Probing individual quantum emitters in bulk semiconductors via photonic nanojets.

Solid-state quantum emitters serve as fundamental building blocks for quantum information processing, quantum telecommunication, and quantum-enhanced sensing, particularly at the nanoscale. However, the high refractive index of the emitters' host materials often result in low-efficiency collection of the photons generated by the emitters. In addition, isolating individual quantum emitters generally requires high-purity samples and precise defect implantation, adding to the fabrication complexity. In this work, we use free-form topology optimization to design broadband monolithic photonic structures within high-refractive index materials hosting a relatively dense ensemble of randomly distributed quantum emitters. Fabricated via standard top-down patterning techniques, these inverse-designed nanostructures generate tightly confined photonic nanojets, which in turn enable selective excitation of individual emitters and improve photon extraction efficiency. The optimized geometries also substantially suppress background photoluminescence from near-surface optically active defects and from randomly distributed emitters in the bulk, boosting the signal-to-noise ratio. We demonstrate this paradigm using negatively charged nitrogen vacancy (NV-) centers in a low-cost diamond sample at room temperature as a case study, achieving selective single emitter excitation with a 10-fold power enhancement and more than 15-fold improvement in the photon extraction efficiency of photoluminescence collection in confocal microscopy. Beyond NV centers and other diamond-embedded quantum emitters, the use of inverse-designed structures generating subwavelength photonic nanojets is applicable to other semiconductor materials containing emitters and can be seamlessly generalized to fiber-integrated platforms.