Theoretical Study On Multi-directional Deflection Of Subwavelength-diameter Optical Fibers

The interaction between light and matter is accompanied by the exchange of momenta,which is the internal reason for the generation of optical force.In general,the optical force is relatively small??p N?,but it can become very significant at the micro-and nanoscale.Subwavelength-diameter optical fiber has became an ideal research platform for optical force due to its excellent wave-guiding characteristics,light weight,and good flexibility.Previous studies have found that the excitation of quasi-linear polarized light only produced the optical force in a certain direction of the subwavelength-diameter optical fiber,thus the end of the optical fiber only produced a deflection in one dimension.This greatly limited the degree of freedom of the optical fiber deflection,thus affecting its application prospects in more fields.In this thesis,we have found a new method to extend the deflection freedom of the end of the subwavelength-diameter optical fiber.At the same time,through comprehensively investigated the optical force and optical force density inside the subwavelength-diameter optical fiber,we have revealed the cause of the multi-directional deflection in the end of the subwavelength-diameter optical fiber.Our research will provide a useful reference for constructing more flexible and controllable sub-wavelength optical-mechanical devices.The specific research contents of this thesis are as follows:1.The first chapter mainly introduced the research background and research significance of optical force and subwavelength-diameter optical fiber.Firstly,we briefly introduced the development history of optical force and its application scenarios,especially the application of optical force in micro-and nanoscale and waveguide.Secondly,we introduced the research progress of subwavelength-diameter optical fiber and its related application of optical force.Finally,we discussed the research purpose and significance of multi-directional deflection of subwavelength-diameter optical fiber.2.The numerical calculation method of optical force based on the finite-difference time-domain method were introduced in Chapter 2.3.In Chapter 3,the distribution of optical force and optical force density inside subwavelength-diameter optical fiber under the excitation of quasi-linear polarization and circular polarization were systematically studied.The results shown that under the excitation of the quasi-linear polarization fundamental mode,the subwavelength-diameter optical fiber was only affected by the optical force in a certain direction;and under the circular polarization fundamental mode excitation,we have observed for the first time that the subwavelength-diameter optical fiber was subjected to the multi-directional optical force?that is,the optical force in the new dimension is added?,which has different characteristics from the quasi-linear polarized fundamental mode excitation.By studying the optical field distribution and optical density distribution of the cross-section of the subwavelength-diameter optical fiber under circular polarization excitation,we have revealed the root cause of the multi-directional optical force being not zero under circular polarization excitation.4.In Chapter 4,the methods of increasing the multi-directional optical force inside subwavelength-diameter optical fiber were thorough studied.The results shown that by increasing the diameter of the subwavelength-diameter optical fiber and reducing the inclination angle of the fiber,the optical force in the new dimension direction of the subwavelength-diameter optical fiber can be effectively increased.What's more,this thesis found that by using the fundamental mode excitation of elliptic polarization and adjusting the ellipticity of the polarization of the input light,it is possible to significantly increase the new dimension optical force without changing the optical force in other directions.5.Chapter 5 studied multi-directional optical force of subwavelength-diameter optical fiber underTM₀₁ andTE₀₁ mode.By observing the linear optical force density curve inside the subwavelength-diameter optical fiber in two higher-order modes,it is pointed out that a hook-shaped deflection may occur at the end of subwavelength-diameter optical fiber under theTE₀₁mode.6.Chapter 6 summarizes the main work and innovations of this thesis,and at the same time prospects the follow-up research.