A Low-Loss Hollow-Core Waveguide Bundle for Terahertz Imaging under a Cryogenic Environment

Optical fiber bundles have been widely used in microendoscopic biomedical imaging, Raman microscopy, and depth-resolved imaging due to their flexibility, long-distance transmission, and high spatial resolution. However, there are few reports on fiber bundles in the terahertz (THz) band, which significantly limits the applications in the biomedical field and nondestructive imaging. Although sapphire dielectric fiber-based bundles have shown promising applications in THz imaging, it is difficult to achieve long-distance and high-quality imaging due to the high absorption coefficient and uneven arrangements of fibers. Here, we propose a copper hollow-core waveguide bundle with a hexagonal arrangement, which is fabricated by a low-cost extrusion and stacking method. Simulation results demonstrate that transmission loss of the TE11 mode increases with the decrease of the inner radius of metallic waveguides, which is consistent with the experimental results. Moreover, under liquid nitrogen conditions, the metallic waveguide bundle exhibits higher output power under different biasing currents of THz quantum cascade lasers (QCLs). The effect of the inner radius of the waveguide on the bundle's spatial resolution has been thoroughly analyzed theoretically and experimentally with a maximum spatial resolution of ∼400 μm, indicating the accuracy of fabrication. In addition, different square tablets with four different mixed ratios of polytetrafluoroethylene powders and silver nanoparticles are well distinguished by the THz waveguide bundle-based transmission images. Moreover, the imaging performance of the THz waveguide bundle is simulated and such a bundle with a length of 6 cm is employed to experimentally demonstrate the remarkable terahertz imaging capabilities. The THz waveguide bundle in this work is well integrated with QCLs to enable long-distance submillimeter near-field terahertz imaging, especially in applications with cryogenic environments.