three-fold generating functions for the DPO and the NDPO are obtained. From this generating function various statistical quantities such as factorial moments and joint photon counting probability distributions are calculated.
Vertical/horizontal aniosotropy in binocular summation
Authors: Cohn Theodore E. , Lasley David J. , Legge Gordon E.
Published: February 2023

Consider a task requiring a subject to detect a binocular luminance change stimulus. Threshold for an increment to one eye and an equal decrement to the other has been shown to be a sensitive indicator of the nature of the binocular interaction.1 It has long been known that vertical and horizontal lines are seen with about equal sensitivity. Nonetheless, we have discovered an orientational anisotropy for binocular summation of lines. Increment–decrement (+-) luminance change pairs for horizontal lines require more contrast than for vertical lines, with foveal viewing and 100-msec duration. The average ratio of +- threshold to ++ threshold is ~2:1 for horizontal lines and 1:7 for vertical lines.
Three-dimensional simulations and analysis of hexagonal pattern formation in a Kerr medium
Authors: Geddes J. B. , Chang R. , Firth W. J. , Indik R. , Moloney J. V. , Wright E. M.
Published: February 2023

We report a detailed series of simulations of counterpropagation of Gaussian beams in a slab of Kerr medium. Inclusion of a second transverse dimension dramatically expands the range of behaviors compared with previous work.1 In particular we observe the formation of stable hexagonal patterns, often with stable or long-lived defect structures. Hexagonal patterns of this type have been observed in sodium vapor2 at frequencies sufficiently detuned from resonance that the Kerr medium approximation may be viable. Severe computer demands are imposed by the need to simulate all three dimensions together with the time, and there is a real need for reduced models retaining only the transverse space variables, which then evolve on a slow time scale.
Anticipation and delay of the bifurcation at threshold in a class b laser
Authors: Bromley D. W. , D'Angelo E. , Alvarez R. , Tredicce J. R.
Published: February 2023

During the past years, theoretical1 and experimental2 studies have shown that the sweeping gains or losses affect the position of the bifurcation point in parameter space by stabilizing dynamically an unstable solution. This process leads to a delayed bifurcation and specific scale laws for the delay time as a function of the sweep rate.2 The systems analyzed previously were lasers for which the adiabatic elimination of the atomic variables holds. Arecchi et al.3 found similar results where the gain was swept across threshold, even if population inversion played a significant role on the dynamic behavior of the laser. Here we show that significant changes occur in sweeping the cavity losses. At low sweep rates, the laser turns on at values of the cavity losses higher than those corresponding to threshold (anticipation). Increasing the sweep rate leads to a delayed bifurcation. We also show that the high of the first laser peak depends on the anticipation or delay time. We also measure the scaling laws for both processes. Statistical analysis shows anomalous peaks on the intensity fluctuations around both the turn-on of the laser.
Imaging and quantum interference in a low light level Raman amplifier
Authors: Duncan M. D. , Mahon R. , Reintjes J. , Tankersley L. L.
Published: February 2023

We have previously reported experimental results of Raman amplification of a low-light-level seed-Stokes beam in the transient limit.1 Here we present results of measurements which used a 10-nsec seed-injected Nd:YAG laser. The 532-nm frequency-doubled output had a duration of 8.5 ns and was used both to generate a 4.5-ns duration seed-Stokes pulse at 683 nm and to pump the 1-m long Raman amplifier. The seed-Stokes beam was spatially filtered, apertured at the Airy minimum, and collimated before the amplifier at a diameter of 5.2 mm. Likewise, the spatial mode of the pump was optimized to be as smooth as possible and was collimated into the Raman amplifier at a diameter of 3.4 mm. With no seed Stokes incident in the amplifier, random spatial patterns characteristic of the quantum-noise initiated, self­generated Stokes light were observed over a spatially gain-narrowed beam size supporting ~ 12 spatial modes. For the Raman gains of ~105 used in these experiments the number of temporal modes was 1.8. The seed-Stokes light was used to illuminate a bar chart of line-width 0.315 mm, and either a direct image of the bar chart, or its Fourier transform, was amplified in the Raman amplifier. The original bar chart image was then observed using an intensified camera. As the level of seed-Stokes was reduced to that approaching one photon per spatial-temporal mode, we were able to observe intensity modulations approaching 100% within the amplified bar chart images. Modulation levels of a few percent persist out to input levels of ~103 photons per mode. This observation is consistent with an interpretation of interference between electric field of the seed-Stokes beam and the vacuum fluctuations present in the amplifier but is inconsistent with a photon number description where the maximum predicted modulation would be only 50%.
Regularized linear method for reconstruction of 3-D microscopic objects
Authors: Preza Chrysanthe , Miller Michael I. , Thomas Lewis J. , McNally James G.
Published: February 2023

