Researches On Topological Phononic Crystal And Particle Manipulation In Acoustic Waves

In the 1980 s,the discovery of integer and fractional quantum Hall effects opened a new era to explore the novel properties of condensed matter.The importance of topological order has begun to be recognized and it has been used to describe the quantum states of a large class of substances.Topological insulators are great applications of topological order in condensed matter physics,and have become a hot topic in physics research during recent years.There are topologically protected edge states or surface states in topological insulators,which are robust with respect to the interference of various defects.Intriguing properties of these states have been widely studied in electronic and photonic systems.At the same time,the related research on topological physics in acoustic system has begun to flourish.In addition,contactless particle manipulation is also one of the hot topics in physics.Compared with light wave,sound wave has special advantages like wider range of wavelength and less damage to biological cells.Therefore,the research on acoustic particle manipulation has recently attracted much attention.In recent years,people have successfully suspended,captured,separated,aggregated and moved particles by using the special sound field produced by the probe or the local sound field produced by the artificial structure.On the other hand,it has been found that even in uniform sound field,the multiple scattering of sound waves by particles can make particles form special distribution.Based on the above background,in this paper,we study topological phononic crystals and acoustic manipulation of particles.Specifically,there are four innovation points as follows:1.Valley states and valley edge states in non-Hermitian phononic crystalsWe studied the valley states and the valley edge states in non-Hermitian phononic crystals.Non-Hermitian properties are introduced into ordinary phononic crystals with valley states and valley edge states by properly applying gain and loss elements to them.As a result,frequency in the band structure of phononic crystals isextended from real number to complex number.Unusual properties appeared in its valley states: one of the valley states is an amplified state,where the energy is obviously increased;the other valley state is a decayed state,where the energy is obviously weakened.At the same time,we verified that the valley edge state with topological protection exists in non-Hermitian systems as well,which can resist the interference of large-angle bending.We find that the energy of valley state and valley edge state changes exponentially with the change of non-Hermitian parameters.Taking advantage of this,we can easily adjust the energy of valley state and valley edge state by simply adjusting the gain and loss coefficient.2.Quantum spin Hall effect in non-Hermitian phononic crystalsWe studied the quantum spin Hall effect in non-Hermitian phononic crystals.It is proved that the edge states induced by acoustic quantum spin Hall effect can be generated in non-Hermitian systems.There is an exponential relationship between the energy of the edge state and the gain and loss coefficient,which can be easily controlled.When there is only loss effect in the system,there will be two attenuation modes in the bulk state of the quantum spin Hall effect.One of them is the attenuation-maximum state,the other is the attenuation-minimum state.Using this attenuation-minimum state,we can reduce the loss of sound wave propagation in the air.3.Topological bulk valley transport in phononic crystalsUsing layered phononic crystals,we have successfully constructed a structure with topologically protected bulk state.We couple trivial phononic crystal with the phononic crystal containing topological protected valley edge state,thus making the bulk valley-polarized state with topological protection appear in the whole structure.The bulk valley-polarized state can polarize the phononic crystals connected to it to a single-valley-polarized state.At the same time,we achieved the beam splitting of the bulk valley state by adjusting the rotation angle of the triangle,which makes it possible for the sound wave to propagate in particular channels.We verified that the bulk state has a strong topological protection property,and has a strong anti-interference ability for defects appearing at the boundary or inside the bulk,where sound waves can continue to propagate unidirectionally.4.Sound-induced particle interactions and stable configurationsThe self-organization effect of polystyrene particles induced by acoustic wave was observed experimentally.The plane wave is used to generate a uniform sound field,which induces the interaction between polystyrene particles.In our experiments,we observed that the interaction force induced polystyrene particles to produce two kinds of stable configurations: one is mechanically equilibrium and featured by nonzero interparticle separations,and the other corresponds to a closely packed assembly.We verify the correctness of the multiple scattering theory by measuring the spacing distribution of two particles and the spacing and angular distribution of three particles.At the same time,through a simple geometric model,we explain the probability of various configurations appearing in closely packed assembly.We hope that our research will contribute to the advances in the manipulation of particles using acoustic interaction forces.