Near-field measurement of infrared coplanar strip transmission line attenuation and propagation constants Peter M. Krenz1,*, Robert L. Olmon2,3,5, Brian A. Lail4, Markus B. Raschke2,5 Glenn D. Boreman1 1College of Optics and Photonics, CREOL, University of Central Florida, Orlando, FL 32816, USA 2Department of Chemistry, University of Washington, Seattle, WA 98195, USA 3Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA 4Department of Electrical and Computer Engineering, Florida Institute of Technology, Melbourne, FL 32901, USA 5Present address: Department of Physics and JILA, University of Colorado, Boulder, CO 80309, USA *krenz@knights.ucf.edu Abstract: Impedance matched and low loss transmission lines are essential for optimal energy delivery through an integrated optical or plasmonic nanocircuit. A novel method for the measurement of the attenuation and propagation constants of an antenna-coupled coplanar strip (CPS) transmission line is demonstrated at 28.3 THz using scattering-type scanning near-field optical microscopy. Reflection of the propagating optical wave upon an open-circuit or short-circuit load at the terminal of the CPS provides a standing voltage wave, which is mapped through the associated surface-normal Ez electric near-field component at the metal-air interface. By fitting the analytical standing wave expression to the near-field data, the transmission line properties are determined. Full-wave models and measured results are presented and are in excellent agreement. ��2010 Optical Society of America OCIS codes: (110.6880) Three-dimensional image acquisition (180.4243) Near-field microscopy (260.2110) Electromagnetic optics (260.3060) Infrared. References and links 1. A. Al��, and N. Engheta, ���Wireless at the nanoscale: optical interconnects using matched nanoantennas,��� Phys. Rev. Lett. 104(21), 213902 (2010). 2. J.-S. Huang, T. Feichtner, P. Biagioni, and B. Hecht, ���Impedance matching and emission properties of nanoantennas in an optical nanocircuit,��� Nano Lett. 9(5), 1897���1902 (2009). 3. C. T. Middlebrook, P. M. Krenz, B. A. Lail, G. D. Boreman , ���Infrared phased-array antenna,��� Microw. Opt. Technol. Lett. 50(3), 719���723 (2008). 4. J. A. Hutchison, S. P. Centeno, H. Odaka, H. Fukumura, J. Hofkens, and H. Uji-I, ���Subdiffraction limited, remote excitation of surface enhanced Raman scattering,��� Nano Lett. 9(3), 995���1001 (2009). 5. A. V. Akimov, A. Mukherjee, C. L. Yu, D. E. Chang, A. S. Zibrov, P. R. Hemmer, H. Park, and M. D. Lukin, ���Generation of single optical plasmons in metallic nanowires coupled to quantum dots,��� Nature 450(7168), 402��� 406 (2007). 6. A. Yariv, Optical Electronics in Modern Communications, 5th ed. (Oxford University Press, New York, 1997). 7. J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, ���Guiding of a one-dimensional optical beam with nanometer diameter,��� Opt. Lett. 22(7), 475���477 (1997). 8. A. Al��, and N. Engheta, ���Optical nanotransmission lines: synthesis of planar left-handed metamaterials in the infrared and visible regimes,��� J. Opt. Soc. Am. B 23(3), 571���583 (2006). 9. T. A. Mandviwala, B. A. Lail, and G. D. Boreman, ���Characterization of microstrip transmission lines at IR frequencies - Modeling, fabrication and measurements,��� Microw. Opt. Technol. Lett. 50(5), 1232���1237 (2008). 10. J.-S. Huang, T. Feichtner, P. Biagioni, and B. Hecht, ���Impedance matching and emission properties of nanoantennas in an optical nanocircuit,��� Nano Lett. 9(5), 1897���1902 (2009). 11. T. A. Mandviwala, B. A. Lail, and G. D. Boreman, ���Infrared-frequency coplanar striplines: design, fabrication, and measurements,��� Microw. Opt. Technol. Lett. 47(1), 17���20 (2005). 12. J. Wen, S. Romanov, and U. Peschel, ���Excitation of plasmonic gap waveguides by nanoantennas,��� Opt. Express 17(8), 5925���5932 (2009). 13. A. C. Jones, R. L. Olmon, S. E. Skrabalak, B. J. Wiley, Y. N. N. Xia, and M. B. Raschke, ���Mid-IR plasmonics: near-field imaging of coherent plasmon modes of silver nanowires,��� Nano Lett. 9(7), 2553���2558 (2009). 14. L. Novotny, ���Effective wavelength scaling for optical antennas,��� Phys. Rev. Lett. 98(26), 266802 (2007). #132881 - $15.00 USD Received 5 Aug 2010 revised 18 Sep 2010 accepted 21 Sep 2010 published 29 Sep 2010 (C) 2010 OSA 11 October 2010 / Vol. 18, No. 21 / OPTICS EXPRESS 21678

