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Physics Colloquium - Dr Alexander Salkeld

Monday, 23 September 2019

Physics Department

Dr Alexander Salkeld

University of Michigan


"Prospects for laser cooling on ED-allowed transitions and saturation effects in laser cooling of crystals"

To date, anti-Stokes fluorescent cooling has been the most successful method in cooling rare-earth-doped crystals, utilizing electric-dipole-forbidden f-f transitions of the dopant. Since the initial demonstration of solid-state anti-Stokes fluorescent cooling utilizing Yb3+:ZBLANP in 1995,[1] advances in experimental methods and the purification of materials have led to cooling of a variety of crystalline media,[2] the reduction of minimum attainable temperatures to the cryogenic range,[3] the demonstration of the first optical cryocooler,[4] and the emergence of the concept of self-cooling or radiation-balanced lasers (RBL).[5]

To date, the only coolant ion for which radiation-balanced lasing has been demonstrated is trivalent ytterbium, and the successful host materials for this have been limited to KGW, YLF, and YAG. All successful solid-state cooling experiments have been limited to electric-dipole-forbidden transitions.

Here we explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions. We analyse the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb2+:SrF2, Ce3+:LiCAF and Ti3+:Al2O3. We also report the observation of laser cooling in a new host (Yb3+:KYW) which is a candidate material for radiation-balanced lasing, and benchmark this against Yb3+:YAG. Finally, we predict improved cooling performance when saturation of background impurities can occur.

[1] Epstein, R. I., Buchwald, M. I., Edwards, B. C., Gosnell, T. R. and Mungan, C. E., “Observation of laser-induced fluorescent cooling of a solid,” Nature 377, 500-503 (1995).
[2] Bowman, S. R. and Mungan, C. E., “New materials for optical cooling,” Appl. Phys. B 71, 807-811 (2000).
[3] Melgaard, S. D., Albrecht, A. R., Hehlen, M. P. and Sheik-Bahae, M., “Solid-state optical refrigeration to sub-100 Kelvin regime,” Sci. Rep. 6, 20380 (2016).
[4] Hehlen, M. P., Meng, J., Albrecht, A. R., Lee, E. R., Gragossian, A., Love, S. P., Hamilton, C. E., Epstein, R. I. and Sheik-Bahae, M., “First demonstration of an all-solid-state optical cryocooler,” Light: Science & Applications 10, 1038 (2018).
[5] Bowman, S. R., “Lasers without internal heat generation,” IEEE J. Quantum Elect. 35, 115-122 (1999).

WHEN: Monday 23 September 2019
WHERE: Room 314, Science 3 Building
TIME: 3.00 pm–4.00 pm

All interested are welcome to attend

Light refreshments to follow in Common Room