Active THz polarization laser imaging

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Light is an electromagnetic wave whose properties are described not only by the amplitude of the wave (i.e. its intensity) and its wavelength (i.e. its color), but also its polarization (i.e. direction of the electric field). Where most types of imaging use the intensity and/or wavelength as contrast mechanisms and ignore the polarization, there is often much information to be obtained by examining the polarization state of the light reflected or transmitted from a scene. Compared to other regions of the electromagnetic spectrum, in the terahertz frequency range (300 GHz-10 THz), polarization has been underutilized as a contrast mechanism for imaging. A major application of terahertz radiation is for non-destructive evaluation due its ability to penetrate many visibly opaque materials for example pharmaceutical tablet coatings, foams, paints, covered art and antiquities. Polarization can potentially provide significant improvement in the utility of terahertz imaging for non-destructive evaluation due to its ability provide information about material birefringence, surface features, shape, roughness, edge detail, and chirality. The theme of this research is the development of a new type of terahertz laser in which the polarization of the emitted light can be chosen and switched rapidly, by electronic control, without moving parts. The approach is based upon the adaptation of antenna design concepts from the microwave frequency range, and combining them with terahertz laser design. The developed lasers will then be used as illumination sources for polarization-based imaging, with a particular emphasis on evaluating their suitability for non-destructive evaluation applications. As a part of the project, the research will train graduate and undergraduate students, and will support recruitment and retention of underrepresented minorities to engineering through the PI's participation in a targeted research project course. The objective of this research is the development of terahertz active polarization imaging based upon THz quantum-cascade lasers (QCLs) as an illumination source. First, an innovative scheme is proposed where the output polarization of QCLs can be modulated electronically. This scheme is based upon terahertz metamaterial antennas or phased arrays of lasers that are designed to radiate with overlapping beams, but with orthogonal polarization. By controlling the relative phase of the laser radiation, the output polarization can be dynamically switched between various linear and circular polarization states. This will allow more rapid and stable modulation of the output polarization compared to current schemes based upon rotating polarizers and waveplates, which in turn will allow faster data collection with reduced noise and a more compact and lightweight system. Second, the developed QCLs will be applied to polarimetric imaging in several schemes, and their performance will be quantified versus conventional techniques. The developed technology has the potential to impact many nondestructive evaluation applications including corrosion detection and monitoring, bioimaging, security screening and illicit material detection, and manufacturing process quality control.