Papers by Author: Ai Qun Liu

Abstract: An on-chip micro-droplet optical filter based on the evanescent wave coupling by fiber taper is presented in this paper. The optical filter consists of a T-junction for micro-droplets generation and an integrated optical fiber taper for light input and output. The light is coupled into the droplet resonator when it has the same wavelengths as the Whispering Gallery Modes (WGMs) of the droplet. The coupling efficiency can be adjusted by tuning the distance between the droplet and the fiber taper using micro fluidic flow. Compared with the solid sphere or ring optical filter using fiber taper coupling, the diameter and refractive index of liquid droplet can be easily tuned, which means the resonant modes can also be tuned. The diameter of the droplet is controlled by the flow rate ratio at the T-junction. The micro-droplet optical filter also has advantages as wide wavelength tuning range, easy fabrication process, simple manipulation, and low cost, which promise for many applications in optical analysis on micro-optical-fluidic chip.

Abstract: Efficient energy interconversion between light propagation and the localized field of light is important for highly-sensitive biological and chemical detectors. Solid optical tip can effectively enhance optical intensity for the measurement of nanoscale single molecule imaging. However, it lacks dynamic control mechanisms and is difficult to realize a smooth interface which may result in serious loss of scattering. Liquid can be dynamically controlled and the interfaces are optically smooth. Recently, liquid waveguides are reported to exhibit various advantages of dynamic, cheap and low optical loss. In this paper, a liquid optical tip in a microchannel controlled by flow rates is reported. In the design of the optofluidic chip, the core flow stream of the liquid waveguide is formed as Calcium Chloride (CaCl2) flow stream, and the cladding flow stream is formed as dioionzed (DI) water flow stream. The diffusion of CaCl2 between the microfluidic laminar flows establishes a gradient refractive index distribution to make an optical tip. For the optical system, laser source with central wavelength of 633 nm is used as input. The microchannel has a height of 80 µm and a width of 100 µm. The diffusion coefficient is 1 × 10-9 m2/s. The original refractive index of CaCl2 solution and deinized (DI) water are 1.442 and 1.332, respectively. The optical intensity at the optical tip is increased by 15 times sing Finite-Difference Time-Domain (FDTD) method. Thus, light be guided to form a sharp optical tip through the control of liquid. The on-chip optical tip has potential applications in biological, chemical and medical solution detectors.

Abstract: Optimization of micro-mixing components is vital for efficient micro-electromechanical systems (MEMS) and lab-on-a-chips. In this area, it is ideal to have a universal micro-mixer design for general purpose multiple-phase fluidic mixing. Numerical methods to compute and analyze the mixing process in the spiral micro-mixer design are presented. This spiral design has also been found to be efficient for intra-droplet mixing as well as continuous particle separation by utilizing its attributing Dean flows. Analysis on the mixing performance of this design serves to present Archimedes’ spiral as a robust solution and exemplifies the effects of minute yet effective alterations to the design.

Abstract: The dependence of the optical properties of spherical gold nanoparticles (AuNPs) on particle size and wavelength were analyzed theoretically using Mie theory, where the complex refractive index of gold was corrected for the effect of a reduced mean free path of the conduction electrons in small particles. The simulation results indicated that larger diameter AuNPs have high sensitivity and the resolution compared to the small diameter AuNPs. Two different fabricated methods for synthesis larger diameter AuNPs have been proposed. One is the directly synthesis 50nm AuNPs, the other method is separated to two steps (prepared 13nm AuNPs seeds, then synthesis 50nm AuNPs). Comparing the UV-vis spectra and SEM image of the AuNPs synthesized by these two method, we find that the symmetry of the directly synthesis method is better than the other method. The experiment results show that the synthesis AuNPs are uniform. The major aim of this work is to provide a simple and fast method to synthesis larger diameter AuNPs and research the optical properties of the nanoparticles.

Abstract: A packaging architecture is developed for MEMS tunable laser (MEMS-TL). The hybrid integration of chips is realized under high accuracy thermal control and precise alignment. The thermal conductivity of the laser chip has been improved up to 60% by the proper substrate design of device and epi-down bonding method. Under the analysis of heat flux, we employ a thermo-electric cooler (TEC) to stabilize the temperature. The substrate is designed to accommodate gain chips with minimal active optical alignment, thus reducing the packaging cost. With this packaging scheme, the insertion loss is greatly induced less than 1 dB due to the advantages of horizontal and vertical alignment in optical fiber fastening and laser diode bonding.

