bluuebunny/crossref_metadata_2025 huggingface dataset:
SLM-specific imperfections and their influence on optical processor performance
Authors: Warde Cardinal
Published: February 2023

Imperfections in today's spatial light modulators (SLMs) include one or more of the following: inactive zones in modulators with spatially discrete pixels, mixed (phase/amplitude) modulation rather than pure phase or pure amplitude modulation, nonlinear transfer characteristics in both optically and electrically addressed SLMs, bandpass spatial frequency characteristics in some devices, and strong readout light absorption losses in certain devices. When SLMs are used in optical processors, these imperfections lead to a variety of problems, including loss of correlation peak sharpness in optical processors, image distortion and blurring, fixed pattern noise in reconstructed images, and low signal throughput in complex systems. Continuous-pixel SLMs with spatially continuous drivers offer sufficient spatial resolution to solve the inactive zone problem. While phase-only and almost-amplitude-only modulators exist, there seems to be no device that offers a linear relationship between modulator output (modulated readout light amplitude or phase) and driver characteristic (e.g., write light intensity or electrode voltage) over the full modulation range of the device. Application-specific modulator choice is therefore an important consideration.
Optical surface quality of spatial light modulators (SLMs)
Authors: Hudson Tracy D.
Published: February 2023

The phase modulating properties of spatial light modulators have been investigated for optical pattern recognition purposes for many years. Researchers have attempted to exploit these properties for phase-only matched filtering and optical associative memories. Early investigations of LCTVs for these applications found that a liquid gate was required because of the nonuniform glass sandwich structure of the devices. Interferometric techniques were recently utilized to investigate the surface uniformity of several commercially available SLMs. These results, to be presented in this communication, verify that the optical uniformity of SLMs should be considered when phase modulating properties of the devices are to be exploited in optical systems.
The influence of ion migration in ferroelectric liquid crystal on the performance of optically addressed spatial light modulators
Authors: Perimutter S. H. , Doroski D. , Barbier P. R. , Wichart S. , Moddel G.
Published: February 2023

Because of the bipolar response of ferroelectric liquid crystals (FLCs) and the asymmetric current-voltage characteristic of most photosensors used to drive optically addressed spatial light modulators (OASLMs), the voltage signal that drives the FLC in an OASLM contains a dc component. This dc component causes ion migration within the FLC and, consequently, ionic buildup at the alignment layers. The separation of ionic charge creates an internal electric field across the FLC that opposes the dc component of the applied field. This phenomenon affects device performance by reducing bistability and contrast ratio. We discuss how the presence of ions in FLC OASLMs leads to these problems and present approaches that we and others have suggested or tried to reduce or circumvent them, including using conducting alignment layers, using a balanced drive voltage, removing ions from the FLC, and using a photosensor with a symmetric current–voltage characteristic.
Field enhancement and reduction and elimination of remanent light flow in electro-optic bulk and thin film displays and spatial light modulators: theory and practice
Authors: Wu Adam Y.
Published: February 2023

Remanent light flow is a very serious problem in electro-optic devices. It will not only degrade the contrast ratio but also shorten the lifetime of the devices. In fact, understanding and solving the remanent light flow problem in an electro-optic device may also help in the understanding and solving of several other problems, such as fatiguing and aging problems in other devices. Efforts have been made to examine the mechanism of the remanent light flow problem in ceramic bulk (such as PLZT) and thin film (such as PLZT, BaTiO3, LiTiO3, and SBN) displays and spatial light modulators and to find effective methods of reducing and eliminating remanent light flow by using a buffer and capping layer on the surface of the devices. Several very effective buffer and capping layers have been tested; they can not only reduce or eliminate the remanent light flow but also enhance the electrooptic effect by 10% to 30%. The physical origin of the remanent light flow, the method of eliminating it, the physical model of the electro-optic enhancement effect, and the experimental results will be presented.
Reducing Berreman’s 4×4 formulation of LCD optics to 2×2 Jones vector equations
Authors: Lu Kanghua , Saleh Bahaa E. A.
Published: February 2023

The propagation of polarized light in inhomogeneous anisotropic media such as twisted nematic liquid crystal displays (LCDs) has been studied by the Berreman's 4×4 formulation and the 2×2 Jones calculus. However, the relationship and equivalence between these two approaches have not been explicitly shown. We derive the 2×2 Jones vector equations from the Berreman's 4×4 equations. We show that the Berreman's formulation can be transformed to a bidirectional Jones vector space described by differential equations that govern the two transmissive waves and the two reflective waves. We use a perturbation expansion to obtain a generalized 2×2 Jones vector description that is equivalent to the Berreman's description. The first-order term can also be directly derived from the conventional Jones calculus approach, and the higher-order terms represent the multiple reflections within the medium. We find that for the LCD the transmissive subspace is almost invariant, i.e., the bulk reflection effects in the LCD are very weak. Therefore, the accuracy of the first-order solution is in general sufficient. This new method significantly increases the speed of computing the overall optical properties of LCD devices under arbitrary conditions.
Model of electro-optic effects by Green’s function and summary representation: applications to bulk and thin film PLZT displays and spatial light modulators
Authors: Wu Adam Y. , Chen Tom C. C. , Chen H. Y.
Published: February 2023

