Silicon-based Nanobeam Cavities For Optical Manipulation

Optical trapping provides a label-free,noncontact and noninvasive method for the manipulation and detection of particles and it is widely used in the biological and materials sciences.Convention free-space optical tweezers based on microscope made a great success in handling with microscale objects,but it is difficult for them to trap nanoscale objects due to the limitation of the natural diffraction of light.Optical near-field of photonic devices can provide higher electromagnetic energy gradient for trapping at the nanoscale.There are strong enhancement and confinement of optical field in photonic crystal nanobeam cavities owing to high quality factor and low mode volume,which greatly improves the trapping efficiency.This thesis mainly study some applications of silicon-based nanobeam cavity in optical manipulation.1.We established a theoretical model of the optical force for a particle trapped in the waveguide/silicon-based nanobeam cavity/waveguide system and systematic analysis of the effect of the properties of the cavity on the optical force.The theoretical analysis shows that the optical trapping force of the cavity is proportional to its QT1/2/V.The performance and optical force of silicon-based nanobeam cavities with different structure parameters are numerical investigated by finite element method(FEM).The numerical results are in accordance with theoretical analysis and show that the maximum optical trapping force for a particle occurs at the transmission of cavity around 0.25 and the maxima are almost the same for cavities with different periods.2.We proposed a hybrid microcavity composed of a silicon-based nanobeam cavity in a microring and applied it for optical trapping.The bandstructure of nanobeam cavity is calculated by finite-difference time-domain method(FDTD)and the resonance modes of the hybrid cavity is analyzed by finite element method(FEM).The optical force based on dipole force approximation for a nanoparticle is obtained and analyzed in detail when nanobeam cavity mode of the hybrid cavity is excited.The numerical results show the particle is stably trapped at the center of the nanobeam cavity.Then the performance and optical force of hybrid cavities with different structure parameters are numerical investigated by finite element method(FEM).The numerical results show the optical force is proportional to.(1-T)Q/V.Furthermore,the quality factor is up to 1.90 × 106,mode volume is as low as 1.92(?/nSi)3 and optical force is as high as-721 pN/mW,which is 12 times enhanced than that of a microring resonator with same radius,when there are just 15 pairs of holes in the nanobeam cavity and the radius of the microring is 3 ?m.3.We proposed a method to individually tune silicon-based nanobeam cavities by electron beam irradiation.Electron beam irradiation compacts the silica substrate,following by inducing a stress on the silicon layer.Both these two effects increase the refractive index,thus a red shift in the resonant wavelength can be observed.By measuring the transmission spectrum of the cavities before and after irradiation,it is shown that resonant wavelength shifts are proportional to the scanning time and the acceleration voltage.Furthermore,larger resonant wavelength shifts can be achieved by scanning the region where the electric field is highly localized.The measurement results show that the resonant wavelength difference can be reduced from 5.5 nm(before irradiation)to 0.4 nm(after irradiation),while the quality factor of the cavities can be maintained.