RECONFIGURABLE METALLODIELECTRIC ELECTROMAGNETIC BAND GAP (EBG) STRUCTURES J. C. Vardaxoglou, A. Chauraya, D S Lockyer, Y. R. Lee Centre for Mobile Communications Research, Department of Electronic and Electrical Engineering Loughborough University, Loughborough, LEICS LE11 3TU, UK E-mail: J.C.Vardaxoglou@lboro.ac.uk ABSTRACT An experimental study of a photonically reconfigurable EBG (based on a switchable dipole array) structure is presented. The level and behaviour of the carrier concentration in the illuminated state is predicted using our model. Optical tuning of the band gap is demonstrated, producing up to 50% frequency shift. The transition from a Silicon based Frequency Selective Surface (SiFSS) under plane wave illumination to a surface wave excitation using a microstrip line is shown and the modal effects are discussed. The reconfigurability of the host SiFSS having embedded square doped patches is measured and predicted. INTRODUCTION Electromagnetic Band Gap (EBG) structures can originate from Frequency Selective Surface (FSS) and resonant periodic arrays in waveguides. [1-2]. Previously, we have proposed one-dimensional band gap structures in microstrip composed of arrays of dipoles or tripoles periodically distributed beneath a microstrip line. [3, 4]. These are particularly suited to planar microstrip circuits and realisations of MMICs. Also, these can be seen as an alternative to recent work on 1-D EBGs, where holes are drilled in the substrate or patterns are etched in the ground plane, see for example [5, 6]. In the first part of this paper we introduce a study of both simulations and measurements of a photonically reconfigurable EBG structure, established from a 50-Ω transmission line over a one-dimensional array of dipoles. In that we introduce a number of switches, inline with the dipoles, applied to the EBG for tunability. The second section deals with an EBG arising from the transition of an FSS band gap response starting from a TEM wave illumination to a surface wave transmission line excitation. The host Silicon FSS (SiFSS) EBG structure is derived from a doped patch FSS array in a high resistivity silicon substrate. Silicon is selected due to the desirable features it possess in electronic devices, photo excitation and the ability to generate doping on the surface. A mode change takes place and three troughs emerge within the bandgap when the ground plane is introduced. The periodically doped patch technique avoids both the drilling and etching process and makes the EBG structure easier to manufacture. We also discuss plane wave measurements carried out on the SiFSS to verify the tuning of the stopband. PHOTONICALLY TUNED EBG Fig. 1a shows the EBG structure with the dipole inline switches. The role of the switch is to extend the dipole length and thus the bandgap of the EBG. The characteristics (insertion loss and isolation) of a 50 Ω optical switch are shown in Fig. 1b. The switch has a 0.4 mm gap, and the line is etched on RT Duroid 5880 substrate material of thickness 1.125 mm ( r ε =2.2). High resistivity silicon (ρ > 6000 Ω cm) of thickness 0.3 mm is placed on the line so as to bridge the gap when optically illuminated. Chamfering the line at the gap decreases the capacitance across the gap, and subsequently the isolation is increased. The structure gives a bandgap response as shown in Fig. 2. Increasing the dipole length lowers the bandgap position without affecting greatly its bandwidth. Since the array becomes an integral part of the transmission line, an optical injection technique is used to tune the bandgap. The transmission line is printed on a polyester dielectric sheet of dielectric constant 3, pasted on top of a 1-D periodic dipole array (dipole length L = 12 mm, width w = 1 mm, and periodicity of 5 mm). The dipole array itself is etched on RT Duroid 5880, and Silicon is placed on the chamfered gap. In the dark state the bandgap (stopband) has a centre frequency at 11.5 GHz. The cut-off frequency is at about 10.0 GHz