J. P. Reithmaier

Long-wavelength InP-based quantum-dash lasers

Self-assembled InAs quantum-dash (QD) lasers with emission wavelengths between 1.54 and 1.78 /spl mu/m based on the AlGaInAs-AlInAs-InP material system were grown by gas source molecular beam epitaxy. Threshold current densities below 1 kA/cm/sup 2/ were achieved for 1-mm-long mirror coated broad area lasers with a stack of four QD layers. The devices can be operated up to 80/spl deg/C in pulsed mode and show a high T/sub 0/ value of 84 K up to 35/spl deg/C. In comparison to quantum-well lasers a much lower temperature sensitivity of the emission wavelength was achieved. The temperature shift of /spl Delta//spl lambda///spl Delta/T = 0.12 nm/K is as low as that caused by the refractive index change.

InP based lasers and optical amplifiers with wire-/dot-like active regions

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 µm. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelling. (Some figures in this article are in colour only in the electronic version)

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.

Broad-band wavelength conversion based on cross-gain modulation and four-wave mixing in InAs-InP quantum-dash semiconductor optical amplifiers operating at 1550 nm

Wavelength conversion based on four-wave mixing (FWM) and cross-gain modulation (XGM) is experimentally demonstrated for the first time in a 1550-nm InAs-InP quantum-dash semiconductor optical amplifier. Continuous-wave FWM with a symmetric conversion efficiency dependence on detuning direction and FWM mediated short-pulse wavelength conversion are demonstrated. Using XGM, we have successfully implemented short-pulse wavelength conversion over 10 THz and error-free data conversion of a 2.5-Gb/s data sequence over 7.5 THz. The pulsed XGM experiments suggest that adjacent regions within an inhomogeneously broadened gain spectrum are partially coupled which increases the operational bandwidth, but at the expense of speed.

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.

On the nature of quantum dash structures

We describe a theoretical model for the linear optical gain properties of a quantum wire assembly and compare it to the well known case of a quantum dot assembly. We also present a technique to analyze the gain of an optical amplifier using bias dependent room temperature amplified spontaneous emission spectra. Employing this procedure in conjunction with the theoretical gain model, we demonstrate that InAs/InP quantum dash structures have quantum-wire-like characteristics. The procedure was used to extract the net gain coefficient, the differential gain, and the relative current component contributing to radiative recombination.

Lasing in high-Q quantum-dot micropillar cavities

We present lasing in optically pumped high-Q micropillar cavity lasers with low thresholds and high β factors. The micropillar cavities with diameters between 1.0 and 4.0μm contain a single layer of low density In0.3Ga0.7As quantum dots as active region. Cavity Q factors of up to 23.000 for 4.0μm micropillar cavities and lasing based on less than 70 quantum dots is demonstrated.

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.

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.