Study Of Silicon-based Nanofluidics Based On Near Field Forces

Recent decades have witnessed the great advancement of modern biology and medical science at single cell level,especially the study of organelles and single molecule such as the expression of protein and the digest imaging of single DNA.Traditional technology like microfluidics is unable to handle such molecule level samples,which requires the more advanced version,nanofluidics.In particular,the tradiaitonal optical tweezers has once become the perfect driving force for nanofluidics due to its noninvasiveness,tiny footprint and parallel processing capability.But its performance is restricted by diffraction limit,short interaction length and the induced heat,which impede the effective manipulation on objects smaller than submicron.To eliminate such problem,another technology based on near field forces,including gradient force and scattering force,is proposed to manipulate quantities of smaller particles at a time.By desigining specific silicon waveguides,various kinds of functions for nanoparticles can be realized in nanofluidics chips,including trapping,transporting,sorting and the like.In this thesis,we have designed several waveguide structures for nano manipulation,namely the wavelength-dependent particle switching unit,the phase-controlled particle switching unit,the standing wave based particle conveyor belt,and the standing wave based particle flow control unit.At the meantime,we evaluate the performance of these structures from multiple angles,specifically theory,simulation and numerical calculation.Further,for the realization of our design,we contact with Suzhou Institute of Nano-Tech and Nano-Bionics(SINANO)and spend long time on exploring the fabrication process of waveguides.The thesis is divided into four parts:1.Proposed a particle switching unit based on wavelength division multiplexing(WDM).When the evanescent field around waveguide interacts with nanoparticles,the induced gradient force and scattering force will capture and transport particles along the waveguide.The trajectory of particle depends on the light path which varies with different wavelengths.Since multi-wavelength excitation has been widely used in silicon based nanofluidics for its convenient operation,we adopt the WDM structure for the switching of particles.Through optical simulation,we obtain the force distribution on particles and calculate the corresponding potential well to predict the trajectory of particle upon the waveguide.This WDM particle switching unit has very compact footprints which improve the efficiency of particle switching.Specifically,the 1×2 and 1×4 WDM structures are about 6 ?m and 20 ?m in length,respectively,so that it takes only a few seconds for particles to travel through them.In addition,our proposal is capable of multiple tasks which holds great promise in nanofluidics systems with high throughput and integration.2.Proposed a particle switching unit based on Mach-Zehnder interferometer(MZI).Though the particle swithching unit based on WDM is capable of multiple tasks by cascading,it is still inappropriate for large scale integration since each stage has to be customized for designated wavelength.Therefore,we propose this particle switching unit based on phase control and highly reduce the length of interferometer arms by introducing a ring waveguide.By electro-optic modulation to induce a refractive index change of about 8.00 ×10-4,the light path through MZI is successfully switched with extinction ratio of about 22 dB.With the input power of 40 mW,the structure can stably trap and transport PS particles with diameter down to 60 nm.Besides,the switching of particle is unaffected even when the interferometer arm is filled with particles,because the induced phase shift and power loss are negligible.Further,features like single wavelength excitation,electro-optic modulation and compact structure greatly improve the efficiency,integration and dynamic configuration capability of our particle switching unit,leading to application of such electro-opto-fluidic platform in nanofluidics.3.Proposed a stepless and high-speed conveyor belt for nanoparticles based on standing wave with position presition in nano scale.By thermo-optic modulation on a ring waveguide coupled to a Sagnac ring,the standing wave can move the trapped particles continuously and controllably which is infeasible in traditional photonic devices.Importantly,the introduction of ring waveguide improves the phase sensitivity of the reflected and input light to the refractive index change.As a result,the length of waveguide is reduced by hundreds of times and the thermal response is significantly increased.By numerically solving the equation of motion,we show that particles near the wavguide can be trapped into the antinodes within tens of seconds and will be moved by the standing wave with position precision in nano scale.Apart from this,the delayed response of particles to the fast moving standing wave can be exploited for particle sorting.This conveyor belt possesses compact dimensions and controllable optical hot spots that make it highly potential for precise manipulation in large scale integrated nanofluidic systems.4.Proposed a particle pausing unit and a particle flow control unit by introducing standing wave into MZI.On one hand,the particle pausing unit combines a Sagnac ring with a MZI in which the light path is switched to control the establishment and removal of standing wave.In this way,particles can be paused and released at will.In order to evaluate the stability of particle trapping,we define the trapping time through which we can decide the reflectance of light for particle pausing and releasing.On the other hand,the particle flow control unit is obtained by incorporating the standing wave based conveyor belt into MZI.Due to the irregular interference pattern in the coupling region of MZI,the behavior of particle cannot be understood directly.Instead,we illustrate the motion of nanoparticles intuitively by optical simulation and numerical calculation.In a highly integrated optofluidic system,our proposal can serve as not only a sample pausing unit at the middle level but also a quantificationally releasing unit at the source end,which has broad prospect in nanofluidics.In addition,with the support of SINANO,we have tried to fabricate the 1 x2WDM particle switching unit and the standing wave based particle conveyor belt.The pattern on the silicon layer is complete but the width of waveguide is significantly reduced due to the proximity effect of electron-beam lithography and the wall broadening of etching.Apart from this,there are many random islands in the etching region which might result from the impurity of photoresist.In a word,more work needs to be done for the exploration of waveguide fabrication in the future.