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While passive illumination schemes often utilize a broadband spectral acceptance, the performance of active illumination with a laser is improved by narrowband spectral filtering at the sensor. We present an experimental demonstration of an optical cavity structure that is capable of toggling between two performance limits: narrowband resonant and broadband omni-resonant transmission. To achieve omni-resonance without modifying the cavity, the incident optical field is pre-conditioned by associating each wavelength with a particular incidence angle that enables a broad continuous spectrum to resonate with the cavity. This strategy can help seamlessly combine passive and active illumination in the same system.

Optical power flow along an active cavity having net gain can differ dramatically from that in a passive cavity. We report the first observation of Poynting’s vector reversal in an active cavity under scattering boundary conditions. At a sub-lasing gain, which we call Poynting’s threshold, a null develops in Poynting’s vector, which divides the cavity into sections with oppositely directed energy flows. Furthermore, we demonstrate that the direction of Poynting’s vector at a fixed point in an active cavity can be controllably reversed without changing its magnitude via an intra-cavity passive element, thereby suggesting a potential methodology for optical switching.