Optics 101

What’s Unique about the Terahertz Spectrum?

It would be reasonable to assume that the entire electromagnetic spectrum is packed with cell phone conversations, television programming, GPS signals, and loads of cosmic background radiation. However, there exists an entire band, sandwiched between the microwave and infrared bands, that is still largely unused. This is the terahertz regime.

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The terahertz spectrum lies between the frequencies of 300 GHz and 3 THz (wavelength between 10 um and 1000 um). Though it has been adopted by scientists since the 1950s, the terahertz spectrum has not found its way into consumer devices yet. Its applications in communications, contraband detection, and medical imaging have been known for decades, however,  producing and detecting terahertz emission has continued to remain a technical challenge.

Cosmic background radiation

Radiation of this wavelength is emitted naturally by any object over a temperature of 2 Kelvin. The Atacama Large Millimeter Array in Chile is one of the first ground-based radio telescopes whose primary purpose is to sense this cold radiation of the universe. The water and oxygen in the atmosphere absorb radiation of this energy, therefore, such observatories are usually constructed at an altitude or in space. It's this composition of the atmosphere on earth that poses another major roadblock to terahertz detectors and devices. Due to these high signal attenuation rates, several next-generation applications are unfeasible today.

Non-invasive medical imaging

Unlike X-rays, terahertz frequencies are non-ionizing, meaning they are not harmful to living tissue or DNA. They are absorbed by water and oxygen, and are, therefore, sensitive to variations in tissue density which permits non-invasive detection of skin cancer. Their low-photon energy emissions also make them an excellent choice for 3D imaging in dentistry. Moreover, they penetrate thin layers of non-conducting material like wood, paper, clothing, plastics. This crucial feature of the THz spectrum leads to its potential applications in newer non-destructive methods of analysis.

Unique spectral identifiers

Many materials generate unique spectral fingerprints in the terahertz regime. Terahertz spectroscopy and tomography techniques have recently been demonstrated to be effective at imaging samples that absorb visible and near-infrared wavelengths. Consequently, in conjunction with its through-wall imaging capabilities, THz waves can accurately detect spectral signatures of specific materials from a distance. This characteristic can be utilized to remotely monitor food quality, or to detect contraband or hazardous substances, or to observe murals and signatures hidden behind coats of plaster on paintings of historical importance.

Ultra-fast Wi-Fi

Every day, millions of smartphones connect to the internet for their first time. The standard lower frequency bands have become congested, and researchers have been looking at ways to design tiny T-ray antennas that can fit into a cell phone. Terahertz transmissions, which are currently unregulated by telecom agencies, can be up to 15 times faster than the current Wi-Fi standard 802.11 ac. When compared to microwaves, the current carriers of our data, terahertz waves have a much larger bandwidth due to their higher frequencies. They are, nevertheless, short-range devices due to high signal attenuation. But, they are still viable for use in satellite-to-satellite and high-altitude communication.

Several research teams are focused on developing and demonstrating the performance of their terahertz devices. In principle, they are groundbreaking but in actuality, the future of terahertz technology rests heavily on the development of high efficiency, low-power, compact sources and that's not a little ask.

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