High Input Intensity Research Articles

Weighted distributed feedback structure for all-optical bistable devices

We proposed to use a distributed feedback structure with a weighted coupling coefficient (W-DPB) in an all-optical bistable device in order to improve on-state transmittance and to reduce threshold intensity. Through numerical calculations, characteristics of the W-DPB structure were discussed compared with those of a conventional DPB structure. The W-DPB optical bistable device exhibits high on-state transmittance and low-threshold input intensity. Fabrication of the W-DPB structure with a periodicity of 169.8 nm is demonstrated on a CdS(x)Se(1-x)-doped glass substrate by means of holographic exposure with a double-coated photoresist and reactive ion etching in CHP(3).

Two-photon absorption in semiconductor-doped glasses

We report the intensity-dependent transmission of a 1054-nm laser in four Schott color glass filters: OG550, OG590, RG665, and RG850. The linear absorption is negligible, and the main mechanism of energy loss is by two-photon absorption. At higher input intensities, additional absorption due to laser-generated carriers was observed. This was confirmed by measuring the absorption of a probe beam delayed by 250 ps relative to a strong pump beam. The electron-hole pair absorption cross sections thus obtained were found to explain quite well the overall intensity-dependent transmission data. The density of laser-generated carriers is much larger in our experiments than in most of the earlier reported one-photon resonant measurements. Even at 1054 nm these glasses suffer accumulative photodarkening at higher fluences. In the photodarkened sample, the value of the two-photon absorption coefficient is approximately half that of the fresh sample.

Water flow characterization and test of a kinematic‐wave model for macropore flow in a highly contrasted and irregular double‐porosi medium

SUMMARYLaboratory experiments to characterize water flow through a column consisting of an artificial double‐porosity medium are described. A gamma‐attenuation device was used to measure water storage in the intra‐ and inter‐aggregate porosity domains after successive infiltrations. Several days after macropore flow had ceased, a significant proportion of the water that had infiltrated was retained within the aggregates.After the column had been brought to its water retention capacity, several rains with different intensities were applied, and the drainage hydrographs were recorded and analysed in order to characterize water transport in the coarser pores and to test the kinematic‐wave approach to macropore flow proposed by Germann (1985). Reasonable fits of calculated hydrographs to experimental data were obtained for high input intensities, but the model was unable to reproduce the hydrographs recorded at low intensities. In particular, the model could not simulate the high dispersion of the input signal observed at low rain intensities.

Theory of a two-cavity gyrotron frequency multiplier.

A frequency multiplying device is analyzed that provides power at selected cyclotron harmonics in a quasioptical gyrotron. The device utilizes a prebunching cavity and an uptapered magnetic field that impose a double-resonance condition on the radiation generated. This selects among cyclotron harmonics in the output cavity even though the cavity structure may support all harmonics. The prebunching is accomplished by modulation of electron orbital magnetic moments by a low-frequency-input rf field. Two design examples are given for quasioptical gyrotron frequency multiplier operation at the third cyclotron harmonic (four or seven times the input frequency) with a 10--15 % electronic efficiency and greater than 33% overall rf conversion efficiency. The drawback of this method of prebunching is that high-input electric-field intensities are required for efficient operation. However, it is shown that the double-resonance method allows complete harmonic selection in the linear regime for zero detuning between the cyclotron and field frequency. When this detuning is nonzero, the selected harmonic still has a higher linear efficiency than all others.

Nonlinear phenomenon of interfacial polarization immittance of a Pt electrode

Description of a computerized, automated method to measure the interfacial polarization immittance of a Pt electrode in nonlinear range is presented. The classical three-electrode setup is used for measurements in conjunction with a special purpose software implemented on a Unix computer using C language. A collection of data at very low frequencies (below 2 Hz) and at high input intensities with various dc biases imposed on the input are presented to show the behavior of the interface in nonlinear range. The instrument also provides on-line harmonic analysis of the output signal, by calculating the first four Fourier series coefficients, in response to a pure sinusoidal input.

