Optical Nonlinearities In Semiconductors Research Articles

Semiconductor nonlinearities for ultrafast all-opticalgating

Various optical nonlinearities in semiconductors areinvestigated in reference to ultrafast all-optical gating. It isshown that a state-filling-type nonlinearity, such as theband-filling effect (BFE) in a semiconductor, is suitable formany ultrafast gating applications: that is, in conjunction witha recently developed differential phase-modulation (DPM)technique, ultrafast and clean gating with a considerably loweroptical excitation than in conventional techniques withferroelectric materials, for example, is possible. Two methods touse the BFE in the DPM regime were experimentally evaluated. Oneis to optically pump a semiconductor in order to generatephotocarriers, and the other is to use a semiconductor opticalamplifier (SOA) to enhance the former BFE by means of stimulatedemission. With optical pumping, a gating speed faster than afew hundred femtoseconds is possible, but a pulse energy of afew picojoules is required. This pulse energy can besignificantly reduced with the use of an SOA, but in this case the gating speed is also reduced. The gating characteristics,including gating speed, gating energy, wavelength, noise, andlinearity in signal intensity, were compared between the twomethods for various applications.

Resonant optical nonlinearities in semiconductors

Semiconductors provide some of the most promising materials for nonlinear optics, because of large resonant nonlinearities, control of recombination time (from milliseconds to femtoseconds), well-developed fabrication technologies, and compatibility with other optoelectronic devices. The paper reviews some of the concepts and results that have come from the study of nonlinear optics in semiconductors. The emphasis is on nonlinear absorption and the refractive index that arises from the photo-induced excitation of free carriers. Mechanisms described include state-filling, carrier transport, and photorefractivity. Devices include optical bistability, reflective asymmetric Fabry-Perot all-optical absorptive switches, optically addressed spatial light modulators, and real-time holography. The paper's approach is to provide a basic engineering understanding of the principles, some of the historical details, and a snapshot of the state of the field today.

Three-Level Description of Optical Nonlinearities in Semiconductors

ABSTRACT A description of the second–order nonlinear optical susceptibility of semiconductor materials is considered. The Lie algebra su(3) arises from a 3–level quantum model for each state in the Brillouin zone, which is obtained by truncation of the true infinite–dimensional basis. The accuracy of several approximations inherent in this approach is analysed, in particular the electric dipole and minimal replacement Hamiltonians which specify the electrodynainical interaction in a finite–dimensional quantum system. The three–level model is extended by adding relaxation processes and utilising the rotating wave approximation to account for resonant nonlinear effects. Analytical forms for the transition rates associated with two–photon absorption and electromagnetically induced transparency are derived.

Chirp-controlled ultrafast optical nonlinearities in semiconductors

We experimentally demonstrate that the differential transmission (DT) response of bulk semiconductors excited well above the band edge can be manipulated by chirping of the broadband excitation and readout pulses. In particular, the maximum transmission change in spectrally integrated DT experiments can be modified on the 20 fs time scale. Spectrally resolved DT studies explain this chirp dependence. Depending on the sign of the chirp, positive or negative DT contributions at low or high photon energies are probed with varying efficiency around zero time delay. These results demonstrate that chirp can become an additional degree of freedom for the optimization of device performance in ultrafast all-optical switching.

Effective Frenkel Hamiltonian for optical nonlinearities in semiconductors: Application to magnetoexcitons

Closed Green-function expressions for the third-order response of semiconductors are derived by mapping the two-band model onto the much simpler molecular (Frenkel) Hamiltonian. The signatures of two-exciton resonances are incorporated through the exciton scattering matrix, totally avoiding the explicit calculation of two-exciton states. Exact expressions for the nonlinear optical response of two-dimensional semiconductor nanostructures in a strong perpendicular magnetic field are derived by truncating at the $n=1$ Landau level, and using the symmetry of the system and a group-theoretical analysis. We find that the nonlinear optical response depends crucially on asymmetries between particle-particle and particle-hole Coulomb interactions.

Fastscan z-scan system for determining optical non-linearities in semiconductors

Traditional z-scan techniques have been used to determine the non-linear optical coefficients of semiconductors. This is usually carried out by scanning a sample along the optical z-axis using a manual or computer-controlled linear translation stage. We report upon a novel z-scan method involving a fastscanning sampleholder: This gives real-time z-scan traces and greatly improves experimental efficiency and applicability. The technique is used here to study the two-photon absorption coefficient of ZnSe.

Carrier-carrier scattering in the gain dynamics of InxGa1-xAs/AlyGa1-yAs diode lasers.

Ultrafast optical nonlinearities in semiconductors play a central role in determining transient amplification and pulse-dependent gain saturation in diode lasers. Both carrier-phonon and carrier-carrier scattering are expected to determine the gain dynamics in these systems. We present a relaxation-time approximation model for carrier-carrier scattering in strained-layer lasers. The carrier-carrier scattering rates are determined using the quasiequilibrium distribution functions for a given background carrier density. The distribution function to which the photoexcited distribution relaxes is a Fermi-Dirac function where the chemical potential and temperature are self-consistently chosen so that both particle number and energy are conserved in the carrier-carrier scattering process. The relaxation approximation makes the problem an effective one-dimensional problem which can then be solved directly for the carrier distributions using an adaptive Runge-Kutta routine. This procedure is less computationally intensive than a full Monte Carlo simulation. The results show that the inclusion of carrier-carrier scattering improves previous results where only carrier-phonon scattering was included and that carrier-carrier scattering is necessary to produce heating of the carriers in the high-energy tails. \textcopyright{} 1996 The American Physical Society.

