THz parallel plate photonic waveguides

Abstract

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...

Guided Wave THz Spectroscopy of Explosive Materials

“…During the past ten years, THz waveguides, which could be applied in various fields including THz communication, have been reported in terms of a rectangular waveguide [5], a circular waveguide [6], a fiber [7], a single wire [8], a coaxial cable [9], planar transmission lines [10], and parallel plate waveguides [11].…”

“…Complex refractive index, complex dielectric properties, and complex conductivity can also be obtained using these techniques. One important application of THz-TDS is the detection of explosive and prohibited materials through identification of vibrational fingerprint spectra [3～4].During the past ten years, THz waveguides, which could be applied in various fields including THz communication, have been reported in terms of a rectangular waveguide [5], a circular waveguide [6], a fiber [7], a single wire [8], a coaxial cable [9], planar transmission lines [10], and parallel plate waveguides [11].Because these THz waveguides can focus the electromagnetic energy on a microsize structure, the use of THz-TDS waveguides allows analysis of traces of powder or thin films that previous THz-TDS was unable to access. Compared with other waveguides, parallel plate waveguides (PPWG) show a particularly rare dispersion of group velocity.…”

Microwire fibers for low-loss THz transmission

“…1 In almost all terahertz time domain spectroscopy setups free space is being used for propagation of terahertz waves due to the lack of low loss terahertz waveguides. Several waveguide solutions coming from either electronics or photonics have been studied such as the hollow metallic circular waveguide, 2, 3 hollow metallic rectangular waveguide, 3 sapphire fiber, 4 plastic ribbon waveguide, 5 air-filled parallel-plate waveguide, 6, 7 plastic photonic crystal fiber, 8 coaxial waveguide, 9 metal wire waveguide, 10, 11 parallel-plate photonic waveguide, 12 and metal sheet waveguide.…”

Quasi-TEM mode propagation in twin-wire THz waveguides (Invited Paper)

“…The strong dispersion scrambles the phase-time information and renders the extraction of the complex spectrum rather difficult. Most studies have been based either on conventional metallic guiding structures such as hollow circular waveguides [2] and hollow rectangular waveguides [3] ; or dielectric waveguides such as sapphire fibers [4] , plastic ribbon waveguides [5] , plastic photonic crystal fibers [6] , and parallel-plate photonic waveguides [7] ; or dielectric waveguides coated with metal sheets [8] . Some structures, such as metallic parallel-plate waveguides [9−11] , are known to support the propagation of almost dispersiveless transverse electromagnetic (TEM) or quasi-TEM modes at the expense of providing confinement in only one dimension, which largely limits their practical impact.…”