1051-8223 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TASC.2016.2535147, IEEE Transactions on Applied Superconductivity EUCAS-15_2A-E-P-04.06 1 Integrated Four-Pixel Narrowband Antenna Array for picosecond THz Spectroscopy A. Schmid, J. Raasch, A. Kuzmin, S. Wuensch and M. Siegel Abstract— One of the most promising sources for pulsed THz radiation is coherent synchrotron radiation (CSR). Due to their response time in the picosecond range, direct THz detectors based on YBa 2 Cu 3 O 7-x hot-electron bolometers are ideally suited for the analysis of electron-beam instabilities occurring in the synchrotron storage ring and affecting the emitted CSR spectrum. Response times of down to 16 ps full width at half maximum (FWHM) have already been achieved with wideband antenna-coupled detectors. We are presenting an integrated array combining this fast detector response time with the capability of single-shot spectral resolution for individual CSR pulses. The detector array consists of an integrated four-pixel array which contains four narrowband double-slot antennas with embedded detectors. Index Terms—Planar arrays, slot antennas, spectroscopy, superconducting THz detectors I. INTRODUCTION he generation of high-intensity THz radiation has always been challenging and a limiting factor in the application and development of electronic devices operating in the THz range. This has changed in recent years as ongoing research in accelerator based sources has established synchrotron-based light sources emitting coherent synchrotron radiation (CSR) [1] as a versatile source for pulsed-broadband THz radiation. CSR is emitted in a storage ring when the bunch length of the electron bunches is compressed by so-called low-α optics [2]. To further improve synchrotron based sources, a clear understanding of the beam mechanics, especially bunch-to- bunch interactions, is needed. This requires a detection system with a fast response time and single-shot detection, which limits the choice of suitable detectors, especially for spectroscopy systems. Electro-optical sampling, based on birefringence introduced in a crystalline material through the electrical fields of the electron beam, is a well-established method for beam diagnostics of CSR and offers response times down to a few femtoseconds [3], [4], but requires a complex setup. A simpler spectroscopic setup based on gratings and pyroelectric direct detectors has been proposed Automatically generated dates of receipt and acceptance will be placed here; authors do not produce these dates. This work was supported in part by the Helmholtz International Research School for Teratronics, the Karlsruhe School of Elementary Particle and Astroparticle Physics: Science and Technology (KSETA) and BMBF Project 05K13VK4 The authors are with the Institut fuer Mikro- and Nanoelektronische Systeme, Karlsruhe Institute of Technology (KIT), 76187 Karlsruhe, Germany (email: alexander.schmid@kit.edu) in [5], but with time constants in the μs range it is too slow for a rigorous analysis of bunch-bunch interaction for pulses in the ps range. Systems based on room-temperature Schottky diodes offer response times down to picoseconds but suffer from a limited dynamic range of typically 25 dB [6]. Compared to the aforementioned concepts, a detector based on superconducting high-Tc YBa 2 Cu 3 O 7-x (YBCO) microbridges offers distinctive advantages. With these detectors coupled to broadband spiral antennas, response times of down to 16 ps (FWHM) for CSR in the THz range have already been achieved and sufficient detector sensitivity was demonstrated at the ANKA synchrotron [7]. In order to use such a detector in a spectroscopic system capable of single- shot bunch-to-bunch analysis, spectral filtering needs to be applied. We propose an integrated planar four-pixel array with narrowband double-slot antennas working in a hybrid lens- antenna configuration. 140 GHz, 350 GHz, 650 GHz, and 1.02 THz have been selected as the design frequencies for the antennas in the array. This array will combine the fast response time of the YBCO detectors with spectral- and single-shot resolution. The considerations that went into the design and optimization process as well as measurement results from continuous-wave THz sources are presented here. II. SIMULATION RESULTS A. Double-slot Antennas Double-slot antennas were selected as the receiving elements because of their high coupling efficiency to Gaussian beams in combination with dielectric lenses [8], [9], [10]. Simulation and optimization of the four individual antennas was performed with CST Microwave Studio (CST MWS) [11]. The simulation model consists of an Au metallization layer (t Au = 100 nm) on a sapphire substrate (ε r = 10.06, tan δ = 8.4·10 -6 at 77 K [12]) with a 300 μm thickness. Both layers were modeled as lossy materials. Due to very small thicknesses of below 100 nm all together, the YBCO thin film and additional buffer layers present in the real structure were not included in the model. All antenna simulations were performed with the antenna in transmitting configuration and a discrete port as the excitation source. Due to the reciprocity theorem, the results derived from transmitting antennas can be equally applied to receiving antennas. An optimization process was performed for each antenna individually. Two main points have to be taken into consideration for the antenna design: Matching of the antenna impedance to the detector and matching of the antenna- detector combination to the readout lines. Both is done in T