Felix Schneider

Plasmonic-photonic metasurfaces enable large-area, spectrally selective field enhancement for surface-enhanced Raman scattering (SERS). However, identifying low-loss hybrid plasmon-photon modes under confocal, near-normal excitation remains elusive. Angle-resolved SERS provides direct access to their dispersion and degeneracies under stationary, ultrahigh numerical aperture excitation (NA ≈ 1.0 in air) by independently scanning polar and azimuthal incidence angles. SERS additionally demands modes that tolerate defects and periodicity fluctuations. We therefore fabricate colloidal metasurfaces across particle concentrations and lattice periods to quantify robustness. Using template-assisted self-assembly, we reveal a direct correlation between nanoparticle concentration, structural order, and the emergence of hybrid modes. At the optimized periodicity, SERS is maximized (up to 2.32× higher SERS intensity), which we can modulate through resonance detuning. Angle-resolved SERS at 633 and 785 nm under oblique incidence shows maximal enhancement when the hybrid guided-mode resonance is tuned to the laser line, yielding up to 36× higher intensity than off-resonant conditions for the same sample. An analytical model attributes these modes to phase-coupled waveguide coupling and collective plasmonic chain resonances. Thus, we yield practical design rules for robust, tunable colloidal plasmonic-photonic SERS metasurfaces via hybrid-mode dispersion engineering.

Tip‐enhanced Raman spectroscopy (TERS) enables molecular vibrational spectroscopy with subnanometer resolution. Although TERS mechanisms and applications continue to be actively explored, the spatial characteristics of far‐field TERS emission, which are critical for quantitative interpretations, reproducibility, and cross‐comparison, have received comparatively less attention. Moreover, although tip orientation and shape are routinely considered, the influences of individual setup configurations on far‐field collection efficiency are often overlooked in data analysis, particularly when comparing experimental results across different research groups. Recent angularly resolved TERS studies have revealed spatial variations in far‐field emission for both photoluminescence and Raman scattering. These findings suggest that spatially selective far‐field signal collection schemes can significantly compromise spectral interpretation in TERS. Given these considerations, this review summarizes fundamental geometries and optical polarization techniques employed in TERS and links them to plasmonic interactions between the tip, the sample, and the substrate. In conclusion, it elucidates how these factors influence the far‐field response. Special emphasis is placed on recent experimental and theoretical work concerning optically excited TERS signal and its angular emission.

Monolayered WS ₂ exhibits a direct band gap, strongly bound excitons, trions and biexcitons as well as no inversion symmetry in the 2D crystal structure ( D _3h ), allowing frequency doubling by second-harmonic generation.

Kurzzusammenfassung: Trotz intensiver Forschung an spitzenverstärkter Ramanspektroskopie (TERS) wurde die Winkelverteilung der Raman‐Streuung bisher nicht experimentell untersucht. Deshalb wird die Relevanz der Winkelverteilung für das gemessene TERS‐Signal selten diskutiert. In dieser Arbeit untersuchen wir die Winkelverteilung des spitzenverstärkten Ramansignals in der Fourier‐Ebene. Dafür verwenden wir ein Modellsystem aus flach liegenden Kobalt(II)hexadecaflouro‐Phthalocyanin Molekülen (CoPcF ₁₆ ), die auf einem glatten Goldfilm adsorbiert sind. Sowohl Vibrationen in der Molekülebene als auch solche senkrecht zur Ebene können beobachtet werden. Die Ramanmoden senkrecht zur Ebene bei 678 cm ⁻¹ und 740 cm ⁻¹ haben dabei andere winkelabhängige Intensitätsverteilungen als die Ramanmoden parallel zur Ebene bei 1309 cm ⁻¹ und 1373 cm ⁻¹ . Wir interpretieren das gemessene TERS‐Signal unter Berücksichtigung der vibronischen Moden, die mithilfe von Dichtefunktionaltheorie (DFT) berechnet wurden, und der direktionalen Ramanstreuung im Bereich (Lücke) zwischen Spitze und Goldfilm, die mithilfe der „Finite‐Difference Time‐Domain (FDTD)“ Methode simuliert wurde. Wir nehmen an, dass die TERS‐Lücke die Ramanschwingungsmoden unterschiedlich weiterleitet, was zu unterschiedlichen Winkelverteilungen in der resultierenden Intensität der Ramanstreuung führt. Unsere Ergebnisse zeigen, dass die TERS‐Detektionsgeometrie für das Verständnis der spektralen Eigenschaften nicht vernachlässigbar ist. Das betrifft beispielsweise die Variation relativer Peakintensitäten bei der Untersuchung molekularer Orientierungen oder die Verfolgung chemischer Reaktionen mithilfe der Ramanspektroskopie.

Despite intensive research in tip‐enhanced Raman spectroscopy (TERS), the angular distribution of Raman scattering in the TERS gap remains experimentally unreported leaving its relevance to the TERS signal formation to be seldomly discussed. Here, we investigate the angular distribution of the tip‐enhanced Raman signal in the Fourier plane using a model system composed of flat‐lying cobalt (II) hexadecafluoro‐phthalocyanine (CoPcF ₁₆ ) molecules physically adsorbed on a smooth gold surface. Both in‐plane and out‐of‐plane vibrational modes are observed, where the out‐of‐plane Raman modes at about 678 and 740 cm ⁻¹ have different angular intensity distributions than those of in‐plane Raman modes at 1309 and 1373 cm ⁻¹ . We interpret the angular spectrum of the TERS signal considering the molecular vibrational modes computed with density functional theory (DFT) for the free and gold‐deposited molecule, and the directed Raman scattering by the gap‐mode predicted by finite‐difference time‐domain (FDTD) simulations. We contend that the TERS gap directs the Raman vibrational modes differently, leading to distinct angularly distributed Raman scattering intensities. These findings emphasize the nonnegligible role of the TERS detection scheme in understanding spectral features, such as the relative peak intensity ratio variations for studying molecular orientations, or for monitoring chemical reactions.

An analytic model is used to calculate the Raman and fluorescence enhancement of a molecule in between two closely spaced gold nanospheres. Instead of using the conventional approach that only the dipolar plasmonic mode is considered, we calculate the electric field enhancement in the nanometre sized gap, by taking account of the higher order modes in one gold sphere, which couples to the dipolar mode of the other sphere. The experimental confirmation is performed by gap‐dependent tip‐enhanced Raman spectroscopy (TERS) measurements. The photoluminescence and Raman enhancement are both observed with different growing trends as the gap width decreases. Red‐shift of the background spectra is observed and implies the increasing coupling between the nanospheres. This analytic model is shown to be able to interpret the enhancement mechanisms underlying gap‐dependent TERS experimental results.