Cooling goes cryogenic

Caren B. Les, caren.les@photonics.com

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,” he said.

“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,” he added.

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.