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.

Resonant optical nonlinearities in semiconductors

By use of an expansion in Gaussian functions we solve the problem of the propagation of a laser beam that has been passed through a thin slab of nonlinearly refracting material. We compare the theoretical results for beam profile with experimental measurements for the semiconductor InSb and confirm that self-defocusing has been observed by D. A. B. Miller, M. H. Mozolowski, A. Miller, and S. D. Smith [Opt. Commun. 27, 133 (1978)]. The deduced value for the nonlinear refractive index at ~5 K is -6 x 10(-5) cm(2) W(-1) at 1886 cm(-1). This implies a third-order optical nonlinearity chi((3)) ~ 10(-2) esu, much larger than any previously reported for a solid.

Effect of low-power nonlinear refraction on laser-beam propagation in InSb

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium-lead-halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

Exciton localization in solution-processed organolead trihalide perovskites

Recently the developments in the field of II?VI-oxides have been spectacular. Various epitaxial methods have been used to grow epitaxial ZnO layers. Not only epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have been grown by laser molecular-beam epitaxy (laser-MBE). We mainly discuss the experimental aspect of the optical properties of excitons in ZnO-based MQW heterostructures. Systematic temperature-dependent studies of optical absorption and photoluminescence in these MQWs were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Based on these data, the localization of excitons, the influence of exciton?phonon interaction and quantum-confined Stark effects are discussed. The optical spectra of dense excitonic systems are shown to be determined mainly by the interaction process between excitons and biexcitons. The high-density excitonic effects play a role in the observation of room-temperature stimulated emission in the ZnO MQWs. The binding energies of exciton and biexciton are enhanced from the bulk values, as a result of quantum-confinement effects.

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/010/pdf

Optical properties of excitons in ZnO-based quantum well heterostructures

Recently the developments in the field of II-VI-oxides have been spectacular. Various epitaxial methods has been used to grow epitaxial ZnO layers. Not only epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have also been grown by laser molecular-beam epitaxy (laser-MBE). We discuss mainly the experimental aspect of the optical properties of excitons in ZnO-based MQW heterostructures. Systematic temperature-dependent studies of optical absorption and photoluminescence in these MQWs were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Based on these data, the localization of excitons, the influence of exciton-phonon interaction, and quantum-confined Stark effects are discussed. 
The optical spectra of dense excitonic systems are shown to be determined mainly by the interaction process between excitons and biexcitons. The high-density excitonic effects play a role for the observation of room-temperature stimulated emission in the ZnO MQWs. The binding energies of exciton and biexciton are enhanced from the bulk values, as a result of quantum-confinement effects.

Optical Properties of Excitons in ZnO-based Quantum Well Heterostructures

Picosecond time resolved luminescence is investigated of excitons localized by alloy potential fluctuations in CdS1–xSex. Time resolved luminescence spectra, as well as time decay curves and time integrated luminescence spectra of a CdS0.36Se0.64 sample are fitted using a model in which transfer to lower energy states occurs through the tunnel effect assisted by acoustic phonons. The density of states is taken to be exponential with a characteristic energy, E0, equal to 2.2 meV. The radiative lifetime is 1.5 ns. The nonradiative lifetime due to transfer is inversely proportional to the number of available states at lower energy. Close agreement with the experimental results can also be found with a phenomenological model.



Nous etudions la luminescence resolue en temps due aux excitons localises par les fluctuations de potential d'alliage dans CdS1–xSex. Les spectres de luminescence resolue en temps, les courbes de decroissance temporelle ainsi que les spectres de luminescence integree en temps d'un echantillon de composition CdS0,36Se0,64 sont ajustes en utilisant le modele de transfert par effet tunnel assiste par phonon acoustique. La densite d'etats decroit exponentiellement avec une energie caracteristique E0 = 2,2 meV. Le temps de vie radiatif est egal a 1,5 ns. Le temps de vie non-radiatif dǔ au transfert est inversement proportionnel au nombre d'etats disponsibles a plus basse energie. Un modele phenomenologique donne egalement un bon accord avec les resultats experimentaux.

Exciton Transfer between Localized States in CdS1–xSex Alloys

A high degree of saturation of optical absorption in InSb is observed at 2 \ifmmode^\circ\else\textdegree\fi{}K by using several lines (between 5.5 and 5 \ensuremath{\mu}m) of a cw CO laser. Luminescence is studied in the same conditions. A model of dynamic Burstein shift is presented which allows us to fit both theoretical transmission and luminescence with the data by taking into account band-gap renormalization and carrier heating. We emphasize that the principal experimental uncertainties arise from inhomogeneous pumping and interference effects in the sample. We show that a weak transverse magnetic field reduces the transmission of circularly polarized light.

P. Lavallard

Papers

Exciton Transfer between Localized States in CdS1–xSex Alloys

Saturation of optical absorption by a nonequilibrium electron-hole plasma in InSb crystals

Excited States of an Exciton Bound to a Neutral Donor