Hanyang Li

Tapered optical fiber waveguide coupling to whispering gallery modes of liquid crystal microdroplet for thermal sensing application

We demonstrate efficient coupling to the optical whispering gallery modes (WGMs) of nematic liquid crystal (NLC) microdroplets immersed in an immiscible aqueous environment. An individual NLC microdroplet, confined at the tip of a microcapillary, was coupled via a tapered optical fiber waveguide positioned correctly within its vicinity. Critical coupling of the taper-microdroplet system was facilitated by adjusting the gap between the taper and the microdroplet to change the overlap of the evanescent electromagnetic fields; efficient and controlled power transfer from the taper waveguide to the NLC microdroplet is indeed possible via the proposed technique. We also found that NLC microdroplets can function as highly sensitive thermal sensors: A maximum temperature sensitivity of 267.6 pm/°C and resolution of 7.5 × 10-2 °C were achieved in a 78-μm-diameter NLC microdroplet.

Synthesis of TiO2-loaded silicate-1 monoliths and their application for degradation rhodamine B

TiO2-loaded silicate-1 monoliths, with TiO2 (Degussa P25) nanoparticles loaded in silicate-1 crystal, were directly synthesized by combining sol–gel method and hydrothermal method. TiO2 nanoparticles were well dispersed in the system and had an intimate contact with silicate-1 zeolite. The physicochemical properties of all samples were characterized by XRD, FTIR, SEM, nitrogen sorption, microstructure, and UV-vis diffuse reflectance spectra. X-ray diffraction and SEM images showed that silicate-1 was synthesized and the surfaces of silicate-1 particles were coated with nanoparticles. UV-vis diffuse reflectance spectra, microstructure and TEM images exhibited that TiO2 nanoparticles were effectively loaded in silicate-1 crystal. BET analysis showed that all samples kept high surface area and micropore size distribution. They also allowed the quick adsorption of reaction molecules outside the surfaces and inside the micropores and showed a high efficient degradation when it is used in the oxidation of rhodamine B (RhB).

Single-fiber tweezers applied for dye lasing in a fluid droplet

We report on the first demonstration of a single-fiber optical tweezer that is utilized to stabilize and control the liquid droplet for dye lasing. In order to trap a liquid droplet with a diameter of 15-30 μm, an annular core micro-structured optical fiber is adopted. By using wavelength division multiplexing technology, we couple a trapping light source (980 nm) and a pumping light source (532 nm) into the annular core of the fiber to realize the trapping, controlling, and pumping of the oil droplet. We show that the laser emission spectrum tunes along the same size as the oil droplet. The lasing threshold of the oil droplet with the diameter of 24 μm is 0.7 μJ. The presented fiber-based optical manipulation of liquid droplet micro-lasers can be easily combined with the micro-fluidic chip technology and also may extend the application of optical fiber tweezers for micro-droplet lasing technology in the biological field.

Dispersing upconversion nanocrystals in a single silicon microtube.

Nanocrystals of Ln3+ (Ln = Yb, Tm and Ho) doped β-NaLuF4 with average diameter about 200 nm are dispersed in silica-based microtube (MT) by a simple flame heating method. The fabricated microtube has a diameter range from 2 μm to 30 μm and lengths up to hundreds microns. The fluorescence of upconversion nanocrystals (UCNCs) can propagate along a single MT and couple into another MT through evanescent field. The guiding performance of the single UCNCs doped MT is measured to prove that it can be used as an active waveguide. Moreover, optical temperature sensing based on the single UCNCs-MT is also demonstrated, and the sensitivity of UCNCs-MT is significantly enough for thermometry applications in the range of 298–383 K.

Humidity-insensitive temperature sensor based on a quartz capillary anti-resonant reflection optical waveguide

A compact, humidity-insensitive, fiber optic temperature sensor based on a quartz capillary, antiresonant reflecting optical waveguide was proposed and experimentally demonstrated. The transmission spectral responses of the proposed sensor were experimentally investigated regarding temperature variation and environmental humidity. Resonant dips exhibited temperature sensitivity as large as 201 pm/°C from -30 to 45 °C in a humid environment. By coating a sufficiently thick gold film onto the sensor surface, the humidity cross-sensitivity issue was effectively resolved. This proposed sensor was anticipated to find potential applications in humid environments, and moreover, immunity to humidity-sensitivity ensures its applicability in marine environments.

