Surface evolution of cylindrical fused silica resonator and its implication on Q factor

Investigating the surface loss mechanism is crucial for further decreasing energy loss in high quality fused silica resonators. Although there are various assumptions on surface loss mechanisms, experimental verification is not enough. In this paper, we present experimental results on the surface evolution and the Q factor change of the cylindrical fused silica resonator. Results on two cylindrical fused silica resonators show that compared with the buffered oxide etch (BOE), the NH4HF2 solution maintained a better surface quality. A series of experiments were performed on the third resonator and a slice manufactured by the same grinding process, both were etched by NH4HF2 solution simultaneously. The surface evolution of the fused silica slice was characterized by a profiler and a scanned electron microscope, and the decay time constant of the cylindrical fused silica resonator was measured by laser Doppler vibrometer. Results show that, as the etching progresses, the surface quality improved, particularly the micro-cracks were completely removed, and the spatial period of the surface increased. The surface roughness first increased, then fluctuated around a certain value. The decay time of the third resonator first increased rapidly, with a peak value of 138.74s at the etching depth of 50.5μm, then decreased with the increase of etching depth. Interesting facts on surface element distributions were discovered that when the decay time reached the maximum, the content of C reached the minimum and the percentage of Si and O was closest to 1:1. Our results suggested that the decay time constant of the macro-scale cylindrical fused silica resonator is not directly correlated with the surface roughness. More efforts are expected to explain the underlying mechanism behind.