PIERS ONLINE, VOL. 6, NO. 2, 2010 153 FDTD Study of a Novel Terahertz Emitter with Electrical Field Enhancement Using Surface Plasmon Resonance Shuncong Zhong, Yao-Chun Shen, Hao Shen, and Yi Huang Department of Electrical Engineering and Electronics, University of Liverpool Liverpool L69 3GJ, UK Abstract— In this work, the finite-difference time-domain (FDTD) technique is used to study novel terahertz (THz) emitter structures. The proposed THz antenna requires less pump power of the femtosecond laser pulse whilst provides higher THz output power. This is achieved by the enhancement of the localized electric field in the THz emitter This electric field enhancement is found to have two origins: One is owing to the enhancement of the static electric field of the bias voltage, and the other is the enhancement of the electric field of the incoming femtolaser pulse. The latter enhancement is caused by the interaction of the pump laser and the surface plasmon resonance at the conical gold structure of the photoconductive emitter. This new terahertz emitter could lead to new applications where high-power and broadband terahertz sources are needed. 1. INTRODUCTION Much of the recent interest in terahertz (THz) radiation stems from its ability to penetrate deep into many organic materials without causing damages since it is not ionizing radiation like X-rays. THz radiation can also help scientists understand the complex dynamics involved in condensed- matter physics and processes such as molecular recognition and protein folding. Recently terahertz pulsed imaging (TPI) has been adopted by the pharmaceutical industry for non-destructive and quantitative characterization of pharmaceutical tablet coatings [1]. The most common device for generating broadband THz radiations is a biased photoconductive antenna, pumped with high- power laser pulses from a femtosecond laser [2, 3]. Photoconductive antenna is one of the most commonly used emitters and detectors for THz radiation. The typical structure of a THz emitter is illustrated in Fig. 1. The gold antenna is fabricated on a low-temperature grown GaAs substrate. The antenna has a gap at the centre, which is biased with a DC voltage and is illuminated with femtosecond laser pulses. After excitation at the photoconductive gap, photo-excited carriers are accelerated under the bias field and create an ultra-short current pulse, which decays with a time constant determined by the carrier lifetime in the photoconductive substrate. The transient current generates ultrashort electromagnetic pulsed radiation (THz radiation) which is collected and colli- mated by Si lens. However, this photoconductive emitter, though widely used for THz spectroscopy and imaging systems, has a limited THz power. This prohibits some important applications of the THz technology. Here we demonstrate a novel THz antenna that requires less pump power of the femtosecond laser pulse whilst provides higher output power of the generated THz pulse. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a gold cone-cylinder structure. Field enhancement is attributed to the collective motion of free electrons confined in narrowly localized regions [4], similar to that observed in colloidal nanoparticles exposed to an external electromagnetic field [5]. Using appropriate fabrication techniques, it is consequently possible to optimize the shape of the nanostructure to be tailored to a particular application [6, 7]. We employ this approach to study a novel THz emitter structure with two different sizes (in nano and micron scales) using the finite-difference time-domain (FDTD) technique. Furthermore, the FDTD is used to study a second novel THz emitter with different antennas fabricated on a GaAs substrate. The enhancement of the static electric field of the bias voltage on the new antennas is demonstrated. 2. FDTD ANALYSIS OF NOVEL TERAHERTZ EMITTERS The FDTD is one of the most popular numerical methods for solving electromagnetic problems with arbitrary geometries and inhomogeneous materials. One of the major advantages of the FDTD is that broadband results can be obtained with the FDTD algorithm run only once, taking a pulse