Doubly resonant Raman electron paramagnetic transitions in ruby (Al
2O
3:Cr
3+) 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.
Experimental investigations of nonlinear characteristics of the transfer coefficient (G) and bistability phenomena in a quasioptical resonator with the ruby layer saturated under the electron spin resonance conditions (ESR) are presented. Experiments have been carried out on the special radiophysical complex BURAN /1/ at the electromagnetic wave frequency ν = 75⋅109H3 and in the temperature range T = 0.9 K + 4 K and with the static magnetic field H<6 T. A two-mirror half-symmetric open resonator has the mirror aperture about 2a=8λ, the radius of a spherical mirror R = 15λ , the quality factor Q ≈8⋅103 and operates on the spectral mode TEM0011. A ruby layer (thickness h = λ/4) is placed near the plane mirror.