Giant mid-IR resonant coupling to molecular vibrations in sub-nm gaps of plasmonic multilayer metafilms

The study addresses the challenge of strengthening light–matter interactions for vibrational spectroscopies (IR absorption and Raman scattering), which are intrinsically weak for dilute analytes. Conventional plasmonic approaches (micron-scale antennas, metamaterials, nanoparticle aggregates) can enhance IR interactions but often lack narrow, tunable resonances in the mid-infrared (MIR), or require highly ordered superlattices that are difficult to fabricate with small gaps and poor analyte accessibility. The authors propose a self-assembled, short-range-ordered Au nanoparticle multilayer on a mirror (NPnML-on-mirror) using a sub-nm molecular spacer to produce robust, strongly absorbing, and highly tunable MIR plasmon-polariton resonances. The goal is to achieve deep subwavelength confinement, large field enhancements for SEIRA and SERS, and efficient free-space coupling to nanogaps to enable sensitive vibrational sensing of very small sample volumes. This platform aims to overcome limitations of order-dependent superlattices and broaden practical SEIRA/SERS applications.

Prior work demonstrates that plasmonic constructs can boost IR and Raman interactions using resonant antennas, metamaterials, and nanoparticle aggregates, enabling SEIRA for diverse applications. Highly ordered nanoparticle superlattices and superparticles support collective resonances shifted into the NIR/MIR but demand stringent fabrication, present tortuous gap networks that hinder analyte delivery, and typically achieve larger mode volumes (on order of (λ)³ in microcavities or ~100(λ)² in patch antennas/superlattices). Chains of nanoparticles create super-radiant modes but saturate near λ ~ 1 µm due to intrinsic coupling. Random Au aggregates and rough Au offer broad IR enhancements but lack specific, tunable resonances. There is a recognized need for improved SEIRA-active substrates and platforms combining SEIRA and SERS. The presented approach leverages self-assembly with a molecular spacer to realize well-defined sub-nm gaps in a disordered multilayer that supports tunable, robust MIR resonances while facilitating analyte access.

