Enhanced absorption Hanle effect on the Fg=F->Fe=F+1 closed transitions

We analyse the Hanle effect on a closed $F_g\to F_e=F_g+1$ transition. Two configurations are examined, for linear- and circular-polarized laser radiation, with the applied magnetic field collinear to the laser light wavevector. We describe the peculiarities of the Hanle signal for linearly-polarized laser excitation, characterized by narrow bright resonances at low laser intensities. The mechanism behind this effect is identified, and numerical solutions for the optical Bloch equations are presented for different transitions.Comment: to be published in J. Opt. B, special issue on Quantum Coherence and Entanglement (February 2001

GaSb-based VECSELs emitting at around 2.35 µm employing different optical pumping concepts

We report on the characteristics of (AlGaIn)(AsSb)-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) emitting at wavelengths around 2.35 µm. For barrier-pumped VECSELs mounted substrate-side down without substrate thinning, typical room temperature cw output powers of 2 mW were achieved, limited by premature thermal rollover due to strong device overheating. The thermal impedance of the VECSEL semiconductor chip could be considerably reduced by bonding an intra-cavity polycrystalline CVD diamond heat spreader to the top surface of the chip. This way, at -18°C a maximum cw output power of 0.6 W and a slope efficiency of 10% were obtained for a multiple transverse mode output beam limited by the available pump power rather than by thermal rollover. Optimising the resonator for TEM(ind 00) mode operation (M(exp 2) about 1.1), an output power exceeding 0.4 W was achieved. To reduce the large quantum deficit of more than 50% inherent to barrier-pumped (1.06 µm pump wavelength) GaSb-based VECSELs which emit at wavelengths above 2 µm, we realized a first in-well pumped (AlGaIn)(AsSb) VECSEL where the pump light is absorbed directly in the quantum wells, with the amount of absorbed light enhanced by a higher order microcavity resonance. Using a pump wavelength of 1.94 µm, the quantum deficit is reduced to only 18% and an output power of 5mW, limited by the available pump power, and a slope efficiency of 10% were achieved. Further optimisation of the pump optics is expected to result in a significant increase in device performance