Integrated Broadband THz Switching Using Photoconductivity Modulation in Si-on-Sapphire Substrates

We report the first experimental demonstration of an alternative approach for achieving integrated broadband terahertz (THz) switching functionality, based on photoconductivity modulation in thin film Si on low-loss sapphire substrates. By employing a silicon thin film mesa (14 × 14 μm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>) on a silicon-on-sapphire (SoS) substrate (with 1.5-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i>m-thick silicon and 600-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i>m-thick sapphire), an optically-controlled THz switch integrated with low-loss coplanar waveguide transmission lines operating from 110 to 220 GHz has been designed, fabricated, and characterized. To control the switch in this prototype demonstration, a 915 nm laser diode was used to illuminate the silicon mesa through an optical fiber. The switch performance was measured on-wafer in both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i>- and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i>-band; an average <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small>-state insertion loss of ∼3.5 dB (with minimum insertion loss of 2.5 and 1.8 dB at 160 and 200 GHz), and an off-state isolation greater than 20 dB across the entire frequency range have been achieved. This performance can be further greatly improved, and is comparable to or better than competing approaches, with the advantage that this approach allows seamless integration of the switch with passive components on low-loss sapphire substrates. This makes the reported approach promising for developing high-performance and compact THz switches for next-generation adaptive circuits and systems.