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Surface-enhanced Raman scattering (SERS) has garnered substantial attention due to its ability to achieve single-molecule sensitivity by utilizing metallic nanostructures to amplify the exceedingly weak Raman scattering process. However, the introduction of metal nanostructures can induce a background continuum which can reduce the ultimate sensitivity of SERS in ways that are not yet well understood. Here, we investigate the impact of laser irradiation on both Raman scattering and backgrounds from self-assembled monolayers within nanoparticle-on-mirror plasmonic nanocavities with variable geometry. We find that laser irradiation can reduce the height of the monolayer by inducing an irreversible change in molecular conformation. The resulting increased plasmon confinement in the nanocavities not only enhances the SERS signal, but also provides momentum conservation in the inelastic light scattering of electrons, contributing to the enhancement of the background continuum. The plasmon confinement can be modified by changing the size and the geometry of nanoparticles, resulting in a nanoparticle geometry-dependent background continuum in SERS. Our work provides new routes for further modifying the geometry of plasmonic nanostructures to improve SERS sensitivity. [ABSTRACT FROM AUTHOR]

Single‐crystalline perovskites are ideal candidates for lasing and other optoelectronic applications. Although significant efforts have been made to grow both bulk single‐crystalline perovskites in liquid solution, their dimensions are still too large to make nanoscale whispering‐gallery‐mode (WGM) resonator based lasers that possess high quality (Q) factor and small volume. Besides, most reported perovskite resonators do not possess atomically smooth surfaces and facets, which limits the Q and thereby increases the lasing threshold. Here, atomically smooth triangular PbI2 templates are fabricated on a mica substrate by the vapor phase deposition method and are converted to atomically smooth perovskites which have regular and unwrinkled facets with average surface roughness less than 2 nm. By using a CH3NH3PbI3 nanoplatelet with a side length of 27 µm and thickness of 80 nm, room temperature WGM lasing with a Q up to 2600 is demonstrated, the highest reported for hybrid organic–inorganic perovskite nanoplatelets. In addition, the volume of the WGM mode is reduced significantly in comparison with the prior reports. The realized high‐quality triangular CH3NH3PbI3 perovskite nanoplatelets with high Q factor and small volume are expected to perform as ideal cavities for long pulse durations lasers and would find potential applications in integrated optoelectronic devices. Resonators that possess atomically smooth surfaces and facets are of critical importance for low‐threshold lasers. Here, atomically smooth CH3NH3PbI3 perovskite nanoplatelets with an average surface roughness less than 2 nm, which exhibit room temperature whispering gallery mode lasing with quality factors up to 2600, the highest reported for hybrid organic–inorganic perovskite nanoplatelets, are fabricated. [ABSTRACT FROM AUTHOR]

Multiple surface lattice resonances generated with nanoparticle arrays are promising to enhance light-matter interactions at different spectral positions simultaneously, and it is important to tailor these resonances to desired frequencies for practical applications such as multi-modal nanolasing. To this end, this study proposes to generate multiple surface lattice resonances using overlapping nanoparticle arrays with different lattice spacing. Both full-wave numerical simulations and analytical coupled dipole approximation calculations reveal that for the overlapping structures composed with two different gold nanosphere arrays, both surface lattice resonances for the element structures are effectively excited. Considering that the optical responses are governed by the dipole-dipole interactions between the nanoparticles, it is interesting to find that the multiple surface lattice resonances are almost invariant by adjusting the relative shifts between the two arrays, which can be useful to tailor the high-quality factor resonances to desired spectral positions. In addition, due to the same reason, it is also shown that the multiple surface lattice resonances can be further finely tuned by selectively removing specific nanoparticles in the array. We anticipate that the tolerance to generate multiple surface lattice resonances and the flexible tunability make the overlapping nanoparticle arrays useful to design high performance linear and nonlinear nanophotonic devices.

Optical microcavities embedded with transition metal dichalcogenide (TMDC) membranes have been demonstrated as excellent platforms to explore strong light-matter interactions. Most of the previous studies focus on strong coupling between excitons of unpatterned TMDC membranes and optical resonances of various microcavities. It is recently found that TMDC membranes patterned into photonic crystal (PhC) slabs can sustain guided-mode resonances that can be excited and probed by far-fields. Here, we present a comprehensive theoretical and numerical study on optical responses of Fabry-Pérot (F-P) cavity-WS 2 PhC hybrid structures to investigate the multi-mode coupling effects between excitons, guided-mode resonances and F-P modes. We show that both the exciton resonance and the guide-mode resonance of the WS 2 PhC can strongly interact with F-P modes of the cavity to reach strong coupling regime. Moreover, a Rabi splitting as large as 63 meV is observed for the strong coupling between the guided-mode resonance and the F-P mode, which is much larger than their average dissipation rate. We further demonstrate that it is even possible to realize a triple mode strong coupling by tuning the guide-mode resonances spectrally overlapped with the exciton resonance and the F-P modes. The hybrid polariton states generated from the triple mode coupling exhibit a Rabi splitting of 120 meV that greatly exceeds the criterion of a triple mode strong coupling (∼29.3 meV). Our results provide that optical microcavities embedded with TMDC PhCs can serve as promising candidates for polariton devices based on multi-mode strong coupling.

The competition mechanism of exciton decay channels in the multilayer TMDs remains poorly understood. Here, the exciton dynamics in the stacked WS 2 was studied. The exciton decay processes are divided into the fast and slow decay processes, which are dominated by the exciton-exciton annihilation (EEA) and defect-assisted recombination (DAR), respectively. The lifetime of EEA is on the order of hundreds of femtoseconds (400∼1100 fs). It is decreased initially, followed by an increase with adding layer thickness, which can be attributed to the competition between phonon-assisted effect and defect effect. The lifetime of DAR is on the timescale of hundreds of picoseconds (200∼800 ps), which is determined by the defect density especially in a high injected carrier density.

In this manuscript, we explored the performance of a hollow thin film array (HTFA) for the detection of HCl vapor based on fluorescence quenching. The HTFA structure was fabricated by manually stacking layers of an active thin film and a supporting film, alternately, with a hollow structure in each supporting film. The total penetration depth of vapor molecules in the HTFA sample is 2n times increased, where n is the layer number of the active thin film. We tested the sensing performance of the HTFA sample using fluorescence emission and laser emission in a Fabry-Pérot (FP) microcavity. In the fluorescence sensing, the sensing efficiency increases with the vapor concentration, and can be as high as 80% with a vapor concentration of 400 ppm. While in the laser sensing, the efficiency can achieve 100% with an external pump intensity three times of the lasing threshold at a vapor concentration of 85 ppm. The HTFA sample is not only suitable for vapor detection but also suitable for molecule detection in liquid.