Nirmalendu Acharyya and Gregory Kozyreff, researchers at the ULB, just published a scientific paper in the Physical Review Applied. More information here.
Abstract of the paper: Efficient microresonators simultaneously require a large quality factor Q and a small volume V. However, the former is ultimately limited by bending losses, the unavoidable radiation of energy of a wave upon changing direction of propagation. Such bending losses increase exponentially as V decreases and eventually result in a drop of Q. Therefore, circular cavities are generally designed with radii that are much larger than the optical wavelength. The same leakage of energy by radiation limits the sharpness of bends in photonic integrated circuits. In this paper, we present a way to reduce bending losses in circular microresonators. The proposed scheme consists of one or more external dielectric rings that are concentric with the cavity. These rings alter the field outside the cavity where radial oscillations set in and thus control the far-field radiation. As a result, the Q factor can be increased by several orders of magnitude while keeping a small cavity volume.