Fig. 1. Highly confined electromagnetic field in the nanogap. (a) Simulated scattering cross-sections of a Au NSoM structure. The schematic of the structure is given in the inset. The diameter of the Au NS is 100 nm. The refractive index of the spacer is taken as a constant (1.45) for simplicity. The thickness is 1 nm. The propagating direction of the excitation light is set to be parallel to the substrate. (b) Contours of the electric field intensity enhancement along the different axes at the resonance wavelength marked by the star in (a). (c) Simulated scattering cross-sections with an in-plane excitation, where the propagating direction of the excitation light is perpendicular to the substrate. (d) Corresponding electric field intensity enhancement at the resonance wavelength marked by the star in (c).

Fig. 2. Plasmon modes in the nanogap. (a) Schematic of a flat-junction NPoM configuration, with a facet width of w. (b) Antenna modes l_n and gap modes s_mn in the NPoM structure, which can be adjusted with the facet width w. The shaded area shows an experimentally accessed facet range. (c) Charge distributions for the l₁, s₁₁, and s₀₂ modes. (d) Schematic showing the interactions between the antenna and gap modes. The plasmon modes in the nanogap are the hybridization (j_n) of the antenna and gap modes. (e) Extinction cross-sections of a faceted spherical gold nanoparticle in the NPoM structure. The diameter is 80 nm and the spacer thickness is varied from 0.6 to 1.4 nm. Figure reproduced with permission from: (b) ref.²³, Copyright 2014 American Chemical Society; (c–e) ref.²¹, Copyright 2015 American Physical Society.

Fig. 3. Construction of NPoM structures. (a–e) Plasmonic nanoparticles for the construction of NPoM structures: Au NSs (a), Au nanorods (b), Ag NCs (c), Au nanoplates (d), and Au nanodisks (e). (f) High-resolution transmission electron microscopy (TEM) cross-sectional image of a SLG-sandwiched NPoM structure. (g) Average count profile across the local area as indicated in the white box in (f). The values of brightness were extracted from the high-resolution TEM image in (f) for analysis. (h) Dark-field scattering spectra and images (insets) of the structure before and after Raman measurements. (i, j) Effect of the mirror quality: Au nanorod-on-(rough gold mirror) (i) and Au nanorod-on-(ultrasmooth gold mirror) (j). GNR: gold nanorod. The surface root-mean-square roughness of the single-crystalline gold microflake (GMF) in (j) is ~ 0.2 nm, which is much lower than that of the deposited gold film in (i). The ultrasmooth gold mirror endows the nanocavities with significantly enhanced quality factors and scattering intensities. Figure reproduced with permission from: (a) ref.²⁵, Copyright 2017 American Chemical Society; (b, i, j) ref.²⁷, Copyright 2022 American Chemical Society; (c) ref.²⁸, Copyright 2018 American Chemical Society; (d, e) ref.³¹, Copyright 2020 Wiley-VCH GmbH; (f–h) ref.¹⁷, Copyright 2019 American Chemical Society.

Fig. 4. Plasmon–exciton coupling in the extreme gap. (a) Schematic of the coupled harmonic oscillator model. (b) Schematic of a NCoM structure with a MoS₂ monolayer. The insets show the TEM image of an individual Ag NC (left) and the cross-sectional schematic of the structure (right). (c) Dependences of the optical mode volume (red) and coupling strength (blue) on the thickness of the spacer. The strong coupling regime is shaded in yellow. (d) Electric field enhancement at different spacer thicknesses. Figure reproduced with permission from: (b–d) ref.¹⁸, Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 5. Strong coupling. (a, b) Schematics of NPoM cavities constructed from WSe₂ flakes. The layer numbers of the WSe₂ flakes are 1 (a) and 12 (b), respectively. (c) Dark-field scattering spectra of the individual NPoMs, showing mode splitting for WSe₂ multilayer. The mode splitting was reproduced by FDTD simulations (dashed line), with two eigen-frequencies (ω_±) of the hybrid system and a Rabi splitting energy (ℏΩ) exceeding 140 meV. Only a single-plasmon peak was observed in the WSe₂ monolayer structure, where ω_p denotes the angular frequency of the plasmon resonance. (d) Schematic and scanning electron microscopy (SEM) image of a plasmonic nanocavity sandwiched with WS₂ monolayer. (e) PL (red) and scattering (cyan) spectra of the hybrid system in the strong coupling regime. Figure reproduced with permission from: (a–c) ref.³⁸, under the terms of the Creative Commons CC BY license; (d, e) ref.³⁹, Copyright 2020 American Physical Society.

