Low Energy Electron Microscopy

In our laboratory, we have developed the TR-PEEM technique, an instrumentation that allows study of charge and carrier dynamics at their fundamental time and length scale, at femtosecond temporal and nanometer spatial resolution. Traditionally, carrier dynamics is studied with ultrafast optics in which time resolution of femtosecond or better is accomplished using very short laser pulses. However, due to the diffraction limit, optic technique has limited spatial resolution and it is insufficient to spatially resolve carrier dynamics in devices or nanostructures at the nanometer length scale.

We overcome this resolution limit by combining ultrafast technique and electron microscopy. With  PEEM, we can image photoelectrons emitted by the ultrafast laser pulse with <30nm spatial resolution, and the combination of optical pump-probe technique allows us to resolve dynamics processes with <200fs temporal resolution. Using this technique, we have made video of electrons flowing within a photovoltaic device structure [1]. We have also shown how we can manipulate the flow of charges on semiconductor surfaces using ultrafast pulses [2]. We have also applied our technique on perovskite photovoltaic materials where we revealed the roles of defects in the trapping of charge [3--5]. This ultrafast nanoscale imaging technique gives us a unique way to investigate novel materials and quantum processes at the nano- and femto- scale.

Before 2016, we used a homebuilt third/fourth harmonic generation setup to generate probe photons of 266nm and 200nm from an Ti:Sapphire oscillator operating at 4MHz. After 2021, we have upgraded our laser system, by using two different NOPAs driven by a high-power (150W) fiber laser, to gain turnability in both our pump and probe wavelength. In 2023, we have finished coupling a high-harmonic beamline to extend our probe wavelength to the sub-100nm 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