process, where an appropriate crystal material is quickly electrically polarized at high optical intensities. This changing electrical polarization emits terahertz radiation. Because of the high laser intensities that are necessary, this technique is mostly used with amplified Ti:sapphire lasers. Typical crystal materials are zinc telluride, gallium phosphide, and gallium selenide. The bandwidth of pulses generated by optical rectification is limited by the laser pulse duration, terahertz absorption in the crystal material, the thickness of the crystal, and a mismatch between the propagation speed of the laser pulse and the terahertz pulse inside the crystal. Typically, a thicker crystal will generate higher intensities, but lower THz frequencies. With this technique, it is possible to boost the generated frequencies to 40 THz (7.5 µm) or higher, although 2 THz (150 µm) is more commonly used since it requires less complex optical setups. THz Detection The electrical field of terahertz pulses is measured in a detector simultaneously illuminated with an ultrashort laser pulse. Two common detection schemes are used in THz-TDS: photoconductive sampling and electro-optical sampling. The power of THz pulses can be detected by bolometers (heat detectors cooled to liquid-helium temperatures), but since bolometers can only measure the total energy of a terahertz pulse rather than its electric field over time, they are unsuitable for THz-TDS. Because the measurement technique is coherent, it naturally rejects incoherent radiation. Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low. The signal-to-noise ratio (S/N) of the resulting time-domain waveform depends on experimental conditions (e.g., averaging time). However due to the coherent sampling techniques described, high S/N values (>70 dB) are routinely observed with 1 minute averaging times. Downmixing The original problem responsible for the “Terahertz gap” (the colloquial term for the lack of techniques in the THz frequency range) was that electronics routinely have limited operation at frequencies at and above 1012 Hz. Two experimental parameters make such measurement possible in THz-TDS with LT-GaAs antennas: the femtosecond “gating” pulses and the < 1 ps lifetimes of the charge carriers in the antenna (effectively determining the antenna's “on” time). When all optical path lengths have fixed length, an effective dc current results at the detection electronics due to their low time resolution. Picosecond time resolution does not come from fast electronic or optical techniques, but from the ability to adjust optical path lengths on the micrometer