Dieter Bimberg

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

Radiative recombination in type‐II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Quantum dot lasers

We presents both edge emitting and surface emitting quantum dot laser research. Growth is by both MOCVD and MBE.

Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN

We have derived consistent sets of band parameters band gaps, crystal field splittings, band-gap deformation potentials, effective masses, and Luttinger and EP parameters for AlN, GaN, and InN in the zinc-blende and wurtzite phases employing many-body perturbation theory in the G0W0 approximation. The G0W0 method has been combined with density-functional theory DFT calculations in the exact-exchange optimized effective potential approach to overcome the limitations of local-density or gradient-corrected DFT functionals. The band structures in the vicinity of the point have been used to directly parametrize a 44 k·p Hamiltonian to capture nonparabolicities in the conduction bands and the more complex valence-band structure of the wurtzite phases. We demonstrate that the band parameters derived in this fashion are in very good agreement with the available experimental data and provide reliable predictions for all parameters, which have not been determined experimentally so far.

Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum dot lasers

Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum-dot lasers are achieved by the application of an annealing and growth interruption step at 600 °C after the deposition of the dots. The transparency current is reduced to below 20 A/cm2 at room temperature. The internal differential quantum efficiency is increased from below 50% to above 90% by improvement of the barrier material and subsequent reduction of leakage current. A peak power of 3.7 W at 1140 nm lasing wavelength in pulsed operation at room temperature is demonstrated.

Excited states in self‐organized InAs/GaAs quantum dots: Theory and experiment

In photoluminescence spectra of nanometer‐scale pyramidal‐shaped InAs/GaAs quantum dots allowed optical transitions involving excited hole states are revealed in addition to the ground state transition. Detailed theoretical calculations of the electronic structure, including strain, piezoelectric and excitonic effects, agree with the experimental data and lead to unambiguous assignment of the transitions.

Multiphonon‐relaxation processes in self‐organized InAs/GaAs quantum dots

We report on optical studies of relaxation processes in self‐organized InAs/GaAs quantum dots (QDs). Near resonant photoluminescence excitation spectra reveal a series of sharp lines. Their energy with respect to the detection energy does not depend on QD size and their energy separations are close to the InAs LO phonon energy of 32.1 meV estimated for strained pyramidal InAs QDs. The shape of the PLE spectra is explained by multiphonon relaxation processes involving LO phonons of the QD as well as of the wetting layer, an interface mode, and low frequency acoustical phonons.

QUANTUM DOT LASERS: BREAKTHROUGH IN OPTOELECTRONICS

Abstract Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for fabrication of self-organized QDs. Optimization of deposition parameters can ensure that the self-organized islands are small (∼10 nm), have a similar size and shape and form dense arrays. Saturation material gain is as high as 150000 cm −1 compared with QW values of about 3000 cm −1 . Maximum differential gain reported for QD lasers approaches 10 −12 cm 2 and exceeds the QW laser values by about three orders of magnitude. Direct observation of relaxation oscillations reveals present cut-off frequencies close to 10 GHz. High internal (>96%) and differential (70%) efficiencies at 300 K are realized. Using the novel concept of electronically-coupled QDs and oxide-defined 10 μm apertures, CW lasing with J th =180 A/cm 2 , is realized in surface-emitting QD lasers (300 K). Wall-plug efficiencies are up to 16%. Total currents as low as 68 μA are measured for 1μm apertures. GaAs-based lasers for the 1.3 μm range with low J th (65 A/cm 2 ) at room temperature (RT) are realized using InAs/InGaAs/GaAs QDs obtained by activated spinodal decomposition. In stripes the lasing occurs via the QD ground state ( J th =90 A/cm 2 ) for cavity lengths L >1 mm (uncoated). Differential efficiency is 55% and internal losses are 1.5 cm −1 . A characteristic temperature near RT is 160 K. 3W CW operation at RT is achieved. The recent progress in lasers based on self-organized MBE QDs already made it possible to fabricate devices with dramatically improved characteristics as compared to recent QW devices for the most important commercial applications.

Ultrafast gain dynamics in InAs-InGaAs quantum-dot amplifiers

The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with /spl sim/100-fs time constant, making devices based on such dots promising for high-speed optical communications.

Quantum dots for lasers, amplifiers and computing

For InAs-GaAs based quantum dot lasers emitting at 1300 nm, digital modulation showing an open eye pattern up to 12 Gb s−1 at room temperature is demonstrated, at 10 Gb s−1 the bit error rate is below 10−12 at −2 dB m receiver power. Cut-off frequencies up to 20 GHz are realised for lasers emitting at 1.1 µm. Passively mode-locked QD lasers generate optical pulses with repetition frequencies between 5 and 50 GHz, with a minimum Fourier limited pulse length of 3 ps. The uncorrelated jitter is below 1 ps. We use here deeply etched narrow ridge waveguide structures which show excellent performance similar to shallow mesa structures, but a circular far field at a ridge width of 1 µm, improving coupling efficiency into fibres. No beam filamentation of the fundamental mode, low a-factors and strongly reduced sensitivity to optical feedback are observed. QD lasers are thus superior to QW lasers for any system or network. Quantum dot semiconductor optical amplifier (QD SOAs) demonstrate gain recovery times of 120–140 fs, 4–7 times faster than bulk/QW SOAs, and a net gain larger than 0.4 dB/(mm*QD-layer) providing us with novel types of booster amplifiers and Mach–Zehnder interferometers. These breakthroughs became possible due to systematic development of self-organized growth technologies.