The Study Of Silicon-Based All-Dielectric Metasurface And Light Emitters

In recent years,metamaterials and metasurface have been widely used in microwave,terahertz and optics due to their unique electromagnetic properties.Different from natural materials,the optical response in metamaterials and metasurfac is mainly determined by the shape,size and arrangement of the unit cells which constitute the structure.Therefore,people can almost freely manipulate light in metamaterials and metasurface and modify its phase,amplitude and polarization characteristics through reasonable structural design.Compared with the three-dimensional metamaterials,metasurface has the advantages of easier fabrication,smaller size,higher integration degree and flexible design.The early researches on metasurface mainly focus on the surface plasma devices based on metal materials.However,when the frequency that devices work in is blueshifted,especially in the near infrared and visible frequency,a series of negative effects caused by metal ohmic loss are hard to be ignored,which seriously affects the efficiency of the devices.At this time,some researchers turn their attention to low-loss all-dielectric materials such as silicon and silicon oxide,and the research on all-dielectric metasurface becomes the focus in the field of metamaterials.The design principle of all-dielectric metasurface is based on Mie scattering theory.According to their function,all-dielectric metasurface can be divided into two broad categories.One is used for wavefront control of light,the other is resonant-type dielectric metasurfaces.The former obeys general Snell's law,and it can achieve beam deflection,focusing and vortex light by introducing phase gradient on the surface of the incident light field.The latter can strongly enhance the resonance of electromagnetic field in the metasurface,and it can enhance the light-matter interaction in the metasurface through the high-quality-factor resonant modes and electromagnetic hot spots.The focus of this thesis is the resonant-type dielectric metasurfaces.Different kinds of high-quality-factor silicon metasurface structures are designed and fabricated.Combining the metasurface and gain medium,we achieve strong emission enhancement and lasing with narrow linewidth.The details of research are as follows:(1)A high-Q metasurface consisted of an asymmetric air hole array in a silicon slab was proposed.We analyzed the origin of Fano resonance and discussed the factors restricting the quality factor of the structure.Through the destructive interference of electric and magnetic dipole radiation,the radiation loss of the structure is reduced,thus improving the quality factor of the device.Moreover,the group effect of dipole resonance in all unit cells will reinforce the resonant behavior and finnaly leads to higher quality factor.(2)An asymmetric Fano metasurface embedded with Ge quantum dots was proposed and fabricated.Strong emission enhancement was achieved due to the high quality factor and ultra-small equivalent mode volume.Over three orders of photoluminescence enhancement was observed at 5K,accompanied by a record-high quality factor of 1011.In addition,the polarization characristics was studied and the dependence between array size and quality factor is explored to verify the coherent oscillation of electric and magnetic dipoles.(3)The relation between the high-Q resonance mode in metasurface with broken inplane symmetry and the bound states in the continuum was explored.We developed an analytical approach to describe light scattering by arrays of meta-atoms based on the explicit expansion of the Green's function of open systems into eigenmode contributions,and demonstrated rigorously that reflection and transmission coefficients are linked to the conventional Fano formula.We found that when the asymmetric parameter becomes zero,Fano resonance corresponds to a collapse,which means a symmetry protected BIC mode.Due to the deliberate asymmetry,the symmetry protected BIC in the system degenerates into quasi-BIC,while the quality factor of the mode is still very high.On this basis,the quasiBIC mode is used to enhance the photoluminescence of G-center defects in silicon material.Clear local stimulated emission in G-center defects is observed in the power dependence test.(4)Based on the deep insight into Fano resonance in the metasurface,a new type of ultra-high-Q metasurface was proposed,which is composed of a shallow-etched air hole array in a silicon slab.The duty ratio of this structure is very small,which can inhibit the influence of structural heterogeneity caused by proximity effect in the farication process.On the other hand,the asymmetry of this structure is introduced by the existence of the air hole,so the device performance is more dependent on the consistency of the hole size,which reduces the requirement for the shape consistency and greatly improves the fabrication tolerance.The high-quality-factor metasurface was used as the external cavity reflector of the fiber laser.The measured reflection bandwidth is about 60 pm,the extinction ratio is about 6.2 dB,the edge-mode rejection ratio is higher than 70 dB,and the linewidth is about 600 Hz.(5)Based on the design principle of ultra-high-Q silicon metasurface,a narrow-band dual-wavelength reflector with high reflectivity was proposed and fabricated by varying the period of air hole array in x and y direction.The fabricated sample was applied in a ringcavity fiber laser as an external cavity mirror.The stable output of the dual-wavelength laser was realized,the edge-mode rejection ratio is greater than 60 dB,the line width is 1.2 kHz.