Ultrafast Active Control of Plasmonic Resonances at THz Frequencies
Semiconductors are promising materials for THz plasmonics. They acquire metallic behavior when sufficient free carriers are present. Plasmonic structures fabricated out of these semiconductor materials can sustain localized surface plasmon polaritons (LSPPs). The plasmonic behavior of such structures is determined by both their geometry and dielectric properties. The carrier density – hence the plasmonic behavior – can be actively controlled for a given geometry by optical excitation of free carriers in the semiconductor.
We have demonstrated that by ultrafast optical pumping of free carriers in a random array of Si bowtie antennas, it is possible to activate LSPPs on picosecond timescales (Berrier et al. Opt Express 18:23226–23235, 2010). Bowtie antennas, Fig. 43.1, have the ability to resonantly enhance the field intensity at the gap separating the two triangular resonators, which allows the concentration of THz radiation beyond the diffraction limit. THz field intensity enhancements of several orders of magnitude in deep subwavelength volumes have been predicted (Giannini et al. Opt Express 18(3):2797–2807, 2010). These large local field enhancements have been already used for the enhanced detection of nanometric inorganic layers and biological films (Berrier et al. Opt Express 20(5):5052–5060, 2012; Berrier et al. Biomed Opt Express 3(11):2937–2949, 2012).
In this contribution we will show the first THz measurements of the enhanced resonant extinction of single semiconductor plasmonic antennas. This demonstration is achieved by placing the antenna at the output aperture of a conically tapered waveguide, which enhances the intensity of the incident THz field at the antenna position by a factor of 10, and suppresses the background radiation that otherwise is transmitted without being scattered by the antenna (Schaafsma et al. New J Phys 15(015006):1–14, 2013). This far-field investigation of single plasmonic antennas may open the possibility of THz time-domain spectroscopy of single nanostructures by bridging together length scales as unequal as the THz wavelengths and the dimensions of nanostructures.