REVIEW ARTICLE Anti-reflection implementations for terahertz waves Yuting W. CHEN (✉) 1 , Xi-Cheng ZHANG (✉) 2 1 IBM Corporations, Poughkeepsie, NY 12538, USA 2 The Institute of Optics, University of Rochester, Rochester, NY 14627-0186, USA © Higher Education Press and Springer-Verlag Berlin Heidelberg 2013 Abstract Undesired reflection caused by impedance mismatch can lead to significant power loss and other unwanted effects. In the terahertz regime, anti-reflection method has evolved from simple quarter-wave anti- reflection coating to sophisticated metamaterial device and photonic structures. In this paper, we examined and compared the theories and techniques of several anti- reflection implementations for terahertz waves, with emphasis on gradient index photonic structures. A comprehensive study is presented on the design, fabrica- tion and evaluation of this new approach. Keywords terahertz, anti-reflection, gradient index, photonic structure 1 Introduction The anti-reflection problem in optics is analogous to the impedance matching problem in microwave circuits. RLC circuits, transformers and transmission lines are used to solve the impedance matching problem, whereas in optics a perfect anti-reflection design can be achieved by infinite layers of materials with refractive indices gradually changing between the two medium of interest. The best example is how the atmosphere acts as an antireflection layer to allow sunlight to pass through. In reality, working with limited space and scarcely available materials, it becomes a difficult task to achieve this goal. Research on anti-reflection techniques in the visible wavelengths proliferated over the years, whereas in the terahertz frequency range researchers are still striving to look for the right materials and implementation method. Take high resistivity silicon as an example–a material suitable for a wide range of terahertz components such as windows, filters and beam splitters because of its high transparency and low dispersion in the entire terahertz range (0.3 to 10 THz). On the other hand, it is associated with high Fresnel loss due to high index of refraction. With its relative refractive index at 3.42 in the terahertz range, it can be shown that every silicon-air interface on an optical component will induce a 30% loss in power; the use of multiple silicon optics will further reduce available power. This disadvantage plus the already limited power that can be generated by conventional terahertz system severely hinder the use of terahertz technology for many applica- tions. Therefore, there is a pressing need to develop anti- reflection methods to remedy the shortcomings of using multiple silicon components in terahertz systems. Effective anti-reflection implementation in the terahertz region should cover a broad frequency range. Among various spectroscopy methods, the widely used terahertz time-domain spectroscopy (THz-TDS) technique has a bandwidth from 0.1 to 3 THz. Anti-reflection technique for silicon components used in these systems should have a similar bandwidth in order to be useful at all. Most of the work being reviewed in this paper achieved this require- ment. However, as new technologies such as terahertz air- biased-coherent-detection (THz-ABCD) is developed, system bandwidth expands to above 10 THz. In such system, some of the techniques presented here will become obsolete due to their limited bandwidth. To illustrate the progress made in terahertz anti-reflection technique, we will start from single layer quarter-wave coating [1,2] and absorptive metallic coating [3–5], and then move on to more advanced methods such as multi-layer coating [6,7], metamaterial device [8] and sub-wavelength structures [9– 11]. In the end, we will focus on gradient index photonic structures, in which we will cover its inception and fabrication as well as its anti-reflection performance evaluated by a THz-ABCD system. 2 Developments of anti-reflection implementation in the terahertz frequency range 2.1 Quarter-wave coating Thin-film anti-reflection coating was first discovered by Received October 2, 2013; accepted October 21, 2013 E-mail: yuting.w.chen@gmail.com, xi-cheng.zhang@rochester.edu Front. Optoelectron. 2014, 7(2): 243–262 DOI 10.1007/s12200-013-0377-z