Hans Christian Schneider

Nonequilibrium model for semiconductor laser modulation response

Presents a laser model for describing the effects of nonequilibrium carrier distributions. The approach is based on the coupled Maxwell-semiconductor-Bloch equations, with carrier-carrier and carrier-phonon collisions treated in the relaxation rate approximation. Using examples involving relaxation oscillation, current modulation, and optical injection, we demonstrate how the model can be used to study the influences of spectral hole burning, dynamic carrier population bottleneck, and plasma heating on semiconductor laser modulation response.

Emission dynamics and optical gain of 1.3-/spl mu/m (GaIn)(NAs)/GaAs lasers

The ultrafast emission dynamics of a 1.3-/spl mu/m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9/spl middot/10/sup -4/ eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment.

Charge-separation effects in 1.3 μm GaAsSb type-II quantum-well laser gain

A microscopic laser theory is used to investigate gain and threshold properties in a GaAsSb quantum-well laser. Depending on the geometry of the type-II quantum-well gain region, there may be appreciable band distortions due to electron–hole charge separation. The charge separation and accompanying band distortions lead to interesting optical behaviors, such as excitation-dependent oscillator strength and band edge energies. Implications to laser operation include significant blueshift of the gain peak with increasing injection current, and inhibition of spontaneous emission, which may result in threshold current reduction.

Temperature dependence of laser threshold in an InGaAsN vertical-cavity surface-emitting laser

We present theoretical and experimental results for the temperature dependence of threshold current in an InGaAsN/GaAs vertical-cavity surface-emitting laser (VCSEL) operating at 1.3 μm under continuous-wave current injection. Using a microscopic many-body laser theory, good agreement with experimental data is obtained. The influence of radiative and nonradiative recombination processes on the threshold current–density is investigated theoretically. Also, comparison to a GaAs/AlGaAs VCSEL emitting at 850 nm is made.

Theory of laser gain in InGaN quantum dots

A theory for gain is developed for wide-bandgap nitride-based quantum-dot laser structures. A semiclassical laser theory is used to describe the optical susceptibility in the presence of strong many-body Coulomb interaction and quantum-confined Stark effect. Application of this theory shows distinctly different gain behavior depending on quantum dot dimensions, because of the interplay of these effects.

Theoretical investigation of laser gain in AlGaInN quaternary quantum wells

Microscopic calculations of laser gain spectra are presented for AlGaInN wurtzite quantum-well structures that are under compressive, zero and tensile strain. It is found that the optical nonlinearities induced by the combination of strain, quantum-confined Stark effect and many-body Coulomb interactions give rise to optical behavior that can differ significantly from that in conventional semiconductor lasers.

Absence of filamentation in quantum-dot lasers. Theory and experiment

We theoretically and experimentally investigate the lateral mode behavior of quantum-dot lasers, and demonstrate the absence of filamentation up to 10/spl times/ threshold. The negative linewidth enhancement factor necessary for filamentation in highly-excited quantum dot lasers originates from the Coulomb coupling between quantum-dot and quantum-well states.

Filamentation in InGaAs quantum-dot lasers: theory and experiment

Summary form only given. One question concerning active media for semiconductor edge-emitting lasers is the behavior of the intracavity laser field (i.e., the beam quality) above threshold. Especially important are self-focusing effects which narrow the laser beam and eventually lead to beam break-up or filamentation. This paper presents theoretical and experimental results on the beam quality of edge-emitting lasers with InGaAs quantum dot active media. We use a screened Hartree-Fock theory to compute the complex susceptibility for a shallow quantum dot for different excitations. The inclusion of Coulomb effects not only influences the gain (imaginary part of the susceptibility), but also changes the dispersive behavior, i.e., the carrier induced refractive index (real part of the susceptibility) of a quantum dot active medium. Our theoretical results show that the filamentation tendency in quantum-dot lasers can be substantially weakened for sufficiently small inhomogeneous broadening, contrary to what is observed in quantum-well lasers. An indication of this is observed in our experimental results which show indeed a weaker filamentation tendency than quantum-well lasers. However, the calculations predict a still better beam quality for lasers fabricated from dot material with reduced inhomogeneous broadening.

Gain and carrier-induced refractive index change in group-III nitride quantum wells

This paper analyses the gain and carrier-induced refractive index change in group-III nitride quantum wells. An approach based on the semiconductor Bloch equations with carrier-carrier collisions treated at the level of quantum kinetic theory is used. The influences of the strong carrier-carrier Coulomb interaction and the quantum-confined Stark effect on laser threshold and output beam quality are discussed.

Origin of emission energy blue shift in a 1.3 /spl mu/m GaAsSb type-II quantum well

Summary form only given. Vertical-cavity surface-emitting laser (VCSEL) gain structures that can be epitaxially grown on GaAs substrates are of particular interest. One such system is the type-II GaAsSb/GaInAs/GaAs quantum well structure. For this gain structure, experiments found significant increase in the optical emission peak energy with increasing excitation. This blue shift is a serious concern because it is detrimental to long wavelength operation.This paper describes experimental and theoretical investigations of the significant blue shift of the emission peak with increasing injection current in a type-II quantum well. We find the blue shift to depend on the interplay of the competing effects of type-II quantum confinement, and the resulting Coulomb attraction and band distortion.