Non-mechanical beam steering with a dynamic lithography of tunable metasurface

Beam steering has been traditionally achieved by mechanical means. Even though these mechanical techniques have evolved over the past few decades, non-mechanical approaches, due to benefits such as increased speed, lower costs and reduced complexity, have gained considerable interest. In this work, we propose a non-mechanical beam steering of terahertz radiation method using a transient, or dynamic lithography process. The experimental setup is based on a pump-probe technique, with a Ti:Sa pulse laser at 800 nm wavelength as laser source. The high power pump beam is used to “write” a metasurface pattern composed of v-shaped antennas on a high purity float zone silicon substrate. The wavefront of the pump beam is modulated by a liquid crystal spatial light modulator (LCSLM), such that, by the time the beam reaches the silicon substrate, the illumination pattern has the appearance of an array of antennas. Upon incidence on the silicon substrate, the beam generates electron-hole pairs in the illuminated areas, therefore creating structures with metallic-like properties. The presence of carriers and implicitly the metallic quality of the structures are ensured as long the radiation is incident on the substrate. This process was named transient or dynamic lithography, due to its non-destructive property relative to the silicon wafer. The probe beam, less powerful, is used to generate the terahertz signal. This is achieved by a photo-conductive antenna. Subsequently, the terahertz beam probes the pattern projected on the silicon substrate by the pump beam. Due to the electron-hole pairs previously generated in the substrate, the antenna structures will respond to the terahertz radiation in a way similar to metallic antennas. The terahertz beam is therefore steered by the pseudo-metallic antenna array. The detection is achieved with an un-cooled bolometer terahertz video camera. This beam steering technique is very promising due to its flexibility in quickly changing the direction of the steered beam, by “rewriting” the antenna arrays on the silicon substrate, without any mechanical movement of optical elements.