Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-7-9815
Timestamp: 2019-04-24 17:51:17+00:00

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We develop a coherent hyperspectral near-field microscope using a combined nano-Fourier Transform Infra-Red (FTIR) spectroscope and a scattering Scanning Near-field Optical Microscope (s-SNOM) illuminated by an ultra-broadband few-cycle femtosecond source, spanning a spectrum from 660 to 1050 nm. Using this spatio-spectral approach, we resolve hyperspectral near-field response of a single plasmonic nano-antennas over 450 nm bandwidth with a spatial resolution of 40 nm and a spectral resolution of 50 cm−1. In particular, we identify the electric near-field spatial distribution of the dipole resonant mode of various nano-antennas and observe, in accordance with previous theoretical reports, that those are spectrally red-shifted from their far-field response. Moreover, we are able to spectrally and spatially differentiate the near-field distribution of the dipole and quadrupole modes at the single nanoparticle level. Being coherent and short-pulsed, our technique opens the path for optical ultrafast characterization and control of light-matter interaction at the nanoscale.
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Fig. 1 Broadband VIS-NIR nano-FTIR characterization of a plasmonic nano-antenna. (a) Schematic of scattering-Scanning Near Field Microscope tip positioned over nanostructures. The nano-antennas are spaced 500 nm apart from eachother. Det: detector, PB: Parabolic Mirror. (b) AFM image of a gold nanoparticle on ITO covered glass. The nanoparticle’s dimensions are 140 × 60 × 40 nm3. Scale bar is 40 nm. (c) Near-field signal of the nanoparticle due to excitation with 660-1050 nm pulse. The double-arrow indicates the polarization of the exciting pulse. The image was acquired with white-light imaging, exhibiting the collective spectral response of the near-field. (d) Near-field spectra taken at two positions, a and b. Point a, shows an enhanced scattered optical signal at 726 ± 10 nm (left y-axis). Point b does not show any spectral feature, as expected. The far-field transmission spectrum, which exhibits a resonance at 702 ± 4 nm, can be seen at red (right y-axis). Spectra were normalized relatively to the background.
Fig. 2 Spatio-spectral near-field mapping of plasmonic nanostructures. (a) SEM image of three different gold nano-antennas. Nano-antennas length, from left to right is 130nm, 150nm and 170nm. Scale bar is 120 nm. (b) Two-dimensional, 2×0.2 μ m 2 , nano-FTIR scan of the three nano-antennas. Images at different wavelengths were then constructed. A clear resonance red-shift is observed due to the lengthening of the nano-antenna. (Visualization 1) (c) Images at three selected wavelengths, in each a different nano-antenna is seen on resonance. White scale bar is 200 nm.
Fig. 3 Hyperspectral near-field imaging of a nano-antenna. (a) Left: AFM image of a gold nanoparticle, with dimensions 60 × 40 × 380 nm3. Scale bar is 100 nm. Right: Near-field imaging of the nano-antenna, using 1580 nm CW excitation. (b) Left: FDTD simulation of a nanoparticle on glass, with light incident at 45°. Right: Simulated transmission. Shaded region is the spectrally-accessible region by our ultra broadband pulse. (c) Simulated (up) and experimental (bottom) near-field imaging of the amplitude and phase of the nanoparticle, at 890 nm. (d) Broadband experimental near-field imaging of the nano-antenna, due to the femtosecond pulse.
Fig. 4 Experimental and simulated hyper-spectral near-field response of an elongated nano-antenna. (a) Experimental amplitude (a.u). (b) Experimental phase. (c) Simulated amplitude. (d) Simulated phase.

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