In-well pumped blue GaN-based vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs), also called thin-disk lasers, combine desirable features from several types of solid-state laser devices. Both high optical output power and a nearly diffraction limited beam quality can be simultaneously achieved. Moreover, the comparatively long resonator with an external cavity allows the insertion of additional optical components, such as non-linear crystals for intra-cavity frequency doubling. In this paper, we demonstrate for the first time in-well pumped blue InGaN/GaN multiple quantum well (MQW) VECSELs. The in-well absorption allows pumping through the GaN substrate and, therefore, the possibility to mount a heat sink close to the active zone (Fig. 1). Furthermore, the approach reduces the thermal energy dissipation due to the low quantum deficit between the pump and lasing emission energies. Finally, homogeneous carrier generation throughout the distributed MQW region can be achieved, which is essential for high-performance VECSEL operation. The gain chips were grown by MOVPE on bulk GaN substrates. Ten periods of InGaN double quantum wells were deposited in a resonant periodic gain scheme. A dual-dielectric DBR with center wavelength of 450 nm was deposited on the epitaxial side of the samples. The substrates were then thinned by polishing to a thickness of less than 100 m. We used a N2/dye laser with emission wavelength at 384 nm as pump source. Lasing was demonstrated for two different types of test samples. Figure 2 (a) shows the lasing emission spectrum for a device where the second DBR was directly deposited on the backside of the GaN substrate. Clearly, several longitudinal laser modes are visible which correlate with the thickness of the laser cavity. Figure 2 (b) shows the laser spectrum for the second type of test sample, where an external mirror is used to provide the feedback. Herewith, single longitudinal laser emission was achieved. The laser line width was < 0.08 nm which is close to the resolution limit of the spectrometer used in this experiment. The threshold peak pump power was determined to about 0.75 W. For an estimated pump spot diameter of about 30 m, the threshold pump power was about 100 kW/cm2. The presentation will include a description and results for an all-optical method to determine the thermal resistance of the packaged gain chip. This knowledge is especially important to realize high-power continuous-wave GaN-based VECSELs.