A nanogap measuring method beyond optical diffraction limit

In this article, we report an optical method for the nanogap measurements beyond optical diffraction limit of a half of wavelength. This method is derived from Fresnel's equations of the traditional Kretschmann-Raether configuration based on surface plasmon resonance. Numerical simulation shows that a decrease of air gap width to be smaller than a half of wavelength of incident light will reveal a larger resonance angle shift. The resonance angle monotonically shifts from 45 degrees to 50 degrees by reducing the air gap width around from 300 to 100 nm. The measured smallest air gap width is about 126 nm by fitting the simulated curves to the experimental data. This gap measurement method provides a simple and real-time measurement system beyond the physical diffraction limit of a half of wavelength. (c) 2007 American Institute of Physics.