Study addresses photon extraction problem

Researchers at the University of Waterloo have reported a nanoscale diamond structure designed to tackle one of solid-state quantum hardware’s persistent bottlenecks: getting light in and out of high-index bulk materials efficiently. The study, published in *Science Advances*, describes inverse-designed photonic structures that create tightly confined “photonic nanojets” inside diamond. (science.org) The authors said the structures let them selectively excite individual quantum emitters and collect more of the photons those emitters produce. The work was demonstrated with nitrogen-vacancy, or NV, centers in diamond at room temperature. The paper is titled “Probing individual quantum emitters in bulk semiconductors via photonic nanojets,” and lists Behrooz Semnani and Michal Bajcsy as corresponding authors. (science.org) The study says the approach is meant for bulk, high-refractive-index semiconductor hosts, where emitted light is often trapped inside the material and difficult to collect with standard optics. ### Why is photon extraction such a problem in bulk quantum materials? High-refractive-index hosts such as diamond make useful homes for quantum defects, but they also trap much of the light those defects emit, the paper said. That reduces collection efficiency and makes optical readout harder for applications such as quantum sensing, communication and computing. (science.org) The same materials problem also complicates targeting one emitter at a time. The authors wrote that isolating individual emitters usually requires high-purity samples and precise defect implantation, which adds fabrication complexity. ### What did the researchers actually build? (science.org) The Waterloo team used free-form topology optimization to design monolithic photonic structures directly inside high-index materials containing relatively dense, randomly distributed emitters, according to the paper. The resulting geometry creates a localized optical beam — a photonic nanojet — that can both focus excitation and improve extraction of emitted photons. (science.org) The University of Waterloo said the structure was sculpted into a diamond surface with an automated inverse-design approach and optimized to act like an antenna for emitted light. Behrooz Semnani said in the university release that the method offers “a powerful and practical route” to improve light-matter interaction in solid-state platforms. (science.org) ### What numbers did the experiment produce? The *Science Advances* paper reported selective single-emitter excitation with a 10-fold power enhancement and more than 15-fold improvement in photon extraction efficiency for photoluminescence collection in confocal microscopy. (science.org) Those results came from experiments on negatively charged NV centers in a low-cost diamond sample at room temperature. The paper also said the optimized structures suppressed background photoluminescence from near-surface defects and from randomly distributed emitters in the bulk, improving signal-to-noise ratio. ### Is this only about diamond NV centers? (uwaterloo.ca) The authors used diamond NV centers as a case study, but they wrote that the same inverse-designed nanojet structures could be applied beyond diamond-embedded emitters. The paper says the method is applicable to other semiconductor materials containing emitters and could be generalized to fiber-integrated platforms. The University of Waterloo release named diamond, silicon carbide and silicon as examples of solid-state platforms that can host atom-like defects, also called color centers, that serve as qubits or single-photon sources. (science.org) ### What comes next from here? The May 19 university release said the device was demonstrated in diamond as a proof of principle. The paper says the next step is extension to other semiconductor hosts and photonic platforms where localized excitation and higher collection efficiency are needed for scalable quantum devices. That article and its supplementary materials are now posted through *Science Advances*, with Semnani and Bajcsy listed as corresponding authors. (science.org) (uwaterloo.ca)