Influence of Upconversion Processes in the Optically-Induced Inhomogeneous Thermal Behavior of Erbium-Doped Lanthanum Oxysulfide Powders

Lasers are commonly known as sources of heat used to burn or cut through tissue and other materials, but when shone on certain solids doped with rare-earth ions, a laser can cool down the material. The basic principle that anti-Stokes fluorescence might be used to cool a material was first postulated by P. Pringsheim in 1929. Twenty years later A. Kastler suggested that rare-earth-doped crystals might provide a way to obtain solid-state cooling by anti-Stokes emission (CASE).

An anti-Stokes emission occurs when a material emits more energy than it absorbs. The key is to shine photons onto the material that fall short of the energy needed to excite the rare earth ions to a higher energy level. The material uses the energy from thermal vibrations to make up the difference. Whenever a quantum of these thermal vibrations is absorbed, an ion is excited to a higher energy state and then fluoresces, carrying energy out of the system and cooling the material. The group is currently investigating Er

The group is currently investigating Er³⁺-doped Lanthanum oxysulphide crystal powders as a promising candidate for all-optical cooling. This material, with maximum phonon energy of about 400 cm^–1 exhibits an efficient infrared to-visible upconversion under excitation in the 800–870 nm spectral region. The team has analyzed the thermal response of Er-doped lanthanum oxysulphide powders by pumping with a tunable femtosecond laser working at 80 MHz. Figure (a) shows a typical video frame of the sample after irradiation. As can be seen, the temperature distribution is rather inhomogeneous showing a sharp distribution of hot spots. Figure (b) displays the average temperature as a function of time measured in the three shown zones.  Initially, the temperature of the sample rises in all the zones from room temperature (~ 24 °C) till it reaches a stationary regime when the thermal load deposited on the material is compensated by the fluorescent losses. As can be seen, in the hot zone, E2, the temperature falls by 2 °C in forty minutes whereas in the wider one, E1, the average temperature drops about 0.5 °C.  These results are in good agreement with the expected random propagation of radiation in a region with a randomly distributed dielectric constant. Finally, it is worthy to point out that the average temperature of the sample may cool after the initial transient heating due to the infrared to visible upconversion processes that can offset the heat load deposited in the doped powder.

(a) Camera video frame showing discrete thermal zones after pumping with 300 mW at 842 nm in a 2 mol% Er³⁺– doped La₂O₂S powder sample. (b) Average temperature as a function of time measured in the three shown zones in (a).