Free Carrier Nonlinearities Research Articles

Hot-carrier-transport-governed nonresonant optical nonlinearity: transient Gunn-domain grating

We review recent investigations of nonlinear carrier transport and related optical nonlinearities in GaAs and InP crystals for the case in which photoexcited carriers are nonuniformly heated by a strong external electric field and are redistributed in a spatially nonuniform internal electric field. We simulated the nonequilibrium carrier and internal field spatial evolution in picosecond and nanosecond time domains and estimated criteria for bipolar high-field Gunn-domain grating formation in dc and ac fields. The coexisting refractive-index modulation mechanisms for free-carrier and electro-optic nonlinearities are analyzed for various external field strengths, grating periods, and excitation levels, thus providing conditions for an efficient and fast electro-optic refractive-index modulation by the transient Gunn-domain grating. Comparison of the experimentally observed enhancement of light’s self-diffraction efficiency with numerical calculations has confirmed that the nonresonant electro-optic Gunn-domain-based nonlinearity may exceed the free-carrier nonlinearity.

Determination of optical nonlinearities and carrier lifetime in ZnO

Bound-electronic and free-carrier optical nonlinearities and relaxation of two-photon-excited free carriers in ZnO have been investigated by use of a single-beam Z-scan technique at 532 nm. Under pulsed radiation of 25-ps duration, the two-photon absorption coefficient, the bound-electron nonlinear refractive index, and the change in the refractive index due to the two-photon generation of an electron–hole pair are determined to be 4.2±0.9 cm/GW,-(0.9±0.3)×10-14 cm2/W, and -(1.1±0.3)×10-21 cm3, respectively. With these values in the Z scans conducted with 7-ns laser pulses, the carrier recombination time and the free-carrier absorption cross section are extracted as 2.8±0.6 ns and (6.5±0.9)×10-18 cm2, respectively.

Measurement of free-carrier nonlinearities in ZnSe based on the Z-scan technique with a nanosecond laser

Self-defocusing and nonlinear absorption resulting from two-photon-excited free charge carriers were observed in polycrystalline ZnSe at the low power density of ~ 30 MW/cm(2) by use of the Z-scan technique with nanosecond laser pulses. The total carrier absorption cross section and the variation of refractive index per unit of photoexcited carrier density were estimated to be 0.80 +/- 0.10 x 10(-18) cm(2) and 0.60 +/- 0.15 x 10(-21) cm(3), respectively.

Low Threshold ZnSe1−x Tex Optical Limiters

AbstractWe report an experimental investigation of optical limiting with bulk ZnSe1−xTex crystals at wavelengths just below the band edge, using nanosecond laser pulses. The limiting threshold as low as a few mJ/cm2 has been demonstrated. The generation of free charge carriers by singlephoton absorption, and subsequent free-carrier nonlinearities (nonlinear absorption and selfdefocusing) are responsible for the limiting behavior.

Determination of bound-electronic and free-carrier nonlinearities in ZnSe, GaAs, CdTe, and ZnTe

We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 μm and in ZnSe at 1.06 and 0.53 μm, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient σr,), the two-photon absorption coefficient β, and the bound electronic nonlinear refractive index n2. The real and imaginary parts of the third-order susceptibility (i.e., n2 and β, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n2 changes from positive to negative as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n2 in solids based on Kramers–Kronig transformations [ Phys. Rev. Lett.65, 96 ( 1990); IEEE J. Quantum Electron.27, 1296 ( 1991)]. We find that the values of σr are in agreement with simple band-filling models.

Theory of high gain transient energy transfer in GaAs and Si

Rate equations for carrier number densities and wave equations for optical fields are solved numerically to investigate transient energy transfer (TET) from a strong pump beam to a weak probe via the free-carrier nonlinearity in GaAs and Si. The calculations of TET include arbitrary ratio of pulse length to diffusion time, pump depletion, free-carrier absorption, and two-photon absorption. The calculations provide guidance on the optimal choice of sample thickness, free-carrier cross section, and two-photon absorption coefficient for obtaining high TET gains in semiconductors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarization-rotation switch using picosecond pulses in GaAs

Photorefractive and free-carrier nonlinearities in GaAs transfer energy from a strong picosecond pump, which is linearly polarized at an arbitrary angle to a weak s-polarized probe, into a p-polarized beam propagating in the probe direction. By placing the GaAs between a crossed pair of high-quality polarizers that are set to extinguish the probe in the absence of the pump, an efficient, high-contrast, high-speed optical switch is realized. By varying the pumpprobe ratio, delay, fluence, and polarization, we can maximize the intensity of the p-polarized component. This intensity with the pump present divided by that without the pump, the on-off ratio, is approximately 2 at fluences as low as 0.03 mJ/cm(2) and approaches 10,000 at fluences of 15 mJ/cm(2) and pump polarizations of 45 degrees .

Free-Carrier Optical Nonlinearity Due to Carrier Scattering and Nonparabolicity

Nonlinear mixing of electromagnetic waves in low-temperature degenerate semiconductors has been analytically investigated. Usual kinetic-theory techniques have been employed for evaluating the mixed-frequency components in the current density. Two types of free-carrier nonlinearity are considered, one arising from carrier scattering processes and the other due to nonparabolicity of conduction band. Numerical results, comparing the two nonlinearities for the special case of mixing of two C${\mathrm{O}}_{2}$ laser beams in indium antimonide, have been presented at the end.