Accelerated molecular vibrational decay and suppressed electronic nonlinearities in plasmonic cavities through coherent Raman scattering

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-color time-resolved coherent anti-Stokes Raman spectroscopy using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM) while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nanometer-wide plasmonic gaps compared to plasmonic nanoparticles. Simultaneous time-resolved incoherent anti-Stokes Raman spectroscopy allows us to separate the contributions of vibrational population decay (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>T</a:mi><a:mn>1</a:mn></a:msub></a:math>) and dephasing (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>T</b:mi><b:mn>2</b:mn></b:msub></b:math>). With increasing illumination intensity, the ultrafast vibrational dephasing rates of biphenyl-4-thiol molecules are accelerated at least tenfold, while phonon population decay rates remain constant. The extreme plasmonic field enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally coupled dynamics and diverse molecular optomechanics phenomena. Published by the American Physical Society 2024