Measurement of the resonant lengths of infrared dipole antennas

The resonant lengths of infrared dipole antennas at 10.6 and 3.39 mu m are experimentally investigated. For this purpose, submicron-sized microbolometers coupled to dipole antennas with lengths between 0.7 and 20 mu m were fabricated on a SiO2-on-Si substrate. The response of the detector to 10.6 mu m radiation shows a first resonance for an antenna length between 1.0 and 2.5 mu m A subsequent zero and a second attenuated resonance are observed as the antenna length increases. Similar behavior is observed for illumination at 3.39 mu m, with a first resonance occurring at a length shorter than 1 mu m. The results permit evaluation of an effective dielectric permittivity and shows the effect of the surface impedance of the metal on the propagation of current-wave on the antenna. The resonance behavior is further studied by changing the irradiation conditions of the detectors. Air-side and substrate-side illumination exhibit identical resonant antenna lengths, but different efficiencies of power collection. The antenna patterns as a function of incident angle have also been measured at 10.6 mu m, showing a transition from a primary broadside lobe to the development of side lobes for longer antennas. Finally, an antenna response is measured at visible frequencies. Our measurements point out similarities, as well as differences, between infrared antennas and their counterparts at microwave frequencies, and provide insights useful for the design optimization of planar infrared antennas. (C) 2000 Elsevier Science B.V. All rights reserved.