Yinzhou Yan

Microsphere-Coupled Scanning Laser Confocal Nanoscope for Sub-Diffraction-Limited Imaging at 25 nm Lateral Resolution in the Visible Spectrum

We report a direct optical super-resolution imaging approach with 25 nm (∼ λ/17) lateral resolution under 408 nm wavelength illumination by combining fused silica and polystyrene microspheres with a conventional scanning laser confocal microscope (SLCM). The microsphere deposited on the target surface generates a nanoscale central lobe illuminating a sub-diffraction-limited cross-section located on the target surface. The SLCM confocal pinhole isolates the reflected light from the near-field subdiffractive cross-section and suppresses the noises from the side lobe and the far-field paraxial focal point. The structural detail of the subdiffractive cross-section is therefore captured, and the 2D target surface near the bottom of microspheres can be imaged by intensity-based point scanning.

Immersed transparent microsphere magnifying sub-diffraction-limited objects

The resolution of an optical microscope is restricted by the diffraction limit, which is approximately 200 nm for a white light source. We report that sub-diffraction-limited objects can be resolved in immersion liquids using a microsphere optical nanoscopy (MONS) technique. Image magnifications and resolutions were obtained experimentally and compared in different immersion liquids. We show that a 100 μm diameter barium titanate (BaTiO(3)) glass microsphere combined with a standard optical microscope can image sub-diffraction-limited objects with halogen light in three different media: water, 40% sugar solution, and microscope immersion oil. In this paper, the super-resolution imaging performance has been described with the three immersion liquid types and the mechanisms are discussed with Mie theory calculation in the field of a Poynting vector.

Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays

Here we report strong enhancement in ultraviolet-photoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 μm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency.

Self-assembled dielectric microsphere array enhanced Raman scattering for large-area and ultra-long working distance confocal detection

Here we report enhanced confocal Raman detection with large-area and ultra-long working distance by capping dielectric microsphere array. Microspheres have been found to provide three channels for Raman scattering enhancement, including localized photonic nanojets, directional antenna effects, and whispering-gallery modes. The maximum enhancement ratio of Raman intensity is up to 14.6 using 4.94-μm-diameter polystyrene (PS) microspheres. Investigation on the directional antenna effect of microsphere reveals that the microsphere array confines electromagnetic (EM) waves to a narrow distribution with small divergent angles, by which the signal-to-noise ratio is retained and the offset of focal plane position from sample surface can be up to ± 7.5 mm. The present work reduces the requirement of focusing in confocal Raman detection and hence makes the large-area detection possible via rapid mapping. It opens up a simple approach for high-sensitivity Raman detection of 3D-structured surface.

Free-Standing Undoped ZnO Microtubes with Rich and Stable Shallow Acceptors.

Fabrication of reliable large-sized p-ZnO is a major challenge to realise ZnO-based electronic device applications. Here we report a novel technique to grow high-quality free-standing undoped acceptor-rich ZnO (A-ZnO) microtubes with dimensions of ~100 μm (in diameter) × 5 mm (in length) by optical vapour supersaturated precipitation. The A-ZnO exhibits long lifetimes (>1 year) against compensation/lattice-relaxation and the stable shallow acceptors with binding energy of ~127 meV are confirmed from Zn vacancies. The A-ZnO provides a possibility for a mimetic p-n homojunction diode with n+-ZnO:Sn. The high concentrations of holes in A-ZnO and electrons in n+-ZnO make the dual diffusion possible to form a depletion layer. The diode threshold voltage, turn-on voltage, reverse saturated current and reverse breakdown voltage are 0.72 V, 1.90 V, <10 μA and >15 V, respectively. The A-ZnO also demonstrates quenching-free donor-acceptor-pairs (DAP) emission located in 390–414 nm with temperature of 270–470 K. Combining the temperature-dependent DAP violet emission with native green emission, the visible luminescence of A-ZnO microtube can be modulated in a wide region of colour space across white light. The present work opens up new opportunities to achieve ZnO with rich and stable acceptors instead of p-ZnO for a variety of potential applications.

Laser crack-free cutting technique for thick and dense ceramics

A new laser crack-free cutting technique for dense ceramics with thickness of ≥ 10 mm based on beam continuous piercing full through the workpieces is presented. The techology can be concluded as a feasible tool for cutting of thick and dense ceramics in complex path, even for 3D cutting.

Sandwich-structure-modulated photoluminescence enhancement of wide bandgap semiconductors capping with dielectric microsphere arrays

Here we investigated the effect of substrate and film thickness on photoluminescence (PL) enhancement of wide bandgap semiconductor (i.e. ZnO) by dielectric microsphere array/luminescence film/substrate (MLS) sandwich structures. The PL enhancement channels in the sandwich structure were revealed, for the first time, including the focusing property of microsphere array (MSA) distinctly enhancing free-exciton recombination, anti-reflection effect of MSA increasing excitation cross-section area, MLS-supported TW-/SW-WGMs inducing ASE and Purcells effect, and optical directional antenna effect for high equivalent NA of objective lens as well as out-coupling efficiency. The enhancement ratio of intensity (ERI) for ZnO UV-PL from free-exciton recombination in the sandwich structure was found to be strongly dependent upon the refractive index of substrate and luminescence film thickness. In order to achieve high ERI for PL emission, the refractive index of substrate should differ from luminescence film and the film thickness needs to be chosen to support WGMs in the sandwich structure. The maximum ERI beyond one order of magnitude for ZnO UV-PL was therefore predicted theoretically and validated experimentally, where 11.25-fold UV PL enhancement ratio was achieved in ~650-nm-thick ZnO film grown on SiC substrate and capped with 5.06-μm-diameter MSA. The ERI could further be increased by improving above-mentioned enhancement channels. The present work provides a novel platform to manipulate light by low-loss dielectric microstructures for enhancing photon-matter interaction, which would be employed for other semiconductors achieving energy-saving luminescence and high-sensitivity photoelectric detection in future.

Porous silicon with double band photoluminescence fabricated by chemical-assisted picosecond laser irradiation

The fabrication of porous silicon with double band photoluminescence (PL) by chemical-assisted 1064 nm picosecond (ps) laser irradiation of polysilicon is presented. The hybrid method includes controlling of the laser scanning interval, number of scans to form dense micropores, and subsequently, short-time acid corrosion to form fine nanostructure. Along with the stable red PL visible to the naked eye, a new violet PL band at 432 nm existed in the fabricated samples. The double band photoluminescence was mainly attributed to the prepared micro/nano hierarchical structure. Moreover, dangling bond defects formed by high-energy photon impinging acted as the catalyzer in the subsequent acid corrosion resulted in good passivation of the nanostructure with the formation of Si-H and Si-O bond, which also contributed to the stable photoluminescence. The maximum surface porosity of the samples was concluded to be 90.48% which also bring good hydrophobicity for the prepared samples.

Parallel-axis positioning device for laser processing: Design and applications

A parallel-axis positioning device with “twice positioning” method is presented to exectly locate the laser spot at a special position on workpiece for laser macro-processing. Position accuracy of the laser spot on workpiece reaches 0.01mm.

Free-standing undoped acceptor-rich ZnO microtubes and their unique optical properties as ultrathin-walled microcavites

The acceptor-rich ultra-thin-wall ed ZnO microtubes were fabricated via a novel technique. It exhibited unusual properties in electronics and optics, e.g. rich shallow Zn-vacancy-related acceptors, temperature-dependent photoluminescence and wave-guided whispering-gallery-modes lasing.