Field distributions and electro-optic effects in ceramic bulk and thin film materials have been calculated by using Green's function and summary representation methods. The specific problems that have been analyzed include displays and spatial light modulators with 1) multiple interdigital planar electrodes on one side of the sample, 2) multiple interdigital planar electrodes on both sides of the sample, with or without misalignment of the electrodes on opposite sides, 3) optimization of the electro-optic effect by optimizing the electrode gap, width, number of electrodes, and sample thickness, 4) electrooptic effects with transparent electrodes, and 5) electro-optic effects in devices that have been coated with a buffer or capping thin film layer of various dielectric constants. Many interesting new effects have been predicted by our model and agree very well with our experimental observations and measurements, including field enhanced electro-optic effects in PLZT bulk and thin film display with a buffer and/or a capping layer.
Signal-to-noise ratio of the diffracted light in an acousto-optic Bragg cell
Authors: Tarn Chen-Wen , Banerjee Partha P.
Published: February 2023

The nature of the first moment ensemble averages of the scattered light fields from an acousto-optic (AO) device in the Bragg regime under the influence of noise is investigated. We start from the wave equation in the presence of the AO interaction term and the noise fluctuation terms. Thereafter, we take the ensemble average and employ the Fourier transform technique to derive the transfer function of the diffracted light. The impulse response at the output can then be found by taking the inverse Fourier transform. The intensity profile at the output has coherent and incoherent contributions from the deterministic and random parts of the impulse response. The random part suggests that the Bragg-diffracted light beam intensity will be attenuated because of the optical imperfection of the AO medium. Furthermore, the light beam profile will be distorted because of the imperfections in the sound profile resulting from the bonding between the transducer and the AO device, random sound scatters, etc. We compute the dependence of the signal-to-noise ratio of the first order light on the mean square values of the distributions of the imperfections and scatters.
New idea for a pulsed electron microscope with high brightness
Authors: Chen J. W.
Published: February 2023

The electron microscope has been widely used in many fields. It is a very useful tool to search the microworld. Unfortunately, it is only used to investigate static state because of the limitation of the low brightness of an electron beam. In this paper, we suggest equipping an electron microscope with a laser driven photocathode and making it operate in both cw and the pulsed states. The brightness and coherence of an electron beam from a photocathode are as perfect as those from a field emission gun, but a photocaathode can operate under short-pulse conditions. These advantages are not possessed by field emission and thermionic emission guns. As for the photocathode materials, many experiments have proved that the photocathodes made of semiconductors may have higher current densities and lower divenrgences. Now the laser to be used to drive a photocathode is Nd:YAG; this laser can work in both cw and pulsed states. In this way, under the cw operation, it is very convenient to align, and under the pulse operation, it is used to make pulse electron holograms.
Divergence of laser beams in regular nonlinearly refracting medium
Authors: Zemlyanov Alexander , Martynko Andrei V.
Published: February 2023

Based on the use of the method of effective beam parameters, we determine the existence conditions for exact aberrational solutions for the effective width, phase front curvature radius and limiting divergence of the beam. The views of these solutions are also obtained. A similarity effect had been revealed in the study of the processes under conditions of strong nonlinear distortions due to thermal blooming of collimated beams of different classes and under the action of different mechanisms of nonlinear interactions. Regimes of limiting beam divergence formation in the initially homogeneous and inhomogeneous nonlinearly refractive media are investigated. In the case of weak nonlinear distortions expressions are derived that relate the parameters of inhomogeneous propagation path to the initial parameters of the laser beam.
Energy transfer lasing from dye-doped microdroplets seeded with fluorescent sol
Authors: Armstrong R. L. , Xie J. G. , Ruskgauer T. E. , Pinnick R. G.
Published: February 2023

We report observations of lasing and fluorescence emission from liquid microdroplets of fluorescein 548 dye in ethanol, seeded with submicrometer sized fluorescent sol. An incident pump laser generates droplet fluo­rescein lasing or fluorescene, which in turn excites either fluorescene or lasing in the sol. All emissions are at wavelengths corresponding to morphology dependent reso­nances (MDR's) of the droplet. Studies of the dependence of these emissions on pump laser intensity and sol concentration suggest they are driven by enhanced radiative energy transfer that occurs when fluorescein lasing or fluorescence emission couples to MDR's of the droplet microcavity. Other noteworthy findings include the absence of sol emission for larger sol and the presence of sol emission, even without any observable fluorescein emission, for smaller sol.
Measurement of single particle size and refractive index in liquids
Authors: Taubenblatt M. A. , Batchelder J. S.
Published: February 2023

