Methods of Measuring the Depth of Penetration of the Field of Terahertz Surface Plasmon-Polaritons into Air

Two methods for measuring the penetration depth of the surface plasmon polariton (SPP) field using quasi-monochromatic terahertz radiation from the Novosibirsk free electron laser (λ = 141 μm) are proposed and tested: a probe method with modulation of the radiation by a shutter or modulation of the diffracting fraction of the SPP field by oscillations of a probe embedded in it and a screening method that records the intensity of SPPs that have passed under a metal screen. In both methods, to reduce the proportion of parasitic illumination from volumetric waves, it is proposed to use a fracture of the sample surface or conversion elements (radiation to SPP and back) of cylindrical shape. The results of experiments on assessing the depth of penetration of the SPP field into the air by both methods are consistent with each other. The advantages and disadvantages of these methods, as well as the conditions for their application when working with samples containing and not containing a dielectric coating, are identified.