This paper reviews recent progress of active millimeter wave (mmW) imaging techniques for personnel security screening. With the ability to penetrate clothing and the promised millimeter scale spatial resolution, mmW imaging has been widely pursued for personnel surveillance without the usual concern for radiation safety. A brief development history of mmW imaging is introduced. An overview of imaging research and developmental achievements, along with antenna considerations based on quasi-optical geometry, phased reflector array, monostatic array, and multistatic array, is provided. Design considerations related to practical applications, including system calibrations, walk-through system (WTS), portable system, and compressive sensing (CS), are also discussed.
INDEX TERMSMillimeter wave imaging, personnel security screening, review.
The ability to break Kirchhoff’s law is of fundamental importance in thermal radiation. Various nonreciprocal emitters have been proposed to break the balance between absorption and emission. However, the thicknesses of the nonreciprocal materials are usually larger than 1/10 times of the wavelength. Besides, the previous proposed nonreciprocal emitters are complex, thus they can hardly be fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials. In this paper, we investigate the nonreciprocal thermal radiation of the magnetic Weyl semimetal (MWSM) film atop of the metal substrate. It is found that the strong nonreciprocal radiation at the wavelength of 9.15 µm can be achieved when the thickness of the MWSM film is 100 nm. The enhanced nonreciprocity is attributed to the Fabry-Perot resonances. The results indicate that the MWSM film is the promising candidate to engineer the ultrathin and simple nonreciprocal thermal emitters. What is perhaps most intriguing here is that the proposed structure can be more easily fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials.
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