1042 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 12, JUNE 15, 2008 Highly Directed Radiation Pattern From a THz Photonic Transmitter With a Two-Dimensional Rampart Slot Array Antenna Yu-Ru Huang, Chung-Chiu Kuo, Chiu-Min Chiu, Hung-Pin Chen, Tzeng-Fu Kao, Yi-Chun Chen, An-Shyi Liu, Ruey-Beei Wu, Senior Member, IEEE, Pei-Chin Chiu, Jen-Inn Chyi, Senior Member, IEEE, and Chi-Kuang Sun, Senior Member, IEEE Abstract—In this study, we investigate the directivity of the terahertz (THz) radiation pattern from a newly designed two-dimensional rampart slot array antenna integrated in an edge-coupled membrane photonic transmitter. The antenna design is based on the array theory which is well-developed in the microwave regime. By means of the array arrangement of rampart slot antennas, we demonstrate that the 3-dB beam width of the THz radiation pattern at 907 GHz is successfully confined within 30 in both and planes. Index Terms—Antenna, array, radiation pattern, terahertz (THz) photonic transmitter. I. INTRODUCTION W ITH the burgeoning terahertz (THz) wave appli- cations such as molecule imaging system [1] and biodetection technology [2], how to collect THz waves more efficiently is a key point to design a THz wave source. We have previously demonstrated an edge-coupled pho- tonic transmitter based on a low-temperature-grown GaAs metal–semiconductor–metal traveling-wave photodectector (MSM-TWPD) and a coplanar-waveguide (CPW)-fed slot dipole antenna. The compact THz source exhibited not only the wide frequency tunability but also the high conversion effi- ciency [3]. However, it is well known that the directivity of the radiation pattern, especially in the plane, from the CPW-fed slot dipole antennas is low [4]. Therefore, the divergent THz Manuscript received November 13, 2007. This work was supported by the National Science Council through NSC 96-2628-E-002-043-MY3 and NSC 96-2120-M-002-014 and by the Division of Genomic Medicine, NTU Center for Medical Excellence. Y.-R. Huang, C.-C. Kuo, C.-M. Chiu, H.-P. Chen, and T.-F. Kao are with the Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan, R.O.C. Y.-C. Chen, A.-S. Liu, and R.-B. Wu are with the Department of Electrical Engineering and Graduate Institute of Communication Engineering, National Taiwan University, Taipei 106, Taiwan, R.O.C. P.-C. Chiu and J.-I. Chyi are with the Optical Sciences Center and De- partment of Electrical Engineering, Nation Central University, Taoyuan 320, Taiwan, R.O.C. C.-K. Sun is with the Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University and Research Center for Applied Sciences, Academia Sinica, Taipei 106, Taiwan, R.O.C. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.924197 wave is not so appropriate for some far-field applications such as direct coupling to THz fibers without high numerical aperture focusing medium. Many groups had been devoted to developing highly directed THz radiation sources with the array design such as photo- conductive photomixer arrays [5] and photodiode-integrated antenna arrays [6]. However, multiple excitation sources are needed to drive each element in the array and this would make the system too complex for use. Arranging multiple devices in the array also makes the final circuit too large (usually several square millimeters to square centimeters) to be integrated with other circuits or devices. To realize a compact highly directed THz wave source, here we demonstrate a new design for the THz photonic transmitter. A newly designed two-dimensional (2-D) rampart slot array an- tenna is adopted to centralize the main beam of the radiated THz waves. Combined with an edge-coupled MSM-TWPD, an ultra- high resonant frequency at 907 GHz could be observed. Under the coherently controlled optical beating excitation, the mea- sured radiation pattern at 907 GHz is successfully concentrated to less than 30 in both the and planes, which is much smaller than the theoretical limit of a single CPW-fed slot dipole antenna [4]. By integrating the array antennas with a single pho- todetector, a highly directed THz wave emitter could be realized rather than combining multiple devices. The fabricated device size is less than 1 mm , with a high capability for future integra- tion in the optoelectronic integrated circuits and biomicrochips [2]. II. DESIGN AND STRUCTURE The top micrographic view of the fabricated THz photonic transmitter is shown in Fig. 1. It is composed of an edge-coupled MSM-TWPD, a low-pass filter, a dc probe pad, and a newly de- signed array antenna. We adopt the MSM-TWPD as our pho- todetector due to its high-power bandwidth product [7] and su- perior microwave properties such as higher propagation velocity and lower high-frequency microwave loss [8]–[10]. The low- pass filter was designed by using a THz-wave photonic bandgap structure to reflect the THz-frequency ac currents back to the antenna to enhance the radiation efficiency. A detailed struc- ture is shown in the inset of Fig. 1. The antenna was designed based on the array theory which is well-developed in the mi- crowave regime. Since the excited THz signal is fed from the CPW after the TWPD, the design of our array antenna can be 1041-1135/$25.00 © 2008 IEEE