bluuebunny/crossref_metadata_2025 huggingface dataset:
Repetitive opening switches using optically activated semiconductors
Authors: Chauchard E. A. , Rhee M. J. , Lee Chi H.
Published: February 2023

Optically activated semiconductor opening switches1 are anticipated to have unique characteristics of repetitive operation, fast opening time, and potential scalability. We have demonstrated the operation of optically activated semiconductors2 as fast opening switches. A semiconductor with high dark resistivity is maintained in a conductive state by continuous illumination with argon laser light. The opening of the switch is obtained by interrupting abruptly the light using a Pockels cell placed between two parallel polarizers. The opening speed is given either by the rise time of the Pockels cell (1 ns in our system) or by the recombination time of the carriers optically formed in the semiconductor, whichever is the longest. To demonstrate its operation, the switch was placed in an inductive charging circuit which delivers the stored energy to a load when the switch opens. The shape of the transient current signal was observed on an oscilloscope for different parameters of the circuit and using different switches. The off-to-on resistivity ratio of the switch was always larger than 103. We have achieved a repetitive 1-ns opening time with an lnGaAs:Fe switch of ~300-ps recombination time. The results obtained with different materials (GaAs, InGaAs, GaAs:Cr) and different gap configurations are presented.
Bidirectional reflectance of snow surfaces in the thermal infrared
Authors: Gimmestad G. G. , Zhou Q. B.
Published: February 2023

The spectral region from 8 to 14 μm is frequently used for thermal imaging and remote sensing of the earth’s surface. To interpret images and radio-metric data taken over snow-covered terrain, it is necessary to know the directional optical properties of the snow surface. Because ice is so highly absorbing in this spectral region, any reflection must take place at the first air-ice interface and therefore reflectance from snow should be modeled as reflection from a rough surface consisting of ice crystals. A simple model for bidirectional reflectance based on Fresnel reflectance coefficients and geometrical considerations has been described by Torrance and Sparrow.1 This model has.only two free parameters, and they are related to surface condition. Preliminary measurements of the 8-14-μm bidirectional reflectance of snow are reported, along with calculations from the Torrance and Sparrow model. The measurements show that snow does behave as a rough surface and that artificially disturbing the snow surface does change its bidirectional reflectance. The model shows qualitative agreement and may be sufficient for describing snow, although better measurements are needed to determine its value.
Detection sensitivity of the laser resonant Voigt effect
Authors: Yamamoto M. , Furuya Y. , Amemiya K. , Hanari J. , Takubo Y.
Published: February 2023

This work is based on previous papers1,2 which analyzed the resonant magnetooptic spectroscopy. These papers presented theoretical expressions for the atomic Voigt effect and the Faraday effect signal intensity. The theory predicts how the analytical detection sensitivity depends on the Zeeman-split resonance line profile and on the polarization state of the incident light beam. To verify the previous analysis we measured the Voigt effect signal profiles of sodium atoms and OH molecules in a chemical flame, using wavelength-scanned narrowband visible and ultraviolet dye lasers. The laser beam passed through the flame, which was placed in the gap of a magnet. The magnetic field was perpendicular to the optical path. The beam passing through the flame was detected with a polarization analyzer. The observed profiles can be interpreted on the basis of the previous theoretical analysis.
Analysis of 81Kr in groundwater using laser resonance ionization spectroscopy
Authors: Kramer S. D. , Hurst G. S. , Chen C. H. , Payne M. G. , Allman S. L. , Phillips R. C. , Lehmann B. E. , Oeschger H. , Loosli H. H. , Willis R. D. , Thonnard N.
Published: February 2023

A new analytical technique based on resonant ionization of krypton with a vacuum ultraviolet (VUV) laser source was used to determine low-level 81Kr concentrations in groundwater. The long half-life (210,000 yr) and low concentration (1.3 × 103 81Kr atoms/liter of modern water) make the detection of 81Kr by radioactive counting techniques extremely difficult. In this method, krypton gas was removed from water taken from an underground Swiss aquifer using standard cryogenic and chromatographic techniques. Stable krypton isotopes were then reduced by a factor of 107 by a two-stage isotopic enrichment cycle using commercial mass spectrometers. The enriched gas containing ~108 stable krypton atoms and ~103 atoms of 81Kr was implanted into a silicon disk. This disk was then placed in the high vacuum final counting chamber and the krypton released by laser annealing. This chamber contained a quadruple mass spectrometer which used a pulsed VUV laser source as the ionizer.1,2 The measured signal indicated that the sample contained 1200 (±300) atoms of 81Kr.
Diffraction catastrophes and inverse scattering: the cusped rainbow of a spheroidal drop
Authors: Marston Philip L.
Published: February 2023

