New two-photon momentum microscopy uncovers ultrafast charge carrier dynamics in energy and momentum space

The ultrafast dynamics and quasi-particle lifetime of optically excited electrons play a crucial role for many fundamental chemical and physical phenomena and are responsible for the performance of next generation nanoscale electronic devices. A comprehensive understanding of the quasi-particle lifetime and the energy and momentum scattering mechanisms of optically excited electrons is therefore essential for scientists to control their dynamics at interfaces or in bulk materials.

One of the most powerful techniques to study the lifetime of hot electrons in solids is time-resolved two-photon photoemission spectroscopy (tr-2PPE). However, due to the limited acceptance angle of conventional photoemission detectors, hot electron dynamics in the entire accessible momentum space has still not been well-understood. In a recent paper, Haag et al. reported a novel approach of time-resolved two-photon momentum spectroscopy to study the quasi-particle lifetime of hot electrons through the entire accessible energy and momentum space in a fixed experimental geometry, demonstrating its capability to reveal hot electron dynamics for a prototypical copper surface oriented in the (111) direction.

The new method combines the scheme of tr-2PPE with a photoemission electron microscope operated in k-space mode, also called a momentum microscope detector. This new combination allows access to the entire available momentum space dynamics, which can be directly translated into a momentum-dependent lifetime map without rotating the sample. The new tool makes it possible to monitor momentum-dependent scattering mechanisms of hot electrons on the femtosecond timescale in real time in all k-space directions. Understanding such fundamental phenomena would advance the discovery of novel functionalities of charge and spin carriers, paving the way for the next generation of information technology and light harvesting applications.

Source: “Time-resolved two-photon momentum microscopy-A new approach to study hot carrier lifetimes in momentum space,” by Florian Haag, Tobias Eul, Philip Thielen, Norman Haag, Benjamin Stadtmüller, and Martin Aeschlimann, Review of Scientific Instruments (2019). The article can be accessed at https://doi.org/10.1063/1.5110049 .