Fluorescence, anti-Stokes

A Jablonski, Efficiency of anti-stokes fluorescence in dyes, Nature, 131 839-841, 1933. 

Anti-Stokes Fluorescence, Cooling of a Dye Solution by (Zander and... 

Anti-Stokes fluorescence depends on the concentration of thermally activated molecules in the ground state. Therefore, its intensity can be reduced by lowering the temperature. This fact is virtually not used in inorganic luminescence analysis. But in several cases of organic analysis, e.g. in the determination of anthracene in the presence of phenanthrenc10, it has been possible to suppress 90% of the luminescence intensity of the blank1 due to anti-Stokes fluorescence of phenanthrene by decreasing the temperature. 

To determine the conditions for such cooling by anti-Stokes fluorescence, the net power emitted by fluorescence may be considered. This is the difference between the power emitted and the power absorbed abs and can be written as... 

Cooling by anti-Stokes fluorescence will occur for P jt > 0. 

Several conclusions can be drawn from the experiments. Cooling by anti-Stokes fluorescence of a dye solution has been observed apparently for the first time. In contradiction to theoretical considerations by Vavilov [10] an energy yield exceeding unity has been observed and thus does not violate fundamental laws of thermodynamics. However, the cooling effect is very small indeed and therefore difficult to measure, as Landau suspected correctly [6]. To observe anti-Stokes cooling a dye solution with a fluorescence quantum yield very close to unity is required. Furthermore, the dye has to be extremely pure, and the intrinsic solvent absorption needs to be very small. Even so, for 100 mW of incident radiation the cooling power is only on the order of some microwatts. 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

Jablohski A (1933) Efficiency of anti-Stokes fluorescence in dyes. Nature 131 839-840... 

It was realized early that a high quantum efficiency was a prerequisite for realizing cooling of a system by laser light. For example, Dewar 1 in Fig. 1 can only achieve net cooling if the two excited states Pi/2 and Ps/2 predominantly decay by radiative relaxation (resonant and anti-Stokes fluorescence)... 

In addition, the 4f electrons are shielded by the outer 5s and 5p orbitals, and the M-X chemical bond is only minimally affected by a 4f electronic excitation. Consequently, equilibrium bond lengths in the ground and excited states are similar as illustrated in Fig. 8 (right). Excitation and relaxation of Ln ions can therefore be dominated by electronic transitions with only minimal involvement of vibrational modes E kEa, no Stokes shift), thereby ehminat-ing much of the internal heating that is inherent to Stokes-shifted parity-allowed transitions. This key feature makes Ln ions particularly attractive for solid-state laser cooUng because it allows anti-Stokes fluorescence to dominate in some special cases. 

---