This paper is concerned with the determination of the shape of a drop of water from the angular location of the cusped rainbow. This approach was stimulated by the recent discovery1 of novel diffraction catastrophes in the rainbow region of horizontally illuminated spheroidal drops. Hyperbolic-umbilic and cusp catastrophes arise because, in addition to the usual set of equatorial rays, skew rays contribute to the scattering near the horizontal plane. The type of scattering depends on the drop’s axis ratio q = D/H where D is the diameter in the equatorial plane and H is the diameter along the vertical symmetry axis. An umbilic focal section was observed when q ≃ 1.31. Nye2 has calculated this critical value from the condition that two infinitesimal-skew rays merge with two equatorial rays at the rainbow angle θ
R
≃ 138°. The cusp in the unfolded catastrophe occurs where the skew rays merge with a single equatorial ray having a scattering angle θ > θ
R
. Ray tracing formalism was used to relate q to the θ of the cusp. This calculation was confirmed by observations with q from 1.21 to 1.37 and D ~ 1 mm.
Why does a BaTiO3 self-pumped phase conjugator produce a frequency-shifted output beam?
Authors: MacDonald Kenneth R. , Feinberg Jack
Published: February 2023

We present theory and experiments to show that the four-wave mixing efficiency in a photorefractive BaTiO3 crystal can, in some conditions, be enhanced by using optical beams of slightly different (~ 1-Hz) frequencies to write a moving grating. This enhancement is an intrinsic property of the four-wave mixing process and is a result of the finite response time of the material to changes in the writing intensity pattern. The optimum frequency shift scales as I0.6, where I is the incident optical intensity. Theory and experiments show the effects of any curvature mismatch between the pumping beams, of the relative beam intensities, and of the magnitude of the nonlinear coupling strength. We believe that our results explain the frequency shifts observed in a self-pumped phase conjugator1,2 and obviate the need to invoke a photovoltaic field or a resonator condition, as previously suggested.
Displacement thresholds across the horizontal meridian as a function of stimulus rate, duration, and length
Authors: Haines Richard F. , Kiefel Steven M.
Published: February 2023

Vertical displacement thresholds (DT) were quantified in 24 observers at the fovea and at each of eight retinal positions out to 80° along the horizontal meridian during binocular viewing. Stimulus length was increased with eccentricity angle in accordance with Virsu and Rovamo’s1 cortical magnification factor; a 1° long stimulus served as a control at each position. Three movement durations (from 0.25 to 4 s) and three angular rates (from 0.04 to 0.42°/s) were also quantified at each position to determine whether peripheral DT is mediated predominantly by an image displacement or rate-sensitive mechanism. It was found that (1) DT is mediated predominantly by a constant rate-sensitive mechanism at each position when stimulus length is increased in proportion to the cortical magnification factor, (2) mean percent correct displacement judgments decrease significantly into the periphery for both stimulus lengths, however, increasing stimulus yields significantly higher accuracy, and (3) at 10° and 30°, increasing rate and duration significantly improve displacement judgment accuracy but beyond 40° only lengthening the stimulus improves accuracy. The findings are related to the design optimization of aircraft dynamic attitude displays and symbology.
Charging and discharging of cones and rods
Authors: Mc Leod Roger D.
Published: February 2023

The effect of light in decreasing outward radial ionic current is to cause an accumulation of positive ionic charge in receptor inner segments, since the inward-flowing ionic current in the inner segment of the receptors is unaffected by light stimulus. That accumulation builds up a positive electrochemical voltage which discharges across the synaptic electrochemical spark gap. This mechanism operates on a proportionality with electric field amplitude values in a spatial Fourier transformation.1 It is optically generated in the diffraction pattern at the eye’s focal surface and conveys all information about the original object. Cones and rods are always at the visual system’s focal surface where field amplitudes are summated and are only coincidentally also at an image surface because all objects viewed from distances of closest distinct vision out to infinity are at an effective infinite distance optically for any eye. Photon uptake is associated with transfer of charge into the interior of the disks inside cones and rods. This function decreases the light-modulated ionic current outflow from receptors and is the equivalent of a gain-decreasing mechanism proportional to energy.
Image processing with an acoustooptic heterodyne scanning system
Authors: Poon Ting-Chung , Indebetouw Guy
Published: February 2023

A two-pupil processing technique with acoustooptic frequency offset has been used to synthesize bipolar point spread functions (PSF) in an incoherent system.1 Spatial filtering operations were demonstrated and implemented in a scanning processor.2 In this presentation, we explore multiplexing techniques in order to perform a number of different operations simultaneously. For example, when the heterodyne transfer function of the system is not even, the two components, in-phase and quadrature of the heterodyne signal, can be used to synthesize two different bipolar PSFs. In contrast, not even transfer functions have this capability. An example is shown where the input and its first and second (approximate) derivatives are obtained in a single scan. The use of multiple-pupil interaction with different frequency offsets in a scanning processor is also discussed.
Pattern discrimination at suprathreshold contrasts
Authors: Wilson Hugh R.
Published: February 2023

