Terahertz (THz) spectroscopy is a scientific technique that uses terahertz radiation to analyze materials. This non-destructive method provides information about a material’s structure, dynamics, and properties. It offers insights into molecular vibrations, rotational modes, and electronic transitions within various substances. The technique is used across many fields due to its ability to probe a wide variety of materials.
Understanding Terahertz Waves
Terahertz waves are a distinct part of the electromagnetic spectrum, positioned between microwaves and infrared light. They span a frequency range from 0.1 to 10 THz, which corresponds to wavelengths between 30 micrometers and 3 millimeters. This placement gives them properties of both neighboring regions, allowing them to penetrate a wide array of non-conductive, non-polar materials such as plastics, textiles, paper, cardboard, wood, and ceramics.
Terahertz waves are non-ionizing, meaning their photon energy is too low to remove electrons from atoms or molecules, unlike X-rays. This property ensures they do not cause damage to biological tissues or the materials being analyzed. Terahertz radiation is strongly absorbed by water, which can be both an advantage and a limitation, as it allows for precise measurements of moisture content but limits penetration through liquid water or dense, water-rich biological tissues. Many biomolecules, proteins, explosives, and narcotics also exhibit characteristic absorption lines, often referred to as spectral “fingerprints,” within the 0.1 to 5 THz range, enabling their identification.
How Terahertz Spectroscopy Operates
Terahertz spectroscopy operates by interacting terahertz radiation with a sample and analyzing the resulting changes. It involves sending short pulses of terahertz radiation through a material or reflecting them off its surface. As the terahertz waves pass through or interact with the sample, they can be absorbed, transmitted, or reflected, depending on the material’s specific properties.
Different materials interact with terahertz waves in distinct ways, creating a distinct “fingerprint” based on their molecular vibrations and rotational states. Terahertz radiation is sensitive to low-energy transitions, such as intermolecular vibrations and lattice modes that occur at THz frequencies. By measuring both the amplitude and the phase of the terahertz electric field after it interacts with the sample, researchers can obtain detailed information about the material’s complex optical properties, including its absorption coefficient and refractive index. This allows for the identification and characterization of substances, even differentiating between different forms of the same molecule, such as crystalline versus amorphous states or different polymorphs.
Applications Across Various Fields
Terahertz spectroscopy has applications across many fields due to its ability to penetrate materials non-destructively and provide molecular fingerprints.
Security Screening
It is employed to detect hidden objects and substances, such as explosives or illicit drugs, concealed within non-metallic packaging like mail envelopes or cardboard boxes. The technique can also be used to identify firearms and controlled knives by analyzing their spectral information.
Quality Control
In manufacturing, terahertz spectroscopy offers a contact-free method for inspecting various products. It can measure the thickness of paint and coating layers in industries like automotive and aviation, ensuring uniform application and protection. The pharmaceutical industry uses it to check the integrity of pills, detect defects, and verify the homogeneity of compounds within medications. This includes differentiating between polymorphic forms, which are different crystal structures of the same chemical compound.
Material Science
Material science benefits from terahertz spectroscopy for characterizing materials. It can analyze polymers, identifying crystalline phase transitions, compositional variations, and porosity within structured materials. The technique is also used to study the electrical properties of 2D materials like graphene and semiconductors, as well as to investigate functional optical materials such as titanium dioxide nanoparticles and yttrium aluminum garnet crystals.
Medical Imaging
Medical imaging is a promising area, where terahertz imaging is explored for non-invasive detection of certain conditions. It can differentiate cancerous tissue from healthy tissue due to differences in water content and structural changes in diseased cells. While water absorption limits penetration depth to about 1 millimeter in tissues, it allows for analysis of excised tissue samples and surface layers.
Cultural Heritage
The preservation of cultural heritage leverages terahertz spectroscopy for analyzing historical artifacts. It enables the examination of multi-layered structures in paintings, revealing underlying images or analyzing pigments and binders. This non-destructive approach allows conservators to understand artwork composition and identify defects like paint delamination or hidden characteristics.
Safety Considerations and Emerging Possibilities
Terahertz radiation is considered safe for biological tissues and materials, as it is non-ionizing. Unlike X-rays, which carry enough energy to ionize atoms and potentially damage DNA, terahertz waves have much lower photon energy in the milli-electron volt range. This property makes terahertz spectroscopy a safe alternative for applications involving human interaction or sensitive materials. While some studies explore potential non-thermal effects, the primary concern of ionization associated with higher-energy radiation does not apply to terahertz waves.
The field of terahertz spectroscopy is evolving, with research focused on improving efficiency, reducing costs, and developing more compact, powerful sources and detectors. Advancements in laser technology are expected to enhance the performance of terahertz systems, leading to wider adoption in various industries. Researchers are exploring new materials like aerogels that can modulate terahertz signals, potentially enabling new applications in advanced medical imaging and communication. As these technological barriers are overcome, terahertz spectroscopy is becoming an increasingly accessible tool across scientific and industrial domains.