GaN microdisk with direct coupled waveguide for unidirectional whispering-gallery mode emission

Microdisks are excellent whispering-gallery mode (WGM) optical resonators, but their emissions are invariably in-plane isotropic due to their circularities and thus difficult to be extracted efficiently. In this work, a waveguide with a width of 0.16 µm directly coupled to a microdisk with a diameter of 10 µm is fabricated on a 0.77 µm thick GaN thin film containing InGaN/GaN multi-quantum wells. This eliminates the need for precision patterning required by evanescent coupling schemes in which coupling gaps of the order of tens of nanometers must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of WGMs to the waveguide from the microdisk is successfully demonstrated.