Generations Of Vector Optical Fields And Sensing Based On Photonic Crystal Cavities

Photonic crystal?PC?is a kind of artificial optical micro-structures with periodically varied dielectric constant.Specifically,the lattice period of PC is the in same order of scale with the optical wavelength.Therefore,PC could modulate light propagating inside it and form a photonic band structure due to Bragg reflection.Relying on the merits of abundant propagating modes in photonic band and strong light localization in the photonic forbidden band-gap,PCs could control light propagation behavior flexibly and realize effective light-matter interactions.By introducing a defect into the PCs,resonant cavity modes could be obtained with the frequencies located in the photonic band-gap.Generally,the defect regions of PC cavities occupy one or several lattice periods and therefore have cubic-wavelength mode volumes,enabling the effective control of light propagation and strong interaction with matter in a wavelength-scale.On the other hand,Bragg reflection induced by the photonic band-gap performs better than that of a total reflection,which promises resonant modes in the PC cavities with even higher quality factors.Hence,PC cavities have photons strongly confined for a long time in a small volume,and the optical fields in PC cavities can be greatly enhanced.In this thesis,the generations of vector optical fields and optical sensing based on PC cavities fabricated in a high refractive index dielectric slab are investigated.The main contents and results are follows:?1?The numerical simulation and experiment platforms for the PC cavities are developed.The calculated methods for PC's band structure and cavity's resonant modes are established based on a finite element technique?COMSOL Multiphysics?.The effects of defect structure,lattice constant,sizes of air-holes on the photonic band-structures,mode distributions,resonant spectra,and quality factors are discussed.A cross-polarized confocal microscope is built,which enable the measurements of PC cavity's resonant modes via the vertically scattering light.By controlling the half wave plate in the microscope setup,a conversion between Lorentzian-and Fano-lineshapes of the resonant mode is studied systemically.In addition,based on the strongly localized resonant modes of the PC cavity,a low-power continuous-wave?CW?pumped SHG in two-dimensional materials is demonstrated by integrating it with a PC cavity.?2?Generations of vector optical fields in PC cavities are investigated both in theory and experiment.A variety of PC cavities are designed.By analyzing the phase structures and polarization distributions of resonant modes using the finite element method,the generation mechanisms of vector optical fields are revealed.The far-field radiation patterns of the cavity modes are calculated via the Sommerfeld diffraction theory,the result shows vector optical fields can be generated in the far-field radiations.Different PC cavities in InGaAs/GaAs quantum wall slab are fabricated,including multiple missing-hole and ring cavities.A low-temperature fluorescence measurement system is constructed,and the fluorescence signals from the PC cavities with resonant peaks are explored.In addition,the stimulated emissions of the vector optical fields are analyzed.?3?Generations of vector optical fields in the photonic crystal fiber?PCF?are studied numerically relying on its flexibly adjustable structure.The field distributions and transmission characteristics of the modes in the PCF are calculated using the finite element method.Four vector modes are obtained in the mulit-core PCF.The designing concept of polarization-maintaining PCF is introduced into a six-core PCF.By shrinking air-holes in the cladding region and enlarging air-holes around the cores,the propagating mode of the fiber core has a specified polarization.In the six-core PCF with the designed polarization-maintaining design,azimuthally and radially polarized modes are obtained separately with low propagation loss and high mode selectivity.?4?By integrating active materials with PC cavities,high-performance optical sensors are achieved relying on the sensitive response of environment by the PC cavity,including sensings of temperature,humidity and organic vapor.With a 220 nm thick silicon slab,the evanescent field of the resonant mode would interact with the coated active materials strongly,which makes the resonant wavelength of the PC cavity very sensitive to the refractive index change of the materials.It is revealed that the silicon PC cavity exhibits a linear temperature-sensitivity of 70 pm/?from 30?to 100?by interrogating its resonant wavelength shift.In a PC cavity coated with720 nm thick PVA,a humidity sensor exhibits a linear spectrum-sensitivity exceeding129 pm/%RH over 40?90%RH,and promises a response time of 300 ms and recover time of 2.7 s.In a graphene oxide-PC cavity humidity sensor,a slope of 0.68nm/%RH in the range of 60%?85%RH is obtained,which presents a response time less than100 ms.A MoS₂-PC cavity sensor shows a highly selectivity to organic vapor and has a fast response time of 300 ms.