Signal amplification of surface-enhanced infrared absorption spectroscopy using virtual complex frequency excitation

Abstract

: Complex frequencywave(CFW)excitationhas recently emergedas apowerfulmethod for recovering information lost due to intrinsic material dissipation. While CFWs have been applied in acoustics, elasticity, and electromagnetic systems, optical implementa tionshaveremainedlimited.Here,wedemonstratethatasyntheticCFW excitationmethod, basedonspectral superposition, enables substantial enhancement in reflection-mode surface-enhanced infrared absorption spectroscopy. By applying this approach to plasmonic nanogap metasurfaces, we achieve near-unity signal enhancement and recover weakmolecularfingerprints, includingCH3subpeaksandprotein-specific Amide I and IImodes. Despite the challenge of phase retrieval in reflectivegeometriesdue tocomplex-planezeros,weproposepractical mitigationstrategies viaminimum-phaseequivalenceandcomplex conjugation.Additionally,we showthat tuning thedamping factorselectivelyamplifiesindividualresonances,allowingdynamiccontroloversubpeakvisibility.Thesefindingsestablishsynthetic CFWexcitation as a robust and tunable platformfor vibrational sensing. Beyond spectroscopy, this technique offers cross disciplinaryrelevanceformanipulatingwave−matter interactionsandmayprovide insights intothermodynamicanalogiessuchas Wickrotationandtime-domaindualities.