Transmission Characteristics of T-ray Multilayer Interference Filters W. Withayachumnankul, B. M. Fischer, S. P. Mickan, and D. Abbott Centre for Biomedical Engineering and School of Electrical & Electronic Engineering, The University of Adelaide Adelaide, SA 5005, Australia ABSTRACT An optical multilayer interference filter is made from two or more different dielectric materials layered in such a way that it promotes constructive or destructive wave interference for a selected frequency in the direction normal to the layers. Usually, each layer has the thickness of a quarter of wavelength at which the stop-band is required. In this paper, a quarter-wavelength multilayer interference filter is realised for T-ray applications. The dielectric materials used are high-resistivity silicon and free space, both of which have high transparency to T-rays and flat all-pass responses over the frequencies of interest. The designed thickness of both materials is in the order of a hundred microns, and thus allows the novelty of a retrofittable assembled structure. An analysis of the affect of the number of layers on the spectral response is given for the first time. The THz-TDS measurement of the fabricated structure is demonstrated to be in agreement with theory. Keywords: THz-TDS, T-rays, terahertz, interference filter, characteristic matrix 1. INTRODUCTION Ultrafast broadband T-ray systems have opened up the previously inaccessible frequency range lying between millimetre waves and infrared. 1 Consequently, a range of T-ray components are required to manipulate the propagation of T-rays, inasmuch as optical components are required to control visible light, infrared, or ultravi- olet. These components may comprise lenses, mirrors, parabolic mirrors, beam splitters, filters, polarisers, and so on. Most of the T-ray components directly adopt principles from optics. This is possible since the T-ray characteristics are quasi-optical. One of the common wave-manipulating components is a filter. A number of T-ray filters have been realised to date, owing to the requirements of either conventional FTIR (Fourier Transform Infrared) spectroscopy 2 or astronomical observations. 3 These filters can be categorized into two major types, according to the applied optical power, as active or passive filters. An active T-ray filter offers more flexibility in frequency and/or energy tuning, but at the expense of complexity and cost. A passive filter, on the other hand, is less complicated, but also less flexible in terms of its function. Several approaches to passive terahertz filters are available, for example, reststrahlen bands, 4 particle scat- tering, 5 photonic bandgap crystals, 6, 7 perforated metal sheets, 8–12 and interference in a multilayer structure. A multilayer interference filter ∗ is an attractive option because of its structural simplicity yet optical functionality. By using alternating thin films of T-ray transparent materials, with a proper index arrangement, full control over a particular frequency band is easily attainable. This work presents a study of the quarter-wavelength multilayer interference filter. The operating frequency range covers the frequency gap of most ultrafast T-ray systems, i.e. between 0.1 and 1.0 THz. 13 Importance is given to the operation in the transmission mode, and the effects of altering the number of filter layers on the transmittance profile. The study of these effects is possible as the submillimetre structure allows a rapid Email addresses: withawat@eleceng.adelaide.edu.au (W. Withayachumnankul); bfischer@eleceng.adelaide.edu.au (B. M. Fischer); spmickan@eleceng.adelaide.edu.au (S. P. Mickan); dabbott@eleceng.adelaide.edu.au (D. Abbott) * For simplicity in the following context an interference filter refers to a multilayer interference filter and not other types of filters exploiting a similar interference mechanism. The same structure may be found in other optical functions, and/or called by different names such as multilayer periodic structure, dichroic filter, 1D photonic bandgap structure, 1D photonic crystal, Bragg mirror, dielectric mirror, etc. Photonics: Design, Technology, and Packaging III edited by Wieslaw Z. Krolikowski, Costas M. Soukoulis, Ping Koy Lam, Timothy J. Davis, Shanhui Fan, Yuri S. Kivshar Proc. of SPIE Vol. 6801, 68011G, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.758811 Proc. of SPIE Vol. 6801 68011G-1