Phononic Transport And Manipulation In Low Dimensional Artificial Microstructure

Phononic transport in low dimensional systems is an intriguing research in condensed physics.Through artifical microstructure,phononic transport can be well manipulated.On one hand,the study of phonons for heat conduction can improve our understanding of the nanoscale thermodynamics.Related researches may have potential applications on new thermoelectric materials,integrated circuit heat dissipation and so on.On the other hand,long-wavelength phonons give rise to acoustic waves and can be manipulated by acoustic metamaterials.Related researches may have potential applications on acoustic devices.In this thesis,we have three sections:the former two are about phononic transport and quantized thermal conductance in low dimensional ionic systems,while the third one is about extraordinary acoustic transmission through acoustic metamaterials in the ultrasonic regime.Firstly,we study the phononic transport and manipulation in ionic composite polymers,and theoretically realize multimode quantized thermal conductance tuned by electric field.The proposed composite polymer consists of three segments,where an ionic polymer is introduced as its central part and the non-polar random n-mer polymers are designed on the left and the right parts,respectively.We demonstrate theoretically that when external electric field is applied on the composite polymer,the dispersion curve of phononic torsion mode is tuned.Thereafter,multiple phononic channels are shut down one by one with increasing external electric field.As a result,the multiple-step quantized thermal conductance tuned by external electric field is realized.The investigations may have potential applications in thermal manipulation and information transfer in mesoscopic phonon systems.Secondly,we focus on the resonant phonon channels tuned by electric field in ionic correlated random systems.We consider two situations,one with doped polar cluser of particles and the other one with doped single polar particle.For the first situation,we study heavily and lightly doped random ionic n-mer systems and find there are two kinds of resonant modes in the system when the electric field is applied.One originates from delocalization of phonon in the correlated random system and exists in both heavily and lightly doped system.While the other one originates from mode matching induced by electric field and only exists in heavily doped system.The two kinds of resonant modes will have blue shifts with increasing electric field.Besides,there exists upper cut-off frequency,which will cut the resonant modes when they exceed the limit with increasing electric field.For the second situation,doped single polar partcile can couple with electric field and induce down cut-off frequecy,which can manipulate the resonant phonon modes originating from correlated random cluster.Thereafter,multimode quantized thermal conductance tuned by electric field can be realized.These researches may have potential applications in designing filter and waveguide for phonons.Thirdly,we have experimentally and theoretically studied extraordinary acoustic transmission through subwavelength gratings in the ultrasonic regime,and find that extremely high and flat transmission of acoustic waves within multiple frequency bands can be achieved under optimal angle incidence.In general,the acoustic transmission properties depend on the coupling of diffractive surface waves and the waveguide modes.For small incident angles,the FP resonance mechanism dominates the resonant transmission peaks.For optimal oblique incidence,the impedance matching mechanism plays an important role.Under this condition,broadband full transmission(100%)can be obtained in the long wavelength range above the first Wood’s anomaly,while in the shorter-wavelength ranges below the first Wood’s anomaly,the grating has lower transmission due to the diffraction effect,but the transmission pattern still consists of nearly flat segments separated by the Wood’s anomalies.Under the condition of low absorption,the transmission efficiency is nearly insensitive to the thickness of the grating.Furthermore,parallel stacking many gratings to obtain a sonic crystal,the broadband full transmission still exists in the long wavelength range for optimal oblique incidence.The effective acoustic impedance of such a sonic crystal is the same with the single-layer grating in the long wavelength range.These researches may open up a field for various novel applications of acoustic gratings,including broadband sonic imaging and screening,grating interferometry,antireflection cloaking and acoustic tunable filter.In summary,we study phononic transport and manipulation in low dimensional ionic systems,which theoretically realize multimode quantized thermal conductance tuned by electric field.These researches can extend our understanding of the nanoscale thermodynamics,and imply new approach to manipulate heat transfer in mesoscopic systems which may have applications in designing novel thermal-conducting and phononic devices.Besides in the ultrasonic regime,we study subwavelength acoustic gratings and find multi-band flat transmission under optimal incidence.Related mechanism exploration may pave the way to broadband design of acoustic metamaterials.Based on this,tunable research may have potential applications to design new acoustic devices.