Measurement of Nanofiber Mechanical Flexural Modes Based on Near-Field Scattering

We systematically investigated the intrinsic mechanical flexural modes of tapered optical fibers (TOFs) with a high aspect ratio up to $3\times {10}^4$. Based on the near-field scattering of the hemispherical microfiber tip to the vibrating TOF evanescent field, we detected more than 320 ordered intrinsic mechanical modes through the TOF transmission spectra which was enhanced by 72 dB compared to without near-field scattering. The trend of the vibration amplitude with the mode order was similar to pendulum waves. Our results open a pathway to study the mechanical modes of photonic microstructures-nanostructures that are expected to be used in waveguide QED, cavity optomechanical, and optical sensing.

Figure 1

Experimental setup for detecting the TOF flexural modes.

Figure 2

The TOF flexural modes are simulated with COMSOL Multiphysics. (a) The vibration amplitude ($x$ axis) as a function along the nanofiber axis ($z$ axis) for the 1st, 5th, 10th, 15th, 20th, and 25th order modes. (b) The vibration amplitudes of various order TOF flexural modes at position $z=0.1\mathrm{mm}$.

Figure 3

(a) TOF flexural vibration enhancement spectrum. Inset: enlarged enhancement spectrum of 47th–50th modes. (b) Frequency (purple open circles) and frequency increment (open black stars) of the flexural modes as a function of the mode order; the red curve is the quadratic results.

Figure 4

(a) TOF mechanical vibration spectra with resonant excitation, off-resonant excitation, and intrinsic vibration spectra. (b) The spectra of the enhancement factor. (c) Ringdown of flexural modes with resonant and off-resonant excitation. (d) $Q$ factors as a function of the pressure with resonant, off-resonant modes’ slow decay and the fast decay of the resonant excitation. Red and cyan indicate resonant and off-resonant excitation, respectively.