Researchers at the University of Cambridge have developed ultra-thin LEDs just a few atoms thick. They constructed the LEDs using stacked layers of graphene, boron nitride, and transition metal dichalcogenides (TMDs).
Microscopic image of ultra-thin LED emitting a stream of single photons. –photo courtesy of Mete Atature
The thin layered structure of TMDs makes them ideal for use in ultra-thin structures on chips. The team first tried used TMD material, tungsten diselenide in the ultra-thin LEDs. The researchers point out that the ability to produce single photons using only electrical current is a key step towards building quantum networks on compact chips.The TMD layer features regions in which electrons and electron vacancies, or holes, are tightly confined. A photon (a particle of light) is released when an electron fills an electron vacancy that is at a lower energy than the electron. In the ultra-thin LED devices, a voltage pushes electrons through the device. Electrons fill the holes and the LED emits a single photon.
Engineers have theorized that a computer based on quantum mechanics (a quantum computer) would be much more powerful and secure than current technologies. Such a quantum computer would be capable of performing calculations that cannot be performed otherwise. However, such a computer would require reliable methods of electrically generating single, indistinguishable photons as carriers of information across quantum networks.
According to the researchers, the ultra-thin LEDs that they developed have the potential for high levels of tunability and integration as well as design freedom.
Typically, generating a single photon requires large-scale optical set-ups using several lasers and precise alignment of optical components. However, the researchers say that the new ultra-thin LEDs brings on-chip single photon emission for quantum communication a step closer. The ultra-thin LEDs also offer an advantage over some other single-photon emitters for feasible and effective integration into nanophotonic circuits.
Ultra-Thin LED Structure
The stack of materials from bottom to top consisted of a silicon/silicon dioxide (Si/SiO2) substrate, a single layer of graphene (SLG), a thin sheet of hexagonal boron nitride (hBN) of 2–6 atomic layers, and a mono- or bi-layer of TMD, such as WSe2. They also tried making the LED with WS2 as the TMD material. They found that both TMDs enabled single photon emission.
The researchers reported the findings in the journal Nature Communications.
“Ultimately, we need fully integrated devices that we can control by electrical impulses, instead of a laser that focuses on different segments of an integrated circuit,” said Professor Mete Atatüre of Cambridge’s Cavendish Laboratory, and one of the senior authors of the paper. “For quantum communication with single photons, and quantum networks between different nodes, we want to be able to just drive current and get light out. There are many emitters that are optically excitable, but only a handful are electrically driven.”
“We chose WS2 because we wanted to see if different materials offered different parts of the spectra for single photon emission,”said Atatüre, who is also a Fellow of St John’s College. “With this, we have shown that the quantum emission is not a unique feature of WS2, which suggests that many other layered materials might be able to host quantum dot-like features as well.”
“We are just scratching the surface of the many possible applications of devices prepared by combining graphene with other materials,” said senior co-author Professor Andrea Ferrari, Director of the Cambridge Graphene. “In this case, not only have we demonstrated controllable photon sources, but we have also shown that the field of quantum technologies can greatly benefit from layered materials. Many more exciting results and applications will surely follow.”
C. Palacios-Berraquero et al. ‘Atomically thin quantum light emitting diodes.’ Nature Communications (2016). DOI: 10.1038/ncomms12978