All-dielectric Subwavelength Gratings Enhance Linear And Nonlinear Optical Properties Of Two-dimensional Materials

With its unique optical,electrical and mechanical properties and broad application prospects,two-dimensional materials have attracted widespread attention in many fields such as optics and materials.Compared with traditional optical materials,two-dimensional materials have excellent material properties such as atomic-scale thickness,ultra-high carrier mobility,strong interaction between light and matter,and excellent mechanical properties,making them widely used in various micro-nano photodetector.Due to the intrinsic atomic-scale thickness of a monolayer two-dimensional material,most of the light irradiated on it is directly transmitted and reflected,resulting in weak optical absorption and second harmonic generation.Therefore,improving the light absorption and second harmonic generation of two-dimensional materials will greatly improve its application prospects in micro-nano optoelectronic devices and nonlinear optical devices.As a simple artificial micro/nano photonic structure,all-dielectric subwavelength grating has a mature preparation process and supports a variety of resonance modes in structure internal.At the same time,it solves the problems of unfavorable thermal effect caused by metal ohmic loss and limiting the upper limit of light absorption of two-dimensional materials.Therefore,it is widely coupled with two-dimensional materials and used to enhance the optical properties of two-dimensional materials.In this paper,by designing an all-dielectric sub-wavelength grating structure,using rigorous coupled wave method and finite element method,the influence on the optical absorption and second harmonic generation of two-dimensional materials is systematically studied.The main content of the full text is as follows:(1)?The theoretical method for calculating the subwavelength grating structure is analyzed,the rigorous coupled wave method is introduced.According to the formula,the calculation program is written,which provides a theoretical basis for the design of the enhanced absorption structure.The finite element method for calculating nonlinear optical effects is introduced.The second harmonic generation in the structure is calculated by introducing the concept of second-order nonlinear polarization,which provides a theoretical basis for the design of enhanced second harmonic generation.(2)?The ultra-narrow band perfect absorption of monolayer Mo S₂ in visible light band is realized by using the coupling effect of guided mode resonance of all-dielectric subwavelength grating.By changing the overall scaling factor of the structure,a large range of tunability of the perfect absorption peak can be achieved.In addition,the structure can also achieve perfect absorption effect on other two-dimensional materials.(3)?Design an all-dielectric sub-wavelength grating structure that satisfies the guided mode resonance conditions at both the fundamental frequency and the doubled frequency,we theoretically achieved four orders of magnitude enhancement of the second harmonic conversion efficiency.Through the overall scaling structure,we can adjust the wavelength of the second harmonic generated in a wide range.At the same time,due to the double resonance effect,the second harmonic generated has a unidirectional radiation effect,which provides a theoretical basis for the realization of miniaturized directional high-efficiency frequency converters.