Study on the Space and Anisotropy of Phonon Thermal Radiation in Metal/Dielectric Thin Films
DONG Xin, ZHANG Xia, SUN Xue-bo, YUAN Shuang-xiu, XU Hui, SU Fu-fang*
School of Physics and Physical Engineering, Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Qufu Normal University, Qufu 273165, China
Abstract：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.
董 欣，张 霞，孙学博，元双秀，徐 慧，苏富芳. 金属/介质薄膜中声子热辐射的空间和各向异性研究[J]. 光谱学与光谱分析, 2021, 41(08): 2404-2408.
DONG Xin, ZHANG Xia, SUN Xue-bo, YUAN Shuang-xiu, XU Hui, SU Fu-fang. Study on the Space and Anisotropy of Phonon Thermal Radiation in Metal/Dielectric Thin Films. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2021, 41(08): 2404-2408.
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