Fig. 6. PL enhancement in the weak coupling regime. (a) PL spectra from a MoS₂ monolayer on a Si/SiO₂ substrate (blue) and in the nanocavity (red). The intensity is measured per unit of excitation power and per unit of integration time. The inset shows the normalized spectra for the two cases along with the scattering spectrum for a typical structure (gray). (b) Schematic of the MoS₂ monolayer-sandwiched NCoM structure. The Ag NC is wrapped with PVP and separated from an ultrasmooth gold film by an alumina layer and a MoS₂ monolayer. (c) Circularly polarized PL spectra under the right circularly polarized (σ⁺) excitation. The degree of circular polarization, defined as ρ = (I(σ⁺) − I(σ⁻))/(I(σ⁺) + I(σ⁻)), was used to evaluate the valley modulation. I(σ⁺) and I(σ⁻) are the measured right-handed and left-handed components of the PL emissions, respectively. A high degree of circular polarization up to 48.7% was obtained. (d) Schematic showing the valley-dependent emissions owing to the chiral Purcell effect. The decay rate of the excitons in the –K (γ_−K) valley is much larger than that in the K (γ_K) valley, leading to enhanced left-handed PL emissions. The intervalley scattering process is denoted using the parameter (γ_s) with a similar expression to the decay rate of the excitons. Figure reproduced with permission from: (a) ref.⁴³, Copyright 2015 American Chemical Society; (b–d) ref.⁵⁰, Copyright 2020 American Chemical Society.

Fig. 7. Plasmonic enhancement for the detection of new exciton complexes. (a) PL spectra from a MoS₂ monolayer on Si/SiO₂ (black) and a MoS₂ monolayer in the nanocavity containing a 65 nm Ag NC (red). The B exciton emissions are largely enhanced. (b) Schematic showing the bright (X₀) and dark excitons (X_D) in the WSe₂ monolayer at the K valley (left) and the coupling between the gap plasmon mode of NSoM and the out-of-plane dipole of the dark excitons (right). (c) PL spectra obtained from the unetched WSe₂-NSoM (top) and etched WSe₂-NSoM (bottom). The insets show the corresponding nanostructures. The PL peak related to the dark excitons is clearly seen in the etched structure. (d) Schematic of a WS₂/MoS₂ heterostructure inserted in a NPoM cavity. A thin h-BN flake acts as a spacer to adjust the resonance wavelength to match the interlayer emissions. The type II band alignment is shown in the right panel, forming the interlayer excitons (IX). (e) PL spectra from the coupled (red) and uncoupled structures (blue). Figure reproduced with permission from: (a) ref.⁵³, Copyright 2017 American Chemical Society; (b, c) ref.⁵⁵, Copyright 2022 American Chemical Society; (d, e) ref.⁵⁷, Copyright 2021 Wiley-VCH GmbH.

Fig. 8. Excitonic upconverted emissions. (a) Schematic of the plasmonic nanocavity. The WSe₂ monolayer is separated from the Au NCs with an organic adhesive layer, i.e., poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) (PAH-PSS), to avoid the hot carrier injection from the plasmonic nanoparticles. (b) Energy diagram showing the photon upconversion process of the 2D excitons. The electrons in the ground state (g) are excited through the absorption of a photon with energy (ℏω₁) and pumped to a higher energy state (X_A) through the absorption of phonons (red arrow). The formed excitons are then recombined through the emissions of upconverted photons with energy (ℏω₂). The plasmonic nanocavities provide two cavity modes for double resonance with the incident and emitted photon energies to achieve both excitation and emission enhancement. (c) Spectra corresponding to the excitation laser light, PL emissions of WSe₂ monolayer, and dark-field scattering of the hybrid structure. (d) Temperature-dependent upconverted emissions, indicating the influence of phonons. Figure reproduced with permission from: ref.⁶¹, under a Creative Commons Attribution 4.0 International License.

