Scientists at the U.S. Naval Research Laboratory (NRL) and the Air Force Research Laboratory (AFRL) have developed a way to directly write quantum light sources, which emit a single photon of light at a time, into monolayer semiconductors such as tungsten diselenide (WSe2). NRL scientists used an atomic force microscope to create nanoscale indents in a single monolayer of WSe2 on a polymer film substrate. Upon application of sufficient mechanical stress using an atomic force microscope tip, the 2D material/polymer composite deformed, resulting in the formation of highly localized strain fields with excellent control and repeatability, and creating a single-photon emitter (SPE) state in the WSe2. (a) Illustration showing an atomic force microscope (AFM) tip indenting the TMD/polymer structure to introduce local strain. (b) Patterned single-photon emission in WSe2 induced by AFM indentation of the letters “NRL” and “AFRL.” (c) AFM indents produce single-photon emitter “ornaments” on a monolayer WSe2 “Christmas tree.” Courtesy of the U.S. Naval Research Laboratory. Researchers at the AFRL performed time-correlated measurements of this light emission to confirm that the state in the WSe2 was a true single-photon state. They showed that SPEs were created and localized at the nano-indents and that the SPEs were bright, exhibiting a single-photon emission up to 60 K, and spectrally stable. The results of this study could pave the way for the use of 2D materials as solid-state hosts for SPEs in applications such as secure communications, sensing, and quantum computation. Such applications could protect communications between distant Department of Defense (DoD) forces from eavesdropping or decryption. The results also indicate that a nano-imprinting approach could be effective in creating large arrays, or patterns, of quantum emitters for wafer-scale manufacturing of quantum photonic systems, said the researchers. “This quantum calligraphy allows deterministic placement and real-time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities, and plasmonic structures,” said researcher Berend Jonker. Researcher Matthew Rosenberger said, “In addition to enabling versatile placement of SPEs, these results present a general methodology for imparting strain into 2D materials with nanometer-scale precision, providing an invaluable tool for further investigations and future applications of strain engineering of 2D devices.” SPEs, also known as quantum emitters, generate exactly one photon on demand, with each photon indistinguishable from another. These characteristics are essential for photon-based quantum technologies under development. The researchers believe that such capabilities should be realized in a material platform that enables precise, repeatable placement of SPEs in a fully scalable fashion compatible with existing semiconductor chip manufacturing. Quantum computation on a chip will provide the capability to rapidly analyze very large data sets, so that the entire data set does not have to be transmitted, reducing bandwidth requirements. The research was published in ACS Nano (http://dx.doi.org/10.1021/acsnano.8b08730).