Ultrasensitive Ultrafast Vibrational Spectroscopy Employing the Near Field of Gold Nanoantennas.

We introduce a novel method to perform nonlinear vibrational spectroscopy on nanoscale volumes. Our technique uses the intense near field of infrared nanoantennas to amplify the nonlinear vibrational signals of molecules located in the vicinity of the antenna surface. We demonstrate the capabilities of the method by performing infrared pump-probe spectroscopy and two-dimensional infrared spectroscopy on 5 nm layers of polymethylmetacrylate. In these experiments we observe enhancement factors of the nonlinear signals of more than 4 orders of magnitude. We discuss the mechanism underlying the amplification process as well as strategies for further increasing the sensitivity of the technique.