Research

Our research program studies the interaction between light and nanostructured materials, focusing on the emergent optical, electrical, and chemical properties of nanoscale materials. We are especially interested in studying plasmonic materials, colloidal semiconductor nanoparticles, and optical metamaterials. We design nanostructures using a combination of experimental and theoretical techniques, synthesize our structures using both bottom-up and top-down methods, characterize the material properties, and ultimately integrate our designs into functional devices and systems. A few highlights of recent research are discussed below.

Efficient photovoltaic operation requires careful management of incident sunlight. We seek to integrate light harvesting structures with photovoltaics to take full advantage of the solar spectrum, co-use land and sunlight, and integrate photovoltaics with the environment.

Luminescent solar concentrators are semi-transparent solar collectors that capture diffuse sunlight and downshift and direct it onto adjacent solar cells. The semi-transparent nature makes these panels ideal for integration with buildings, greenhouses, and other locations. Over the past several years we have developed various luminophore and composite materials and light management strategies for LSCs, and considered their integration into agricultural applications. We have also designed spectrally-selective mirrors for module thermal management in both monofacial and bifacial modules. Our work has shown that these mirrors can be made with low complexity and act as “thermally-aware” structures that increase energy yield through a combination of anti-reflection and operating temperature reduction. Another recent study on tandem photovoltaics revealed the optical role of surface roughness on efficiency in CdTe/Si tandem solar cells, which is critical to achieving high efficiency in state-of-the-art devices.

Reducing the operating temperature of silicon photovoltaic modules increases both the module energy yield and lifetime. Our research focuses on designing, characterizing, and fabricating nanophotonic structures which reduce module temperature by reducing parasitic absorption of sub-bandgap photons in the module. We consider designs with reflection, transmission, or scattering properties which are controlled over the wavelength range of the solar spectrum. Furthermore, we focus on achieving significant temperature reduction via inexpensive processing techniques to decrease levelized cost of the electricity.

Chirality, the symmetry property of objects that are not superimposable on their mirror image, is ubiquitous in nature and engineering. Nanoscale chiral materials have attracted attention for numerous applications, from sensing to optoelectronic devices to anti-counterfeiting strategies. 

Our research group has studied a variety of different chiral materials, including lithographically patterned structures comprised of both plasmonic and semiconductor nanocrystal elements, chiral nanocrystals, and single and double gyroids that can be assembled using block copolymers. Our work has shown various strategies for tuning and switching these materials, by modifying the refractive index of nearby materials, changing ligand-nanoparticle interactions, or controlling the surface structure of single and double gyroid thin films. We have demonstrated a design process for circularly polarized luminescence that consists of both patterned plasmonic structures and patterned quantum dots that achieves simultaneously high photoluminescence intensity and degree of polarization. 

The ability to tune and switch the optical properties of materials is essential for many optical devices, including tunable filters, beam steering, dynamic windows, and others. Our research group is studying electrolyte gating as a route to tunable optics, as part of our MRSEC research. Our recent paper on La_1-xSr_xCoO_3-d measured the complex refractive index before and after electrochemical reduction across a range of compositions, showing significant refractive index modulation across the near and mid-infrared. 

Many of the advances in both understanding and applications of nanoscale optical materials require precise, sub-wavelength nanopatterns with excellent control over shape, feature size, and proximity. We work on an interesting type of nanopatterned material formed from patterned semiconductor nanocrystals using direct write electron beam lithography, where the nanocrystals retain their photoluminescence after patterning. This offers opportunities to both create light-emitting metasurfaces and complex, hierarchical optical structures.