Design of a controllable attenuator with high dynamic range for THz-frequencies based on optically stimulated free carriers in high-resistivity silicon

As the variety of passive circuit components for the 0.3-5 THz domain is rarely developed, we examine the possibilities to use electro-optically stimulated high resistivity float-zone silicon for the design of a tunable attenuator for quasi-optical systems operating at 580 GHz. Possible applications also include switches and modulators for millimeter and submillimeter-waves. A fiber-coupled 1W CW near infrared (NIR) diode-laser is used to illuminate a silicon slab and creates electron-hole pairs in the bulk silicon. At the operating frequency, the silicon slab acts as a Fabry-Perot resonator to minimize insertion loss due to unwanted reflections from the air-silicon interface. A second silicon slab with micromachined anti-reflection coating is used to shield the NIR to ensure safe operation. We achieve more than 35 dB dynamic range with an insertion loss of less than 0.5 dB.