Trapping of terahertz waves in a wide bandwidth

Abstract

Until recently, most of the THz components such as terahertz (THz) sources, detector, modulators, filters, and absorbers, etc, are still large, which hinder the large-scale integration of THz system. Taking THz absorber for example, it can be directly applied into THz indoor wireless communication system, and extended to other system such as chip THz time domain spectroscopy. Ultra-compact, polarization-independent and wideband THz absorbers are now pursuit goals of many researchers. As a candidate for realizing such a THz component to manipulate THz waves, periodical metal structure is being wide studied. However, most of these metal-based absorber devices can just manipulate THz waves in single-frequency, leading to the lack of THz technology platform on which THz waves can be manipulated with sufficient confinement in a wide bandwidth. Recently, as a practical solution, R. Kakimi et. al., proposed a photonic-crystal (PC) slab, which can be easily fabricated with doped silicon, to capture of THz waves1. Based on guide-resonance and Fabry–Pérot resonance, a PC-based THz absorber with just 50 GHz bandwidth is realized. Here, we propose a periodical structure consisting of doped-silicon array to demonstrate and visualize terahertz (THz) wave trapping in a much more wide bandwidth. First, we design and fabricate a square-shaped doped-silicon array to capture of THz waves in a wide bandwidth. Our theoretical and experimental results demonstrated that such a periodical structure can achieve over 95% absorption in a broad frequency range from 1 to 2 THz (Fig. 1) due to the destructive interference effects and the first-order grating diffraction2. Then, we propose a periodical structure consisting of two crossed dumbbell-shaped doped-silicon array to break the degenerate (Fig. 2) of first-order diffraction, and thus the absorption bandwidth is enhanced to 1.5 THz with more than 95% absorption3. As another method to obtain broadband THz absorber, multi-Fabry–Pérot resonance is proposed combing with grating diffraction to realize such ultra-broadband THz absorber, by adding a mirror underneath the substrate4.