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The science of light

How the field of photonics is creating a brighter future.
Graduate student Qi Zhou in Dan Congreve’s lab prepares materials for 3D printing … with light.

Most of us don’t spend much time thinking about light, beyond whether or not we can see what we’re doing. But to researchers in the field of photonics, light is a tool that does much more than simply illuminate our world. Photonics is the science and technology of controlling light – manipulating its behavior with precise crystalline structures, encoding information in its wavelengths, using it to trigger chemical reactions, or applying any other number of techniques. Photonics researchers are putting light to work to improve our electronics, help us live more sustainably, and provide a better understanding of our health and well-being.

“One of the great things about light is that it’s ubiquitous,” said Dan Congreve, an assistant professor of electrical engineering at Stanford working in photonics. “It’s everywhere; we know how to generate it; we know how to harness it. And as we continue to develop finer and finer control over these things, light becomes an enormously powerful tool.”

Graduate student Arynn Gallegos and Assistant Professor Dan Congreve view light emission from perovskite nanocrystals using a UV lamp. Perovskite nanocrystals are tiny crystals 1000x smaller than a human hair that emit light when excited, and Congreve and his lab specialize in engineering them to emit the right wavelength of light efficiently for a variety of applications such as communications, televisions, and room lighting.

Photonics research at Stanford has been ongoing since the 1960s. Joseph Goodman, an early pioneer in the field and now professor emeritus, made significant strides in optical imaging and wrote a foundational textbook on the subject. The university was also at the center of several initial developments in laser technology – particularly in the Electrical Engineering groups of Anthony Siegman and Stephen Harris – and continued to develop the field as it has become increasingly relevant to modern life.

“It is exciting to see the progression of the field from something that was a little bit marginal in engineering a few decades ago to something that’s one of the main strategic directions,” said Jelena Vučković, the Jensen Huang Professor in Global Leadership at Stanford. “Electrical engineering used to be all about electrons, but that’s not the case anymore.”

Bringing photons into electronics

For decades, our computing technology has been based on the movement of electrons, but there are certain areas where photons – particles of light – have proven to be more effective. For example, fiber-optic cables, which send data with pulses of light instead of electricity, can transmit more information faster and over longer distances than copper wiring. In a world where we are increasingly dependent on digital communications, photonic devices could help provide faster, more efficient, more secure computing.

“Some estimates show that close to 10 percent of all electricity is used for information processing and that number is only growing,” said David Miller, the W.M. Keck Foundation Professor in Electrical Engineering. “We’re sending increasingly large amounts of information around inside chips for artificial intelligence and machine learning. That’s not sustainable. We need new solutions.”

Vučković, Miller, and their colleagues are working on ways to use light to replace the copper wire connections between electronic pieces inside a computer, like the processor and memory. Unlike wiring, photonic communication doesn’t produce heat, so data could move faster and use significantly less energy.

At chip scale, where engineers hope lasers will one day transform computer circuitry, effective optical isolators have proven elusive. This is a close-up photo of a chip-scale isolator that Vučković’s lab has created to work towards overcoming the challenges involved in using lasers to effectively transform computer circuitry.

There are also opportunities to use light’s quantum nature in computing and information processing, Vučković said. Information encoded in the quantum states of light would be extremely secure, because the signal couldn’t be intercepted without being disrupted (a result of the intrusive effects of measurement in quantum physics). Quantum properties of light could also be harnessed to facilitate building powerful quantum computers.

Miller is also working with a group of researchers on light-based technologies that reduce the amount of energy needed to train neural networks. The Stanford team recently designed a photonic chip that can do an aspect of this training entirely optically.

“You can take light and propagate it through a structure and that process actually does the computation,” said Shanhui Fan, the Joseph and Hon Mai Goodman Professor in the School of Engineering.

PhD students Hope Lee (right) and Yakub Grzesik (left) align a diamond quantum photonics experiment in Jelena Vuckovic’s Lab
PhD students Yakub Grzesik (left) and Hope Lee (right) align a diamond quantum photonics experiment in Jelena Vučković’s Lab

The researchers have applied similar photonic technology to blockchain – a sort of digital ledger for securely transferring data. The work paves the way for efficient, optical computing and has applications in cryptocurrency, medical records, voting, and more.

“The point is to lower energy costs,” said Olav Solgaard, the Robert L. and Audrey S. Hancock Professor in the School of Engineering. “We can’t do every type of calculation in the optical domain, but when we can, there’s a clear-cut advantage in energy consumption.”

control center
A close-up look at the photonic integrated circuit designed to implement optical neural network training. Optical interfaces to the chip are managed via a fiber array approaching from the left and a single mode fiber approaching from the right side of the chip. Electrical signals that allow researchers to adjust the settings of the circuit are interfaced using wire bonds from a printed circuit board at the top face of the chip.
integrated circuit
The control station for operating photonic integrated circuits. The station consists of optical inputs and outputs, electrical inputs, and mechanical micropositioners, as well as a microscope for monitoring the status of the chip.

Steps in sustainability

Energy-efficient computing is just one of the ways that photonics could help us live more sustainably. Researchers are using photonics to improve solar panels, develop efficient home cooling systems, and tackle plastic recycling as well.

Solar arrays – perhaps the most visible application of photonic technology – already collect light and turn it into usable power. Last year, Solgaard and fellow researcher Nina Vaidya, who is now at the University of Southampton, UK, developed a device that could gather and concentrate light from any angle, capturing light that would otherwise bounce off a solar panel. Taking a different approach, Congreve and his colleagues are working on coatings for solar panels that would combine unusable low-energy photons into high-energy photons that can be turned into power. Both developments could help solar panels capture more energy and reduce our reliance on fossil fuels.

