Methods for laser cooling of fluorescent materials

Abstract

Methods for cooling fluorescent material are provided. A first method includes providing a sample of the material having an elongated direction of light propagation, exhibiting fluorescence at a mean fluorescence wavelength and capable of emitting superradiant pulses with a formation delay time. The method then involves generating a pump pulsed laser beam having a wavelength longer than the mean fluorescence wavelength, a pump power at which superradiant pulses are emitted and a pulse duration shorter than the formation delay time. The pulses are directed onto the sample along the direction of light propagation to produce the superradiant pulses in an anti-Stokes process inducing a cooling of the sample. A second laser cooling method includes a combination of a traditional anti-Stokes cooling cycle and an upconversion cooling cycle, wherein the two cooling cycles act cooperatively to cool the sample.

1 . A method for cooling a fluorescent material, the method comprising the steps of:
 a) providing a sample of the fluorescent material, the sample having an elongated light propagation direction, the fluorescent material exhibiting fluorescence at a mean fluorescence wavelength and being capable of entering a superradiance regime wherein superradiant pulses are emitted with a formation delay time;   b) generating a pump laser beam comprising laser pulses, the generating comprising the substeps of:
 i) selecting a pump wavelength of the pump laser beam that is longer than the mean fluorescence wavelength of the fluorescent material; 
 ii) selecting a pump power of the pump laser beam so as to reach the superradiance regime of the fluorescent material; and 
 iii) selecting a pulse duration of the laser pulses that is shorter than the is formation delay time of the superradiant pulses; and 
   c) directing the laser pulses of the pump laser beam onto the sample of the fluorescent material along the elongated light propagation direction thereof so as to produce the superradiant pulses in an anti-Stokes process inducing a cooling of the sample.   
     
     
         2 . The method according to  claim 1 , comprising a step of mounting the sample of the fluorescent material in a vacuum chamber prior to directing the laser pulses thereonto. 
     
     
         3 . The method according to  claim 1 , wherein the fluorescent material is a solid material comprising a host material doped with ions of a rare-earth element. 
     
     
         4 . The method according to  claim 3 , wherein the host material is one of a glass and a crystal. 
     
     
         5 . The method according to  claim 4 , wherein the host material is selected from the group consisting of a fluoride glass, a fluoro-chloride glass, an oxide crystal, a fluoride crystal and a chloride crystal. 
     
     
         6 . The method according to  claim 3 , wherein the rare-earth element is selected from the group consisting of ytterbium, thulium and erbium. 
     
     
         7 . The method according to  claim 1 , wherein the sample of the fluorescent material is shaped as a cylinder. 
     
     
         8 . The method according to  claim 1 , wherein the sample of the fluorescent material is shaped as a rectangular parallelepiped. 
     
     
         9 . The method according to  claim 1 , wherein the sample of the fluorescent material is a sphere supporting whispering-gallery modes. 
     
     
         10 . The method according to  claim 3 , further comprising adjusting a temperature of the sample of the solid material so as to adjust a threshold value for a number of excited ions in the host material beyond which the solid material enters in the superradiance regime. 
     
     
         11 . A method for cooling a fluorescent material, the method comprising the steps of:
 a) providing a sample of the fluorescent material, the fluorescent material having an absorption spectrum comprising at least one absorption band, each of the at least one absorption band having a corresponding maximum absorption wavelength;   b) illuminating the sample of the fluorescent material with a first pump laser beam having a first pump wavelength that is longer than the corresponding maximum absorption wavelength of one of the at least one absorption band, so as to generate an anti-Stokes cooling cycle; and, simultaneously,   c) illuminating the sample of the fluorescent material with a second pump laser beam having a second pump wavelength, the second pump wavelength being selected for exciting electrons of the fluorescent material so as to generate an upconversion cooling cycle,   
       wherein the anti-Stokes and the upconversion cooling cycles act cooperatively to induce a cooling of the sample of the fluorescent material. 
     
     
         12 . The method according to  claim 11 , comprising a step of mounting the sample of the fluorescent material in a vacuum chamber prior to steps b) and c). 
     
     
         13 . The method according to  claim 11 , wherein the fluorescent material is a solid material comprising a host material doped with ions of a rare-earth element. 
     
     
         14 . The method according to  claim 13 , wherein the host material is one of a glass and a crystal. 
     
     
         15 . The method according to  claim 14 , wherein the host material is selected from the group consisting of a fluoride glass, a fluoro-chloride glass, an oxide crystal, a fluoride crystal and a chloride crystal. 
     
     
         16 . The method according to  claim 13 , wherein the rare-earth element is selected from the group consisting of ytterbium, thulium and erbium. 
     
     
         17 . The method according to  claim 11 , wherein the upconversion cooling cycle comprises an excited state absorption process and a cooperative energy-transfer upconversion process. 
     
     
         18 . The method according to  claim 13 , wherein the solid material is a potassium-lead chloride crystal doped with trivalent erbium ions. 
     
     
         19 . The method according to  claim 18 , wherein the first pump wavelength is selected so that the anti-Stokes cooling cycle comprises  4 I 15/2  and  4 I 13/2  absorption bands of the potassium-lead chloride crystal doped with erbium, and wherein the second pump wavelength is selected so that the upconversion cooling cycle comprises  4 I 15/2 ,  4 I 13/2  and  4 H 912  absorption bands. 
     
     
         20 . The method according to  claim 19  wherein the first and second pump wavelengths are equal to about 1567 and 860 nanometers, respectively.