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We investigate triangular surface-relief gratings for reducing reflection at a planar silicon surface for light in the terahertz frequency region of 0.3-3.0THz. Structural parameters of the one- and two-dimensional (1D 2D) subwavelength gratings required for the antireflection (AR) layer were obtained numerically. Experimental results for the AR effects agreed well with those obtained numerically, except for gratings fabricated with laser ablation, which causes structural fluctuations of the grating. In the 1D grating, a high transmittance exceeding 90% for the TM wave was obtained. 2D gratings comprised of arrayed micropyramids were experimentally confirmed to be polarization insensitive.

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