Raman-scattering transitions between the excited states Ē (2E) and 2Ā (2E) of Cr3+ in αAl2O3 are studied, with the laser radiation (λ=514.5 nm) simultaneously serving as an optical pump and a Raman probe. This gives rise to a quadratic dependence of the Raman signal on the intensity of the incident radiation at low laser powers, which tends to become linear at higher power levels due to the onset of saturation effects. An analysis is made of the tensorial symmetry properties of the scattering, based on group-theoretical selection rules as well as on a perturbative expansion in the spin–orbit coupling parameter. In comparing the observed polarized spectra with the results of the theoretical analysis, proper account is taken of the dichroic features of the broad absorption bands in ruby, which affect the pumping process and the accompanying population in the excited state. An alternative method for measuring any asymmetry in the scattering tensor is described and employed, making use of the relation between Stokes and anti-Stokes Raman intensities for a fixed scattering geometry.
Raman-scattering transitions between the excited states Ē (2E) and 2Ā (2E) of Cr3+ in αAl2O3 are studied, with the laser radiation (λ=514.5 nm) simultaneously serving as an optical pump and a Raman probe. This gives rise to a quadratic dependence of the Raman signal on the intensity of the incident radiation at low laser powers, which tends to become linear at higher power levels due to the onset of saturation effects. An analysis is made of the tensorial symmetry properties of the scattering, based on group-theoretical selection rules as well as on a perturbative expansion in the spin–orbit coupling parameter. In comparing the observed polarized spectra with the results of the theoretical analysis, proper account is taken of the dichroic features of the broad absorption bands in ruby, which affect the pumping process and the accompanying population in the excited state. An alternative method for measuring any asymmetry in the scattering tensor is described and employed, making use of the relation between Stokes and anti-Stokes Raman intensities for a fixed scattering geometry.
Studies of the resonant interaction of radiation from a liquid-nitrogen-cooled ruby laser with a ruby crystal in the temperature range 4.2°–100°K are described. The interaction was measured by observation of the change which occurs in the Cr3+ ground state EPR absorption on laser irradiation. At low radiation intensities, the temperature dependence of this change is shown to be determined by the well-known thermal tuning behavior of the R lines. Precise measurement of the relative laser and ruby-sample absorption frequencies by this technique of double-resonance spectroscopy is described. Under optical saturation conditions, additional temperature-dependent pumping effects occur at low temperatures. In particular, as the ruby temperature is increased from 4.2° to 40°K, the laser-induced change in EPR absorption is observed to undergo a large increase. This increase cannot be explained by a relative thermal shift of the laser and absorption frequencies which is known to be very small in this temperature range. The phenomena are attributed in part to the temperature-dependent homogeneous linewidth which determines the widths of the holes burnt into the inhomogeneous strain-broadened R lines. Calculations indicate that the related temperature-dependent thermalization process which redistributes excited Cr3+ among the 2E levels quantitatively accounts for most of the increase.
The general method developed in a previous paper (see ibid., vol.10, p.345 (1977)) is applied to the problem of resonance Raman scattering in very intense laser fields. At very low intensities the spectrum consists of three monochromatic lines (Rayleigh, Raman Stokes and anti-Stokes) which split into a triplet and two doublets when the laser light, tuned on one of the two transitions connecting the upper state to the two lower sublevels, has a sufficiently high intensity to saturate this transition but not the second one. The Raman and Rayleigh lines are completely mixed when the intensity becomes so high that both transitions are saturated. The corresponding evolution of the spectrum is determined. Optical-pumping effects leading to asymmetries in the fluorescence spectrum are taken into account.
Doubly resonant Raman electron paramagnetic transitions in ruby (Al2O3:Cr3+) have been investigated using dye laser excitation tuned across the range of the Zeeman components of its well-known R1 emission line. With magnetic field B normal to c, the optic axis of ruby, it is seen that the Raman electron paramagnetic resonance (EPR) lines exhibit significant intensity enhancements due to the simultaneous occurrence of ``in resonance'' and ``out resonance,'' as visualized in the Kramers-Heisenberg formalism of inelastic light scattering. The specific Raman EPR features observed in the present study, together with their resonances, however, differ markedly from those observed in our previous investigation with $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$. These differences can be traced to level mixing effects within the Zeeman multiplet of the 4A2 (ground) state on the one hand, and the negligible value of ${g}_{\ensuremath{\perp}}$ for the $\mathrm{E\ifmmode \bar{}\else \={}\fi{}}$ (excited) state of the R1 emission line on the other. Furthermore, we note that with B\ensuremath{\perp}c and under resonant conditions, the experimentally observed \ensuremath{\Delta}m= \ifmmode\pm\else\textpm\fi{}1 pair of Stokes and anti-Stokes Raman lines are not connected by time-reversal symmetry.
The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.
An experimental study of normal and stimulated Raman emission in diamond is presented, including measurements of linewidth, intensity, and angular distribution of radiation having a frequency shift of 1332.0 ${\mathrm{cm}}^{\ensuremath{-}1}$. The diamond crystal formed a Raman resonator; excitation was by a giant-pulse ruby laser. The dependence of normal Stokes emission on laser intensity and the threshold for oscillation were investigated. A value for the Raman gain ($g=6.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ per MW/cm2) was calculated from the measured linewidth ($\ensuremath{\Delta}\ensuremath{\nu}=2.04$ ${\mathrm{cm}}^{\ensuremath{-}1}$) and cross section for scattering, and found to be in good agreement with a value determined from the threshold for oscillation. The angular distributions of anti-Stokes and Stokes emission and absorption for parallel and convergent laser light are also in good agreement with theory: The emission cone angles are strongly dependent on the angle of convergence of the incident light, and it is established that the preferred directions for emission are those making use of the most intense first-order Stokes radiation which is peaked in the forward direction within the Raman resonator.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.