a–b Micrographs of the microresonator and tungsten tip. c Illustration of the real part of the backscattered field (cavity size and wavelength not to scale); the scatterer is represented by one effective scatterer here (black dot). d Introducing a second scatterer at an azimuthal angular distance ϕ from the effective scatterer, the backscattered field changes depending on ϕ. e Illustration of wave profiles along the azimuth showing the counter-clockwise-propagating backscattered waves due to the clockwise-propagating pump field. The backscattered field amplitudes are small compared to the pump, as only a small fraction of the light is backscattered. The tip is critically coupled (equal amplitudes for intrinsic and induced backscattered waves). f Backscattered waves and their sum under critical coupling (equal amplitudes) for different azimuthal positions ϕi of the tip, corresponding to phase offsets between the effective intrinsic scatterer and the induced scatterer (2 m + q) π for integer m and q = 1/3, 5/6 and 1, showing both constructive and destructive interference
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