Understanding light-matter interactions in deeply subwavelength materials with strong resonant properties is crucial for both fundamental science and nano-optoelectronic applications. Typically, light doesn't significantly alter a material's electronic dispersions because the material dimensions exceed the wavelength (λ), or lack an electronic dipole resonance at λ, resulting in weak coupling. However, stronger coupling between light and matter's dipoles leads to the formation of polaritons—quasiparticles that are part-light, part-matter. Different types of dipoles (plasmonic in metals, excitonic in semiconductors, and phononic in dielectrics) create distinct polaritons. Strong light-matter coupling requires confining light within a low-dimensional material or interface to efficiently interact with dipoles. Surface plasmon polaritons (SPPs) confine light to a dielectric/metal interface, creating propagating electromagnetic waves along the surface. Exciton-polaritons, conversely, need an intrinsic optical resonance overlapping with trapped light wave packets in an optical cavity (like a Bragg mirror or plasmonic cavity). Propagating exciton-polariton modes have also been observed using scattering-type nanoprobes and microcavities. Reduced-dimensional materials exhibit strong exciton-medium interactions due to minimal dielectric screening. The high binding energies of excitons in 2D WSe2 (0.78 eV), 1D carbon nanotubes (0.3-0.4 eV), and 0D CdSe quantum dots (0.2-0.8 eV) significantly exceed room-temperature thermal energy (0.025 eV). This large binding energy makes excitons, not free carriers, the dominant excited species, enhancing light-matter interaction. Strong light-matter coupling in excitonic nanomaterials has been studied using various methods, such as exciton-polaritons in 2D MoS2 within optical cavities, exciton-plasmon polaritons in 2D WSe2, CdSe/ZnS quantum dots in plasmonic cavities, and SPPs at 2D MoS2/Al2O3/Au interfaces. Most prior studies used diffraction-limited optical setups or non-optical excitation techniques (electron energy loss spectroscopy). However, imaging strong light-matter coupling in nanoscale materials under optical near-field excitation remains largely unexplored, along with the influence of nano-probes and complex nano-optical fields on dipole interactions and energy transduction at these subwavelength scales. Tip-enhanced nano-spectroscopy enables direct nano-resolution spatio-spectral imaging of nanomaterial emission at optical frequencies. By utilizing the plasmonic gap mode confined between a plasmonic tip and substrate, it visualizes optical responses from subwavelength semiconductor structures (quantum dot fluorescence patterns, strain-induced Raman/fluorescence shifts, van der Waals semiconductor heterostructures, and localized excitonic emission from nanobubbles). Most studies use contact mode to maximize plasmonic gap mode confinement, achieving strong light-matter coupling normal to the surface (e.g., exciton-plasmon polariton formation in CdSe/ZnS quantum dots, and brightening of dark excitons in transition metal dichalcogenides via the Purcell effect). However, tapping-mode studies of inelastic emission or scattering remain limited. While tapping mode maintains subwavelength resolution and avoids charging, the tip's role in emission is unclear. This paper investigates in-plane light-matter interactions using tapping mode tip-enhanced spectroscopy.

The study builds upon existing research on strong light-matter interactions and polariton formation in various materials and geometries. Previous work has demonstrated the existence of exciton-polaritons in 2D materials like MoS2 and WSe2, often utilizing optical cavities to enhance light-matter coupling. Research on quantum dots coupled to plasmonic structures has explored the formation of exciton-plasmon polaritons and the effects of strong coupling on emission properties. The use of tip-enhanced spectroscopy to study near-field interactions in nanomaterials has also been established, with several studies focusing on the normal direction interactions. However, the authors note a lack of comprehensive research on in-plane near-field interactions and the role of the tapping mode AFM tip in these interactions, which motivates their current investigation. This paper expands the existing knowledge by directly visualizing and characterizing in-plane light propagation launched by excitonic emission from nanoscale emitters, providing a detailed understanding of the underlying mechanisms.

The researchers synthesized quasi-2D CdSe/CdxZn1-xS core-shell nanoplatelets (NPs) with dimensions of 40.2 ±2.9 nm (length) × 16.1 ± 1.7 nm (width) × 2.8 ± 0.5 nm (thickness), as confirmed by TEM. To capture in-plane near-field signals, they employed hyper-spectral mapping using tapping mode operation in a tip-enhanced nano-spectroscopy setup. The use of tapping mode is crucial, as the authors demonstrate in supplementary information that contact mode leads to different types of strong light-matter coupling. They also investigated the effects of charge-induced quenching, demonstrating its absence in their system. The gold tip acts as a near-field scatterer, allowing the collection of light propagating along the dielectric/metal interface. The emission from the NPs launches SPPs along the dielectric/metal interface, and these SPPs interfere upon reflection from the tip, creating standing waves. This is supported by extensive electromagnetic wave simulations using COMSOL Multiphysics. The simulations showed that the fringes only appear when both the NP and the tip are present and close to the substrate. As the tip-NP distance increases, the electric field intensity changes periodically. The fringe period is determined by the standing wave condition (λSPPN = Ln, where λSPP is the SPP wavelength, N is an integer, and Ln is the distance between the tip and emitter), and calculated using the dispersion relation of SPPs (λSPP = λ0√(εd + εm)/(εdεm), where λ0 is the PL emission wavelength, and εd and εm are the dielectric constants of the dielectric and metal, respectively). Hyperspectral tip-enhanced photoluminescence (TEPL) maps with ~20 nm spatial resolution were obtained to verify the interference pattern, showing spatially localized emission at 664 nm in near-field TEPL but a distinct boomerang-shaped fringe pattern in tapping mode TEPL. The authors demonstrate the influence of the NP orientation (edge-up vs. face-down) on fringe formation, attributing the fringes to SPPs launched by excitons in the NPs and scattered by the tip. They also investigated the effects of polarization of the excitation laser, showing that only TM-polarized light excites out-of-plane transition dipoles in the edge-up assembled NPs that strongly couple to the plasmons and launch SPPs, while TE-polarized light shows negligible SPP generation. The shape of the fringe patterns (e.g., boomerang-shaped versus circular) is related to the transition dipole orientation of the NPs, with inclined dipoles creating parabolic patterns. The effect of dielectric permittivity and thickness on SPP propagation was studied by using different dielectrics (Al2O3, TiO2, WSe2) and thicknesses, with both simulations and experimental measurements demonstrating that a larger permittivity difference between the dielectric and metal leads to stronger light confinement. Finally, the authors extended their findings to monolayer WSe2 nanobubbles and TiO2-NP structures on SiO2/Si substrates, demonstrating the universality of the fringe pattern formation for various localized emitters and substrates.

