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Extensive studies, including 3-D simulations and experimental measurements, were carried out to investigate the effect of the aperture diameter on light transmission for a cantilever-based pyramidal probe. A series of Au-coated nano-apertures were fabricated at the apexes of pyramidal probes and the transmission efficiency of light through those apertures was measured and compared with theoretical results. The relationship between the aperture diameter and the transmission efficiency, both in simulations and experiments, can be divided into three groups depending on the aperture diameter. When the aperture diameter becomes very small (d/$\lambda <$ 1/10) compared with the input wavelength, the transmission efficiency follows Bethe's theoretical result. It increases rapidly when the diameter is around half the wavelength of the input laser and finally approaches the classical expectation with increasing diameter above that wavelength.