Antenna-based near-field scanning optical microscopy

Some time ago a near-field optical imaging technique had been introduced (Appl. Phys. Lett. 73, 1669 (1998)), which achieves high spatial resolution and excellent sensitivity by exploiting the highly localized and mutual near-field interactions between a Au-nanosphere and a sharp Si-probe under evanescent field illumination. Specifically, the scattering of Au-nanoparticles is significantly enhanced by the presence of a sharp nanoscopic probe demonstrating that the probe acts as an efficient antenna. The present study focuses on the underlying physics of the original results by investigating more systematically nanoparticle-probe interactions: (1) The polarization pattern of the scattered field of an evanescent wave excited Si-probe is studied, which demonstrates that the probe scatters as a single dipole. (2) The enhanced scattering signal is measured as a function of sample size, which allows us to predict the signal strength for different size samples. (3) The wavelength dependence of the probe-sample scattering is investigated by exciting Au-nanospheres on (@543 nm) and off plasmon resonance (@633nm). The data shows a pronounced wavelength dependence reflecting the near-field spectrum of the Au-nanocrystals. (4) Finally, a simple, but intuitive model describing these mutual near-field interactions is presented, which explains qualitatively both the size and wavelength dependence of the enhanced scattering signals.