On the Validity of the Judd-Ofelt Theory for Two-Photon Absorption in the Rare-Earths
The one-photon absorption theory of JUDD and OFELT has been rather succesfui in describing the oscillator strength of many forced electric-dipole f-f transitions in the trivalent rare-earths.[2,3] Such a success has very often been taken as a vindication of all the assumptions behind the Judd-Ofelt theory when really it shows only that the formalism of representing the intensity of an f-f transition by the sum of phenomenological parameters multiplied by the squares of tabulated reduced matrix elements U(λ)(λ = 2, 4, 6) is valid. There are cases where the Judd-Ofelt theory of one-photon absorption does not lead to accurate prediction Some well-known examples are given by the hypersensitive transitions. These transitions are characterized by large values of U(2) matrix elements and are in general rather sensitive to the environment of the lanthanide ions. Since the Judd-Ofelt theory has also its weak points, it is very important to design new experiments to pinpoint its limitations. We would like to report here on a completely new test of the Judd-Ofelt theory. It is based on a theoretical calculation of AXEon two-photon absorption in the rare-earths. His work is a direct extension of the Judd-Ofelt theory for one-photon absorption to two-photon absorption. The theoretical predictions on relative two-photon absorption cross-sections are even simpler and involve only the U(2) reduced matrix elements. It should be noted here that both the Axe and the Judd-Ofelt theories ignore the band character of the 5d states. In our work, we have observed direct two-photon f-f transitions in the Gd3+ ions in a LaF3 host, using a CW linearly polarized laser beam of about 100 mW power.
KeywordsLine Strength Reduce Matrix Element Polarize Laser Beam Hypersensitive Transition Absorption Theory
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