J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 269
Figure 32.( a) Schematic illustration of the mechanical modes of the cantilever-microsphere structure, the tapered fiber injecting a temporally-modulated laser beam into the WGM, and an acoustic source.( b) Oscillation spectrum of different mechanical modes from the tapered fiber“ string”: each mode has been verified by FEM.( c) Demonstration of the dual channel communications. Each row represents a combination of waves acting on the system( the magenta color represents the generating frequency). OM: optical modulation. AM: acoustic modulation. From [ 74 ].
displacement sensor [ 150 ] with a shot-noise limited sensitivity of 10 �19 mHz �1 / 2. In 2013, microtoroids and / or microdisks were also proposed as mass sensors [ 23 ] and managed to detect sub-pg masses by reading the shift of the oscillation frequency.
In 2019, Basiri-Esfahani et al. [ 151 ] achieved a noiseequivalent-pressure( NEP) between 8 and 300 lPa Hz �1 / 2 for frequencies from kHz to MHz with a dynamic range greater than 120 dB using a spoked silica microdisk. This kind of microdisks were first studied by Anetsberger [ 152 ]. Such a structure can boost the quality factor by an order of magnitude, reaching values of about 5 10 4. Someyears later, Yang et al. [ 153 ] performed a more systematic study to optimize the device by varying the geometrical dimensions( namely the radius and the thickness) and using a trench structure around the microdisk. The best result was obtained for a microdisk with a radius of 300 lm and a thickness of 2 lm, resulting in sensitivity of 1.18 lPa Hz �1 / 2 at 82.6 kHz in air. Tamayo’ sgroup [ 154 ] detected a single Staphylococcus epidermidis deposited on the surface of a microdisk by designing a device with eigenfrequencies close to the vibration frequencies of the bacteria. Sentre-Arribas et al. [ 155 ] detected temperature and relative humidity changes exploiting both optical and mechanical modes, reaching limit-of-detection( LOD) of 0.6 mK and 0.05 % for an independent detection, whereas the LODs for an entangled detection were 1 mK and 1 %.
5.6 Magnetometry
Another interesting sensing application is magnetometry. Microtoroids have mechanical oscillations in the MHz range, which makes them highly sensitive for magnetic resonance imaging( MRI). Instead, magnetic anomaly detection, magnetoencephalography( MEG) and mineral exploration need magnetometers with lower frequency range. Forstner et al. [ 156 ] demonstrated the first cavity magnetometer based on a microtoroid with a micrometric