STIMULATED RAMAN SCATTERING OF AQUEOUS DROPLETS CONTAINING IONS: CONCENTRATION AND SIZE DETERMINATION

ABSTRACT The dependence of the detected Raman scattering intensity as the input laser intensity is increased can be separated into four regions: (1) spontaneous Raman scattering at low input intensity; (2) stimulated Raman scattering (SRS) with an exponential growth; (3) saturated SRS; and (4) laser-induced breakdown (LIB) within the droplet at high input intensity. Within the exponential growth region, the relative SRS intensity can be related to the relative concentration of the ions. At high nitrate and sulfate ion concentrations, the series of SRS peaks centered around the v1 vibrational mode of the ions provide information on the droplet size.

Comparison between theory and experiment in self-pumped Brillouin-enhanced four-wave mixing

We report on a theoretical and modeling study of self-pumped four-wave mixing in a Brillouin medium. A theoretical analysis gives expressions for the threshold intensity and for the steady-state reflectivity at small input intensities. Operation over a range of input intensities is also investigated by using a computer model that predicts that, for high input intensities, the conjugate can become unstable and oscillate chaotically in time. Experimentally we observe phase conjugation with an input threshold 25 times less than that required for phase conjugation by stimulated Brillouin scattering.

Optical bistability in bulk GeSe 2

We report the observation of intrinsic dynamic optical bistability in bulk GeSe 2 semiconductor glass. The transmitted intensity is studied as a function of the incident intensity at 0.6328 μm. At high input intensities a nonlinear dependence of the transmitted signal is observed. Moreover the experimental results suggest non linear and self diffraction effects in the material.

Picosecond all-optical switching in single-mode GaAs/AlGaAs strip-loaded nonlinear directional couplers

Room-temperature all-optical switching has been achieved in single-mode strip-loaded nonlinear directional couplers with a GaAs/AlGaAs multiple quantum well guiding layer. For low input intensities nearly complete (1:3–1:5) cross coupling occurs, while at high input intensities switching occurs so that most (>3:1) of the light stays in the input guide. The effects of carrier diffusion are minimized by the use of picosecond pulses. The response time and the recovery time of the nonlinear directional couplers are measured with pump-probe experiments, and the origin of the nonlinearity is attributed to fast electronic effects.

Propagation velocity of laser-induced plasma inside and outside a transparent droplet

The supersonic propagation velocity of the emission front of plasma produced by laser-induced breakdown of a micrometer-sized transparent droplet flowing in a gas was measured with a streak camera at three intensity levels. At low input intensity, the plasma velocities in the gas away from and toward the shadow face were determined. At medium input intensity, the plasma velocities in the gas outside the shadow face and within the liquid (traveling toward the illuminated face) were measured. At high input intensity, the plasma velocities in the gas outside the shadow face, within the liquid, and in the gas outside the illuminated face were deduced.

Temporal transfer and nonlinearity properties of turtle erg: Tuning by temperature, pharmacology, and light intensity

ERG impulse response, amplitude and phase temporal spectral transfer functions, and nonlinearities were measured in turtle retina under different retinal temperatures, pharmacological treatments, and light intensities.b-Wave amplitude is strongly temperature dependent; amplitudes of thea-wave and slow P-III are less sensitive. Their time courses all slow markedly as temperature decreases. ERG amplitude transfer function is bimodal bandpass with narrow low-frequency peak (below 1 Hz) and broader mid-frequency peak (5 Hz at 8°C). Both peaks broaden and their frequency increases (low, 1 Hz; mid, 15 Hz, at 23°C) as temperature increases. Phase transfer function slope decreases (from −70°/Hz at 8°C to −25°/Hz at 23°C) as temperature increases. Nonlinear properties of ERG at high input intensity are modelled by a quadratic nonlinearity, low-pass prefilter with cutoff above 12.4 Hz, and low-pass postfilter broadly peaked at 6–10 Hz with cutoff above 20 Hz. For low input intensity, ERG exhibits linear properties with low-pass filtering sharply cut off above 6 Hz.a-Wave and slow P-III were isolated by aspartate treatment; depolarizing bipolar cell activity was examined using ethanol/GABA treatment of retina. High-frequency components, including broad mid-frequency peak, were attenuated and low-frequency components were enhanced with aspartate. Transfer function narrows and peaks at a lower frequency with ethanol/GABA.

