268
J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025)
Figure 31.( a) Sketch of the microsphere with a flake of graphene placed at 30 ° angle from the equatorial plane. The hybrid WGMR is pumped by an external cavity diode laser( ECDL) that excites soliton combs, which belong to distinct mode families. Adapted from [ 146 ].( b) Sketch of the Brillouin lasing device used for gas detection. BLs: Brillouin laser, PL: pump laser, Gr: Graphene. Adapted from [ 15 ].
laser within moving cells could be tracked continuously with high signal-to-noise ratios for 2 h, while conventional microlasers exhibited frequent signal loss causing tracking failure [ 140 ]. Schubert et al. proposed the WGMR lasers for all-optical recording of transient cardiac contraction profiles with cellular resolution [ 141 ]. The same group has recently published the protocol to integrate microlasers into living cells and to perform massive cellular tracking by means of hyperspectral confocal imaging [ 142 ]. In 2014, Yang’ s group proposed an active detection scheme based on Raman lasing to detect single nanoparticles down to 10 nm in size [ 143 ] in a dry environment, whereas Li et al. [ 144 ] demonstrated the technique in both aqueous and dry environment with a similar LOD.
Several groups have been exploring new sensor architectures based on spectroscopic techniques for gas detection. Vahala’ s group developed a soliton based dual comb spectroscopic technique using silica microtoroids [ 145 ]. Two microtoroids are used to generate two combs with slightly different repetition rates: one comb is used as a reference, whereas the other one goes through a gas cell. Then, both the reference arm and the sensing one are combined and the interferogram spectrum is observed. Yao’ s group [ 146 ] coated asymmetrically a microsphere with graphene to generate different Stokes solitons, each of them belonging to different family modes. In this configuration the exfoliated graphene deposited on the surface only affects combs with large spatial distributions. The authors measured three beat signals through self-heterodyne detection and they were able to detect simultaneously ammonia, carbon dioxide, and water vapor at the picomolar per liter level in a mixture. The same group lowered the detection of ammonia, carbon dioxide and nitrogen dioxide down to 10ths of parts per trillion [ 15 ]. The authors were also able to identify multiple species in gas mixtures with an error below the 6 %. Figure 31 show the conceptual working principle of the sensor.
5.5 Mechanical and photoacoustic sensing
Yang et al. [ 74 ] reported on a Brillouin lasing scheme for detecting optical, acoustic and micro waves. In this case, the authors used the Brillouin lasing shifts upon external stimulus such as radiation pressure, acoustic waves and microwave excitation. In this particular approach, the microsphere acts as a cantilever. This cantilever WGMR sensor detected a force of about 1.61 pN for 1 mW of external laser power; in the case of acoustic sensing the minimum detectable force is around 18 pN Hz �1 / 2. There are, however, some limitations for the simultaneous detection of three waves. The first limit is the use of modulated light, rather than continuous light, since the sensor is based on the oscillation from the cantilever-microsphere structure. The second limit is the amplitude variation of the Brillouin lasing caused by microwaves, since it can affect the amplitude of the light-actuation channel and the sound-actuation channel. Figure 32 shows the dual excitation of the microsphere-based cantilever and the spectra collected for different perturbations.
Sun et al. [ 147 ] proposed an interesting approach to use a microsphere as an ultrasonic probe. The authors encapsulated a microsphere and a taper into an optical glue droplet, creating a compact and environmental robust probe for PA imaging. Figure 33 shows a sketch of the microsphere probe, the detection scheme and the PA image of a zebrafish. In this experiment, a pulsed laser source induces an ultrasound( US) wave in the blood vessels of the tissue, which is then detected by the WGMR-based probe. In terms of working principle, the US signal shifts the resonance by acting on the droplet-packaged system, and the readout of the resonance position reconstructs the typical PA oscillation.
Tang et al. [ 3 ] demonstrated the use of a microspherical WGMR as ultrasound sensor for real-time vibrational spectroscopy of single mesoscopic particles. The authors explored the feasibility of biomechanical fingerprinting of microbial cells. Three types of microorganism – the living cyanobacteria( Synechocystis sp. PCC6803), Aspergillus sydowii and Aspergillus niger spores were deposited on the surface of the WGMR and excited with a pulsed laser light. The cyanobacteria generated an US wave that shifted the resonance of the WGMR, allowing the readout.
Hollow resonators like MBR or quasi-suspended devices like microtoroids are more sensitive to deformations compared to solid microspheres. For this reason, they have been widely used as optomechanical and ultrasound sensors together with low-viscosity microdroplets [ 148 ]. Optomechanical studies in microtoroids are almost two decades old, Vahala’ s group studied first the fundamental physics behind optomechanical oscillations( OMO) [ 17, 18, 149 ] and then Kippenberg proposed a microtoroid as a