A free-space electro-optic sampling system for real-time two-dimensional (2-D) imaging of a terahertz (THz) beam has been demonstrated by researchers at Rensselaer Polytechnic Institute (Troy, NY). Time-resolved far-infrared (IR) images are converted to time-resolved optical images using a zinc telluride (ZnTe) sensor and a high-performance digital charge-coupled-device (CCD) camera with a temporal resolution of 50 fs and submillimeter spatial resolution.
The use of terahertz beams for imaging applications has been demonstrated previously with far-IR images of tree leaves and semiconductor integrated-circuit chips, for example (see Laser Focus World, July 1995, p. 15). In these demonstrations the imaging system used a single photoconductive dipole antenna as the detector so the samples were scanned in two dimensions relative to the fixed beam during the measurement. As a result, the acquisition time of an image was typically on the order of minutes or hours, depending on the total number of pixels and the lowest terahertz frequency components of interest.
The Rensselaer real-time 2-D imaging system is based on the linear Pockels effect in electro-optic crystals in which a pulsed microwave signal acts as a transient bias to induce a transient polarization in the sensor crystal. This polarization induces a birefringence, which then modulates a synchronously pumped laser beam. The amplitude-modulated output can be captured by the CCD camera. The laser source is a Ti:sapphire laser with pulse duration of less than 50 fs. An unbiased GaAs wafer is used to generate the pulsed radiation.
This terahertz radiation is focused onto a 0.9-mm-thick, 6 × 8-mm ZnTe crystal with parabolic mirrors, while an optical readout beam with a diameter larger than that of the terahertz beam probes the electric field distribution in the crystal via the Pockels effect. The 2-D field distribution in the sensor crystal is converted into a 2-D optical intensity distribution after the readout beam passes through a crossed polarizer; the optical image is then recorded by the camera. The 8.4 × 6.3-mm imaging area of the 384 × 288-pixel CCD camera is comparable to that of the ZnTe sensor, and no focusing optical element is used between the sensor and the camera.
According to Rensselaer researcher X.-C. Zhang, the data-acquisition rate and signal-to-noise ratio of the system are limited by the thermoelectrically cooled CCD camera operated in frame-transfer mode. The high sensitivity of the camera allows use of low optical readout power, typically less than 300 µW, for operation of the CCD chip near its full-well capacity. The frame-transfer limit of 7.5 frames/s at a 1-MHz scan rate can potentially be increased up to 38 frames/s for true video operation by reducing the frame readout time from 133 to 27 ms at a 5-MHz scan rate. "Such high data rates are unprecedented for a terahertz imaging system," says Zhang.