Two-photon fluorescence correlation spectroscopy with high count rates and low background using dielectric microspheres

Large-Scale Fabrication of Photonic Nanojet Array via Template-Assisted Self-Assembly

A large-scale homogenized photonic nanojet array with defined pattern and spacing facilitates practical applications in super-resolution imaging, subwavelength-resolution nanopatterning, nano objects trapping and detection technology. In this paper, we present the fabrication of a large-scale photonic nanojet array via the template-assisted self-assembly (TASA) approach. Templates of two-dimensional (2D) large-scale microwell array with defined pattern and spacing are fabricated. Melamine microspheres with excellent size uniformity are utilized to pattern on the template. It is found that microwells can be filled at a yield up to 95%. These arrayed microspheres on the template serve as microlenses and can be excited to generate large-scale photonic nanojets. The uniformly-sized melamine spheres are beneficial for the generation of a homogenized photonic nanojet array. The intensity of the photonic nanojets in water is as high as ~2 fold the background light signal. Our work shows a simple, robust, and fast means for the fabrication of a large-scale homogenized photonic nanojet array.

Enhancing Upconversion Fluorescence with a Natural Bio-microlens

Upconversion fluorescence has triggered extensive efforts in the past decade because of its superior physicochemical features and great potential in biomedical and biophotonic studies. However, practical applications of upconversion fluorescence are often hindered by its relatively low luminescence efficiency (<1%). Here, we employ a living yeast or human cell as a natural bio-microlens to enhance the upconversion fluorescence. The natural bio-microlens, which was stably trapped on a fiber probe, could concentrate the excitation light into a subwavelength region so that the upconversion fluorescence of core-shell NaYF₄:Yb³⁺/Tm³⁺ nanoparticles was enhanced by 2 orders of magnitude. As a benefit of the fluorescence enhancement, single-cell imaging and real-time detection of the labeled pathogenic bacteria, such as Escherichia coli and Staphylococcus aureus, were successfully achieved in the dark fields. This biocompatible, sensitive, and miniature approach could provide a promising powerful tool for biological imaging, biophotonic sensing, and single-cell analysis.

Influence of the background medium on imaging performance of microsphere-assisted super-resolution microscopy

We demonstrated the importance of the refractive index of the background medium containing the microsphere in microsphere-assisted super-resolution microscopy. We performed numerical simulation and showed that the photonic nanojet properties of a microlens can be optimized by adjusting the index of the background medium surrounding it. Our simulation results, in qualitative agreement with experimental observations, demonstrated that for the same refractive index contrast between the microsphere and its surrounding medium, a high-index microsphere embedded in a medium provides sharper focusing and better imaging quality compared with its low-index counterpart working in air. Our results are of interest for various applications that require optimization of nanojet properties of microspheres, and will inform on the design of novel microparticle-embedded devices for high-resolution imaging.