We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior.Our twisted bilayer photonic device,which has an operating wavelength in the C-b...We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior.Our twisted bilayer photonic device,which has an operating wavelength in the C-band of the telecom window,uses two crystalline silicon photonic crystal slabs separated by a methyl methacrylate tunneling layer.We numerically determine the magic angle using a finite-element method and the corresponding photonic band structure,which exhibits a flat band over the entire Brillouin zone.This flat band causes the group velocity to approach zero and introduces light localization,which enhances the electromagnetic field at the expense of bandwidth.Using our original plane-wave continuum model,we find that the photonic system has a larger band asymmetry.The band structure can easily be engineered by adjusting the device geometry,giving significant freedom in the design of devices.Our work provides a fundamental understanding of the photonic properties of twisted bilayer photonic crystals and opens the door to the nanoscale-based enhancement of nonlinear effects.展开更多
Remarkable geometrical similarities are found in digging claws of soil burrowing animals,in spite of the fact that they evolved independently.Based on convergent evolution theory,this study innovatively proposed a bio...Remarkable geometrical similarities are found in digging claws of soil burrowing animals,in spite of the fact that they evolved independently.Based on convergent evolution theory,this study innovatively proposed a bionic engineering perspective that focuses on general and analogous geometrical characteristics of soil animals.It was observed that soil animals with powerful burrowing ability have analogous serrated structures on their digging claws.Taking soil imprinting toothed wheel as the research object,the hypothesis that special serrated structures have the potential of reducing penetrating resistance from soil and enhancing digging efficiency for soil engaging component was investigated.The convergent evolution inspired bionic serrated structures were utilized for the design of cutting edge on toothed wheel.Then,a toothed wheel that mounted with the conventional tooth and a bionic tooth were manufactured and tested in the soil bin.Results showed that special bionic serrated structure could reduce the required draft force for toothed wheel;meanwhile increase the depth and volume of prepared micro-basin.It was found that the soil-penetrating mechanism of the bionic toothed wheel behaved as saw cutting that similar to the digging behavior of soil burrowing animals.Geometry of serrated structure has the ability to maximum stress concentrations in soil,thus increased the tendency of soil material to fail.These results indicate that the convergent evolution inspired bionic approach is novel and advantageous for the design of new soil engaging implements for working quality optimization and forward resistance reduction.展开更多
文摘We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior.Our twisted bilayer photonic device,which has an operating wavelength in the C-band of the telecom window,uses two crystalline silicon photonic crystal slabs separated by a methyl methacrylate tunneling layer.We numerically determine the magic angle using a finite-element method and the corresponding photonic band structure,which exhibits a flat band over the entire Brillouin zone.This flat band causes the group velocity to approach zero and introduces light localization,which enhances the electromagnetic field at the expense of bandwidth.Using our original plane-wave continuum model,we find that the photonic system has a larger band asymmetry.The band structure can easily be engineered by adjusting the device geometry,giving significant freedom in the design of devices.Our work provides a fundamental understanding of the photonic properties of twisted bilayer photonic crystals and opens the door to the nanoscale-based enhancement of nonlinear effects.
基金Thank for the financial support for this research project by the National Natural Science Foundation of China for Young Scholars(Grant No.51605210)by Scientific Research Project of Yunnan Provincial Education Department(Grant No.2018Y018).
文摘Remarkable geometrical similarities are found in digging claws of soil burrowing animals,in spite of the fact that they evolved independently.Based on convergent evolution theory,this study innovatively proposed a bionic engineering perspective that focuses on general and analogous geometrical characteristics of soil animals.It was observed that soil animals with powerful burrowing ability have analogous serrated structures on their digging claws.Taking soil imprinting toothed wheel as the research object,the hypothesis that special serrated structures have the potential of reducing penetrating resistance from soil and enhancing digging efficiency for soil engaging component was investigated.The convergent evolution inspired bionic serrated structures were utilized for the design of cutting edge on toothed wheel.Then,a toothed wheel that mounted with the conventional tooth and a bionic tooth were manufactured and tested in the soil bin.Results showed that special bionic serrated structure could reduce the required draft force for toothed wheel;meanwhile increase the depth and volume of prepared micro-basin.It was found that the soil-penetrating mechanism of the bionic toothed wheel behaved as saw cutting that similar to the digging behavior of soil burrowing animals.Geometry of serrated structure has the ability to maximum stress concentrations in soil,thus increased the tendency of soil material to fail.These results indicate that the convergent evolution inspired bionic approach is novel and advantageous for the design of new soil engaging implements for working quality optimization and forward resistance reduction.
