Electromagnetic waves at terahertz frequency have significant prospects for the development of scanning and imaging technologies as well as communications. However, using their potential is associated with obstacles. A team from Tohoku University has made a breakthrough by creating a new type of tunable filter specifically for the terahertz wave spectrum. Their findings were published in a journal Optical Letters.

Waves in the terahertz range occupy the region of the electromagnetic spectrum between microwave and infrared frequencies. They have a higher frequency (shorter wavelength) than radio waves, but a lower frequency than visible light. The increasingly concentrated radio spectrum transmits large amounts of data transmitted via WiFi, Bluetooth, and modern mobile phone (cell phone) communication systems.

The clustering of signals in the lower parts of the electromagnetic spectrum is one incentive to explore options in the terahertz region. Another is the ability to support ultra-high data rates. But the main challenge in using terahertz signals for common applications is the ability to tune and filter signals at specific frequencies. Filtering is necessary to prevent interference from signals outside the desired frequency range.

Breakthrough in terahertz filtering

“We created and demonstrated a frequency-tunable filter for terahertz waves that achieves higher transmission rates and better signal quality than traditional systems, unlocking the potential of terahertz wireless communications,” says Yoshiaki Kanamori from the Tohoku team. He adds that the work could be more broadly applicable beyond the terahertz frequency range.

The new terahertz filter is based on a device called a Fabry-Perot interferometer; This device, like all other interferometers, is based on the interference patterns created when different waves of electromagnetic radiation interact with each other as they bounce between mirrors. The researchers’ version uses fine-grained gratings with sub-wavelength gaps that interact like material between mirrors. Variable extension of the gratings allows precise control of refractive indices, which is necessary to tune the filtering effect of the interferometer. This allows you to transmit only the frequency you want. Using different grids allows you to control the different frequency ranges selected.

Expansion of applications and benefits

The team demonstrated the application of their system to frequencies suitable for next-generation (6G) mobile phone signals.

“In addition to applications of our method in communications systems, we also envision applications in scanning and imaging technologies in medicine and industry,” says Kanamori.

One of the advantages of terahertz waves in scanning and imaging is that they can easily penetrate materials that block the passage of light, including biological tissues. Besides medical applications, this can offer opportunities for materials analysis, safety systems and quality control in production.

“Overall, our work provides a simple and cost-effective method for filtering and active control of terahertz waves, which could facilitate their use in many applications,” concludes Kanamori.