- Fabrication: Au nanoparticles (D = 20–100 nm) are aggregated using cucurbit[5]uril (CB[5]) and concentrated in chloroform; drop-casting onto a substrate yields millimeter-scale 2D AuNP monolayers formed at the chloroform–water interface with ~65% fill fraction. Sequential deposition after drying builds NPnML multilayers (n = 1–9). CB[5] binding sets uniform nanogap sizes d = 0.9 ± 0.05 nm; alternative aggregation (salt/acid) leads to broader gap distributions and broader modes. - Structural characterization: Optical profilometry maps film thickness (regions with 1–2 layers) and roughness; SEM reveals disordered NP networks with connected clusters and vacancies. The prevalent structural motif resembles AuNP septamers within the amorphous network. - Optical measurements: Reflectance/extinction spectra from visible to near-IR measured using a custom spectrometer for NP1ML-on-mirror as a function of D; FTIR extinction spectra collected for NPnML (n = 1–9) with D = 100 nm to track MIR resonances and molecular vibrational signatures of CB[5]. Spectra are reported in absorbance units (−log10 R). Near-complete extinction observed at MIR resonances (R < 5%). - Modeling: A generalized circuit model (small-gap limit) estimates the collective plasmon ω₁, primarily determined by nanogap capacitance. Hybridization of ω₁ with light in the effective medium yields a bulk plasmon–polariton ωBPP with Hopfield-type dispersion. For finite-thickness multilayers on a mirror, standing-wave Fabry–Pérot-like resonances form with the boundary condition enforced by the mirror field null, giving λ1 = 4L with L = nD and in-plane wavevector k|| = 2π/(4L). Experimental resonance positions for n = 1–9 are fit with an effective index neff ≈ 1.3 and coupling parameters (ε1 ≈ 0.71 eV, ε2 ≈ 0.51 eV), capturing the redshift with increasing D and n. - Vibrational coupling analysis: When ωBPP approaches the CB[5] carbonyl stretch (ωm = 1765 cm⁻¹), the spectra exhibit asymmetric Fano lineshapes. Anticrossing behavior is quantified by tracking the molecular Fano dip versus plasmon resonance position as n varies near 7–8 ML. Coupling strength g = 102 ± 8 cm⁻¹ is extracted. Linewidths: plasmon γc is broad (~770 cm⁻¹) versus molecular γm ~70 cm⁻¹. A modified coupled-oscillator/Fano model is used to fit line shapes: P(ω) ∝ [(ν + q)² + B]/(ν² + 1) with ν the normalized detuning; fits yield Fano asymmetry parameter q, molecular Lamb shift δωm, and multipolar plasmonic contributions (δωp, γc) as functions of detuning Δ = ωBPP − ωm. - Purcell and mode volume: From coupling parameters and radiative rates, the radiative Purcell factor Fp = 4g²/(γcγm) is evaluated. The spontaneous radiative decay rate of CB[5] outside the cavity is estimated from oscillator strength (γm ≈ 1.1 × 10⁻⁶ cm⁻¹). Fitting near-resonant spectra gives in-cavity rates γm = 62 ± 5 cm⁻¹ and γc = 49 ± 4 cm⁻¹ for Purcell evaluation, yielding Fp ≈ (6 ± 1) × 10⁶ and an effective mode volume Vm ≈ 1.5 × 10⁻⁸ λ³ (~2900 nm³), consistent with confinement across ~7 nanogaps in a septamer motif with ~25 nm facets and 0.9 nm gaps. - SEIRA vs SERS measurements: SEIRA spectra (NP7ML-on-mirror) highlight the CB[5] portal C=O stretch; SERS spectra emphasize the 830 cm⁻¹ CB[5] ring-breathing mode and citrate signatures, reflecting different selection rules. Hyperspectral IR and Raman maps of the same substrate show SEIRA variability tied to local layer number (tuning ωBPP), while SERS is more homogeneous due to uniform nanogaps and local septamer structures. For analyte demonstration, a 4'-cyanobiphenyl-4-thiol (BPTCN) SAM is formed on the mirror prior to NPML deposition, yielding strong SERS with an enhancement factor ~1.1 × 10⁶. Evidence for SEIRA of flowed analytes through CB[5]-scaffolded gaps is noted (SI).

- The NPnML-on-mirror architecture supports dual resonances (visible and MIR) with near-total extinction (>90%, reflectance <5%) and strong tunability by nanoparticle diameter (D = 20–100 nm) and number of layers (n = 1–9), reaching MIR wavelengths beyond 11 µm (~900 cm⁻¹). - Uniform sub-nm gaps defined by CB[5] (d = 0.9 ± 0.05 nm) drive deep subwavelength confinement, enabling efficient free-space coupling into gaps even in disordered films and practical SEIRA/SERS sensing of sub-picolitre volumes. - Model–experiment agreement: A Hopfield-type plasmon–polariton dispersion with λ1 = 4L (L = nD) accurately describes resonance evolution; fitted neff ≈ 1.3 and coupling parameters reproduce the observed redshifts with D and n. - Vibrational coupling: Strong Fano interference of the CB[5] C=O stretch at 1765 cm⁻¹ with the MIR plasmon produces giant Fano dip strengths (comparable to the plasmon mode extinction) and large molecular Lamb shifts up to ~40 cm⁻¹. Coupling strength g = 102 ± 8 cm⁻¹; q flips sign with detuning and the largest Fano response occurs off zero detuning (Δ ~ 200 cm⁻¹) due to multipolar near-field contributions. - Coupling regime: Despite large g, broad plasmon linewidth (γc ~ 770 cm⁻¹) vs narrow molecular linewidth (γm ~ 70 cm⁻¹) keeps the system in the weak coupling regime (2g/γc ~ 0.3), but with high cooperativity C ≈ 0.8, comparable to visible single-molecule strong coupling and MIR collective strong coupling reports. - Enhancement metrics: Lower-bound SEIRA enhancement factor EF ≈ (1.2 ± 0.1) × 10⁶ (from comparison to 5 mM CB[5] transmission), exceeding many state-of-the-art SEIRA platforms (typically 10²–10⁴). SERS EF for a BPTCN SAM on the mirror under NP1ML is ~1.1 × 10⁶, demonstrating simultaneous strong SEIRA and SERS on one substrate. - Cavity QED figures: Radiative Purcell factor Fp ≈ (6 ± 1) × 10⁶ (exceeding 10⁷ in some fits) and mode volume Vm ≈ 1.5 × 10⁻⁸ λ³ (~2900 nm³), implying confinement across ~7 nanogaps within a septamer motif. These values align with abstract-level metrics (Purcell ~10⁶, mode volume compression ~10⁸). - Robustness: The MIR resonance is remarkably robust to vacancy disorder; disorder enhances coupling into the mirror–NPML gap compared to perfectly ordered superlattices which exhibit field nulls at the mirror interface.

