Integrated waveguide-coupled terahertz microcavity resonators

Abstract

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...

Terahertz spectroscopy and imaging – Modern techniques and applications

“…The development of the parallel-plate waveguide for THz frequencies has overcome some of the problems associated with the recording of absorption spectra with high spectral resolution. The development of the PPWG as a useful spectroscopic tool for the THz region was pioneered by Grischkowsky et al [39,[243][244][245][246][247] and Mittleman et al [149,248,250] In the following we will discuss the relevance of the PPWG for sensitive spectroscopy with high spectral resolution.…”

Terahertz microfluidic sensor based on a parallel-plate waveguide resonant cavity

“…8 Additionally, the resolution of this type of sensing strategy is limited by the intrinsic linewidth of the resonant line, since sublinewidth shifts are difficult to detect. [9][10][11][12][13] Here, we describe a THz resonator, integrated with a parallel-plate waveguide ͑PPWG͒, which addresses these issues. It exhibits among the narrowest linewidths reported for an artificial resonance in the THz range.…”

“…Although the quality factor ͑Q ϳ 98͒ is not the highest measured for an artificial structure in the THz range, 10 narrowest ever directly measured for a PPWG-based resonant cavity, 17 and is among the narrowest measured for any resonator in the THz range. [9][10][11][12][13] The increase in linewidth for the filled cavity can be attributed to the higher refractive index of the fluid, and also due to its non-negligible absorption. The dramatic redshift is a signature of the refractive index of the material inside the resonator, and demonstrates how this system can be used as a highly sensitive refractiveindex sensor.…”

A fully integrated MEMS tera hertz interaction circuit with multi wafer packaging

“…Recently, many groups are exploring kinds of devices for radiating or amplifying electromagnetic beams [1][2][3]. The main problem is that there are a few high power mm-wave or sub-mm-wave radiation sources.…”