Effect of Double-Atom Vacancy Defects on the Elastic Properties of Single-Layered Graphene Sheets

A molecular structural mechanics approach is used to study the effect of double-atom vacancy defects (DAVD) on the elastic properties of zigzag and armchair single-layered graphene sheets (SLGS). To this end, the space frame structure is also adopted to model the interatomic forces of the C–C bonds. The numerical simulation results obtained via the finite element method strongly suggest that double-atom vacancy defects reduce the elastic module of SLGS, which effect weakens with graphene size. Finally, the elastic modulus and Poisson’s ratio of SLGS are found to decrease with the number of DAVDs.

Multiscale Modeling-Based Assessment of Elastic Properties of SLGS-Polymer Nanocomposites with Double-Atom Vacancy Defects

In this study, which is a continuation of our earlier work, the effect of double-atom vacancy defects (DAVD) on the elastic properties of single-layered graphene sheets (SLGS)-polymer nanocomposites is assessed by the multiscale modeling. According to the latter approach, the polymer matrix is modeled by finite element approach, while the SLGS and interphase layer are simulated at the atomistic scale by the molecular structural mechanics approach. In view of the Lennard–Jones potential concept, it is assumed that the SLGS and polymer matrix are related by van der Waals reciprocity. The fact that the elastic modulus of a polymer with 5% volume fraction of SLGS is increased by 17 times is demonstrated by numerous simulation results. It is also shown that the elastic modulus of SLGS-polymer nanocomposites with DAVDs is deteriorated with an increase in the number of DAVDs and improved with the increased volume fraction of SLGS.

Dispersing upconversion nanocrystals in PMMA microfiber: a novel methodology for temperature sensing

This paper reports the synthesis of a β-NaYF4:Yb3+/Tm3+ phosphor by a thermal decomposition method and focuses on the fabrication of microfibers by the co-doping of nanocrystals with PMMA solution via a facile drawing method. The structural characteristics of the nanocrystals are studied by XRD and TEM techniques. Meanwhile, the optical properties of the microfibers are probed by wave guiding performance and upconversion spectroscopy. With the excitation of a 980 nm laser source, the microfiber presented blue upconversion emission of Tm3+ ions. The fluorescence intensity ratio (FIR) method is utilized for the non-thermally coupling transition (1D2 → 3F4 (452 nm) and 1G4 → 3H6 (476 nm)) levels to carry out the optical thermometry. The maximum sensitivity is recorded at 298 K and is 0.00157 K−1. The results suggest that the microfibers have potential applications in thermometry with high sensitivity.

Concentration dependent optical transition probabilities in ultra-small upconversion nanocrystals

Transition probability is of vital importance for luminescence process, whereas the effects of doping concentration have not been explored in the Er3+:NaGdF4. In this work, we investigate the radiative transition probabilities of Er3+ highly doped NaGdF4 sub 10 nm nanocrystals using J-O theory. It is found that the transition probabilities vary with changing Er3+ concentration, especially altering the ratio of Er3+ 2H11/2 to 4S3/2 level, which is highly useful for optical thermometers as they are thermally coupled. To validate the concentration dependent transition probabilities, significant enhancements of upconversion luminescence are achieved by epitaxial growth of the inert shell, and thermal sensing behaviors are investigated using the improved samples.

Thermal sensing based on whispering gallery modes in tapered-fiber-coupled liquid crystal microdroplets

We report the efficient coupling of optical whispering gallery modes (WGMs) in liquid crystal microdroplets suspended in immiscible aqueous environment. Individual nematic liquid crystal (NLC) microdroplet is confined at the tip of a microcapillary used to generate the microdroplets and coupled through a tapered optical fiber waveguide positioned in the vicinity of the microdroplets. Efficient coupling of WGMs is observed in the NLC microdroplets with a diameter of 50–150 μm. In addition, the wavelengths of the WGMs can be tuned by temperature, making such NLC microdroplets suitable for thermal sensors. A temperature sensitivity of 0.244 nm/°C is achieved in a 75-μm-diameter microdroplet. The estimated thermal resolution of the microdroplet sensor is 8.2 × 10−2 °C.