Important breakthrough in photonics first full photonic integration at visible to near-infrared wavelengths

Photonics technology uses light rather than electricity: it is fast and versatile and promises to be the next major advance in human technology. However, photonics research requires expensive, complex equipment: for example, precision lasers and customised circuits. To realise its full potential, the technology must become smaller, cheaper and easier to produce. Researchers have made progress in these areas, but still face the challenge of getting their circuits to work at shorter wavelengths of light.

A team from Nexus Photonics, the University of California, Santa Barbara (UCSB) and Caltech has developed a technique that allows photonic chips to operate in the visible to near-infrared spectrum, a discovery that promises to make these components smaller and more powerful; the technique also makes use of a method common in electronics manufacturing, making them easy to produce cheaply and on a large scale.

The results were published in Nature under the title 'Extending the spectrum of fully integrated photonics to submicron wavelengths' [1].

01Fabrication of photonic circuits using fabrication techniques for electronic circuits

Laser light on the surface of a photonics chip. (Photo: Matt Perko)

"This is a breakthrough that could open up possibilities that no one has thought of before." Co-author Ted Morin, a PhD student at the University of California, Santa Barbara, says the technology will take high-performance photonics into new markets and applications such as augmented and virtual reality, healthcare and atomic clocks for visible and near-infrared wavelengths. More importantly, mass production will cut the price of lasers and photonic circuits.

Over the past half century, society has progressed as electronics have become smaller and more powerful, and Morin says: "Human beings have collectively done an amazing job in making smaller, faster and more reliable electronic circuits. We're figuring out how to use the manufacturing technology of electronic circuits to make photonic circuits."

Light information and energy are transmitted through waveguides in photonic circuits, much like wires in electronic circuits. A huge obstacle to miniaturization is connecting the laser to the photonic circuit itself, but plugging it into every pathway is simply not practical. says Morin[2], "Imagine someone hand-plugging wires into every few transistors of your computer processor."

02The problem: light is absorbed by silicon waveguides

This fully machined 4-inch wafer contains thousands of devices.

In 2005, researchers at the University of California, Santa Barbara, led by Professor John Bowers, solved the "laser connection" problem for silicon circuits very well: they overcame this obstacle by bonding the laser material directly to the silicon and bending the light into the waveguide. Since then, this technology and its variations have been developed by several industrial and research institutions and commercialised by Intel on a multi-million dollar per year scale.

Unfortunately, there is a problem with these solutions. They only work on light with wavelengths above 1100 nm, which is deep in the infrared. Every semiconductor has a so-called "band gap energy", and photons with higher energy than this (i.e. smaller wavelengths) are absorbed by the material. Silicon has a band gap of about 1,100 nanometres," says Nexus Photonics research director Minh Tran, "so anything shorter than that can't be used in current technology. Visible light ranges from 380 to 750 nanometres, which means that ultraviolet, visible light and even some infrared are absorbed by silicon waveguides."

So while silicon is great for electronics, it can't transmit every wavelength of light that scientists and engineers want. tran explains, "If we want to extend our applications to shorter wavelengths, we need to use a different material to guide the light."

03Silicon nitride: a fully photonic integrated solution for visible to near-infrared wavelengths

Silicon nitride emerged as the best candidate material. This material has a band gap of around 250 nm, well into the ultraviolet part of the spectrum. And, because it is a silicon compound, it can be easily integrated into electronics manufacturing practices. Its components, silicon and nitrogen, are also abundant and inexpensive, says Morin: "Elementally, it's sand plus air. And the whole process involves conventional manufacturing steps that can easily be scaled up with existing infrastructure."

But now the challenge of connecting the laser to the waveguide must again be addressed, as the original technology on silicon nitride does not work on silicon nitride because of its optical properties. Light travels at different speeds, depending on the substance it passes through. Scientists use a number called the refractive index to describe the speed of light in a material, and the refractive index of silicon nitride is very different from that of the laser material. This makes it difficult for the beam from the laser layer to bend into the silicon nitride waveguide beneath it.

To solve this problem, the team added an intermediate material with a refractive index close to that of silicon nitride to the same plane as the laser. This allows the laser to enter the transition waveguide head-on and then be guided down into the silicon nitride from a material with similar optical properties. While the design was a step forward, the real challenge was to make the process compatible.

This achievement marks a major breakthrough for the team. "In 2018, several of us from UCSB founded Nexus Photonics to address the challenges of fabricating short-wavelength photonic integrated circuits," said co-founder and CEO Tin Komljenovic: "Now, we've finally got the technology optimized to exceed the performance of large commercial systems while being smaller than a one-cent coin."

"This is an important milestone in the field of semiconductors and photonics," added Chong Zhang, a UC Santa Barbara graduate, co-first author, and co-founder and vice president of engineering at Nexus Photonics, "It provides the first ever full photonic integration for visible light to near-infrared wavelengths to provide a viable and scalable solution for all-photonic integration."

Coherent, widely tunable lasers integrated on silicon nitride.

04Cost reduction for commercial silicon casting

This laser coupling technology will reduce the cost of high-powered precision photonic technologies by several orders of magnitude. As a result, they will be available for discoveries by researchers, for engineers to build better technologies, and ultimately even for consumer electronics. After all, mass-produced, miniaturised photonic circuits are exactly the kind of component that could be incorporated into consumer products.

There are a variety of applications, the authors explain. The technology could open up new avenues into biomedicine, in atomic physics and quantum research through applications such as biosensing and DNA sequencing. "Using a commercial silicon foundry would mean that every university professor in every school in the world could afford to buy equipment and conduct experiments that are now only feasible at major research institutions," Morin says. "

"We are democratising quantum physics." Tran added.

Photonics could also revolutionise virtual and augmented reality. with integrated photonics, it is possible to take light from a small chip and send it in a precisely controlled direction: a quick sweep across the direction allows images to be projected dynamically; it is also possible to detect where the light is coming from on the same chip," says Morin. So it's possible to shine a light somewhere and see what comes back, all in a tiny 'package'." That's the idea behind LIDAR, the radar equivalent, which is revolutionising our driving experience.

The team has already successfully integrated photonics into electronics manufacturing and they are already focusing on their next challenge. Eventually, they plan to integrate photonic and electronic circuits onto the same chip, achieving greater efficiency in terms of cost and capability.

Reference links：

[1]https://www.nature.com/articles/s41586-022-05119-9

[2]https://www.nanowerk.com/nanotechnology-news2/newsid=61966.php