The results of a series of spatial frequency and orientation masking studies in my laboratory have recently been incorporated into a nonlinear mathematical model for suprathreshold pattern discrimination tasks.1,2 The model contains six distinct spatial frequency tuned mechanisms, each of which consists of an orientation selective linear filter followed by a nonlinear contrast transfer function. Pattern discrimination thresholds are computed by pooling the difference in mechanism responses to each of the two patterns in a two alternative forced choice paradigm, with the pooling being over spatial frequency mechanisms, orientations, and spatial nearest neighbors. Thus, the model is formally analogous to line element models for color discrimination. Following a summary of the model, it will be applied to a number of suprathreshold pattern discrimination tasks including spatial frequency discrimination, curvature discrimination, a variety of hyperacuity tasks, and phase or position discrimination between pairs of spatial patterns with bandwidths of 1.0 octave. The model will also be applied to the effect of contrast on spatial frequency discrimination, Vernier acuity, and phase discrimination. Some discrepancies between model computations and data may be attributed to individual differences among subjects, differing temporal presentations, and differing mean luminances in various studies.
Advances in the theory of phase conjugation by stimulated Brillouin scattering
Authors: Suni Paul , Falk Joel
Published: February 2023

Present theories of phase conjugation by stimulated Brillouin scattering in waveguide geometry disagree on key results. Hellwarth1 ignores incoherent scattering contributions to the nonconjugate fraction f1 of the high gain mode and calculates a nonconjugate output f1 that is independent of pump intensity. Zeldovich2 introduces statistical quantities to account for the incoherent scattering and arrives at an intensity-dependent output f1. We argue that these statistical quantities are not measurable under normal experimental circumstances and lead to conceptual problems. We show that both coherent and incoherent scattering contributions can be taken into account by deterministic methods. We focus attention on the spatial dependence of the gain coefficient. If its fluctuations can be ignored, f1 can be derived as a sum of term. The largest term has the same form as that of Zeldovich’s result but involves only deterministic quantities. When incoherent contributions are neglected our solutions duplicate Hellwarth’s. We emphasize that the different results of the Hellwarth and Zeldovich theories are of physical origin and are not due to different field expansions.
Efficient phase conjugation by Brillouin-enhanced four-wave mixing
Authors: Scott A. M. , Hazell M.
Published: February 2023

Andreev et al.1 have shown that Brillouin-enhanced four-wave mixing can be used to achieve phase conjugation with high reflectivities (~106). In this paper we develop a theoretical model which effectively describes the four-wave mixing process and compare the results of this model with the experimental results we have obtained.
Nonclassical states of the radiation field
Authors: Hillery Mark
Published: February 2023

A nonclassical state of the electromagnetic field is a state whose P representation is not non-negative definite.1 Examples of such states are squeezed states and states with sub-Poissonian statistics. These states demonstrate the necessity of the quantum mechanical description of the radiation field.
Laser resonator theory with variable aperture phase conjugate mirror
Authors: Bochove Erik
Published: February 2023

The ability of a phase conjugate mirror (PCM) to correct for intracavity phase aberrations, when substituted for a conventional spherical mirror (CM) in an optical resonator, is compromised by the presence of apertures located inside the cavity.1 We show that aperture effects in the entrance plane of a four-wave mixing cell, as caused by the use of finite-width pump beams, may be greatly reduced through proper design of the laser cavity. The basic concept is the matching of cavity modes to those of the PCM. Only the phases are of primary importance, and the matched modes have focus inside the mixing cell.2 We tested two methods of optimized cavity design as based on a simple model: (a) maximization of PCM effective spot diameter, and (b) a wave front matching criterion applied to the CM end of the cavity. Both techniques reduce intracavity phase aberration effects on the output beam by typically 2–3 orders of magnitude compared to predictions based on cavity modes which have the beam waist located in the PCM entrance plane.
Simulation of thin-film growth
Authors: Song Dar-Yuan , Liao Bangjun , Macleod H. A.
Published: February 2023

Hard disk, mechanical simulations of film growth date back to Henderson et al.1 and Dirks and Leamy2 and were successful in predicting the tangent rule of columnar orientation in conditions of limited mobility, that is, relaxation of the adatoms to the nearest stable position. In the simple model, packing density is lower than normal films but a scheme of increased mobility gives higher, more realistic values. With higher mobility the tangent rule no longer necessarily holds good, but this is in accordance with measurement.
Brillouin spectra of Langmuir-Blodgett films
Authors: Zanoni R. , Naselli C. , Bell J. , Stegeman G. , Seaton C.
Published: February 2023

The Brillouin spectra of thin films of cadmium arachidate have been measured. These films are multiple layers of ordered monolayers (26.8 Å thick) of the cadmium salt of arachidic acid deposited one monolayer at a time onto aluminum and glass substrates by the Langmuir-Blodgett technique.1 The elastic properties of these films can be determined from these Brillouin scattering spectra. Light from a single-mode argon-ion laser at 0.5145 m was scattered by multiple, high frequency, surface acoustic waves guided by LB films of thickness 100, 35, and 11 monolayers. The scattered light was collected and spectrum analyzed with a 9-pass tandem Fabry-Perot interferometer to obtain the velocities of the surface acoustic waves. The use of the tandem interferometer provided the high contrast needed to detect the weak Brillouin signals in the presence of the intense elastically scattered light. The Brillouin spectra are now being analyzed to determine the elastic properties of the Langmuir-Blodgett films.
Stimulated rotational Raman scattering in nitrogen in long air paths
Authors: Henesian M. A. , Swift C. D. , Murray J. R.
Published: February 2023

