Joining the dots to boost integrated photonics

Defects on atomically thin semiconductors produce light-emitting quantum dots that could be useful in integrated photonics – the integration of quantum photonics with solid-state electronics.

The quantum dots serve as a source of single photons, according to researchers at the University of Rochester, New York, US.

Scientists are interested in integrated solid-state devices for quantum information processing uses. Quantum dots in atomically thin semiconductors could not only provide a framework to explore the fundamental physics of how they interact, but also enable nanophotonics applications.

Quantum dots are often referred to as artificial atoms. They are artificially engineered or naturally occurring defects in solids that are being studied for a wide range of applications.

Atomically thin 2D materials such as graphene have also generated interest among scientists who want to explore their potential for optoelectronics, said University of Rochester assistant professor of optics Nick Vamivakas. However, until now, optically active quantum dots have not been observed in 2D materials.

In a paper published in Nature Nanotechnology, the Rochester researchers show how tungsten diselenide (WSe2) can be fashioned into an atomically thin semiconductor that serves as a platform for solid-state quantum dots. The defects that create the dots do not inhibit the electrical or optical performance of the semiconductor and can be controlled by applying electric and magnetic fields.

The brightness of the quantum dot emission could be controlled by applying the voltage, Vamivakas said in a statement, adding that the next step would be to use voltage to “tune the color” of the emitted photons, making it possible to integrate the quantum dots with nanophotonic devices.

A key advantage is how much easier it is to create quantum dots in atomically thin tungsten diselenide, compared with producing quantum dots in more traditional materials such as indium arsenide.

“We start with a black crystal and then we peel layers of it off until we have an extremely thin later left, an atomically thin sheet of tungsten diselenide,” said Vamivakas, the senior author on the paper.

The researchers take two of the atomically thin sheets and lay one over the other. At the point where they overlap, a quantum dot is created. The overlap creates a defect in the otherwise smooth 2D sheet of semiconductor material. The extremely thin semiconductors are much easier to integrate with other electronics.

The quantum dots in tungsten diselenide also possess an intrinsic quantum degree of freedom ­– the electron spin. This can act as a store of quantum information and provide a probe of the local quantum dot environment.

“What makes tungsten diselenide extremely versatile is that the colour of the single photons emitted by the quantum dots is correlated with the quantum dot spin,” said the paper’s first author, Chitraleema Chakraborty.

The ease with which the spins and photons interacted should make these systems ideal for quantum information applications, as well as nanoscale metrology, Chakraborty said.