In general, photonic nanojets (PNJs) occur only when the refractive index (Ri) difference between the microparticle and background media is less than 2. The minimum full width at half-maximum (FWHM) of the PNJ is ~130 nm (approximately one-third of the illumination wavelength λ = 400 nm) formed within the evanescent field region. This paper proposes and studies a method to overstep the Ri upper bound and generate ultra-narrow PNJs. Finite element method based numerical investigations and ray-optics theoretical analyses have realized ultra-narrow PNJs with FWHM as small as 114.7 nm (0.287 λ) obtained from an edge-cut, length-reduced and parabolic-profiled microparticle with Ri = 2.5 beyond evanescent decay length. Using simple strain or compression operations, sub-diffraction-limited PNJs can be flexibly tuned on the order of several wavelengths. Such ultra-narrow PNJs offer great prospects for optical nonlinearity enhancements of greater enhancing effect, optical nanoscopy of higher spatial resolution, optical microprobes of smaller measurement accuracy, nano/micro-sized sample detections of higher sensing sensitivity, nanoscale objects of more accurate control, advanced manufactures of smaller processing size, optical-disk storage of larger data capacity and all-optical switching of lower energy consumption.

A novel method of detecting single nanoparticles (NPs) in a microfluidic channel directly using a photonic nanojet (PNJ) was investigated. The theoretical model comprised a plane wave-illuminated, liquid-filled hollow-microcylinder (LFHM) and a single Au NP. Relevant studies were implemented and demonstrated with a finite element method (FEM)-based numerical simulation and explained physically through a ray-optics theoretical analysis with the assistance of energy flow line shifts. When depicting the optical-field distribution by gradually altered contour lines for LFHMs with or without a single Au NP, the outward distances of the specific points on the right end of each contour line, for a LFHM with a single Au NP relative to a LFHM without a NP, increased exponentially with decreasing contour levels. By dividing the contour levels into ten levels, the detectable NP of size of a few nanometers can be reflected through the outward distance of the contour points. The key parameters of the PNJ (the maximum light intensity, decay length and lateral beam waist), combined with the electric field distribution and focal point offset, can provide information on NP location. This work showed the PNJ itself to be a powerful and promising tool for the detection and identification of single NPs.

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Recently, photothermal therapy (PTT) has been proved to have great potential in tumor therapy. In the last several years, MoS2, as one novel member of nanomaterials, has been applied into PTT due to its excellent photothermal conversion efficacy. In this work, we applied fluorescence lifetime imaging microscopy (FLIM) techniques into monitoring the PPT-triggered cell death under MoS2 nanosheet treatment. Two types of MoS2 nanosheets (single layer nanosheets and few layer nanosheets) were obtained, both of which exhibited presentable photothermal conversion efficacy, leading to high cell death rates of 4T1 cells (mouse breast cancer cells) under PTT. Next, live cell images of 4T1 cells were obtained via directly labeling the mitochondria with Rodamine123, which were then continuously observed with FLIM technique. FLIM data showed that the fluorescence lifetimes of mitochondria targeting dye in cells treated with each type of MoS2 nanosheets significantly increased during PTT treatment. By contrast, the fluorescence lifetime of the same dye in control cells (without nanomaterials) remained constant after laser irradiation. These findings suggest that FLIM can be of great value in monitoring cell death process during PTT of cancer cells, which could provide dynamic data of the cellular microenvironment at single cell level in multiple biomedical applications.