Large fusion lasers propagate high intensity beams over long distances in air. The propagation of high intensity beams through long air paths has also been proposed for other purposes. Averbakh et al.1 have analyzed stimulated Raman processes in air and have shown that for pressures below ~2 atm the process of lowest threshold is stimulated rotational Raman scattering in nitrogen.
Excited state spectroscopy of H2 via XUV-UV doubly resonant H− production
Authors: Kung A. H. , Page R. H. , Larkin R. J. , Shen Y. R. , Lee Y. T.
Published: February 2023

Recent developments on coherent XUV sources by frequency tripling of tunable dye laser radiation provide new means of preparing atomic and molecular states for studying their spectroscopic proper ties, especially in the region <100 nm. We have studied the photoionization of H2 in the region of 71 nm by a stepwise two-photon excitation technique using the

B
1

Σ
u
+

and the C1π
u
states as the resonant intermediate. The first photon at ~96 nm is derived from a tunable XUV source recently developed in our laboratory. The second photon is the UV output of a second dye laser. This excitation coupled with the detection of negative H− ions has uncovered several new spectroscopic features that could not be observed in positive ion detection due to an intense continuum in this region. In contrast to previous one-photon studies1 our spectrum suggests that a H− continuum that couples to the excited H2 states exists. Intense spectral features in this continuum are assigned as heretofore unobserved Rydberg series converging to the high vibrations of

H
2
+

(

v
+
≥
9
,
 

N
+

=
1
,
3

)

levels. The efficiency of H− production and the effect of the nature of the intermediate state on this efficiency are discussed.
Multiphoton Ionization spectroscopy of atomic fluorine
Authors: Bischel William K. , Jusinski Leonard J.
Published: February 2023

The study of the electronic spectrum of atomic fluorine is extremely difficult since all the single photon transitions from the ground state require wavelengths that are below the LiF window cutoff at 106 nm. This situation presents an ideal application for the techniques of multiphoton excitation, followed by either fluorescence or ionization detection. We report here the first observation of three-photon resonant excitation of the
2
P1/2,3/2 excited states in atomic fluorine at 105,000 cm-1. This state is then ionized by the absorption of two additional photons forming a five-photon process. The experiment consists of focusing a doubled YAG-pumped dye laser at 286.3 nm into a flowing cell containing a mixture of F2 and He. The UV radiation completely dissociates the F2 in the focal volume and simultaneously ionizes the atomic fluorine. The ions are collected with a pair of biased electrodes and the charge is amplified. We estimate that ~104 ions are produced in the focal volume for a 1:1 signal/noise ratio. We also observe three-photon resonant four-photon ionization of molecular F2 in the same region. This is the first observation of the MPI spectrum of F2 and most of the observed features have been assigned using previous data.1 We have carefully measured the intensity of our laser in the focal volume and present estimates of the effective five-photon ionization cross section for detection applications.
Time-resolved density measurements in a laser-produced indium plasma
Authors: Lacy Robert A. , Nilsson Alan C. , Byer Robert L. , Silfvast William T.
Published: February 2023

In 1983, Silfvast et al.1 demonstrated a high gain of 4416-Å cadmium II laser pumped by inner-shell photoionization. We propose an 1850-Å laser on the isoelectronic

4

d
9

5

s
2

2

D

5
/
2

→
4

d

10

5
s
5

p
2

P

3
/
2

∘

transition in indium III. Our atomic structure calculations indicate near-equal line strengths for the CD II 4416-Å and in in 1850-Å transitions. Our calculated natural lifetime of 80 ns for the In III 4d95s2 2D5/2 state makes an inner-shell photoionization-pumped laser seem feasible.
Temporal response of laser-induced photoacoustic pulses in H2O, measured as a function of incident beam energy
Authors: Bennett G. T. , Fry E. , Sogandares F.
Published: February 2023

The second harmonic, 530 nm, of a pulsed YAG laser was used to generate photoacoustic pulses in water and water doped with potassium permanganate. The photodeflection technique1 was used to measure the pressure pulse shape as a function of time. Studies were made for a range of incident pulse energies from several microjoules to the onset of dielectric breakdown in water. The discussion compares the observed time responses to the theory of photoacoustic pulses by Lai and Young.2
Optically stimulated sound from gas bubbles in water: radiation pressure and resonance response
Authors: Unger Bruce T. , Marston Philip L.
Published: February 2023

