By Dr. Richard I Epstein, ThermoDynamic Films, LLC, richard.epstein@gmail.com; and Dr. Mansoor Sheik-Bahae, University of New Mexico, msb@unm.edu

Cryocooling devices can be divided between mechanical devices that compress and expand a working fluid and solid-state devices that have no moving parts or fluids. Solid-state cryocoolers have the advantages of eliminating vibrations and frictional losses and wear. On the other hand, currently available thermoelectric solid-state refrigerators have limited cooling ranges and cooling efficiencies.

Over the last couple of decades researchers around the world have been working on optical refrigeration, a new solid-state cooling scheme based on light-matter interactions, and a team of researchers from the University of New Mexico (UNM) and ThermoDynamic Films, LLC (TDF) is currently working to harness the physics of optical refrigeration to develop practical solid-state cryocoolers.

While there are no fundamental physics or thermodynamics barriers to cooling a load, there are some interesting engineering challenges. These challenges mainly focus around thermally coupling the cooling element to the cold load while preventing the vast amount of fluorescent emission from generating unwanted heating. Since the power of waste fluorescence is perhaps 50 times greater than the cooling heat lift, this coupling requires some care. As a rough analogy, it is as though one wants to use a 10 W refrigerator that had a 500 W refrigerator light bulb that can’t be turned off.

Figure 1 illustrates the general approach to building the optical refrigerator the UNM/TDF team is pursuing to reach 91 K. The refrigerator has to connect the cooling element to the load in a way that minimizes heating by waste fluorescence or by heat leakage from the warm environment. The cooling element is attached to a thermal link that has a high thermal conductivity and very low absorptivity for both the fluorescence and the pump light. The thermal link is given a sharply kinked shape so that fluorescence or scattered pump light that enters the thermal link will exit it at the bend and not reach the heat spreader connected to the load.

Optical refrigerators bring a set of valuable capabilities to cryogenic cooling that are not met by traditional approaches such as mechanical refrigerators or cryogens [1]. The most important advantages of an optical refrigerator are its complete lack of vibrations and its high reliability. In some applications it may be significant that optical refrigerators do not generate electromagnetic interference (nor are they affected by it), and that optical units can be used in the presence of strong magnetic fields.

For applications that require reliable vibration-free cooling in the temperature range between 180 K and 90 K, optical refrigerators may be the best solution. Examples include cooled IR cameras, gamma-ray spectrometers, ultra-stable lasers and interferometers and electron cryo-microscopy.

Cooled IR cameras are needed for taking fast or high-resolution images. In recent years, the development of high operating temperature detectors, HOT detectors, has made it possible to use high-performance imagers at temperatures as high as 150 K. This temperature is well within the operating range of optical refrigerators but still far below what is achievable with Peltier devices. Reliable vibration-free optical refrigerators would be especially well suited for satellite-borne systems.

Gamma-ray spectrometers with high-energy resolution and high-detection efficiency are needed to detect and identify nuclear materials. This capability is critical for homeland security applications and for the interdiction of smuggled nuclear material. High-purity germanium (HPGe) gamma-ray spectrometers, cooled to at least 120 K, are the best devices for these applications. Additionally, the cooling system must not produce significant vibrations as even minor mechanical motion generates noise that blurs the gamma-ray spectra.

Ultrastable lasers and interferometers are critical for the most accurate measuring science including gravitational wave detection, cavity quantum electrodynamics, quantum optomechanics and precision tests of relativity. Commercially, the ability to measure and precisely control laser frequencies is important for navigation and communication standards.

Researchers at NIST created an ultrastable laser interferometric cavity from single-crystal silicon that is cooled to 124 K, where the coefficient of thermal expansion of silicon is zero [2]. The cooling system for these cavities has to be essentially vibration-free. Currently, the cavity is cooled with nitrogen vapor, but optical refrigeration is the only cooling technology on the horizon that can be used to make this type of laser cavity portable and rugged.

Electron cryo-microscopy and the related cryo-electron tomography allow investigators to determine the structure of biological macromolecules in the cell. These structures are measured at near-atomic resolution (a few angstroms) by averaging thousands of electron microscope images of cryocooled samples. The samples are generally cooled by liquid nitrogen, with the attendant complexities and size associated with dewars and flow controls. Vibration-free optical refrigerators could be used instead of liquid nitrogen to reduce the complexity and increase the stability of electron cryo-microscopes.