Amplitude Self-modulation of Intracavity Second-harmonic Generation

We consider second-harmonic generation by a non-linear crystal placed in an empty resonant cavity and pumped by an external coherent field. It was recently predicted that the stationary state loses its stability through a Hopf bifurcation when the input intensity reaches a critical value. A new amplitude self-modulated state then arises. We construct explicitly this new time-periodic solution and prove its stability in the vicinity of the transition point. As the input intensity is further increased, numerical investigations indicate a new transition to another time-periodic solution; this transition involves an hysteresis cycle. The two periodic solutions differ by the number of maxima in one period: at low input intensity, there is one maximum per period, whereas at high input intensity there are two.

Near resonant third harmonic generation

Third harmonic generation has been investigated theoretically in the vicinity of a two-photon resonance. Optimum achievable conversion efficiencies are given for a fixed column density as well as for an optimized column density for input intensities ranging from 10 6–10 10 W/cm 2. It is shown that the resonant enhancement in the small signal limit is turned into a minimum of the conversion efficiency at high input intensities. Under optimum conditions the minimum is partially filled up. This is due to the varying AC-Stark shift of the two-photon resonant transition over the length of the nonlinear medium.

Electrical and gain characteristics of a simple compact XeCl laser

The electrical and gain characteristics of a uv preionized XeCl rare gas halide laser have been investigated over a laser pressure range of 1 to 6 atmospheres. Values for the small-signal gain as high as 30% per cm have been observed while measurements at high input intensity yielded an approximate value of 250 kW/cm2 for the optical intensity saturation parameter, Is.

Erratum: Multiphoton ionization of Na atoms in third-harmonic generation by a mode-locked Nd: YAG laser

Effects of multiphoton ionization on third-harmonic generation in phase-matched Na and Xe mixtures have been investigated using a mode-locked Nd: YAG laser with a pulse duration of 28 psec and peak-power densities up to 3 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ W/${\mathrm{cm}}^{2}$. Analytical expressions accounting for refractive-index changes due to the depletion of neutral Na atoms and the plasma formation are presented which are used to estimate the wave-vector mismatch created by the multiphoton ionization of Na. The multiphoton-ionization probability $W$ for Na has been obtained from measurement of the number of ions formed by the focused laser beam: $W=6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}49}{I}_{0}^{5}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ at $\ensuremath{\lambda}=1.064$ \ensuremath{\mu}m, where ${I}_{0}$ is the field intensity in W/${\mathrm{cm}}^{2}$. A time-dependent calculation using the value of $W$ obtained shows that multiphoton ionization of Na atoms brings about advancing and shortening of the third-harmonic pulse at high input intensities. The experimental results for the third-harmonic generation are in good agreement with calculated results which incorporated the effects of ionization. A comparison of the ionization effects with the quadratic Kerr effect is also presented.

Multiphoton ionization of Na atoms in third-harmonic generation by a mode-locked Nd: YAG laser

Effects of multiphoton ionization on third-harmonic generation in phase-matched Na and Xe mixtures have been investigated using a mode-locked Nd: YAG laser with a pulse duration of 28 psec and peak-power densities up to 3 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ W/${\mathrm{cm}}^{2}$. Analytical expressions accounting for refractive-index changes due to the depletion of neutral Na atoms and the plasma formation are presented which are used to estimate the wave-vector mismatch created by the multiphoton ionization of Na. The multiphoton-ionization probability $W$ for Na has been obtained from measurement of the number of ions formed by the focused laser beam: $W=6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}49}{I}_{0}^{5}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ at $\ensuremath{\lambda}=1.064$ \ensuremath{\mu}m, where ${I}_{0}$ is the field intensity in W/${\mathrm{cm}}^{2}$. A time-dependent calculation using the value of $W$ obtained shows that multiphoton ionization of Na atoms brings about advancing and shortening of the third-harmonic pulse at high input intensities. The experimental results for the third-harmonic generation are in good agreement with calculated results which incorporated the effects of ionization. A comparison of the ionization effects with the quadratic Kerr effect is also presented.