Fig. 9. Surface-enhanced Raman scattering. (a) Schematic (top) and cross-sectional TEM image (bottom) of a 1L MoS₂-NPoM structure. The inset in the bottom panel shows the enlarged image of the marked area (red square). (b) Atomic displacements of the A_1g, E2g1, and LA modes in the unit cell of MoS₂ monolayer. (c) Normalized Raman spectra of 1L MoS₂-NPoM, 1L MoS₂ on a Au film, and 1L MoS₂ on a quartz substrate. In the 1L MoS₂-NPoM cavity, the MoS₂ monolayer is coated with a 32-nm-thick Al₂O₃ layer. (d) Schematic showing the pumping process of a MoS₂-spaced NCoM system under laser light excitation (ω_L). The plasmonic nanocavity is designed to selectively enhance the Stokes (ω_L − ω_phon) or anti-Stokes (ω_L + ω_phon) processes. (e) Scattering and laser light spectra (blue line, 784 nm; orange line, 821 nm) in the blue-detuned and red-detuned measurements, respectively. The inset shows the SEM image of the tested sample (scale bar: 100 nm). (f) Power-dependent SERS intensity for the A_1g mode, showing a drastic increase in the blue-detuned case. Figure reproduced with permission from: (a–c) ref.²⁴, under a Creative Commons Attribution 4.0 International License; (d–f) ref.¹⁹, Copyright 2022 American Chemical Society.

Fig. 10. Quantum tunneling through the atomic spacer. (a) Measured (Exp. EF¯z) and simulated maximal field enhancement in response to the gap distance in a 1L MoS₂-NPOM structure. The simulated field enhancement factors were calculated using the E⁴ model (Sim. E⁴) and the two-study model (Sim. TSM), respectively. The reduction of the measured plasmonic enhancement in the atomic gap suggests the emergence of the quantum tunneling effect. (b) Schematics showing two types of structures, Au NS on a Au film without (top) and with (bottom) SLG as a spacer. (c) Single-particle dark-field scattering spectra of the two types of structures in (b). The corresponding dark-field images (left) and SEM images (right) are given in the insets. The transverse (T) and charge transfer (P_CTP) plasmon modes are seen when the Au NSs are in direct contact with the Au film. Interestingly, the plasmon doublet (P₊ and P₋) appears in the SLG-sandwiched structure, which is believed to arise from the hybridization of the charge transfer plasmon mode and the gap mode. The scale bar is 1 μm. Figure reproduced with permission from: (a) ref.²⁴, under a Creative Commons Attribution 4.0 International License; (b, c) ref.²⁰, Copyright 2013 American Chemical Society.

Fig. 11. 2D material-gapped nanocavities for nanoscale light sources. (a) Schematic of WSe₂ monolayer coupled to a Au NCoM array for single-photon emissions. The inset shows the cross-sectional view of the structure. The WSe₂ monolayer is separated from the Au NCs and the Au film by a 2 nm Al₂O₃ layer (grey shading in the inset) on each side to prevent quenching and short-circuiting of the nanoplasmonic gap mode. (b) Simulated (dashed line), measured (grey solid line) extinction spectra, and emission spectrum (red solid line) of an individual quantum emitter. (c) Second-order photon-correlation function g⁽²⁾(τ) recorded under pulsed excitation, indicating single-photon emissions. (d) Single-photon purity values at zero delay time for 15 quantum emitters. (e) Schematic showing an electroluminescence device. A PVP-coated Ag NC is separated from a Au film by a graphene (top) and h-BN (bottom) stacking. (f, g) Measured spatial (f) and spectral (g) photon distributions. The inset in (f) shows a line-cut of the emission spot, featuring a linewidth of ~ 460 nm. Figure reproduced with permission from: (a–d) ref.¹³, Copyright 2018 The Author(s), under exclusive licence to Springer Nature Limited; (e–g) ref.⁸⁷, Copyright 2019 The Author(s), under a Creative Commons Attribution 4.0 International License.