| For nearly half a century,solar energy,as one of the alternatives to traditional fossil energy,has been widely explored because of its clean,rich and stable characteristics.According to the distribution of solar radiation energy in visible band,near infrared band and ultraviolet band,the ideal solar absorber should have high absorptivity in ultraviolet,visible and near infrared range.On the other hand,the research on the absorbing structure of metamaterials has developed from the initial microwave band to the near infrared and visible bands.Because of the consistency between the resonant pattern size and the wavelength in the working electromagnetic band,the optical absorption metamaterials in infrared,visible and higher frequency bands require sub-micron or even nanometer pattern design and fabrication,which is highly dependent on micro-nano processing methods such as electronic or ion beam etching.The traditional lithography technology often has the problems of high cost and insufficient exposure efficiency,but the colloidal sphere lithography method has attracted wide attention because of its low cost and simple technology.therefore,this paper mainly focuses on how to use colloidal sphere lithography to design and fabricate large area metamaterial solar absorbers.The main contents of this paper are as follows:(1)A truncated hollow conical shell array structure is designed and fabricated,thus the ultra-wideband near-perfect absorption in UV-visible near-infrared band is realized.The electromagnetic simulation of the structure is carried out by using the finite-difference time-domain method.The results show that the average absorptivity of the nano-conical shell array metamaterials to positive incident light is 96.11% in the wavelength range of 100-2500 nm.The effects of the geometric parameters of the structure on the absorption properties are studied.the results show that the optical absorptivity can be increased by increasing the radius,height and thickness of the upper and lower bottom or decreasing the period.The simulated electromagnetic field intensity distribution shows that the wide spectrum absorption is caused by local surface plasmon resonance,magnetic polarization resonance and cavity resonance in the structure.The electric field intensity electromagnetic simulation of polystyrene nanospheres with different particle size in 365 nm lithography process was carried out.It is found that the colloidal spheres with particle size more than 600 nm can effectively complete the lithography.Colloidal microspheres with a thickness of 1 micron were self-assembled on a negative photoresist SU8 2000 film with a thickness of 1 micron,and a photoresist nano-round table array with a height of about 500 nm was prepared by photolithography and development.The atomic layer deposition(ALD)resist array was used to coat platinum with different thickness.the average absorptivity of 200 cycles atomic layer platinum deposited samples was 78.6% in the band from deep ultraviolet to visible light(260-780 nm).The experimental results show that the colloidal sphere lithography is suitable for the preparation of truncated hollow nanoconical shell array structure metamaterials,which have objective wide-spectrum absorption properties.(2)The dome nanoconical shell array structure is designed and fabricated,so that the broadband super-absorption which is insensitive to incident angle and polarization is realized.The main factors affecting the structural absorption spectrum are studied by the finite-difference time-domain method,including the geometric parameters,shape and arrangement of the structural elements.The simulation results show that the increase of the radius,height and thickness of the upper and lower bottom or the decrease of the period can effectively improve the light absorption capacity of the structure,while increasing the refractive index of the dielectric in the cavity will cause the red shift of the absorption spectrum.The simulation results of structural geometry and coating materials show that the dome conical shell structure has stronger absorption performance in the visible near infrared band and does not depend on the selection of specific materials.The average absorptivity of square close row is slightly higher(0.19%)than that of hexagonal close row.The average absorptivity of the designed structure to the polarized light of TE and TM modes in the range of incident angle from0° to 65° is 92.7% and 94.6% respectively,which proves that the absorption performance of the structure is almost not affected by polarization and incident angle.The lift-off photoresist AZ 5214 E was lithographed with 1 μm diameter PS sphere,and the dome cone photoresist array with a period of 1 μ m was prepared.Titanium thin films with a thickness of 30 nm were deposited by evaporation.The average absorptivity was 81.8% in the deep ultraviolet to visible band(220-780 nm).Metal platinum films of different thickness were coated by 2 μm PS Sphere lithography and atomic layer deposition,in which the absorptivity of 160 cycles of samples was 77.35%.The test results of the samples with different periods are basically consistent with the simulation,which further verifies the broadband super-absorption characteristics of the designed model.(3)Based on the colloidal sphere lithography suitable for positive photoresist,the porous photoresist array template was obtained.Two kinds of cylindrical metamaterial structures suitable for positive photoresist were designed and studied,and their absorption characteristics in visible band were explored.The positive photoresist S1805 was lithographed with a monolayer polystyrene microsphere template with a particle size of 800 nm,and a pore array template with a diameter of about 405 nm was prepared.The solid titanium nitride nanowires and zinc oxide nanowires coated with titanium nitride were designed respectively,and the effects of different periods,diameters and heights on the absorption properties were simulated.In the range of 300-900 nm band,the optical absorptivity of the two cylindrical metamaterial absorbers is more than85.7% and 90.5% respectively,and both of them are insensitive to polarization and angle in a certain wavelength range. |
