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Applied Physics Letters

TWO-PHOTON STEPWISE ABSORPTION IN Er3+ DOPED SALTS

Isaac Richman, N. C. Chang

Published Apr 15, 1967• 5 citations
Publisher

Abstract

The ordinary fluorescence of Er3+ in Y2O3 and LaF3 crystals is observed when the crystals are excited with 6328-Å, He-Ne laser radiation. From the linear power dependence of the 6800-Å fluorescence and the quadratic power dependence of the 5640- and 3900-Å fluorescence on the 6328-Å excitation, a single-photon excitation for the 6800-Å fluorescence and a double-photon stepwise excitation for the 5640- and 3900-Å fluorescence were proposed. The calculated photon flux of the 5640-Å fluorescence based on the two-photon stepwise excitation agrees with the estimated observed value.

DOI
10.1063/1.1754920

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International Conference on Luminescence - 1984•Jan 1, 1984•0 citations
The ordinary fluorescence of Er3+ in Y2O3 and LaF3 crystals is observed when the crystals are excited with 6328-Å, He-Ne laser radiation. From the linear power dependence of the 6800-Å fluorescence and the quadratic power dependence of the 5640- and 3900-Å fluorescence on the 6328-Å excitation, a single-photon excitation for the 6800-Å fluorescence and a double-photon stepwise excitation for the 5640- and 3900-Å fluorescence were proposed. The calculated photon flux of the 5640-Å fluorescence based on the two-photon stepwise excitation agrees with the estimated observed value.
The absorption, fluorescence, and excitation spectra of Er3+(4f11) in LaF3 have been investigated between 3500 and 10 000 Å. Based on an energy-level scheme consistent with all observed spectral lines, a number of states have been identified. The parameters giving the best over-all agreement between the predicted ``free-ion'' spectrum and the spectrum of Er3+ in LaF3 are F2=428.708 cm-1, F4=67.784 cm-1, F6=6.751 cm-1, and ζ=2389.60 cm-1. The average deviation for 11 of these levels is 35.3 cm-1. The possible use of Er3+ in LaF3 for the generation of coherent green light is also discussed.
The analysis of the absorption spectrum of Er3+ in LaF3 is extended into the ultraviolet region to 2000 Å. A number of fluorescence lines recently observed in the region from 3000 to 25 000 Å is also reported. Based on the best over-all agreement between the predicted ``free-ion'' 4f11 SLJ states and 21 experimentally observed levels for Er3+ in LaF3, one finds a set of atomic wavefunctions for the ion based on F2 = 435.7 cm—1, F4 = 67.21 cm—1, F6 = 7.388 cm—1, and ζ = 2350.8 cm—1. Laser experiments are described and the details of coherent emission found at 1.61 μ are presented.
Using the energy level shift induced by a Nd3+ ion on a neighbour ion, the selective excitation of a class of (Nd, Nd) pairs is possible in weakly doped samples with a spectrally narrow laser. The observation of an anti-Stokes fluorescence shows that a pair can absorb two photons. From experiments with a magnetic field, it is shown that using a single laser, the double excitation of moderatly coupled pairs is essentially induced by a double quantum process by which two photons are simultaneously absorbed. The strongly coupled pairs, on the contrary, can only be doubly excited by two lasers of different frequencies. Orders of magnitude of the coupling strength between the two ions are deduced from the difference between these frequencies.
The intensities of optical absorption spectra of Er 3+ in single crystals of yttrium gallium garnet at 700 °K have been shown to fit Judd's theory of 4f-4f transitions with the following parameters: T 2 = 1·44 × 10 -21 s, T 4 = 1·12 × 10 -21 s, T 6 = 1·42 × 10 -21 s. These have been used to estimate Einstein coefficients useful in a discussion of the quantum efficiency of the fluorescence from the 4 S 3/2 state, and the efficiency of infra-red quantum counter action in the material. For pure spontaneous radiation an efficiency of 71% for transitions to the 4 I 15/2 ground state is expected. The quantum efficiency has also been directly measured and a value of 12% was obtained. This is lower owing to competing phonon processes. Using the experimental value for the quantum efficiency, and the above values of T , a theoretical photon gain for quantum counter action of 0·55 × 10 -8 has been calculated.
In this paper, we report the first laser-excited optical double-resonance experiments in rare-earth-doped crystals. Infrared upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+ has been achieved by means of a tunable cw dye laser pump. The quantum efficiency of the upconversion process at 2.03 μ in LaCl3 : Pr3+ is 1.2×10−3, which is an improvement of 4×105 over the highest efficiency found previously with incoherent sources. It is shown that an efficiency of 1 can be easily obtained with higher fluxes and improved crystal quality. Direct excitation of the output fluorescence from the 3P0 level is observed in both LaCl3 : Pr3+ and LaF3 : Pr3+ when the dye laser frequency is resonant with the states of the lower-lying 1D2 manifold in the absence of the infrared input. Detailed assignments of the observed double-resonance transitions have been made from wavelength and polarization measurements.
We excite the (4f2)1s0 state directly through a two photon excited state absorption (from the 1D2 level) and indirectly through excitation of the 4f5d band (using an ArF excimer laser). In Pr3+:LaF3 the 1S0 state lies below the band, and only 1S0 emission is observed. In most hosts examined, it is above the band, and only 4f5d emission is observed. In Pr3+:CaF2 there are sites with each of these properties, and both 1S0 and 4f5d emission are observed, with a relative strength that is concentration dependent.