In 1929, the German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter. Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption. In solids, the thermal energy is mostly due to the vibrational modes (phonons) of the lattice. Thus, using lasers to cool a solid, one has to irradiate a sample with laser light in the red tail of the absorption spectrum. The material to be cooled would then absorb a photon and absorb extra energy from a phonon to emit a blue-shifted photon of higher energy. By removing these phonons, the material is cooled.

Today, this technique is known as laser cooling of solids or optical refrigeration. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids. Laser cooling also allows for the possibility of portable lasers that require no or smaller external cooling systems because the pump wavelength can be adjusted such that spontaneous anti-Stokes luminescence cooling compensates for the stimulated quantum defect heating. A thermally balanced laser such as this would not suffer from thermal defocusing or heat damage; therefore, such solid-state-lasers could achieve higher output powers. Unfortunately, for cooling to occur in solids, the quantum efficiency of the material must be high and nearly all the anti-Stokes luminescence must leave the material without being reabsorbed.

Recently, much research has been performed on rare-earth ions for laser cooling. However, the cooling efficiency of rare-earth ions approaches zero as the temperature decreases. As a result, they have a theoretical cooling limit about 90 K. On the other hand, semiconductors provide more efficient pump light absorption, the potential for lower temperatures of 10 K, and the ability to directly integrate the material into electronic and photonic devices. This coupled with the strong electron-phonon coupling of Group II-VI semiconductors and core-shell quantum dot structure minimizing the reabsorption of the anti-Stokes luminescence, makes direct bandgap semiconductors viable laser cooling candidate materials.

Owing to the possibility for near-unity quantum efficiency of CdSe/ZnS, we have chosen to investigate CdSe/ZnS quantum dots for laser cooling applications. Recently, we have utilized photothermal deflection techniques to show that CdSe/ZnS can be cooled at a microscopic level. Moreover, we developed a new photothermal technique that is capable of discerning heating or cooling inside a material using nothing but the probe deflection thereby eliminating the need for a Stokes and anti-Stokes laser. Using the information gained from the photothermal results, we have begun to cool CdSe/ZnS polymers. This talk will go over the cooling results as well as our new photothermal technique.