Detection and identification of submicrometer contaminant particles in processing liquids is of critical importance in semiconductor manufacturing. Typical particle detection instruments sense scattered intensity and offer no means to determine particle composition. By detecting both the magnitude and phase of the forward scattered light from a particle, refractive index information can be obtained in addition to size. The size is also computed more accurately since the refractive index is determined, rather than assumed. This measurement is analogous to the use of ellipsometry to determine the thickness and refractive index of thin films. A laser inter­ferometer using Nomarski optics is used to detect the phase shift and extinction in one beam relative to the other as a particle traverses both beams. These measurements are corrected for particle trajectory (off-axis and defocus) by judicious alignment of the beam shear direction and are then used to compute the forward scattered phase and magnitude from the particle. This instrument has been used to accurately size and bin particles into four composition classes: low dielectrics, medium dielectrics, metals, and bubbles.
Tomographic analysis of line of sight spectra of low pressure burner flames
Authors: McNesby Kevin L. , Fifer Robert A.
Published: February 2023

Computed tomography is used to obtain species profiles within a low pressure premixed CH4/N2O flame as a function of height above the burner surface and lateral position at that height. This information is extracted from parallel line of sight absorbance data by using Abel inversion. This process yields two dimensional slices of the flame. At any point within each slice the infrared spectrum of the species present may be obtained. Abel inversion may be applied only to systems possessing axial symmetry. Species profiles for inverted and normal line of sight absorbance spectra are compared and contrasted with data taken on similar systems by different methods.
Spectroscopic probes of laser-produced microplasmas
Authors: Simeonsson J. B. , Forch B. E. , Miziolek A. W.
Published: February 2023

Microplasmas produced by lasers operating in the ultraviolet are unusual because of their low threshold requirements on laser pulse energy and their short lifetimes (microplasma lifetimes as short as 100 ns have been observed). These laser-produced microplasmas have been used as ignition sources for reactive gases and as the working media in techniques for chemical analysis (particularly in conjunction with gas chromatography). In order to better understand the details of microplasma initiation and growth and to extend their range of usefulness, time-resolved spectroscopic measurements are being conducted. Specifically, atomic and ionic emissions from hydrogen, carbon, oxygen, and chlorine are being analyzed to determine the microplasma temperatures and electron densities. Knowledge of these parameters provides important insight into the excitation and equilibrium characteristics of the microplasmas, information that is neces­sary for their analytical applications.
Multiple exposure interferometric lithography
Authors: Zaidi Saleem H. , Brueck S. R. J.
Published: February 2023

Volume manufacturing of dense, large-area arrays of small feature size (≅ 100-500 nm) structures is of considerable importance to a range of present and developing technologies including microelectronics and displays. Substantial effort is being expended on improvements in conventional optical lithography (including shorter wavelengths and phase-shift masks), on x-ray lithography, and on alternate patterning techniques, such as e-beam and focused ion-beam lithographies. The optical and x-ray techniques have limitations in terms of depth-of-focus that become more severe as critical dimensions decrease. The particle lithographies, while providing the necessary resolution, are generally more complex serial processes and are ill-suited to volume production. We report on a simple use of multiple exposure interferometric lithography using readily available laser sources to produce large areas of submicrometer structures. This technique has an essentially infinite depth-of-field and is well matched to volume manufacturing constraints. The use of multiple exposures, of zone plates within the optical paths, and the combination of this technique with conventional (but lower resolution) lithography offers the potential of fabricating a wide variety of structures. Multiple exposures result in moire patterns. A simple, but important, example of these moire patterns is a two dimensional array of submicrometer holes on uniform grids in both x and y directions. Another useful structure is a large area interdigitated comb. Linewidths to as small as 100 nm and periods to 250 nm are readily achieved by using a 488 nm Arion laser source.
Image degradation in a soft x-ray projection lithography system due to diffraction phenomena in the condenser
Authors: Sweatt William C. , Lawrence George N.
Published: February 2023

We are doing physical optics modeling of a soft x-ray projection lithography system. The system being modelled consists of projector and condenser optics. The object mask is imaged onto the photoresist with the projector, an off-axis f/5 Schwarzs-child reflective microscope. The condenser is an off-axis section of an elliptical mirror. It images the incoherent, 14-nm laser-plasma source into the entrance pupil of the Schwarzschild, as in Kohler illumination. It is widely understood that the f/5 Schwarzschild must have wave-front errors that are small relative to the 14 nm wavelength if 200 nm features are to be sharply imaged onto the chip. How about the condenser mirror? How good does it have to be? We generate specifications for the mirror, as well as alignment tolerances. In addition, we study the effects on the image of edge diffraction in the condenser system.
Pellicle photodegradation as a cause of localized magnification error within individual chip subarrays
Authors: Rosenbluth A. E. , Gortych J. , Bergendahl A. , Fullenbaum M. , Bakeman P. , Doany F. , Chastang J. C. , Maculuso D. , Sturtevant J.
Published: February 2023

