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 E(z) 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.
Highlights
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 [1,2,3,4,5]
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
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]
Summary
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 [1,2,3,4,5]. 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 [10,11,12]. Metals in this frequency range exhibit a high sensitivity to geometrical details, with critical dimensions in the nanometer range, 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. With the open and short loads, we provide the first practical steps towards impedance matching in the IR by demonstrating how Received 5 Aug 2010; revised 18 Sep 2010; accepted 21 Sep 2010; published 29 Sep 2010 11 October 2010 / Vol 18, No 21 / OPTICS EXPRESS 21679 the impedance mismatch between the CPS and load is visualized in the resulting standing wave
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