Terahertz time-domain spectroscopy

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the effect of a material on both the amplitude and the phase of the terahertz radiation. In this respect, the technique can provide more information than conventional Fourier-transform spectroscopy that is only sensitive to the amplitude.

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.

Typically, the terahertz pulses are generated by an ultrashort pulsed laser and last only a few picoseconds. A single pulse can contain frequency components covering the whole terahertz range from 0.3 to 4 THz. On the detection side, the electrical field of the terahertz pulse is sampled and digitized, conceptually similar to the way an audio card transforms an electrical voltages in an audio signal into numbers that describe the audio waveform. In THz-TDS, the electrical field of the THz pulse interacts in the detector with a much-shorter laser pulse (e.g. 0.1 picoseconds) in a way that produces an electrical signal that is proportional to the electric field of the THz pulse at the time the laser pulse passed through the detector. By repeating this procedure and varying the timing of the laser pulse, it is possible to scan the THz pulse and construct its electric field as a function of time. Subsequently, a Fourier transform is used to extract the frequency spectrum from the time-domain data.

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Generation

There are two widely used techniques for generating terahertz pulses, both based on ultrashort pulses from titanium-sapphire lasers.

Photoconductive emitters

In a photoconductive emitter, a short optical laser pulse (100 femtoseconds or shorter) creates electron-hole pairs in a semiconductor material to which a voltage is applied. Effectively, the semiconductor changes abruptly from being an isolator into being a conductor, leading to a sudden electrical current. This changing current emits terahertz radiation, similar to what happens in the antenna of a radio transmitter. Terahertz pulses can be 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 (such as InP) substrate. In a commonly used scheme, the electrodes are formed into the shape of a simple dipole antenna with a gap of a few micrometers and have a bias voltage of approximately 40 V between them. This scheme is suitable for illumination with a Ti:sapphire oscillator laser with pulse energies of about 10 nJ. For use with amplified Ti:sapphire lasers with pulse energies of about 1 mJ, the electrode gap can be increased to several centimeters with a bias voltage of up to 10 kV.

Absorption of the laser pulse, which is typically 100 fs or shorter and should have a wavelength that is short enough to excite electrons across 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, thus emitting radiation 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.^{[citation needed]}

Pulses produced by this method generally have a power level on the order of nanowatts average, although the peak power during the pulses can be many orders of magnitude higher. The bandwidth is mostly limited by how quickly the charge carriers can accelerate in the material, rather than by the duration of the laser pulse.

Optical rectification

Main article: Optical rectification

In optical rectification, a high-intensity ultrashort laser pulse passes through a transparent crystal material that emits a terahertz pulse without any applied voltages. It is a nonlinear-optical 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 or higher, although 2 THz is more commonly used since it requires less complex optical setups.

Detection

The electrical field of the terahertz pulses is measured in a detector that is simultaneously illuminated with an ultrashort laser pulse. Two common detection schemes are used in THz-TDS: photoconductive sampling and electro-optical sampling. THz pulses can also be detected by bolometers, heat detectors cooled to liquid-helium temperatures. Since bolometers can only measure the total energy of a terahertz pulse, rather than its electrical field over time, it is not suitable for use in THz-TDS.

In both detection methods, a part (called the detection pulse) of same the ultrashort laser pulse that was used to generate the terahertz pulse is lead to the detector, where it arrives simultaneously with the terahertz pulse. The detector will produce a different electrical signal depending on whether the detection pulse arrives when the electric field of the THz pulse is low or high. An optical delay line, is used to vary the timing of the detection pulse.

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.

Photoconductive Detection

Photoconductive detection is similar to photoconductive generation. Here, the bias electrical field across the antenna leads is generated by the electric field of the THz pulse focused onto the antenna, rather than being applied externally. 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.

Electro-optical sampling

The materials used for generation by optical rectification can also be used for detection by using the Pockels effect, where certain crystalline materials become birefringent in the presence of an electric field. The birefringence caused by the electric field of a terahertz pulse leads to a change in the optical polarization of the detection pulse, proportional to the electric-field strength. With the help of polarizers and photodiodes, this polarization change is measured.

As with the generation, the bandwidth of the detection is dependent on the laser pulse duration, material properties, and crystal thickness.

References and notes

• C. A. Schmuttenmaer (2004). "Exploring dynamics in the far-infrared with terahertz spectroscopy" (pdf). Chemical Reviews 104: 1759-1779.

See also

• optical rectification