TERAHERT¥IMAGING

A free-space electro-optic sampling system for real-time two-dimensional (2-D) imaging of a terahert¥(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.

TERAHERT¥IMAGING

Researchers ma¥real-time 2-D terahert¥images

Stephen G. Anderson

A free-space electro-optic sampling system for real-time two-dimensional (2-D) imaging of a terahert¥(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 terahert¥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 terahert¥frequency components of interest.

Imaging system

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 (see Fig. 1). An unbiased GaAs wafer is used to generate the pulsed radiation.

This terahert¥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 terahert¥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 (see Fig. 2). 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 chi¥near its full-well capacity. The frame-transfer limit of 7.5 frames/s at a 1-MH¥scan rate can potentially be increased u¥to 38 frames/s for true video operation by reducing the frame readout time from 133 to 27 ms at a 5-MH¥scan rate. "Such high data rates are unprecedented for a terahert¥imaging system," says Zhang.

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