Scientific Research

In my work with my PhD students, we study how to pattern materials to change how they interact with light. I am interested both in the basic physics and the applications to solar cells, infrared cameras, and thermal control of satellites.
For a more technical version, visit my university website.

The Force of Light on Objects
As a 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. 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 recombine 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.

Infrared Detectors and Cameras
In the last decade, my group has developed new types of infrared sensors. Infrared sensing and imaging are important for 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 eyes. My work has introduced a toolkit for making infrared detectors highly selective, allowing them to measure specific wavelengths or angles of incoming light. The ability to collect specific information could 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 goal will be to develop 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. Under ordinary conditions, Kirchhoff’s law of thermal radiation says that a material absorbs and emits light in equivalent ways. My group has pioneered a strategy for departing from that conventional relationship by varying a material’s properties in space and time. This work opens new possibilities for controlling energy emission, transfer, and conversion.