Thermal Effect And Trapping Properties Of Silicon-based Optical Tweezers

Precious metal nanostructures are widely used in the field of optical nanotweezers technology due to their unique surface plasmon resonance characteristics,which can enhance near-field energy and break through diffraction limits.However,precious metal materials have large dissipation and light absorption in the visible and near-infrared regions,the photothermal effect produced by the optical tweezer structures made by them will greatly affect the stability of the trap,which is not conducive to trapping.In recent years,studies have shown that nanostructures made of high refractive index dielectric materials can also produce optical resonance and break through diffraction limits.Therefore,this paper replaced precious metal materials with silicon material as the main material of optical tweezers,and designed a silicon-based optical tweezer structures of double nano columns plus nano ring.The detailed simulation and experimental analysis of optical properties,photothermal properties and trapping properties of the structure were carried out in this paper.In this paper,we firstly introduced the basic principle of optical tweezer and the basic properties of optical resonance in nanostructures.Secondly,we explained the related calculation methods and theories.Then,the optical properties of the silicon-based optical tweezer were analyzed by numerical algorithm of three-dimensional frequency domain finite element method,and the enhanced field distribution are obtained（Electric field enhancement is 8.76 times,magnetic field enhancement is 4.23 times）.We also comparatively analyzed the near-field distribution in the gold-based optical tweezer（Electric field enhancement is 16.1 times,magnetic field enhancement is 4.37 times）.It was found that the near-field enhancement effect of the gold-based structure is higher than that of the silicon-based structure under the same incident light field intensity.Thirdly,by controlling the two structures under the same ability in trapping polystyrene spheres（d=20 nm）,we obtained the required incident light field intensity(I_Si=36.32mW/μm²,I_Au=6.67 mW/μm²).Under the respective incident light field intensities,the temperature changes and thermal convection of the two structures under steady state were obtained.The temperature of the silicon-based structure only increased by 0.83 K,the thermal convection velocity was 3.2×10^-3 nm/s;and the temperature of the gold-based structure increased by 29.7 K,the thermal convection velocity was 5.3 nm/s.It can be seen from the simulation results that the thermal effects of the gold-based structure are much larger than that of the silicon-based structure under the same trapping capability,so the silicon-based structure has better capture stability under high light field intensity.Finally,through the quantitative analysis of the trapping force,trapping potential energy,trapping stiffness of polystyrene spheres by silicon-based structure and its trapping experiment,the detailed trapping properties are obtained.The results show that the structure of the silicon-based optical tweezer can trap the polystyrene particles well.