InP quantum dot based semiconductor disk laser emitting at 655 nm

Abstract

Summary form only given. Semiconductor disk lasers (SDLs), also known as vertical-external surface-emitting-lasers (VECSELs) are an ideal choice to reach high output powers. Further advantageous characteristics [1] of this laser type, as e.g. superior beam quality with a radial symmetric TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> beam profile, are provided by the external cavity. A further big advantage of VECSELs is given by the possibility of bandgap engineering. By proper adjustment of the semiconductor material and their composition, many different wavelength areas can be covered. The use of quantum dot (QD) layers instead of quantum wells (QWs) should further lead, according to theory [2], to broader gain spectra as well as to lower laser thresholds accompanied by decreased temperature sensitivity.We present a continuous-wave VECSEL system, based on a RPG structure with multiple InP QD layers (see Fig. 1a), emitting around 655 nm. All samples of this study were fabricated by metal-organic vapor-phase epitaxy. The seven single InP QD layers are embedded in a separate confinement heterostructure (SCH) which consist of tensile strained (Al0.1Ga0.9)0.52In0.48P barriers and (Al0.55Ga0.45)0.52In0.48P cladding layers. Below the active region an Al0.45GaAs / AlAs distributed Bragg reflector (DBR) consisting of 55 λ/4 pairs to generate a reflectivity of R>99.9 % is fabricated. The QD characteristics in ensemble and micro-photoluminescence investigations indicate that really the QDs are contributing to this emission. Auto-correlation measurements on a sample with a single QD layer, proves that the luminescence consists of emission of individual QDs. Measurements of the standard laser parameters reveal maximum output powers of 1.4 W at a low emission wavelength ~ 654 nm with a slope efficiency of ηdiff = 25.4% (Fig. 1b). Laser characteristics like high output power at the mentioned wavelenth and the possibility of inserting optical intra-cavity elements for wavelength selection, frequency doubling and tuning, given by the external cavity, make the here introduced VECSEL a very well suited laser source for medical applications like photodynamic therapy (in the red sprctral range) or for scientific and bio-technological applications as coherent light source (frequency doubled to ultraviolet spectral range) for micro-photoluminescence of nitride structures and for luminescence microscopy on biological samples.