Active terahertz beam steering using photo-excited thin-?film semiconductors: Changing the transmission directionality by optically inducing blazed refractive index gratings in gallium arsenide

Abstract

Changing the directionality of electromagnetic radiation has gained much attention from both industry and the research community in the last century. In the infrared and terahertz regime beam steering is conventionally achieved by fabricating multiple sub-wavelength metallic antennas in a unit cell. In this work a novel approach is used that photo-excites carriers locally in a thin-film semiconductor using a spatial light modulator. In this way photo-excited blazed refractive index gratings are achieved. This active technique allows to change the directionality of the transmitted THz radiation by changing the illumination pattern on the sample, without the need of physically structuring the sample. We show a factor 2.0 increase in the amount 1 THz intensity in the first diffraction order of the blazed grating by changing the photo-excited free carrier profile in the semiconductor. This increases to a factor 2.4 when a broadband THz probe beam is used. The experimental results show a good agreement with theoretically calculated values derived from diffraction theory.