Free spectral range magnetic tuning of an integrated microcavity

Abstract

Tunable whispering-gallery-mode (WGM) microcavities are promising devices for reconfigurable photonic applications such as widely tunable integrated lasers and reconfigurable optical filters for optical communication and information processing. Scaling up these devices demands the ability to tune the optical resonances in an integrated manner over a full free spectral range (FSR). Here we propose a high-speed full FSR magnetic tuning scheme of an integrated silicon nitride (Si3N4) double-disk microcavity. By coating a magnetostrictive film on the spokes and the central pad of the Si3N4 cavity, magnetic tuning can be realized using a microcoil integrated on the same chip. An FSR tuning can be achieved by combining magnetostrictive strain with strong optomechanical interactions provided by the double-disk microcavity. We calculate the required magnetic flux density to tune an FSR (BFSR) as a function of several key geometric parameters, including the air gap, radius, width of the spokes and ring of the double-disk cavities, as well as the thickness of the magnetostrictive film. The proposed structure enables a full FSR tuning with a required magnetic flux density of milli-Tesla (mT) level. We also study the dynamic response of the integrated device with an alternating current (AC) magnetic field driving, and find that the tuning speed can reach hundreds of kHz in the air.