Terahertz Time-Domain Spectroscopy

The pump-probe principle is a method for measuring ultrashort processes. It enables temporal resolutions of less than 100 fs, although the actual measurement duration is significantly longer. A temporal resolution of this magnitude is achieved by scanning the process to be measured. In order to be able to measure a complete process, it must be excited (»pumped«) several times in order to scan (»probe«) small sections one after the other. In terahertz TDS, this means that the emitter emits several terahertz pulses in succession, while the detector scans the terahertz pulse in short time windows. It is crucial that the detector is only active for a very short moment in order to record precise measured values. To be able to detect a certain range, the time delay between the emitter and detector pulse must be variable (see figure).

An example of a measurement result is shown on the right-hand side of the figure. Taking into account the time delay between the emitter and detector pulses, a time curve can be generated from the measured data points. In order to obtain information about the frequency-dependent amplitude and phase, the measurement can be translated into frequency space using the Fourier transformation.

Generation of Terahertz Waves

Photoconductive switches are often used to generate terahertz waves. In the university environment, surface emitters or non-linear optics methods are also used.

The first pulsed terahertz beams were emitted and received in 1984 by H. Auston et al. using photoconductive antennas. The antenna structure of a photoconductive antenna is shown in figure three: Two symmetrical conductor tracks on a semiconductor material are connected to a DC voltage. The resulting electric field has the greatest field strength in the gap of the dipole antenna.

This gap is illuminated by a femtosecond laser. If the photon energy of the laser is greater than the band gap of the semiconductor material used, electrons are lifted into the conduction band. They are accelerated by an applied bias voltage, which results in a current flow through the antenna. The electrons excited in the semiconductor material leave electron holes in the valence band, resulting in recombination between the electron and the electron hole after a certain time.

This temporal change in current density – caused by the generation and recombination of the charge carriers – leads to the emission of electromagnetic waves according to Maxwell's equations.

Detection of Terahertz Waves

A photoconductive antenna can also be used to detect terahertz waves. In contrast to the generation of terahertz beams, however, the electrons are not accelerated by an external bias voltage, but by the electric field of the incident terahertz pulse. The resulting current flow is converted into a voltage using a transimpedance amplifier and then measured.

Typical Properties of a TDS System

With mechanical delay lines, typical measurement rates of 40 terahertz pulses per second are possible, with electronic delay concepts up to 1,600 terahertz pulses per second. The fourth figure shows a typical terahertz pulse and the associated spectrum.

The bandwidth is typically more than 4 THz with a dynamic range of approx. 60 dB and measurement times in the range of 1 s.

Application of Terahertz Technology in Material Analysis

The most important application is the measurement of reflected signal transit times. Thanks to the short terahertz pulses and high temporal accuracy in pulse detection, this method is particularly well suited for measuring the thickness of thin layers that are not transparent in the visible range. This is the basis of industrial coating thickness measurement.

Terahertz time-domain spectroscopy is a reliable, non-contact and non-destructive method for characterizing a wide variety of materials, such as powders, solids, liquids and gases. Many molecules show characteristic signatures in their absorption spectra in the spectral range – their own chemical fingerprint.

Gases exhibit rotational spectra with narrow absorption lines. Only crystalline solids show broad absorption bands due to the phonon vibrations of the crystal. Powdery or solid substances must have at least one crystalline component in order to be identified via these absorption bands, which is why liquids do not exhibit distinct absorption bands. Spectra are analyzed using chemometric methods for reliable substance identification, for example in the case of the post-scanner, in which terahertz spectroscopy is combined with chemometric analysis for drug and explosives detection.

In contrast to IR and Raman spectroscopy – which are sensitive to intramolecular vibrational and rotational movements – terahertz spectroscopy provides information on intermolecular movements. In addition to the mere detection of macromolecules, statements can be made about the aggregate state, polymorphic structures and crystallinity of the substances.