An optical resonator can be described as just a box made of mirrors. If strong enough laser light is shone on the outside it will transmit through one of the mirrors and the photons will start bouncing back and forth inside the box, known as a cavity. As all lasers must have some divergence eventually this bouncing light is destined to spread out and fade away. It is however possible to form lasting stable reflections within the cavity and when this happens the set up is known as an optical resonator. This can be imagined as a standing wave of light in a fixed shape. If a gain medium is present then this design can be turned into a laser with the present photons generating more through stimulated emission. When an active medium is present the resonator is called active and unsurprisingly passive is the name given to when there is no contributing medium.
This study has aimed to examine exceptional points (exceptional points is the technical term) in optical resonators; places where the regular calculus used to describe the behaviour appears not to work and so a different technique needs to be applied. The set up for the experiment was to connect an active cavity with a passive cavity through a mechanical resonator (phonon-photon interactions) which is known as a three mode system. It was found that when the driving frequency of the external electromagnetic field was above or below the mechanical frequency of oscillator, exceptional points of order 2 were found (these are quite common). But when the laser was matched to the frequency then the resonant structure of the oscillators changed to incorporate a very high order exceptional point. When this occurred it was also observed that the mechanical dampening went through the roof as well as the coupling between the lasers and the optical cavities becoming enhanced. These are the first controllable high order exceptional points to be generated in an optical cavity and hopefully it will lead to greater understanding of the mechanical quantum interface.
Paper links: High-order exceptional points in optomechanics