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Abstract Herein, the role that point defects have played over the last two decades in realizing solid‐state laser refrigeration is discussed. A brief introduction to the field of solid‐state laser refrigeration is given with an emphasis on the fundamental physical phenomena and quantized electronic transitions that have made solid‐state laser‐cooling possible. Lanthanide‐based point defects, such as trivalent ytterbium ions (Yb3+), have played a central role in the first demonstrations and subsequent development of advanced materials for solid‐state laser refrigeration. Significant discussion is devoted to the quantum mechanical description of optical transitions in lanthanide ions, and their influence on laser cooling. Transition‐metal point defects have been shown to generate substantial background absorption in ceramic materials, decreasing the overall efficiency of a particular laser refrigeration material. Other potential color centers based on fluoride vacancies with multiple potential charge states are also considered. In conclusion, novel materials for solid‐state laser refrigeration, including color centers in diamond that have recently been proposed to realize the solid‐state laser refrigeration of semiconducting materials, are discussed.
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Photoluminescent cooling with incoherent light
Optical refrigeration using anti-Stokes photoluminescence is now well established, especially for rare-earth-doped solids where cooling to cryogenic temperatures has recently been achieved. The cooling efficiency of optical refrigeration is constrained by the requirement that the increase in the entropy of the photon field must be greater than the decrease in the entropy of the sample. Laser radiation has been used in all demonstrated cases of optical refrigeration with the intention of minimizing the entropy of the absorbed photons. Here, we show that as long as the incident radiation is unidirectional, the loss of coherence does not significantly affect the cooling efficiency. Using a general formulation of radiation entropy as the von Neumann entropy of the photon field, we show how the cooling efficiency depends on the properties of the light source, such as wavelength, coherence, and directionality. Our results suggest that the laws of thermodynamics permit optical cooling of materials with incoherent sources, such as light emitting diodes and filtered sunlight, almost as efficiently as with lasers. Our findings have significant and immediate implications for design of compact all-solid-state devices cooled via optical refrigeration.
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- Award ID(s):
- 1952841
- PAR ID:
- 10594380
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Photonics
- Volume:
- 9
- Issue:
- 8
- ISSN:
- 2378-0967
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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