J. Hader

On the importance of radiative and Auger losses in GaN-based quantum wells

Fully microscopic many-body models are used to study the importance of radiative and Auger carrier losses in InGaN∕GaN quantum wells. Auger losses are found to be negligible in contrast to recent speculations on their importance for the experimentally observed efficiency droop. Good agreement with experimentally measured threshold losses is demonstrated. The results show no significant dependence on details of the well alloy profile.

Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes

It is shown that a carrier loss process modeling density-activated defect recombination can reproduce the experimentally observed droop of the internal quantum efficiency in GaN-based laser diodes.

Temperature-dependence of the internal efficiency droop in GaN-based diodes

The temperature dependence of the measured internal efficiencies of green and blue emitting InGaN-based diodes is analyzed. With increasing temperature, a strongly decreasing strength of the loss mechanism responsible for droop is found which is in contrast to the usually assumed behavior of Auger losses. However, the experimental observations can be well reproduced assuming density activated defect recombination with a temperature independent recombination time.

High-Power Optically Pumped Semiconductor Laser at 1040 nm

We demonstrate near-diffraction limited (M 2 ¿ 1.5) output up to 23.8 W with optical-to-optical efficiency 27% and slope efficiency 32.4% and 40.7 W of multimode output from an optically pumped semiconductor laser at 1040 nm. Temperature-dependent photoluminescence measurements confirm accurate epitaxial growth according to the design thereby enhancing the effective gain.

Microscopic theory of gain and spontaneous emission in GaInNAs laser material

Abstract A fully microscopic model is used to calculate absorption/gain and spontaneous emission for GaInNAs quantum-well laser gain media. It is demonstrated how this approach can be used to derive the optical properties for the regime of semiconductor laser operation from low density photo luminescence spectra which can be obtained from simple experiments. Numerical results are presented showing that increased well depth leads to strongly increased differential gains and gain amplitudes and pronounced shifts of the gain maximum with increasing density. On the basis of a quantum Blotzmann model for the incoherent carrier dynamics it is shown, that high carrier confinement can lead to unusually long carrier capture times. Furthermore, temperature dependent bandstructure parameters for GaInNAs for the applied 10-band k · p -model are presented that have been derived from comparison to recent experimental data.

Gain spectra of (GaIn)(NAs) laser diodes for the 1.3-μm-wavelength regime

Optical gain spectra of (GaIn)(NAs)/GaAs quantum-well lasers operating in the 1.3-μm-emission-wavelength regime are measured and compared to those of a commercial (GaIn)(AsP)/InP structure. Good agreement of the experimental results with computed spectra of a microscopic many-body theory is obtained. Due to the contributions of a second confined subband, a spectrally broad gain region is expected for (GaIn)(NAs)/GaAs at elevated carrier densities.

Microscopic evaluation of spontaneous emission- and Auger-processes in semiconductor lasers

A fully microscopic approach is used to compute the losses in semiconductor lasers due to spontaneous emission and Auger recombination. The model is based on the semiconductor-Bloch equations and generalized quantum-Boltzmann type scattering equations in the second Born-Markov approximation. As input the theory only needs the structural layout and fundamental bulk-bandstructure parameters. It is demonstrated that such a comprehensive model that calculates gain/absorption, spontaneous emission and Auger processes on the same microscopic level can reliably predict these usually dominant loss processes. Examples of the results are compared to measurements on lasers in the 1.3-1.5 /spl mu/m range demonstrating very good agreement without empirical fitting.

Gain in 1.3 μm materials: InGaNAs and InGaPAs semiconductor quantum-well lasers

The absorption and gain for an InGaNAs/GaAs quantum-well structure is calculated and compared to that of a more conventional InGaAs/InGaPAs structure, both lasing in the 1.3 μm range. Despite significant differences in the band structures, the gain value is comparable for high carrier densities in both structures and the transition energy at the gain maximum shows a similar blueshift with increasing carrier density. For low and intermediate carrier densities, the calculated gain in the InGaPAs system is significantly lower and the bandwidth smaller than in the InGaNAs system.

Tunable high-power high-brightness linearly polarized vertical-external-cavity surface-emitting lasers

We report on the development and the demonstration of tunable high-power high-brightness linearly polarized vertical-external-cavity surface-emitting lasers (VECSELs). A V-shaped cavity, in which the antireflection-coated VECSEL chip (active mirror) is located at the fold, and a birefringent filter are employed to achieve a large wavelength tuning range. Multiwatt cw linearly polarized TEM00 output with a 20nm tuning range and narrow linewidth is demonstrated at room temperature.

Experimental and theoretical analysis of optically pumped semiconductor disk lasers

We describe the experimental cw power scaling of optically pumped semiconductor disk lasers OPS-DLs and give a detailed insight into the physical mechanism of this type of high-power surface-emitting semiconductor laser with external cavity. Minimizing the thermal resistance between active region and heat sink enables improved efficiency and gives access to high power and excellent beam quality of OPS-DL at 1000 nm. Results from initial numerical modeling are in good agreement with the experimental data, and show that thermal management is a critical parameter for the temperature-driven power shutoff in such devices. The computations are based on the macroscopic thermal transport, spatially resolved in both the radial and longitudinal directions, and coupled to the carrier density rate equations. A quantitative microscopic approach is used for the quantum-well gain and absorption dependence on wavelength, carrier density, and lattice temperature. The dependence of the computed output power on the substrate thickness and detuning are discussed.