Caren B. Les, email@example.com
ALBUQUERQUE, N.M. – All-solid-state laser cooling, or “optical refrigeration,”
a technique that can be applied to airborne and space-borne sensors, has been demonstrated
by a team at the University of New Mexico under the direction of Mansoor Sheik-Bahae,
a professor in the physics and astronomy department.
“Currently, standard multistage thermoelectric (Peltier)
coolers are capable of cooling a device (i.e., a detector) only to 170 K with diminishing
efficiency and cooling power. We have broken this barrier by laser cooling [an ytterbium-doped
LiYF4 crystal] to 155 K and 90 mW of heat lift under nonoptimal conditions,”
Sheik-Bahae said. In laser cooling of solids, heat is removed through the annihilation
of lattice vibrations in the process of anti-Stokes fluorescence.
“Based on our ongoing modeling efforts and supporting spectroscopic
measurements, cooling to near liquid nitrogen (77 K) temperatures should be possible
once optimum conditions and reasonable improvements in material purity have been
implemented,” added Denis Seletskiy, lead author, senior graduate student
and major contributor to the research.
Graduate students Seth Melgaard (left) and Denis Seletskiy (right),
members of Mansoor Sheik-Bahae’s research group at the University of New Mexico,
are conducting a spectroscopic study of cooling efficiency of the ytterbium-doped
YLF crystal. Photo courtesy of Mansoor Sheik-Bahae.
Infrared photon detectors and focal plane arrays must be cooled
to become highly sensitive (low dark current), Sheik-Bahae explained. He added that
many such detectors currently use mechanical cryocoolers (e.g., Stirling coolers),
which are relatively bulky and introduce microphonic noise due to vibrations. Solid-state
optical cryocoolers will be compact and vibration-free and have long lifetimes with
very low thermal jitter, he said. The lightweight requirement makes such cryocoolers
specifically suited for space-based and airborne applications.
Seletskiy noted that they have started working on proof-of-principle
experiments where they are aiming to demonstrate cooling of the detectors. “We
have already demonstrated an ytterbium-based cryocooler to lower a temperature of
5 micrograms of gallium arsenide semiconductor heterostructure to 165 K,”
“The key insight was to exploit the sharp Stark manifold
resonances in an ytterbium-doped YLF crystal (crystal field splitting),” Sheik-Bahae
said. Seletskiy added that, unlike in glass hosts previously used in laser cooling,
ytterbium resonances are preserved due to long-range order of the host crystal. “By
tuning the excitation laser to the lowest-energy resonance, we were able to utilize
maximum cooling efficiency of the process and thus achieve cryogenic operation,”
he said. In addition, cavity enhancement of the pump absorption and careful thermal
management played important roles in the advance.
To minimize parasitic heat load from the environment, the researchers
conducted cooling experiments in high vacuum and used a sample chamber with coating
designed to minimize radiative (blackbody) load on the sample. They also designed
and implemented a noncontact temperature measurement technique.
“Another essential factor in reaching this milestone was
the high quality of the crystal that was grown by collaborators at the University
of Pisa in Italy under professor Mauro Tonelli,” Sheik-Bahae said. “They
managed to grow relatively high-concentration Yb-doped crystals with extreme purity
and essential requirements for low-temperature operation in laser cooling,”
The researchers said they will be addressing challenges such as
the need for higher-quality (purity) material synthesis in rare-earth doped crystals
as well as semiconductors with high quantum efficiency and low parasitic absorption.
“Our modeling predicts that factor-of-four improvement in purity of rare-earth
doped crystals can lead to cooling below 100 K,” Seletskiy said. Possible
applications of solid-state laser cryocooling include superconducting electronics,
solid-state spintronics and, eventually, quantum computing.
The researchers will also be working on integration with photovoltaic
devices to recycle waste fluorescence to improve overall efficiency and on development
of compact, efficient (diode-based) pump laser sources.
The team, which included researchers from the Los Alamos National
Laboratory in New Mexico, conducted its investigation under a multi-university grant
from the U.S. Air Force Office of Scientific Research, based in Arlington, Va. The
study was published online in Nature Photonics on Jan. 17, 2010.