Size and morphology dependent evolution of resonant modes in ZnO microspheres grown by hydrothermal synthesis.

We report the correlation between inner morphology, size and whispering gallery mode (WGM) behavior in ZnO microspheres (MSs) grown by hydrothermal method. WGMs in different ZnO microspheres with diameters in the range of 2 - 6 µm were analyzed by a modified refractive index (MRI) scheme. We found that the size dependence of WGMs in our system is more complicated than others because of the appearance of porosity inside each sphere. Such features might account for the refractive index change and peak shift. Despite that, our MRI scheme can detect such complex features and reproduce universal relations between all important quantities of a microsphere WGM resonator.

Near-field spectroscopic imaging of exciton quenching at atomically sharp MoS2/WS2 lateral heterojunctions.

Heterojunctions made by laterally stitching two different transition metal dichalcogenide monolayers create a unique one-dimensional boundary with intriguing local optical properties that can only be characterized by nanoscale-spatial-resolution spectral tools. Here, we use near-field photoluminescence (NF-PL) to reveal the narrowest region (105 nm) ever reported of photoluminescence quenching at the junction of a laterally stitched WS2/MoS2 monolayer. We attribute this quenching to the atomically sharp band offset that generates a strong electric force at the junction to easily dissociate excitons. Besides the sharp heterojunction, a model considering various widths of the alloying interfacial region under low or high optical pumping is presented. With a spatial resolution six times better than that of confocal microscopy, NF-PL provides an unprecedented spectral tool for non-scalable 1D lateral heterojunctions.

Sensing of Streptococcus mutans by microscopic imaging ellipsometry.

Microscopic imaging ellipsometry is an optical technique that uses an objective and sensing procedure to measure the ellipsometric parameters ? and ? in the form of microscopic maps. This technique is well known for being noninvasive and label-free. Therefore, it can be used to detect and characterize biological species without any impact. Microscopic imaging ellipsometry was used to measure the optical response of dried Streptococcus mutans cells on a glass substrate. The ellipsometric ? and ? maps were obtained with the Optrel Multiskop system for specular reflection in the visible range ( ? = 450 to 750 nm). The ? and ? images at 500, 600, and 700 nm were analyzed using three different theoretical models with single-bounce, two-bounce, and multibounce light paths to obtain the optical constants and height distribution. The obtained images of the optical constants show different aspects when comparing the single-bounce analysis with the two-bounce or multibounce analysis in detecting S. mutans samples. Furthermore, the height distributions estimated by two-bounce and multibounce analyses of S. mutans samples were in agreement with the thickness values measured by AFM, which implies that the two-bounce and multibounce analyses can provide information complementary to that obtained by a single-bounce light path.