Research On The Application Of Graded-index Optics In Microfluidic Chips

Conventional optics are usually fabricated using solid processing techniques with very limited tunability.In contrast,liquid media can be easily reconfigured and replaced to achieve a great range of reconstruction and tunability in shape,refractive index,spectral absorption coefficient,and so on.In the early days,liquid media have been involved in some of optical devices with limited applicability,such as immersion microscopy,liquid crystal display,etc.These cases are incomplete,coarse and independent.Liquid media has not been systematically studied in optical devices.Microfluidics is a multidisciplinary field that performs precise manipulation and control of micro-fluids at micro/nano scales.The designed micro-structure and functional components are integrated into a chip through the traditional micromachining process,to achieve the driving,mixing,separation and other operations of the trace liquid,with less reagent consumption,low cost and fast fluid response.Also,it shows good controllability on the composition and spatial distribution of liquid.This has inspired the creation of optofluidic technology that combines microfluidic technology with optics to develop liquid-based tunable devices and systems.Due to the fluidity of the liquid,it always shows the gradual changes in optical properties,which are mainly classified into three categories according to the way that micro-fluids effect the light: temporal graded-index,spatial graded-index,and stepwise graded-index.In this paper,these graded-indices are respectively applied to different optical devices for developing new optofluidic devices and methods.Each chapter of this paper is briefly described below:The first chapter is the research basics of optofluidics.First of all,we briefly introduce the initiation and development of optofluidic technology,while the advantages and problems brought by the combination of microfluidic technology and optics are analyzed in detail.Secondly,the main methods for fabricating microfluidic chips and optical devices are summarized,and the possible combinations of them are pointed out.After that,the main differences between micro-fluids and macro-fluids are emphasized,and the gradient concentration distribution formed by dynamic equilibrium of convective diffusion is described in detail.At the end of this chapter,we classify the regulatory mechanism into three main categories according to the way that micro-fluids effect the light,summarize their respective features and application cases.On this basis,a general description of our research objectives is given.The second chapter is an optical application of temporal graded-index property.Based on the principle of phase-reversal zone plate,we design a solid-liquid hybrid zone plate in the microfluidic chip,which consists of an alternating half-wave zones of solid and liquid,and liquid exerts the phase modulation of lights.The refractive index of liquid is slowly changed by the microfluidic mixer,so that the zone plate periodically satisfies the condition of phase-reversal,thereby achieving the continuous adjustment of focusing properties of the zone plate.In the experiment,the focusing properties can be quickly switched between strong focusing,defocusing,and collimation.The characteristics of the focus,such as position,size,and intensity,also achieve a wide range of adjustments.Besides,the dispersion of the zone plate has also been proven to be tunable.The third chapter is an optical application of spatial graded-index property.Based on the diffusion of laminar flows,we create a novel hybrid waveguide in a simple micro-channel: a graded-index liquid-liquid waveguide at the front end and a step-index solid-liquid waveguide at the back end,which are naturally formed,transitioned and transformed by convective diffusion.We have successfully realized the self-imaging effect based on multimode interference in this waveguide with great tunability.It is found that the graded-index waveguide can not only effectively reshape the light source and eliminate the additional light diffraction,but also further change the multi-mode interference,resulting in controllable spatial modulation of the self-imaging pattern.Compared with the solid counterparts,this hybrid waveguide is very suitable for inplane optical applications on the microfluidic chip,with simple structure,easy processing and strong adjustability.The fourth chapter is an optical application of stepwise graded-index property.We use the microfluidic network as the reaction chamber,while the sample being tested and the indicator are continuously redistributed and color-developed through the microfluidic network,and the colored product forms a unique stepwise graded concentration profile.Based on this concentration profile,differential colorimetry is proposed for the fast nitrite detection.Unlike conventional colorimetric method,this method can quickly detect the nitrite concentration in one step,without the need of system's standard curve.The measured data has the self-calibration ability,which can effectively eliminate errors caused by bubbles,impurities in the micro-fluid.The differential method not only improves the detection accuracy and stability by several times,but also has strong resistance to system drift and change in absorption coefficient caused by various factors.In the fifth chapter,I will give a brief summary of my work during my Ph.D.time.The design procedure,working mechanism and performance of each optofluidic device are explained.Then,a perspective of the future work that extends my present research,is provide.