Optical refrigeration inches toward liquid-nitrogen temperatures

Superconductivity, longand mid-wave IR detectors, and ultrastable laser cavities that operate in the 77–150K temperature range can all benefit from vibration-free cooling.1 Currently, such low temperatures can only be achieved using cryogenic gases or liquids, solid cryogens, or mechanical refrigerators. Unfortunately, these coolers require regular attention, introduce vibrational noise, and are subject to mechanical wear over time. Many space-based applications (particularly ultra-stable laser cavities) cannot tolerate these drawbacks. All-solid-state cryocoolers are therefore desirable because of their inherent vibration-free operation and potentially long lifetime. Optical refrigeration (i.e., anti-Stokes fluorescence cooling) is the only solid-state cooling technology capable of reaching cryogenic temperatures. Anti-Stokes cooling—in which a doped crystal is excited by a laser with a wavelength that is longer than the average wavelength of the resulting fluorescence, thus leading to cooling of the crystal—was first suggested by Peter Pringsheim almost 90 years ago.2 It was not actually observed, however, until years after the invention of lasers and the availability of high-purity host materials. The first demonstration of optical refrigeration, reported in 1995, used a fluorozirconate glass doped with ytterbium (Yb). The resulting material is known as Yb3C: ZBLANP.3 Cooling occurs when low-entropy laser light (tuned to a slightly lower energy than the mean fluorescence of a material) is absorbed, thus giving rise to efficient fluorescence generation and escape. On average, each pump photon removes vibrational energy (i.e., phonons) from the cooling sample after being absorbed and re-emitted. Figure 1. Schematic of our astigmatic Herriott cell. The geometry of the cell enables laser light (red) to be trapped inside of the crystal, ensuring more than 95% absorption. R1x;y D 50cm, R2x D 1, and R2y D 50cm, where R1 and R2 are the radii of curvature of the spherical and cylindrical mirrors, respectively. x;y : Launching angle. W : Crystal length, width, and height (Wx D Wy D W ).