Nanoparticle-on-mirror pairs: building blocks for remote spectroscopies

Abstract

Surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS), fluorescence (SEF), circular dichroism, etc., are powerful tools for investigating nano-entities with high sensitivities. Owing to the giant local electric field confined in a plasmonic nanogap, nanogap-enhanced spectroscopies could detect samples with ultralow concentrations, even down to the single-molecule level for SERS and SEF. This great ability to detect analytes with ultralow concentrations provides opportunities for early diagnosis and monitoring in modern biomedicine. However, local laser excitations would inevitably bring about unwanted disruptive background perturbations, local heating, and the consequent geometry reshaping and biological analyte damages. Remote spectroscopies avoiding direct laser exposure to the samples can be treated as remarkable solutions. Here, we combined the nanoparticle-on-mirror (NPoM) family with the philosophy of remote spectroscopy to construct so-called “NPoM pairs” structures. They consist of two identical NPoMs with matched resonances yet separate functions either as receiving or transmitting antennas. A figure of merit for evaluating the remote spectroscopies was put forward, which accounts for the efficiencies in three processes, i.e., receiving, transporting, and transmitting. In addition, we experimentally demonstrated the performances of these NPoM pairs by proof-of-principle applications on the remote SERS and SEF. The optical access of the spectral information in these NPoM pairs both locally and remotely manifests themselves as fundamental building blocks for remote spectroscopies.

Tunable Couplings of Photons with Bright and Dark Excitons in Monolayer Semiconductors on Plasmonic-Nanosphere-on-Mirror Cavities

“…A longer-wavelength mode highlighted by the blue-shaded area is only accessible under oblique incidence, while the areas shaded in pink and gray highlight two modes dominant at normal incidence. We label them as an antenna mode l 01 (blue) and waveguide modes S 11 (gray) and S 12 (pink) based on their mode profiles in Figure c,d, in agreement with the literature. ,− As illustrated in Figure c, S 11 mode is defined by the field distributions at the AuNS–spacer interface and shows an in-plane dipole moment in the gap. S 12 mode is the higher-order mode featuring a quadrupole in the AuNS (Figure c) but with a weaker local E -field intensity (Figure d).…”

“…Note that the resonance wavelengths of the bright and dark excitons in monolayer WS 2 are ∼612 and 630 nm, respectively, , and the material and size/thickness of the nanosphere/spacer are designed accordingly for the spectral overlap. ,− Figure b shows the simulated total scattering efficiency of such a well-designed NSoM cavity with a SiO 2 spacer of 9 nm thickness. The excitation-angle dependence is evaluated by introducing transverse magnetic (TM)-polarized incident light, considering the rotational symmetry of AuNSs. The typical spectra under normal incidence and oblique incidence at 56° are plotted in dashed-dotted and solid lines for comparison.…”

“…31,36−38 Figure 1b shows the simulated total scattering efficiency of such a well-designed NSoM cavity with a SiO 2 spacer of 9 nm thickness. The excitation-angle dependence is evaluated by introducing transverse magnetic (TM)-polarized incident light, 36 consid- ering the rotational symmetry of AuNSs. The typical spectra under normal incidence and oblique incidence at 56°are plotted in dashed-dotted and solid lines for comparison.…”

“…Plasmonic NSoM cavities are made of a metallic nanosphere standing on a dielectric-coated mirror, which support highly confined light field down to deep-subwavelength scales in the gap between the nanosphere and mirror. − The complex hybridization of plasmons in such a configuration leads to various cavity modes, − providing more degrees of freedom in design to match the in-plane and out-of-plane dipole moments of bright and dark excitons in monolayer TMDs. − Moreover, in comparison with other nanoparticle-on-mirror counterparts such as structures based on nanocubes or nanowires, nanosphere-based cavities can generate an extremely excitation-sensitive out-of-plane antenna mode in contrast to the in-plane waveguide modes because of their unique morphology and symmetry. , This makes NSoM a good candidate for us to design tunable couplings with dark excitons as well as the dynamic interaction between bright- and dark-exciton–photon hybrids.…”

“…39−41 Moreover, in comparison with other nanoparticle-on-mirror counterparts such as structures based on nanocubes or nanowires, nanosphere-based cavities can generate an extremely excitation-sensitive out-of-plane antenna mode in contrast to the in-plane waveguide modes because of their unique morphology and symmetry. 36,38 This makes NSoM a good candidate for us to design tunable couplings with dark excitons as well as the dynamic interaction between bright-and dark-exciton−photon hybrids.…”