A. K. Kar

Near-resonant third-order optical nonlinearities in p-toluene sulfonate polydiacetylene

We report single-wavelength subpicosecond and nanosecond excite-probe and four-wave mixing experiments near the exciton resonance in p-toluene sulfonate polydiacetylene. Contributions to the third-order susceptibility χ(3) resulting from phase-space filling, photoinduced absorption, and two-photon absorption are identified. A three-level rate-equation model that accounts for the change in sign of the nonlinear absorption observed at near-resonant frequencies is discussed.

Time resolved self-defocusing in InSb at room temperature

We report a detailed study of time resolved self-defocusing in InSb at room temperature, using 3 μs long CO 2 laser pulses. We have determined that the intensity dependence of the refractive index varies as intensity to the two-thirds power. The measured values correspond to an equivalent dn/dI of -0.11 cm2/MW at an incident intensity of 210 kW/cm2.

Spectrally resolved third-order nonlinearities in polydiacetylene microcrystals: influence of particle size

The on- and near-resonant third-order nonlinear optical response of polydiacetylene microcrystals in suspension was studied by the Z-scan technique with tunable picosecond pulses. The absorptive and refractive contributions to the excitonic-dominated nonlinearity were fully resolved at several wavelengths. Microcrystals of two different average sizes were studied, and a comparison of their nonlinear coefficients was made. A three-level model for the nonlinear response is presented that shows good agreement with the experimental data.

Quasi-CW optical bistability in InSb at room temperature

We report the first observation of quasi-steady-state optical bistability in an InSb etalon at room temperature as a result of the generation of free carriers through two photon absorption of 10 μm radiation. Nonlinear tuning of the etalon through two free spectral ranges has been achieved, with bistability observed around the two cavity resonances. Switching from a low to high transmission state has been achieved in times less than 10 ns while switching in the opposite direction occurs within 50-100 ns. The experimental results have been successfully fitted to a numerical model of the device.

Photogenerated carrier recombination time in bulk ZnSe

We have measured the free‐carrier recombination time in polycrystalline bulk ZnSe to be in the range 7–14 ns at room temperature. We used an excite and probe technique with near‐band‐edge nanosecond laser pulses to measure the decay of induced free‐carrier absorption. We estimate the free‐carrier‐pair absorption cross section to be in the range (3–6)×10−17 cm2.

Near-resonant refractive nonlinearity in polydiacetylene 9-BCMU thin films

The spectral dependence of the third-order nonlinear susceptibility χ(3) (ω; ω, −ω, ω) of the polydiacetylene 9-BCMU has been measured, and the imaginary component is found to change sign in the exciton absorption tail. In the red-phase material, a significant refractive nonlinearity was observed around the 1.93-eV input energy range, whereas the absorption change was too small to quantify. A magnitude for the nonlinear refractive index coefficient |n2| = 1.9 (±1.0) × 10−10 cm2 W−1 was measured at this Im χ(3) = 0 position, indicating good figures of merit for all-optical switching.

Photon-recycling and optically driven plasma-expansion techniques applied to lifetime experiments on molecular-beam-epitaxy ZnSe

Transient-grating techniques are used to investigate the picosecond dynamics of photoexcited carriers in a range of n- and p-doped ZnSe thin-film layers grown by molecular-beam epitaxy. Two-photon absorption is employed experimentally to generate known excitation levels, comparable with those under laser action in II–VI diode structures. The analysis of the measured grating decays, incorporating photon recycling, reveals carrier lifetimes of the order of 300 ps and radiative recombination coefficients of the order of 10−8 cm3 s−1. The influence of photon-assisted plasma expansion at excitation levels above the threshold for stimulated emission is modeled and quantified.

ELECTRONIC SELF-DEFOCUSING OF NANOSECOND LASER PULSES IN BULK ZNSE

A large refractive nonlinearity (−2×10−20 cm3) has been observed in polycrystalline ZnSe across a range of wavelengths just below the band edge, using nanosecond laser pulses. The transmitted spatial profiles are measured, and theoretical fits give the spectral response of the nonlinear refractive cross section at low irradiances. A comparison with band filling theory indicates that the dynamic Burstein–Moss shift is the dominant mechanism responsible for this refractive nonlinearity.

Theory of elliptical defocusing in anisotropic nonlinear-optical media

We have determined the far-field beam profiles of an initially Gaussian laser beam that traverses a thin slab of nonlinear-optical material in which the spatial distribution of the refractive nonlinearity is anisotropic by an expansion in terms of elliptical Gaussian functions. In marked contrast to the annular structures observed with isotropic media, it is shown that the transmitted beam breaks into two beams in the far field. This nonlinear beam-splitting behavior has been confirmed experimentally.

Polarization-state beating in four-wave-mixing experiments on ZnSe epilayers

The polarization angle of the time-integrated four-wave-mixing signal from a ZnSe epilayer is found to oscillate as a function of the delay time between the two incident pulses, the modulation depth being dependent on the polarization angle between the incident pulses. The oscillation is shown to be a feature of heavy-hole–light-hole quantum beating, and is described by a solution of the optical Bloch equations.