JJ Hu

One glance at an outline of Juejun Hu’s research interests makes it clear that my usual ice breaker—“What are some of the potential applications of your research?”— isn’t going to cut it. For Hu, whose lab focuses on optics and photonics, that question is like asking, “What can you do with a microchip?” Where does one even begin to answer?

Microelectronics have enabled virtually all of the technological advances we’ve seen in our lifetimes. Hu, associate professor of materials science and engineering at MIT, believes that micro-optics are the next wave.

Traditional bulk optics—curved lenses, filters, mirrors—are mechanical, rigid, costly to manufacture, and difficult to scale without decreasing performance. “The overarching goal of my group’s work,” Hu says, “is to replace conventional bulk optic elements with miniaturized counterparts that are mass produced using standard microfabrication technology.”

Biopharmaceuticals, food and agriculture, energy, consumer goods, defense, medical sensing, manufacturing, virtual reality... It’s hard to imagine an industry that couldn’t utilize mass producible, chip-scale, micro-optics components to develop dramatic new technologies.

Hu’s research into micro spectrometry, metasurfaces, and phase-change materials brings the micro-optics revolution into ever-sharper focus.

A New Architecture for Micro Spectrometry

Spectrometers measure the intensity of light as a function of wavelength or of color. They allow researchers to determine, among other things, the composition of a material at the atomic level, even in complex environments.

Traditional bench spectrometers are large, expensive, and fragile. But they’re reliable and precise. Chip-scale spectrometers that are currently on the market can’t compete. “Fundamental physical scaling loss means that as spectrometers get smaller, their performance usually gets worse,” says Hu.

To transcend these limits, Hu’s Photonics Materials Group has developed a new architecture, digital Fourier-Transform (dFT) spectroscopy, that enables exponential scaling in a chip-scale platform. dFT spectroscopy gives researchers exponentially large points of light intensity data. An algorithm that the group developed can quickly process that data to back-infer the optical spectrum.

The technology can help improve pharmaceutical manufacturing by allowing real-time quality control monitoring. “[The pharmaceutical industry] is moving toward manufacturing processes that use continuous flow chemistry instead of batch processing,” Hu says, “but the challenge is that you want a sensor that gives you the chemical composition and reaction kinetics in real time.” Devices that use dFT spectroscopy can provide that information. [cf: video of JJ showing this device in the lab]

The food processing industry has similar quality-control needs. Hu is collaborating with a fishing company that wants to monitor the quality of their catch while it’s still fresh, rather than heading back to port and sending samples to a lab for analysis. “By the time that readout comes back, it can be too late to act on,” Hu says. “But a miniaturized spectrometer that can be carried like a smartphone will let them examine the quality of food products right on the ship.”

From Fishing Boats to Fish-Eye Lenses

In the pursuit of micro-optics breakthroughs, dFT spectroscopy addresses one of the challenges of miniaturization. What about the challenges imposed by mechanical moving parts? Hu and his collaborators have shown that phase change materials and metasurfaces hold the key.

“Metasurfaces are essentially artificial electromagnetic materials,” Hu says. “The paradigm is usually a flat substrate, like a piece of glass. On top of that we put micro-scale, wavelength-scale antennas that can bend light in much the same way as a geometric curved surface does.”

Think of a fish-eye lens. In photography, fish-eye lenses allow you to capture wide-angle panoramic views. “The price you pay for high quality fish-eye images,” says Hu, “is that the system becomes very complicated.” Traditional fisheye lenses may require five to ten individual lenses. More lenses weigh more, cost more, and require more complex optical alignment assemblies. “Metasurfaces allow us to transform one single piece of thin glass into a fish-eye lens while maintaining very high image quality.”

This wide-angle imaging technology has attracted the interest of the defense sector, as well as consumer electronics, for example, in the development of 3D sensors with a 180-degree field of view.

Now You See It

When metasurfaces are built using phase change materials, the transformational possibilities of micro-optics become even clearer.

Optical phase change materials (O-PCMs) switch from an amorphous state to a crystal state with the application of heat. Blu-ray discs and DVDs are the most common example of O-PCMs. The phase change affects the disc’s reflectivity, which is how data is read. “The challenge of using these materials for optics is that conventional phase change materials are optically opaque,” says Hu. In DVDs where you’re relying on reflectivity, that’s not a problem. But in devices where transparency is critical, “when you try to send light through, you lose most of it.”

So Hu’s group, in collaboration with colleagues at Lincoln Laboratory, developed the first transparent phase changing materials. If metasurfaces and their light-bending antennae enable smaller, more lightweight versions of complex optical devices, the addition of O-PCMs makes those devices instantly reconfigurable.

“Let’s think of a conventional reconfigurable optical device, the optical zoom lens,” he explains. “The way you achieve optical zoom is to mechanically move the lens.” That movement increases the device footprint and the cost. “If we have a phase change material, we can modulate the optical properties of the antenna and change the optical zoom without any moving parts.” Instead of rotating a lens barrel or a gimbal mount, you would apply thermal energy to induce a phase change; instead of moving the lens, you reconfigure the surface of the lens.

Stay Focused, but Flexible

Surpassing the mechanical constraints of traditional bulk-optics components opens the door to further innovation. The micro spectrometer on the fishing boat or the thin fish-eye lens on a satellite are still fabricated on mechanically rigid substrates: silicon, semiconductors, glass.

“My group has developed ways to transform these devices into something mechanically flexible and stretchable, yet maintaining the same optical performance,” Hu says. These flexible devices could be attached to curved surfaces like a lens, or even biological tissue, such as human skin. “We could then perform sensing functions to do continuous monitoring on biological inputs from a human.”

So what’s next?

How about developing optical device technologies that can be applied in high-speed data communications, like a new universal optical interface that allows low-loss broadband coupling of light and transfer between different kinds of components?

Or a collaboration with Caroline Ross’s group in the MIT Center for Materials Science and Engineering, where they’re developing optical isolators that can be directly integrated with photonic circuits to realize optical diode functions?

Or research into micro concentrated solar cells that could replace conventional photovoltaic systems with ones that increase solar conversion efficiency by 30 percent or more?

Hu says he’s drawn to optics and photonics research in part because of how idea driven it is. “You have innovative ideas. You perform rigorous simulations and predict what you are going to do to get your research. Then you fabricate the devices and integrate them into a system,” he explains. MIT’s strong industry connections can accelerate that process. “Laboratory curiosity can quickly transform into something that makes a real impact,” he says. “That’s very, very fulfilling.”

So back to that ice breaker question: What are some of the applications of Hu’s optics and photonics research? The spectrum of possibilities is limitless.