Confocal fluorescence microscopy is widely applied for the study of point-like emitters such as biomolecules, material defects, and quantum light sources. Confocal techniques offer increased optical resolution, dramatic fluorescence background rejection and sub-nanometer localization, useful in super-resolution imaging of fluorescent biomarkers, single-molecule tracking, or the characterization of quantum emitters. However, rapid, noise-robust automated 3D focusing on point-like emitters has been missing for confocal microscopes. Here, we introduce FiND (Focusing in Noisy Domain), an imaging-free, non-trained 3D focusing framework that requires no hardware add-ons or modifications. FiND achieves focusing for signal-to-noise ratios down to 1, with a few-shot operation for signal-to-noise ratios above 5. FiND enables unsupervised, large-scale focusing on a heterogeneous set of quantum emitters. Additionally, we demonstrate the potential of FiND for real-time 3D tracking by following the drift trajectory of a single NV center indefinitely with a positional precision of < 10 nm. Our results show that FiND is a useful focusing framework for the scalable analysis of point-like emitters in biology, material science, and quantum optics.


S. Sahoo, J. Jiang, J. Li, K. Loehr, C. Germany, J. Zhou, B.K. Clark and S.I. Bogdanov, arXiv:2311.06479 (2023)

2.  Hybrid quantum nanophotonic devices with color centers in nanodiamonds

 Optically active color centers in nanodiamonds offer unique opportunities for generating and manipulating quantum states of light. These mechanically, chemically, and optically robust emitters can be produced in mass quantities, deterministically manipulated, and integrated with a variety of quantum device geometries and photonic material platforms. Nanodiamonds with deeply sub-wavelength sizes coupled to nanophotonic structures feature a giant enhancement of light-matter interaction, promising high bitrates in quantum photonic systems. We review the recent advances in controlled techniques for synthesizing, selecting, and manipulating nanodiamond-based color centers for their integration with quantum nanophotonic devices.


S. Sahoo, V.A. Davydov, V.N. Agafonov, and S.I. Bogdanov, Opt. Mater. Exp., 13, 191 (2023)


3.  Rapid absolute sizing of deeply subwavelength dielectric nanoparticles by confocal scanning optical microscopy

 Accurate sizing of individual nanoparticles is crucial for the understanding of their physical and chemical properties, and for their use in nanoscale devices. Optical sizing methods are noninvasive, rapid and versatile. However, the low optical response of weakly absorbing subwavelength dielectric nanoparticles poses a fundamental challenge for their optical metrology. We demonstrate scalable optical sizing of such nanoparticles based on confocal scanning microscopy. The method is absolutely calibrated by correlating the optical signatures in the scattered pump laser signal to the ground truth nanoparticle sizes measured by an atomic force microscope. Using an air objective with a numerical aperture of 0.9, we measured the sizes of nanodiamond particles ranging from 35 to 175 nm, with an average error of ± 12.7 nm compared to the ground truth sizes. This technique paves the way for the metrology of a wide range of weakly scattering nano-objects for applications in biomedicine, catalysis, nanotechnology and quantum optics.


S. Sahoo, H. Azzouz, and S.I. Bogdanov, Appl. Phys. Lett. 118, 241105 (2021)