Antenna-enhanced nonlinear infrared spectroscopy under internal reflection geometry

Nonlinear infrared (IR) spectroscopy, including pump-probe IR spectroscopy and two-dimensional IR spectroscopy, is a powerful tool to study detailed molecular structures and dynamics. In particular, these techniques have great potential to characterize protein conformation and its dynamics. However, small oscillator strengths of molecular vibrations and strong IR absorption bands of liquid water limit the application of nonlinear IR spectroscopy to biomolecules in aqueous environments. One promising approach is to amplify the interaction of molecular vibrations with IR light by using IR-resonant metal nanoantennas and detect the vibrational signals in reflection from nanoantennas. Here, we apply this approach to `nonlinear' IR spectroscopy for the first time. We obtain a dramatic signal amplification of ~107and show that the enhanced nonlinear signals reflect the intrinsic vibrational dynamics of sample molecules.