THz plasmonics

The terahertz (THz) range is the region of the electromagnetic spectrum bridging the frequencies that can be generated with high-frequency electronics (microwaves) and the frequencies generated optically (infrared). Terahertz radiation, also called T-rays or submillimeter waves, has low energy (few meV) and it is non-ionizing for biological matter. In the group Surface Photonics, we investigate resonant plasmonic structures at THz frequencies to locally enhance the electromagnetic field. We also use these field enhancements for the selective and sensitive detection of biological and chemical agents.

Plasmonics deals with the excitation and control of surface plasmon polaritons (SPPs). SPPs are electromagnetic waves coupled to charge carriers at the interface between a dielectric and a conductor. This coupling leads to a strong confinement of the electromagnetic energy to the interface, characterized by an evanescent decay of the field away from the surface. SPPs can propagate along the surface with a propagation length determined by the Ohmic losses in the conductor, by absorption in the dielectric and by scattering with inhomogeneities on the surface. SPPs can be also localized into small conducting particles. Scientists have started using plasmonics in the optical and infrared range for biosensing applications, essentially because the enhanced electromagnetic field at the surface gives unprecedented sensitivity and allows label-free detection. The very large permittivity of metals at THz frequencies leads to a weak coupling between the electromagnetic field and the free charge carriers. In this case SPPs on flat surfaces are referred to as Zenneck waves and they are loosely bound to the surface. For a gold-air interface the decay of a Zenneck wave into air at 1 THz is several centimeters. Doped semiconductors have a much lower permittivity than metals at THz frequencies, offering an interesting alternative to metals for THz plasmonics.

In the Surface Photonics group, we investigate the behavior of resonant plasmonic structures with sizes of the order of magnitude of the wavelength of THz. These structures act as plasmonic antennas, catching and localizing THz radiation . One particular type of antenna is the dimer antenna (formed by two adjacent rods) or the bowtie antenna (formed by two adjacent triangles). We have demonstrated theoretically that semiconductor materials, in particular InSb with high mobility electrons, are suitable to sustain localized surface plasmon polaritons. We have shown that, with an appropriate choice of the shape and dimension of the semiconductor antennas, it is possible to obtain large electromagnetic field enhancement. A remarkable property of semiconductor materials is that they offer the possibility to tune the response of the plasmonic antenna by varying the carrier concentration. We also investigate the active control of plasmonic resonances by optical excitation of free charge carriers.

RELATED PROJECTS: ULTRA project