THz Laser Beam Profiling by Homogeneous Photodoping of High Resistivity Silicon in a Compact Single-Pixel Detection Setup

A compact single-pixel detection scheme for terahertz (THz) beam profiling is demonstrated. It incorporates an optical amplitude modulator that uses photodoping of a high resistivity Silicon (HR-Si) wafer with near-infrared (NIR) binary intensity masks, to modulate transmission of THz radiation through the semiconductor. A 270 nm thick indium-tin-oxide thin film deposited on glass by radio-frequency magnetron sputtering is used as dichroic mirror, such that the NIR radiation is brought at the Si wafer and the THz radiation is directed toward the detector, in a compact and efficient design. Given that homogeneous and well-defined binary masks in the form of structured intensity profiles of the NIR laser beam were needed, a beam homogenizer based on the Kohler integrator is also implemented. The profile of a THz laser beam with the wavelength of 118.8 μm was recorded using a resolution of 32 × 32 pixels and a square imaging area of 169 and 400 mm
²
, respectively. Also, due to the tunable nature of the laser system and symmetry of the laser cavity, more than one transverse modes of the laser cavity were imaged, namely, TEM
₀₀
and TEM
₀₁
. A beam diameter of 3.25 mm in the TEM
₀₀
mode was recorded and a modulation depth of 52% at 2.5 THz was achieved with the present setup. With this paper, a THz spatial-domain imaging setup based on single-pixel method and having a compact design is proposed. Its functionality and possibility of further development are shown.