We detected high-frequency sound radiated by individual gas bubbles in response to modulated green light from an Ar laser. The bubbles were surrounded by clear water. They were prevented from rising by contact with a thin glass plate or with a clear Mylar film. Bubble radii were in the 20-100-μm range. Each bubble was first illuminated by short pulses, well separated in time, with a peak power ≃2 W. Sound radiated from the bubbles exhibited the ringing of a damped oscillator. This is evidence of monopole oscillations of the bubble's surface in which the surrounding water provides the inertia and the gas within impedes compression and acts like a spring.1 The resonance frequency f
R
was found by Fourier trans forming this signature; typically f
R
< 150 kHz and f
R
was close to the predicted monopole resonance frequency. Bubbles were subsequently illuminated with bursts of pulses with a frequency f ≃ f
R
. The resulting sound was characterized by an initial buildup of its amplitude which is the expected resonance response of an oscillator. The mechanism for driving the oscillations appears to be the compression of the bubble by modulated optical radiation pressure rather than thermal expansion. The transfer of momentum is thought to occur at the total reflection region of the bubble’s surface.
Small angular displacement monitoring using diffraction
Authors: Langlois P.
Published: February 2023

When a laser beam impinges on the edge of a razor blade, it produces on an observation screen a bright line perpendicular to the edge. According to Takenaka and Fukumitsu1 the intensity along this line varies as 1/x2 for relatively large x (where x is the distance from the geometrical shadow boundary). The precise mathematical relation of this intensity distribution is a consequence of diffraction theory. We illuminated a small mirror fixed on a tuning fork with this bright line (produced by a He-Ne laser) and we detected a part of the reflected light power with a small photodiode. The angular displacement of the tuning fork was thus monitored by means of the light intensity variations produced at the detector. It was possible to detect very small vibrations of the tuning fork and also to measure the angular displacement by evaluating the ratio of the ac component of the signal to the dc component. The latter corresponds to the x value of the detector position at the chosen operation point when the tuning fork is at rest. Thus light intensity measurements of a diffracted laser beam constitute an efficient and simple method to monitor small angular displacements. This new method could advantageously replace the more elaborate ones based on interferometry when a moderate precision is required. Moreover the time resolution of this diffraction method is limited only by the response time of the detector since there are no moving mechanical parts in the system.
Coherent ensemble of diode lasers with feedback in the Fourier plane
Authors: Rediker R. H. , Anderson K. K. , Corcoran C.
Published: February 2023

Five external-cavity diode lasers (diode lasers with one facet antireflection coated and feedback provided by the mirror at the other end of the external cavity) have been made to operate coherently by placing a spatial filter at the Fourier plane of the lens system in the cavity.1 For cw operation, the spectral linewidth of the emission from this coherent ensemble was determined to be less than the 7.5-MHz (1.7 × 10−5-nm) resolution of the Fabry-Perot spectrum analyzer used in the measurement. Preliminary results for pulsed-mode operation are presented for dc biases both below and above threshold. Spectral behavior of the coherent ensemble is compared with that of external-cavity lasers (no spatial filter) and of similarly constructed diode lasers that were not antireflection coated. Operating behavior of the ensemble with all five elements pulsed simultaneously, and also with one or more elements providing cw feedback while the others are pulsed, are presented.
Color contrast modeling and CRTs
Authors: Schuchard Ronald A. , Pokorny Joel , Smith Vivianne C.
Published: February 2023

Many researchers assume that the CIE uniform color spaces, CIELUV and CIELAB, can model color contrasts for CRT displays. These color spaces were developed for surface colors, not self-luminous colors as generated on CRTs. CIELUV and CIELAB combine, by vector summation, a luminance component (L*) and chromaticity components weighted by L*. The chromaticity components for CIELUV and CIELAB have limited areas of incompatibility when modeling data from self-luminous objects like MacAdam’s ellipses. The luminance component L*, a function of Y1/3, is based on the Munsell value function and has not been compared with data from self-luminous objects. We tested the accuracy of L* and Y1/3 in modeling the Weber fraction data. Over a typical CRT luminance range, Y1/3 is adequate while L* is not. Over an extended luminance range, neither Y1/3 or L* model the data. L* predicts the same number of just noticeable differences for any luminance range. A further problem is that previous psychophysical studies (e.g., MacAdam1) used vector summation of independent chromatic and luminance contrast components to model color contrasts, an approach not possible with CIELUV and CIELAB since L* appears in the chromaticity component.
Reflectances giving volumetric color constancy in daylight
Authors: Brill Michael H.
Published: February 2023

