Ivan S. Manak

Optoelectronic properties and characteristics of doping superlattices

Optical and electric properties of doping superlattices, or n-i-p-i crystals, can be varied in a wide range under excitation and through the choice of the thicknesses and doping of the crystal layers. Some basic results concerned the transformation of the electron energy spectrum of doping superlattices are summarized. Parameters and characteristics of doping superlattices related to optoelectronics devices, such as photodetectors, laser diodes, and optical modulators, are presented.

Effects of energy-spectrum broadening in alloyed semiconductor superlattices

The density of energy states is calculated for alloyed semiconductor superlattices for different excitation levels with cllowance for the effects of screening and fluctuations of impurity concentrations. The influence of the state density tails on spontaneous-emission spectra is investigated for cross-over transitions and in a model without a selection rule for the electron wave vector. Account for the state density tails allows one to describe the longwave wing and shape of spontaneous-emission spectra in accordance with experimental data.

Gain and luminescence spectra of broadband emitters based on asymmetric quantum-well heterostructures

Laser diodes and amplifiers of a new type based on asymmetric quantum-well heterostructures having a set of effective layers of different thickness are considered. In distinction to conventional laser heterostructures, for such modified quantum-well systems the gain spectrum and the set of lasing frequencies can be varied over a wide range by choosing the widths and the component composition of quantum wells and barrier regions. The transformation of the amplification bands for the TE and TM modes with the excitation current was studied. Calculations were performed for the GaAs−AlxGa1−xAs system. In a model of direct transitions the subbands of heavy and light holes as well as changes in the polarization factor with the frequency of light are considered. A number of questions associated with spectral broadening of lines and determination of radiation localization and distribution of quasi-Fermi levels in the effective layers are discussed.

Quantum-well heterostructure laser diodes with flat widely tunable gain spectra

For a novel type of asymmetric multiple-quantum-well heterostructure lasers it is shown that a flat modal gain spectrum is obtained in a wide spectral range. It occurs since the quantum wells varied in widths and chemical compositions give a definite contribution to the total gain in different intervals of the spectrum. A certain design of the laser structures (chemical composition, thickness, doping, and arrangement of active and barrier layers) provides the conditions of non-uniform excitation of the quantum wells that results in the broad-band flat gain spectrum. Output power characteristics of the tunable laser diodes with a grating external cavity are examined in detail. For the spectral interval near the wavelength of 820 nm, the GaAs-AlGaAs system is preferred. In this case, the width of the gain band reaches up 50 nm and the tuning curve is practically flat at the output power about 10 mW in a single-mode regime without mode hops. Use of the other ternary or quaternary semiconductor compounds transfers the tuning range to a necessary spectral region. The described quantum-well heterostructures are suitable to make effective tunable laser diodes for a wide variety of applications, such as WDM optical networks, coherent spectroscopy, chemical analysis, metrology, and environment monitoring.

Polarization characteristics of quantum-well semiconductor structures

For low-dimensional semiconductor systems, matrix elements of optical dipole transitions versus different directions of the radiation polarization vector have been analyzed in detail. Analytical and numerical calculations are performed for quantum-well heterostructures in III-V semiconductor compounds. An influence of the spectral broadening due to intrasubband relaxation of current carriers on the transformation of light emission spectra in TE and TM modes with excitation has been studied. Distributions of electromagnetic wave fields and the optical confinement factor for TE and TM modes in multiple quantum-well layer structures, including a novel type of asymmetric heterostructures, have been determined. For quantum-wire structures, including a novel type of asymmetric heterostructures, have ben determined. For quantum-wire structures, the degree of light emission polarization has been examined and effects in porous Si luminescence are explained.

NONLINEAR OPTICAL PROCESSES IN DOPED SEMICONDUCTOR SUPERLATTICES

For doped semiconductor superlattices we have analyzed the effects of absorption saturation and change in the refractive index depending on the level of their excitation and structure parameters. The calculations were carried out with account for the tails of the density of states and screening of the electrostatic potential. It is shown that doped superlattices may display “shading,” i.e., the increase in the absorption coefficient at a fixed frequency with increase in light intensity. For δ–doped superlattices a stronger nonmonotonicity of the change in the refractive index with increase in the excitation level in comparison with typical structures can manifest itself.

Regular-pulsation asymmetric multiple-quantum-well heterostructure laser radiator

Regimes of regular pulse generation at remote optical wavelengths in asymmetric quantum-well heterostructure lasers have been examined in detail. Influence of the heterostructure parameters and pump current density on frequency, duration, and magnitude of light spikes and on phase shift between pulses of radiation at different wavelengths has been studied. Design of optimal band energy structures for novel type of laser sources is proposed.

Saturation of absorption in n-i-p-i crystals

Peculiarities of the absorption saturation in doping superlattices with n-i-p-i crystal type structure are established. Calculations are performed in the k-selection rule model taking into account the screening of the electrostatic potential by current carriers and the density state tails.

Design and characteristics of widely tunable quantum-well heterostructure lasers in the Littman and Metcalf cavity configuration

Analysis of light-current and tuning characteristics of a quantum-well laser in an external cavity is performed. The band diagram of the proposed tunable asymmetric multiple quantum-well heterostructure laser source under the forward bias is shown. The heterostructure consists of the GaAs-AlGaAs system.

Current injection and recombination processes in asymmetric triple quantum well lasers

For lasers based on asymmetric quantum-well heterostructures comprising three different quantum wells and barrier layers with a complex potential profile, current carrier injection and radiation emitting processes have been considered. The analysis performed by using rate equations has shown that a regime of regular pulse generation at remote wavelengths is practicle. The radiation pulsation process is accompanied by oscillations of the carrier injection efficiency into quantum wells. At certain parameters of the laser structures it is possible to realize radiation bistable switching-on or the regime where relative powers at different wavelengths change with increasing the pump current. The injection efficiency has been determined by solving the Poisson equation and continuity conditions for electron and hole currents. Carrier tunneling through potential barriers between the quantum wells has been taken into account. Calculations have been performed for the GaAs - AlxGa1- xAs system.