The band gap of silicon is 1.12 eV. Therefore, silicon is transparent to light in the 1310 nm and 1550 nm telecom bands. Although silicon photodetectors are widely used in laboratories and various applications, they generally cannot detect light in the 1310 nm and 1550 nm telecom bands.
Our group recently published a paper, entitled "Infrared photoconductor based on surface-state absorption in silicon" in Optics Letters [46, 2577-2580 (2021)], using surface state absorption (SSA) effect of semiconductor and lock-in readout. This work realized the normal-incidence silicon photodetector working in the 1310 nm and 1550 nm telecom bands, and demonstrated its application in imaging and beam characterization. Because SSA is a very weak light-absorption effect, there is almost no lost of light during the detection process, so it is a non-invasive photodetector.