Research On Ultra-low Mode Volume And Silicon Nanobeam Cavity For Optical Sensing

Silicon photonic devices are playing an increasingly important role in many applications such as optical interconnection and optical sensing.Among them,silicon nanobeam cavity has attracted great attention due to its advantages such as high quality factor,small structure size,low mode volume,and no free spectral range.This thesis proposes two silicon nanobeam cavities based on silicon-on-insulator(SOI)platform,one is a bowtie nanobeam cavity with ultra-low mode volume,and the other is a suspended multi-slot nanobeam cavity with very high sensitivity.This thesis first introduces the basic concepts,basic theories,model types,research status and applications of silicon nanobeam cavities.Two numerical simulation methods for studying silicon nanobeam cavities are introduced:finite-difference time-domain methods and finite element methods.Taking the air-mode silicon nanobeam cavity as an example,this thesis describes the design and the fabrication process flow of silicon nanobeam cavity.Moreover,the vertical fiber coupling setup for device characterization is described.We design and fabricate an on-substrate bowtie photonic crystal(Ph C)cavity in silicon.By optimizing the bowtie shapes in the unit cells of the Ph C cavity,the maximum of the electric field can be confined in the bowtie tips.Due to such confinement,an ultra-low mode volume of?0.1(?/n_Si)³ is achieved,which is more than an order of magnitude smaller than the previous on-substrate nanobeam cavities.Benefitting from the Gaussian-shaped field profile within the cavity,an optimized Q factor of 1×10⁶ in simulations and 1.4×10⁴ in fabricated devices has been achieved.The total size of the bowtie silicon nanobeam cavity is only 26.2?m×0.8?m.Furthermore,thermal-optical nonlinear effect of the bowtie silicon nanobeam cavity is explored.We propose and design a silicon suspended multi-slot nanobeam cavity sensor.By optimizing the structure of the silicon nanobeam cavity,most of the light can be distributed in the lower index region;thus,the sensitivity can be dramatically improved.Q factor of our cavity is 2.3×10⁴,the mode volume V_m is 1.7(?/n_si)³.Using the finite element method to simulate the characteristic frequency of the sensor under different refractive indices,the sensitivity of the device is?820nm/RIU.Furthermore,the total sensing area of the silicon suspended multi-slot nanobeam cavity sensor is only 24?m×1.6?m.