Optical second-harmonic generation of Janus MoSSe monolayer

The transition metal dichalcogenides (TMD) monolayers have shown strong second-harmonic generation (SHG) owing to their lack of inversion symmetry. These ultrathin layers then serve as the frequency converters that can be intergraded on a chip. Here, taking MoSSe as an example, we report the first detailed experimental study of the SHG of Janus TMD monolayer, in which the transition metal layer is sandwiched by the two distinct chalcogen layers. It is shown that the SHG effectively arises from an in-plane second-harmonic polarization under paraxial focusing and detection. Based on this, the orientation-resolved SHG spectroscopy is realized to readily determine the zigzag and armchair axes of the Janus crystal with an accuracy better than ±0.6°. Moreover, the SHG intensity is wavelength-dependent and can be greatly enhanced (～ 60 times) when the two-photon transition is resonant with the C-exciton state. Our findings uncover the SHG properties of Janus MoSSe monolayer, therefore lay the basis for its integrated frequency-doubling applications.     

Improvement of InGaAs/GaAs vertical-cavity surface-emitting lasers by post-oxidation annealing

InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) are fabricated by a thermal selective wet-oxidation confinement technique. Post-oxidation annealing in a nitrogen environment at high temperatures is then conducted to improve the performance of the oxide-confined InGaAs/GaAs VCSELs. The optimum post-oxidation annealing conditions are determined by changing the furnace temperature and annealing time. Compared with a unannealed laser device, the light output power increases by about 12%. An aging test is carried out to examine the reliability of the annealed oxide-confined VCSEL device. The temperature dependence of the lasing wavelength of the annealed oxide-confined VCSELs is also investigated.