The volumetric theory of color constancy1 provides illuminant-invariant estimation of reflectance spectra from reflected-light tristimulus values. The basis of the computation is the tristimulus volume ratio. Tristimulus volume ratios are illuminant-invariant when (a) the illuminant spectrum is a linear combination of N functions si(λ) free to vary only in their coefficients; and (b) each reflectance spectrum r(λ) is a linear combination of three basic functions rk(λ) plus a residual term R(λ) which is orthogonal to the 3N products fm(λ) of si and the tristimulus functions qj(i.e., r must be orthogonal to a forbidden subspace spanned by fm except for the projections of the functions rk). The color-constant reflectance spectrum r′(λ) closest (in a least-squares sense) to r(λ) can be obtained by subtracting off the forbidden-subspace components of r(λ). The present paper displays for N = 3 the r′(λ) for several real reflectances, based on si and rk drawn from principal-component analyses. We compute r′(λ) using the Gram-Schmidt orthogonalization on the forbidden-subspace functions. Each r′(λ) is compared with the function derived from the corresponding r(λ) by subtracting off the forbidden subspace of von Kries color constancy.2 The comparison favors the volumetric theory.
Heuristic analysis of von Kries color constancy
Authors: Worthey James A. , Brill Michael H.
Published: February 2023

If the eye had three photopigments with narrow, nonoverlapping spectral sensitivities, integrated-reflectance (von Kries) adaptation could achieve perfect color constancy. However, human spectral sensitivities are broad and overlapping, which impairs constancy.1 A broad receptor sensitivity enables two lights to be metameric to that receptor system (i.e., two lights differing over the bandwidth of the receptor can produce the same response). Lights that are metameric to a trichromatic observer must be metameric to at least one receptor system. Hence the breadth of receptor sensitivities permits metamerism. When this takes the form of object-color metamerism, it will limit constancy by any mechanism. If the receptor sensitivities were broad but nonoverlapping, restricting the illuminant to three narrow bands2 in the respective spectral-sensitivity domains would eliminate object-color metamerism and would also permit von Kries color constancy. However, an overlap in human spectral sensitivities incurs off-diagonal elements in the (invertible) transformation between object reflectance and receptor stimulus. Hence, adjustment of a three-narrow band illuminant cannot be compensated by a von Kries (= diagonal) transformation. Since human red and green spectral sensitivities overlap greatly, the red-green opponent system shows mediocre color constancy even for three-band illuminants.1
Fluorescence studies of helium double excitation
Authors: Samson James A. R.
Published: February 2023

The importance of two-electron transitions in the photoionization of atoms and molecules is well established and the photoionization of helium provides the simplest example of electron correlations in atomic physics. The simultaneous photoexcitation and photoionization of He occurs at photon energies >65.4 eV and has been studied by photoabsorption, photoelectron, and fluorescence spectroscopy. Each method is important in establishing absolute partial cross sections for ionizing neutral helium to He+ in the ns and np levels (n ⩾ 2) and for the study of the various autoionizing transitions that take place. The technique of fluorescence spectroscopy has proved to be a particularly simple and sensitive method to study the double excitation process.1 Transitions into the 2p levels, either by direct photoionization or through autoionizing transitions, produce fluorescence radiation at 304 Å. Results obtained by this technique are discussed and compared with other methods.
Narrowband picosecond cascade-pumped dye laser
Authors: Hsu S. C. , Chiu P. H. , Kwok H. S.
Published: February 2023

Recently, the Roess-Lin scheme for the generation of picosecond dye laser pulses has been demonstrated in cascade.1 This cascade-pumped dye laser (CDL) was shown to produce dye laser pulses of 10-ps duration starting with 350-ps nitrogen laser pump pulses. The bandwidth was also measured to be ≳ 100 Å. For the CDL system to be more useful, the pulse duration stability has to be characterized and the bandwidth has to be narrowed. Because of the nature of the Roess-Lin scheme, intensity variations in the pump pulse translate into fluctuations in the duration of the output pulse. We have completely redesigned the CDL laser by replacing the first stage with a side-pumped dye laser. The thickness of the second stage was also decreased to produce a shorter cavity lifetime and wider mode spacing. By using a spectrometer to pass only one longitudinal mode, it was found that the bandwidth was 0.5 Å. Using a 2-ps streak camera to monitor 100 laser pulses, it was found that the average duration was 7.6 ps with a standard deviation of 0.7 ps. These near transform-limited ultrashort pulses can be amplified to high energy for further pulse compression or for performing nonlinear optics experiments.
Raman scattering or four-wave mixing
Authors: Kepros John G.
Published: February 2023

An attempt was made to prove the identity of the set of equations describing the Raman phenomenon with the set describing four-wave mixing. The result is a classic proof by contradiction: for the two phenomena to be the same, 1 would have to equal 3/2, 1 would have to equal 5/2. The Raman phenomenon is described by Hertzberg as the vibronic coupling of monochromatic light with the quantized vibrational states of a molecule, producing sidebands from integral quanta. This process is considered one of the initial proofs of quantum mechanics. Four-wave mixing is the optical application of a radar model: two signals of frequency f1 and f2 are transmitted through a radar amplifier withfour (or more) waves emitted, of the form f1, f2, jf1, + kf2, and jf1−kf2. When the Raman equations are equated with the CARS, HORSES, SCIS-SORS…equations2 where j = 1,2,3 … and f2 is greater than f1, the result is inconsistent with logic. Thus, one is led to the conclusion that the Raman effect and the collection of acronyms do not describe the same effect. It is recommended that the Optical Society of America take the editorial position that submissions which label their topic contrary to the proposed definitions be returned to the author for revision.
Nature of the putative π4 mechanism of W. S. Stiles
Authors: Kirk D. B.
Published: February 2023

