Researches On Manipulation Of Optical Properties In Subwavelength Metallic Materials And Low-dimensional Nanostructures

In the last several decades, the semiconductor low-dimensional quantum structureshave become one of frontier research topics in condensed matter physics and materialscience as they exhibit some novel physical properties and potential application values.Simultaneously, the rapid advances and maturing in the ultrathin growth, such as molecu-lar beam epitaxy, and nanoscale lithography techniques have made it possible to fabricatevarious nano-devices. In this thesis, we deeply investigate the features of acoustic phonontransmission and thermal conductance in low-dimensional nanostructures and some use-ful results are obtained. It is expected to be helpful theoretically for the design and man-ufacture of quantum devices.At first, by using the continuum elastic approximation model and the transfer matrixmethod, we investigate the effect of diffusion layers and defect layer on acoustic phononstransport through the structure consisting of different films. Our work show that mostacoustic phonons can easily pass the structure, but some only have much less transmissionprobabilities and form corresponding dips in the transmission spectrum. With the changeof the structure parameters such as the width of diffusion layers and defect layer, thenumber of unit cell and the density of containing Al in diffusion layers and defect layer,the magnitude of the frequencies of acoustic phonons corresponding to the dips almostremain unchanged, but the transmission coefficients corresponding to the dips change atdifferent degree, and the transmission probabilities of some frequencies are very sensitiveto the variation of the above-mentioned structure parameters. These results can providesome references in controlling the transmission coefficients of acoustic phonons, devisingparts of acoustic apparatus and theoretical investigation related.Then, we investigate acoustic phonon transmission and thermal conductance in afour-perpendicularity-bend quantum waveguide at low temperatures using the scatteringmatrix method. The calculated results show some interesting features. The transmis-sion spectrum of the quantum waveguide displays a series of resonant peaks and dips;and when one of the bend heights is larger than or equal to the minimum of the dimen-sions of the phonon channel in the quantum waveguide, a stop-frequency gap will ap-pear; and some single four-perpendicularity-bend quantum waveguides with larger bendheights exhibit narrower width or smaller number of the stop-frequency gaps than thatwith smaller bend heights. When only the mode 0 is excited in the bend section, the transmission spectra can be seen to vary periodically with the bend heights. The ther-mal conductivity is much sensitive to the change of the smaller heights and longitudinallengths of the bend section; and the thermal conductivity decreases with the increasingof the temperature first, then increases after reaches a minimum. The investigations ofmultiple four-perpendicularity-bend waveguides connected in series indicate that the firstadditional four-perpendicularity-bend waveguide to the single one suppresses the trans-mission coefficient and forms stop-frequency gap; and two additional resonance peakswill be formed when each four-perpendicularity-bend waveguide is added in the series.The results could be useful for controlling thermal conductance artificially and the designof phonon devices.The acoustic phonon transport in a four-channel quantum structure is also investigated by use of the scattering matrix method. It is found that different acoustic phonon modes transport selectively into different channels, standing waves can be formed owning to acoustic phonons interfering with each other in the quantum structure, the transmission coefficients of acoustic phonon through different channels depend sensitively on the parameters of the structure, and the channels all exhibit the noninteger quantized thermal conductance at very low temperitures due to the splitting of the quantum structure. The structure may be used as a split device for acoustic phonon modes and controlling the acoustic phonon transport.Finally, we investigate the effect of two different kinds of double defects embeddedin a nanowire on acoustic phonon transport at low temperatures by using scattering matrixmethod. When acoustic phonons propagate through the nanowire, the total transmissioncoefficient versus the reduced phonon frequency exhibits a series of resonant peaks anddips, and acoustic waves interfere with each other in the nanowire to form standing wavewith particular wavelengths. In the nanowire with void defects, acoustic phonons whosefrequencies approach zero can transport without scattering. The acoustic phonons prop-agating in the nanowire with clamped material defects, the phonons frequencies must belarger than a threshold frequency. It is also found that the thermal conductance versustemperature is qualitatively different for different types of defects. At low temperatures,when the double defects are void, the universal quantum thermal conductance and a ther-mal conductance plateau can be clearly observed. However, when the double defectsconsist of clamped material, the quantized thermal conductance disappear but a thresholdtemperature where mode 0 can be excited emerges. The results both help people to knowsome important features on defects and give another way to control the transmission co-efficients of acoustic phonons and devise parts of acoustic apparatus.