Green's-function approach to the optical nonlinearities in semiconductors and quantum-confined structures.

We introduce a method to calculate the linear and nonlinear optical properties of semiconductors and quantum-confined semiconductor structures. The approach is based on a rigorous and simple formula describing the interband density of states, eventually in the presence of a carrier plasma. It allows us to implement numerical algorithms that run faster than previous methods and to write compact analytical expressions. We show analytically that our approach yields the correct optical properties of quantum wells at vanishing plasma densities (two-dimensional Elliott formula).

Third-order optical nonlinearities in semiconductors: The two-band model.

We calculate the coherent electronic contributions to the third-order optical response ${\mathrm{\ensuremath{\chi}}}^{(3)}$(-\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\Omega},-\ensuremath{\Omega}) of bulk semiconductors in the independent-particle approximation using a simple two-band model. The formalism used to derive this response coefficient naturally accounts for all relevant contributions and, in contrast to existing results in the literature, leads to physically realistic, nondivergent expressions in the limits \ensuremath{\omega},\ensuremath{\Omega}\ensuremath{\rightarrow}0. Such well behaved infrared limits imply that the imaginary part of our ${\mathrm{\ensuremath{\chi}}}^{(3)}$ correctly describes the dispersion of nondegenerate absorption; indeed for \ensuremath{\Omega}=0 our results are consistent with predictions from Franz-Keldysh theory. Complementing these results, we can now also unambiguously extract from the real part of ${\mathrm{\ensuremath{\chi}}}^{(3)}$ the below band gap, two-band model predictions for the nonlinear refractive index, the dc Kerr effect, and the virtual photoconductivity; all of these predict a finite, real ${\mathrm{\ensuremath{\chi}}}^{(3)}$(0;0,0,0) as physically expected for clean, cold semiconductors. Finally, our specific results help expose more general consequences of the gauge choice when employing common approximate band-structure models.

Free-carrier-induced third-order optical nonlinearities in semiconductors

We have studied a variety of dissipative, free-carrier-induced optical nonlinearities in semiconductors with CO2 lasers and observed third-order nonlinear susceptibilities as large as 2 × 10−3 esu with picosecond recovery times. These fast nonlinear processes are capable of inducing huge perturbations in the dielectric function because they do not saturate at high intensities, as slower processes do. At laser intensities above a few tens of kilowatts per square centimeter, fast nonlinear processes dominate over the slower processes, which are stronger at low intensities. The large nonlinearities that we report can operate at room temperature. They are nonresonant, requiring no exact matching of the material parameters to the light frequency, and are insensitive to ambient-temperature changes. Hence they offer considerable versatility for design and stability in operation.

Measurements of Ultrafast Optical Nonlinearities in Semiconductors

AbstractThe response of a bulk semiconductor platelet under the influence of nonresonant pulse excitation below the band gap is measured using femtosecond laser techniques. These optical Stark effect measurements are analyzed using the effective Bloch equations for semiconductors. These equations are solved (i) dynamically in the large detuning, low intensity limit and (ii) for arbitrary intensities in the adiabatic limit.

Transient and steady-state optical nonlinearities in semiconductors

Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed. A simple description of the physical processes underlying the nonlinearities is given. The discussion is focused on the spectral region around the fundamental absorption edge, and it covers coherent oscillations, the optical Stark effect as well as the bleaching of the exciton resonance with increasing excitation intensity, plasma screening and band-filling phenomena.

Exciton-related optical nonlinearities in semiconductors and semiconductor microcrystallites

We report on large optical nonlinearities due to specific many-body effects in high-density electron-hole pair systems. On bulk CdS we study the effect of free-carrier screening on the absorption and refraction in the vicinity of the band-gap at room temperature. Steady-state saturable absorption at mW-power levels and four-wave mixing with first-order efficiencies as large as 2% of the incoming light are demonstrated on a 1μm slab. On CuBr microcrystallites embedded in glass we investigate the changes of the exciton absorption caused by many-exciton effects both at cryo and room temperature. In contrast to bulk semiconductors, we observe a blue shift of the exciton peak at resonant optical excitation. In addition, strong saturation of absorption with very large contrast shows up. We find Lorentzian saturation intensities in the 100 kW/cm2 range.

Optical Nonlinearities and Instabilities in Semiconductors

The main mechanisms for the observed large optical nonlinearities in semiconductors are briefly discussed and illustrated for the example of the exciton ionization by plasma screening. These nonlinearities cause various types of optical bistability, either of intrinsic nature or evoked by an additional resonator feedback. Under certain operation conditions also higher instabilities such as oscillations and chaos can be obtained. As an example, the self-pulsing of an induced absorber in a ring cavity is treated. The resulting locked oscillations are shown to follow a Farey-tree scenarium.

Nonequilibrium many-body theory of optical nonlinearities of semiconductors

Laser excited semiconductors are in a state far from equilibrium. Close to intrinsic resonances they show large optical nonlinearities due to many-body effects. Nonequilibrium Green's functions are best suited to describe highly excited semiconductors, because they allow a consistent determination of the eigenmodes and the occupation numbers of the excitations involved.

Non-perturbative many-body theory of the optical nonlinearities in semiconductors

We discuss the various many-body effects that contribute to the large optical nonlinearities of semiconductors close to the band edge. Such effects are: band filling, screening due to the excited electron-hole pairs, band-gap shrinkage, energy broadening and excitonic enhancement. Compared to the Coulomb self-energy corrections, the radiative self-energies are normally small. However, they determine the transition rates in the rate equations of the photons and electrons.