Seth D. Melgaard

Optical refrigeration to 119 K, below National Institute of Standards and Technology cryogenic temperature

We report on bulk optical refrigeration of Yb:YLF crystal to a temperature of ~124 K, starting from the ambient. This is achieved by pumping the E4-E5 Stark multiplet transition at ~1020 nm. A lower temperature of 119±1 K (~-154C) with available cooling power of 18 mW is attained when the temperature of the surrounding crystal is reduced to 210 K. This result is within only a few degrees of the minimum achievable temperature of our crystal and signifies the bulk solid-state laser cooling below the National Institute of Standards and Technology (NIST)-defined cryogenic temperature of 123 K.

Local Laser Cooling of Yb:YLF to 110 K

Minimum achievable temperature of ~110 K is measured in a 5% doped Yb:YLF crystal at λ = 1020 nm, corresponding to E4-E5 resonance of Stark manifold. This measurement is in excellent agreement with the laser cooling model and was made possible by employing a novel and sensitive implementation of differential luminescence thermometry using balanced photo-detectors.

Solid-state optical refrigeration to sub-100 Kelvin regime.

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.

Identification of parasitic losses in Yb:YLF and prospects for optical refrigeration down to 80K

Systematic study of Yb doping concentration in the Yb:YLF cryocoolers by means of optical and mass spectroscopies has identified iron ions as the main source of the background absorption. Parasitic absorption was observed to decrease with Yb doping, resulting in optical cooling of a 10% Yb:YLF sample to 114K ± 1K, with room temperature cooling power of 750 mW and calculated minimum achievable temperature of 93 K.

Demonstration of an optical cryocooler

We present the first observation of cryogenic operation in an all-solid-state refrigerator. A temperature drop of ~150 K is demonstrated in a 0.2 cm3 rare-earth doped fluoride crystal (Yb:YLF) using anti-Stokes fluorescence, at a record cooling power of 110 mW. Lowest electronic transition within Yb3+ Stark manifold along with cavity enhanced absorption and thermal-load management were key in achieving this operation. We show that temperatures down to 100 K are achievable in this arrangement given sufficient absorbed laser power at 1020 nm.

Materials for Optical Cryocoolers

Vibration-free cooling of detectors to cryogenic temperatures is critical for many terrestrial, airborne, and space-based instruments. Cooling of solids by anti-Stokes fluorescence is an emerging refrigeration technology that is inherently vibration-free and compact, and enables cooling of small loads to cryogenic temperatures. In this Highlight, advances in laser-cooling of solids are discussed with a particular focus on the recent breakthrough laser cooling of Yb3+-doped YLiF4 crystals to 114 K. The importance of the material structure, composition, and purity of laser-cooling materials and their influence on the optical refrigerator device performance is emphasized.

First solid-state cooling below 100K

Abstract : Advances in material purity and laser light absorption offer new possibilities for vibration-free cryogenic cooling. Material properties change as a function of temperature, and cryogenic refrigeration allows us to obtain very useful properties not available at higher temperatures. For instance, in the temperature range 77 150K, superconductivity, long- and mid-wave IR detectors, and ultra-stable laser cavities become usable.1 Currently, these low temperatures are reached using liquid or solid cryogens or mechanical refrigerators. Unfortunately, liquids and solids require regular attention to refill after evaporating away, and mechanical refrigerators introduce vibrational noise and mechanically wear over time. Space-based applications, and ultra-stable laser cavities in particular, cannot tolerate these drawbacks. A solid-state solution is preferable for its inherent vibration-free operation and potentially long lifetime. Optical refrigeration via anti-Stokes fluorescence is currently the only solid-state cooling technology capable of reaching cryogenic temperatures.

Optical refrigeration progress: cooling below NIST cryogenic temperature of 123K

We have achieved cryogenic optical refrigeration with a record low temperature in optical refrigeration by cooling 5% wt.Yb:YLF crystal to 119K ±1K (~-154 C) at 1=1020 nm corresponding to its E4-E5 Stark manifold resonance with an estimated cooling power of 18 mW. This demonstration confirms the predicted minimum achievable temperature (MAT). Further cooling is achievable as shown by measurements of a doping study where a 10% wt. Yb:YLF crystal with reduced parasitic heating has predicted cooling below 100K (~-173K).

Laser cooling of a semiconductor load to 165 K

We demonstrate cooling of a 2 micron thick GaAs/InGaP double-heterostructure to 165 K by means of an optical refrigerator. Cooler is comprised of Yb3+-doped YLF crystal, pumped by 9 Watt near E4-E5 Stark manifold transition

Laser cooling of a semiconductor load using a Yb:YLF optical refrigerator

We demonstrate cooling of a 2 micron thick GaAs/InGaP double-heterostructure to 165 K from ambient using an all-solid-state optical refrigerator. Cooler is comprised of Yb3+-doped YLF crystal, pumped by 9 Watt near E4-E5 Stark manifold transition.