Based on the developed method of self-consistent calculations the results of description of tunable optical spectra versus temperature and excitation level for the GaSb doping superlattices of different design are presented. Account of the appearing tails of the density of states allows describing the long-wavelength edges and shape transformation in the spectra of absorption, gain, and luminescence and peculiarities in the optical transitions characteristics. Possible laser diode structures with n-i-p-i crystals in the active region are suggested including ordinary and δ-doped superlattices. Effects of tunable lasing are examined and ways for control of the radiation wavelength are discussed.
The knowledge of the spectral, power, polarization, and temporal characteristics of laser diodes is of great importance for
the development of efficient methods of the light emission control which is required for various applications. In the
paper, results of a comprehensive study of the output performance characteristics of quantum-well heterostructure lasers
are presented. The main attention is given to asymmetric multiple-quantum-well heterolasers which are possessed of
unique functional properties, such as wide-band tuning the emission spectra, multi-wavelength lasing, power switching,
and insensitivity to light polarization. The influence of cavity detuning on the nonlinear dynamics of the heterolasers
with harmonic modulation of the pump current is analyzed in detail. Four different models of the active medium which
allow to simulate different practical situations with respect to the modulation frequency and tuned lasing wavelength are
described. Effects of the radiation polarization state on the performance of the heterolasers are studied and operation
parameters of the laser diodes required for optoelectronic applications are determined.
Theoretical analysis of the emission line broadening in quantum-well lasers is carried out taking into account the Coulomb interaction of current carriers in the approach of two-dimensional electron-hole gas. The principal idea of the used method consists in the determination by means of the perturbation theory for many-body systems the functional behavior of tails of the emission line and in the subsequent extrapolation of the central part of the line according to a normalization requirement. Based on the obtained in the parabolic band approximation analytical shape function for the
homogeneously broadened spectral line, the influence of various factors on the optical spectrum is analyzed. An explanation of the experimental data, including the spectral line asymmetry and the linewidth change versus temperature and power excitation, is given. Results of the numerical calculations are presented for quantum-well heterostructure laser diodes in the GaInAsSb-AlGaAsSb-GaSb system. Spectra of luminescence and gain in dependence on the quantum well
width, temperature, and excitation level are examined. Spectrum transformation in the long-wavelength range and tuning curves for the GaSb-based laser emitters are also discussed. New possibility to overlap the spectral emission diapason of 2.2-2.9 μm is examined due to asymmetric multiple-quantum-well heterostructure configuration of the active region.
Optical properties of photonic crystal heterostructures with embedded n-i-p-i superlattices are studied. Nonlinear behavior of the transmission and reflection spectra near the defect mode is investigated. Self-consistent calculations of the output performance characteristics are performed using the transfer-matrix method and taking into account the gain saturation. Features and characteristic parameters of the nonlinear gain in active n-i-p-i layers are determined. Detail analysis of the gain saturation and accompanying nonlinear refraction effects is carried out for one-dimensional photonic crystal heterostructure amplifiers in the GaAs-GaInP system having at the central part an active "defect" from the doubled GaAs n-i-p-i crystal. The gain saturation in the active layers in the vicinity of the defect changes the index contrast of the photonic structure and makes worse the emission at the defect mode. Spectral bistability effect which can be exhibited in photonic crystal heterostructure amplifiers is predicted and the hysteresis loop and other attending phenomena are described. The bistability behavior and modulation response efficiency demonstrate the potential possibilities of the photonic crystal heterostructures with n-i-p-i layers as high-speed optical amplifiers and switches.
The temperature dependence of the lasing threshold in the GaInAs-GaInAsP-InP bi-quantum-well heterolasers with different widths of the quantum wells (4 and 9 nm) has been determined. Processes of non-radiative Auger recombination which occur in the active region of the quantum-well lasers have been included into consideration. The analytical approach for the evaluation of the characteristic temperature of the lasing threshold is presented. For described asymmetric quantum-well heterostructure lasers, it is shown that the influence of Auger recombination processes on the temperature behavior of the lasing threshold is not essential until the temperature of the active region is lower than 360 K and the cavity losses do not exceed 80 cm-1.
For the account of the propagation speed of radiation, a precision laser rangefinders based on a two-frequency opto-electronic recirculation and the phase method of measurements have been developed. A semiconductor laser diode with asymmetric quantum-well heterostructure is used as the source of radiation at two different optical wavelengths. It allows to receive information on the propagation speed of laser radiation along the line of supervision and to take into account the exact value of the radiation speed in the distance calculations.
A continuously operating route meter of low concentrations of gases, based on optoelectronic recirculation, is proposed. A semiconductor laser diode with an asymmetric quantum-well heterostructure providing emission at two different wavelengths is used for the radiation source. The regime of opto-electronic recirculation simultaneously at two wavelengths is realized in the system, that makes it possible to perform route measurements of the gas concentration and determine the length of the controlled route with a high accuracy.