An unusual overlay error has been seen in test chips exposed at A ~ 250 nm through pellicles made from polyvinyl butyral (PVB). The error manifests itself as a local excess magnification within each subarray of an integrated circuit (IC) pattern. In the worst case we have seen misregistration increase over time to 600 nm. The phenomenon appears to be the same as recently reported at λ = 365 nm. We have found the cause of the problem to be the steady pellicle thickness loss known to arise under 248 nm irradiation. The lifetime of these pellicles is acceptable by the usual criterion of dose to punch-through, but the image displacement effect is of far greater concern. The pellicle thickness loss is not uniform, since gross features of the mask layout remain visible after several millimeters of defocus. When the resulting phase modulation reaches ~λ/10 during wafer exposure, the sensitive overlay tolerances of photolithography can be exceeded. This can occur at cumulative pellicle doses about 2 orders of magnitude below punch-through. The effect is highly pattern dependent, so different IC levels show placement errors of widely different severity. Within a level, large shifts typically occur near kerfs. We will discuss the theory of pellicle image formation and show interferograms of the phase modulation. We will show limited etch rate measurements from pellicles made of two different materials. The problem can for all practical purposes be eliminated by using pellicles made from fluorocarbon.
Advances in the modeling of diffraction from latent image gratings
Authors: Bishop K. P. , Naqvi S. S. H. , Milner L. M. , McNeil J. R.
Published: February 2023

As the photolithography field strives to obtain sub-half-micrometer geometries, the interrogation of latent images in photoresist is becoming increasingly important. A latent image is defined as the distribution of photoactive compound (PAC) concentration after exposure to radiation. The PAC concentration present in a photoresist layer after exposure determines the index of refraction of this layer. Photolithography simulation tools, such as PROLITH or SAMPLE, that model the PAC concentration can be utilized to provide a complete description of the index grating structure. Rigorous coupled-wave diffraction analysis is then used to model the scattered light from the latent image grating. The effects of focus and exposure conditions on the final line shape of the grating structure are of increasing importance in photolithography. By using the photolithography simulation tools, the effects of the focus and exposure conditions on the scat­tered light can now be considered. Also, processes that affect the latent image after the exposure step, such as post-exposure bake (PEB), can be considered. The theoretical results obtained illustrate how examination of scatter from a latent image grating enables the tracking of the effect of the focus, exposure, and PEB conditions.
Rare-earth-doped waveguide devices
Authors: Sanford N. A. , Malone K. J. , Aust J. A. , Larson D. R.
Published: February 2023

Rare-earth-doped integrated optic waveguide lasers have been demonstrated in glass, LiNbO3, and other host materials. This technology offers a variety of new components that include diode-pumped amplifiers and lasers. The planar geometry is particularly attractive because selective doping permits the integration of passive and active devices on the same substrate and it is also compatible with pumping by laser diodes. Ion exchanged channel waveguides have been demonstrated to lase near 1060 nm and 1320 nm in Nd-doped silicate and Nd-doped phosphate glass, respectively. Nd-doped waveguide lasers have also been fabricated by chemical vapor deposition; Er-doped waveguide lasers have been similarly fabricated. Y-branch splitters with gain at 1060 nm have been reported in Nd-doped silicate glass waveguides. Ion implantation has been used to form waveguides in Nd-doped YAG and Nd-doped GGG. Nd and Er-doped LiNbO3 waveguide lasers operating near 1060 nm and 1550 nm, respectively, have also been reported. Attempts have been made to demonstrate visible upconversion lasing in Er-doped LiNbO3 and LiTaO3 waveguides. We will review the status of this technology and also highlight some of the more promising applications.
15-dB amplification at 1.06 μm in Nd3+-doped ion-exchanged silicate glass waveguides
Authors: Miliou A. N. , Cao X. F. , Srivastava R. , Ramaswamy R. V.
Published: February 2023

Recently, interest has emerged in developing glass waveguide amplifiers. Such components are useful in monolithic integration of sources, amplifiers, and other optical circuits. Ion-exchanged waveguide lasers operating at 1.06 and 1.3 μm have already been demonstrated. However, there are very few reports of amplification in glass waveguides. By using Ag+-Na+ exchange, 3 dB gain has been reported in two different glasses at 1.06 μm wavelength. In this work, we report 15-dB amplification in K+-Na+ ion-exchanged channel waveguides fabricated by thermal diffusion from KNO3 melt in a 2% Nd3+-doped silicate glass. We used the 532 nm radiation from a frequency-doubled, Q-switched and mode-locked Nd:YAG laser as the pump. At this wavelength, the 8 mm long waveguides absorb 5% of the pump power. Apart from the strong fluo­rescence at 1.06 μm, a weak fluorescence at 1.3 μm and some visible light due to the upconversion processes were also observed. For amplification measurements, the residual 1.064 μm radiation from the laser was attenuated and served as the signal. The two collinear beams were simulta­neously coupled into the guides polished at both the ends, and the output signal was measured with and without the pump. We obtained up to 15-dB amplification with 100 mW peak pump power incident at the input without observing any saturation. To the best of our knowledge this is the largest amplification yet reported in these guides. The amplification factor is likely to increase further with an 808 nm diode pump where the absorption is larger.
Neodymiun-doped ion-exchanged waveguide laser in germaniun-rich soda-lime silicate glass
Authors: Roman J. E. , Winick K. A.
Published: February 2023

