Experimental observation of scatterer-induced asymmetric backscattering in a WGM resonator side-coupled to two waveguides.
(A) Illustration of a WGM resonator side-coupled to two waveguides, with the two scatterers enabling the dynamical tuning of the modes. cw and ccw are the clockwise and counterclockwise rotating intracavity fields. acw(ccw) and bcw(ccw) are the field amplitudes propagating in the waveguides. β is the relative phase angle between the scatterers. Inset shows the optical microscope image of the microtoroid resonator, the tapered fiber waveguides (horizontal lines), and the two silica nanotips denoting the scatterers (diagonal lines on the left and right sides of the resonator). (B) Varying the size and the relative phase angle of a second scatterer helps to dynamically change the frequency detuning (splitting) and the linewidths of the split modes revealing avoided crossings (top) and an EP (bottom). (C and D) When there is no scattering center in or on the resonator, light coupled into the resonator through the first waveguide in the cw (C-i) [or ccw (D-i)] direction couples out into the second waveguide in the cw (C-i) [or ccw (D-i)] direction: the resonant peak in the transmission and no signal in the reflection. (C-ii and D-ii) When a first scatterer is placed in the mode field, resonant peaks are observed in both the transmission and the reflection regardless of whether the light is input in the cw (C-ii) or ccw (D-ii) direction. (C-iii and D-iii) When a second scatterer is suitably placed in the mode field, for the cw input, there is no signal in the reflection output port (C-iii), whereas, for the ccw input, there is a resonant peak in the reflection, revealing asymmetric backscattering for the two input directions. Inset in D-iii compares the two backscattering peaks in C-iii and D-iii .