A new method makes it possible to generate ultraviolet (UV) light on a photonic chip, with power levels high enough for practical applications. In the process, it has succeeded for the first time in producing milliwatt-level UV light on a chip. The breakthrough is an important step for quantum technology, optical atomic clocks and advanced measurement devices, according to the researchers involved.
The method was developed by researchers at the University of Twente and Harvard University. Integrated light sources are essential for various technologies. Data, for example, is transported as infrared light over glass fibres. However, other applications, such as sensors and quantum computers, require visible or UV light. Until now, chips were mainly suited to longer wavelengths.
"Each application requires a specific colour of light," explains Kees Franken, one of the authors of the study. "At short wavelengths like UV, the quality of integrated light sources was simply not good enough."
Smart conversion process
The researchers solve this with a clever conversion process. They start with red light, which has long been relatively easy to generate on a chip. They then convert this red light into UV light, converting two red photons into one UV photon.
Until now, this method produced only minimal light output on chips. In the new study, however, researchers have managed to generate a usable amount of UV light. This amounts to several milliwatts, which is about 100 times more than in previous studies.
Thin-film lithium niobate
This is possible in part by using a thin-film lithium niobate. The chip version of this material was developed by a group affiliated with Harvard University, where the research was carried out in collaboration with the University of Twente.
Using the material, the researchers built a waveguide: an on-chip nanostructure that conducts and traps light. They manipulated the waveguide along its entire length of almost two centimetres. To do this, they measured the shape with a precision of several tens of atomic diameters.
About 10,000 electrodes per waveguide
Using electrodes along the sides of the waveguide, they periodically reversed the orientation of the material's crystal structure, up to a thousand times per millimetre. Alternating voltage on and off along the waveguide creates a pattern that enables the conversion. Each of the approximately 10,000 electrodes per waveguide is unique and tailored to the exact shape of the waveguide at that specific point on the chip.
In previous work, the electrodes were placed at some distance from the waveguide. "In our design, they are directly on it," says Franken. "That required a fabrication process with an accuracy of fifty nanometres over a chip several centimetres long. But it gives us much more control, and the conversion from red to UV is much more efficient as a result."
Relevant for quantum computers, among others
The results are particularly relevant for technologies that are currently bulky, expensive and difficult to scale. Quantum computers are a case in point. "If you want to scale up systems like quantum computers, you need light sources on a chip," Franken said. This also applies to optical atomic clocks, which are so precise that they can even detect differences in gravity. Putting them on a chip makes them compact and practical enough for applications in satellites.
The technology is housed in the UT spin-off Sabratha. This company focuses on thin-film lithium niobate and scaling up these photonic chips for telecoms and wireless communications.