Fabrication and characterization of neodymium-doped planar channel waveguide lasers in GeO2-rich, soda-lime silicate glass are reported. Measured fluorescence and absorption specta for the glass are identical to those of soda-lime silicate glass containing no GeO2. Single mode chan­nel waveguides are fabricated in the glass by standard photolithography and KNO3 thermal ion exchange. Thin dielectric mirrors are placed in direct contact with the polished ends of the waveguide by using index-matching fluid, and a 13 mm long device is pumped at 800 nm by using a Ti-sapphire laser. The slope efficiency is 1.6%, and the lasing threshold is 30 mW. Lasing is achieved simultaneously at 1058.8 nm, 1060 nm, 1063 nm, and 1065 nm at high pump powers. The use of a GeO2-rich glass may facilitate the integration of multiple com­ponents, such as distributed feedback reso­nators, onto a single substrate. This integration can be performed by direct-write techniques using UV light (245 nm) tuned to the germanium-oxygen valence defect band. Experiments to form Hill gratings in the GeO2-rich laser glass by two-photon absorption at the 488 nm line of an argonion laser are reported.
Doped spin-on glass for integrated active waveguides
Authors: Ashby C. I. H. , Neal D. R. , Vawter G. A. , Hohimer J. P.
Published: February 2023

Rare-earth-doped polysiloxane spin-on-glasses (SOG) are being investigated as new optical materials for forming active channel waveguides directly on semiconductor wafers (GaAs). Use of a SOG matrix for rare earth ions, such as Nd+3 and Er3+, permits direct integration of the active waveguide and a laser-diode pump source on the same semiconductor wafer. Inclusion of the polarizable rare-earth ions increases the refractive index of the doped SOG relative to undoped SOG; this permits one to use undoped SOG as a cladding layer between the doped SOG and the underlying III—V substrate. Materials preparation and optical properties, fabrication techniques, and the development of SOG channel waveguides will be presented.
Quantum theory of driven multi-level systems
Authors: Manka A. S. , Narducci L. M. , Scully M. O. , Keitel C. H. , Zhu S.-Y. , Fleischhauer M.
Published: February 2023

We discuss a generalized version of the Scully-Lamb laser master equation for a collection of driven multilevel atoms in a lambda-type configuration. We are especially interested in the effect of the external driving field on the laser process with or without a population inversion between the lasing levels. We provide an exact solution of the photon probability distribution in steady state and discuss regimes of operation that can lead to sub-Poissonian statistics, as measured by the ratio between the variance and the average photon number. We solve the field master equation in the transient regime and analyze the effects of the external driving field on the linewidth of the emitted radiation. The largest reduction of the photon variance relative to the mean number corresponds to the injection into the cavity of atoms prepared in the nonlasing level. Injection of atoms in an incoherent mixture of the three energy states normally leads to super-Poissonian statistics.
High resolution molecular beam spectroscopy of yttrium monosulfide
Authors: Simard B. , James A. M. , Hackett P. A.
Published: February 2023

The YS molecule has been produced in a free jet molecular beam apparatus by laser vaporization of yttrium metal in the presence of He carrier gas doped with 1–2% of CS2, and its visible spectrum has been studied by laser induced fluorescence. The B2Σ
+
−X2Σ
+
(0,0) and A2Π−X2Σ
+
(0,0) bands have been studied at sub-Doppler resolution, and rotational, fine, and hyperfine parameters have been derived. Both hands are complicated by local perturbations, which also have been analyzed in detail. The first rotational lines of the B2Σ
+
−X2Σ
+
(0,0) band have been used to determine the permanent dipole moment of both the X and B states. Several other bands not belonging to the B−X or the A−X systems and that to our knowledge have never been reported before have also been observed and are currently the subject of intensive studies. The data will be compared with the predictions of ab initio calculations and the ligand field model and will be used to discuss aspects of the bonding.
Three-photon excitation of Ar with a tunable KrF excimer laser
Authors: Siegler Frank , Andresen Peter , Kim Gyung-Soo , Rothe Erhard W. , Reck Gene P.
Published: February 2023

The highly excited 4d′[5/2],3 state of Ar is populated via a three photon excitation by 248.44 nm light, a wavelength that is near the center of the KrF excimer range. We have measured a) its radiative lifetime, b) its collisional rate constant to a specific lower state, and c) the dependence of the ac Stark broadening of the excitation line shape upon laser intensity. Multiphoton resonances in rare gases have often been used to create coherent light in the UV or VUV. We describe the effort to create such light within a pulsed Ar molecular beam by using ≈400 mJ pulses of ≈ 10 ns width from a narrow-band (≈½ cm−1) KrF laser and to detect fifth harmonic (49.69 nm, 24.95 eV) light via photoionization of He (IP = 24.58 eV).
Nonlinear optics of isotropic opto mechanical media
Authors: Rogovin D. , Shen T. P. , Scholl J.
Published: February 2023

