Review of condensed matter laser cooling using electric-dipole-allowed transitions

The cooling of solids via laser-excited anti-Stokes luminescence has reached the cryogenic regime and is now being considered for several applications that benefit from vibration-free cryogenic refrigeration. 4f-4f transitions in rare-earth-doped crystals with low phonon energies currently offer the best laser-cooling performance. These transitions, however, are electric-dipole (ED) forbidden and thus have low cross sections, limiting the minimum achievable temperatures in practical materials to the 50-100 K regime. In contrast, ED-allowed transitions may offer greater cross sections and potentially enable lower minimum temperatures or more compact devices. The search for laser cooling with ED-allowed transitions has been an active field of research over the past decades with a wide variety of materials having been studied. This paper reviews the fundamentals of solid-state laser cooling, provides a preliminary theoretical assessment showing that laser-cooling with ED -allowed transitions is in principle possible, and reviews the experimental results reported to date. While cooling with ED-allowed transitions has not yet been realized in the solid state, the existing experimental body of work along with the theoretical considerations presented here suggest that further research on this topic may be fruitful.

Automated summary

Solid-state laser cooling using anti-Stokes luminescence has been studied extensively, with rare-earth-doped crystals showing the best performance. However, the current best materials have low cross sections due to their electric-dipole forbidden transitions, limiting the achievable temperatures. Research on laser cooling with electric-dipole allowed transitions has been active, and this paper provides a theoretical assessment and reviews experimental results for this approach. While laser cooling with electric-dipole allowed transitions has not been achieved yet, further research in this area shows promising potential.