📝 SummarXiv

Abstract

Vibrational sum- and difference-frequency generation (SFG and DFG) spectroscopy is a technique to probe the nonlinear response of interfaces at mid-infrared wavelengths while detecting the upconverted signal at visible wavelengths. It has recently transitioned from large-area molecular films and colloidal suspensions to nanoscale structures, enabled by dual-resonant plasmonic nanocavities that confine light and matter in the same deep-subwavelength sensing volume. Here, we implement high-resolution ($<1$~cm$^{-1}$), continuous-wave ultra-broadband SFG and DFG spectroscopy from $860$ to $1670~\mathrm{cm}^{-1}$ on a variety of dual-resonant antennas under ambient conditions. The use of a commercial, broadly tunable quantum-cascade laser and the absence of geometric phase-matching requirements speed up and simplify data acquisition. At the same time, simultaneous access to SFG and DFG sidebands permits ratiometric signal analysis, enabling both common-noise rejection and field-normalized characterization of coherent response. The resulting spectra harbour the signatures of coherent interference between the resonant (vibrational) and nonresonant (electronic) contributions to the $\chi^{(2)}$ susceptibility, previously only observed under fs- and ps-pulsed excitation. We demonstrate the versatility and reproducibility of our approach across several molecules that yield distinct resonant and nonresonant mid-infrared responses. Moreover, we identify robust differences between our data and DFT calculations that hint at the modification of the infrared molecular response by the nanocavity environment. Together, these features position our platform as a scalable route toward multiplexed, high-resolution mid-IR sensing and a potential basis for coherent single- or few-molecule vibrational spectroscopy at the chip level.