The Quest for Uni-Directionality with WGMs in µ-Lasers: Coupled Oscillators and Amplifiers

Uni-directional emission emerges perpendicularly to the notch of a 2-d spiral-shaped mu-disk and not with 30deg tilted, as is the case for poorer quality spiral-shaped mu-disks. While wanting to preserve the uni-directionality of the spiral, we also wanted to extract as much of the stored energy from the mu-disks at the expense of very high Q-value whispering gallery modes (WGMs). In this paper we present experimental results of directly coupling the flat edge of a spiral mu-cavity with the flat edge at one corner of a semicircle mu-cavity. The uni-directional emission appears normal to the flat edge of the opposite corner of a semicircle. The roles of the spiral as an oscillator and the semicircle as an amplifier are investigated with separate currents pumping the spiral and the semicircle. The competition between lasing modes (with narrow spectral peaks) and broadband amplified spontaneous emission (ASE) in the spiral for the inverted population in the semicircle limits the amplification process. The feedbacks provided by the Fresnel reflection on one side and the reflection from the channel on the coupled side cause the semicircle mu-disk to self oscillate above some current value. The directly coupled 2-element mu-disk provides an interesting system to explore as a means of achieving higher output power from mu-cavities