Regulation Of Thermal Radiation For Optoelectronic Confrontation Via Dielectric Properties And Micro/Nano Structures

With the military optoelectronic technology grows day by day.the optoelectronic confrontation technology also develops rapidly.It is a vital passive jamming way to change the radiation properties by optoelectronic confrontation materials.Microstructure.composite coating and smoke screen are three important methods.All-dielectric metasurface resonator is a kind of microstructure with excellent ability of thermal radiation regulation.Wide bandgap semiconductors such as 4H-SiC and wurtzite GaN support surface phonon polaritons(SPhPs)with low optical loss.which greatly enhances the control ability of microstructures on radiative intensity.spectrum and direction.It is of great significance to study the influence of temperature and structure size on the thermal radiation of metasurface resonators.The addition of metal particles in the coating can effectively adjust the surface radiation of the target.The geometric parameters such as the shape,size and orientation of the pigmented particles have an important influence on the radiative properties.The smoke screen interferes with the radiative transfer of electromagnetic waves,whose transmission performance is affected by particle size and concentration.At present the lack of infrared dielectric functions of SiC and GaN at high temperatures limits their application and development in infrared optoelectronic confrontation technology.The effects of temperature and structure size on the excitation and coupling of polaritons of metasurface resonators are not clear.The effect of particle size distribution on transmission performance of SiC smoke screen is not clear.The composite coating is usually assumed to be uniformly distributed and isotropic in the literature,ignoring the influence of anisotropic scattering of particles on the radiative properties of the coating.The effect of particle orientation on the ratio of absorptivity to emissivity of the coating has not been thoroughly explored yet.The dielectric function of materials is the foundation of radiation property calculation.Experimental measurement is a direct way to obtain the dielectric functions of solid materials.The infrared dielectric functions of 4H-SiC and wurtzite GaN crystals are measured by spectroscopic ellipsometry under different temperatures.The results show that as the temperature increases from 300 K to 800 K,the amplitudes of the real and imaginary parts of the infrared dielectric functions of 4H-SiC decrease and the linewidth and Reststrahlen band broadens,which greatly reduces the propagation length of SPhPs by more than half.The main reason is that the phonon relaxation lifetime is significantly reduced at high temperatures,which disturbs the SPhPs’ hybridization.As GaN crystal is heated from 250 to 800 K,the peak value of the imaginary part for ordinary dielectric function is significantly reduced by 71.49%.The linewidth broadens and the Reststrahlen band varies little.The propagation length of SPhPs is significantly shortened.indicating the strong influence of lattice vibration on the SPHPs.Theoretical calculation is helpful to understand the underlying physics of experiments.It can break through the limitation of experiments which generally can not be carried out in hightemperature conditions,and can provide an effective prediction of the high-temperature dielectric functions.The phonon dispersion relationship is calculated by the first-principles method,and the influence of temperature on the phonon relaxation lifetime is discussed.The results predict the phonon dispersion relation well and confirm that the strong coupling of coherent atom’s motions with incident photons leads to the formation of SPhPs.High-order anharmonicity terms greatly influence phonons’ dynamical scattering at elevated temperatures,which verifies that high temperatures lead to the strengthening of anharmonic phonon scattering and obvious decline of phonon relaxation lifetime.So the amplitudes of semiconductor dielectric functions decrease and the linewidth broadens.which explains the trend of measurement results.Using the measured high-temperature dielectric functions of semiconductors,the influence of temperature and structure size on the excitation and coupling of polarizations of wide bandgap semiconductor metasurface resonators is studied by computational electromagnetics.The regulation mechanism of thermal emission intensity and spatial coherence of metasurface resonators is revealed.The results show that several electric resonance modes emerge due to the coupling between propagation and local SPhPs.And they gradually weaken with increasing temperature due to the shortened phonon relaxation lifetime.Moreover,the height of nanopillars and filling factor are shown to greatly tailor the resonant optical absorption of 4HSiC nano-resonators.There is an extra dip located around 522 cm-1 in the pp-polarized reflection spectroscopy of nano-porous GaN thin films,which suggests the excitation of the localized surface phonon resonances.The SPhPs of 4H-SiC metasurface resonators induce the coupling between optical modes and dominate the regulation of coherent thermal emission.The geometric period and temperature affect the excitation and coupling of each emission mode.The increase of the period enhances the spatial coherence of the emission but weakens the temporal coherence.High temperature weakens the excitation of surface waves of materials and weakens the coherence of thermal emission.The period and temperature mainly adjust the coherence of thermal emission by changing the effective propagation length of surface waves.The geometric parameters such as size of particles in smoke screens and coatings are important parameters for regulating the transmittance of smoke screens and the absorption and emission ratio of coatings.The influence of particle size,concentration and size distribution on the transmittance of SiC smoke screen is studied by the Monte Carlo method using the measured room-temperature dielectric functions.The results show that the particle size distribution has a great influence on the transmittance of SiC smoke screen.The radiative properties of aluminum sphere and flake are calculated by Mie theory and geometric optics.respectively.A two-flux model considering anisotropic scattering is established to calculate the absorption and emission characteristics of composite coatings.The regulation mechanism of geometric parameters such as particle shape,size and orientation angle on the radiative properties of the coating is studied.Moreover.the effects of binder type,particle volume fraction and coating thickness are also investigated.The results show that the ratio of absorptivity to emissivity of the coating pigmented with aluminum flakes is larger than that pigmented with aluminum spheres,and the ratio of absorptivity to emissivity of the coatings is smaller when the diameter of the sphere is larger.The ratio of absorptivity to emissivity of the coating increases first and then decreases with the increase of particle volume fraction.The coating thickness has a great influence on the radiative properties of the coating pigmented with aluminum flakes.For aluminum flakes,the ratio of absorptivity to emissivity of PMMA coating is the largest,while that of TPX coating is the smallest among the selected four types of resins.The type of resin has little effect on the ratio of absorptivity to emissivity of the coating pigmented with aluminum spheres.The ratio of absorptivity to emissivity of coating can be regulated in the range of 0.48-1.69 by adjusting the flake orientation.The average absorptivity and emissivity of the coating increase significantly when the orientation angle of aluminum flakes exceeds 45°.The ratio of absorptivity to emissivity reaches a minimum value at the orientation angle of around 45°,and decreases with the increase of the volume fraction of aluminum flakes.In this thesis,the effects of the temperature on the infrared dielectric functions of wide bandgap semiconductors and the coupling of optical phonons and SPhPs of microstructure are studied by experiments and multi-scale simulations.The regulation of the radiation characteristics of metasurface resonators via temperature and structure size,as well as the regulation of composite coatings via geometric parameters such as particle shape,size and orientation,are revealed.The transmittance of SiC smoke screens is also studied.This thesis provides a theoretical guide for the design and implementation of optoelectronic jamming means.