The candidacy of π4 for the middle-wave cone fundamental was evaluated with the tests of shape invariance and field additivity for two observers. Field additivity was rejected: addend field sensitivities obtained in the presence of 649-nm fields displayed superadditivities near 460 nm when compared with the (π4) field sensitivities obtained without an auxiliary background. Similar findings were made when a brief duration (10-msec) test flash was used. Shape invariance was also rejected for one of the two observers for 461- and 649-nm fields. The failure of field additivity was further investigated with bichromatic mixture curves for 461- and 649-nm fields. Superadditivities were consistently found for mixture fields causing ≥ 0.5 log units threshold elevation. The possibility that these failures were due to intrusion of the shortwave mechanism (π1) was rejected. Analysis indicates that the failures of field additivity and shape invariance are due to intrusion of the longwave mechanism (π5).
Multimode instabilities in homogeneously broadened lasers
Authors: Koch Karl , Hillman Lloyd W. , Stroud Carlos R.
Published: February 2023

Hillman et al.1 reported a series of thresholds in which a cw dye laser with a high Q-cavity discontinuously changes the power and the spectrum of its output. These original observations are now extended to investigate the effect of cavity Q, detuning from line center, and other parameters. The regime of higher pump intensities is also investigated. A self-consistent solution to the Maxwell-Bloch equations is presented which describes the laser field in terms of an envelope propagating without change around the cavity. This theory predicts the observed spectral splitting as a function of output power.
Measurements of two-time correlations of the dye laser below threshold
Authors: Lett P. , Mandel L.
Published: February 2023

Time-varying fluctuations of the light intensity of a single-mode dye laser have been investigated experimentally by photoelectric correlation measurements. The experiments cover the region from the nominal laser threshold to about 1000 times below threshold, where pumping fluctuations have previously been found1 to result in a relatively sharp maximum of over 100 in the relative intensity fluctuations. The two-time correlation functions are found to be of crude exponential form in this region. However, there are indications that the initial slope at zero time delay is near zero, as predicted for colored noise pumping fluctuations.2 The correlation time of the laser light is found to be relatively insensitive to the working point over a wide range. The results are compared with computer solutions of the equations of motion with both delta-correlated spontaneous emission noise and Ornstein-Uhlenbeck pumping noise.3,1
Saturation effects in multiphoton ionization
Authors: Yeazell John A. , Malcuit Michelle S. , Stroud Carlos R. , Boyd Robert W.
Published: February 2023

We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Inferring material changes with double-opponent operators
Authors: Rubin John M.
Published: February 2023

An important goal of early vision is to determine which edges in the image correspond to boundaries between different materials. When there is a “spectral cross-point” at a luminance edge, it can be reliably inferred that, regardless of illuminant color, the edge is due to a material change.1 [If L and S denote two spectral samples of image intensity, there is a cross-point when (Lx − L
y
)(S
x
− S
y
) < 0, where subscripts x and y denote the two regions on either side of the edge.] There will almost never be a cross-point at a nonmaterial edge, such as a shadow, highlight, or surface orientation discontinuity. Another simple condition at an edge is opposite slope sign or (L
x
− S
x
)(L
y
− S
y
) < 0. When the illuminant is white (or equivalently, after the image has been spectrally normalized), the opposite slope sign condition also reliably indicates material changes2; this condition also does not arise at nonmaterial edges. The cross-point and opposite slope sign conditions are independent. Detecting these two conditions requires two distinct kinds of double-opponent operators. Biological double-opponent cells may detect material changes. It can be tested whether there are two varieties of such cells, one computing the cross-point condition, the other, the opposite slope sign.
Temporal response of the Franz-Keldysh effect
Authors: Sarid Dror , Gibbons Wayne M.
Published: February 2023

External perturbations such as temperature, stress, or electric fields, which can modulate the band gap of a semiconductor, play a major role in the study of energy band structures. In electroabsorption, in particular, the modulation of the band gap of a semiconductor by an externally applied electric field, referred to as the Franz-Keldysh effect, can be detected as a change in the sample's reflectivity. By measuring the relative change in the reflectivity due to a modulating external perturbation as a function of wavelength, one can study the band structure of semiconductors. The relative change in the reflectivity as a function of wavelength is given by the third derivative of the unperturbed dielectric constant.1 Yacoby2 discussed the case where the duration of the perturbing electric field is comparable to or shorter than the characteristic time of the system. Here we show, using the time gauge and the Fourier time transform of the time-dependent current operator, that if the duration of the perturbing electric field is shorter than the characteristic time of the system, the relative change in the reflectivity due to the Franz-Keldysh effect will be proportional to the first-order rather than the third-order derivative and will therefore yield results similar to those obtained in piezoreflectance, thermoreflectance, and wavelength modulation.1
Curious features of nonlinear pulse propagation in single-mode optical fibers
Authors: Tomlinson W. J. , HAUS H. A. , Stolen R. H.
Published: February 2023