The next few years should see optical refrigerators deployed to cool electronic and optical devices where avoiding vibrations is critical. Additional applications will become viable as the efficiency of optical refrigerators improves and their mass and cost decrease.

Much of the recent work on optical refrigeration is summarized in two recent volumes [3,4]. In optical refrigeration, a nearly transparent solid, either a crystal or glass, absorbs laser light at one frequency and re-emits light at a higher average frequency; this is called anti-Stokes fluorescence or luminescence. When dissipative processes are minimized, the outgoing higher-energy fluorescent photons remove more energy than the cooling element absorbed. The energy difference is due to the removal of thermal vibrations or phonons in the solid, causing the solid to cool.
An example of a material that has shown good optical refrigeration properties is yttrium-lithium-fluoride crystals doped with ytterbium ions: Yb:YLF [5]. Figure 2 shows the seven low-lying energy levels of the ytterbium ions (not to scale).

Pump laser light is tuned to the energy difference between the top of the low-energy group of states and the bottom of the higher energy group; i.e., from level E4 to level E5. The excited ytterbium ions thermalize to levels E5, E6 and E7 by absorbing phonons and then decay to the low-energy group of states.

For laser photons of energy hv and fluorescence photons with average energy hvf, the simplified cooling efficiency nc of the optical refrigeration can be expressed in the equation [6]:

When one considers loss processes such as light absorption by unavoidable impurities in the cooling crystal, the cooling efficiency falls below that of the equation. The left panel of Figure 2 shows a plot of the predicted cooling efficiency when loss processes are taken into account. This efficiency plot is for a particular high purity Yb:YLF crystal that was subsequently cooled to record low temperatures [7].

The vertical axis is the temperature of the crystal and the horizontal axis gives the wavelength of the pump laser. In the bluest region the cooling efficiency is nc= 5 percent and in the red regions the crystal heats (negative cooling efficiency). This plot shows that the minimum achievable temperature for this crystal is 89 K.

The right panel shows the cooling achieved with this crystal [7]. The Yb:YLF crystal was suspended by optical fibers in vacuum in a temperature-controlled “clamshell.” The clamshell absorbs the emitted fluorescence and limits the thermal radiation that can heat the crystal. An Acktar coating on the inside of the clamshell has high absorptivity at the fluorescent wavelengths and low emissivity at thermal wavelengths around 10 microns.

In this experiment, 40 watts of 1020 nm laser light passed through the crystal multiple times to ensure that it was mostly absorbed. The researchers measured the temperature of the crystal by comparing the spectral shape of the fluorescence with that from carefully calibrated reference spectra. As the temperature curve shows, after 15 minutes the Yb:YLF cooled to 91 K, far below the ~170 K that can be achieved with the best thermoelectric coolers.

The Air Force Research Laboratory, the Air Force Office of Scientific Research and DARPA have supported this work through small business and regular research awards.

1) R.I. Epstein, M.P. Hehlen, M. Sheik-Bahae and S.D. Melgaard, “Optical cryocoolers for sensors and electronics” ed. B.F. Andresen, G.F. Fulop, C.M. Hanson and P.R. Norton, 2014.
2) T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle , J. Ye, L. Chen, M.J. Martin, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 2012.
3) R.I. Epstein and M. Sheik-Bahae. Optical Refrigeration: Science and Applications of Laser Cooling of Solids, Wiley-Vch Verlag, 2009.
4) G. Nemova, Laser Cooling: Fundamental Properties and Applications, Pan Stanford, 2016.
5) D.V. Seletskiy, S.D. Melgaard, S. Bigotta, A. Di Lieto, M. Tonelli and M. Sheik-Bahae, “Laser cooling of solids to cryogenic temperatures,” Nature Photonics 4, p 161 – 164, 2010.
6) R.I. Epstein, M.I. Buchwald, B.C. Edwards, T.R. Gosnell and C.E. Mungan, “Observation of laser-induced fluorescent cooling of a solid,” Nature 377. p 500–503, 1995.
7) S.D. Melgaard, A.R. Albrecht, M.P. Hehlen, M. Sheik-Bahae, “Solid-State Optical Refrigeration to sub-100 Kelvin Regime,” Nature Scientific Reports, vol. 6, Article Number: 20380, 2016. 