High-Resistivity Silicon THz Windows

Overview

High-resistivity silicon has the flattest refractive index and among the lowest absorption of any terahertz window material, which makes it the reference choice where optical performance and mechanical strength both matter. These windows are finished to 80/50 scratch-dig with diameter held to 0.2 mm and curvature to 0.01 mm.

Where silicon is the right answer

Silicon is rigid enough to hold a pressure differential, so it is the standard window for vacuum chambers and cryostats, where a polymer window would flex or fail. It is also the usual material for terahertz beam combiners, because a thin silicon plate can transmit a near-infrared probe while reflecting the terahertz beam, or the reverse, depending on geometry.

The trade-off to plan for

The refractive index of about 3.42 is high, so roughly 30% of the incident power is reflected at each uncoated surface and the internal etalon is strong. In a time-domain measurement this shows up as a clear delayed replica of the main pulse. Either specify the thickness so the replica falls outside the scan window, use a wedged part so the internal reflections walk off axis, or ask about anti-reflection treatment.

Practical notes

Silicon is opaque in the visible, so a visible alignment laser cannot be used through it; set the axis first with a TPX part or align mechanically. Handle by the edge and keep the faces clean, since surface contamination shows up more strongly in a high-index material. For any window in the terahertz path, state the beam diameter at that plane along with the aperture you need, so the part is not undersized in a collimated section.