Stimulated and Fluorescent Optical Emission in Ruby from 4.2° to 300°K: Zero-Field Splitting and Mode Structure

The spectral character of the stimulated emission from ruby has been investigated; we found that the zero-field splitting $\text{(0.38\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}})$ of the ${}^4{A}_2$ ground level and the cavity optical length and its reflectivity determine the spectral character of this emission. At 300°K the system is induced into oscillation in a number of cavity modes separated by ${\text{Δv̄\hspace{0.17em}=\hspace{0.17em}(2lε}}^{\frac{1}{2}}{)}^{\text{−1}}$ , where l is the length of the resonator and ${\epsilon }^{\frac{1}{2}}$ is the index of refraction for the ordinary ray. At $\text{T≅80°}\mathrm{K}$ the ${\mathrm{Ē(}}^2\mathrm{E)\hspace{0.17em}\to \hspace{0.17em}\pm }\frac{3}{2}{\mathrm{(A}}_2)$ and the ${\mathrm{Ē(}}^2\mathrm{E)\hspace{0.17em}\to \hspace{0.17em}\pm }\frac{1}{2}{(}^4{A}_2)$ transitions were identified in the stimulated emission spectrum. Each can oscillate in an axial cavity mode (or modes) plus an off-axis mode at the center of the transition. At 4.2°K the system oscillates in off-axis modes, unless the length of the cavity l is such that it has axial resonances coincident with the transition frequencies. An analysis of the temperature dependence of the oscillation threshold for the separate transitions is presented and is compared with the observed behavior.