Abstract: In the broadband communication network, the wavelength-division-multiplexed (WDM) system is widely used to maximize the information that the signals can carry. As a result, the number of channels which are carried by different optical wavelengths in the WDM optical fiber network also keeps increasing. To separate the huge number of different wavelength signals, optical filter is required. The optical filter based on semiconductor has been widely studied due to the maturation of semiconductor fabrication technology and that it is possible to integrate the filter with the stable semiconductor devices such as laser diodes and MOSFETS. The tunable optical filter is basically a selective optical resonator that only allows the resonant modes passing through. Various mechanical methods are studied to achieve the tunable effect by tuning the physical structure of the filter; however, there is not much research on how the semiconductor material will affect the tuning function. In this paper, the author studied the influence of refractive index of the multi-silicon-slabs on the filter, whereby the tuning of refractive index is reached by thermal effect. It is found by simulation that when heating the silicon slabs, the increasing refractive index of silicon will lead to a shift of the resonant mode wavelength. This shift is almost linear with the change of the temperature, which is about 1nm with every 20K temperature increase. For certain devices, the result of the simulation showed it is possible to tune the resonant mode from C band to L band in the Fiber Optical Communication.

Abstract: The optical sensor achieves its sense function generally by measuring the change of intensity or the phase of the light beams. Compared to conventional sensor types, the optical sensor enjoys the advantages such as great sensitivity, wide dynamic sense range and multiplexing capabilities. In recent years, MEMS technology is widely used on the design of optical sensor due to the small feature of MEMS, which could be high sensitive with tiny change of the detected objective. In this paper, instead of observing the change of intensity and phase, the author studied how the shift of wavelength spectrum can be utilized to sense the physical change of the object such as the pressure, vibration, velocity and electrical field. The author designed and simulated a FP-resonator-like structure which consists of 2 resonant cavities with 2 multi-layer mirrors located on two ends and 1 multi-layer mirror in the middle of the two cavities. The mirror in the middle is movable by physical pressure. In the simulation, it was found that this structure is able to have certain wavelength light resonate inside the cavities. The resonant mode will shift 1nm when the mirror in the center moves forward/backward for every 14nm, which is sensitive enough to detect the small change of the objectives.

Abstract: This paper reports the development of a miniature pressure sensor on the optical fiber tip for in vitro measurements of rodent intradiscal pressure. The sensor element is biocompatible and can be fabricated by simple, batch-fabrication methods in a non-cleanroom environment with good device-to-device uniformity. The fabricated sensor element has an outer diameter of only 366 μm, which is small enough to be inserted into the rodent discs without disrupting the structure or altering the intradiscal pressures. In the calibration, the sensor element exhibits a linear response to the applied pressure over the range of 0 - 70 kPa, with a sensitivity of 0.0206 μm/kPa and a resolution of 0.17 kPa.

Abstract: Protein plays key role in cellular processes and diseases diagnosis [1-3]. This paper reports a label-free detection of proteins through localized surface plasmon resonance (LSPR) and realized Au nanoparticles (AuNPs) immobilized on glass slide. Glass slide is first sonicated with 5% v/v glassware detergent in ultrapure water for 30 mins, and then in ultrapure water for another 30 mins. A 5% v/v aminopropyltriethoxysilane (APTES) solution in ultrapure water is prepared and the glass slide is immersed in the 5% v/v APTES solution for 20 mins. The glass slide is sonicated for 60 mins in ultrapure water. The purpose of the sonication is to remove the multi-layers of APTES which are formed on the glass surface. The silanized glass slide is immersed in AuNPs solution for 40 mins. Finally, the protein solution is added on the surface of the slide. The surface absorption of proteins induces a significant difference of environmental refractive index around the AuNPs. The shift in the wavelength of the LSPR spectra is very sensitive to the change in the surface refractive index of the nanoparticles. The shifts in the LSPR spectra are primarily determined by the volatility and refractive indices of the protein species. In this paper, the LSPR-nanobiosensor is designed and fabricated. The LSPR band responses are measured by a real-time UV–vis spectrometer with a CCD array detector. The response time of the protein–LSPR spectrum is less than 3 seconds and the response is reversible and reproducible.

Abstract: As the field of Micro-Fluidic-Systems (MFS) is still at it early stage of development, only some simple optical devices such as liquid waveguide and liquid optical switches have been reported. The lack of in-depth analysis makes some novel photonic phenomena unexplained and uninvestigated in MFS. Solutions to these problems require a lot of imagination and ingenuity. In this paper, dynamic liquid optical splitters and interferometers have been demonstrated by diffusion according to flow rates are reported. In the case of the Y splitters, light from a single source can be split into two directions with angle-controlled dynamic. To the interferometers, light from a single source can be split and then combined to interference. The lengths of the interferometers can be controlled from via control of diffusion coefficient and flow rate. It implies that the Micro-Photonic-Fluidic –Systems may establish a new branches to MFS and has potential applications in biological, chemical and medical solution detectors.