Broadband Mid-infrared Absorptions And Their Mechanisms Of Artificial Structured Thin Films

Due to many unique electromagnetic properties,the mid-infrared spectral region has received increasing interest in the the fields of fundamental research,and applied military and civilian technologies.Mid-infrared absorbers enable the effective collection and utilization of mid-infrared light,which are of potential use in the areas of infrared detection,infrared and thermal energy harvesting,radiative cooling and camouflaging.In recent years,artificial structured thin films have attracted extensive attentions due to their designable optical properties and relatively simpler fabrication.This dissertation has studied broadband mid-infared absorptions and their mechanisms in artificial structured thin films.Broadband mid-infrared absorbers including dielectric-metal multilayered thin film,tunable vanadium dioxide composite multilayered film and porous silicon film,were demonstrated.The mechanisms of broadband mid-infrared absorption and its dynamic tuning were also revealed.These research results are valuable for understanding the fundamental interaction between artificial structured materials and light,and hold great promise for the development of high-efficiency mid-infrared functional devices.The main results obtained in this dessertation include:1.Based on the principle of impedance matching,a mid-infrared broadband absorber made of dielectric-metal multilayer was designed and fabricated.The impedance matching method has been widely used in traditional microwave Salisbury screens,and in broadband absorbers recently reported in the near infrared and visible regions.In this dissertation,a new combination of Si3N4 and Ti multilayer was developed to extend the near-perfect broadband absorption of planar multilayered structure to the mid-infrared range.The multilayer structure consists of four layers made of Si3N4(400 nm)/Ti(40 nm)/Si3N4(500 nm)/Cu(300 nm),whose effective impedance was matched with vacuum in wavelength range of 2.2?6.2?m.As a result,reflection on the structure surface was suppressed to be near zero,and the incident infrared light was absorbed by the conductive Ti metal layer.Our experimental measurements showed that the multilayer structure absorbed more than 85%of the incident light in the wavelength range of 2.2?6.2?m,which was independent of the polarization of incident light.The broadband absorption was also shown to be insentitive for incident angles of less than 45o.The demonstrated dielectric-metal multilayered absorber does not require any lithographic patterning and thus has the advantage of simpleness in fabrication.2.Based on the phase transition of vanadium dioxide,a dynamic tunable mid-infrared broadband absorber made of multilayered thin film was demonstrated.Dynamic control over infrared absorption is of great value for infrared modulating and infrared camouflaging applications.In this dissertation,phase change material vanadium dioxide was introduced into the multilayered thin-film absorber designed with impedance matching method.Two types of absorbers were designed and demonstrated by using either the insulating or metallic state of vanadium dioxide at the impedance matched condition.The two tunable absorbers were composed of VO2(200 nm)/Al2O3(550 nm)/Ni Cr(15 nm)/Al2O3(1300 nm)/Cu(300 nm)and Zn Se(500 nm)/VO2(55 nm)/Al2O3(900 nm)/Cu(300 nm),respectively.Dynamic temperature-tuned absorptions were achieved at 5?9.3?m and 3.9?8.2?m bands,respectively.The measured modulation depths of absorption were 30%and 88%,which is the highest value among reported similar vanadium dioxide based infrared absorbers.3.Mid-infrared broadband absorber made of porous silicon film was demonstrated with its scattering-enhanced absorption mechanism illustrated.Structured silicon,including porous silicon,has been widely reported to exhibit enhanced infrared absorption and photoresponse.However,due to the complexity and randomness of the surface morphology of structured silicon,the scattering-enhanced mechanism remains elusive.In this dissertation,highly doped and intrinsic porous silicon samples were fabricated.Experimental measurements showed that the highly doped porous silicon exhibited over 70%light absorption in the broadband range of 0.4?16?m.Meanwhile,the scattering spectra of the intrinsic porous silicon showed that the relationship between the scattering intensity of the nanopores and the wavelength satisfied I?1/?~nwith n falling in the range of 1?3.7,which is smaller than n=4 as in the classical Rayleigh scattering theory.Moreover,due to the asymmetry of the structure of porous silicon,the forward diffuse transmission of the light by the nanopores was 2?4 times stronger than the backward diffuse reflection,indicating that most of the infrared light scattered by the nanopores entered the silicon substrate with higher refractive index,resulting in the scattering-enhanced light absorption.These revealed light scattering properties are helpful for understanding the interaction process between microstructured silicon and light,which clarified the mechanism of mid-infrared broadband absorption enhanced by scattering in the porous silicon films.