Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires

Abstract

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm^-1 for 2.5-3.9-μm-long NWs reveal a series of resonances due to the Fabry-Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry-Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry-Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Keywords: Fabry–Pérot modes; photothermal imaging; plasmons; single-particle spectroscopy.

Fig. 1.

(A and B) IR-PHI spectra of individual Au NWs with lengths L = 2.50–3.80 μm. Asterisks denote positions of the (A) m = 3 and (B) m = 4 resonances from Eq. 1. Solid red lines are Lorentzian fits to the data. (C and D) False-color IR absorption maps obtained at 3,074 and 3,393 cm⁻¹ for Au NWs with lengths of L = 2.90 and 3.75 μm, respectively. Averaged line profiles are presented above each IR absorption map. (Scale bars, 0.5 μm.)

Fig. 2.

(A) Resonance frequencies as a function of inverse length times mode order $\left(m/L\right)$ and (B) average linewidths $\left(\mathrm{\Gamma }\right)$ versus frequency for the m = 3 (blue circles) and m = 4 (red squares) Fabry–Pérot resonances of the gold NWs. Data collected from at least three NWs with error bars representing SDs. The dashed black line in A shows the frequencies calculated from Eq. 1. The shaded area in B shows the range of values for the Drude relaxation parameter for Au (55, 56). The solid red and blue lines in A and B are the results from the 3D FEM simulations. (C) Spectra for different length NWs calculated from FEM simulations. The lowest-frequency features are the m = 1 Fabry–Pérot resonances. (Inset) An expanded view of the higher-energy m = 2, 3, and 4 resonances (the spectra have been offset for clarity).

Fig. 3.

FEM simulation maps of the resistive heating and time-dependent temperature changes for an L = 3.1-μm-long Au NW on a glass substrate for the (A) m = 3 and (B) m = 4 Fabry–Pérot modes. (Top) Images in each panel shows maps of the IR absorption of the NWs. (Bottom) Images show the time-dependent temperature changes in the system. The excitation wavelength for the simulations is chosen to be at the maximum for each resonance. Note that the temperature profiles in the right-hand panels in A and B have been offset for clarity.