Metasurface Optics With On-Axis Polarization Control for Terahertz Sensing Applications

Nondestructive testing (NDT) and imaging of materials with unknown properties are important applications of terahertz technology. Different contrast-forming methods are available to obtain diverse information on the imaged region, including intensity, color, phase, and polarization. Polarimetric imaging is relatively under-investigated owing to the difficulty in its implementation with complex optical arrangements. In this article, we present the design and monolithic fabrication of an anisotropic metasurface that incorporates several beam-forming functions in a single layer with high efficiency thus simplifying instrument construction. The probe beam generated by the metasurface consists of an orthogonally polarized pair of collinear Bessel beams that create a well-defined on-axis change of polarization. The metasurface is integrated into a polarimetric imaging microscope for 3-D beam profiling and sensing experiments at 2.52 THz. An analytical model to describe the on-axis polarization variation was found to be in excellent agreement with both finite-difference time-domain (FDTD) simulations and the experimental results. The setup was used to measure the refractive index and diattenuation of homogeneous sample regions in a proof-of-principle application. We anticipate this technology will find future applications in NDT and polarimetric imaging of polymer composites and 3-D profilometry of reflective surfaces.

I. Introduction

The terahertz region of the electromagnetic spectrum lies in the wavelength range of 10 µm to 3 mm [1]. Terahertz imaging [2], [3] is a major opportunity for applications such as security [4], [5], [6] or bio-medical imaging [7], [8], astronomy [9] and nondestructive testing (NDT) [10], [11], [12]. Spectroscopy is also well advanced, offering the potential for spectroscopic imaging [9], [10]. Terahertz radiation is non-ionizing, hence suitable for human body imaging [5], [8], delivers sub-milimeter resolution [10], [11], [12] and offers several molecular absorption bands [9]. Furthermore, many industrial materials including polymers [11], cardboard [6], and textiles [4], [5] are relatively transparent at terahertz wavelengths so that obscured objects can be reliably imaged.