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Home Events Seminars Archive 2013 Qihua Xiong, Ph.D.

Qihua Xiong, Ph.D.

Laser Cooling of Semiconductors

When Feb 05, 2013
from 04:00 PM to 05:00 PM
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Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of
matter, a phenomenon known as laser cooling or optical refrigeration proposed in 1929 by
Peter Pringsheim. In solid state materials, the cooling is achieved by annihilation of lattice
vibrations (i.e., phonons). Since the first experimental demonstration in rare-earth doped
glasses, considerable progress has been made particularly in ytterbium-doped glasses or
crystals with a recent record of ~110 K cooling from ambient, surpassing the thermoelectric
Peltier cooler. On the other hand, it would be more tantalizing to realize laser cooling in
direct band-gap semiconductors. Semiconductors exhibit more efficient pump light
absorption, much lower achievable cooling temperature and direct integrability into
electronic and photonic devices. However, so far no net-cooling in semiconductors has been
achieved despite of many experimental and theoretical efforts in the past few decades, mainly
on III-V group gallium arsenide quantum wells. Here we demonstrate the first net laser
cooling in semiconductors using cadmium sulfide (CdS) nanobelt facilitated by multiple
longitudinal optical phonon assisted upconversion due to strong and enhanced Fröhlich
interactions. Under a low power excitation, we have achieved a ~40 K and ~20 K net cooling
in CdS nanobelts starting from 290 K pumped by 514 nm and 532 nm lasers, respectively.
The cooling effect is critically dependent on the pumping wavelength, the blue shifting
parameters and the absorption, the latter of which can be evaluated from photoconductivity
measurement on individual nanowire level. Detailed spectroscopy analysis suggests that
cooling to even lower temperature is possible in CdS nanobelt if thermal management is
optimized. Our findings suggest alternative II-VI semiconductors for laser cooling compared
to III-V GaAs-based heterostructures and may find promising applications in the field of
cryogenics with the advantage of compactness, vibration- and cryogen-free, high reliability
and direct integrability into nanoscale electronic and photonic devices.

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