15. R. L. Olmon, P. M. Krenz, A. C. Jones, G. D. Boreman, and M. B. Raschke, ���Near-field imaging of optical antenna modes in the mid-infrared,��� Opt. Express 16(25), 20295���20305 (2008). 16. D. M. Pozar, Microwave Engineering, 3rd ed., (J. Wiley, Hoboken, NJ, 2005). 17. M. Rang, A. C. Jones, F. Zhou, Z.-Y. Li, B. J. Wiley, Y. Xia, and M. B. Raschke, ���Optical near-field mapping of plasmonic nanoprisms,��� Nano Lett. 8(10), 3357���3363 (2008). 18. R. Esteban, R. Vogelgesang, J. Dorfm��ller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, ���Direct near-field optical imaging of higher order plasmonic resonances,��� Nano Lett. 8(10), 3155���3159 (2008). 19. A. Bek, R. Vogelgesang, and K. Kern, ���Scanning near-field optical microscope with sub 10-nm resolution,��� Rev. Sci. Instrum. 77(4), 043703 (2006). 20. F. Keilmann and R. Hillenbrand, ���Near-field microscopy by elastic light scattering from a tip,��� Philos. Transact. A Math. Phys. Eng. Sci. 362(1817), 787���805 (2004). 21. B. C. Wadell, Transmission Line Design Handbook (Artech House, Boston, 1991). 22. R. L. Olmon, M. Rang, P. M. Krenz, B. A. Lail, L. V. Saraf, G. D. Boreman, and M. B. Raschke, ���Determination of electric field, magnetic field, and electric current distributions of infrared optical antennas: A nano-optical vector network analyzer,��� Phys. Rev. Lett. (to be published). 23. S.-N. Lee, J. I. Lee, W. B. Kim, J. G. Yook, Y.-J. Kim, and S. J. Lee, ���Conductor-loss reduction for high- frequency transmission lines based on the magnetorheological-fluid polishing method,��� Microw. Opt. Technol. Lett. 42(5), 405���407 (2004). 24. J. E. Chan, K. Sivaprasad, and K. A. Chamberlin, ���High-frequency modeling of frequency-dependent dielectric and conductor losses in transmission lines,��� IEEE Trans. Compon. Packag. Tech. 30(1), 86���91 (2007). 1. Introduction An integrated optical or plasmonic nanocircuit consists of three basic components: an optical collector or receiver, a waveguide or transmission line, and a nonlocal nanoscale load in the form of, e.g., a molecule, a plasmonic nanoparticle, a detector, or a re-radiating antenna [1���5]. In particular, the efficient energy transduction through that antenna ��� transmission-line ��� load system as provided by an impedance matched and low loss transmission line is essential for optimal delivery of energy to the load. It is furthermore of interest to reduce the size of such a system, therefore prohibiting the use of dielectric waveguides, since their cross-sectional size is limited to a minimum on the order of the guided wavelength [6]. The size of the guiding structure can be reduced by either replacing the dielectric core with a negative index dielectric material to reduce the spatial extent of the guided mode [7], or by replacing the dielectric waveguide by a transmission line. Applying concepts adapted from the radio frequency (RF) regime, the transmission line can be constructed of nanoinductors and nanocapacitors based on metamaterials [8], a metal strip separated from a ground plane by a dielectric standoff layer creating a microstrip [9], or two parallel metal wires forming a coplanar strip (CPS) transmission line. The latter stands out for its structural simplicity and the ease with which it can be integrated with planar antenna designs [10���12]. A lack of detailed understanding of the relationship between geometrical or material parameters, and the corresponding transmission line attenuation and propagation constants has made the practical design of properly matched optical circuits challenging so far. In particular, at optical and infrared (IR) frequencies, transmission lines, typically patterned from thin metal films, are subject to loss resulting from damping associated with a large complex permittivity and surface roughness, and the influence of an inhomogeneous dielectric environment [13,14]. In addition, metals in this frequency range exhibit a high sensitivity to geometrical details, with critical dimensions in the nanometer range, thus on the order of the size of fabrication imperfections. These effects make design of transmission lines in the optical regime using theory alone challenging, and create a need for a systematic experimental approach. Extending previous efforts based on bolometric measurements [9,11], here we demonstrate the measurement of the attenuation and propagation constants of individual antenna-coupled CPSs at long-wave IR frequencies using scattering-type scanning near-field optical microscopy (s-SNOM), which we have previously used to characterize IR dipole antenna modes [13,15]. Reflection of propagating optical excitation upon an open-circuit or short-circuit load at the terminal of the CPS provides a standing wave [16], which is mapped through the phase-sensitive measurement of the associated surface-normal Ez electric near- field component at the metal-air interface. In addition, with the open and short loads, we provide the first practical steps towards impedance matching in the IR by demonstrating how #132881 - $15.00 USD Received 5 Aug 2010 revised 18 Sep 2010 accepted 21 Sep 2010 published 29 Sep 2010 (C) 2010 OSA 11 October 2010 / Vol. 18, No. 21 / OPTICS EXPRESS 21679