Excitation of Terahertz Plasmonic Resonance on a Subwavelength Metallic Grating in an Attenuated Total Reflection Scheme Using a Monochromatic Radiation Source

Sensors based on surface plasmon resonance excited in the scheme of attenuated total reflection (ATR) on thin metal films have undergone a revolution in the visible frequency range. It is possible to study very weak interactions, up to single molecules, using these sensors. Such an approach, which has already become classical, cannot be implemented in the terahertz-frequency range. Terahertz waves barely penetrate metals due to their high dielectric constant, which makes it difficult to maintain resonance at the metal–dielectric interface. One way to overcome this problem is to modify the metal surface with a structure with a characteristic size much smaller than the radiation wavelength. The structured region of the metal behaves as an effective layer of the medium, the permittivity of which is a function of the response of the dielectric and the metal. On these structures, spoof-surface plasmon resonances, which are similar in properties to surface plasmon resonances in the visible range, can be excited. By combining spoof-surface plasmon resonance and attenuated total reflection, it is possible to implement a sensor that, in the future, will make it possible to detect the smallest concentrations and observe small changes in the boundary dielectric medium in the terahertz-frequency range. The excitation of a spoof-surface plasmon resonance is studied using the method of attenuated total reflection on a flat one-dimensional subwavelength grating with gold sputtering in the Otto scheme. For the first time, the angular reflection spectra are measured according to this scheme using monochromatic terahertz radiation from the Novosibirsk free-electron laser (the wavelength is 141 μm). The plasmon resonances observed in the spectra are consistent with the simulation results using COMSOL Multiphysics.