56 Annual report 1999/2000 Flexible All-Plastic Mirrors for the THz Range P. Knobloch, T. Dobbertin, D. Turchinovich, and M. Koch We present a simple and cheap approach to fabricate large-area stopband filters and mirrors for the THz range. This approach extends the well known concept of dielectric mirrors to the far- infrared. We use alternating layers of different materials to build a flexible all-plastic mirror. The films with a typical thickness of several tens of micrometers are arranged to form pairs of quarter-wavelength-layers. The structures are characterized by THz time-domain spectroscopy. The experimental results are in good agreement with transfer matrix simulations. 1. Introduction THz technology is a very rapidly growing field of scientific and technological interest. In partic- ular, the future of telecommunication will involve sub-mm systems for in-door communications. This forecast triggers a high demand for active and passive devices operating in this frequency range. Recently several approaches have been reported to build filters for sub-mm waves. All these ap- proaches which include photonic band-gap crystals [1] and metallic meshes [2] require a sophisti- cated and expensive technology. Here we present an easier and cheaper method to fabricate the large area filters and mirrors. We produce dielectric mirrors for sub-mm range simply by stacking alternating thin layers of poly- meric materials with different refractive indices (Fig. 1). The materials relevant for commercial dielectric mirrors should obviously satisfy the following conditions: a sufficient step in the re- fractive index, low absorption and dispersion, mechanical stability, processing simplicity and low cost. For a first demonstration we use commercial polymer films of polyethylene (PE), low density polyethylen (LDPE), polypropylene (PP) and Styrolux TM (SX). They satisfy the above mentioned criteria, have a thickness ranging from a few tens to more than a hundred microns and are widely available. Fig. 1: Sample holder