Enming Zhao

Whispering gallery mode temperature sensor of liquid microresonastor

We propose and demonstrate a whispering gallery mode (WGM) resonance-based temperature sensor, where the microresonator is made of a DCM (2-[2-[4-(dimethylamino)phenyl] ethenyl]-6-methyl-4H-pyran-4-ylidene)-doped oil droplet (a liquid material) immersed in the water solution. The oil droplet is trapped, controlled, and located by a dual-fiber optical tweezers, which prevents the deformation of the liquid droplet. We excite the fluorescence and lasing in the oil droplet and measure the shifts of the resonance wavelength at different temperatures. The results show that the resonance wavelength redshifts when the temperature increases. The testing sensitivity is 0.377 nm/°C in the temperature range 25°C-45°C. The results of the photobleaching testing of the dye indicate that measured errors can be reduced by reducing the measured time. As far as we know, this is the first time a WGM temperature sensor with a liquid state microcavity has been proposed. Compared with the solid microresonator, the utilization of the liquid microresonator improves the thermal sensitivity and provides the possibility of sensing in liquid samples or integrating into the chemical analyzers and microfluidic systems.

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.

Dual-truncated-cone structure for quasi-distributed multichannel fiber surface plasmon resonance sensor.

We propose and demonstrate an effective method to adjust the dynamic range of a fiber surface plasmon resonance (SPR) sensor by introducing a multimode fiber-sensing probe with a dual-truncated-cone (DTC) structure. When the grind angle of the DTC structure increases, the dynamic range redshifts. Based on this result, we fabricate a quasi-distributed two-channel multimode fiber SPR sensor by cascaded-connecting a DTC-sensing probe of 14° grind angle and a traditional transmitted multimode fiber (TMF)-sensing probe in the same fiber. The corresponding sensitivities of two sensing probes are 3423.08 nm/RIU and 2288.46 nm/RIU. By using this quasi-distributed multichannel fiber SPR-sensing approach, we may improve the detecting accuracy by extracting, calibrating, and compensating for the signals caused by the nonspecific bindings, other physical absorptions, and temperature changes in detecting samples, truly achieving dynamic detection in real-time. The excellence of this multichannel fiber SPR sensor is that the sensitivity of each subchannel-sensing probe stays unreduced after it is cascaded-connected in the main-channel fiber; the sensor is based on the multimode fiber, which is inexpensive, accessible, and convenient to be universalized in applications.

A SPR sensor based on twin-core fiber

We propose and demonstrate a novel fiber surface plasmon resonance (SPR) sensor based on a twin-core fiber (TCF). We grind the TCF tip into a frustum wedge shape, and plate a 50nm sensing gold film on the end face, two 500nm reflected gold films on the side faces of the wedge. We launch light source into the core of the TCF by using the high accuracy three-dimensional adjusting mount and microscope objective system. This SPR probe can be combined with microfluidic chip, and realize the real-time monitoring of the refractive index (RI) sensing of flow liquid in the microfluidic channel. The probe successfully monitors the refractive index of liquid ranged from 1.33 to 1.37 and the average sensitivity reaches to 5213nm/RIU in the solution.

Phase transformation and controllable size of γ-Al2O3 nanocrystals through Li doping using sol–gel method

ABSTRACT The effects of Li, as the ‘additive’, on the structures of nano-sized Al2O3 were investigated. Li evidently increased the crystallite size of γ-Al2O3, whereas it showed evident effects on neither LiAl2(OH)7 precursors nor α-Al2O3 nanocrystals. α-Al2O3 and γ-Al2O3 co-existed in samples calcined at 1000 °C and 1100 °C, and the composition can be adjusted by altering Li concentration. When the calcination temperature reached 1200 °C, high purity of α-phase Al2O3 formed and Li showed no effects. In addition, mechanisms of doping Li on the structrure of Al2O3 were discussed.

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.

Localized surface plasmon resonance sensing structure based on gold nanohole array on beveled fiber edge

This paper proposes a simple, stable, sensitive, and angle-dependent localized surface plasmon resonance (LSPR) sensing structure based on multi-mode optical fiber. We adopted the template transfer method to integrate a nanohole array onto a fiber tip with beveled angle. Experimental results indicated that beveled angle structured probe sensor outperform the flat optical fiber tip structured LSPR sensor in our experiment. We tested the sensitivity and the figure of merit (FOM) of the probe beveled angle from 5°-22°, with refractive index ranging from 1.333-1.385, to find that sensitivity and FOM were optimal at fiber tip bevel angle of 7°, reaching 487 nm/RIU and 29 respectively.

Light-induced thermal convection for size-based microparticle sorting

Separating and sorting of micrometer-sized particles in liquids are important subjects in current analytical chemistry, colloidal chemistry, environmental chemistry, and biological technology. In this paper, we propose and demonstrate a microparticle separating and sorting method based on light-induced thermal convection (LITC). The thermal convection generated as a result of frequency-dependent absorption of optical energy plays an important role in microparticle trapping and manipulation. By using the LITC, we can separate and sort microparticles of different sizes. Manipulation of microparticle sorting is noncontact, which contributes to keeping the natural characteristics of the microparticles. In addition, compared with traditional sorting methods, the microparticle sorting method based on the LITC effect is much easier to implement and convenient to manipulate. This possibility adds a light-induced thermal convection effect as an additional degree of freedom to microparticle separating and sorting.

A novel method to generate a self-accelerating Bessel-like beam based on graded index multimode optical fiber

We propose and demonstrate a transverse self-accelerating Bessel-like beam generator based on a graded index multimode optical fiber(GIF). The single-mode fiber and the graded-index multimode fiber are spliced with a defined offset. The offset Δx and the GIF length L affect the final properties of the Bessel-like beam, here we choose the offset Δx=20μm and the GIF length L=430μm to be optimal. The beam accelerates along the designed parabolic path up to 250μm in z direction and 40μm in x direction, the curvature of bending is 16% (40μm/250μm, x/z). This transverse self-accelerating Bessel-like beam generator based on the graded index multimode optical fiber constitutes a new development for high-precision micro particles experiments and manipulations because of its simple structure, high integration and small size.

A method to gather/arrange particles based on thermal convection

We propose a novel method to gather or arrange multiple micro particles by using the thermal convection effect in the water. We fabricate the fiber tip to be a nonadiabatic-tapered shape and then plate a gold film on the fiber tip. The gold film coated on the fiber tip absorbs the light output from the fiber and then generate lots of heat in the water, which causes the thermal convection. The convection forces bring the micro particles moving towards the fiber tip where the temperature is much higher. By using this thermal convection effect, we can realize the multiple micro particles gathering or arranging quickly, easily and simply.