SPIE Optics + Photonics 2025: Plenary speakers outline technologies for sustainable energy

05 Aug 2025

Shanhui Fan and Harry Atwater present research on photonic structures for manipulating thermal radiation, and the science of light-matter interactions.

By William G. Schulz

Perspectives on the integral role of photonics for sustainable energy by two leading researchers in the field were presented at the 3 August evening plenary session at SPIE Optics + Photonics 2025, which is taking place through 7 August in San Diego, California.

The speakers, Stanford University’s Shanhui Fan and Caltech’s Harry A. Atwater, respectively, discussed research efforts regarding photonic structures for manipulating thermal radiation, and the science of light-matter interactions that open possibilities for new energy conversion technologies.

Fan, a professor of electrical engineering at Standford U., noted that standard thermal radiation is broadband, wide angle, and that absorption and emission are related by Kirchoff’s Law. Today, with photonics engineering, he said, “everyone of these characteristics can be altered and that opens many interesting opportunities in thermal radiation control [including] some that have important practical implications.”

‘Send excess heat to the coldness of space’

For example, Fan’s group has developed radiative cooling technologies that take advantage of the spectral and angular characteristics of thermal radiation. He noted that all thermal emitters, say office buildings, can send excess heat to the coldness of space—as long as they aren’t also being heated by sunlight beyond their radiative cooling capacity.

To tackle the direct sunlight problem and allow radiative cooling of buildings, developed a multilayer optical film that reflects about 97% of the sunlight while simultaneously being able to emit the surface’s thermal energy through the atmosphere. Without heat from sunlight, Fan said, the radiative sky cooling effect can enable cooling below the air temperature even on a sunny day.

He says the technology could be used to improve the efficiency of air conditioning systems, and his group has looked at combining systems, for example, adding a system of water-filled pipes that, when also cooled by radiative cooling, would act as a sort of chiller to further lower building temperatures sans electrical power.

Atwater noted three grand challenges for photonics in meeting sustainability goals: thermal radiation (for example Fan’s work), solar energy to produce liquid fuels, and capturing solar energy from space and beaming it back to Earth.

As to Kirchoff’s Law—that absorptivity and emissivity within a given wavelength and angular range and temperature will be equal—he said researchers are beginning to see potential in breaking that reciprocity with new engineered structures. “This has some very interesting implications,” he said, including capabilities like recovery and re-use of waste heat from industrial processes.

The second challenge Atwater cited—making fuels from sunlight—is basically a process of artificial photosynthesis. His lab has been involved in efforts to create fuel cells that would convert sunlight and CO2 into combustible fuels. If research efforts at his lab and elsewhere are successful and these technologies can be scaled, he noted such possibilities as sustainably replacing the complex fossil fuels needed to power jet aircraft.

Atwater’s third challenge, he noted, has been the stuff of science fiction. That is, capturing the ever-present sunlight of space and beaming it safely back to Earth. For solar power, we would no longer be stuck with the variability of our planet’s day/night variation in solar power availability. His team has worked on a variety of new collectors, transmitters, and receivers, he said, that might one day lower the cost of the technology to make it competitive with other forms of energy.

William G. Schulz is Editor in Chief of SPIE Photoncis Focus.