Two-Dimensional Terahertz Wave Imaging

Abstract

The terahertz, or far-infrared, region of the electromagnetic spectrum is an important region for the spectroscopy of condensed matter, vibronic spectroscopy of polar liquids and gas phase analysis. In the past, terahertz spectroscopy was hindered by the low brightness of incoherent far-infrared sources and the limited sensitivity of bolometeric detectors. However, these difficulties are completely overcome with the advent of terahertz time-domain spectroscopy (THz-TDS). THz-TDS is based on electromagnetic transients generated opto-electrically by the femtosecond laser pulses. These terahertz transients are single-cycle bursts of electromagnetic radiation of typically less than 1-ps time duration. Their spectral range spans from below 100 GHz to more than 5 THz. The brightness of the THz transients exceed that of conventional thermal sources, and the gated detection technique adopted in THz-TDS is, in magnitude, more sensitive than conventional bolometric detection. The gated detection is coherent — both the amplitude and phase of the THz waveform can be measured. In general, many chemical compounds show very strong, highly specific frequency-dependent absorption and dispersion in the THz range, which leads to characteristic time-domain waveforms when THz wave transmits through different materials. Accordingly, highly sensitive imaging of chemical compositions in THz range is possible. The value of a practical far-infrared imaging system has been recognized in a wide range of applications for some time. A THz imaging system based on the TDS technique excludes any cryogenics or shielding for the detector; it has the potential to be portable, compact, and reliable far-infrared imaging system, which shall find its practical application in “real-world” environments.