Nonlinear Sensing with Whispering-Gallery Mode Microcavities: From Label-Free Detection to Spectral Fingerprinting

Thermo-Optoplasmonic Single-Molecule Sensing on Optical Microcavities

Whispering-gallery-mode (WGM) resonators are powerful instruments for single-molecule sensing in biological and biochemical investigations. WGM sensors leveraged by plasmonic nanostructures, known as optoplasmonic sensors, provide sensitivity down to single atomic ions. In this article, we describe that the response of optoplasmonic sensors upon the attachment of single protein molecules strongly depends on the intensity of WGM. At low intensity, protein binding causes red shifts of WGM resonance wavelengths, known as the reactive sensing mechanism. By contrast, blue shifts are obtained at high intensities, which we explain as thermo-optoplasmonic (TOP) sensing, where molecules transform absorbed WGM radiation into heat. To support our conclusions, we experimentally investigated seven molecules and complexes; we observed blue shifts for dye molecules, amino acids, and anomalous absorption of enzymes in the near-infrared spectral region. As an example of an application, we propose a physical model of TOP sensing that can be used for the development of single-molecule absorption spectrometers.

Ultrasound sensing with optical microcavities

Ultrasound sensors play an important role in biomedical imaging, industrial nondestructive inspection, etc. Traditional ultrasound sensors that use piezoelectric transducers face limitations in sensitivity and spatial resolution when miniaturized, with typical sizes at the millimeter to centimeter scale. To overcome these challenges, optical ultrasound sensors have emerged as a promising alternative, offering both high sensitivity and spatial resolution. In particular, ultrasound sensors utilizing high-quality factor (Q) optical microcavities have achieved unprecedented performance in terms of sensitivity and bandwidth, while also enabling mass production on silicon chips. In this review, we focus on recent advances in ultrasound sensing applications using three types of optical microcavities: Fabry-Perot cavities, π-phase-shifted Bragg gratings, and whispering gallery mode microcavities. We provide an overview of the ultrasound sensing mechanisms employed by these microcavities and discuss the key parameters for optimizing ultrasound sensors. Furthermore, we survey recent advances in ultrasound sensing using these microcavity-based approaches, highlighting their applications in diverse detection scenarios, such as photoacoustic imaging, ranging, and particle detection. The goal of this review is to provide a comprehensive understanding of the latest advances in ultrasound sensing with optical microcavities and their potential for future development in high-performance ultrasound imaging and sensing technologies.

Transferable microfiber laser arrays for high-sensitivity thermal sensing

Functional microfibers have attracted extensive attention due to their potential in health monitoring, radiation cooling, power management and luminescence. Among these, polymer fiber-based microlasers have plentiful applications due to their merits of full color, high quality factor and simple fabrication. However, developing a facile approach to fabricate stable microfiber lasing devices for high-sensitivity thermal sensing is still challenging. In this research, we propose a design of a stable and transferable membrane inlaid with whispering-gallery-mode plasmon hybrid microlaser arrays for thermal sensing. By integrating plasmonic gold nanorods with polymer lasing microfiber arrays that are embedded in the polydimethylsiloxane matrix, whispering-gallery-mode lasing arrays with high quality are achieved. Based on the thermo-optical effect of the membrane, a tuning range of 1.462 nm for the lasing peak shift under temperature variation from 30.6 °C to 38.7 °C is obtained. The ultimate thermal sensing sensitivity can reach up to 0.181 nm °C-1 and the limit of detection is 0.131 °C, with a high figure of merit of 2.961 °C-1. Moreover, a stable laser linewidth can be maintained within the tuning range due to plasmon-improved photon confinement and PDMS-reduced scattering loss. This work is expected to provide a facile approach for the fabrication of high-sensitivity on-chip thermometry devices.

Optically pumped Milliwatt Whispering-Gallery microcavity laser

Whispering-gallery-mode microcavity lasers possess remarkable characteristics such as high Q factors and compact geometries, making them an essential element in the evolution of microlasers. However, solid-state whispering-gallery-mode lasers have previously suffered from low output power and limited optical conversion efficiency, hindering their applications. Here, we present the achievement of milliwatt laser emissions at a wavelength of 1.06 µm from a solid-state whispering-gallery-mode laser. To accomplish this, we construct a whispering-gallery-mode microcavity (with a diameter of 30 µm) using a crystalline Nd: YAG thin film obtained through carbon-implantation enhanced etching of a Nd: YAG crystal. This microcavity laser demonstrates a maximum output power of 1.12 mW and an optical conversion efficiency of 12.4%. Moreover, our unique eccentric microcavity design enables efficient coupling of free-space pump light, facilitating integration with a waveguide. This integration allowed for single-wavelength laser emission from the waveguide, achieving an output power of 0.5 mW and an optical conversion efficiency of 6.18%. Our work opens up new possibilities for advancing solid-state whispering-gallery-mode lasers, providing a viable option for compact photonic sources.

