Lasers are the pillars of modern optics and sensing. Microlasers based on whispering-gallery modes (WGMs) are miniature in size and have excellent lasing characteristics suitable for biosensing. WGM lasers have been used for label-free detection of single virus particles, detection of molecular electrostatic changes at biointerfaces, and barcode-type live-cell tagging and tracking. The most recent advances in biosensing with WGM microlasers are described in this review. We cover the basic concepts of WGM resonators, the integration of gain media into various active WGM sensors and devices, and the cutting-edge advances in photonic devices for micro- and nanoprobing of biological samples that can be integrated with WGM lasers.

Plasmonic metal nanostructures provide a promising strategy for light trapping and therefore can dramatically enhance photocurrent in optoelectronics only if the trapped light can be coupled effectively from plasmons to excitons, whereas the reverse transfer of energy, charge, and heat from excitons to plasmons can be suppressed. Motivated by this, this work develops a scheme to implement a metafilm with Ag nanoparticles (NPs) embedded in 10 nm thick silica (Ag NPs–silica metafilm) to the active device channel of a hybrid perovskite film/graphene photodetector. Remarkably, an enhancement factor of 7.45 in photoresponsivity, the highest so far among all the reports adopting plasmonic metal NPs in perovskite photodetectors, has been achieved on the photodetectors with the Ag NPs–silica metafilms. Considering that the synthesis of the Ag NPs–silica metafilms can be readily scaled up to coat both rigid and flexible substrates, this result provides a low-cost metaplatform for a variety of high-performance optoelectronic device applications.

We study control of optical coupling of plasmon resonances in metallic nanoantenna arrays using ultrathin layers of silicon. This technique allows one to establish and tune plasmonic lattice modes of such arrays, demonstrating a controlled transformation from the localized surface plasmon resonances of individual nanoantennas to their optimized collective lattice modes. Depending on the polarization and incident angle of light, our results support two different types of the silicon-induced plasmonic lattice resonances. For s-polarization these resonances follow the Rayleigh anomaly, while for p-polarization an increase in the incident angle makes the lattice resonances significantly narrower and slightly blueshifted.

We study biological sensing using the hybridization phase of localized surface plasmon resonances (LSPRs) with diffraction modes (photonic lattice modes) in arrays of gold nanoantennas. We map the degree of the hybridization process using an embedding dielectric material (Si), identifying the critical thicknesses wherein the optical responses of the arrays are mainly governed by pure LSPRs (insignificant hybridization), Fano-type coupling of LSPRs with diffraction orders (hybridization state), and their intermediate state (hybridization phase). The results show that hybridization phase can occur with slight change in the refractive index (RI), leading to sudden reduction of the linewidth of the main spectral feature of the arrays by about one order of magnitude while it is shifted nearly 140 nm. These processes, which offer significant improvement in RI sensitivity and figure of merit, are utilized to detect monolayers of biological molecules and streptavidin-conjugated semiconductor quantum dots with sensitivities far higher than pure LSPRs. We further explore how these sensors can be used based on the uncoupled LSPRs by changing the polarization of the incident light.

We study the impact of structural features of Si/Al oxide junctions on metal-oxide plasmonic metafilms formed via placing such junctions in close vicinity of an Au/Si Schottky barrier. The emission intensity and dynamics of colloidal semiconductor quantum dots deposited on such metafilms are investigated, while the surface morphology and structural compositions of the Si/Al oxide junction are controlled. The results show the conditions wherein the Si/Al oxide junction can reshape the impact of plasmonic effects, allowing it to increase the lifetimes of excitons. Under these conditions, the plasmonic metafilms can quarantine excitons against the fluctuating trap environments of the quantum dots, offering super-plasmonic emission enhancement that includes enhancement of the spontaneous emission decay rate combined with the suppression of Auger decay.

We show that under certain conditions the plasmonic field of a hybrid system consisting of a metallic nanoparticle and a semiconductor quantum dot can be converted into ultrashort stationary pulses with temporal widths as short as 300 ps. This happens as this system interacts with an infrared and visible laser fields, both with time-independent amplitudes. These fields generate quantum coherence via simultaneous interband and intersubband transitions of the quantum dot, forcing the polarization dephasing rate of the quantum dot to become negative during the plasmon pulses. This makes the amplitudes of such pulses time-independent (undamped), indicating total suppression of quantum decoherence of the quantum dot. These results suggest that hybrid quantum dot-metallic nanoparticle systems can act as undamped coherent-plasmonic oscillators.

We use localized surface plasmon resonances in metallic nanoantennas to suppress defect environment of colloidal quantum dots and to enhance and polarize their spontaneous emission. For this we study the interaction of such quantum dots with functional metal-oxide plasmonic metastructures consisting of an Au/Si Schottky junction in close vicinity of a Si/Al oxide charge barrier. We show that optically excited quantum dots can couple with such metastructures via their electric dipole fields, offering super-plasmonic processes that include the impact of hot electron generation and fixed negative charges at the Si/Al oxide junction. For metallic nanoantennas with small aspect ratios our results show that these metastructures can reduce the defect-induced nonradiative decay rates of quantum dots such that their lifetime can become significantly longer than those in the absence of such nanoantennas. For metallic nanoantennas with larger aspect ratios these structures lead to ultrahigh enhancement of exciton-plasmon coupling, making the spontaneous emission of quantum dots strongly polarized while increasing their emission intensities by about 50 times. These results show by keeping excitons in the cores of quantum dots, the metastructures suppress migration of photo-excited electrons to surface defects and, therefore, reduce Auger recombination. Coherent dynamics associated with the quantum dot-induced exciton-plasmon coupling is theoretically investigated.

This work explores superposition of the localized surface plasmonic resonance (LSPR) effect of Au nanoparticles (AuNPs) with that on transition metal dichalcogenide (TMD) WS 2 nanodomes (WS 2 −NDs) enabled by enhanced dipole−dipole interaction at van der Waals (vdW) interfaces in AuNP/WS 2 −ND/graphene heterostructures for surface-enhanced Raman spectroscopy (SERS) with high-sensitivity, The confirmation of such a superposition is first demonstrated in the enhanced graphene Raman signatures, such as the G-peak intensity by approximately 7.8 fold on the AuNP/WS 2 −ND/graphene over that of reference graphene sample, in contrast to 4.0-and 5.3-fold, respectively, on AuNP/graphene and on WS 2 −ND/graphene. Furthermore, Raman spectra of probe molecules of fluorescent Rhodamine 6G (R6G) were hired to quantify the enhanced SERS on AuNP/WS 2 − ND/graphene SERS substrates. At the R6G concentration of 5 × 10 −5 M, enhancement factors of ∼2.0 and 2.4 based on the R6G 613 cm −1 peak intensity are detected on the AuNP/WS 2 −ND/graphene with respect to that on WS 2 −ND/graphene and AuNP/ graphene, respectively. The benefit of the superposition of the LSPR effects from the WS 2 −NDs and AuNPs results in high SERS sensitivity up to 1 × 10 −12 M on AuNP/WS 2 −ND/graphene, which is about an order of magnitude better than what's on WS 2 −ND/ graphene, and several orders of magnitude better than that on the AuNP/graphene and metal nanostructure/TMD (continuous layer) substrates. This result reveals the advantage of superposition of the LSPR effects from different nanostructures through design of vdW heterostructures. In addition, considering the AuNP/WS 2 −ND/graphene vdW heterostructures can be fabricated in the layer-by-layer growth developed in this work, the high-sensitivity SERS substrates are scalable and low cost for marketable devices in optoelectronics and biosensing.