A standard THz time-domain spectroscopy/imaging (THz-TDS) system is built around an ultrafast laser system and two photoconductive antennas (Figure 1a). The ultrafast laser pulse (< 100 fs) is split into pump and probe beams. The transmitter antenna consists of a photoconductive semiconductor substrate that has an electrode structure on one surface. This structure consists of two parallel metal lines (~100 micron separation) with two tabs at the center extending toward one another. These tabs are separated by a gap that can vary from 5 to 100 microns. The pump laser beam is focused onto this gap. The laser wavelength is chosen to match the photoconductive substrate's bandgap energy. Gallium arsenide (GaAs) is one of the most-often used substrate materials because its bandgap is well suited for the 800 nm light generated by Ti:Sapphire lasers. Charge carriers (electron-hole pairs) are created when the light is incident on the GaAs. When a dc bias is placed across the metal electrodes, these carriers are accelerated, thus inducing a time-varying current. The system is now a linear dipole antenna and a linearly polarized THz pulse is emitted. Figure1b shows both time- and frequency-domain plots of the emitted pulse. A high-resistivity silicon dome is placed on the opposite surface of the GaAs substrate in order to minimize reflection losses and to collimate or focus the THz pulse. Conventional free-space optics are then used to guide and manipulate the beam. A sample can be placed in the THz beam for spectroscopy and/or imaging.

Detection of the THz pulse is accomplished using a receiving photoconductive antenna nearly identical to the transmitter. The receiver substrate is a material that has a fast carrier trapping time so as to achieve the best obtainable temporal resolution. Low-temperature grown GaAs is often used. The probe beam is focused onto the receiver's antenna gap, thus generating charge carriers (Figure 1a). The THz pulse is incident on the receiver's silicon dome, which focuses the pulse onto the antenna gap. The electric field of the THz pulse biases the gap accelerating the carriers, inducing a current surge. A complete time-domain measurement is obtained by varying the arrival time of the probe laser pulse relative to the THz pulse by varying the path length of the probe beam using a mechanical delay line.

The difficulty and bulk associated with most THz systems confines them to academic and research laboratories and away from “real-world” applications. This difficulty is lessened by the use of conventional fiber optics for manipulation of the ultrafast laser beam, but it is not eliminated because there is no equivalent fiber-optic technology for THz frequencies. The requirement that the sample or area of interest have direct line-of-sight access is difficult if not impossible to address with free-space optics. This precludes the development of THz applications such as biomedical endoscopes or security wands for inspection purposes.

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