Resonant optical transmission through a single sub-wavelength aperture for near field applications

Spatial resolution of any far field optical system is limited by the diffraction limit, which is ∼λ/2. In the near field, by making use of evanescent waves, spatial resolution beyond the diffraction limit can be achieved. A simple way to achieve this high resolution is to use a sub-wavelength aperture in a metallic plate, in which the spatial resolution is generally determined by the aperture size. High spatial resolution is extremely important for applications such as nano-microscopy and spectroscopy, ultra-high density optical data storage, single molecule detection, single quantum dot applications, nano-particle studies, among others.; Unfortunately, a conventional small aperture has the devastating problem of extremely low power transmission. For a small circular aperture, calculations and experiments show that power throughput decays as the fourth power of the aperture size. This low transmission problem greatly hinders the applications of small apertures for solving significant problems.; Using both Finite-Difference-Time-Domain (FDTD) simulations and microwave experiments, systematic metallic aperture transmission studies were carried out for both transmission efficiency and near-field characteristics. Three aperture transmission regimes were identified: the large aperture regime, the very small aperture regime, and the resonant transmission regime. At resonant transmission, high power transmission with a transmission cross-section larger than the aperture area can be achieved. Furthermore, the aperture's geometry was found to have great impact on its transmission. In particular, a unique “C”-shaped aperture was discovered. The “C”-aperture provides ∼1000x higher power transmission than a conventional square aperture with a similar near-field spatial resolution of ∼λ/10. This transmission enhancement was quantitatively demonstrated in microwave experiments at a frequency of 6GHz. The high transmission through the “C”-aperture is attributed to both a propagating TE10 mode inside the aperture and an efficient coupling of the incident beam to the propagation mode. Modified “C”-geometries with further improved spatial resolution and transmission were designed as well. A new concept of using a resonant transmission sub-wavelength aperture with a long resonant wavelength for high spatial resolution applications was proposed. Other results such as the aperture near field characteristics, polarization effects on aperture transmission, thickness induced resonant transmission, and other fundamental issues are also discussed.