The Force of Light on Objects

As I PhD student, I was curious about the force of light on objects. Surprisingly, light pushes on the objects all around us, an effect called radiation pressure. In everyday life, we do not notice this tiny effect; the intensity of light is not strong enough, and the objects are too heavy, for motion to occur. However, this situation changes dramatically when a powerful laser is aimed at a tiny, nano-scale object.

When I started my work, no one had yet considered how light might be used to push on the tiny waveguides that make up photonic chips. Photonic chips are similar to computer chips, but instead of controlling the flow of electrons, they control the flow of light. I was curious: could light be used to push and pull photonic waveguides, and would that force be significant enough to move them?

I wrote a key simulation paper showing that with the right waveguide design, the push and pull of light could be significant, causing significant relative motion. This led to a flurry of further work in the field. Later, as an assistant professor, I used optical forces to capture nanoparticles from solution and form them into regular arrays. Today, I work with colleagues to use optical forces in more sensitive infrared detectors, useful for chemical sensing.

Nanostructured Solar Cells

Shortly after starting as an assistant professor, a second fundamental question led to a new line of research. Could patterning a solar cell improve its ability to absorb light and convert it to electricity? My work was the first to examine this question in nanowire silicon solar cells. Over a series of papers, I examined when it is beneficial to pattern the solar absorber, and when it is not – for example, when patterning causes electrons generated by solar absorption to disappear before they can be collected in an external circuit. I further studied the effect of varying the solar absorber material and developed detailed models for predicting solar efficiency. This work led to a sequence of influential papers, and the PhD student who led the work was immediately hired after graduation to lead modeling research at a nanostructured solar cell startup! Moreover, the insights developed during this work led directly to my more recent work on nano-patterned photodetectors.

Infrared Detectors and Cameras

In the last decade, my group has invented new classes of infrared sensors. Infrared sensing and imaging is key to a wide range of applications, from detection of heat leaks from buildings to remote monitoring of air quality and pollution. Drawing on my insights from solar cells, I asked how nanoscale patterning could be used to improve infrared detector performance. This led to a research partnership between my group and NASA.

Standard infrared photodetectors and bolometers measure wide ranges of the infrared spectrum, a region invisible to our eye. My work has introduced a toolkit for making infrared detectors highly selective, measuring specific wavelengths or angles of incoming light. The ability to collect specific information will directly benefit emerging applications, such as remote monitoring of crop health.

Thermal Radiation Engineering

Warm objects emit light, albeit in the portion of the spectrum invisible to the eye. A major part of my work in the last decade has been the discovery of new techniques for changing how materials radiate in this portion of the spectrum.

For example, a key paper we wrote proposed a concept for “smart skins,” coatings that could automatically regulate their temperature. Over a series of papers, I advanced this idea first theoretically and then in experiment, further optimizing the approach through the study of alternative geometries and nanoscale patterns. This work translated into a long-running collaboration with the aerospace industry. The ultimate impact will be smart coatings that help regulate the temperature of satellites without any need for external power. A similar approach may benefit residential and industrial heating and cooling.

I have also studied more exotic thermal radiation properties, such as nonreciprocity. Almost all known materials are governed by Kirchoff’s Law, which dictates that they absorb light in the same way that they emit it via thermal radiation. My work has pioneered a new strategy for breaking this constraint, by tuning the material’s properties in space and time. While the most practical long-term application is not yet known, this fundamental change in thermal radiation properties will likely have wide- ranging impact on energy storage, conversion, and harvesting.