262
J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025)
Figure 18. Sketch of a mass sensor based on a microspherical WGMR. When a bovine serum albumina( BSA) molecule binds to the surface of the microsphere, the optical mode shifts, as well as the mechanical frequency of the system. The acoustic mode involved in the detection is a radial breathing mode. From [ 72 ].
surface-acoustic waves showed much lower frequencies than that of typical Brillouin phenomena [ 76 ].
Figure 19a shows a sketch of a microspherical Brillouin laser. The pump laser launched into the spherical WGMR is sufficient to excite forward SBS lasing, which propagates resonantly into the microsphere since it fulfills the mode matching condition. In this situation, the cavity is triply resonant: the pump and the Stokes photon together with the acoustic phonon( SBS) are resonant( see Fig. 19b). This triple resonant condition can be easily perturbed by the environmental conditions( temperature, mechanical vibrations, etc.), making the system an exceptional platform for sensing. All these properties( i. e. narrow linewidth and easy-to-measure MHz signals) combined with the characteristic molecular vibrations of the target are be very helpful for establishing a molecular bar-code. Even though the principle is quite simple, measuring the shifts can be quite challenging. For this reason, Li et al. have proposed a pump and probe optical system [ 77 ].
As discussed in Section 1, ultrasound or PA sensing based on all-optical detection is a great alternative to piezo-electric transducers. WGMR can optically detect acoustic waves through the photo-elastic and photothermal effects. The first demonstrations of PA detection were achieved with hybrid set-ups. The authors were resorting to piezo-electric transducers to excite the acoustic wave with the disadvantage of having the WGMR and the transducer immersed or partially immersed in water [ 78 – 80 ]. Figure 20a shows a picture of an initial setup used in our laboratories to verify the PA emission of gold nanorods( GNRs) hosted inside an MBR( the MBR position is highlighted by a green circle). In this initial setup, the PA response is detected with a piezo-electric transducer( visible on the left side), with the MBR was placed in the transducer acoustic focus. Figure 20b shows two PA signals measured with the advanced setup described in [ 19 ], where
Figure
19.( a) Sketch of the forward SBS generation in a microsphere and the associated acoustic WGM( the acoustic mode is a wineglass mode),( b) optical spectrum of the triply resonant condition. Adapted from [ 74 ].
Figure 20.( a) Picture of an initial setup used to verify the PA generation from the the GNR inside the MBR, using a piezoelectric transducer for the detection,( b) Ultrasound signal detected through an all-optical reading of the PA wave, using an advanced all-optical setup [ 19 ].
Frigenti et al. were the first one to demonstrate an alloptical generation and detection of an ultrasound signal. In this second setup, the all-optical status was achieved since the PA wave was triggered by a light pulse and the wave were detected by the shift induced into the MBR optical resonances. With respect to the initial setup and the hybrid configurations, the system is not submerged in water and therefore the taper is physically separated from the fluidics, which is confined within the MBR and its capillary stem.