In Fan’s lab, the team is working on tiny photonic structures that could cool a house without electricity. They have built a device that reflects all the sunlight that hits it and radiates infrared light, which we feel as heat, back out into space.

“It looks like a very, very good mirror to human eyes,” Fan said. “When we put it on the roof of the Packard Electrical Engineering Building facing the sky, we got a temperature that was about 5 degrees Celsius [9 degrees Fahrenheit] below the ambient air temperature, even under direct sunlight, without the need of any energy input.”

Cooling panel
Postdoctoral scholar Ming Zhou (right) holds up a panel that cools down even when placed in direct sunlight. The panel reflects sunlight and simultaneously emits heat in the form of infrared radiation. Designed in Professor Shanhui Fan’s (center) lab, the panel is fabricated by coating an aluminum sheet with PDMS, a transparent silicone polymer. Postdoctoral scholar Sid Assawaworrarit (left) holds up an insulation box designed for measuring the cooling performance of the panel.

Meanwhile Jennifer Dionne, an associate professor of materials science and engineering, is using light as a catalyst to break down various types of plastics into uniform building blocks so they can be more easily recycled.

“We can use light-driven catalysts to snip plastic polymers at well-defined molecular intervals,” Dionne said. “Compared to mechanical or thermal-based approaches to break down plastics, light promises a more homogeneous distribution of hydrocarbons. The goal is to enable more efficient plastic upcycling and help create a zero-waste system.”

Photonics meets biology

Photonics has also worked its way into health and biological applications. Dionne has devised a way to capture light-induced vibrational-scattering signatures created by cells and use them like a fingerprint to identify bacteria. Her lab is applying this work to rapidly identify strains of tuberculosis and the antibiotics they would be most susceptible to.

“Our goal is to replace the lengthy, culture-based approaches for drug susceptibility testing with a more rapid test that can detect pathogens in samples like mucus or wastewater,” Dionne said. “Photonics gives us new tools to interrogate the molecular composition of samples, while advances in artificial intelligence allow us to interpret those signals.”

Members of the Dionne Lab (L–R: Varun Dolia, Yirui Zhang, Liam Herndon, Jennifer Dionne, and Sajjad Abdollahramezani) work with a home-built microscope setup for rapid detection of respiratory pathogens and bacteria. Their innovative method capitalizes on ultra-densely-patterned silicon sensors (over 5 million per square centimeter) and acoustic bioprinting of surface chemistry to realize highly multiplexed, molecular-to-cellular detection at the point of care.

Dionne is also developing photonic systems to help us learn more about how our bodies work. With Miriam B. Goodman, the Mrs. George A. Winzer Professor in Cell Biology at Stanford Medicine, she has built biocompatible nanoparticles that emit different colors of light depending on the amount of pressure they are subjected to. The nanoparticles, which have been tested in a small species of worm, can provide real-time information about the mechanical forces involved in cellular interactions.

H. Tom Soh, a professor of electrical engineering, of bioengineering, and of radiology, is working on light-based molecular sensors that can continuously operate in our bodies. To this end, his lab has created “molecular switches” that bind to specific molecules and change their shape, giving off a bit of light when they do. By detecting changes in that light, the researchers could monitor the levels of a wide range of target molecules, such as drugs, hormones, and other disease markers.

Postdoctoral research fellow Yasser Gidi holds a flow cell used for DNA sequencing. As a part of Tom Soh's lab, Gidi and his colleagues repurpose and use this DNA sequencing instrument to study short pieces of DNA, also known as aptamers. Soh's lab has developed new approaches to rapidly generate aptamers and aptamer switches, with the ultimate goal of sensing biomarkers that are associated with human health and disease.

“Wouldn’t it be amazing if we could continuously measure our molecular state in real time?” Soh said. “It opens up the capability to detect health problems before they get serious, and extremely sensitive photonic detectors could allow us to capture these signals. Keeping people healthy, instead of waiting until they get sick, is where I believe the future of medicine and our society has to go.”

Lighting the way forward

There are still hurdles to overcome before many of these photonic developments make it to mainstream production. Devices on current photonic chips are larger than electronic ones, limiting where and when they can be used; nanoparticles and sensors will need further testing before they can be used in humans; photonic structures for concentrating or reflecting light need to be precise to the nanometer, but built in large enough layers to, say, coat a solar panel.

In the case of precision structures, Congreve is working on a photonics-based solution. Using the same low-energy to high-energy photon conversion that his lab has applied to solar cells, Congreve has designed a light-based method of precision 3D printing. The goal is to be able to print high-resolution objects over larger areas, enabling nanostructures to be produced at a scale that allows them to be put to work outside the lab.

“Whether you’re trying to print a photonic crystal on the back of a solar cell or a scaffold that you could grow cells and tissues on, the ability to print in high resolution over large scale really enables these applications,” Congreve said.

Photonics certainly can’t solve every problem, but as we face new challenges in computing, the environment, health, and other areas, it’s becoming an increasingly important tool in our belt. It’s not unreasonable to imagine a near future where light-based technologies are smoothly integrated into daily life and the improvements they bring feel as commonplace as electricity.

“Countless labs here at Stanford and around the world are working hard to figure out all of the little details that are necessary to translate cutting-edge photonics research into practical applications,” Congreve said. “I’m excited to see these technologies make a real impact in people’s lives.”