Electronically Reconfigurable Liquid Crystal Based Mm-wave Polarization Converter E. Doumanis, Member, IEEE, G. Goussetis, Senior Member, IEEE, R. Dickie, R. Cahill Senior Member, IEEE, P. Baine, M. Bain, V. Fusco, Fellow IEEE, J. A. Encinar, Fellow IEEE, G. Toso Senior Member, IEEE Abstract—An electronically tunable reflection polarizer which exploits the dielectric anisotropy of nematic liquid crystals (LC) has been designed, fabricated and measured in a frequency band centered at 130 GHz. The phase agile polarizing mirror converts an incident slant 45º signal upon reflection to Right Hand Circular (RHCP), orthogonal linear (-45 o ) or Left Hand Circular (LHCP) polarization depending on the value of the voltage biasing the LC mixture. In the experimental set-up this is achieved by applying a low frequency bias voltage of 0 V, 40 V and 89 V respectively, across the cavity containing the LC material. Index Terms—mm-wave, submm-wave, tunable, polarizer, liquid crystals, space communications, imaging, remote sensing, earth observation, polarimetric systems, interferometry. I. INTRODUCTION atellite polarimetric imaging systems, such as those used for passive and active Earth observation to measure the surface wind vector from space [1, 2] and vegetation properties [3], commonly involve dedicated receive and transmit chains for each polarization state. Tunable polarizers can reduce redundancy, volume/mass budget and cost. Dynamic polarization agility is also desirable in radar applications for defense and remote sensing to enhance detection and measurement of a feature in a radar scene [4] as well as wireless and satellite telecommunications to minimize feed losses and polarization purity impairments. Polarization agility in quasi-optical mm-wave systems can also be used to create tunable isolators (switches) [5] as well as frequency tunable interferometers for filtering and diplexing [6]. Traditional tunable polarizer technology relies on mechanical motors or rotors [4] leading to increased energy consumption and mass as well as compromised reliability. In This Manuscript received August 29, 2013. This work was supported by the European Space Agency, under project 4000103061. G. Goussetis would like to acknowledge support by FP7 project DORADA (IAPP-2013-610691). E. Doumanis is with Bell-Labs Alcatel-Lucent, Blunchardstown Industrial Park, Dublin 15, Ireland (e-mail: efstratios.doumanis@alcatel-lucent.com) G. Goussetis is with the School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK (e-mail: g.goussetis@ieee.org) R. Dickie, R. Cahill, P. Baine, M. Bain and V. Fusco are with the Institute of Electronics, Communications and Information Technology, Queen’s University Belfast, Belfast BT3 9DT, Northern Ireland, UK, (e-mail: r.cahill@qub.ac.uk) J. A. Encinar is with the Department of Electromagnetism and Circuit Theory, Universidad Politécnica de Madrid, E-28040, Madrid, Spain (e-mail: jose.encinar@upm.es). G. Toso is with the European Space Agency ESA–ESTEC, 2200 AG Noordwijk, The Netherlands, (e–mail: giovanni.toso@esa.int). order to address such limitations, integrated solutions based on e.g. MEMS [7] and piezoelectric ultrasonic motors [8] have been proposed. Despite their compact physical dimensions, these technologies are suitable for switched-based architectures offering discrete polarization states. Moreover, such technologies are mostly relevant to waveguide-based polarizers which are difficult to scale to (sub)mm-wave frequencies; free-space polarizers would typically require multiple actuators which is undesirable for practical implementation. Fig. 1. Schematic of the proposed reconfigurable polarizer that exploits a tunable Liquid Crystal substrate biased by a low frequency quasi-static source. Recently the use of nematic liquid crystals as a dielectric with electronically tunable permittivity has emerged as a technology suitable for continuous tuning of (sub)mm-wave and THz devices [9]. By applying an external field dynamic adjustment of the effective dielectric properties can be achieved [10], [11]. A waveguide-based liquid crystal polarizer was proposed in [12] as means to deliver continuous tuning of the polarization state between -90 o to 90 o and was demonstrated at Q–band. Significantly, the cost and complexity of integrating liquid crystals in multi-wavelength free-space devices does not increase proportionally with the size; this integration is also compatible with mature semiconductor technologies hence enabling scaling to the (sub)mm-wave range. Liquid crystals have therefore been successfully employed in the development of tunable frequency selective surfaces [13]. They have also been employed in the development of tunable reflectarray cells for sum and difference monopulse radiation patterns [14] and dynamic beam steering at W-band [15]. In this communication we present a mm-wave tunable quasi-optical reflection polarizer that exploits liquid crystal technology to dynamically change the polarization state of an incoming wave. In particular we demonstrate the first reported free-space polarizer operating at frequencies larger than 100 GHz with the capability to convert a linearly polarized incoming wave to a reflected wave with a polarization state that can be selected to be RHCP, LHCP or the orthogonal linear polarization, thus covering an entire meridian on the Poincare sphere and, when considering the polarization of the incident signal, covers the four key canonical polarization S