F. Klopf

Tunable photonic crystals fabricated in III-V semiconductor slab waveguides using infiltrated liquid crystals

Microcavity structures formed by two-dimensional photonic crystal mirrors with triangular lattice and a crystal period of 280 nm suitable for transmission experiments at about 1 μm were fabricated in III-V semiconductor planar waveguides. The photonic crystals were filled with a liquid crystal of type E7. The wavelength of the resonance peak can be shifted by the temperature-dependent refractive index of the liquid crystal. The temperature shift follows the typical refractive index behavior of liquid crystals, with a jump in wavelength at the clearing point (about 60 °C) and a continuous shift below. The wavelength jump is about 4 nm within a few Kelvin, while the total shift amounts to 9 nm between 20 and 70 °C. The experimental results agree well with the calculated temperature dependent photonic band structure by assuming a preferential alignment of the liquid-crystal molecules parallel to the holes.

Highly efficient GaInAs/(Al)GaAs quantum-dot lasers based on a single active layer versus 980 nm high-power quantum-well lasers

Highly efficient 980 nm GaInAs/(Al)GaAs quantum-dot (QD) and quantum-well (QW) lasers based on a single active layer have been fabricated and compared in view of high-power applications. QD lasers show a significantly reduced temperature shift of the emission wavelength and achieve external quantum efficiencies of 80% (>1 W/A for 1-mm-long devices). For longer cavity lengths (>2.5 mm), QD lasers show lower threshold current densities than QW lasers. Threshold current densities as low as 54 A/cm2 are reached.

Correlation between the gain profile and the temperature-induced shift in wavelength of quantum-dot lasers

The influence of several design parameters on the temperature stability of the emission wavelength of 980 nm GaInAs/(Al)GaAs quantum-dot lasers was studied. The results obtained agree well with a simplified model based on the inhomogeneously broadened transitions of a quantum-dot ensemble. Using this model, the optimum cavity design for a given gain function can be determined. Following this approach, quantum-dot lasers with low wavelength shifts of 0.16 nm/K were realized, which is only half the value of a typical GaInAs/(Al)GaAs quantum well laser.

High-temperature operating 1.3-μm quantum-dot lasers for telecommunication applications

High-performance 1.3-/spl mu/m-emitting quantum-dot lasers were fabricated by self-organized growth of InAs dots embedded in GaInAs quantum wells. The influence of the number of quantum-dot layers on the device performance was investigated. Best device results were achieved with six-dot layers. From the length dependence; a maximum ground state gain of 17 cm/sup -1/ for six dot layers could be determined. Ridge waveguide lasers with a cavity length of 400 /spl mu/m and high-reflection coatings show threshold currents of 6 mA and output powers of more than 5 mV. Unmounted devices can be operated in continuous wave mode up to 85/spl deg/C. A maximum operating temperature of 160/spl deg/C was achieved in pulsed operation for an uncoated 2.5-mm-long ridge waveguide laser.

Wide range tunable laterally coupled distributed-feedback lasers based on InGaAs-GaAs quantum dots

The authors have investigated tunable distributed feedback (DFB) lasers based on InGaAs quantum dots grown by molecular beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side-mode suppression ratios of up to 40 dB have been achieved. The tuning range covers 30 nm.

Optical spectroscopy of single InAs/InGaAs quantum dots in a quantum well

We have grown self-assembled InAs quantum dots embedded in the center of an InGaAs quantum well by molecular-beam epitaxy. Using electron-beam lithography and wet etching techniques, small mesas with only a few quantum dots were fabricated. At room temperature, the quantum dots have an emission wavelength of 1.3 μm. By photoluminescence spectroscopy at low temperatures, we observe the emission lines of excitons and biexcitons in single-dot structures. The assignment of exciton and biexciton recombination is based on the characteristic excitation intensity dependence of these states. A biexciton binding energy of about 3.5 meV is obtained for the present dots.

Deeply etched two-dimensional photonic crystals fabricated on GaAs/AlGaAs slab waveguides by using chemically assisted ion beam etching

High quality two-dimensional photonic crystals were fabricated by electron-beam lithography and a combination of reactive ion etching (RIE) and chemically assisted ion beam etching (CAIBE) in GaAs/AlGaAs slab waveguides. With optimized parameters, etch depths of more than 2 μm were achieved. This technique was used to fabricate three-dimensionally confined optical microcavities defined by a slab waveguide and a two-dimensional photonic crystal mirror structure. The optical quality of the deeply etched structures were tested by optical spectroscopy. The Q-factor of the cavity mode is dependent on the structural quality and depth of the holes in the photonic crystals. Due to the large etch depth and smooth surfaces, high quality planar microcavities could be realized with Q-factors of about 1000.

High Performance 1.3 µm Quantum-Dot Lasers

InAs/GaInAs-quantum-dot lasers emitting at 1.3 µm have been grown on GaAs substrates by solid source molecular beam epitaxy, and the device performance has been studied. Laser structures with 6 dot layers in the active region yield the best results. They show low transparency current densities and high characteristic temperatures. Ridge waveguide lasers with a cavity length of 400 µm and high reflectivity coatings exhibit threshold currents as low as 4.4 mA and output powers of 12 mW in continuous wave (cw) operation at room temperature. Unmounted devices can be operated in cw mode beyond 80°C. In pulsed mode laser operation above 150°C could be achieved with uncoated 800 µm long devices. By application of lateral chromium gratings, distributed feedback (DFB) lasers were fabricated. 800 µm high reflectivity (HR) coated devices display threshold currents of 20 mA, high side mode suppression ratios well above 40 dB and stable single mode operation.

High brightness GaInAs/(Al)GaAs quantum-dot tapered lasers at 980 nm with high wavelength stability

High brightness (2 W with M2=3.4) is demonstrated at 980 nm using a gain-guided tapered GaInAs/(Al)GaAs quantum-dot laser. A remarkable low temperature shift (0.09 nm/K) of the emission wavelength is observed. Moreover, at 20 °C, the emission wavelength is quasiconstant as a function of the injected current.

Low threshold high efficiency MBE grown GaInAs/(Al)GaAs quantum dot lasers emitting at 980 nm

Using solid source molecular beam epitaxy, quantum dot lasers with a single active layer have been fabricated. The influence of the waveguide design on basic device properties has been investigated. Through the use of a large vertical cavity a significant reduction in threshold current density could be achieved. Slope efficiencies of more than 1 W/A and absorption levels as low as 2.6 cm -1 could be realized. First high power devices with wall-plug efficiencies of up to 50% were fabricated, demonstrating the high quality of these quantum dot lasers.