Observation of evanescent-wave filtering in super-resolved THz images

Summary form only given. According to rigorous diffraction theory, monochromatic electromagnetic waves reflected from a material surface form a mixture of propagating and evanescent waves. During propagation, light fields suffer a progressive spatial filtering that depends exponentially on the size of the subwavelength image detail, a peculiar multiscale distortion in the image that spans the near-field of the sources, occurs in vacuum and with no dissipation, and has never been reported before.Here we explore this transition from super-resolved to diffraction-limited imaging for THz frequencies (? = 1 mm). The experiment is carried out using a knife-edge scan, that allows subwavelength beam profiling in the near-field of emitters [1], here enacted remotely to produce super-resolved images on planes at different distances from the emitters [2]. Compared to visible light imaging, the millimetric wavelength expands the near-field region to the point that single near-field imaging planes can be scanned using a mechanical free-standing blade.