Multi-refractive Index Particles Capture And Information Detection Based On Single Fiber Optical Tweezers

Optical tweezers have been an important tool since it was discovered in 1986 by Ashkin.Three-dimensional trapping of transparent particles with a higher refractive index than the surrounding medium（so-called high-refractive particles）was first demonstrated many years ago with strongly focused Gaussian beams of light.This type of trap is known as optical tweezers.Because of its ability to manipulate and isolate microscopic objects in a precise and non-destructive manner,it has found a wide range of applications in different fields such as acoustics,interface science,biology,and biophysics.However,low-refractive-index particles are repelled by the focusing point of conventional optical tweezers and are usually captured by a light field with a dark cage.The capture and manipulation of microscopic particles with refractive indices below that of solution is of great importance in applications such as colloidal physical chemistry,targeted drug delivery,and ultrasound contrast agents.Moreover,the simultaneous manipulation of high and low refractive index particles is important for the synergistic study of colloids,hollow glass microspheres,biological cells and other particles.In this thesis,we have successfully achieved the capture of low refractive index biological cells and the simultaneous capture and information detection of high and low refractive index particles by combining optical fiber probe structure and mode multiplexing technology.First,the theoretical models and calculation methods of the optical forces on different particles are analyzed by the principle of optical tweezers technology for the different particles manipulated,providing theoretical support for the subsequent simulation and experimental studies.Meanwhile,the structural properties of the mode field of LP₀₁ and LP₁₁ modes and the excitation principles are analyzed,and the specific excitation methods of the beams of the two modes are studied.Second,in order to simplify the device for trapping low-refractive-index particles,a single fiber-based dark trap is proposed to achieve stable trapping of low-refractive-index biological cells.When the incident laser is focused by a specially structured fiber optic probe drawn by the thermal fusion method,a dark trap is generated in the axial direction to achieve stable trapping of low refractive index biological cells.In addition,this hot-melt drawn fiber optic probe is not only simple and inexpensive,but its all-fiber optic structure can be used as a micro-optical probe for in situ characterization and manipulation of biological cells with refractive indices lower than that of the base fluid.Finally,a single-fiber optical tweezer based on the coexistence of LP₀₁ and LP₁₁modes is proposed to achieve the simultaneous capture of high and low refractive index particles because the current study of simultaneous capture of high and low refractive index particles using an all-fiber structure is not sufficient.And the F-P（Fabry-Perot）interference structure is combined to achieve the detection of different particle displacements.The structure of the special fiber probe was constructed according to the different optical field properties of different modes to achieve not only multiplexed capture near the fiber tip,but also capture of particles with different refractive indices.After that,the signal changes of different particles at the moment of arrest were detected by using the F-P interference structure.The proposed optical trap can simultaneously trap particles with different refractive indices,which makes the manipulation of cells or molecules with different properties and the exploration of multi-molecular interactions easier and beneficial for biologically and chemically relevant studies.