Mapping local field distribution at metal nanostructures by near-field second-harmonic generation

摘要

We study nonlinear optical response of nanofabricated gold particles with sub-100-nm spatial resolution by means of non-linear near-field scanning optical microscopy (NSOM). In our instrument, femtosecond pulses at 800 nm wavelength are coupled to hollow-pyramid aperture sensors. Such probes show high throughput and preserve pulse duration and polarization, enabling the achievement of sufficiently high peak power in the near field to perform nonlinear optics on the nanoscale. We study second-harmonic generation (SHG) from gold nanoparticles of two different kinds, namely, closely-packed gold triangles and nanoellipsoids. We find a strong dependence of SHG efficiency on the shape and the fine structure of the nanoparticles. Near-field SHG is therefore a subwavelength resolution probe of local field enhancements occurring at specific sites of the particles. This work is focused on the dependence of NSOM linear and nonlinear images on the aperture size and linear polarization direction of light. Our measurements give strong evidence that SHG is mainly excited by a high field concentration at the rims of the metal NSOM aperture. This conclusion is supported by the high spatial resolution obtained for SHG even with apertures so large that FW imaging shows much poorer resolution.