We present a theoretical analysis of light pulse delay in resonant photonic bandgap structures made from Bragg-spaced semiconductor quantum wells. Quantum well Bragg structures offer the possibility for parametric manipulation of the polariton band structure. This, in turn, may be used for stopping, storing, and releasing of light pulses. Based on a theoretical model utilizing a time-dependent transfer matrix approach to the solution of Maxwell's equations, we study light pulse propagation, light pulse trapping and releasing, and light pulse deformation in these structures. We discuss the photonic band structure concepts relevant to our light delay scheme and present numerical simulation results of pulse delay through presently existing quantum well Bragg structures.
Quantum interference of single and two photon absorption pathways connecting valence and conduction band states in a semiconductor allow one to generate spin currents with or without charge currents. The underlying principles for these generation processes are outlines. We offer experimental demonstration of pure spin currents in GaAs using two color beams configured collinearly to produce spatially homogeneous currents, or non-collinearly to produce spin current gratings.
We have investigated the nonlinear optical mechanisms responsible for optical limiting of both picosecond and nanosecond 532-nm optical pulses in the organometallic compound cyclopentadienyliron carbonyl tetramer (King's complex). For fluences below ~200 mJ/cm2, picosecond pump-probe measurements in solutions of the King's complex reveal a prompt reverse saturable absorption (RSA) that recovers with a time constant of 120 ps. We attribute this RSA to excited-state absorption within the singlet system of the King's complex, and we demonstrate that the RSA is completely characterized by a simple three-level model. We find, however, that the material parameters extracted from these picosecond measurements cannot account for the strong optical limiting previously observed in identical solutions of this compound using nanosecond excitation at higher fluences. Picosecond measurements at fluences greater than 200 mJ/cm2 reveal the onset of an additional loss mechanism that appears ~1 ns after excitation. The magnitude of this loss depends on both the laser repetition rate and the solvent, indicating that the loss is not directly related to the intrinsic properties of the King's complex but is most likely thermal in origin. Using nanosecond excitation pulses, we have performed angularly resolved transmission and reflection measurements, which reveal strong forward- and backward-induced scattering at these fluences. Furthermore, when the King's complex is incorporated in a solid host, we observe negligible induced scatter and the response is completely described by the singlet parameters extracted from the picosecond measurements. These observations indicate that the nanosecond optical limiter response of solutions of King's complex is dominated by thermally induced scattering.
We demonstrate a proof-of-principle, GaP optical energy limiter for 532 nm, 25 ps, pulsed radiation that exhibits an output limiting level of less than 1 (mu) j/cm2. Optical limiting at this level is significant for use in systems designed to protect the human eye from laser radiation damage. The device employs a standard configuration that is realized by placing the GaP at the intermediate focal plane of a 1-to-1 inverting telescope, which is followed by an aperture set to clip the linearly transmitted beam at the 1/e point of the irradiance profile. The linear (indirect) absorption in GaP at this wavelength results in the optical generation of very large densities of free carriers. We therefore anticipate the performance of this device to be highly dependent on free carrier nonlinearities. This is confirmed by performing both two beam and single beam measurements on the material itself in order to determine both the absorptive and refractive parameters at this wavelength. We find that the free carrier absorption cross section is is congruent to 2 X 10-18 cm2, and the change in index per photogenerated electron hole pair is is congruent to -2.4 X 10-22 cm3.
We have measured the photodynamics of reverse saturable absorption (RSA) in solutions of cyclopentadienyliron carbonyl tetramer (King's complex) using picosecond pump-probe techniques. Similar preliminary measurements in solutions of synthesized variations of the King's complex indicate that the excited state transition responsible for the observed RSA is most likely a second d-d transition within the metal core of the molecule. On time scales of hundreds of picoseconds, the observed RSA in the King's complex is well characterized by a three-level rate-equation, singlet-state absorption model, where the excited-state cross section is greater than that of the ground state. On nanosecond timescales and at fluences above 200 mJ.cm-2, however, we observe the onset of a response that is consistent with a thermally induced scattering process. Further evidence of this scattering is provided by angularly-resolved measurements of the transmitted and back-scattered signals for nanosecond excitation. When the King's complex is incorporated in a solid host negligible scatter was observed and the response is completely described by the singlet parameters extracted from the picosecond measurements. The observation of, scatter from solution, together with a time- resolved decay to the ground state that is rapid (approximately 120 ps) and largely nonradiative in this molecule, indicate that solutions of King's complex may provide a mechanism for efficiently generating thermal nonlinearities on a subnanosecond timescale.
In this paper, we discuss recent results on the propagation of dark spatial solitons (DSS). Dark spatial solitons are particular solutions of the nonlinear Schroedinger (NLS) equation modeling propagation of light beams in optical Kerr media. Experimental results are presented for three systems, including sodium vapor, various thermally nonlinear liquids, and the bulk semiconductor system ZnSe. The results of these investigations indicate that experimental dark spatial solitons obey the conservation laws of the NLS equation, possess collision properties characteristic of the theoretical DSS solutions, and are stable to external perturbations induced by the experiment. In addition, through an interferometric technique, we investigate the phase profile of the dark spatial solitons and show that it is in good agreement with the NLS solution. In addition to the fundamental DSS, we have performed experiments where nonfundamental DSS are excited in pairs by making use of an even initial field profile as originally discussed by Zakharov and Shabat. The transverse velocities of the solitons excited in the is manner are measured and found to be in good agreement with those predicted theoretically.
We report our investigations of single- and multiple-beam optical limiter configurations using GaAs and Si as the nonlinear optical materials. Three distinct multiple-beam geometries are discussed. One of these, in which two beams interfere within the semiconductor to produce a grating, takes advantage oftransient energy transfer and photorefractive beam coupling to deplete the signal beam. The other two configurations exploit the whole-beam absorptive and refractive index changes induced in the semiconductor by a strong control beam that arrives at the sample before the signal. For one of the latter two configurations, nonlinear absorption and induced defocusing are used to attenuate the signal; in the other, nonlinear absorption and induced deflection are used. We discuss the relative merits of each configuration and compare them to singlebeam results obtained under identical experimental conditions.
We report our investigations of multiple-beam optical limiter configurations using GaAs and Si as the
nonlinear optical material. Three distinct multiple-beam geometries are discussed. One of these, in which
two beams interfere within the semiconductor, takes advantage of transient energy transfer and
photorefractive beam coupling to deplete the signal beam. The other two configurations exploit the
absorptive and refractive index changes induced in the semiconductor by a strong control beam that
arrives at the sample before the signal. For one of these configurations, nonlinear absorption and induced
defocusing are used to attenuate the signal in the other, nonlinear absorption and induced deflection are
used. We discuss the relative merits of each configuration and compare them to single beam results
obtained under identical experimental conditions.
We describe picosecond single- and multiple-beam measurements of the nonlinear absorption and
refraction in a variety of semiconductors. Single-beam and pump-probe transmission measurements are
used to isolate instantaneous nonlinearities from cumulative processes. These techniques, together with a
simple rate equation model, have allowed us to extract information regarding mid-gap levels and to
measure both the two-photon absorption coefficients and the free carrier absorption cross sections in these
samples. Our model, together with Z-scan and beam deflection measurements of the nonlinear refraction,
has provided the change in index due to each photogenerated electron-hole pair.
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