Simultaneous temperature and pressure sensing based on a single optical resonator

We propose a dual-parameter sensor for the simultaneous detection of temperature and pressure based on a single packaged microbubble resonator (PMBR). The ultrahigh-quality (∼10⁷) PMBR sensor exhibits long-term stability with the maximum wavelength shift about 0.2056 pm. Here, two resonant modes with different sensing performance are selected to implement the parallel detection of temperature and pressure. The temperature and pressure sensitivities of resonant Mode-1 are -10.59 pm/°C and 0.1059 pm/kPa, while the sensitivities of Mode-2 are -7.69 pm/°C and 0.1250 pm/kPa, respectively. By adopting a sensing matrix, the two parameters are precisely decoupled and the root mean square error of measurement are ∼ 0.12 °C and ∼ 6.48 kPa, respectively. This work promises the potential for the multi-parameters sensing in a single optical device.

Phase-Transition Microcavity Laser

Liquid-crystal microcavity lasers have attracted considerable attention because of their extraordinary tunability and sensitive response to external stimuli, and because they operate generally within a specific phase. Here, we demonstrate a liquid-crystal microcavity laser operated in the phase transition in which the reorientation of liquid-crystal molecules occurs from aligned to disordered states. A significant wavelength shift of the microlaser is observed, resulting from the dramatic changes in the refractive index of liquid-crystal microdroplets during the phase transition. This phase-transition microcavity laser is then exploited for sensitive thermal sensing, enabling a two-order-of-magnitude enhancement in sensitivity compared with the nematic-phase microlaser operated far from the transition point. Experimentally, we demonstrate an exceptional sensitivity of -40 nm/K and an ultrahigh resolution of 320 μK. The phase-transition microcavity laser features compactness, softness, and tunability, showing great potential for high-performance sensors, optical modulators, and soft matter photonics.

Cascade Brillouin Lasing in a Tellurite-Glass Microsphere Resonator with Whispering Gallery Modes

Brillouin microlasers based on microresonators with whispering gallery modes (WGMs) are in high demand for different applications including sensing and biosensing. We fabricated a microsphere resonator with WGMs from a synthesized high-quality tellurite glass with record high Q-factors for tellurite microresonators (Q ≥ 2.5 × 10⁷), a high Brillouin gain coefficient (compared to standard materials, e.g., silica glasses), and a Brillouin frequency shift of 9 ± 0.5 GHz. The high density of excited resonance modes and high loaded Q-factors allowed us to achieve experimentally cascade Stokes-Brillouin lasing up to the 4th order inclusive. The experimental results are supported by the results of the theoretical analysis. We also theoretically obtained the dependences of the output Brillouin powers on the pump power and found the pump-power thresholds for the first five Brillouin orders at different values of pump frequency detuning and Q-factors, and showed a significant effect of these parameters on the processes under consideration.

Integrated Molecular Optomechanics with Hybrid Dielectric-Metallic Resonators

Molecular optomechanics describes surface-enhanced Raman scattering using the formalism of cavity optomechanics as a parametric coupling of the molecule's vibrational modes to the plasmonic resonance. Most of the predicted applications require intense electric field hotspots but spectrally narrow resonances, out of reach of standard plasmonic resonances. The Fano lineshapes resulting from the hybridization of dielectric-plasmonic resonators with a broad-band plasmon and narrow-band cavity mode allow reaching strong Raman enhancement with high-Q resonances, paving the way for sideband resolved molecular optomechanics. We extend the molecular optomechanics formalism to describe hybrid dielectric-plasmonic resonators with multiple optical resonances and with both free-space and waveguide addressing. We demonstrate how the Raman enhancement depends on the complex response functions of the hybrid system, and we retrieve the expression of Raman enhancement as a product of pump enhancement and the local density of states. The model allows prediction of the Raman emission ratio into different output ports and enables demonstrating a fully integrated high-Q Raman resonator exploiting multiple cavity modes coupled to the same waveguide.

Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications

Surface plasmonic sensors have been widely used in biology, chemistry, and environment monitoring. These sensors exhibit extraordinary sensitivity based on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) effects, and they have found commercial applications. In this review, we present recent progress in the field of surface plasmonic sensors, mainly in the configurations of planar metastructures and optical-fiber waveguides. In the metastructure platform, the optical sensors based on LSPR, hyperbolic dispersion, Fano resonance, and two-dimensional (2D) materials integration are introduced. The optical-fiber sensors integrated with LSPR/SPR structures and 2D materials are summarized. We also introduce the recent advances in quantum plasmonic sensing beyond the classical shot noise limit. The challenges and opportunities in this field are discussed.

Operando monitoring transition dynamics of responsive polymer using optofluidic microcavities

Optical microcavities have become an attractive platform for precision measurement with merits of ultrahigh sensitivity, miniature footprint and fast response. Despite the achievements of ultrasensitive detection, optical microcavities still face significant challenges in the measurement of biochemical and physical processes with complex dynamics, especially when multiple effects are present. Here we demonstrate operando monitoring of the transition dynamics of a phase-change material via a self-referencing optofluidic microcavity. We use a pair of cavity modes to precisely decouple the refractive index and temperature information of the analyte during the phase-transition process. Through real-time measurements, we reveal the detailed hysteresis behaviors of refractive index during the irreversible phase transitions between hydrophilic and hydrophobic states. We further extract the phase-transition threshold by analyzing the steady-state refractive index change at various power levels. Our technology could be further extended to other materials and provide great opportunities for exploring on-demand dynamic biochemical processes.