  Traditional imaging systems, such as the eye or cameras, image scenes that
lie in the direct line-of-sight (LoS). Most objects are opaque in the optical
and infrared regimes and can limit dramatically the field of view (FoV).
Current approaches to see around occlusions exploit the multireflection
propagation of signals from neighboring surfaces either in the microwave or the
optical bands. Using lower frequency signals anatomical information is limited
and images suffer from clutter while optical systems encounter diffuse
scattering from most surfaces and suffer from path loss, thus limiting the
imaging distance. In this work, we show that terahertz (THz) waves can be used
to extend visibility to non-line-of-sight (NLoS) while combining the advantages
of both spectra. The material properties and roughness of most building
surfaces allow for a unique combination of both diffuse and strong specular
scattering. As a result, most building surfaces behave as lossy mirrors that
enable propagation paths between a THz camera and the NLoS scenes. We propose a
mirror folding algorithm that tracks the multireflection propagation of THz
waves to 1) correct the image from cluttering and 2) see around occlusions
without a priori knowledge of the scene geometry and material properties. To
validate the feasibility of the proposed NLoS imaging approach, we carried out
a numerical analysis and developed two THz imaging systems to demonstrate
real-world NLoS imaging experiments in sub-THz bands (270-300 GHz). The results
show the capability of THz radar imaging systems to recover both the geometry
and pose of LoS and NLoS objects with centimeter-scale resolution in various
multipath propagation scenarios. THz NLoS imaging can operate in low visibility
conditions (e.g., night, strong ambient light, smoke) and uses computationally
inexpensive image reconstruction algorithms.
