We present an effective approach to calculating the low-frequency part of the spectrum of uniaxially patterned periodic structures. In this approach we ignore to zeroth-order the Bragg scattering by crystalline planes but include local field effects in first order perturbation theory. Bragg reflections are shown to be important only near points of symmetry-induced spectral degeneracy, where they can be taken into account by the degenerate perturbation theory. We apply this approach to waveguiding by thin patterned slabs embedded in a homogeneous medium. This results in an effective medium approximation, similar to the Maxwell Garnet theory but modified for the local field corrections specific to 2D geometry. Slab spectra are well described by a single frequency-independent parameter, which we call the guiding power. Simple analytic formulae are presented for both TM and TE polarizations. Comparing these formulae with similar expressions for homogeneous uniaxial slabs of same thickness, we derive the principal values of the effective homogeneous permittivity that provides identical waveguiding. We also discuss the extinction of waves due to the Rayleigh-like scattering on lattice imperfections in the slab. The TE waves that are normally better confined are scattered out more effciently, in part because of the higher scattering cross-section and in part because the better confinement leads to higher exposure of TE waves to lattice imperfections in the slab.
We propose a novel design of an electrically tunable type II mid-IR light source based on a InAs/AlSb/GaInAsSb/GaInSb heterostructure. The design combines the advantages of strong wavelength tuning due to the linear Stark effect and the presence of separate charge accumulation layers, which enables laser wavelength tuning without a change of the optical loss. Experiment shows a blue shift of the electroluminescence (EL) line at increasing bias current, commensurate with that expected form the linear Stark effect. The laser generation was observed at higher currents. The EL wavelength shifts from 2.79um to 2.38um ( ~ 80meV) at T=80K as the bias current increases from 97mA to 418mA, which provides the record combination of the wide tuning range and low relative change of the bias current.
We develop a general approach to including the internal optical loss in the description of semiconductor lasers with a quantum-confined active region. We assume that the internal absorption loss coefficient is linear in the free-carrier density in the optical confinement layer and is characterized by two parameters, the constant component and the net cross-section for all absorption loss processes. We show that the free-carrier-density dependence of internal loss gives rise, in general, to the existence of a second lasing threshold above the conventional threshold. Above the second threshold, the light-current characteristic is two-valued up to a maximum current at which the lasing is quenched. We show that the presence of internal loss narrows considerably the region of tolerable structure parameters in which the lasing is attainable; for example, the minimum cavity length is significantly increased. Our approach is quite general but the numerical examples presented are specific for quantum dot (QD) lasers. Our calculations suggest that the internal loss is likely to be a major limiting factor to lasing in short-cavity QD structures.
Gain in broad area mid-infrared diode W lasers ((lambda) =3- 3.1micrometers ) has been measured using lateral mode spatial filtering combined with the Hakki-Paoli approach. The internal optical loss of approximately equals 19cm-1 determined from the gain spectra was the same for devices with either 10- or 5-period active regions and nearly constant in the temperature range between 80 and 160K. Analysis of the differential gain and spontaneous emission spectra shows that the main contribution to the temperature dependence of the threshold current is Auger recombination, which dominates within almost the entire temperature range studied (80-160K).
Different approaches to the design of a genuinely temperature-insensitive quantum dot (QD) laser are proposed. Suppression of the parasitic recombination outside the QDs, which is the dominant source of the temperature dependence of the threshold current in the conventional design of a QD laser, is accomplished either by tunneling injection of carriers into the QDs or by bandgap engineering. Elimination of this recombination channel alone enhances the characteristic temperature T0 above 1000 K. Remaining sources of temperature dependence (recombination from higher QD levels, inhomogeneous line broadening, and violation of charge neutrality in QDs) are studied. Tunneling injection structures are shown to offer an additional advantage of suppressed effects of inhomogeneous broadening and neutrality violation.
Temperature dependencies of the threshold current, device slope efficiency and heterobarrier electron leakage current from the active region of InGaAsP/InP multi-quantum-well lasers with different profiles of acceptor doping were measured. We demonstrate that the temperature sensitivity of the device characteristics depends on the profile of p- doping, and that the variance in the temperature behavior of the threshold current and slope efficiency for lasers with different doping profiles cannot be explained by the change of the measured value of the leakage current with doping only. We show that doping of the p-cladding/SCH layer interface in InGaAsP/InP multi-quantum-well lasers leads to improvement of the device temperature performance. We also show that doping of the active region increases the value of the optical loss without degradation of characteristic temperature T0.
Leakage of electrons from the active region of InGaAs/InP laser heterostructures with different profiles of acceptor doping was measured by a purely electrical technique together with the device threshold current. Comparison of the obtained results with modeling data and SIMS analysis shows that carrier leakage of electrons over the heterobarrier depends strongly on the profile of p-doping and level of injection. In the case of a structure with an undoped p- cladding/waveguide interface the value of electron leakage current can reach 20% of the total pumping current at an injection current density of 10 kA/cm2 at 50 C. It is shown that carrier leakage in InGaAsP/InP multi-quantum-well lasers can be minimized and the device performance improved by utilizing a p-doped SCH layer.
Large signal analysis of dual modulation of semiconductor lasers (by a simultaneous high-frequency control of the pumping current I and an additional intrinsic parameter) shows that the method allows suppressing the relaxation oscillations for an arbitrary shape of the pumping current signal I(t). Because of that, the rate of information coding can be enhanced to about 80 Gbit/sec. Moreover, we demonstrate that dual modulation allows us to maintain a linear relationship between I(t) and the output optical power in a wide frequency band.
We review recent theoretical studies of the symmetry properties of charge injection transistors. These studies, based on continuation modeling and transient device simulation, incorporate self-consistently the electron energy and real-space transfer currents over heterojunction interfaces. Inspection of the full device phase—space reveals a variety of instabilities and a striking novelty of multiply-connected current-voltage characteristics. We have found anomalous steady states in which hot-electron injection occurs in the absence of any voltage between the emitter electrodes. In these states, some of which are not only stationary but also stable with respect to small perturbations, the electron heating is due to the fringing field from the collector electrode. Some of the anomalous states break the reflection symmetry in the plane normal to the channel at midpoint. The study elucidates the formation of hot-electron domains in real-space transfer.