Michael A. Kneissl

Novel monolithic waveguide-based smart pixel for high-contrast, high-gain, and high-speed optical switching

We present results on optical switching of a novel monolithic waveguide-based smart pixel. In this smart pixel two surface-normal optical input beams control an optical output beam propagating in-plane through a waveguide modulator. For the operation only DC biases are required. The optical front end of our waveguide-based smart pixel consists of a specially designed n-i-p photoconductive detector in series with a reference n-i-p photodiode. Together, both devices are forming a digital opto-electronic switch, which is directly controlling the waveguide modulator. All components are based on the Franz-Keldysh effect. For a first demonstration of our monolithic waveguide-based smart pixel all components have been processed from the same GaAs/AlGaAs double hetero n-i-p structure grown by MBE on a semi-insulating GaAs substrate. With a non-optimized sample design we obtain an output contrast ratio of 17 dB and an optical gain in excess of 320. The optical input energy is estimated to be 2.6 pJ (detector area 20 X 20 micrometers 2). In this case the wavelength of the input beams was 783 nm, while the waveguide modulator was operated at 910 nm. An evaluation of the switching dynamics indicates that high-speed operation in excess of 400 Mbit/s can be achieved. This waveguide-based smart pixel could for example be used in optical routing networks.

In-situ structured MBE-grown crystals for applications in optoelectronics

We present the shadow mask molecular beam epitaxial (MBE) growth technique which allows an in situ lateral structuring of the doping profile and the growth rate on a micrometers scale. The electrical dc characteristics show that excellent quality selective contacts have been achieved on devices with lateral dimensions down to the micrometers range. The leakage currents are, even for highly doped structures, in the nA range. High reflectivity Bragg mirrors and pronounced exciton peaks observed on MQW structures confirm the high quality of the regrown material. The influence of the aspect ratio on the growth rates is very small. We have applied this novel technique to fabricate various selectively contacted optoelectronic devices based on n-i-p-i doping superlattices. For GaAs Franz Keldysh n-i-p-i modulators with selective contacts an on/off ratio of 6:1 has been achieved. High frequency results obtained on medium size devices indicate that 3 dB frequencies in the GHz range should be possible for n-i-p-i devices with dimensions < 4 micrometers fabricated with this technique. By selectively contacting the QWs in a modulation doped hetero n-i-p-i structure constructive superposition of field and carrier induced absorption changes have been achieved.

Ultraviolet InGaN, AlGaN, and InAIGaN multiple-quantum-well laser diodes

Design and performance characteristics of InGaN, AlGaN and InAlGaN multiple quantum well (MQW) laser diodes emitting in the ultraviolet spectral region are reported. The nitride laser diodes were grown on quasi-bulk GaN and c-plane sapphire substrates by metalorganic chemical vapor deposition. By reducing the indium content in the InGaN MQW, we have realized laser diodes on GaN substrates emitting at wavelength as short as 366.9 nm with pulsed threshold current densities around 8kA/cm2. Improved performance characteristic with differential quantum efficiencies of 7.7% and light output powers of close to 400mW were obtained. We also demonstrate room-temperature pulsed operation of AlGaN and InAlGaN MQW laser diodes grown on sapphire substrates emitting at a record short wavelength of 357.9nm. Light output powers greater than 80mW under pulsed current-injection conditions and differential quantum efficiencies of 4.2% have been achieved.

Advances in InAlGaN laser diode technology toward the development of UV optical sources

We report on ultraviolet (UV) InGaN and GaN multiple quantum well (MQW) laser diodes grown on c-plane sapphire substrates by metal organic chemical vapor deposition. By reducing the indium content in the InGaN/InAlGaN MQW, we have systematically pushed the room-temperature laser emission to a record low wavelength of 363.2nm. Pulsed threshold current densities around 5 kA/cm2 have been achieved for laser diodes with emission wavelength between 368 nm and 378 nm. Light output powers greater than 400mW under pulsed current-injection conditions (pulse duration 500 ns, repetition frequency 1 kHz) and differential quantum efficiencies of 4.8% have been achieved. We also demonstrate room-temperature continuous-wave operation of ridge-waveguide devices with threshold currents of 85 mA for an emission wavelength of 377.8 nm and output power of more than 3 mW.