Spatially, Temporally, and Spectrally Resolved Electrical Probing of Optical Excitations

Nontechnical description

In semiconductors, a photon of sufficient energy can generate an electron-hole pair, which forms a bound excitonic state due to the attractive Coulomb force. The recombination of excitons is critical for light-emitting applications, whereas the dissociation of excitons into free carriers plays a key role for photovoltaics and photodetection. Combining the illumination in the optical regime and detection in the low-frequency microwave regime, the research team performs variable-temperature electrical probing of optically excited quasiparticles in a spatially, temporally, and spectrally resolved manner.

This work contributes to the fundamental understanding of low-frequency response of optical excitations and exotic quantum phases in moire superlattices. An integrated research and education program at University of Texas at Austin is established such that students at different levels are trained to master modern nanofabrication techniques, microwave engineering and laser optics, and scanning probe microscopy, which prepares them for future careers in science or engineering.

Technical description

The interaction between light and semiconductors is dominated by the generation, recombination, and dissociation of excitons. Traditionally, studies of bound excitons and mobile charges are performed in different spectral regimes, with either insufficient spatial or temporal resolution. In this project, the researchers conduct cryogenic laser illuminated microwave impedance microscopy experiments for the study of transport and diffusion of excitons and trions in two-dimensional monolayers and heterostructures, as well as local properties of exciton insulators and topological moire excitons, with high spatial, temporal, and spectral resolutions.

As a result, the team can extract the effective permittivity of excitons in monolayer semiconductors, visualize the transport and diffusion of excitons and trions with high spatial resolution, evaluate the electrical properties of intralayer and interlayer excitons, and probe the correlation and topology of moire excitons. The work is important for optoelectronic applications of two-dimensional materials and heterostructures under intense illumination.

The research is at the cutting edge of nano-optoelectronic materials systems and aligned with the mission of the NSF Electronic and Photonic Materials Program.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.