Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling, or optical refrigeration, which was proposed by Pringsheim in 19291. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling2, and laser cooling of ultradense gas has been demonstrated by collisional redistribution of radiation3. In solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence. Since the first experimental demonstration in glasses doped with rare-earth metals4, considerable progress has been made, particularly in ytterbium-doped glasses or crystals: recently a record was set of cooling to about 110 kelvin from the ambient temperature, surpassing the thermoelectric Peltier cooler5,6. It would be interesting to realize laser cooling in semiconductors, in which excitonic resonances dominate7,8,9, rather than in systems doped with rare-earth metals, where atomic resonances dominate. However, so far no net cooling in semiconductors has been achieved despite much experimental10,11,12 and theoretical7,8,9,13,14 work, mainly on group-III–V gallium arsenide quantum wells. Here we report 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. We attribute the net laser cooling in cadmium sulphide nanobelts to strong coupling between excitons and longitudinal optical phonons (LOPs), which allows the resonant annihilation of multiple LOPs in luminescence up-conversion processes, high external quantum efficiency and negligible background absorption. Our findings suggest that, alternatively, group-II–VI semiconductors with strong exciton–LOP coupling could be harnessed to achieve laser cooling and open the way to optical refrigeration based on semiconductors.
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We thank M. Sheik-Bahae and R. Merlin for helpful discussions. Q.X. acknowledges the support from the Singapore National Research Foundation through a fellowship grant (NRF-RF2009-06). This work was also supported in part by the Singapore Ministry of Education via a Tier 2 grant (MOE2011-T2-2-051) and start-up grant support (M58113004) from Nanyang Technological University.
The authors declare no competing financial interests.
This file contains Supplementary Text and Data comprising: Section 1- Electrical and Optoelectronic Properties of CdS Nanobelts; Section 2- Optical Properties of CdS Nanobelts; Section 3- Laser Cooling of CdS Semiconductor Nanobelts and Section 4-Estimation of the Luminescence Extraction or Escape Efficiency ηe. It also includes Supplementary Figures 1-9 and Supplementary References. (PDF 1718 kb)
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Zhang, J., Li, D., Chen, R. et al. Laser cooling of a semiconductor by 40 kelvin. Nature 493, 504–508 (2013). https://doi.org/10.1038/nature11721
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