Third-harmonic generation of mode-locked Nd:glass laser pulses in phase-matched Rb-Xe mixtures

Third-harmonic generation in phase-matched Rb-Xe mixtures has been produced using 7- and 300-psec pulses of a mode-locked Nd:glass laser with peak powers up to 200 MW. Maximum energy conversion efficiencies of 2.8% have been achieved. A theoretical analysis is given which includes the transient excitations of the system and the linear and nonlinear susceptibilities up to seventh order in the perturbation. Agreement between theory and experiment has been obtained up to the highest input intensities demonstrating the importance of the transient excitation due to the adiabatic following. The phase-matching condition for parallel and focused beams has been investigated showing the influence of density gradients at the boundaries of the nonlinear medium (Rb vapor) and of the field-dependent changes of the refractive index at high input intensities. The limiting processes of self-focusing and multiphoton ionization are discussed.

On the development of feature detectors in the visual cortex with applications to learning and reaction-diffusion systems

Developmental mechanisms for tuning of visual cortex are derived from adult learning mechanisms: an adaptational property of shunting on-center off-surround networks that prevents saturation of parallel processed patterns at high input intensities, a contrast enhancement and short-term memory mechanism, and plastic synaptic strengths that compute a time average of presynaptic signals and postsynaptic activities and multiplicatively gate signals. The mechanisms can generate fields of feature detectors; e.g., line or picture detectors. A developing hierarchy of such fields can be synthesized in which successive critical developmental periods are triggered as a dynamic equilibrium is established between shortterm memory and long-term memory at each stage. Shunting adaptation can account for some data on spatial frequency adaptation. Shunting network properties resemble properties of certain reaction-diffusion systems that have been used to model developmental data in various species; e.g., Hydra, Xenopus retina, slime molds. For example, positional information due to regulation in reaction-diffusion systems is analogous to constancies due to network adaptation, firing of a developmental gradient is analogous to contrast enhancement, and maintenance of a pattern of morphogens is analogous to short-term memory.

Superradiant Effects on Pulse Propagation in Resonant Media

A theoretical model for the propagation of coherent light pulses in resonant media where a large number of atoms cooperate in spontaneous emission (superradiance) is discussed. The medium is supposedly free of inhomogeneous broadening, and no lossy relaxation mechanisms are taken into account. The conditions for the emission of all spontaneous radiation in the direction of pulse propagation are described. A semi-classical model is used and, at each point in the medium, the propagation field and the spontaneous damping field are considered separately. The equation governing the evolution of a superradiant system in the presence of an applied pulse is derived. A mechanism of energy transfer from the traveling pulse to the medium is described which includes superradiant spontaneous emission effects. As a result of the neglect of all lossy relaxation processes, all energy stored in the medium is eventually returned to the pulse by coherent spontaneous emission. Therefore, in this theory, all pulses travel through the medium without energy loss. Numerical results for square input pulses are discussed which show two possible types of propagation behavior: (a) Low input intensities produce a negative tail to the input (positive) pulse and sharpening of the tail with the advance in the medium; (b) higher input intensities produce, after sufficient penetration, a steady-state pulse envelope somewhat similar to the $2\ensuremath{\pi}$ pulses of self-induced transparency. The steady-state solutions of the equations of motion are found numerically and the pulse shapes are computed for different degrees of superradiance. In the limit of strong superradiance, very asymmetric pulse envelopes of large area are found. For negligible superradiance, approximately $2\ensuremath{\pi}$ hyperbolic-secant pulses are obtained. The effects of inhomogeneous broadening are briefly considered, and the consequences of important superradiant damping in photon-echo experiments are qualitatively discussed.