Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 μm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different line shape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points to the interference's origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in a nanoconfined environment.

The development of spectroscopic approaches to study molecules at interfaces is important as the molecular properties often differ from those in the bulk. Implementation of surface-enhanced two-dimensional infrared (SE 2DIR) spectroscopy using lithographically fabricated plasmonic nanoarrays is demonstrated for nanometer thick films. The sample, 4-azidobutyrate-N-hydroxysuccinimide ester (azNHS), dispersed in polystyrene was deposited onto the nanoarray. Raw enhancements in the SE 2DIR spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the CO and NN peaks, respectively. The field enhancement provided by the nanoarray was sufficient to record cross-peaks in 1 nm thick samples under dilute conditions for azNHS (∼0.1 M). Note that the cross-peaks were recorded for vibrational modes frequency separated by ∼350 cm–1 with the enhancement factor of 4.1 × 104. The effective electric field enhancement factors, measured for NN and CO modes via linear and two nonlinear IR techniques, have similar sample-thickness dependences, which permit using linear spectroscopy for enhancement evaluation. High-quality cross-peak waiting-time dependences were recorded for samples as thin as 1 nm involving several IR reporters demonstrating the applicability of an arsenal of 2DIR approaches, including spectral diffusion, chemical exchange, and relaxation-assisted 2DIR, to interrogate samples in nanometer thick films. The study opens new opportunities in analyzing structures and dynamics of molecules at interfaces.

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Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule-plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions.

Arrays of subwavelength plasmonic nanoparticles exhibiting narrowband lattice resonances are referred to as open cavities because of their ability to strongly couple with electronic excitations in molecular chromophores. However, realization of these ideas in the mid-infrared spectral region has been limited. We demonstrated a dramatic reduction in the bandwidth of lattice resonances in large-area arrays of half-wavelength mid-infrared antennas, reaching resonance quality factors above 200. By tuning the wavelength of the antenna-lattice resonances (ALR) to match the transition frequency of the molecular vibrational modes, we achieved a strong coupling between the ALR and the carbonyl stretching excitation in a thin film of (poly)­methyl methacrylate (PMMA) polymer deposited on the array. Splitting of the polaritonic transitions, reduction of their bandwidth below that of the bare molecular transition, and characteristic dispersion confirmed the strong coupling regime. Our results pave the way for exciting research on the many-body correlated dynamics of vibrational polaritons.