We examine optical phase conjugation and harmonic phase conjugation at microwave and millimeter wavelengths using an isotropic opto-mechanical array as the active medium. Such a medium, which can be regarded as a specialized class of artificial dielectrics, consists of a three-dimensional array of di-electric, metal, or metal coated spheres that are mechanically supported but free to roll or slide on either flat, transparent planes or strings. Theory asserts that these media form very deep translational index gratings, due to electrostrictive forces, that are suitable for nonlinear optical processes, such as phase conjugation. In particular, it is found that such a medium can easily achieve gain by using very low write beam powers, typically on the order of milliwatts per square centimeter. We examine medium dynamics from the perspective of the Langevin equations and find that typical optical index grating formation times are on the order of a few seconds at atmospheric pressures. However, these response times can be reduced to several milliseconds if the medium is maintained at an air pressure of a few torr.
Harmonic generation in thin film doped silica waveguides
Authors: Kester John J. , White William R. , Wolf Paul J.
Published: February 2023

Second harmonic generation (SHG) has been produced in waveguiding films of doped silica. The film was prepared by simultaneous illumination at 1064 and 532 nm radiation by using a prism coupling technique. Subsequent illumination by 1064 nm irradiation produced 532 nm radiation. The intensity of the film-generated 532 nm grew as a function of time until it saturated with an efficiency of about 1 %. Film-generated SHG produced by p-polarized 1064 nm incident light grew an order of magnitude faster than that produced by s-polarized light. The saturated SHG intensities for these input polarizations also differed by an order of magnitude. SHG could be produced only from specific modes of the waveguiding film. This technique allows direct observation and interaction of the spatial modulation of the index of refraction within the waveguide. Connections of this work with harmonic generation in doped silica fibers will be made.
Detection thresholds for chromatic pulses at loci removed from the adaptation locus
Authors: Krauskopf John , Gegenfurtner Karl R.
Published: February 2023

Thresholds for discriminating chromatic pulses are elevated when the discriminations are made at a locus differing from that to which the observer is adapted. Our previous results were equivocal with regard to the hypothesis that the threshold elevating processes operate independently within the "cardinal mechanisms." The results suggested that performance, in some cases, was improved by the utilization of information from additional mechanisms similar to those postulated by Hurvich and Jameson. We will present results of experiments on the detection of pulses at loci removed from the adaptation locus designed to eliminate the influence of the additional mechanisms.
Properties of holographic polarization beam splitters
Authors: Campbell Eugene , Kostuk Raymond K.
Published: February 2023

Many important photosensitive holographic materials used to form phase holograms are absorptive, which leads to variations in the average index and index modulation. The effect of such variations on the polarization properties of holographic optical elements is shown theoretically by using rigorous coupled wave theory and are compared to experimental data. Specifically, polarization beam splitters that operate on the principal of diffracting the first order beam at an angle that is normal to the incident beam have limited polarization selectivity in a medium with index variations. For transmission holograms, the variation in average index causes the angles of the incident and diffracted beams to vary within the medium and prevents Bragg matching throughout the medium. Polarization selective holograms that operate via the coincidence of the maximum diffraction efficiency for TM polarization with the minimum diffraction efficiency for TE polarization are also considered for media with average-index and index-modulation variations.
Confocal imaging by extended source interferometer
Authors: Sun Pang-Chen
Published: February 2023

The confocal scanning imaging technique offers significant advantages over conventional imagery, including superior resolution and rejection of scattered light. We describe a non-scanning system that achieves results essentially equivalent to those of confocal scanning. The basic theory of the process is developed, and the theoretical performance is compared with that of conventional confocal scanning. Such factors as signal to noise ratio, scattered light rejection, lateral resolution, depth resolution, and light utilization efficiency are considered. The practical advantages of eliminating the need for synchronized scanning by the illuminating beam and by the image sensing detector are compared with the complications arising from two beam interferometry. Experimental results demonstrate the ideas. In particular, we demonstrate the capability of the non-scanning confocal technique for imaging objects embedded in scatterers and to reject the scattered light from other depth positions.
Measurement of the angular correlation of speckle patterns
Authors: Syratt R. , Dainty J. C.
Published: February 2023

Following a study by Leger and Perrin in 1975, we have derived a general expression for the correlation of two speckle patterns produced by illuminating a randomly rough surface at two angles and observing at two arbitrary angles. The expression is valid only for single scattering surfaces. Measurements of the angular correlation for particular angles of illumination and viewing broadly confirm the theory. This provides a method of estimating the surface roughness by a non-contact means for singly scattering surfaces. Of more interest to us are surfaces that exhibit multiple scattering: in this case, there is no simple theory and measurements show that the angular correlation is quite different from the single-scattering case.
Error diffusion algorithm using pixel and halftone based error corrections
Authors: Eschbach Reiner
Published: February 2023