Computational optical sectioning is a method for reconstructing 3-D images of living biological structures from data acquired via light microscopy as a series of 2-D images taken along the optical axis of the microscope. Each 2-D image is corrupted with light from out-of-focus regions. This effect can be modeled as the 3-D convolution of the specimen with the microscope’s point-spread function. The problem is to determine the object’s intensity in terms of the measured data quickly enough for interactive use and for time-lapse analyses. The linear least-squares solution can be obtained rapidly by inverse filtering, but the problem is ill-posed in view of the inversion of small eigenvalues of the point-spread function operator. We have regularized the problem by application of the linear-precision-gauge formalism of Joyce and Root.1 The linear least-squares solution is constrained to lie in a subspace corresponding to a selected number of large eigenvalues. The tradeoff between the variance of the solution and the regularization error determines the number of inverted eigenvalues. The resulting linear method is a fast one-step algorithm with computational complexity of the order of the FFT. The method is robust to noise and to underestimation of the width of the point-spread function.
Ferroelectric liquid crystal SLMs for beam steering, field-of-view steering, and computer generated holograms
Authors: O'Callaghan Michael J.
Published: February 2023

There are many light manipulation uses that would benefit from the availability of suitable high-speed low-power electrooptic spatial light modulator (SLM) technologies. Ferroelectric liquid crystal (FLC) SLMs may perform these functions through modulation of optical wavefronts. In effect, these devices act as optical phased arrays. FLC-SLMs have been fabricated by patterning electrodes onto glass substrates and by using VLSI active backplanes.1 Both binary and analog modulation are possible by exploiting the range of FLCs available.
Invisible bodies and the uniqueness of inverse scattering problems
Authors: Wolf Emil
Published: February 2023

In an interesting paper published in 1975, Kerker1 described small, nonabsorbing, compound ellipsoids which have the property that when they are illuminated in certain directions they do not scatter any radiation. Such bodies are examples of so-called nonscattering scatterers, which play an important role in connection with questions of uniqueness of solutions to inverse scattering problems. A class of deterministic model media that behave, within the accuracy of the first-order Bom approximation, as nonscattering scatterers for any finite number of directions of incidence was found by Devaney2. The present paper is concerned with the question of whether Devaney’s result may be generalized for all possible directions of incidence. We show that this is not possible. Specifically, we demonstrate that within the accuracy of the first-order Born approximation there are no media that are nonscattering scatterers for all directions of incidence. Both deterministic and random media are considered.
Study of light scattering by spherical particles on various substrates
Authors: Gao Harry , Johnson Carl E. A. , Broude Sergey V.
Published: February 2023

Several models of light scattering from a spherical particle on a flat surface have been proposed in the last five years.1,2 We are studying this phenomenon experimentally to check the validity of the existing theories and to solve some technical problems in the semiconductor surface inspection applications. We have set up an optical system with a scanned laser beam. Polystyrene spherical particles with ~0.1-3.0 µm in diameter on different substrates have been studied. The comparisons of scattering matrices for conducting and dielectric surfaces have been made.
Using flow-induced birefringence to visualize the flow
Authors: Chao Y. F. , Hsieh W. F.
Published: February 2023

