The terahertz (THz) and sub-THz frequency region (100 GHz ¨C 10 THz) of electromagnetic spectrum bridges the gap between microwaves and infrared. The ¡°THz gap¡± is very attractive because of many possible applications in spectroscopy and imaging of biological objects, and in novel communication systems as well.
THz Time Domain Spectroscopy
The typical THz-TDS setup is shown in Fig. 2. Subpicosecond pulses of THz radiation are detected after propagating through a sample and an identical length of a free space. A comparison of Fourier transforms of these pulse shapes gives the spectra of absorption and dispersion of the sample under investigation. Such measurements can be successfully performed for investigation of gases and organic materials.
Pump-Probe THz Experiments
Femtosecond lasers make it possible to investigate ultrafast nonequilibrium dynamics in semiconductors. For this aim, optical-pump-optical-probe techniques are usually employed. An intense optical pump pulse is used to excite free carriers in a sample, while a weaker probe beam monitors changes in sample optical properties. In contrary to the optical probe, terahertz pulses are non-resonant with the semiconductor band gap and therefore can be used to directly probe free-carrier dynamics, avoiding numerous experimental artefacts typical for optical-pump-optical-probe systems.
THz Imaging
THz radiation has the ability to penetrate deep into many organic materials, what makes THz imaging attractive when dealing with biological samples. An image of the sample can be obtained by raster-scanning with a focused THz beam. Sub-millimeter resolution has been reported in scientific literature.
THz Spectroscopy Kit
The standard kit consists of photoconductor antenna THz emitter and detector, pump laser steering optics, motorized delay line and chopper with controller, THz beam guiding mirrors, sample holder and lock-in amplifier. The kit configuration can be easily modified: for example, the sample holder can be mounted on a motorized X-Y stage for imaging experiments.
Typical examples of data collected are shown in Fig. 1. The THz pulse waveform and its Fourier spectrum were measured without sample inserted between emitter and detector.
THz Components
THz emitter and THz detector consists of microstrip antenna integrated with LTG-GaAs photoconductor and THz lens, both mounted on X-Y stage. The performance of photoconductive antenna as a THz emitter and detector depends on carrier mobility and their trapping time in antenna semiconductor layers. Low-temperature grown GaAs (LTG-GaAs) is one of the best materials for THz applications because of high carrier mobility, fast carrier capture time, high breakdown voltage and high resistivity. The technology of LTG-GaAs growing allows to control the photoexcited carrier lifetime within a very wide region: from less than ~100 fs to 100 ps. Photoconductor antenna geometry, parameters of THz lenses, as well as the properties of LTG-GaAs epitaxial layers were optimized for the highest THz radiation output efficiency while preserving optimal bandwidth.
THz lenses of various radiuses can be made from various materials such as teflon and silicon.
Gold-coated mirrors are used for THz beam collimation and focusing.
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