Development of an organic dye solution for laser cooling by anti-Stokes fluorescence
- PDF / 48,839 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 95 Downloads / 174 Views
Development of an organic dye solution for laser cooling by anti-Stokes fluorescence Jarett L. Bartholomew, Peter A. DeBarber, Bauke Heeg, and Garry Rumbles1 MetroLaser, Inc. Irvine, CA 92614, U.S.A. 1 Center for Electronic Materials and Devices, Imperial College, London, U.K. ABSTRACT Several independent groups have observed optical cooling by means of antiStokes luminescence in condensed media. The most promising materials are grouped into two categories: ion-doped glasses and organic dye solutions. It is this latter group that we focus our efforts on. Recent studies by our group show that irradiating a solution of rhodamine 101 in the long wavelength wing of the absorption spectrum results in the observation of optical cooling. To improve upon the initial observation of a few degree drop in temperature requires a better understanding of the conditions and phenomena leading to anti-Stokes luminescence in dye solutions. We develop a thermal lensing experiment to obtain fluorescence quantum yields of various dye solutions. The importance of concentration, choice of solvent, deuteration, and acidification are discussed. INTRODUCTION It is easy to see why the concept of cooling by anti-Stokes luminescence is so counter-intuitive. Not only does our experience with high power lasers teach us that laser light is very capable of heating, cutting, drilling, and vaporizing, early debate concerning the validity of anti-Stokes fluorescence generated much heat. Arguments for the existence of anti-Stokes fluorescence began when Pringsheim rebuffed Lenard’s statement that anti-Stokes fluorescence violates the second law of thermodynamics [1]. This excited exchange heated up in the 1940s concerning the possibility of cooling by anti-Stokes fluorescence [2-4]. Landau settled the argument when he addressed the entropy change that occurs in the process of photoluminescence. Landau showed that the increase in the entropy for photoluminescence lies in the photon gas; the emitted photons are incoherent and occur over 4π steradians, thus accounting for the necessary increase in entropy. Today, several groups have observed cooling by anti-Stokes luminescence [5-9]. In this paper, we examine specific factors affecting the anti-Stokes cooling performance including concentration, choice of solvent, deuteration, and acidification. We first present a brief synopsis of the theory underlying the phenomenon of cooling by anti-Stokes luminescence. This is followed by development of a thermal lensing experiment to measure fluorescence quantum yields for various dye solutions. The results of the thermal lensing experiments are presented and discussed. And finally, we conclude by making recommendations for developing optical refrigeration based on dye compounds.
G1.7.1
BACKGROUND THEORY The physics behind laser cooling by anti-Stokes luminescence in dyes relies on excitation into the low energy absorption wings of the dye spectrum. The excited states are deactivated by a combination of decay paths. It is the non-radiative pathways that lead to an
Data Loading...