By introducing a quarterwave plate into a video polarimeter, we are able to extract all Stokes parameters of light scattered from any sample.2 After mapping out the degree of polarization of a linearly polarized light transmitted through a polycarbonate plate, we can observe the flow lines from the contour of its degree of polarization. In flows, the axisymmetrical particles tend to be oriented along the direction of flow, and are usually used as seeds for flow visualization. Since a PC-plate is constructed by long-chain polymers, the contour of its polarization properties can provide a visible flow with embedded seeds. Besides, the solid structure of the PC-plate is an ideal frozen flow for studying. A numerical simulated flow is used for comparison.
Twin beams, squeezed light, and their uses
Authors: Giacobino E. , Mertz J. , Fabre C. , Reynaud S. , Heidmann A.
Published: February 2023

In recent years, several experiments have established the optical parametric oscillator (OPO) as a practical source of highly correlated twin beams. These beams have an intensity difference noise substantially below the associated shot noise, for noise frequencies within the OPO cavity bandwidth. They present the additional feature of being laserlike and intense, making them convenient for experimental applications. Refinements on our OPO characteristics and photodiode efficiencies have yielded a reduction of the noise power in the intensity difference of 86% below the shot noise level.1 A first use of twin beams is the production of a single light beam having an intensity squeezed noise. This can be achieved by monitoring the intensity of one beam with a photodiode and using this information to control the intensity fluctuations of the second beam with an electrooptic modulator.2 Other applications to high sensitivity spectroscopic measurements such as linear and nonlinear absorption or dispersion can now be considered for practical use.
Tunable squeezed-light generation from twin beams using an optical-phase feed-forward scheme: a broadband calculation
Authors: Kim Chonghoon , Kumar Prem
Published: February 2023

Recently we analyzed an optical-phase feed-forward scheme to produce a squeezed-light beam from the twin beams generated by nondegenerate optical parametric amplification.1 Our interest in this scheme stems from the possibility it allows for the generation of tunable squeezed light. If the input signal beam to the parametric amplifier is tunable, a tunable squeezed output beam can be obtained for spectroscopic applications.2 In this paper we present a wideband model of this scheme. Noise characteristics of the homodyne detector, that measures squeezing properties of the phase-controlled output signal beam from the parametric amplifier, are analyzed in detail. To minimize the squeezed-noise power at an arbitrarily given sideband frequency, the optimal phase feed-forward transfer function is calculated. This feed-forward transfer function is achievable with a bandpass filter and an electronic amplifier. For certain frequency profiles of the gain and phase of the parametric amplifier, the squeezed-noise power spectral density can be minimized over the whole electronic bandwidth of the photodetector that is employed in homodyne detection.
Quantum fluctuations in an optical parametric amplifier
Authors: Pereira S. F. , Ou Z. Y. , Peng K. C. , Kimble H. J.
Published: February 2023

The nondegenerate optical parametric amplifier (OPA) has many uses for investigating quantum fluctuations of light. Among them are EPR-type quadrature-phase correlations,1 QND measurement,2 and phase-sensitive quantum amplification.3 To study these problems, a reliable high-gain low-loss optical parametric amplifier (OPA) is crucial. Although OPAs operated at 1064 nm with KTP crystals pumped by a frequency doubled YAG laser exist,1 their performance is compromised because of problems with beam walk-off and polarization mixing for pairs of critically phase-matched KTP crystals. To avoid such problems, we have constructed a different nondegenerate OPA, which is operated at 1080 nm and consists of a single KTP crystal with type II noncritical phase matching. The pump beam at 540 nm for the amplifier is obtained from intracavity frequency doubling of a single-mode Nd: YAP laser with a similar KTP crystal as in the OPA.
Multimode analysis of single photon nonlocality in parametric down-conversion processes
Authors: Ou Z. Y. , Kimble H. J.
Published: February 2023

With regard to the system of parametric downconversion, we analyze a recently proposed experiment for demonstrating the nonlocal property of a single photon.1 In the original proposal by Tan et al., a light field in a single photon-number state is split into two beams, which are then homodyned with two spatially separated weak local oscillators. The two-photon detection probability with the two homodyned beams provides evidence for nonlocality. It is shown that this phenomenon is in essence a phase-sensitive fourth-order interference effect. A single photonnumber state can be achieved from the pair of photons produced in parametric downconversion processes.2 A multimode model for the downconversion process is used to carry out the calculation of three-photon detection probability that is associated with nonlocality of a single photon.
Characterization of polymer integrated-optical devices
Authors: Culbert D. L. , Ermer S. P. , Ticknor A. J. , Van Eck T. E. , Lipscomb G. F. , Lytel R.
Published: February 2023

