Collective Behaviour of Cr³⁺ ions in Ruby Revealed by Whispering Gallery Modes

We present evidence for collective action of Cr$^{3+}$ ion impurities in a highly doped ruby crystal at microwave frequencies. The cylindrical geometry of the crystal allows for the creation of a superradiant, or "spin-mode" doublet, with spatial structure similar to that of microwave whispering gallery modes (WGMs). This results in a strict criteria of selection rules regarding the interaction of resonant WGMs and spin-modes; namely that only modes with the same wavenumber and azimuthal phase may interact. What results is an avoided level crossing between the two, in which both WGM doublet constituents are seen to interact with the spin resonance. We demonstrate that a four harmonic oscillator model is necessary to accurately describe this result.