Anti-Stokes phosphors

As should be apparent by now, the rare earths are one of the few cases where such Stokes processes can be studied at room temperature. If we were to study phonon processes using other activators, we would find that we must study them at 4.2 K. We would also find that the phonon spectra were rather complex and difficult to interpret, instead of being simple. Now we will address the so-called Anti-Stokes phosphors where the dominant mechanism is absorption of several phonons to produce a single photon of higher energy. We will find that only certain trivalent lanthanides eire suitable when combined with a selected host lattice if we wish to have high efficiency, i.e.- brightness . 

The unique energy levels of the rare earths have been utilized to produce Anti-Stokes phosphors. These are essentially infra-red to visible light converters. Two, or more, infra-red photons are absorbed to produce one visible photon. Reexamine the energy levels of Pr3+, N d +, Ho3+ and Er3+ again. All of these ions absorb in the infra-red region of the spectrum. If we started with two ions, say Er3+, each of which absorbed an infra-red photon of 6600 cm i, we would end up with two ions in the Iis/2 excited 

It drops off in relation to l/r3, the radius of the ion in the crystal. A three-body exchange process has too low a probability to be of practical value in an Anti-Stokes phosphor. If we reexamine the calculated free-ion energy level diagram, we notice that Yb3+ has a single energy level with no adjacent levels to which relaxation could occur. Therefore, if we add this ion to our material, we should be able to obtain a phosphor which functions as an Anti-Stokes material. The energy level of Yb3+ lies at 10,300 cm-i (9710 A). It just happens that the output of a GaAsrSi laser diode occurs at 9500 500 A. 

The up-conversion (Anti-Stokes) phosphors have been used in solid state indicator lamps incorporated within various electronic equipment for several years before being supplanted by the visible-emitting GaAlAs Si solid state diode. 

A large number of hosts were studied in which Yb3+ and Ln3+ were combined. Obviously, the phonon spectrum will be critical to the overall operation of up-conversion phosphors. Some of the hosts studied are given in the diagram shown on the next page as follows  

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The lanthanides (rare earths) have found use in regard to phosphors, solid state lasers (which are specialized phosphors) and Anti-Stokes phosphors. Indeed, if rare earths were not available, we would not have useful lasers,... 

Since we cited Anti-Stokes phosphors as one area dominated by rare earths, we need to address this technological field. We will find that these phosphors are unique and only made possible by the unusual energy levels encountered in these 14 components of the Periodic Table. 

The following diagram shows the excitation routes that have been cited for both green emission from Er3+ and for blue emission from Tm3+ in Anti-Stokes phosphors ... 

Relative Efficiencies of Anti-Stokes Phosphors Using Er ... 

In this case one ion at Site-1 spin-couples to another ion at Site-2, In the above diagram, i refers to the initial state of each and f is the final state. Thus, the "2-ion" goes from the "B" level to the "A" level, while the "1-ion" simultaneously transforms fi"om the "B" level to the "C" level. Thus, from two excited Ions, we end up with one in the ground state and the other in an energy state double that of the tnltial state. This process has been observed in rare earth activated phosphors which can absorb infrared radiation and convert it to visible light (an Anti-Stokes process). 

Since the spectroscopy of this phosphor is incorrectly described in the book on lamp phosphors [2], we add here, also as an illustration of the theory, a few comments on the spectroscopy. In view of its electron configuration (d ), the Mn ion will be octahedrally coordinated. The emission lines are tabulated in Table 6.3. There is a zero-phonon transition (Sect. 2.1) which at low temperatures is followed by vibronic lines due to coupling with the asymmetric Mn -0 deformation and stretching modes, 1/4 and 1/3, respectively. These uneven modes relax the parity selection rule. At room temperature there occur also anti-Stokes vibronics (Pigs. 6.21 and 6.22). The vibrational modes in the excited state and ground state are equal within the experimental accuracy as is to be expected for the narrow A2 transition [25,26]. The intensity ratio of the Stokes and anti-Stokes vibronic lines agrees with the Bose-Einstein distribution [26]. 

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