Selective photobleaching has recently been suggested as a technique for fabricating channel waveguides in electrooptic (EO) polymers.1,2 Both active and passive integrated-optical devices were fabricated to evaluate this technique. First, three layers of polymer are spin-coated on metallized silicon: acrylate buffer/active organic polymer/acrylate buffer. Next, the top metal layer is deposited. The stack is then poled to induce a nonzero EO coefficient. Finally, the top electrode is patterned as a mask for selective photobleaching of channel waveguides. The exposure to ultraviolet light lowers the index of refraction in the exposed regions of the organic layer. The wafer is then cleaved and ready for endfire testing, which was done at both 0.8 μm and 1.3 μm. The device structures included straight waveguides, angled splitters and combiners, s-bend splitters and combiners, directional couplers, and six-port evanescent couplers. Waveguide widths ranged from 1 μm to 7 μm, and the model characteristics of each size waveguide were evaluated. Overall loss was measured for each type of device, as well as splitting ratios for the branching devices, and switching voltages for the coupling devices. More than one hundred devices were tested, demonstrating that photobleaching is a viable waveguide fabrication technique for certain EO polymers.
Radiometric ray-tracing
Authors: Jannson Tomasz , Sadovnik Lev , Aye Tin , Tengara Indra
Published: February 2023

A new method of ray-tracing for optical system design (including imaging and nonimaging system) is proposed. The method can be used for computing radiometric functions such as brightness (or phase-space density), emissivity (or optical intensity), radiant intensity, and optical flux (power), for quasihomogenous fields,1-3 with arbitrary intensity distribution and arbitrary degree of spatial coherence. Based on phase space formalism, the method can adapt geometrical ray-trajectories, weighted by optical intensity and 2-D Fourier transform of coherence degree, according to the generalized brightness theorem.
Measurement and theory of nondegenerate bound electronic nonlinearities in semiconductors
Authors: Sheik-Bahae M. , Wang J. , Hutchings D. C. , DeSalvo J. R. , Said A. A. , Hagan D. J. , Van Stryland E. W.
Published: February 2023

We recently obtained the dispersion and band-gap scaling of the bound electronic Kerr effect (n2) from a nonlinear Kramers-Kronig transformation of the nondegenerate nonlinear absorption spectrum calculated including the AC Stark, electronic Raman, and two-photon absorption effect.1 The results for the degenerate case gave excellent agreement with the experimental results of semiconductors as well as large gap dielectrics.2 We have extended this formalism to obtain a general expression for the nondegenerate n2(ω1; ω2) and 2PA coefficient β(ω1; ω2). This allows us to calculated the change of refractive index (Δn) and absorption (Δα) at a probing frequency ω1, due to the presence of a pump beam at ω2 < ω
g
. Experiments were performed using a excite-probe (two-color) Z-scan technique.2,3 Picosecond pulses from a mode-locked Nd:YAG laser at λ2 = 1.06 μm were used as the pump beam while the second harmonic of these pulses at λ1 = 0.532 μm probed the induced nonlinearities in two-color Z-scan geometry. Here the pump beam induces a Kerr lens in the sample, which changes the transmittance of the probe beam as transmitted through an aperture in the far field. This change is a strong function Z position (along the propagation distance) of the sample.
Induced focusing of optical pulses in a self-defocusing semiconductor medium
Authors: Jin R. , Khitrova G. , Gibbs H. M. , Xu J. , Boggavarapu D.
Published: February 2023

