research

THz plasmonics

                                                       

Our activities in THz plasmonics are performed within the European project ULTRA (Ultra-fast Electronics for Terahertz Rapid Analysis in Compact Lab-on-Chip Applications. You can find more information about this project and the consortium at the web pages of ULTRA.

Plasmonics or surface plasmon polariton optics is a recent development of electromagnetism dealing with the control of the propagation 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. This scattering can be controlled to modify the propagation of SPPs in a similar way as it is done with mirrors, lenses and waveguides with free space electromagnetic radiation. 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. However, 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 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 as large as 5 cm. Doped semiconductors have a much lower permittivity than metals at THz frequencies, leading to strongly coupled SPPs to semiconductor-dielectric interfaces. The decay length of THz SPPs on semiconductor surfaces is on the order of a few hundred micrometers. An alternative to semiconductors for efficient THz plasmonics are metallic surfaces perforated with arrays of holes much smaller than the wavelength of the THz radiation. These structures have a larger effective skin depth or penetration of the field into the effective medium defined by the structured metal, leading to a surface mode that mimics a SPP on a flat surface.

One of the aims of the ULTRA project is to use THz radiation instead of infrared or visible light for selective detection of biological and chemical agents by exploiting their peculiar spectral signatures in the THz range. We plan to use THz SPPs as a probe in order to increase the sensitivity. Terahertz SPPs will increase the sensitivity for detection by a factor proportional to the field enhancement at the surface, which in optimized structures can be of several orders of magnitude. This increased sensitivity will have a large impact on the realization of accurate and reliable THz analysis using small amounts of analyte.