Apertureless Near-Field Probes

Abstract

Apertureless probes made of metal, dielectrics, and semiconductors were investigated for use in near-field scanning optical microscopy (NSOM) [1-5]. In the configuration, the electric field localized near the tip of the sample is scattered by the fine structure of the sample or the tip and is detected with external collection optics. From the viewpoint of electromagnetism, the intensity of the scattered field detected in the far-field changes on a nano-metric scale as the boundary conditions of the electromagnetic field vary on a nanometric-scale while scanning the tip or sample of a fine structure with nanometric resolution. Apertureless probes have the following advantages compared to probes with small apertures:1Since a probe does not have an aperture or opaque coating surrounding the aperture but just the scattering point at the apex, the resolution of an apertureless-probe NSOM can be much higher and can reach a few nanometers.2Since the metal-coated dielectric waveguide is not used for sending (or receiving) photons to (or from) the probe apex, large optical throughput can be gained without any loss in waveguide propagation near the apex where the diameter of the dielectric is much shorter than the wavelength.3By using metal as a probe material, field enhancement is anticipated with the local mode of the surface plasmon-polariton at the apex of the probe [6].4The spectral response of near-field detection with a metallic tip ranges from ultraviolet to infrared because of the use of external optics (e.g., a Cassegrain objective mirror or a lens using an appropriate material), whereas the spectral response of an apertured probe is limited by the component material of the waveguide. The scattering efficiency of a metal is higher in the infrared region than in the visible, then the use of a metallic probe tip can be beneficial in infrared microspectroscopy [5,7].