Numerical simulations of the propagation of high-intensity pulses in single-mode optical fibers, and the subsequent compression of such pulses in a grating-pair compressor, have shown some curious features. For a wide range of parameters the pulses exhibit a phenomenon which we describe as optical wave breaking, in which the inner parts of a pulse overtake the pulse tails, resulting in a high-frequency modulation on the pulse, and the generation of new frequency components. Numerical simulations with sufficient resolution to model this effect predict some previously unexplained features in the spectrum of the pulses which have been used to achieve the highest compression yet reported.1 Simulations including the wave-breaking effect, and a linear fiber loss, quite accurately predict the results of those experiments, thus extending the range of validity of the nonlinear Schroedinger equation for describing nonlinear pulse propagation in single-mode fibers. Simulations including the effects of a finite response time for part of the nonlinear index of the fiber have demonstrated the existence of an instability, better known as stimulated Raman gain, which limits the usefulness of this technique for long input pulses.
Efficiency of computation in neural networks
Authors: Abu-Mostafa Yaser S.
Published: February 2023

A neural network with threshold logic and feedback1 is a plausible model for associative memory. It can be used directly to store and retrieve information with robustness and error-correction capability. It can also perform certain computations naturally, such as nearest-neighbor search. However, when a general computational problem is addressed, a special network is designed for the problem based on some energy function. The network is usually of large size, and the solution is approximate. By the efficiency of computation, we mean a quantitative measure of how cost-effective this approach is and how far it is from the theoretical limits. The efficiency of information storage in neural networks has been determined,2 but the computational aspect involves essentially different ideas. The assessment of the raw computational power of a network, the complexity of a problem, and the efficiency of embedding the problem in the network are the main issues.
Optical fiber pulse compression at 1.319 μm
Authors: Tai K. , Tomita A.
Published: February 2023

Optical fiber pulse compression1 using an external grating pair has been shown to be a practical technique to generate ultrashort optical pulses. Here we report pulse compression in the near-infrared region using a cw mode-locked Nd:YAG laser operating at 1.319 μm, which gives 125-ps FWHM pulses with the peak power of a few tens of watts coupled into the fiber. The difficulty due to the low power and long duration in the starting pulse can be overcome by the low loss in the fiber at the operating wavelength. We used a few kilometers long dispersion-shifted fiber to achieve enough self-phase modulation and dispersion. Pulses with twice initial width and few nanometers frequency chirping are typically seen at the output end of the fiber. Using a grating pair separated by 7 m in a double-pass configuration, 30× compression, i.e., 4 ps in final pulse, has been obtained. Numerical simulations using a fast-Fourier-transform technique are also performed. The results are in good agreement with the experimental observations.
Picosecond study of charge-carrier-induced nonlinear refraction in intermediate band-gap semiconductors
Authors: Guha Shekhar , Van Stryland Eric W. , Soileau M. J.
Published: February 2023

Free carriers generated by absorption of light in semiconductors change the complex dielectric constant and give rise to irradiance-dependent refraction and absorption. In addition to the refractive-index change arising from the low frequency dipole moment induced by the charge carriers (the Drude contribution), there can be contributions to the refractive-index change arising from the band-gap resonant saturation of the conduction band states. The amount of this additional contribution has been theoretically predicted.1 Verification of this prediction for two-photon absorption-generated charge carriers needs accurate determination of the two-photon absorption coefficient (β) of the semiconductors as well as careful measurement of the induced irradiance-dependent refractive index. We present a study of this nonlinear refraction; the method of measurement is external self-action. From the changes in the near-field shapes of spatially and temporally Gaussian picosecond laser beams incident on the semiconductor samples, measured as functions of the incident irradiance and pulse width, the total induced nonlinearity is estimated. Both 1.06- and 0.53-μm radiation are used to study two-photon absorption in a variety of semiconductors. This knowledge of the relative roles of different mechanisms contributing to the nonlinearity is not only of theoretical interest but also is important for practical applications in optical devices.
Architectures for optical implementation of 2-D content addressable memories
Authors: Farhat Nabil H. , Psaltis Demetri
Published: February 2023

Neural networks are known to be massively interconnected and to process information in parallel. Parallelism and massive interconnection capability are recognized strengths of optics. Increasing attention is therefore being given to the optical modeling of neural processing. Optoelectronic implementation and performance evaluation of a neural net consisting of thirty-two neurons based on the Hopfield model which is suitable for use with binary 1-D input vectors was reported recently.1 In this paper we discuss extension of the model to 2-D formats. The underlying principle and detail of two schemes for optical implementation of the required 4-D memory matrix (synaptic mask) are given. These are based on the use of either spatial multiplexing or spatial frequency multiplexing. Practical design considerations are covered.