We study the effect of a dielectric film attached to the surface of a metal hole array ͑MHA͒ on the reflection spectrum in the terahertz ͑THz͒ region. The frequency of the reflection dip, attributed to the excitation of surface waves in the vicinity of the MHA surface, shifts to lower frequencies with increasing dielectric film thickness. This resonant characteristic of MHAs can be applied to highly sensitive THz sensing for samples attached to the MHA surface. We also investigate the dependence of the reflection spectrum on the MHA's thickness and the side to which the dielectric film is attached. These sensing applications occupy an important place in THz technological research because many substances have distinct electromagnetic features in the THz region, allowing their detection and identification. In particular, highly sensitive detection of extremely small amounts of substances or small variations in a sample's condition is crucial and has the potential to advance the development of label-free sensing of biomolecules or gases. Electromagnetic resonant structures are useful for highly sensitive detection and sensing. 7,8 The sensitivity of sample detection could be increased significantly around the resonant frequency, realizing the detection of very small amounts of sample. In a previous study, we used metal hole arrays ͑MHAs͒, which are metal films perforated periodically with circular holes, as surface wave ͑SW͒ resonant sensors in the THz region. 9,10 By illuminating an MHA, a SW can be excited on the metal surface at a certain resonant frequency as a result of the interference of light scattered from the aperture array. 11,12 Due to strong localization of the electromagnetic field of SWs in the vicinity of the MHA surface, the resonant transmission characteristics depend strongly on the dielectric distribution near the surface. 13 This characteristic allows us to achieve highly sensitive detection of small amounts of sample placed at a subwavelength distance from the MHA surface. 9,10In our previous study, we applied this highly sensitive sensing to transmission measurement. In this study, for the purpose of highly sensitive sensing with MHAs by reflection measurement, we investigate the effect of a dielectric film attached to the MHA surface on the reflection spectrum of the MHA. Reflection measurement has an advantage over transmission measurement for THz sensing; i.e., electromagnetically opaque substances are available as detected samples.The MHAs used in our study are 100 and 300 m thick metal films perforated with a triangular lattice structure of circular holes, as shown in the inset of Fig. 1. For both MHAs, the diameter of the holes is 500 m and the period of the triangular lattice is 1000 m. The reflection spectra of the MHAs, at normal incidence, are measured by a THz time-domain spectroscopic system that allows us to measure the waveform of the THz wave directly in the time domain. Using a Fourier transformation, we can calculate the reflection and phase spectra from the time-domain wavef...

Abstract:We have observed resonant terahertz transmission peaks in samples comprising perforated periodic hole array in a metal film, covered with a high dielectric substrate. These resonant transmissions arise from the interplay between waveguide modes in dielectric substrate and the periodic hole array in the metal film. Finite difference time domain (FDTD) simulations show good agreement with the data, in support of the proposed mechanism. Inducing additional resonant transmissions using guided modes can lead to the ease in tuning the transmission peak frequencies that are potentially useful to terahertz (THz) bio-sensing. ©2010 Optical Society of America