Measuring the Interaction of Nanoplatelets with Light Could Advance Operation of Quantum Optic Networks

Quantum optic networks have been moved one step closer to reality, thanks to the efforts of researchers. The potential to accurately control the interactions of light and matter at the nano level could enable such a network to transmit enormous amounts of data more rapidly and securely compared to an electrical network.

Scientists at Argonne, the University of Chicago and Northwestern University have moved quantum optic networks a step closer to reality with their latest work on semiconducting nanoplatelets that act as tiny light switches. The Argonne team is pictured here. Left to right: Xuedan Ma, Benjamin Diroll, Richard Schaller, and Gary Wiederrecht. (Image credit: Argonne National Laboratory)

A group of scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the University of Chicago, and Northwestern University has been successful in overcoming the difficulties faced while measuring the way in which nanoplatelets—comprising of two-dimensional layers of cadmium selenide—interact with light in three dimensions. The operation of quantum optic networks could be improved by progress in this area.

In order to integrate nanoplatelets into, say, photonic devices, we have to understand how they interact with light or how they emit light.

Xuedan Ma, Nanoscientist, Center for Nanoscale Materials (CNM), Argonne National Laboratory.

The CNM is a DOE Office of Science User Facility at Argonne. Ma and six co-authors published the outcomes of their study in Nano Letters in a paper titled “Anisotropic photoluminescence from isotropic optical transition dipoles in semiconductor nanoplatelets.”

The project ultimately targets the unique optical properties of quantum materials and the fact that they emit single photons,” stated Gary Wiederrecht, a co-author who also heads the CNM’s nanophotonics and biofunctional structures group. “You have to be able to integrate the quantum emitter with the optical networks.”

Single-photon sources such as these are required for applications in long-distance quantum communications and information processing. These sources serve as signal carriers in quantum optical networks and emit light as single photons (light particles). Single photons are perfect for various quantum information science applications since they travel at the speed of light and do not lose much of their momentum over long distances.

Upon absorbing light, the nanoplatelets form subatomic particle-like entities known as excitons. The excitons experience quantum confinement along the vertical dimension of the nanoplatelets, where the quantum confinement is a phenomenon that governs their energy levels and parcels electrons into discrete energy levels.

For this study, some of the nanoplatelets with astonishingly uniform thickness were produced in the laboratory of chemistry professor Dmitri Talapin at the University of Chicago. Talapin has a joint appointment with Argonne and is another co-author of the study.

They have precise atomic-level control of nanoplatelet thickness,” Ma told about Talapin’s research team.

The thickness of the nanoplatelets is roughly 1.2 nm (spanning four layers of atoms) and their width is about 10–40 nm. The thickness of a stack of over 40,000 nanoplatelets would be less than that of a piece of paper. This renders it difficult to measure the interactions of the material with light in three dimensions.

Ma and her team could trick the two-dimensional nanoplatelet material into disclosing the way they interact with light in three dimensions using the special sample preparation and analysis capabilities offered by the CNM.

The transition dipole moment is a critical three-dimensional parameter that operates on organic molecules and semiconductors. “It defines, basically, how the molecule or the semiconductor interacts with external light,” stated Ma.

However, it is challenging to measure the vertical component of the transition dipole in a material as flat as the semiconducting nanoplatelets. The team overcame that challenge with the help of the dry-etching tools of the CNM’s nanofabrication cleanroom to slightly roughen the flat glass slides on which the nanoplatelets are positioned for close investigation through laser scanning and microscopy.

The roughness is not so large that they distort a laser beam, but enough to introduce random distributions of the nanoplatelets.

Xuedan Ma, Nanoscientist, Center for Nanoscale Materials (CNM), Argonne National Laboratory.

The researchers made the most of the random orientations of the nanoplatelets to evaluate the three-dimensional dipole characteristics of the material using exclusive optical techniques to develop a doughnut-shaped laser beam within a distinctive optical microscope at the CNM.

The next step of the researchers is to combine the nanoplatelet materials with photonic devices for the transmission and processing of quantum information. “We’re proceeding in this direction already,” stated Ma.

The U.S. Department of Energy’s Office of Science and the National Science Foundation supported this study.

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