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...

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A one-dimensional ͑1D͒ photonic metal parallel plate waveguide is presented in the spectral range of 0.5-3 THz that has high throughput and stop bands with up to the experimental limit of 40 dB of dynamic range. By incorporating a defect into the periodic bottom plate of the waveguide, a transmission resonance is generated in the first stop band with a Q value of 120 and a dynamic range of over 17 dB. The 1D geometry allows the utilization of the mode matching technique to analytically calculate the transmission of the photonic waveguide. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2710002͔The metal parallel plate waveguide ͑PPWG͒ is the ideal structure for single transverse electromagnetic ͑TEM͒ mode guiding of subpicosecond pulses with frequencies in the terahertz regime due to its low loss, undistorted propagation and ease of coupling.1,2 Waveguide terahertz time-domain spectroscopy ͑THz-TDS͒ has detected water layers as thin as 20 nm. 3 The sensitivity of waveguide terahertz TDS can be further increased by adiabatic compression of the waveguide plates, combining the benefits of a closer plate spacing to increase sensitivity along with higher coupling at optimal plate spacing. 4 More recently, cooling the PPWG to liquid nitrogen temperatures has enabled the measurement of linewidths as much as five times narrower than has been previously observed for a variety of thin organic films. 5In addition to the above spectroscopic work, there have also been studies of the effect of photonic band gap ͑PBG͒ materials inside the waveguide with transmission through the PBG structure, 6,7 and alternatively in an air gap adjacent to the PBG structure. 8,9 The strong band gaps produced by these structures are useful for their filtering capabilities and also allow the opportunity to integrate defects. A high Q filter inside a PPWG would couple the waveguide sensitivity enhancement with a longer interaction time between the sample and the resonating frequency for increased sensitivity. By placing a sample film over the PBG surface, small changes in the defect resonance frequency would occur, indicating the potential utilization as a sensor. As the Q value increases, smaller frequency shifts can be observed, and the more sensitive the sensor becomes.Previous experiments in which a two-dimensional metallic PBG structure was used as the bottom plate of a PPWG were promising, demonstrating sharp band gap turn-ons and high dynamic range.8 Despite these intriguing results, the ratio of component size to wavelength prevented the use of circuit theory approximations for theoretical predictions, as used on similar structures in the microwave regime.10-12 Additionally, the two-dimensional periodicity blocked the calculation of the theoretical output of such a structure. As such physical insight into the locations and strength of the band gaps was not possible.Here, in order to obtain a tractable theoretical problem in our frequency range of 0.5-3 THz and to enable the design of filters with specified properties, the periodicit...

Numerical simulations were used to design a variety of high-Q resonant cavities for integration into a terahertz 2D photonic crystal waveguide. After fabrication, the transmission characteristics of each integrated cavity were explored. These photonic waveguide-coupled cavities demonstrate resonances with linewidths approaching 10 GHz. The results compare favorably to previous observations of rectangular waveguide cavities. Good agreement between the experimental results and the numerical simulations was obtained.

We demonstrate integrated square terahertz microcavity resonators side coupled to waveguides. We present the microcavity transmission spectra for different resonator sizes and coupling strengths. The measured quality factors due to external coupling and cavity loss are found to be between 40 and 90 and between 30 and 40, respectively, for cavity resonance frequencies between 1.077 and 1.331 THz. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2809608͔Integrated terahertz photonic components are important for performing terahertz spectroscopy of vibrational modes in biological and chemical molecules, 1 realizing lab-on-achip detection platforms with high sensitivity, 2-4 and developing compact sources of coherent terahertz radiation. 5,6 Although several types of passive terahertz waveguides and resonators have been experimentally demonstrated, 7-9 they have typically been fabricated using bulk micromachining techniques and are not suitable for on-chip integration. In this letter, we experimentally demonstrate for the first time integrated terahertz air-core metallic square microcavity resonators side coupled to metal waveguides. At near-IR wavelengths, evanescent-field coupling is commonly used to excite dielectric microcavities via waveguides.10 Here, we demonstrate aperture coupling of air-core resonators to waveguides at terahertz frequencies. Strong field enhancement in air-core microcavity resonators can be used for narrowband sensing and the study of terahertz nonlinearities. We present measurements of the cavity quality factors resulting from external coupling and intrinsic cavity loss for different resonator sizes and find good agreement with calculations. Figure 1 shows a schematic of a square terahertz microcavity resonator, with side d, coupled to a waveguide via an aperture with width w and thickness t. The ŷ-polarized electric field propagates in the TE 10 waveguide mode and excites the fundamental TE 101 mode of the microcavity. Because the spatial profiles of the TE 10 and TE 101 modes are constant in the ŷ direction, the electromagnetic fields in these modes can be simulated using a two-dimensional finite-difference timedomain ͑2D-FDTD͒ method. The simulated resonant terahertz electric field in the structure is shown in Fig. 1.The terahertz resonators and waveguides under test were fabricated from highly doped 3 in. Si wafers ͑N A =10 20 cm −3 ͒ to avoid measuring terahertz radiation coupled to the substrate. First, one wafer was patterned and etched 150 m in an inductively coupled plasma deep-Si etcher. The 150 m etch depth established the waveguide, resonator, and aperture height ͑dimension in the ŷ direction͒. A scanning electron microscopy ͑SEM͒ image of a typical etched structure is shown in Fig. 2͑a͒. As can be seen, the coupling aperture was etched anisotropically and was undercut by 20 m. Next, both the etched and blank Si wafers were oxidized with 750 nm of thermal oxide. After oxidation, 50 nm of Ti and 400 nm of Au were uniformly deposited on the wafers using an e-beam evaporator...