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Optothermal effects are ubiquitous in all kinds of WGMR and have been extensively studied in the last decade [ 129, 171 ]. These studies are mainly focused on the characterization and compensation of the optothermal effects of the cavity as a whole [ 172, 173 ], since the thermal shift of the WGMs is highly detrimental in label-free sensing approaches [ 116 ]. On the other hand, such extreme sensitivity can be used to develop efficient thermometers. Ultra high-Q microtoroids were the first WGMR transducer proposed to develop photothermal microscopy measurements of non-luminescent nano-objects such as carbon nanotubes [ 174 ], conductive polymers [ 175 ] and a single gold nanorod [ 67 ], even though the tuning range was quite small. These results were obtained in dry environment, which is not ideal in sensing applications [ 176 ]. For that reason, Goldsmith’ s group in collaboration with Nic Chormaic’ s group resorted to MBR and in combination with a double-modulation detection scheme. The groups measured the chemical etching dynamics of GNRs through polarization sensitive photothermal absorption microscopy [ 177 ]. Frigenti et al. proposed the MBRs for scattering-free absorption spectroscopy in an opaque medium like milk [ 178 ] and, after implementing a mode-locked technique, the LOD was calculated as a concentration of about 62pM GNRs [ 179 ]. Figure 36 shows the experimental set-up using a supercontinuum laser source for photothermal spectroscopy( Fig. 36a), together with the optical shifts recorded in an aqueous solution containing milk powder and GNRs( Fig. 36b). Figure 36c shows the absorption spectra normalized at the 740 nm values, together with a reference profile from a spectrophotometer( cyan curve).
6 Conclusions
WGMR based devices are an ideal platform for sensing since they posses remarkable properties such as high quality factors and small mode volumes. In this review, we have described the most recent progress in the field, including the sensing principles, the WGMR fabrication and the coupling techniques. We have given an overview of different applications based either in optical or mechanical interactions. WGMR have a superior sensitivity, a broadband operation and low power consumption, unfortunately they are still lab-based sensors since all of them necessitate careful alignment. New approaches for having ready-to-use robust sensors are still needed to become real world devices.
Acknowledgments
Mr. Franco Cosi from IFAC-CNR is gratefully acknowledged for the manufacturing of the tapered fibers, crystaline microdisks and the active spheres shown in Figures 3, 4, 5, and7. Dr. Romeo Bernini from CNR-IREA is gratefully acknowledged for providing pictures of the polymeric microbottles( Fig. 8).
Funding
D. F., S. P., G. N. C. and S. S. H. acknowledge the funding of the bilateral agreement CNR / CNPq( Brazil), Photothermal spectroscopy enhanced by polymeric optical microcavities( 2025- 2026). G. F. and S. S. H. acknowledge the support of the European Union – Next Generation EU, Mission 4 Component 2 Inv. 1.5
CUPB83C22003930001( Tuscany Health Ecosystem, Spoke 4: Nanotechnologies for diagnosis and therapy). Conflicts of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Data availablity statement
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request. Author contribution statement
All authors contributed to the article and approved the submitted version.
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