In order to be able to print an image on a binary output device, the image has to be rendered as a sequence of on/off (black/white) states. The image quality of the printed binary image strongly depends on the actual distribution of the black/white pixels. In order to increase the fidelity of the reproduction it is desirable that a feedback algorithm in the form of error diffusion be used. This type of algorithm, however, produces patterns that are not reproducible by standard xerographic printers, where a clustering of pixels into dots is required. This talk demonstrates a method to combine error diffusion with standard halftoning by first using a pixel based error correction followed by a halftone cell based error correction. This combination allows the generation of printable patterns that incorporate characteristics of feedback algorithms.
Optimal use of SLMs in correlators
Authors: Lanteigne David J.
Published: February 2023

Conventional correlators with similar SLMs in the input and filter planes are generally designed such that the highest spatial frequency in the input object is mapped to the edge of the filter plane SLM. Simple analysis shows that the diffraction limited spot size of such a system is substantially larger than the pixel size in the filter plane. The consequences of this phenomenon for the design of optimal filters and correlators will be discussed, including the role of optical aberrations.
Time-resolved second-harmonic generation as a probe of surface dynamics
Authors: Tom Harry W. K. , Prybyla Judith A.
Published: February 2023

Understanding surface dynamics requires knowing the microscopic details of the dynamical pathways open for surface chemical reactions. The time scale of atomic motion, bond breaking, electronic vibrational coupling, and substrate-adsorbate coupling all require using surface probes with 100 fs time resolution. The 100 fs laser pulse pump-probe technique provides such a tool if the second-harmonic generated by the probe pulse (SHG) is used as the probe of the surface structure. SHG is sensitive to surface symmetry, adsorbate coverage, and adsorbate orientation. Because the electronic- vibrational coupling time is >100 fs, the 100 fs pump pulse can heat the electronic system without heating the vibrational system. Such excitation leads to a new class of reaction mechanisms that are non-thermal. We have used time-resolved SHG to observe a 100 fs order-disorder transition in bulk Si following the 100 fs excitation of ~4% of the valence band. We have also observed CO molecules desorb from Cu(111) <350 fs after a 100 fs pulse excites the substrate electronic temperature to 3600 K while leaving the lattice temperature rise <50 K. These reactions occur too fast to be thermally or lattice- vibrationally driven and are probably driven by the cooperative pumping of vibration excitation upon repeated electronic excitation-deexcitation cycles. These observations suggest that lasers can process materials and stimulate chemical reactions in new and unexpected ways.
Surface temperature measurements on picosecond time scales using surface second-harmonic generation
Authors: McVeigh V. , Hoffbauer M.
Published: February 2023

Phenomena involving laser-materials interactions are playing an increasingly important role in a number of disciplines. To understand laser-induced surface processes, it is essential to know both the actual surface temperature increase and the time scales required for thermalization of the deposited energy. We have measured the time-resolved transient surface temperature rise induced by a sub-nanosecond laser pulse incident on a Cu(110) surface in UHV by using surface second-harmonic generation (SHG). Transient surface heating was induced by a 1064 nm infrared pulse having a temporal width of ~100 ps. An amplified dye laser operating near 600 nm with a 6 ps pulse width was used as the SHG probe. The non-resonant SHG signal was found to decrease rapidly as the surface temperature was increased by resistive heating. The physical origins of this decrease will be discussed. The strong SH temperature dependence was used to convert the time-resolved decrease in the SH intensity observed with the infrared heating pulse into a surface temperature profile showing a maximum surface temperature rise of ~50°C. The observed transient surface temperature increase will be compared to calculations using both an equilibrium one-dimensional heat diffusion model and a model that accounts for a hot-electron distribution that is not in equilibrium with the lattice.
Observation of fast electromagnetic transients from GaAs surfaces
Authors: Keiding S. R. , Pedersen J. E.
Published: February 2023

We report the observation of fast electromagentic transients with terahertz band widths, radiated perpendicular to a GaAs/air interface. With a femtosecond laser pulse at normal incidence, free carriers are generated at the GaAs surface. These carriers radiate a fast EM transient when accelerated by the electric field present at the GaAs/air interface. In a simple dipole model no radiation is emitted perpendicular to the interface, along the direction of the driving field. We, however, observe a fast transient radiated perpendicular to the surface, and we ascribe this effect to the strong change in the angular distribution of the radiating dipoles due to the presence of the interface and the presence of a small dielectric lens near the interface.
Optical and mass spectroscopy of single microparticles using the electrodynamic trap
Authors: Dale J. M. , Yang M. , Whitten W. B. , Ramsey J. M.
Published: February 2023

