Octave-Spanning Mid-Infrared Passive Optical Resonator

Passive optical resonators are widely used for power enhancement, high harmonic generation and sensitive absorption spectroscopy [1] - [4] . The usual way of "filling" such a resonator with light is to couple in through a slightly transparent mirror, via constructive interference. We demonstrate an alternative method by using a thin wedged plate in the cavity, at an angle of incidence (AOI) close to Brewster’s angle. By adjusting the AOI on the plate, the cavity can be operated at different values of finesse without changing components. The main advantage, however, is the possibility of using low-dispersion mirrors, e.g., gold mirrors for the cavity, allowing for ultrabroadband resonant enhancement. We note that related ways of input coupling have been previously demonstrated. A parallel plate, one side of which was antireflection coated [5] and a plate with a slight wedge angle have been applied for input coupling [6] . However, both of these experiments were strongly limited in bandwidth and were conducted with continuous-wave lasers.