Sub-nanometer measuring ellipticity of suspended optical nanowaveguides based on nondegenerate mechanical modes

Optical waveguides with miniature dimensions to the nanoscale can facilitate the development of highly integrated photonic devices, integrated optical circuits, and hybrid quantum system coupling with emitters. Nondegenerate intrinsic flexural mechanical modes of nanowaveguides provide unique insights into the mechanical properties and structural integrity of materials, which is of great significance to the applications of nanowaveguides. Here, we propose and implement a scheme to measure the nondegenerate intrinsic flexural mechanical modes of a suspended optical nanowaveguide, a tapered optical fiber (TOF). A TOF with an elliptical cross-section can support two nondegenerate intrinsic flexural mechanical modes (IFMMs) because the two orthogonal modes vibrate along the principal axes (major or minor axis) of the elliptical TOF cross-section with splitting vibration frequencies. The frequency ratio for the two IFMMs approaches a constant with increasing mode order, which is equal to the inverse of the TOF ellipticity. Thus, the TOF ellipticity can be determined on the basis of the splitting vibration frequencies of the nondegenerate modes with subnanometer-level accuracy, 0.16 nm, for a TOF radius of 260 ± 5 nm. The elliptical TOF's nondegenerate IFMMs offer a novel pathway for research on nanoscale structures and vector measurement in fields such as quantum optics, atom physics, sensing, optical communications, and micro/nanomechanics.