Laser Cooling of CdSe/ZnS Quantum Dots
Authors: Fontenot Ross , Mathur Veerendra , Barkyoumb John , Mungan Carl , Tran Thanh 
Published: February 2020

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 of 100 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 determined that the optimum laser excitation wavelength for CdSe/ZnS anti-Stokes emission is 647 nm. This wavelength indicates that the second-harmonic longitudinal optical phonon (2LO) is strongly coupled to the laser photons. Using this wavelength, we were able to cool a 4-mL colloidal solution containing 2 mg of CdSe/ZnS per 1 mL of toluene by 2.3°C from room temperature. However, it took several hours to cool the liquid owing to the convectional cooling produced by the small-diameter laser beam, which was only cooling 3% of the volume of the solution.

To reduce this problem, we added a beam expander to irradiate the entire sample area and speed up the cooling process. In addition, we split the beam to simultaneously measure the changes in temperature of CdSe/ZnS colloid and of pure toluene. Photothermal techniques have also been explored to accurately measure the temperature of the CdSe/ZnS material. In addition, polymer hosts have been prepared for the CdSe/ZnS quantum dots.