F. Schäfer

Low-threshold high-quantum-efficiency laterally gain-coupled InGaAs/AlGaAs distributed feedback lasers

We have developed gain-coupled lasers based on metal gratings patterned laterally to the laser ridge. For narrow ridge waveguides, the evanescent field of the laser mode couples to the grating. The fabrication requires no overgrowth steps and can be applied to all material systems. Ridge-waveguide gain-coupled lasers with threshold current densities of 600 A/cm2 were obtained from InGaAs/AlGaAs GRINSCH structures. The continuous wave threshold currents are around 9 mA for a cavity with 600 μm length and 2.5 μm width. Monomode emission up to output power levels of 64 mW and sidemode suppression ratios of over 45 dB have been obtained.

HIGH-PERFORMANCE GAINAS/GAAS QUANTUM-DOT LASERS BASED ON A SINGLE ACTIVE LAYER

GaInAs/GaAs quantum-dot lasers were fabricated by self-organized growth in a molecular beam epitaxy system. By using a single active layer, lasers with low-threshold current densities (Jth=144 A/cm2 for a 2 mm long device) and high internal quantum efficiencies (>90%) were obtained. Ground-state lasing of the quantum dots was observed up to a device temperature of 214 °C.

Lateral coupling – a material independent way to complex coupled DFB lasers

We have developed a new technique for the fabrication of complex coupled distributed feedback laser (DFB) lasers. A metal grating patterned lateral to a narrow ridge waveguide laser couples to the evanescent part of the guided mode. No regrowth is required for the processing, which makes this approach applicable to all material systems. DFB lasers fabricated from InGaAs/AlGaAs quantum well and dot lasers, GaInNAs DFB lasers emitting at 1.3 μm and long wavelength lasers at 2 μm based on InGaSbAs/AlGaSbAs structures are described. The lasers show low thresholds, good efficiencies and a high sidemode suppression ratio.

High-temperature properties of GaInAs/AlGaAs lasers with improved carrier confinement by short-period superlattice quantum well barriers

The influence of the carrier confinement on the output characteristics of GaInAs/AlGaAs lasers was investigated. To improve the carrier confinement, AlGaAs/GaAs short-period superlattices were used as quantum well barriers. In comparison to lasers with GaAs barriers the structures with the modified barriers show improved temperature properties at low threshold current densities without deterioration of the internal quantum efficiency (>95%). High characteristic temperatures (T0) well above 300 K were measured between 20 and 75 °C and laser operation up to 238 °C could be achieved. The large improvement in T0 is mainly attributed to the reduced thermionic emission of carriers out of the quantum well due to the increased barrier height and the carrier reflection above the barrier by the short-period superlattice.

Laser emission from photonic dots

Laser emission was observed in photonic semiconductor dots with a discretized optical mode spectrum. The photonic dots with lateral sizes between 1 and 5 μm provide a three-dimensional optical confinement by using in the vertical direction AlAs/GaAs Bragg mirrors and in the lateral directions the refractive index discontinuity at the etched surfaces. In the optically pumped structures, the laser emission takes place on the fundamental mode of the microcavities. External threshold excitation densities of 200 W/cm2, which correspond to a very low internal optical excitation power of 0.15 μW per microcavity post, were measured for microcavity structures with a lateral size of 2.7 μm.

Carrier and light trapping in graded quantum-well laser structures

We investigated the carrier and light trapping in GaInAs/AlGaAs single-quantum-well laser structures by means of time-resolved photoluminescence and Raman spectroscopy. The influence of the shape and depth of the confinement potential and of the cavity geometry was studied by using different AlGaAs/GaAs short-period superlattices as barriers. Our results show that grading the optical cavity improves considerably both carrier and light trapping in the quantum well, and that the trapping efficiency is enhanced by increasing the graded confining potential.

Capture and confinement of light and carriers in graded-index quantum well laser structures

Abstract We investigated graded-index separate-confinement heterostructure lasers and compared their performance with a conventional quantum well laser by using Raman and time-resolved photoluminescence spectroscopy. We found that grading the index of the Ga 1− x Al x As/GaAs waveguide and increasing the Al content improves considerably both, the light and carrier trapping and reduces carrier escape from the active region.