Researchers pioneering a revolutionary new method to directly observe dark excitons

Heralding the end of a decade-long quest, in a promising new class of extremely thin two-dimensional semiconductors, scientists for the first time directly visualized and measured elusive particles, called dark excitons, which cannot be seen. by light.

The powerful technique, described in the main journal Science, could revolutionize research on two-dimensional semiconductors and excitons, with profound implications for future technological devices, from solar cells and LEDs to smartphones and lasers.

Excitons are excited states of matter found in semiconductors, a key ingredient in many current technologies. They form when electrons in the semiconductor material are excited by light to a higher energy state, leaving behind a “hole” at the energy level where the electron previously resided.

“Holes are the absence of an electron and thus carry the charge opposite to an electron,” explained lead author Professor Keshav Dani, who heads the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology. Technology Graduate University (OIST). “These opposing charges attract, and the electrons and the holes bind together to form excitons which can then move throughout the material.”

In ordinary semiconductors, excitons turn off within a few billionths of a second after being created. In addition, they can be “fragile”, which makes them difficult to study and handle. But a decade ago, scientists discovered two-dimensional semiconductors, where the excitons are more robust.

“The robust excitons give these materials truly unique and exciting properties, so there have been many intense studies around the world aiming to use them to create new optoelectronic devices,” said co-first author Dr Julien Madéo, OIST Femtoseconde Spectroscopy Unit scientist. “But at the moment, there is a major limitation with the standard experimental technique used to measure excitons.”

Currently, researchers are using optical spectroscopy techniques – essentially measuring the wavelengths of light absorbed, reflected, or emitted by the semiconductor material – to uncover information about the energy states of the excitons. But optical spectroscopy only captures a small part of the image.

Scientists have long known that only one type of exciton, called light excitons, can interact with light. But other types of excitons exist as well, including dark ones forbidden by momentum. In this type of dark exciton, the electrons have a different impulse than the holes they are attached to, preventing them from absorbing light. It also means that electrons in dark excitons have a different momentum than electrons in bright excitons.

“We know they exist, but we can’t see them directly, we can’t probe them directly, and therefore we don’t know how important they are, nor how much they impact the optoelectronic properties of the material. “Said Dr Madéo.

Bright light on dark excitons

To visualize dark excitons for the first time, scientists have modified a powerful technique that was previously widely used to study single, unbound electrons.

“It was not clear how this technique would work for excitons, which are composite particles in which electrons are bound. There was a lot of theoretical work in the scientific community to discuss the validity of this approach,” said Professor Dani.

Their method proposed that if a beam of light containing photons of high enough energy were used to strike excitons in the semiconductor material, the energy of the photons would break down the excitons and expel electrons from the material.

By measuring the direction in which electrons fly out of the material, scientists would then be able to determine the initial momentum of electrons when they were part of excitons. Scientists would therefore not only be able to see, but also to differentiate bright excitons from dark excitons.

But the implementation of this new technique required to solve enormous technical challenges. Scientists had to generate pulses of light with extreme high-energy ultraviolet photons capable of splitting excitons and chasing electrons out of the material. The instrument then had to be able to measure the energy and angle of these electrons. In addition, the excitons being of such short duration, the instrument had to work on time scales of less than a thousand billionths of a second. Finally, the instrument also required a sufficiently high spatial resolution to measure 2D semiconductor samples, which are typically only available at the micron scale.

“When we solved all the technical issues and turned on the instrument, there was basically on our screen turning them on – it was really amazing,” said co-first author Dr Michael Man, also of the unit. of OIST femtosecond spectroscopy.

The researchers found that, as expected, bright and dark excitons were present in the semiconductor material. But to their surprise, the scientists also found that dark excitons dominated the material, outnumbering bright excitons. The team further observed that under certain conditions, when the excited electrons scattered through the material and changed momentum, the excitons could go from brightness to darkness.

“The dominance of dark excitons and the interplay between dark and bright excitons suggest that dark excitons are having an even greater impact than expected on this new class of semiconductors,” said Dr. Madéo.

This technique is a real breakthrough, “concluded Professor Dani.” Not only does it allow the first observation of dark excitons and illuminate their properties, but it inaugurates a new era in the study of excitons and other excited particles. ”