Sequential two-photon excitation processes ofNd3+ions in solids

The frequency-doubled and -tripled outputs of a high-power pulsed Nd-doped yttrium aluminum garnet laser were used to excite ${\mathrm{Nd}}^{3+}$ ions in hot crystals of ${\mathrm{Y}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$ and ${\mathrm{Y}}_3$ ${\mathrm{Ga}}_5$ ${\mathrm{O}}_{12}$ and in a commercially available Nd-doped lithium silicate glass (ED-2). The fluorescence emission occurring as a result of two-photon excitation processes was studied for both of these excitation wavelengths. The variations of the intensities of the emission processes with laser pulse width show that the excitation processes are sequential stepwise absorption processes with real resonant intermediate states. A time-resolved spectroscopy technique developed previously was used to determine the excited-state absorption cross sections which are shown to be consistent with theoretically predicted values. The frequency-tripled laser excitation process terminates on a 5d level of ${\mathrm{Nd}}^{3+}$, leading to a cross section over two orders of magnitude greater than that associated with the process occurring after frequency-doubled laser excitation which terminates on a level of the 4f configuration. Radiative and radiationless relaxation processes between 5d and 4f levels are also observed in these systems.