In our laboratory, we developed the TR-PEEM technique, enabling the study of charge and carrier dynamics at fundamental time and length scales, with femtosecond temporal and nanometer spatial resolution. Traditionally, carrier dynamics have been studied using ultrafast optics, achieving femtosecond or better time resolution through very short laser pulses. However, optical techniques are diffraction-limited and lack the spatial resolution needed to resolve carrier dynamics within devices or nanostructures at the nanometer scale.
We overcame this limitation by combining ultrafast optics with electron microscopy. Using PEEM, we image photoelectrons emitted by an ultrafast laser pulse with spatial resolution below 30 nm. Integrating this with an optical pump-probe technique enables us to resolve dynamic processes with a temporal resolution below 200 fs. This technique has allowed us to create videos of electron flow within photovoltaic device structures [1], manipulate charge flow on semiconductor surfaces using ultrafast pulses [2], and explore perovskite photovoltaic materials, where we revealed the role of defects in charge trapping [3–5]. This ultrafast nanoscale imaging technique provides a unique way to investigate novel materials and quantum processes at both nano- and femtosecond scales.
Before 2016, we used a homebuilt third/fourth harmonic generation setup to produce 266 nm and 200 nm probe photons from a Ti oscillator operating at 4 MHz. Since 2021, we upgraded to a laser system with two different NOPAs driven by a high-power (150W) fiber laser, giving us tunability in both pump and probe wavelengths. In 2023, we completed the coupling of a high-harmonic beamline, extending our probe wavelength capabilities to the sub-100 nm range.
[1] M. K. L. Man, et al., Nature Nanotechnology 12, 36-40 (2017).
[2] E. L. Wong, et al.,Science Advances 4, eaat9722 (2018).
[3] T.A.S. Doherty et al., Nature 580, 360 (2020).
[4] S. Kosar et al., Energy Environ. Sci. 14, 6320-6328 (2021).
[5] K. Frohna et al., Nat. Nanotechnol. 17, 190-196 (2022).
Combination of ultrafast pump-probe scheme with PEEM allows imaging of charge carriers at their fundamental time and length scale