Abstract-MEMS terahertz-to-infrared band converter using frequency selective planar metamaterial

Fabio Alves, Leroy Pimental, Dragoslav Grbovic,  Gamani Karunasiri

Terahertz to infrared imaging system concept. Schematic diagram of the imaging system from the illumination source (left) to the infrared camera (right). The transmissivity, absorptivity and emissivity of the components are represented by the Greek letters τ, η and ε with the appropriate indices. The THz-to-IR FPA is placed inside a vacuum chamber with a THz-transparent window (receiving side) and a IR-transparent window (emitting side). The THz pixel area is represented by A0 while the IR camera optical aperture area is represented by A1. The object is depicted as an envelope with a concealed metallic key inside, showing in the THz image. Each THz pixel (dark square on the THz FPA) is mapped on a subarray of pixels on the IR camera FPA (follow the dashed red line).

https://www.nature.com/articles/s41598-018-30858-z


A MEMS terahertz-to-infrared converter has been developed based on the unique properties of metamaterials that allow for selective control of the absorptivity and emissivity of the sensors. The converter consists of a sensing element structurally made of planar metamaterial membranes, connected to a substrate frame by four symmetrically-located thermal insulators. Upon THz absorption, the temperature of the sensing element increases and the outward infrared flux from the backside of the element is read by a commercial long-wave infrared camera. Two configurations were designed and fabricated with metamaterial absorptivity optimized for 3.8 THz and 4.75 THz quantum cascade lasers. The first sensor, fabricated with an oxidized aluminum backside, exhibits higher responsivity, but lower conversion efficiency than the second sensor, fabricated with a metamaterial backside. The spectral characteristics of the metamaterial on the two sides can be optimized to improve both responsivity and sensitivity, while keeping the sensors’ thermal time constant sufficiently small for real time imaging. No dedicated electronics or optics are required for readout making metamaterial-based MEMS THz-to-IR converters very attractive for THz imaging as means of a simple attachment to commercial IR cameras.

Abstract-Metal-organic hybrid resonant terahertz absorbers with SU-8 photoresist dielectric layer

Grbovic, Dragoslav and Alves, Fabio and Kearney, Brian and Waxer, Benjamin and Perez, Rolando and Omictin, George (2013) Metal-organic hybrid resonant terahertz absorbers with SU-8 photoresist dielectric layer. Journal of Micro/Nanolithography, MEMS, and MOEMS, 12 (4). Art. No. 041204. ISSN 1932-5150

.http://resolver.caltech.edu/CaltechAUTHORS:20140318-105658578

We report on the characterization of metal-organic hybrid metamaterials for MEMS-based terahertz (THz) thermal sensors and on the characterization of refractive index of SU-8 in the THz band. This type of metamaterial, coupled with the applicability of SU-8 as a structural material, offers possibilities for quick, simple microfabrication of THz imagers. SU-8, a negative photoresist, is a low-cost material that can quickly be spun onto a substrate at a wide range of thicknesses, and then photolithographically patterned into a variety of structures. It is also transparent to THz radiation and thus a suitable choice for a dielectric spacer in metamaterials. We investigated metamaterials consisting of a 0.18 μm Al ground plane and 0.18-μm layer of patterned Al separated by a dielectric spacer of ∼0.5  μm of SU-8. Absorption close to 70% at around 6.1 THz was observed. A model was developed to simulate absorption spectra of several metamaterials, agreeing well with experiments. Matching simulation to measurements was used to determine the refractive index of SU-8 at THz frequencies, extending the known values from 0.1 to 1.6 THz to as far as 10 THz. Finally, Kirchoff’s law for these metamaterials was verified and their use as THz emitters demonstrated with about 0.8  mW/cm^2 output.

Abstract-Bi-material terahertz sensors using metamaterial structures

Fabio Alves, Dragoslav Grbovic, Brian Kearney, Nickolay V. Lavrik, and Gamani Karunasiri »View Author Affiliations

In this paper we report on the design, fabrication and characterization of terahertz (THz) bi-material sensors with metamaterial absorbers. MEMS fabrication-friendly SiO_x and Al are used to maximize the bimetallic effect and metamaterial absorption at 3.8 THz, the frequency of a quantum cascade laser illumination source. Sensors with different configurations were fabricated and the measured absorption is near 100% and responsivity is around 1.2 deg/μW, which agree well with finite element simulations. The results indicate the potential of using these detectors to fabricate focal plane arrays for real time THz imaging.
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