Bridging the “terahertz gap” relies on synthesizing arbitrary waveforms in the terahertz domain, enabling applications requiring both narrowband sources for sensing and few-cycle drives for classical and quantum objects. However, a realization of tailored waveforms required for these applications needs improvement due to the limited flexibility for optical rectification of femtosecond pulses in bulk crystals.

Researchers at EPFL’s School of Engineering’s Laboratory of Hybrid Photonics (HYLAB), led by Cristina Benea-Chelmus, have made significant progress in effectively exploiting the so-called terahertz gap, which is between about 300 and 30,000 gigahertz (0.3 and 30 THz). ) on the electromagnetic spectrum. This frequency range, which describes frequencies that are too fast for modern electronics and telecommunications equipment but too slow for optical and imaging applications, is currently considered a technical dead zone.

The HYLAB researchers and their collaborators at ETH Zurich and Harvard University have now achieved not only the production of terahertz waves, but also the engineering of a method to specifically modify their frequency, wavelength, amplitude and phase. This was made possible by a fragile chip with an integrated photonic circuit made of lithium niobate. Terahertz radiation can now potentially be used for next-generation applications, both electrical and optical, thanks to such fine control over it.

Co-first author Alexa Herter, currently a Ph.D. student at ETH Zurich, said: “Seeing the devices emit radiation with properties we predefined was confirmation that our model was correct.”

Co-first author Amirhassan Shams-Ansari, a postdoctoral researcher at Harvard University, said: “This was made possible by the unique characteristics of lithium niobate integrated photonics.”

integrated photonic circuit
The HYLAB chip with integrated photonic circuit made of lithium niobate © Alain Herzog

Benea Chelmus said: “While such terahertz waves have been produced in a laboratory setting, previous approaches have primarily relied on bulk crystals to generate the correct frequencies. Using her lab’s lithium niobate circuit, finely etched at the nanometer scale by Harvard University collaborators, makes their new approach much more streamlined. The use of a silicon substrate makes the device suitable for integration in electronic and optical systems.”

“Generating waves at very high frequencies is extremely challenging, and there are few techniques that can generate them with unique patterns. We can now design the exact temporal shape of terahertz waves – to say, essentially, ‘I want a waveform that looks like this.”

To do this, Benea-Chelmus’ group created the chip configuration of waveguides, or channels, from which tiny antennas emit terahertz waves produced by light from optical fibers.

Benea Chelmus said: “The fact that our device already uses a standard optical signal is an advantage because it means these new chips can be used with traditional lasers, which work very well and are well understood. It means that our device is compatible with telecommunications.”

“Miniaturized devices that send and receive signals in the terahertz range could play a key role in sixth-generation (6G) mobile systems.”

Benea-Chelmus believes that small lithium niobate chips hold special promise for spectroscopy and imaging in optics. Terahertz waves are non-ionizing and have a much lower energy than many other types of waves (such as X-rays) that are now used to reveal the chemical composition of a substance, be it an oil painting or a bone. Therefore, a less invasive alternative to current spectrographic methods could be offered by a small, non-destructive device such as the lithium niobate chip.

Benea Chelmus said: “You can imagine sending terahertz radiation through a material you are interested in and analyzing it to measure the reaction of the material depending on its molecular structure. All this with a device smaller than a matchstick.’

Researchers further plan to modify the properties of the chip’s waveguides and antennas to develop waveforms with larger amplitudes and more finely tuned frequencies and decay rates. They also expect their terahertz technology to be useful for quantum applications.

Magazine reference:

  1. Herter, A., Shams-Ansari, A., Settembrini, FF et al. Terahertz waveform synthesis in integrated thin-film lithium niobate platform. Nat Commun 14, 11 (2023). DOI: 10.1038/s41467-022-35517-6