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J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025)
Figure 33. Top panel: sketch of the optical microcavity ultrasound probe and detection scheme. Bottom panel: PA image of a zebrafish. Adapted from [ 147 ].
piece of Terfenol-D affixed to its top surface. The authors reported a peak sensitivity of 400 nT Hz �1 / 2. The same authors improved the device by embedding the material into the microtoroid in order to overcome the poor coupling between the affixed magnetostrictive material and the toroid. In this second paper [ 157 ], a sensitivity of 200pT Hz �1 / 2 was achieved in 100 kHz – MHz range( maximum bandwidth of 40 MHz), whereas in the range of 2 Hz – 1 kHz the reported sensitivity was 150 nT Hz �1 / 2. The device was further optimised, first by improving the fabrication method [ 158 ]( see Fig. 34c) and then by improving the geometrical
design to reach a sensitivity of 26 pT Hz �1 / 2( see Fig. 34b) [ 159 ]. However, these results were obtained with advanced readout systems and highly controlled environment. Yang et al. [ 160 ] designed a robust polymer-packaged optomechanical magnetometer with microcavity readout, where the micromagnet was integrated into a soft polymer( see Fig. 34a). Their device worked in the hertz-to-kilohertz frequency range and achieved an improved sensitivity of 880 pT Hz �1 / 2 at 200 Hz. Another way to reach lower frequency detection is to use larger WGMR such as crystalline disks. Yu et al. [ 161 ] used a CaF 2 WGMR with a diameter of about 16 mm with a Terfenol-D cylinder on top of it. The authors reported a sensitivity of 131 pT Hz �1 / 2 in a non-cryogenic environment without magnetical shielding( see Fig. 34d). MBR were also proposed as magnetometers by Freeman et al. [ 162 ]. The authors reported a sensitivity of 1.9 GHz / mT with a borosilicate MBR on chip.
5.7
Viscometry
Carmon et al. [ 163 ] pioneered the use of optomechanical oscillations and Brillouin lasing modes in MBRs for detecting viscosity. The authors measured the shift in the optomechanical modes in high density and high viscosity liquids, paving the way to the use of acoustic properties to characterise viscous biological fluids. One year later, Bahl et al. demonstrated an optomechanofluidic viscometer by using an MBR and measuring the line broadening of the mechanical mode [ 164 ]. The same group also developed a pressure sensor based on MBRs and demonstrated that it can work in harsh environment conditions, such as a high temperature environment. The pressure increment modifies geometrically the MBR( namely, increases the radius) which in turn modifies the mechanical and optical frequency of the MBR. The shift of the WGM resonance changes the coupled laser power and therefore the temperature of the cavity. Since the mechanical modulus depends on the temperature, the mechanical frequency also changes, in a self-sustained way.
As we mentioned in Section 4.4, Kim et al. [ 80 ] reached
a NEP of 215 mPa Hz �1 / 2 and 41 mPa Hz �1 / 2 at 50 kHz and 800 kHz in air, using an hybrid approach. Tu et al. [ 165 ] have encapsulated the MBR and the taper to maintain stable performance under varying temperatures and static pressures. The authors detected acoustic waves at low frequencies in 10 Hz – 100 kHz range, achieving a NEP of 2.2 mPa Hz �1 / 2.
5.8 Thermometry
Frigenti et al. have been studying extensively the use of MBR as US sensors, where the walls can serve as ultrasound transducers, while the hollow structure inside can act as a sample container [ 19, 166 ]. The first experiment [ 19 ] demonstrated the feasibility of the measurement, verified the transducer readings through the characterisation of a known PA agent and allowed to gain insight on the transduction mechanism, which is strongly influenced by the mechanical properties of the MBR. The second experiment [ 166 ], instead, challenged the MBR transducer through the characterisation of a constantly flowing and more diluted PA agent, simulating the configuration of a flow cytometry experiment. A sketch of the experimental set-up is shown in Figure 35. Our approach allows to avoid contact between the nanoparticle solution and the waveguide, and, with the implementation of a tunable excitation source, to reconstruct the absorption spectra of the nanoparticles. Our approach differs from the previous and pioneering results of Han et al. [ 167 ] andWardetal.[ 168 ]. Han et al. used radiation pressure and Brilluoin scattering to activate the long range phonons and use the mechanical mode to sense different kinds of particles in the micrometric range in an open flow circuit. Ward et al. used a quasi droplet MBR to extend the optical mode deep inside the MBR and be able to sense nanoparticles using the optical shift without additional methods such as mode-locking or interferometry [ 169 ] or special lasers [ 170 ]. Frigenti et al. [ 166 ] were also able to detect 0.5 mM Au solution as the limit concentration in a flow-cytometry like configuration. Since the liquid flow caused an increased noise level in the time-domain read-out trace, the analysis moved to Fourier space exploiting the filtering action of the MBR mechanical modes. With this approach, signal-to-noise ratio( SNR) was greatly increased and the height of the most prominent peak was used as the measurement of the PA intensity. As in the first experiment [ 19 ], the PA response trend of the contrast agent was reconstructed and the LOD was estimated to be a 1 nM GNR concentration.