This research presents several key findings: 1. **Direct Visualization of In-Plane SPP Propagation:** The study successfully visualized the in-plane propagation of surface plasmon polaritons (SPPs) launched by excitonic emission from nanoscale emitters. The SPPs manifest as distinct fringe patterns in near-field photoluminescence maps, extending up to 1.7 µm from the emitter, demonstrating the ability to image near-field interactions over substantial distances at deep subwavelength scales. 2. **Standing Wave Formation:** Extensive electromagnetic simulations confirmed that the observed fringe patterns are standing waves formed by the interference of SPPs launched by the nanoplatelets and reflected by the AFM tip. This highlights the crucial role of the tip not only as a detector but also as a reflector that shapes the observed interference patterns. 3. **Influence of Nanoparticle Orientation and Polarization:** The orientation of the nanoplatelets significantly impacts SPP generation and fringe pattern formation. Edge-up assembled nanoplatelets, with their transition dipole moments oriented perpendicular to the substrate, generate strong SPPs and clearly defined fringe patterns under TM-polarized excitation. Face-down NPs, and TE-polarized excitation yield significantly weaker or absent fringe patterns, demonstrating a strong dependence on exciton dipole orientation and polarization. 4. **Dielectric Engineering of Light Confinement and Emission:** The surrounding dielectric environment plays a critical role in controlling both light confinement and in-plane emission. Both simulations and experimental data show that increasing the dielectric constant or thickness of the surrounding layer enhances the strength and propagation length of SPPs, leading to more prominent and extended fringe patterns. This demonstrates the potential to engineer the light-matter interaction by controlling the dielectric properties of the surrounding medium. 5. **Universality of the Fringe Phenomenon:** The formation of fringe patterns was observed not only for CdSe/CdxZn1-xS nanoplatelets but also for monolayer WSe2 nanobubbles and even on non-plasmonic SiO2/Si substrates with a high-index dielectric waveguide layer. This suggests that the phenomenon is a general feature of near-field interactions between localized nanoscale emitters and a reflective surface, regardless of the specific material system or the presence of plasmonic effects. 6. **Quantitative Analysis of Fringe Patterns:** The study provides detailed quantitative analysis of the fringe patterns, including their period, decay length, and relationship to the dielectric properties of the surrounding medium and the transition dipole orientation of the emitters. This allows for a precise understanding of the energy transfer mechanisms and light confinement in these nanoscale systems. These findings collectively demonstrate the potential for developing novel nano-optoelectronic devices and exploring quantum phenomena at the nanoscale.

The findings of this research significantly advance our understanding of light-matter interactions in nanoscale excitonic emitters. The direct visualization of in-plane SPP propagation and the identification of the standing wave formation mechanism provide crucial insights into the near-field energy transfer processes. The demonstrated control over light confinement and in-plane emission via dielectric engineering opens new avenues for designing advanced optoelectronic devices. The universality of the fringe phenomenon expands the applicability of the technique to a wide range of nanoscale emitters and substrates. The quantitative analysis provides valuable tools for characterizing nanoscale light-matter interactions and for studying the properties of nanoscale emitters. The ability to infer transition dipole orientation from fringe patterns offers a unique method to characterize nanoscale systems. The extension of the findings to non-plasmonic substrates suggests the potential to observe similar phenomena in diverse systems beyond plasmonics. This work, therefore, bridges the gap between fundamental understanding and practical applications, paving the way for future investigations into quantum nanophotonics and the development of novel nanoscale devices.

This study presents a comprehensive investigation of near-field light-matter interactions in nanoscale excitonic emitters. The use of tapping-mode tip-enhanced nano-spectroscopy allowed for the direct visualization of in-plane SPP propagation, revealing standing wave patterns influenced by emitter orientation, polarization, and dielectric environment. The universality of this phenomenon across different emitter types and substrates highlights its fundamental nature. This research provides a new tool for characterizing nanoscale light-matter interactions, offering insights into energy transfer and the potential for advanced nano-optoelectronic device development. Future work could explore the application of this technique to investigate quantum emitters and further refine the understanding of light-matter interactions at deep subwavelength scales.

While this study provides significant insights, some limitations should be noted. The large error bars in the fringe period measurements, attributed to the lossy plasmonic cavity, highlight the challenges of precisely quantifying SPP propagation in complex systems. The study focuses primarily on a specific range of nanoplatelet and nanobubble sizes, and further investigation is needed to confirm the generality of the observations across a broader range of sizes and shapes. The reliance on simulations for interpreting the results implies the need for experimental validation and refinement of the theoretical models. Future studies could address these limitations by exploring larger datasets, investigating different nanomaterial systems, and refining the experimental and theoretical models.