We report and characterize sub-kHz linewidth operation of an AlGaInP-based VECSEL system suitable for addressing the narrow cooling transition of neutral strontium atoms at 689 nm. When frequency-stabilized to a standard air-spaced Fabry-Perot cavity (finesse 1000) via the Pound-Drever-Hall (PDH) technique, it delivers output power >150 mW in a circularly-symmetric single transverse mode with low frequency and intensity noise. The optical field was reconstructed from the frequency noise error signal via autocorrelation and the Wiener-Khintchine theorem, leading to an estimated linewidth of (125 ± 2) Hz. Optical beat note measurements were performed against a commercial locked laser system and a second, almost identical, VECSEL system resulting in linewidths of 200 Hz and 160 Hz FWHM, respectively. To the best of our knowledge, this is the first demonstration of a VECSEL compatible with the narrowest of lines (few hundred Hz) used for cooling and trapping atoms and ions.

Coherent backscattering is a coherence effect in the propagation of waves through disordered media involving two or more scattering events. Here, we report on the observation of coherent backscattering from individual atoms and their mirror images. This system displays two important advantages: First, the effect can be observed at low optical densities, which allows to work in very dilute clouds or far from resonance. Second, due to the fact that the radiation of an atom interferes constructively with that of its own image, the phenomenon is much more robust to dephasing induced by strong saturation. In particular, the contribution of inelastically scattered photons to the interference process is demonstrated.

Stable laser sources at 461 nm are important for optical cooling of strontium atoms. In most existing experiments this wavelength is obtained by frequency doubling infrared lasers, since blue laser diodes either have low power or large emission bandwidths. Here, we show that injecting less than 10 mW of monomode laser radiation into a blue multimode 500 mW high power laser diode is capable of slaving at least 50% of the power to the desired frequency. We verify the emission bandwidth reduction by saturation spectroscopy on a strontium gas cell and by direct beating of the slave with the master laser. We also demonstrate that the laser can efficiently be used within the Zeeman slower for optical cooling of a strontium atomic beam.