Terahertz devices will significantly increase the speed of data transmission and processing. In this study, researchers developed a new strategy to convert the frequency of a terahertz wave passing through a waveguide to another arbitrary frequency in the terahertz range, which is an important transformation in practical applications. The innovative approach improves terahertz technology, contributing to faster data transmission and wider application.

Terahertz technology has the potential to meet the need for increasingly high data rates, but converting terahertz signals to lower frequencies remains a challenge. Recently, Japanese researchers invented a new approach to up- and down-converting terahertz signals in a waveguide. This is achieved by dynamically changing the conductivity of the waveguide with light, thus creating a time boundary. These breakthroughs could lead to advances in optoelectronics and increased telecommunications efficiency.

As we move deeper into the information age, the demand for faster data transfer continues to grow rapidly, driven by rapid developments in fields such as deep learning and robotics. Against this background, more and more scientists are investigating the potential of using terahertz waves to develop high-speed telecommunications technologies.

But to effectively use the terahertz range, we need frequency division multiplexing (FDM) techniques to transmit multiple signals simultaneously. Of course, the ability to up- or down-convert a terahertz signal to another arbitrary frequency is a logical prerequisite for FDM. Unfortunately, with modern technologies, this has proven to be quite difficult. The fundamental problem is that terahertz waves are extremely high-frequency waves in the context of traditional electronics and very low-energy light in the context of optics, exceeding the capabilities of most devices and configurations in both areas. Therefore, a fundamentally different approach will be needed to overcome the current limitations.

An innovative solution for frequency conversion

Surprisingly, in a recently published study Nanophotonics On May 20, 2024, a research team including Associate Professor Keisuke Takano of the Faculty of Science at Shinshu University in Japan reported an innovative solution to reduce the frequency of terahertz waves. Co-authors of their paper are Fumiaki Miyamaru of Shinshu University, Toshihiro Nakanishi of Kyoto University, Yosuke Nakata of Osaka University, and Joel Perez-Urquizo, Julien Madeo, and Keshav M. Dani of the Okinawa Institute of Science and Technology.

The proposed strategy is based on frequency transformations that occur in time-varying systems. Just as a waveguide confines a propagating wave packet in space, there is a similar concept that occurs in time, known as a temporal waveguide. Simply put, changes that occur throughout the system over time will act as a “time boundary.” Similar to spatial boundaries (such as the interface between two different media), temporal boundaries can also change the dispersive properties of a waveguide, creating different propagation modes at new frequencies.

Experiments and potential applications

To create this time boundary, the researchers first placed a GaAs waveguide on top of a thin metal layer. Terahertz waves passed through the waveguide in the transverse magnetic (TM) mode, scattering light on the bare GaAs surface. The resulting photoexcitation of the top surface instantly changed its conductivity, effectively transforming the bottom metallized waveguide into a parallel, bimetallized waveguide. This transition from one waveguide structure to the other served as a temporal boundary where the incident TM modes of the bare waveguide merged with the transverse electromagnetic (TEM) mode of the bimetallized waveguide. Given that the dispersion curve of the TEM mode occupies a lower frequency range than the curve of the incident TM mode, this approach produces a terahertz wave with a downward frequency shift.

The research team conducted experiments that finally validated their comprehensive theoretical analysis of the proposed frequency conversion method. The results of this study therefore paint a bright future for future terahertz technologies. Dr. Takano is excited about the results: “Frequency conversion devices for terahertz waves have the potential for future ultra-high-speed wireless communication applications. For example, they could enable information copying between terahertz frequency channels carrying different data. There could also be devices where terahertz wave information processing circuits are integrated with various optical processing components.” The increase in conversion with the proposed approach is “F. Miyamaru and others., Phys. Rev. Lett., 127, 053902 (2021). In addition, up- and down-conversion can be changed by changing the polarization of the incident terahertz waves, which will help make FDM more useful in the terahertz range.

In addition, the current frequency conversion method is not limited to terahertz waveguides and could also have significant implications in optics. “It is important to note that the concept of this research goes beyond the terahertz frequency range and can also be applied to the optical frequency range. Based on recent discoveries, ultrafast frequency conversion devices incorporating optically modulated indium tin oxide waveguides may also be possible,” Dr. Takano points out.

Further developments in this area could lead to faster and more energy-efficient telecommunications, helping us build a more interconnected and sustainable society.