THz

Terahertz (THz) refers to the portion of the electromagnetic spectrum with frequencies from about 0.1 to 10 terahertz, corresponding to wavelengths roughly 3 millimeters to 30 micrometers. This band sits between microwaves and infrared radiation and is sometimes called the terahertz gap because early sources and detectors were scarce. THz radiation is non-ionizing and can probe low-energy vibrational and rotational transitions in molecules and solids, making it useful for spectroscopy and imaging.

The interaction of THz radiation with matter is highly material-dependent. Many non-metallic solids and polymers are partially transparent to THz waves, while water and some polar substances strongly absorb THz radiation. Atmospheric transmission is also wavelength-dependent, with significant absorption by water vapor that limits long-range propagation but allows short-range imaging and spectroscopy in controlled environments.

Generation and detection techniques have advanced in recent decades. Common THz sources include photoconductive antennas and optical rectification in nonlinear crystals, as well as quantum cascade lasers and frequency multipliers. Detection methods range from bolometers and Schottky diodes to superconducting detectors, often used in time-domain spectroscopy. THz time-domain spectroscopy (THz-TDS) is a widely used technique that yields amplitude and phase information for materials.

Applications span science and industry. THz spectroscopy helps characterize solids, semiconductors, pharmaceuticals, and biomolecules; THz imaging enables non-destructive testing, quality control, and security screening; and THz communications research aims at high-bandwidth wireless links. Ongoing challenges include developing compact, efficient, room-temperature THz sources and detectors, mitigating atmospheric losses, and integrating THz systems into practical devices.