Resonant light transmission through a nanohole in a metal film

The maximum wavelength at which light can propagate through a rectangular hole in a metal is dramatically increased due to the existence of surface-plasmon (SP) waves along the edges of the hole. For example, a 15-nm-wide hole in silver can have a cutoff wavelength more than double that of a perfect electric conductor. Furthermore, we show that there is a Fabry-Pe´rot (FP) resonance for light transmission close to the cutoff wavelength, which gives a peak in the transmission. Impedance mismatch between the hole and free space provides the reflection required for the FP resonance. Due to the SPs in the hole, the reflection coefficient is typically larger in amplitude and has a smaller phase-shift than what has previously been observed in microwave systems. Our findings using analytic theory, finite-difference mode calculations, and finite-difference time-domain simulations agree well with recent experiments on the transmission through subwavelength rectangular holes. The transmission resonances found in a single subwavelength hole are of interest to nanolithography, biosensors, subwavelength microscopy, and photonic integrated circuits.