Absorption of 6328-A radiation by Cr(+3) ions in the (2)E level is used to determine the population density of this level in a ruby crystal under optical pumping. This method allows a much more accurate and simple determination of the excited state population than has been available previously.
Absorption transitions between two excited optical states in ruby have been studied by comparing the absorption coefficients measured under optically pumped and unpumped conditions. The spectrum has been determined in the π-and σ-polarizations in the photon energy range from 5,500 to 45,000 cm -1 . Several absorption bands have been found which arise from the transitions from the excited states t 2g 3 2 E g and t 2g 3 2 T 1g of the Cr 3+ ion. From the locations and intensities of these bands, the terminal states of the observed transitions are assigned to the charge transfer state, the t 2g 3 2 T 2g state, and the doublet states belonging to the electron configuration t 2g 3 e g . Calculated energies between these states and the metastable states, t 2g 3 2 E g and t 2g 3 2 T 1g , are in good agreement with the peak energies of the observed absorption bands. The present experiment gives new information inaccessible by the ordinary ground state absorption measurement, such as the locations of several doublet states with the configuration t 2g 2 e g and the intensities of doublet-doublet transitions in ruby.
Depopulation of ground-state Cr ions in ruby (DGS) under optical pumping with a Xe flash light has been measured as a function of time. Experiments have been made by means of measuring the decrease in absorbing power for a probe light (5790 Å or 5461 Å) from a Hg lamp. DGS reaches its maximum value after some delay from the peak of Xe light, and decreases after discontinuation of optical pumping with a time constant of 3.4 ms, which is the lifetime of metastable-state 2E. During laser oscillation mean value of DGS is forced to take a constant value, which varies with the condition of laser oscillation. So long as the input energy to the Xe lamp is not altered, the shorter the duration of Xe flash light is, the higher becomes the maximum value of DGS. On the basis of the rate equation, these experimental results have been explained.
The absorption cross sections of the Cr3+ ion in the metastable states 2E(Eand2A) of lightly doped ruby (\ensuremath{\sim}0.01%) have been measured as a function of polarization at temperatures of 30, 105, and 296\ifmmode^\circ\else\textdegree\fi{}K in the region from 2300 to 7000 \AA{}. The lifetime of these states was measured by observing the decay rate of the excited-state absorption, and agreed with the value of the measured fluorescence decay rate at various temperatures. The population of the excited states was measured by the change of the area under the first two vibronic sidebands at low temperatures. At room temperature the population was measured by the change of transmitted intensity of a ruby laser.
A spatially periodic distribution of excited electronic states is produced in ruby which is placed into the interference region of two light beams derived from a pulsed laser (wavelength 530 nm). The spatially modulated population density acts like an optical grating and is detected by diffraction of a quasi-cw laser beam (488 nm). The lifetimes of the population density gratings are measured for ruby crystals with 0.05 to 5% Cr-doping and compared with the fluorescence lifetimes of the R-lines (694 nm). For the 0.05% ruby both lifetimes are nearly equal. At higher Cr-concentrations the grating decay is considerably faster than the fluorescent decay. The grating lifetimes are independent of the grating period A = 1 to 100 μm. From this observation an upper limit of the diffusion constant for energy transfer between excited electronic states in ruby is estimated.
The degree of the decrease in the population of ground-state Cr ions in ruby brought about by optical pumping with Xe flash light has been measured as a function of time under Q-switching operation. The measurement has been made by observing the decrease in absorbing power of the ruby crystal for a probe light (Hg 5461Å), (γ), while Q-switching with a rotating prism or saturable absorber (Cryptocyanine). Observations of rapid change of (γ) due to the emission of giant pulse laser light have been carried out, with the very satisfactory results. At the occurrence of a giant pulse, the higher the initial value of (γ) is, the lower becomes the final value of (γ).