Tunable broadband antireflection structures for silicon at terahertz frequency

We report enhanced transmission of broadband terahertz electromagnetic waves in the frequency range from 0.2 to 3.15 THz using silicon with micropyramid surface structures. We observe a maximum 89% reduction in reflectivity of terahertz power when the sample with 60-μm-period micropyramids is used, compared to a planar silicon substrate. By varying the period of micropyramid structures from 110 to 30 μm, the cutoff frequency of enhanced transmission is tuned from 0.74 to 2.93 THz and the bandwidth of enhancement increases from 0.91 to 3.15 THz, respectively.