金属/介质薄膜中声子热辐射的空间和各向异性研究

基于理论、实验和仿真相结合的方式，着重研究了金属/介质(MD)薄膜中声子热辐射的空间特性和各向异性。声子是由于晶格振动产生的元激发，是物质的内在属性。尽管声子不易调控，但是声子与其他光学激发的耦合会产生奇异的光学现象。特别是红外到太赫兹范围内的光子与极性介质中的声子强耦合产生表面声子激元（SPhP）。SPhP具有强局域、低损耗等特点，与等离子体（plasmon polaritons）形成互补，使得深亚波长光学成为可能。为了进一步了解声子吸收的内在理论基础，首先通过黄昆方程和超晶格连续介电模型在理论上分析了声子吸收。实验上，主要以SiO₂声子作为研究对象，利用等离子体增强化学气相沉积(PECVD)方法，分别在Si/Al（150 nm）薄膜和Si衬底上制备出500 nm厚的SiO₂薄膜。基于傅里叶红外光谱仪(FTIR)，在垂直入射下得到热辐射光谱，通过热辐射光谱分析，并结合由时域有限差分算法（finite-difference time-domain，FDTD）计算得出的仿真光谱图，对比了MD薄膜结构和非MD薄膜结构中声子的热辐射，发现MD薄膜结构更能够有利于声子和SPhP的激发。根据Berreman效应，纵光学波（LO）声子只在倾斜入射时产生。光谱线没有呈现洛伦兹线型，因此，虽然LO声子在垂直入射时测得的热辐射图中不辐射，但同样影响横光学波（TO）声子辐射谱的线型。另外，利用FTIR对金属（Si/Al）/介质（SiO₂薄膜）进行热辐射转角测试，对热辐射转角图分析证明，Si/Al/SiO₂薄膜中SiO₂声子遵循LST(lyddano-sachs-teller)关系，纵横声子成对出现，且两种声子的空间辐射特性不同。改变偏振，发现在S偏振和P偏振下，声子热辐射呈现不同的模式, 体现出声子的空间各向异性。并且，声子与光子耦合可以激发SPhP，反过来，SPhP可以增强声子的吸收。基于MD结构，能够激发并调控SPhP和声子辐射行为，为红外器件的实现奠定了基础。

Study on the Space and Anisotropy of Phonon Thermal Radiation in Metal/Dielectric Thin Films

Based on the approach to combine theory, experiment and simulation, this paper highlights the space and anisotropy of phonons thermal radiation utilizing metal/dielectric (MD) structure. Phonons, the elementary excitation characterizing the vibrations of lattice, are the eigenstates of materials. Albeit phonons controlled difficultly, the couplings between phonon modes and other photonic excitations enable exotic optical phenomena. Notably, surface phonon polaritons (SPhP) emanate from the coupling between phonon modes of polar dielectrics and photons in the infrared to terahertz. SPhP is featuring tight electromagnetic field confinement, low optical loss, and complementary to those provided by plasmon polaritos, facilitates access to deep subdiffraction optics. Firstly, the paper theoretically analyzed phonons absorption based on the Huang-kun equation and superlattice continuous dielectric model to further understand the internal theoretical foundation of phonons absorption. Experimentally, the research object we took into account was SiO₂ phonons, and then, the 500 nm-thick SiO₂ thin films respectively were synthesized on Si/Al（150 nm）thin films and Si substrate utilizing plasma chemical vapor deposition( PECVD) approach. The thermal radiation spectra were obtained at normal angle, which fourier transform infrared (FTIR) implemented. Phonons thermal radiation spectra in MD structure and in the non-MD structure were compared by analysing thermal radiation spectra and simulation spectra calculated by finite-difference time-domain (FDTD), demonstrating that MD structure was more conducive to stimulate phonons and SPhP. Longitudinal optical (LO) phonons emerge merely at oblique-incident in accordance to Berreman effect. LO phonons was obviously non-radiation due to the thermal radiation spectra obtained at normal- incidence. However, it made a difference on the liner-shaped of transverse optical (TO) phonons. What’s more, from the metal（Si/Al）/dielectric（SiO₂ thin films）thermal radiation angle diagram of two kinds of polarizations, we could observe that the SiO₂ phonons in Si/Al/SiO₂ thin films abided by Lyddano-Sachs-Teller (LST) relation, LO phonons and TO phonons appeared in pairs, and the spatial radiation characteristic of the two phonons differed. In addition, the difference between the phonons modes appearing under S polarization and under P polarization existed, verifying the spatial anisotropy of phonons. Especially, the coupling of phonons and photons could stimulate SPhP, in turn, SPhP could enhance the absorption of phonons. Strikingly, the phonon modes and SPhP enable to be stimulated and tuned based on MD structure, which set the stage for the implementation of these appealing concepts in infrared optical devices.