Mid-infrared-perturbed molecular vibrational signatures in plasmonic nanocavities

Recent developments in surface-enhanced Raman scattering (SERS) enable observation of single-bond vibrations in real time at room temperature. By contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient slow detection. Here we develop a new method for MIR sensing using SERS. This method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both visible and MIR plasmonic hotspots in the same nanogap formed by a monolayer of molecules. Molecular SERS signals from individual NPoF nanocavities are modulated in the presence of MIR photons. The strength of this modulation depends on the MIR wavelength, and is maximized at the 6–12 μm absorption bands of SiO2 or polystyrene placed under the foil. Using a single-photon lock-in detection scheme we time-resolve the rise and decay of the signal in a few 100 ns. Our observations reveal that the phonon resonances of SiO2 can trap intense MIR surface plasmons within the Reststrahlen band, tuning the visible-wavelength localized plasmons by reversibly perturbing the localized few-nm-thick water shell trapped in the nanostructure crevices. This suggests new ways to couple nanoscale bond vibrations for optomechanics, with potential to push detection limits down to single-photon and single-molecule regimes.

: Measurements of MIR laser spot size at = 10 µm. Laser spot is scanned across an 80 µm wide Au stripe fabricated on a glass cover slip placed on a computer-controlled stage. The reflection from the Au stripe is monitored and fit with a sigmoid function across the stripe edge to extract the MIR laser spot size focused by the Cassegrain objective.

Phenomenon considered
Why it does not fit experiments

Photothermal optical deflection
The time dynamics of MIR absorption-induced temperature rises in the system are estimated from = a − d , where and represent the mass and specific heat capacity of the absorber, / is the change in temperature over time, and a , d are absorbed and dissipated heat energies. The heat absorption a = MIR and heat dissipation is governed by the gradient of heat d = ℎ [ ( ) − 0 ], where MIR is intensity of MIR light, is the absorption cross-section and ℎ and represent the heat transfer coefficient and effective transfer surface area from specimen to environment, respectively.
The rate of change of temperature in the system is given by Considering the illumination volume (20 µm) 3 and = 0.7 J (gK) -1 (for SiO2), the thermal decay rate ( ℎ ) is 0.3 s which is much slower than observed in our experiments. Even using instead the specific heat capacity of Au, = 0.1 J (gK) -1 ) will not give rates of a few 100 ns as observed in experiment.
Light scattering in NPoMs is determined by the nanocavity modes and SERS signals are out-scattered at high angles though (10) and (20) modes of the nanocavity. The change in temperature of AuNP do not affect these scattering angles.
Overall, the conventional photothermal signal mechanism is thus an unlikely explanation here.
-Rates of heat absorption and dissipation do not match with experiment.

Optical forces
Optical forces in the dipole approximation are given by the polarizabilities of AuNP ( Au ) and substrate ( sub ) opt ∝ ( Au × sub ) 0 2 / 4 At 1 nm separation, with = 4 0 3 ( −1) ( +2) and using the complex for SiO 2 and Au, the wavelength dependent optical forces are dispersive line shapes.
-Wavelength-dependent line-shapes are dispersive, contrary to observed data.

Thermal expansion
MIR light-induced thermal changes in the SiO2, Au, and BPT are considered here.
Thermal expansion can change the visible-wavelength optics in two ways: A. thermo-optic coefficient ( ) B. linear expansion coefficient ( ) A. For SiO2, = -1×10 -4°C-1 , which suggests that the change in refractive index (Δ ) of the medium is <10 -4 for a 1 K increase in temperature. The thermal simulations imply ΔT < 1 K (Fig. 6d). At the same time full-wave optical simulations show that the Δ needed for a 10% change in SERS signal is Δ > 0.2, a thousand times larger than this estimated Δ from .
B. For SiO2, = 80×10 -6°C-1 , which suggest that a 1°C increase in temperature results in a small expansion of SiO2 perpendicular to the substrate. The amplitude of this undulation ( ), considering the calculated MIR heated spot size of 500 nm from the hot NP (Fig. 6d), is 0.04 nm. This is smaller than the radius of Au atom.
-Refractive index change needed is >0.2 which can only be achieved by laser powers >10 W.
-Undulations underneath the foil are too small, as the curvature needs to be on the scale of the AuNP radius to have any effect. Estimated local expansion of SiO2 is much smaller than the size of a single Au atom.