The optical response of atomically thin materials has been successfully controlled on very short timescales by a group of researchers at Columbia University in New York and Stanford University in California, US. The finding advances our understanding of many-body phenomena in low-dimensional systems. They could also help in the development of photonic devices, such as light emitters and lasers made from novel 2D transition metals.

The team, led by Tony Heinz, studied the 2D material tungsten selenide (WS₂). This semiconductor belongs to the family of the transition metal dichalcogenides (TMDCs) – these have the chemical formula MX₂, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se or Te).

The materials go from being indirect band-gap semiconductors in the bulk, to direct band-gap semiconductors when scaled down to monolayer thickness. These monolayers efficiently absorb and emit light, and so might find use in a variety of optoelectronics device applications, such as light-emitting diodes, lasers, photodetectors and solar cells. TMDCs might also be used to make circuits for low-power electronics, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a variety of surfaces.

Strong photoexcitations

To explore the optical response of these materials in more detail, especially when they are strongly photoexcited, Heinz and colleagues subjected monolayers and bilayers of WS₂ to very short and intense laser pulses, which lasted just 250 fs. They probed the resulting photoresponse over a range of wavelengths, using a technique known as “spectrally resolved ultrafast pump–probe spectroscopy”. When the sample absorbs an ultrafast laser pulse, an extremely high density of excited charge carriers (electrons and holes) is injected into the material (up to roughly one electron per square nanometre).

“In this so-called electron–hole plasma regime, the semiconductor material starts to behave somewhat like a metal,” explains team-member Alexey Chernikov, “although the comparison should not be taken too literally, since the band gap in the material is still present.”

Transition time

The presence of these carriers strongly modifies the character of the optically excited states in the material, he says. “In the unexcited material, when a photon is absorbed, it creates an exciton (a bound electron–hole pair), formed by the Coulomb attraction between the oppositely charged carriers,” explains Chernikov. He adds that “at the limit of high excitation density, however, the photogenerated charges mutually screen one another, and a plasma of free electrons and holes is produced. Going from a regime of excitons to one of free carriers is known as a Mott transition, and is of fundamental interest in many-body physics. Understanding the Mott transition in these materials is also important for applications involving high excitation densities of charge carriers”.

That these monolayer materials can sustain very intense light pulses is a property that would benefit devices operating at high intensity, such as lasers, concentrator solar cells and intra-cavity saturable absorbers and modulators, adds team-member Claudia Ruppert, who is currently at the Technical University of Dortmund in Germany.

Electron behaviour

“From a more fundamental point of view, the Mott threshold of the transition from semiconducting to metal-like behaviour identified in our study roughly defines the limit of the regime where phenomena associated with stable exciton particles can be observed,” says Chernikov. “Excitons have attracted a lot of attention in our field, thanks to their large binding energies and their peculiar ‘spin-valley’ and related properties. It is thus important to determine when these particles exist and when they are unstable and ionized.”

The Columbia–Stanford team says that it is now trying to better understand how interacting electrons behave in this class of material. “We will do this by both mapping the ‘phase-space’ diagram for the electronic many-body states in 2D films such as WS₂ and by finding out how to efficiently manipulate them,” says Heinz.

The research is published in Nature Photonics 10.1038/nphoton.2015.104.

• This article first appeared on nanotechweb.org