Flexible guided-mode resonance cantilevers for non-contact optical sensing of sub-millinewton magnetic forces

Abstract

Flexible guided mode resonance (GMR) devices offer a potential platform for sensitive optical detection of small-scale mechanical disturbances. This sensitivity is utilized in this study using a flexible PET-based GMR device built as a cantilever for small force sensing. A small mass permanent magnet is mounted to the free end of the cantilever to provide magnetic actuation and amplify displacement response to a chirped sinusoidal signal applied on an external coil, with oscillations at the cantilever resonance frequency detected through variation of the GMR response as the surface tilt changes relative to the source. An external magnet is slowly introduced to create a pulling force that shifts the cantilever resonance frequency and hence the GMR response, allowing force measurement by optical means. Across probe magnet masses of 0.18 g, 0.36 g, and 0.54 g, the force-distance response follows the nonlinear trend predicted by the two-dipole magnetic force model, with a root-mean-square error of 0.176–0.40 mN relative to the analytical model and an average local sensitivity of 0.55–3.83 mN/cm, obtained from power-law fits to the experimental force-distance data. Using repeated resonance-frequency measurements to establish the sensor's noise floor independently of model residuals, a minimum detectable force of 0.12–0.67 mN was obtained, corresponding to a distance resolution of 0.06–0.17 cm and a dynamic range of 7.3–13.2 dB depending on magnet configuration. These results indicate the potential of magnetic gradient tuning as a sensitive, non-contact force measurement technique, expandable towards vibration and acceleration measurement applications.