ABSTRACT
Passive millimeter and terahertz wave imaging is a powerful way for personnel security inspection and scene monitoring. The existing systems usually have a single polarization mode. To obtain more information, polarimetric imaging has been preliminarily explored recently. However, there is no work exhibiting high-performance polarimetric imaging to analyze and interpret polarization characteristics. In this Letter, we report on the development of a W-band passive polarimetric imaging system for human body screening and present the polarization characteristics analysis of several typical scenarios. The experimental system has a spatial resolution of better than 2 cm at 2.5 m distance and has a thermal sensitivity of better than 0.3 K. The system can display polarization properties of human bodies and concealed objects. The experimental results demonstrate that passive polarimetric imaging has a great potential for object contrast enhancement, detection, segmentation, and recognition.
ABSTRACT
Active millimeter-wave (MMW) imaging is of interest because it has played an important role in the area of personnel surveillance over the past decades. Through reconstructing reflectivity, potential threats can be recognized based on shape. Besides reflectivity, diverse physical characteristics should be explored so that supplementary information can be used to assist recognition. This paper presents a surface-roughness measurement method using holographic data that has already been acquired during security checks. Based on scalar diffraction theory and speckle metrology, a simple mathematical relation between the correlation of holographic fields and surface roughness has been derived. Consequently, another kind of information, that is, the surface-roughness estimate, can be used to help differentiate similarly shaped articles. Results of simulations and laboratory experiments have shown its validation and potential in application to MMW imaging systems.
ABSTRACT
Direct millimeter-wave (MMW) holographic imaging, which provides both the amplitude and phase information by using the heterodyne mixing technique, is considered a powerful tool for personnel security surveillance. However, MWW imaging systems usually suffer from the problem of high cost or relatively long data acquisition periods for array or single-pixel systems. In this paper, compressive sensing (CS), which aims at sparse sampling, is extended to direct MMW holographic imaging for reducing the number of antenna units or the data acquisition time. First, following the scalar diffraction theory, an exact derivation of the direct MMW holographic reconstruction is presented. Then, CS reconstruction strategies for complex-valued MMW images are introduced based on the derived reconstruction formula. To pursue the applicability for near-field MMW imaging and more complicated imaging targets, three sparsity bases, including total variance, wavelet, and curvelet, are evaluated for the CS reconstruction of MMW images. We also discuss different sampling patterns for single-pixel, linear array and two-dimensional array MMW imaging systems. Both simulations and experiments demonstrate the feasibility of recovering MMW images from measurements at 1/2 or even 1/4 of the Nyquist rate.
ABSTRACT
We investigate the optoelectronic properties of a carbon nanotube (CNT)-metal heterostructure in the terahertz range. On the basis of terahertz time-domain spectroscopy characterization of a double-walled CNT (DWNT) film, we present and analyze the photocurrent measurement for a DWNT-nickel heterojunction illuminated by continuous-wave terahertz radiation. A significant current across the junction directly induced by terahertz excitation is observed and a negative photoconductivity behavior is found to occur in the device. The photocurrent shows a linear response to the bias voltage and the illumination power within the examined range. These phenomena support the feasibility of using CNT-metal heterojunctions as novel terahertz detectors.
ABSTRACT
The usability of many high-throughput lab-on-a-chip devices in point-of-care applications is currently limited by the manual data acquisition and analysis process, which are labor intensive and time consuming. Based on our original design in the biochemical reactions, we proposed here a universal approach to perform automatic, fast, and robust analysis for high-throughput array-based microfluidic immunoassays. Inspired by two-dimensional (2D) barcodes, we incorporated asymmetric function patterns into a microfluidic array. These function patterns provide quantitative information on the characteristic dimensions of the microfluidic array, as well as mark its orientation and origin of coordinates. We used a computer program to perform automatic analysis for a high-throughput antigen/antibody interaction experiment in 10 s, which was more than 500 times faster than conventional manual processing. Our method is broadly applicable to many other microchannel-based immunoassays.
