Research On The Basic Characteristics Of Mid-infrared Metasurface Gratings

Diffraction grating,as a dispersive light splitting element,has a very wide range of applications in the fields of spectrometer,laser,aerospace,etc.However,the traditional diffraction gratings,due to the limitation of their physics nature,have trade-off between efficiency and working bandwidth.Their performances are dependent on incident polarization states.To address the above shortcomings of traditional gratings,we design a reflective blazed grating based on metasurface,which has an efficiency greater than 80% in the8.5?m-12?m band.And it is insensitive to the polarization state of incident light.Metasurface is a two-dimensional metamaterial,which is artificially designed to manipulate electromagnetic waves.By adjusting dispersion of micro/nano unit structures,arbitrary control of the electromagnetic wave beam can be realized.Another advantage of metasurface is that it can be densely integrated.We select the mid-infrared metasurface grating as the research topic.Dispersion control of the metasurface grating is theoretically analyzed and verified through experiments.The specific work is divided into the following parts:1.Dispersion control of metasurface grating is theoretically analyzed.A twodimensional harmonic oscillation can be decomposed to two orthogonal oscillations in one dimension,which determine the dispersion of two-dimensional harmonic oscillation.A 45° C-shape resonator is designed by applying this phase dispersion model to the field of optics.Oscillations of the symmetric and asymmetric mode can be controlled by changing the geometric parameters of the C-shape resonator.Therefore arbitrary regulation of the metasurface dispersion is achieved.2.The design principle of metasurface grating is proposed.The phase gradient of metasurface is obtained by combining the generalized Snell's law and grating equation.Unit structures are selected and then arranged to form the metasurface grating through this dispersion control method.An electromagnetic simulation software is used for calculating the metasurface grating efficiency.3.The fabrication process of metasurface grating is proposed.Firstly,we use an electron beam evaporation coating machine to successively evaporate copper,zinc selenide,and gold on the monocrystalline silicon and then spin-coat the photoresist.Secondly,the photoresist is exposed with stepper lithography machine,and then developed and hardened.Thirdly,ion beam is used to etch the uppermost layer of gold with an etching depth of 100 nm.Finally,the photoresist is removed.4.The metasurface grating is characterized by a proposed experiment setup.In this experiment,the Fourier transform infrared spectrometer(FTIR)is used as broadband light source.The incident light vertically incident on the metasurface grating.Next,we change the position of the detector by rotating the stages to measure output light intensity of metasurface grating at different angles.Then output light intensity is compared with the reference light intensity which was measured by the Au mirror to obtain the efficiency.The proposed metasurface grating is evaluated by comparing the experimental and simulation results.The difference between the experimentally and theoretical measured diffraction efficiency is only 5%,and the mean square error of the diffraction efficiency for different polarizations is 36,which is only 1/37 of the commercial gratings.Its diffraction efficiency is higher than 80% in the 8.5 ?m-12 ?m wavelength range,and its bandwidth is 65% higher than commercial gratings.Its performance is better than the current commercial blazed gratings(the diffraction efficiency is more than 80% at 8-10 ?m wavelength range,and the mean square error of the diffraction efficiency for different polarizations is 1334.7).