Abstract
Spatiotemporal nanofocusing of ultrafast surface plasmon polaritons (SPPs) coupled on a metal Au tapered tip with a curvature radius of a few tens of nanometers is deterministically controlled based on the measured plasmon response function. We control the SPP pulse shape and the second harmonic generation at the apex of the Au tapered tip by shaping the excitation femtosecond laser pulses based on the response function. We also adapted a similar control scheme for coherent anti-Stokes Raman scattering (CARS) and achieved selective CARS excitation of a single Raman mode of carbon nanotubes with only a single excitation laser pulse at the apex of the tip.
Highlights
Since the development of fine processing technology is increasing the demand for high-capacity, highly-integrated, and power-efficient opt-electronics devices, it is necessary to confine light into space beyond diffraction limits
Berweger et al improved the signal-to-noise ratio of near-field optical images with surface plasmon polaritons (SPPs) nanofocusing on a tapered tip and surpassed a result obtained by a typical metallic apertureless near-field scanning optical microscopy (NSOM) probe
When such complex waveguide geometries as the metallic conically tapered waveguide employed in this study are used as SPP waveguide geometries, a major challenge is to optimize the coupling of the incident light to the SPPs on the complex waveguide surface
Summary
Since the development of fine processing technology is increasing the demand for high-capacity, highly-integrated, and power-efficient opt-electronics devices, it is necessary to confine light into space beyond diffraction limits. Nanofocusing SPPs that are coupled on a metal tapered waveguide provide superior functions to focus optical energy on subwavelength scales.. Nanofocusing SPPs that are coupled on a metal tapered waveguide provide superior functions to focus optical energy on subwavelength scales.1,2 When combining this plasmonic nanofocusing scheme with ultrashort laser technology, ultrafast optical excitation in subwavelength scales can be achieved. SPP nanofocusing on a metallic tapered tip enables background-free localized optical excitation because SPPs are coupled to far-field excitation light a few tens of micrometers from the tip’s apex. This scheme is a powerful tool for achieving a localized light source for scattering type near-field scanning optical microscopy (NSOM). Schmidt et al obtained topographic and optical images of an Au nanoparticle with a 40 nm diameter at a spatial resolution of 10 nm by SPP nanofocusing on the taper with a 10-nm tip radius that was excited by femtosecond laser pulses. Berweger et al improved the signal-to-noise ratio of near-field optical images with SPP nanofocusing on a tapered tip and surpassed a result obtained by a typical metallic apertureless NSOM probe. The characteristics of the SPP pulses on the apex of an Au tapered tip were experimentally analyzed with interferometric frequency-resolved optical gating (IFROG) or interferometric frequencyresolved autocorrelation (IFRAC) using an second harmonic (SH) generated at the apex of the tip. Berweger et al controlled the SPP pulses by shaping the spectral phase of the excitation femtosecond laser pulses with feedback by the SH intensity at the apex.
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