A number of researchers have shown that sensitive optical measurements can be performed on single microparticles levitated in particle traps. In fact, we have shown that single fluorescent molecules can be detected in microdroplets held in an electrodynamic trap. Further chemical characterization of a microparticle can be achieved by mass spectroscopically analyzing the species on the surface or in the bulk of a particle. Our approach to single particle chemical characterization is to first probe a particle levitated in an electrodynamic trap by using optical spectroscopy. Following the nondestructive investigation, an intense laser pulse is used to desorb ions from the particle while simultaneously converting the particle trap to an ion trap. The laser generated ions are confined in the ion trap and subsequently mass analyzed. A number of different species have been desorbed from particles and mass analyzed by using this approach, including metals, quaternary salts, and explosives. Results and discussion will concentrate on the laser desorption/mass spectrometry aspects of these experiments.
Contrast polarity and letter identification
Authors: Alexander Kenneth R. , Xie Wei , Derlacki Deborah J. , Fishman Gerald A.
Published: February 2023

Evidence indicates that the spatial tuning properties of luminance increments and decrements may not be equivalent and that stimulus onset-offset characteristics can differentially affect contrast thresholds. The present study investigated whether this is the case for letter identification. Individual Sloan letters were presented as luminance increments or decrements on a gray-scale display of a Macintosh IIfx computer, with contrasts controlled by an ISR video attenuator. The temporal mode of presentation was either a rapid onset with a Gaussian offset, or the reverse. Contrast sensitivity for letter identification was measured with a forced-choice staircase procedure. When plotted in Rayleigh (Michelson) units, contrast sensitivity was higher for letters of positive than of equivalent negative contrast, and the contrast sensitivity function for letters of positive contrast extended to smaller letter sizes. In Weber units, however, contrast sensitivity was equivalent for increments and decrements of equal magnitudes. Performance at Weber contrasts greater than unity was predictable from a negative exponential fit to the letter contrast sensitivity functions. Onset-offset characteristics had no differential effect on letter contrast sensitivity. These results indicate that the Weber definition provides the better metric for specifying letter contrast, and the findings provide a framework for predicting the effect of contrast polarity on tasks that involve letter identification.
Contrast sensitivity and efficiency as a function of grating area and spectral density of spatial noise
Authors: Rovamo J. , Luntinen O. , Nasanen R.
Published: February 2023

We measured contrast sensitivity for cosine gratings by using various grating areas and spectral densities of white two-dimensional spatial noise. Contrast sensitivity increased with grating area similarly at all noise spectral density levels. Also, contrast sensitivity decreased with increasing noise spectral density similarly at all grating areas. The experimental data was described by Eq. (1): S = Smax(1 + A
c
/A)
−
0.5(1+N
e
/N
o
)
−
0.5, where RMS contrast sensitivity S is the dependent variable, grating area A and spectral density of noise N
e
are the independent variables, and the maximum contrast sensitivity Smax, critical area A
c
of spatial integration, and critical spectral density of noise N
o
are the parameters of Eq. (1). The ideal detector for a signal known exactly is a matched filter. Its energy threshold is d'
2
(N
o
+ N
e
), whereas the human energy threshold is S
−2
A. Efficiency is their ratio, which is equal to d'
2
Smax
2
N
o
(A
c
+ A)
−
1, where d' is the detectability index. The maximum efficiency was 30%. Our results can be explained by a suboptimal matched filter with a sampling aperture equal to A
c
.
Latent image characterization using diffracted light
Authors: Milner Lisa M. , Bishop Ken P. , Naqvi S. Sohail H. , McNeil John R.
Published: February 2023

Our goal is to use the light diffracted from a latent image grating to improve CD uniformity. The experimental technique involves using a laser at a non-exposing wavelength to illuminate a grating pattern in exposed, undeveloped resist (a latent image). Photodiodes are employed to measure the intensity in the orders diffracted from the image. Data has been taken on a latent image focus-exposure matrix produced on Si. The data indicates that the first order is a function of both exposure and focus, and the second order is more strongly effected by focus. The zeroth order is a function of the thickness of the resist and any underlying films. The effects of post exposure bake (PEB) temperature have also been investigated. The zeroth order changes as a function of PEB temperature because of the resist thickness shrinking, and the first and second orders are also functions of PEB temperature. We have generated a program that uses rigorous diffraction theory to predict the intensity in the diffracted orders as a function of exposure, focus, and PEB. This modeling program is discussed in a separate paper.
Spectral analysis algorithms for measurements of critical dimensions
Authors: Franco P. R. G. , Jordan R. , Naqvi S. S. H.
Published: February 2023

Measurement of critical dimensions is accomplished by means of thresholding the intensity profile of an image. What we present here is an alternative technique that utilizes the spectrum of the intensity profile in order to perform these measurements. It is well known that as we move away from the object we lose important information that the evanescent modes contain (e.g., near field microscope). As feature sizes are reduced, limitation in the number of propagating modes that are gathered is a fundamental problem for classical optical microscopes. Utilizing the fact that the remaining frequency components still have valuable information, we performed simulations by utilizing a strip and a relief grating. The results obtained are very good even for submicrometric features in the case of the strip gratings. For the case of a 90° sidewall angle, results are very promising. Studies on structures that have side-wall angles that are different from 90° are under way.