(Invited) Laser Cooling in Semiconductors

Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling; in solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence.

Here we will show my studies how to use laser to cool whole semiconductor by using unresolved sideband anti-Stokes luminescence upconversion and how to use laser to cool specific optical phonon vibration by using resolved sideband Raman cooling technique. We demonstrate a net cooling by about 40 kelvin in a semiconductor using group-II–VI cadmium sulphide nanoribbons, or nanobelts, starting from 290 kelvin. We use a pump laser with a wavelength of 514 nanometres, and obtain an estimated cooling efficiency of about 1.3 per cent and an estimated cooling power of 180 microwatts. At 100 kelvin, 532-nm pumping leads to a net cooling of about 15 kelvin with a cooling efficiency of about 2.0 per cent. In resolved sideband regime, we experimentally demonstrate spontaneous Raman cooling and heating of longitudinal optical phonon with a 6.23 THz frequency in polar semiconductor zinc telluride nanobelts. We use the exciton to resonate and assist photo-elastic Raman scattering from LOPs due to the large exciton-LOP coupling. The cooling (heating) is mediated by detuning the laser pump to lower (higher) energy sideband, and spontaneous scattering photon resonates with exciton at anti-Stokes (Stokes) side, that beat and photo-elastically attenuate (enhance) the dipole oscillation of the optical phonon. 

References:

1. J. Zhang, D. H. Li, R. J. Chen, Q. H. Xiong, Laser cooling of a semiconductor by 40 Kelvin, Nature, 493, 504-508 (2013)
2. J. Zhang, L. C. Kwek, Q. H. Xiong, Resolved Sideband Raman cooling of optical phonons in semiconductor materiaals, Nature Photonics, under review （2015）