The work targets enhanced MIR light–matter interaction for vibrational sensing. By leveraging self-assembly with a precise molecular spacer (CB[5]), the authors achieve consistent 0.9 nm gaps across amorphous AuNP multilayers on a mirror, enabling a collective plasmon–polariton that is both highly tunable and strongly absorbing. This addresses limitations of ordered superlattices (fabrication difficulty, analyte access) and random aggregates (lack of discrete resonances). The mirror-backed multilayer establishes a Fabry–Pérot-like condition (λ1 = 4L) that, together with plasmon–polariton dispersion, explains the continuous shift of resonances into the MIR as D and n increase. The dense, uniform gaps compress the mode volume to ~10⁻⁸ λ³, producing large coupling strengths (g ~ 100 cm⁻¹), giant Fano dip strengths, significant Lamb shifts, and very high Purcell factors. Although the system remains in the weak coupling regime due to plasmon damping, its cooperativity approaches unity, indicating near-strong-coupling behavior for molecular vibrations. Importantly, structural disorder—often detrimental in photonics—here aids in coupling free-space light into the deeply subwavelength mirror–NPML gaps, enabling practical, large-area SEIRA and SERS. Together, these results demonstrate a robust, scalable platform for MIR vibrational spectroscopy with record enhancements among self-assembled systems and compatibility with simultaneous Raman/IR sensing.

The authors introduce a self-assembled, short-range-ordered Au nanoparticle multilayer-on-mirror platform that supports giant, tunable MIR plasmon–polaritons via uniform sub-nm molecular gaps. The system achieves near-total extinction, tuning beyond 11 µm, SEIRA and SERS enhancements up to ~10⁶, strong vibrational–plasmon coupling (g ~ 100 cm⁻¹), Purcell factors ~10⁶, and ultralow mode volumes (~10⁻⁸ λ³). The MIR resonance is robust to disorder, and disorder itself facilitates efficient in/out-coupling to nanogaps, enabling sensitive measurements in tiny volumes. This work establishes a practical route to high-performance MIR vibrational sensing using scalable self-assembly. Future directions could include: (i) pushing into the strong coupling regime by reducing plasmon damping or optimizing gap architectures; (ii) extending to different molecular spacers and analytes, including flow-through sensing and selective host–guest capture; (iii) integrating with microfluidics for quantitative trace detection; (iv) exploring other metals/geometries to tailor dispersion and bandwidth; and (v) employing active tuning (electro-optic, thermal) to dynamically sweep resonances across target vibrational lines.

- Coupling regime: Despite large g, the broad plasmon linewidth keeps the system in the weak coupling regime (2g/γc < 1), limiting clear Rabi splitting under current conditions. - Dependence on precise gap formation: Performance relies on maintaining uniform ~0.9 nm gaps via CB[5]; alternative aggregation methods broaden gap distributions and degrade spectral quality. - Material/geometry constraints: The platform uses Au and a mirror backplane; changes in materials or significant deviations in layer uniformity could alter resonance quality and coupling. - Generalizability to diverse analytes: While strong CB[5] signatures and a SAM example (BPTCN) are shown, comprehensive analyte studies (e.g., flow-through sensing) are only briefly indicated (SI) and warrant further validation.