Valley Topological Transport of Lamb Waves in Solid Phononic Crystal

Author:Yan Mou

Supervisor:liu zheng zuo li feng


Degree Year:2019





In the past three decades,research on artificial structural materials,including photonic crystals(PTC),phononic crystals(PC),sub-wavelength metamaterials for electromagnetic waves and sonic waves,have made great progress.With the birth and development of topological band theory,the concepts such as Quantum Hall Effect and Quantum Spin Hall Effect have also been introduced into the study of artificial structures,causing extensive research interest and making many breakthroughs.Among them,valley topology materials have attracted more and more attention.Similar valley-like dispersion relationship are common in two-dimensional PTC and PC.Valleys in momentum space can provide additional freedom.Due to the protection of the topological properties of the valleys,there are valley edge states that are resistant to scattering transmission at the interfaces of different valley topological phases.Considering the small attenuation of Lamb waves in solid systems,the research of elastic valley topological materials is of great significance.It can be used to prepare various surface acoustic wave devices with excellent performance that has application value in many fields such as communication,sensing and non-destructive testing.This paper mainly studies the topological transport properties of Lamb wave in solid PC,including the following three parts:1、Combined Micro-Nano processing technology,we fabricate an on-chip elastic valley PC.A triangular-arranged periodic structure elastic PC plate is designed on a thin silicon wafer,and the symmetry of the system can be broken by a simple rotating scatterers to form energy valley near the K point.In contrast to the valley states in the PC with fluid backgrounds,which carry the clockwise or anticlockwise vortices in the interstitial spaces of the lattice,the valley states appear here as a distribution of gyros,with each triangular pillar convoluting around its vertical central axis.Using micro-etching technology,we fabricate an on-chip elastic valley PC.The displacement and phase information is obtained by the Doppler laser vibrometer.We clearly observed the valley-like dispersion relationship in the momentum space.It is confirmed by simulation that the chiral source can realize the selective excitation of a single valley state in the PC according to the distribution characteristics of the intrinsic field of different valleys.With the directional angle of the scatterers is gradually changed from negative to positive,the Dirac point opens,closes and reopens,accompanied by the chirality of the upper and lower valley states reverses,indicating that the system has undergone a topological phase change during this process.2、We explore the robust properties and anomalous current partition phenomena of the valley edge states.Due to the protection of the valley topological properties,there are edge states at different valley topological material interfaces which resistant to defect scattering.We made the valley PC on the silicon wafer,and directly observed the field distribution and dispersions of the edge states by the laser measuring vibrometer.It is confirmed that the edge state has the ability to resist large corner scattering by measuring the "Z" type interface structure.In addition,we study the influence of disorder and defects at the interface on the edge states.There is little effect on the transport of the valley edge state even if the structure of the scatterers is slightly deformed or inconsistent in size during sample processing.Finally,our experiments confirmed the anomalous current partition of the edge states,that is,alternating the topological properties of different valleys to form a four-channel topological interface intersection.The smaller the angle between the channels,the stronger the edge state coupling,and the energy tends to be transmit toward the channel with a sharp angle.3、We studied the transport of the valley edge state in a bendable elastic PC plate,and designed a three-dimensional cylindrical PC waveguide to achieve multi-channel transport.The topological properties of elastic PC plates are studied in a two-dimensional up to now.Here we study the topological properties of the bending PC plates,especially the transport properties of the edge states.We study influence of the disorder and defect structure at the curved interface on the edge state transmission by simulation.As a possible application example,we designed a cylindrical surface waveguide device that can realize multi-channel information transmission without affecting each channel,which improving the space utilization of the waveguide device.