J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 261
Figure 16. Cascaded Raman generated in a MBR pumped at 779 nm, showing the first and second order Raman laser lines.
otherwise the fluorescence of the acceptor could be directly excited from the pump laser. FRET combined with WGMR detection was first introduced by Wang et al. [ 64 ], who showed that the interaction between FRET and WGMs enhanced the sensing performance by four orders of magnitude.
Frequency comb( FC) generation or cascaded Raman lasing are by far more interesting for molecular fingerprinting than non coherent techniques. Frequency combs are a broadband and stable coherent source with equally separated lines that can be used to interrogate molecular fringerprints. FC and Raman lasing can be used in different spectroscopic configurations [ 65 ]. FC generation can be either solitonic or non-solitonic, with the first producing an ultra-broadband laser source. For a detailed description of the nonlinear phenomena occurring in microcavities, we refer to Chembo et al.[ 7 ]. Figure 16 shows the generation of cascaded Raman laser lines in a MBR, pumped at 779 nm in the near infrared band( NIR).
4.3
Optoplasmonics
Optoplasmonics can be described as the combination of optical microcavities with metallic nanoparticles( MNPs) that sustain localized surface plasmon resonances( LSPRs). A surface plasmon resonance occurs at the interface of a dielectric medium and a metal, and it originates from the coupling of the electronic charge density with an electromagnetic mode. The hybrid system highly benefits from the coupling of both resonances( i. e. the cavity resonance and the LSPR) since it increases the overall sensitivity. The MNPs can be either labels of the analytes or nanoantennae [ 66 ]. MNP act as plasmonic hotspots and, near resonance, they can increase the scattering and radiation losses and therefore change the cavity resonance. Coupling MNP to WGMR can reduce the resonance linewidth and improve sensitivity [ 67 ] and this approach can be implemented in surface enhanced Raman scattering( SERS) spectroscopic measurements. Plasmonic nanoparticles can be grown directly on the surface [ 68 ] or using a controlled deposition by means of carousel forces [ 69 ].
Figure 17. Sketch of the transmission signal associated with optomechanical vibrations in WGMRs.
4.4 Optomechanical and photoacoustic detection
In the last decade optomechanical sensing has gained a well deserved recognition. Optomechanical sensing can be split into two categories, based on its working principle. It is passive, if the detection is based on radiation pressure; it is active, if the detection is based on SBS. Usually, radiation pressure involves mechanic vibrations in the megahertz range, whereas SBS or Brillouin lasers frequency vibrations are in the gigahertz range. In the case of radiation pressure, the coupled light induces a parametric instability [ 18, 70 ] and obviously requires the coexistence of high optical and mechanical Q-factors. A detailed theoretical description of the dynamical back-action was given by Kippenberg et al. [ 17 ]. Another interesting approach can be found in the paper by Li et al. [ 71 ]. In this paper, Li et al. describe the dynamical system as quantum Langevin equations, picturing the mechanical resonator and the cavity fields as quantized bosonic fields. Figure 17 shows an illustration of the self-induced radiation pressure oscillation. When the laser frequency is locked on the side of a resonance, a huge circulating power is confined into the WGMR and this induces a mechanical bending of the cavity due to radiation pressure. This bending translates into a periodical modification of the optical path, shifting the resonance and periodically modulating the transmission with a frequency X m.
When a particle is adsorbed into the surface, the oscillation will change its frequency due to the mass change of the resonator, in analogy with traditional optical sensing. Subpg mass sensing was first demonstrated using a microtoroid WGMR [ 27 ]. Figure 18 shows a render of the working principle of a mass sensor based on a microspherical WGMR developed by Yu et al. in 2016 [ 72 ].
When the mechanical oscillations are due to SBS, the sensing is made by measuring the changes in the Brillouin structural mode. In 2009, Tomes and Carmon demonstrated that silica microspheres can generate both a Stokes photon and an acoustic phonon through an electrostrictiveinduced SBS process in the backward direction [ 73 ], a decade later Yang et al. [ 74 ] demonstrated the possibility of exciting SBS in the forward direction. Liquid droplets can also amplify hypersonic waves and allow forward-scattering processes [ 75 ]. In this particular case, the generated