Dark excitons outnumber bright ones

When a valence electron jumps across a bandgap to the conduction band, it doesn’t always escape the hole it left behind. In some materials, Coulomb forces are strong enough to hold negatively charged electrons and positively charged holes together in neutral electron–hole pairs known as excitons. The prevalence and dynamics of those quasiparticles affect the electronic and optoelectronic properties of the materials that host them.

Excitons come in two types, bright and dark. A bright exciton forms when a single photon is absorbed. In the case of dark excitons, the electron and hole are connected by an optically forbidden transition, meaning the electron didn’t reach the conduction band through photon absorption alone—it also needed phonon scattering. Because they may form or recombine through many possible pathways, dark excitons are difficult to study through the usual optical methods.

Julien Madéo and Michael Man, both at Okinawa Institute of Science and Technology in Japan, and their coworkers turned instead to angle-resolved photoelectron spectroscopy (ARPES) for observing dark excitons. Their novel application of the technique detected both bright and dark excitons and monitored the populations over time. Two different exciton-creation protocols produced the same result: Dark excitons were twice as prevalent as their bright companions. The finding shows the importance of dark-exciton dynamics to understanding the optoelectronic properties of exciton-rich materials.

In ARPES, incident photons kick electrons out of a target material and the electrons’ energies and momenta are measured. It’s a go-to technique for probing electronic structures. But experimental challenges have prevented its application to excitons: The experiment must have extreme-UV (XUV) photon energies to break apart the tightly bound electron–hole pairs and eject photoelectrons, subpicosecond temporal resolution to follow the exciton population’s evolution, and micron-scale spatial resolution to probe tiny samples of high-quality exciton-hosting materials. Madéo, Man, and coworkers are the first to unite those capabilities in one apparatus.

The researchers formed excitons by striking a tungsten diselenide monolayer with an ultrashort pulse of photons whose energy, 1.72 eV, was just enough to create excitons. Pulses of XUV photons then broke the excitons apart, as illustrated in the figure. At first, the ejected electrons’ momenta peaked at a value consistent with a bright exciton. But once the delay between pulses reached about 400 fs, the peak shifted to a momentum value corresponding to a dark state. After about 1 ps, the ratio of dark to bright excitons remained at a steady-state value of approximately 2:1.

In a second experiment, higher-energy 2.5 eV photons pushed the valence electrons across the bandgap. Some of the electrons escaped their holes, but those left in excitonic states were again about twice as likely to be dark as bright. Now that their apparatus is up and running, the researchers see many directions for future study, including understanding the mechanisms of dark-exciton recombination, probing few- and many-body excitonic states, and tracking long-time exciton decay dynamics for potential qubit applications. (J. Madéo et al., Science 370, 1199, 2020 .)