Terahertz time domain spectroscopy
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In terahertz time domain spectroscopy (THz-TDS), picosecond pulses of terahertz radiation are used to probe different materials. The radiation has several distinct advantages over other forms of spectroscopy: many materials are transparent to THz, THz radiation is safe for biological tissues because it is non-ionizing (unlike for example x-rays), and images formed with terahertz radiation can have relatively good resolution (less than 1 mm). Also, many interesting materials have unique spectral fingerprints in the terahertz range, which means that terahertz radiation can be used to identify them. Examples which have been demonstrated include several different types of explosives as well as several illegal narcotic substances.
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[edit] Generation
Terahertz pulses are typically produced by conduction between two electrodes patterned on a low temperature gallium arsenide (LT-GaAs), semi-insulating gallium arsenide (SI-GaAs), or other semiconductor (including InP) substrate. Usually the electrodes are formed into the shape of a simple dipole antenna. The antenna leads have a bias voltage of approximately 40 V imposed between them.
Absorption of a ~100 fs laser (often a titanium sapphire laser (Ti-Saph laser)) pulse whose center frequency exceeds the bandgap of the semiconductor substrate generates carriers there between the antenna leads. These charge carriers quickly accelerate due to the bias across the antenna leads. Accelerating charges emit photons, in this case in the THz frequency range. Very short THz pulses (typically ~2 ps) are produced due to the rapid rise of the photo-induced current in the gap and, in short-lifetime materials such as LT-GaAs, the fall of the photocurrent as well. This current may persist for only a few hundred femtoseconds or up to several nanoseconds, depending on the material of which the substrate is composed. This is not the only means of generation, but is currently (as of 2006) the most common.
Pulses produced by this method generally have a power level on the order of nanowatts. As one would imagine, the blackbody radiation levels present at room temperature in this frequency range overpower this by a few orders of magnitude so an advanced detection system is also required.
[edit] Detection
The detection of the resulting THz field may be accomplished in a similar manner as generation. The initial Ti-Saph laser pulse is split into two halves and the second portion is used to generate carriers in the THz receiver semiconductor substrate. However, for the receiver, the "bias" across the antenna leads is generated by the electric field of the THz pulse focused onto the antenna (rather than a DC bias as in the THz emitter). The presence of the THz electric field generates current across the antenna leads, which is usually amplified using a low-bandwidth amplifier. This amplified current is the measured parameter which corresponds to the THz field strength. Again, the carriers in the semiconductor substrate have an extremely short lifetime. Thus, the THz electric field strength is only sampled for an extremely narrow slice (fs's) of the entire electric field waveform.
To collect an entire THz time waveform an optical delay line is used to sweep the delay between the generation and detection laser pulses. In this manner, points of the THz electric field waveform can be measured for increasing delays and the whole time waveform can be reconstructed (analogous to the method used in sampling digital oscilloscopes). This method of THz detection yields both the amplitude and phase of the THz pulse, and thus is said to be coherent. Because the measurement technique is coherent, it also naturally rejects incoherent radiation (which points just as often in one direction as in another, and therefore generates zero current on average). Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low. As a result, this advanced detection system allows for very good Signal-to-Noise (S/N) levels for the measurement (S/N >10,000 is typical).
