Shape-sensitive reflectance by nanostructured metal attached on an optical waveguide-mode sensor.

The optical reflectance of He-Ne laser light on a waveguide-mode sensor was measured as a function of light incident angle, in the case of either a metal (Au, Cr or Pt) film or nanoparticles being attached to the waveguide surface of the sensor. A dip appears in the reflectance spectrum as a function of incident angle at the angle where waveguide-mode excitation is induced. It is found that the dip moves toward a lower angle in the case that the attached metal is of a film shape, while it shifts toward a higher angle when the metal is an ensemble of nanoparticles. This difference in the direction of shift can be explained well by theoretical calculations using average refractive indices of the metal-containing layers. The present result indicates that one can estimate whether a metal nanostructure is film-like or an ensemble of spherical nanoparticles by the sensor.