We demonstrate extraordinary THz transmission of an array of subwavelength apertures patterned on ultrathin highly doped silicon by reactive ion etching. The zero-order transmission spectra exhibit well-defined maxima and minima which are attributed to the excitation of surface-plasmon polaritons and Wood's anomaly. The transmission anisotropy is investigated with respect to the orientation of the elliptical hole array in the frequency range from 0.2 to 3.5 THz, and we notice that the transmission increases significantly when the major axis of the elliptical hole is perpendicular to the polarization of THz beam. In addition, redshift and reduction in transmission amplitude are observed when the surrounding dielectric permittivity is increased.
We study the influence of dielectric function of metals on the transmission properties of terahertz pulses through periodically patterned subwavelength holes. Because of a drastic increase in the value of dielectric constants, most metals become highly conductive at terahertz frequencies. Extraordinary terahertz transmission is observed in subwavelength hole arrays made from both good and poor electrical conductors. The measured transmittance of terahertz pulses is found to be enhanced with increasing ratio of the real to the imaginary dielectric constant of the constituent metals, for which the dielectric function follows the Drude model.
We have observed narrowband transmission or rejection in the frequency spectra of THz pulses transmitted through air-spaced parallel plate photonic waveguides. These waveguides have one of the metal plates covered by a silicon plate with a metallic photonic band gap ͑PBG͒ surface precisely fabricated by lithographic techniques. We use two different PBG surface types: an array of metallic cylindrical pillars, and an array of metallic cylindrical holes. With the inversion of the PBG structures from cylinders to holes, the output spectra changes from narrow bandpass to narrow band-reject filtering. These photonic waveguides show extremely sharp spectral responses in regions as large as 1 THz, with stop bands or transmission bands having contrasts of as much as 90 dB. Over the past few years the metal parallel plate waveguide ͑PPWG͒ has received much attention for its use at THz frequencies, where it offers transverse electromagnetic ͑TEM͒ mode propagation with no modal dispersion, loss determined by the conductivity of the metal plates, and consequent very low group velocity dispersion.1,2 With its excellent coupling to free space THz radiation, and ease of fabrication, the PPWG is the ideal structure for the undistorted guiding of sub-ps THz pulses. Recently other guided wave components have been integrated into the PPWG for further control over pulse propagation, e.g., metallic mirrors, 3 transmitters, 4 and dielectric lenses. 5 The next technical challenge involves frequency filtering inside the waveguide.While many groups have demonstrated various filters for THz applications, most of the recent work has focused on THz photonic band gap ͑PBG͒ structures. A subset of the THz PBG research has included the integration of these structures into waveguides resulting in a plastic photonic fiber, 6 and a dielectric waveguide grating. 7 More recently there has been the first demonstration of a dielectric PBG structure filled PPWG, 8 giving THz frequency filtering inside the PPWG.Due to the lack of spatial dependence of the TEM mode in the direction perpendicular to the plates of the PPWG, three-dimensional PBG cylinder structures can be replicated in the two-dimensional ͑2D͒ geometry of the PPWG.5 The initial conceptual plan for the experiment presented here was to incorporate such 2D-PBG cylinder structures into the metal PPWG to control the frequency dependent transmission. This goal was to be achieved with high-precision 2D-PBG cylinder structures, fabricated by our new cleanroom based lithographic technique. Initially, we used a 2D-PBG structure of metal cylinders to fill the space between the two metal plates of the waveguide. However, the transmission through this structure was negligible. In order to increase the transmission, we broke the 2D-PBG symmetry by increasing the space between the waveguide plates to let the tops of the 70-m-long metal cylinders ͑pillars͒ form the surface of a 100 m air gap to the second plate. For this theoretically more complex asymmetric system we observed strong PBG transmission eff...
2-D metallic photonic crystals without defects, with point defects, and with a Fabry-Perot (F-P) defect are characterized by terahertz time-domain spectroscopy. The metal parallelplate waveguide (PPWG) with single TEM-mode propagation is used as a tool to simulate 2-D photonic crystals in free space. The 2-D metallic photonic-crystal structures were fabricated by coating Au on an SU-8 polymer cylinder array. Wide terahertz bandgaps were observed in the photonic crystals within the PPWG. The experimental measurements have excellent agreement to 2-D photonic-crystal theory without defects. Defect modes are observed in the samples with defects and show the F-P defect has a strong localization effect.
We demonstrate effectively two-dimensional (2D) terahertz (THz) photonic bandgap (PBG) structures for transverse electromagnetic (TEM) mode propagation within metal parallel plate waveguides (PPWG). The 2D-PBG structures consisting of square arrays of dielectric cylinders were characterized by THz time-domain spectroscopy (THz-TDS). THz photonic bandgaps were observed, as determined by the 160 mum lattice constant, the 65 mum diameter, and the dielectric constant of the cylinders. The experimental measurements were fitted with excellent agreement to 2D theory, confirming that for TEM mode propagation, effectively 2D propagation experiments can be achieved within the bounded space of the PPWG.
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