In situ experimental study of a near-field lens at visible frequencies

Frequency dependent near-field scanning optical microscopy (NSOM) measurements of plasmon-mediated near-field focusing using a 50 nm thick Au film are presented. In these studies the tip aperture of a NSOM probe acts as a localized light source, while the near-field image formed by the metal lens is detected in situ using nanoscale scatterers placed in the image plane. By scanning the relative position of object and probe, the near-field image generated by the lens is resolved. NSOM scans performed at different illumination frequencies reveal an optimum near-field image quality at frequencies close to the surface plasmon resonance frequency