Terahertz surface plasmon polaritons imaging system

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Surface plasmon polaritons (SPPs) have a lot of applications such as near-field sensing, super-resolution imaging, and nanolithography. Fundamental devices for exciting, guiding, and focusing the SPPs have been extensively investigated. However, the primary ways used to characterize the functionality of SPPs devices in the visible spectral range can obtain only the intensity information and the phase information is lost completely, which is unfavorable for the design and characterization of the SPPs devices. In the terahertz (THz) spectral range, metals are no longer described by the Drude model but are considered as the perfect electrical conductors (PEC). The SPPs excited by the THz are weakly confined in the dielectric side. The 1/e field extension of the THz-SPPs from the metal surface can arrive several millimeters into the air. The weak confinement and the scale of the wavelength enable the detection of the THz-SPPs with the techniques which are available in the THz spectral range. In this presentation, a comprehensive system is constructed for imaging the THz SPPs. Both the amplitude and phase information of the focusing THz-SPPs excited by a semicircular plasmonic lens are achieved using this system. The amplitude images present the focusing profiles of THz-SPPs for different frequency and the phase images unveil the Gouy phase shift as the THz-SPPs evolve through the focus. The simulations are also performed and a good agreement between the experiment and simulation results has been found.