Micro/nano Manipulation And Fluorescence Detection Technology In Integrated Optofluidic Chips

With the development of medicine and biochemistry,the demand for manipulating precision has reached the level of cells or even molecules,such as the molecular-level dose control.Due to the decrease of the sample size,the conventional operation methods can no longer achieve the required accuracy,so the microfluidic technology emerges.The mechanisms used in microfluidics includes mechanical contact,electricity,acoustics,optics,and so on.The technique of applying optical methods to control micro or nano particles is also called optofluidcs.Optical tweezer is one of those methods using optical force to manipulate tiny particles.Traditional optical tweezers produce strong enough optical force to trap particles by focusing laser beam to a micro-level light spot.But Traditional optical tweezers demands huge and complicated equipment,and because of diffraction limit,the radius of the light spot cannot be reduced infinitely,which makes it hard to control submicron particles.Besides,traditional optical tweezers can only control one or several samples at a time,and the thermal effect of the highly focused light spot may damage the sample or decline the trapping effect.Another kind of optofluidic method developed in the recent decades based on waveguides,can overcome the diffraction limit,because it utilizes near-field optics.Using the evanescent on the surface of the waveguide fabricated on the chip,it is easy to realize the control of submicron-level particles,and batch manipulation of a large number of particles can be realized.Also it avoids thermal effects.The most important is that the whole structure are fabricated on a chip,so it is stable,tiny,and easy to integrate with other functional units.Based on the near-field optical force,we proposed a controllable particle trapping and release structure and studied it by FDTD algorithm.The structure can shift between two states without losing control of the particles,and is able to realize manipulation of a single particle.Our structure designed here requires only a monochromatic source,which makes it convenient to integrate with other functional units.That produces infinite possibilities for optofluidics.The structure is designed by cascading on output port of a ring assistant Mach-Zehnder interferometer(RAMZI)with a Sagnac loop.By thermal-optical modulation,the refractive index of the ring can be changed by 4.3×10-4,making the output of the electromagnetic waves shifting between the two ports of the RAMZI.When it is in release state,the waves in the waveguides outputs from the port not cascaded the Sagnac loop,so the evanescent waves will drive particles moving along the waveguides,until released.As for trapping state,the waves are guided to the other port of the RAMZI,and the Sagnac loop there induces a 20%reflectance.That produces standing waves in the waveguide,and the 'hot spots' of the standing waves are discrete potential wells for the particles loaded on the waveguides,which will trap particles.We calculated the optical force using Maxwell Stress Tensor(MST).Then we simulated the motion of the particles,considering drag force of the fluid and Brownian movement.The result shows that if the diameter of the particle is similar to the cycle length of the standing waves,the standing wave cannot trap it.That is because the gradient force applied on one particle is balanced in that condition,while the scattering force still exists,which drives the particle moving along the waveguides.At last,we demonstrated that our structure is able to realize manipulation of a single particle.Gallium nitride(GaN)is a well-known light emitting semiconductor material,but in fact,it can also be used to fabricating waveguides in visible light range,because it almost do not absorb visible lights.Based on that,we designed a GaN waveguides structure on optofluidic chips.It can be used for fluorescence excitation and detection.These GaN waveguides are fabricated on sapphire substrates,utilizing slot mode.The nanoparticles can be trapped in the slot and excited to fluorescence,and the fluorescence can be collected to the slot waveguides.We used FDTD algorithm to simulate the working state of our structure,and calculated the optical force applied on the samples,so that we can find the most possible position the samples will stay.Then we simulated the fluorescence collecting process by placing three dipoles whose polarization orientation are in three mutually perpendicular directions,we got a collecting efficiency of 22.05%.Also we designed a wavelength division multiplexing(WDM)structure to eliminate the source light.First a V-shaped structure guided the lights in slot mode to the strip waveguides,heading a directional coupler(DC).By adjusting the coupling length of the DC,we eliminated 99.23%light of the source,making the detection easier.Besides,because GaN itself is a good semiconductor material,we may integrate the light source and the detection system to the chip,realizing active and portable fluorescence detection system.Our design may open a new door for fluorescence detection and optofluidics.In summary,in this paper,we designed two kinds of on-chip waveguide structures.The first on can achieve the capture and release of individual particles at the sub-micron level,and has a very good application prospect in molecular-level measurement control.The second structure is designed for fluorescence excitation and collection,achieving a good fluorescence collection rate,and promising to enable future portable,active bio fluorescence detection.