Three-photon absorption in Nd-doped yttrium aluminum garnet

This paper presents the results of a study of resonantly enhanced, three-, photon absorption in Nd-doped yttrium aluminum garnet (Y3Alg0~2) or Nd: YAG. The trivalent ions of the rare-earth elements, such as Nd'+, are characterized by energy levels which remain relatively sharp even in a crystalline environment, and thus crystalline solids doped with rare-earth ions present interesting systems in which to study resonantly enhanced optical nonlinearities. While a number of different types of optical nonlinearity can be studied in impurity-doped crystals, such as two-photon absorption, "saturated absorption, ' degenerate four-wave mixing, ' and polarization spectroscopy, this paper focuses on. the process of three-photon absorption. 'Through the absorption of three visible photons we are able to excite Nd'+ energy levels which lie approximately 40000 to 70000 cm ' above the ground state, and hence belong to the 4f'Sd' electron configuration. Since Nd: YAG is essentially opaque for radiation of wave numbers greater than 42000 cm ', the technique of three-photon absorption allows us to probe regions of the 4 f '5d' configuration which are inaccessible using one-photon techniques. Some of the single=photon spectroscopic properties of Nd: YAG that are needed for the interpretation of the three-photon-absorption results are presented here. An absorption spectrum of our 1 wt. %, Nd:YAG taken on a Carey model 14 recording spectrophotometer is shown in Fig. 1. The numerous sharp absorption features in the range 2500 to 9000 A are due to the Nd'+ ion. Since the fundamental absorption edge of YAG occurs' at 1920 A, the observed 2400 A cutoff wavelength in Nd:YAG has been interpreted by Krupke' as due to a distortion of the host lattice by the Nd dopant. Figure 2 shows an energy-level diagram of Nd: Y AG which combines our results of Fig. 1 with previously published data. The region between 0 and 70000 cm ' contains a large pumber of energy levels belonging to the 4f 3 electron configuration. Since the 4f electrons are shielded from the crystalline electrostatic field by the closed outlying Ss and Sp shells, energy levels in this region are very narrow. For energies below 21000 cm ', we have plotted the energy levels determined by Koningstein and Geusic including their level identifications. For energies in the range 27000 and 40000 cm ', we have plotted