Controlled Changes in Spectra of Open Quasi-optical Resonators

Free electromagnetic oscillations in a quasi-optical open resonator, i.e., in a resonator whose basic dimensions are much grater than the operating wavelength λ = 2π/k, are generated by the beams of plane waves propagating in opposite directions between resonator’s mirrors. By moving along the closed paths, the beams recover their phase characteristics when returning at some fixed point thus ensuring stability of the field pattern. There exist many ways for changing spectral characteristics of resonators with simple dielectric or metal mirrors [1]. Thus for example, a low-sized scattering object being placed in the antinode of some high-Q oscillation destroys this oscillation. The corresponding complex eigenfrequency k̄ vanishes from the spectral set Ωk (pure spectrum sparseness). By changing mirror’s curvature, one can control a degree of beams defocusing, and hence, the Q-factor of the corresponding oscillations. In such a situation, with slight growth (reduction) in ∣∣Imk̄∣∣ we can observe actual sparseness (crowding) of the spectrum — nominally the eigenfrequencies k̄ keep their places in the spectral set Ωk but the actual ‘status’ of the oscillations decreases (increases) substantially. A similar result can be obtained by varying the mirror transmission or reflectivity. Through rotating the mirrors or replacing the flat mirrors by the mirrors of curved or broken shape,