Quantum photonics, a technology that utilizes the unique properties of quantum optics, has the potential to revolutionize fields like secure communications, metrology, sensing, and quantum computation. These applications require stringent control of quantum emitters (QEs), which includes deterministic placement, high single photon purity (90-100%), and a mechanism for modulating the emission for practical use. By controlling the emission characteristics of QEs, it is possible to encode information on a single photon stream, supporting the advancement of secure communications and encryption systems. The team's findings were recently published in ACS Nano 18, 25349-25358 (2024).
Quantum photonics involves the generation, manipulation, and detection of light in ways where quantum effects are significant, allowing for coherent control of individual light quanta. Single photon emitters, also known as quantum emitters, play a critical role in this technology.
"Two-dimensional materials such as monolayer tungsten disulfide and tungsten diselenide serve as hosts for QEs, and their planar, atomic-layered structure offers many advantages as material platforms for quantum photonic circuits," said Berend Jonker, Ph.D., senior scientist and principal investigator at NRL. "They can be readily integrated with other materials and substrates, and the proximity of the QE to the surface facilitates both extraction of the light as well as control of the emission by external effects."
The NRL team devised a nonvolatile and reversible method to control single photon emission in monolayer tungsten disulfide (WS2) by integrating it with a ferroelectric material. The researchers created an emitter in WS2 and were able to switch the emission between high purity quantum light and semi-classical light by modulating the ferroelectric polarization with a bias voltage. Emitters located over "up-domains" of the ferroelectric film produced high purity quantum light, whereas emitters over "down-domains" emitted semi-classical light.
"This novel heterostructure introduces a new paradigm for control of quantum emitters by combining the nonvolatile ferroic properties of a ferroelectric with the radiative properties of the zero-dimensional atomic-scale emitters embedded in the two-dimensional WS2 semiconductor monolayer," Jonker explained.
The researchers studied samples consisting of monolayer WS2 films grown by chemical vapor deposition, which were then mechanically transferred onto a 260-nanometer film of an organic ferroelectric polymer. This polymer film had previously been transferred onto a highly doped silicon substrate. The team used atomic force microscope (AFM) nanoindentation-a technique developed and patented by NRL-to create and position the quantum emitters within WS2 precisely.
"Achieving intimate contact between WS2 and the ferroelectric film is crucial, and requires an ultra-smooth ferroelectric film surface," said Sungioon Lee, Ph.D., an American Society for Engineering Education (ASEE) postdoctoral fellow working with Jonker. "Therefore, a spin-coating and flip-over process was used for the film."
"The organic ferroelectric polymer serves as a deformable polymer," added Ben Chuang, Ph.D., a research physicist with the NRL Materials Science and Technology Division. "When the AFM tip is removed, the WS2 conforms to the contour of the nanoindent, and the local strain field activates single photon emission from atomic scale defect states in the WS2."
The researchers then transferred graphite as a top electrical contact, which partially covered the WS2, and used a conductive piezo force microscopy tip to apply bias voltage and switch the polarization of the ferroelectric polymer beneath the WS2.
Quantum emitters are foundational elements in materials science and quantum technologies that will enhance the future capabilities of the Navy. The Naval Science and Technology Strategy states that "quantum science will play a crucial role in naval warfare by enabling breakthrough technologies such as faster computation speeds, ... robust encryption and innovative sensors." The Office of the Undersecretary of Defense (Research and Engineering) and the National Defense S and T Strategy 2023 have identified quantum science and advanced materials as critical areas of focus.
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