Terahertz (THz) radiation represents a fascinating and increasingly useful portion of the electromagnetic spectrum. It occupies the region between microwaves and infrared light, spanning frequencies from 0.1 to 10 terahertz. Historically, generating and detecting these waves was challenging due to a “gap” between conventional electronic and optical technologies. Recent advancements have bridged this gap, opening new avenues for measurement and application.
Unique Properties of Terahertz Waves
Terahertz waves possess distinct characteristics that make them suitable for various measurement tasks. One significant property is their non-ionizing nature, meaning they do not carry enough energy to remove electrons from atoms or molecules. This makes them safe for biological tissues and human exposure, unlike X-rays.
These waves can readily penetrate many common non-conducting materials that are opaque to visible light, such as plastics, paper, cardboard, wood, ceramics, and textiles. This allows for non-destructive inspection, as they can “see through” various enclosures and materials. However, terahertz radiation is strongly absorbed by water and metals, a characteristic that can be exploited for specific detection and imaging purposes.
Many materials exhibit unique “spectral fingerprints” in the terahertz range. Different substances absorb THz waves at varying frequencies, creating distinct patterns that allow for their identification and characterization. This provides information about a material’s composition and structure.
Principles of Terahertz Measurement
Generating and detecting terahertz waves for measurement relies on specific physical principles and specialized equipment. One primary approach is Terahertz Time-Domain Spectroscopy (THz-TDS), which probes materials with short pulses of terahertz radiation. An ultrashort pulsed laser is split into a pump beam and a probe beam.
The pump beam is directed to a terahertz source, which converts the optical pulse into a terahertz pulse. This pulse then interacts with the sample. The probe beam then samples the electric field of the terahertz signal after it has interacted with the sample.
Fourier analysis transforms the collected time-domain signal to yield the sample’s frequency-dependent absorption coefficient and refractive index. Another method involves Continuous-Wave (CW) terahertz systems, which generate THz waves by combining two frequency-stabilized lasers. These systems measure the THz signal using specialized detectors.
Diverse Applications of Terahertz Measurement
Terahertz measurement techniques are finding widespread practical uses across various sectors due to their unique properties. In security screening, THz technology offers a safe alternative to X-rays for detecting concealed objects. Its ability to penetrate clothing, paper, and plastics allows for the identification of hidden weapons, explosives, or illicit substances without direct contact. For example, full-body scanners at airports utilize THz waves to detect items hidden under clothes.
Industrial quality control benefits from terahertz non-destructive testing. Manufacturers can inspect materials like ceramics, plastics, and wood for defects. In the pharmaceutical industry, THz imaging verifies the consistency of tablets and capsules. The technology also supports quality assessment in the food industry, such as detecting moisture content or foreign objects.
Medical imaging applications are emerging due to THz radiation’s sensitivity to water content in tissues. Researchers are exploring its use for early detection of skin cancer, as THz waves can differentiate between healthy and cancerous tissues based on hydration levels. Dental diagnostics are also being investigated for detecting cavities.
Beyond these specific applications, terahertz measurement is a tool in scientific research for material characterization. It enables the study of molecular vibrations and rotations, providing insights into the fundamental properties of various substances. This supports advancements in fields from fundamental physics to new material development.