Unique Imaging Sensors That Will Advance Terahertz Imaging Systems
Terahertz Sensors Use Metamaterial Structures for Imaging
Due to some very unique features uncommon to other sensing technologies, Terahertz (THz) imaging has emerged as a major focus in NPS research.
One major element, peaking the minds of government and industry researchers, is its nonionizing radiation properties. These properties places the technology high on the list of applications that involve human exposure, such as medical diagnoses and security screening.
Also, some explosive constituents display unique THz absorption characteristics, presenting itself with an opportunity for potential bomb detection. And due to the transparent nature of dry and non-metallic materials in THz frequencies, concealed objects can be identified using non-threatening Terahertz imaging systems.
Despite the availability of some Terahertz sources and detectors, highly sensitive THz imaging systems are a major focus of research. Researcher and NPS Physics Professor Gamani Karunasiri and his team have developed a real-time THz imaging system using a conventional microbolometer-based camera that’s optimized for infrared (IR) wavelengths (8-12 μm) and is coupled with a quantum cascade laser (QCL) used for illumination.
“Although microbolometer cameras are not optimized to operate in the THz, the results were encouraging and we were able identify hidden objects in Styrofoam, plastic, paper and cloth,” said Prof. Karanasiri.
The focus of the teams current research is to develop a highly sensitive focal plane array (FPA) optimized for THz frequencies. One approach is to employ microelectromechanical system (MEMS) based bi-material sensors with integrated high absorption THz metamaterial structures tuned to the same frequency as the QCL illuminator.
The bi-material sensors are fabricated using high thermal mismatch materials, such as silicon oxide and aluminum. Temperatures rise in these bi-material sensors when bombarded by THz radiation. The temperature spike causes a deformation in the material, which is then detected by a light beam.
The Teams initial work involved the design and fabrication of metamaterial structures tuned to the same frequency as the QCL. Their metamaterial structures showed a near 100% absorption at the designed frequency and matched well with finite element modeling carried out using COMSOL Multiphysics software.
Next, the team fabricated sensors and focal plane arrays (FPA's) of various configurations and integrated their metamaterial absorbers. The sensors and FPAs were characterized using an optical readout system consisting of a QCL-THz illuminator and a CCD camera, which was combined with a visible light source to capture the thermomechanical deformation of sensors.
The Teams research strongly suggests that metamaterial-based MEMS bi-material sensors have great potential forbeing integrated into high sensitive THz imaging systems.
The results of the research are published in high impact Journals, such as, Applied Physics Letters, Optics Letters, Optical Engineering and Optics Express.
Thesis Opportunities Finite element modeling of THz sensor characteristics; Analytical and numerical modeling of metamaterial structures; Characterization of THz sensors; Design and characterization of optical readout for THz sensors; Processing of images from THz focal plane arrays;
Acknowledgments This work was supported in part by grants from the AFOSR, ONR and NRO. The fabrications of sensors were carried out at Oak Ridge National Laboratory.
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