Induced focusing in a self-defocusing medium1 is observed in a GaAs/AlGaAs waveguide. The 1.2-μm guiding region of the 127-μm long waveguide consists of sixty periods of 9.7-nm GaAs/9.9-nm AlGaAs multiple quantum wells. Observation of induced focusing is made using 857-nm 10-ps pulses. The pump pulse leads the orthogonally polarized probe pulse by ~30 ps to allow for complete index-profile buildup while minimizing effects of interference and carrier diffusion. Two induced focusing effects corresponding to two different pump-intensity profiles are observed. In the first effect2 150-pJ Gaussian-profile self-defocusing pump copropagates with a 1.5-pJ probe of the same profile but displaced by 1.2 FWHM diameters. Compared to the guide-exit profile in the absence of the pump, the probe profile in the presence of the pump becomes 45% narrower, is deflected away from the pump, and has a 50% higher peak intensity. In the second effect, induced focusing without deflection is achieved using a twin-peak pump followed by a Gaussian probe. The probe-output profile is ~55% narrower with the peak intensity increased by 60%. These results are consistent with two-transverse-dimension beam-propagation modeling using the plasma theory.
Interferometric scheme for diode laser mode-locking
Authors: Langlois Patrick , Piche Michel
Published: February 2023

Mode-locking of many laser systems can be achieved by interferometric techniques.1,2 When a pulse chirped after traveling through a material with a nonlinear index of refraction is interferometrically recombined with an unchirped pulse, an effective temporal modulation is produced on the time scale of the pulse. In this paper, we describe an interferometric scheme suitable for the mode-locking of diode lasers. It consists of an optical cavity where a Michelson interferometer is used as a cavity mirror; a semiconductor laser amplifier is inserted in one arm of the interferometer. The amplifier provides both the gain to sustain laser action and the nonlinearity needed for pulse chirping. An analysis of the problem shows that conditions can be found where cw oscillations are quenched and where an intense short pulse is above threshold. Simulations have shown that, in the absence of frequency filtering and dispersion, the pulse duration keeps decreasing at each round trip. However, the inclusion of a Lorentzian filter allows the pulse duration to stabilize to a value close to the inverse of the filter bandwidth.
Stimulated Brillouin scattering in optically amplifying fiber
Authors: Pannell C. N. , Russell P. St. J. , Newson T. P.
Published: February 2023

Backward stimulated Brillouin scattering (BSBS) is reassessed for the novel case of an optical fiber with gain instead of loss. In these conditions (which are achievable in optical fibers lightly doped with rare earth ions) the threshold powers can become exceedingly small both directly because of the gain and indirectly because the amplified spontaneous noise augments the usual spontaneous noise that normally seeds the process. One consequence of this is that the assumptions made by Smith1 to predict the thresholds for the onset of the process no longer always hold. Further new effects include: (1) the appearance of strong higher Stokes orders that must be taken into account in an accurate treatment of the problem, (2) augmentation or suppression of the line narrowing process normally associated with BSBS in a lossy medium, and (3) the possibility of bistability, instability, and oscillation owing to the low threshold powers. Such effects are of particular importance if optical amplifiers are used in sensor systems employing Sagnac loops or ring resonators.
Power scaling in high-power optically pumped vertical cavity surface-emitting semiconductor lasers
Authors: Wu Cun-kai , McInerney John G.
Published: February 2023

Vertical cavity surface-emitting semiconductor lasers (VCSELs) are potentially useful for fast optical interconnection free-space communication or remote sensing, and formation of coherent 2-D laser arrays. The use of resonant-periodicgain (RPG) media, in which a series of gain regions with half-wave optical periodicity is aligned with the antinodes of the optical standing wave in the laser cavity, provides enhanced gain along with cavity axis and at the designed lasing wavelength. The result is a considerable reduction of the laser threshold.1 Such structures should also have greatly reduced parasitic amplified spontaneous emission (ASE) in the transverse (epitaxial) plane. Here we describe the results of experiments using a pulsed (7 ns at 10 Hz) dye laser at 740 nm to pump a VCSEL structure with 20 GaAs/AlGaAs quantum wells.2 Pumped spot sizes varied from 10 μm up to 0.1 mm (limited by available pump power), and the threshold power per unit area did not vary significantly over this range, indicating negligible parasitic ASE. The slope efficiency and lasing wavelength varied due to nonuniformities in the MOCVD-grown structure. These data indicate that the compact and efficient multiwatt diode-pumped VCSELs are feasible.