Infrared Thermal Emission Mediated By Polar Materials

Thermal emission,as a common and easily overlooked phenomenon in life,is prominent in many military and civil applications including infrared camouflage,thermal management,energy harvesting and infrared detection.In recent years,with the rapid development of micro-nano photonics,research on various micro-nano structures(e.g.Fabry-Perot cavities,gratings,photonic crystals and metasurfaces)is becoming mature.These structures are utilized to manipulate thermal emission,which could improve the miniaturization of the devices as well as provide more choices and freedom for the spectral and spatial control.Metal is the most commonly used material in the current study.However,the broadband thermal emission induced by the high optical loss in metal is impossible to be ignored.Polar material,featuring as a new type of material in the infrared and terahertz regime,has become a better substitute.It behaves like metal in some special wavelength ranges named as Reststrahlen bands(charactered by a negative dielectric permittivity).Moreover,its internal phonons have the merits of long lifetime and low optical loss,which could support narrowband thermal emission.In addition,polar material has high subwavelength confinement and thus reduces the thickness of micro-nano structures.However,due to the rich resonant modes in polar materials,some undesired parasitic radiation peaks can be observed in the thermal emitters based on polar materials,rendering a large amount of energy waste.In addition,the reported ultranarrowband thermal emitters have the drawback of either large thickness or complex nanopatterning process.Finally,thermal emitters based on polar materials still remain at the level of static control,and hence their optical responses can only be modulated through redesigning and refabricating.However,most of the reported dynamic control methods(e.g.electrical,optical,mechanical and magnetic control)are volatile.In light of the above,this thesis theoretically and experimentally demonstrates several approaches to achieve the static and dynamic spectral control of thermal emission based on two types of polar materials-isotropic crystal SiC and anisotropic crystal ?-MoO3.1.Regarding the static spectral control of thermal emission based on isotropic polar material SiC,a single-peak narrowband thermal emitter is obtained by combining SiC with periodic metallic(Au)grating array.On the one hand,low-loss SiC could provide narrowband thermal emission.On the other hand,Au gratings enable the redistribution of the surface charges in SiC and thus excite transverse dipole mode.Single-peak narrowband thermal emission over a broad spectral range between 2.5?m and 25?m can be achieved,which suppresses parasitic peaks at both ends of the Reststrahlen band and improves energy efficiency.The central wavelength lies at around 11.2?m with the emissivity up to 0.94.The quality factor is around 19.Moreover,the top Au layer is replaced with the refractory material Mo and the working temperature is increased up to 600?.This paves the way for the design of infrared detectors,thermal sources and thermophotovoltaics systems.2.Regarding the static spectral control of thermal emission based on anisotropic polar material ?-MoO3,an ultrathin narrowband thermal emitter is obtained by designing a bilayer structure consisting of ?-MoO3 and metal(Au)reflective mirror.?-MoO3 shows anisotropy along three crystalline directions due to the different lattice vibrations.Along the out-of-plane crystalline direction z[010],Berreman mode can be excited at 9.9 ?m(near the longitudinal optical phonon frequency)with the dielectric permittivity close to zero.The measured quality factor can reach up to 164;along the in-plane crystalline direction x[100]and y[001],two different modes can be found(transverse optical phonon mode and Fabry-Perot cavity mode)with the central wavelength varying with the crystalline direction;all of these above modes can maintain stable radiation performances over a wide viewing angle from 1 °to 60°.Meanwhile,this device meets the requirements for ultrathin layer(on the order of hundreds of nanometers)as well as simple fabrication(thin-film deposition and mechanically exfoliation only).Berreman mode and transverse optical phonon mode/Fabry-Perot cavity mode can operate as the measurement wavelength and reference wavelength respectively in nondispersive infrared detecting technology for ozone gas.Furthermore,the natural in-plane birefringence property of ?-MoO3 provides the possibility to control the polarization of thermal emission via a planar configuration.3.Regarding the dynamic spectral control of thermal emission based on isotropic polar material SiC,a tunable thermal emitter is obtained by depositing the phase-change material(GST)thin film onto a SiC substrate.By thermally switching GST from amorphous state to intermediate state and then to crystalline state,the dynamic and continuous control of emissivity from 0.3 to 1 is obtained over the wavelength range from 11.1?m to 12.5?m.The extinction ratio is up to 10.7 dB.This emission performance is insensitive to the viewing angle.Moreover,the non-volatile nature of GST without external power sustaining has greatly reduced the energy consumption.This structure is promising in infrared camouflage,active thermal management and thermal sources.At the end,we suggest more possible future directions.Meanwhile,we analyze and prospect the development and applications of thermal emitters based on polar materials.