AFM and Nonlinear Optics Combine to Improve Optical Defect Detection

A technique for detecting nanosize imperfections in optical materials could lead to improvements in detector technologies for applications ranging from cellphones to solar cells. The technique developed by a McGill University research team uses atomic force microscopy (AFM) to better understand and control imperfections in optical materials. These nanosize optical defects are difficult to identify and characterize.

The McGill team combined nonlinear optical methods with AFM to detect ultrafast forces that arise when light and matter interact. Using AFM, the researchers measured the electrostatic force originating from light-induced electron motion in a dielectric. They observed changes in the force, caused by second-order nonlinear optical interactions in the sample, on a sub-15-nm scale and 100-femtosecond (fs) time resolution. The time resolution was set by the light pulse characteristics (not by the properties of the force sensor).

“To understand and improve materials, scientists typically use light pulses faster than 100 femtoseconds to explore how quickly reactions occur and determine the slowest steps in the process,” researcher Zeno Schumacher said. “The electric field of a light pulse oscillates every few femtoseconds and will push and pull on the atomic-sized charges and ions that comprise matter. These charged bodies then move, or polarize, under these forces and it is this motion that determines a material’s optical properties.”

The team showed that forces arising from two time-delayed light pulses could be detected with sub-fs precision and nm spatial resolution in a wide range of materials. The researchers demonstrated their technique on an insulating nonlinear optical material (LiNbO₃) and a nanometer-thin, 2D semiconducting flake of molybdenum diselenide (MoSe₂), an inorganic compound used in optical and scanning-probe microscopy.

The researchers believe their technique will allow the correlation of nm structure with light-induced time-resolved kinetics. “Our new technique is applicable to any material, such as metals, semiconductors, and insulators,” professor Peter Grutter said. “It will enable use [of] high spatial and temporal resolution to study, understand, and ultimately control for imperfections in photovoltaic materials. Ultimately, it should help us improve solar cells and the optical detectors used in a wide range of technologies.”

The research was published in the Proceedings of the National Academy of Sciences (www.doi.org/10.1073/pnas.2003945117).