Optical properties of one-dimensional heterostructures having optical forbidden gap are described and photonic structures with n-i-p-i superlattices are designed for laser applications.
Dependence of the nonlinear refraction coefficient on light intensity in the GaAs-AlGaAs quantum-well heterostructures is established. Effects of spectral broadening and light polarization are taken into account in the calculations based on the Kramers-Kronig relation.
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.
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.
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.
Band engineering conception of asymmetric quantum-well (QW) heterostructures widens possibilities to control functional performances of semiconductors lasers and other semiconductor optoelectronic devices. We have analyzed a new type of laser diodes and amplitudes based on asymmetric multiple-QW heterostructures having active layers of different thicknesses and component compositions. For such QW systems, it is possible to change the optical gain spectrum in a wide range and to control the set of definite amplification frequencies due to selecting the width, component composition, and doping of QW and barrier regions.
The rate of spontaneous radiative recombination and gain coefficient for the high-doping superlattices versus the pump current are calculated in the model with no k-selection rule. Results for the inversion current density and differential gain at suitable design of superlattice parameters are presented.
A novel type of laser diodes and amplifiers based on asymmetric quantum-well heterostructures having a set of active layers of different thickness and/or compositions has been considered. In contrast to ordinary laser heterostructures, for such modified quantum-well systems, it is possible to change the gain spectrum in a wide range and to control the set of definite amplification frequencies due to the selecting of the width and composition of quantum wells and barrier regions and of their doping and arrangement as well. New heterostructure schemes for tunable spectrum light-emitting sources have been proposed.
In the work we investigated the influence of the dimensional quantization on the probability of optical transitions with no k-selection rule. It is shown that calculations of the recombination rate and gain coefficient will be inaccurate if they are performed with using the probability of optical transitions determined for bulk semiconductors. Successive examination of radiative transitions with no the k-selection rule leads to a qualitative coincidence of the dependence of the spontaneous recombination rate on the quantum well width with the results obtained in the model of direct transitions.With the suppression of the constant injection efficiency the inversion current value is practically independent of the quantum-well layer thickness. Various approaches for the calculation of the spontaneous recombination rate are discussed.
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.
Use of asymmetric quantum-well heterostructures is a new conception of band engineering for semiconductor optoelectronics. In this manner there are developed semiconductor photomultipliers, infrared detectors, optical modulators and amplifiers, generators of harmonics, and other modem nanodevices.
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.
The model of spectral broadening involving the composition fluctuations having a normal distribution law in barrier layers of the GaAs-AlxGa1-xAs quantum-well system is presented. Dependence of the energy broadening on the distribution law parameters and quantum well width is determined. A method of calculation of the density of states and optical gain in undoped quantum-well heterostructures having quantum layers or quantum wires is also proposed.
Laser diodes and amplifiers of a nevv type based on asymmetric quantum-well heterostructures having a set of active layers different in thicknesses are considered. For such modified low-dimensional systems, in contrast to conventional laser heterostructures, the gain spectrum and a series of stimulated emission frequencies are varied through a wide range by choice of the widths and component compositions of quantum wells and barrier regions.
Keywords: asymmetric quantum-well heterostructures, gain spectra, spectral broadening
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.
Theoretical consideration of lasing regimes of asymmetric semiconductor heterostructures has been made. The conditions for bistable power switching and periodical radiation pulsations have been determined. The efficiency of injection of current carriers into quantum wells has been investigated. Doping levels of the layers forming the asymmetric heterostructure and its band configuration to realize the request inhomogeneous excitation of the quantum wells have been defined. The asymmetric heterostructure waveguide properties have been considered and the optical confinement factor for various lasing modes has been established. Calculations have been performed for the GaAs-AlxGa1-xAs system.
The properties and characteristics of asymmetric quantum-well heterojunction lasers having a set of active layers of different thickness and composition are described. The regimes of bistable switching and regular radiation pulsations are analyzed and optimal schemes of the laser structures are proposed.
The transient radiation oscillation process and generation instabilities in quantum-well laser systems are investigated in detail. A novel type of laser with quantum-well layers of various thickness is considered. The conditions for lasing switching with increasing the excitation current and for regular optical pulse generation have been determined. The calculations have been performed for the GaAs-AlGaAs system. The developed mathematical models and rate equations can be used to describe dynamics of laser action in quantum-well systems on other materials too.
The stimulated emission frequencies of laser QW heterostructures are analyzed here as a function of the size and composition of the active region and wide bandgap emitters, the excitation level, and the spectral characteristics of low-dimensional systems with varying thickness of QW layers. The simultaneous lasing conditions at two remote frequencies are determined for varying widths of the reactive region. In a GaAs-AlGaAs system with layer thickness of 50 and 100 A, the wavelength shift can attain 170-250 A.
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