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.

Figure 1

(a) SEM image of one of the antenna arrays. The scale bar corresponds to a distance of $4\mu \mathrm{m}$. (b) Infrared absorbance spectra (light polarized along the long antenna axis) of randomized nanoantenna arrays coated with a 14 nm layer of PMMA. The spectra are offset vertically for clarity. (c) Schematic representation of the coupled-dipole model described in the text. Both the nanoantenna and the PMMA patch on top are represented as point dipoles.

Figure 2

(a) Top: Linear infrared spectrum (blue line) of the carbonyl vibration of a 150 nm film of PMMA on a ${\mathrm{CaF}}_2$ substrate. The red line represents a Gaussian fit. Bottom: Transient spectrum of the same sample (isotropic signal). The solid red line is a fit to a linear combination of two Gaussian bands (dashed lines) whose widths are taken from the linear spectrum. (b) Delay scan of the transient signal at a frequency of $1735{\mathrm{cm}}^{-1}$. The red line is a monoexponential fit.

Figure 3

(a) Transient spectrum of a 10 nm layer of PMMA on ${\mathrm{CaF}}_2$ (isotropic signal). (b) Transient signal for a 5 nm PMMA film deposited on top of nanoantennas. The pump polarization is along the long axis of the antenna while the probe polarization is along the short axis. (c) Transient signal for the same sample as in (b) with the pump and probe polarizations set parallel to the long axis of the antennas. (d) Normalized delay traces at $1735{\mathrm{cm}}^{-1}$ for the reference measurement on a 150 nm layer of PMMA (blue line) and for a 5 nm layer of PMMA on a nanoantenna array ($L=2000\mathrm{nm}$) with a perpendicular probe (red dots) and a parallel probe (green dots).

Figure 4

2DIR spectra of 5 nm PMMA on nanoantennas of (a) 2200 nm and (b) 1900 nm. The thin solid lines represent equidistantly spaced contours. Positive absorption changes are shown in red and negative absorption changes in blue. The spectra correspond to a pump-probe delay of 0.5 ps. The thick solid lines represent the central lines and are calculated by fitting the locations of the local maxima of the 2D spectra to a straight line.