Scientists have made a new discovery in the quest to control extremely low intensities of light using atomically thin materials.
The international team of researchers, including Professor Alexander Tartakovskii from the University of Sheffield, has shown that a three-atom thick monolayer of material MoSe2 becomes very sensitive to light when it is highly charged with electrons.
Not only are very low intensities of light required to change the material’s response, but also the material is very sensitive to the polarisation of light – the way the electric field vector rotates within the electromagnetic wave.
This breakthrough marks an important next step towards a new generation of quantum sensing and computing technologies.
This effect, described today (7 July 2022) in Nature Photonics, is achieved when a monolayer sheet of semiconducting MoSe2 is first charged from the underlying thin film. A combination of two, called a heterostructure, is then placed in an optical microcavity made of two highly reflecting mirrors placed within a few hundred nanometers of each other.
New hybrid particles called polaritons are formed that combine properties of light that bounce between the mirrors and matter, represented by the electronic states within MoSe2 monolayer.
The research group in Sheffield led by Professor Tartakovskii found that when such a highly charged sheet of MoSe2 in a microcavity is placed in magnetic field, polaritons can only form for light of a specific polarisation of light.
When the intensity of light that illuminates the sample is slightly increased (still in micro-Watt regime) the way light interacts with the monolayer semiconductor changes dramatically, and polaritons can form for light in two opposite circularly polarised states. This changes the energy spectrum of the monolayer, and is described in terms of very strong optical nonlinearity.
The mechanism of this nonlinearity, described as ‘polarisation selective strong light-matter interaction regime’ has previously not been observed and will pave the way to the use of this system in quantum sensing and computing, where optical nonlinearity at the level of few photons (extremely low light intensity) is sought.
Previously, highly doped MoSe2 monolayers were difficult to make within a microcavity, but have been made possible by providing an underlying layer of another semiconductor EuS, which was made for this experiment by researchers in Tokyo.
Professor Tartakovskii said: “This is a very interesting regime in the strong light-matter interaction. The next step should be to learn how to control the charging of the monolayer, for example by changing the properties of the EuS film or inserting other atomic layers, for example of hexagonal boron nitride between MoSe2 and EuS.
“Understanding the dynamics of electrons in such heterostructures is also an important task. All this combined will allow making controllable devices eventually giving us optical nonlinearities at a few-photon level.”
This research is the result of a successful international collaboration between the University of Sheffield where the microcavity experiments were carried out; Université Clermont Auvergne (Clermont-Ferrand, France) who provided theoretical interpretation; RIKEN Center for Emergent Matter Science and University of Tokyo (Japan) who provided interesting substrates made from EuS for the MoSe2/EuS heterostructures; and Technische Universität Dortmund (Germany) who carried out important ultra-fast optical spectroscopy experiments.
