Investigation of impedance-matching techniques for infrared antennas

In this study, we used scattering-scanning near-field optical microscopy (s-SNOM) to experimentally characterize several structures relevant to infrared (IR) antenna technology, by measuring the electric near-field strength as a function of position on metallic antennas and transmission lines. Having spatially resolved measurements of electric field amplitude allows assessment of the wave impedance at any location on a standing-wave structure. We improved the usual s-SNOM data-processing method using a principal-components decomposition to allow unambiguous phase retrieval. We demonstrated the efficacy of this technique on IR bow-tie antennas of continuous and discrete designs, allowing comparison of their polarization dependence and spatial response distribution. This phase-retrieval procedure was used throughout our investigations. An IR sensor of particular interest is the antenna-coupled metal-oxide-metal (MOM) diode, which rectifies IR-frequency current waves collected by the antenna to produce an output voltage proportional to the incident irradiance. These sensors are appealing because they have a fast response and do not require cryogenic cooling. IR antennas have a typical impedance in the range of tens of Ohms at resonance, while MOM diodes have impedance in the range of thousands of Ohms. This impedance mismatch is a limiting factor in the detection sensitivity that can be achieved with antenna-coupled MOM diodes. To address this issue, we studied two impedance-matching techniques. The first is based on the fact that a MOM diode under DC bias exhibits a change in its dynamic resistance. We obtained measurements that demonstrate modification of IR-frequency current waves using diodes contained in the antenna structure. The ability to tune the operating point of a MOM diode and thereby modify antenna or transmission-line impedance at IR frequencies offers the possibility of active impedance-matching networks. The second technique we investigated involved tailoring of the feed-point geometry to obtain an antenna with higher impedance that offers better matching. We designed, fabricated and demonstrated several new IR-antenna designs that have impedance in the range of 1000 Ohms. This new class of antennas stands to improve signal-transfer efficiency to high-impedance IR sensors such as MOM diodes.