Active terahertz beam steering by photo-generated graded index gratings in thin semiconductor films
We demonstrate active beam steering of terahertz radiation using a photo-excited thin layer of gallium arsenide. A constant gradient of phase discontinuity along the interface is introduced by an spatially inhomogeneous density of free charge carriers that are photo-generated in the GaAs with an optical pump. The optical pump has been spatially modulated to form the shape of a planar blazed grating. The phase gradient leads to an asymmetry between the +1 and -1 transmission diffracted orders of more than a factor two. Optimization of the grating structure can lead to an asymmetry of more than one order of magnitude. Similar to metasurfaces made of plasmonic antennas, the photo-generated grating is a planar structure that can achieve large beam steering efficiency. Moreover, the photo-generation of such structures provides a platform for active THz beam steering.
Steinbusch, T.P, Tyagi, H.K, Schaafsma, M.C, Georgiou, G, & Gómez Rivas, J. (2014). Active terahertz beam steering by photo-generated graded index gratings in thin semiconductor films. Opt. Express, 22(22), 26559–26571. doi:10.1364/OE.22.026559