Fourier Transformed Terahertz Spectroscopy Inspired Detection of Evanescent Orders in All Dielectric Sub-Wavelength Grating

Although evanescent orders offer interesting physics, the experimental detection of these modes are very challenging task because of their inherent characteristics. Evanescent orders are non-propagating in nature making it impossible to detect in the far-field. At the same time, near-field detection is also complicated because of the concurrence of propagating and non-propagating diffraction orders. In order to overcome these hindrances, we introduce Fourier transformed terahertz spectroscopy (FTTS) through near-field scanning terahertz microscopy (NSTM) to experimentally detect the non-propagating evanescent orders originating from a 1-dimensional dielectric grating for both the TE and TM configurations. Our studies demonstrate exponential decay of mode strengths away from the grating-air interface confirming them as evanescent waves. Identification of evanescent orders demonstrate the sound competences that FTTS can offer in exploring near-field systems. We believe, FTTS based approaches can pave new avenues on studying other near-field phenomena in terahertz photonic systems.