Research On The Characteristics Of Super-resolution Imaging Based On Tunable Hyperlens

Since the invention of the optical imaging system represented by the optical microscope,people have greatly broadened their horizons and contributed to many important discoveries in the fields of life science and microelectronics.However,due to the existence of the diffraction limit,it is difficult for conventional optical imaging systems to break half the wavelength of incident light,which greatly hinders the in-depth exploration of the micro-nano field.Based on the unique non-closed dispersion curve of metamaterials,the hyperlens could make the evanescent waves be reconstructed in the far field,thus breaking the diffraction limit and realizing super-resolution imaging.Higher resolution capability,better imaging quality and the realization of three-dimensional(3D)imaging have always been the key points of the super-resolution imaging field based on hyperlens.However,many problems such as weak radial resolution,serious side lobe interference and difficulty in fabrication due to too many layers seriously restrict the development and application of hyperlens.In order to improve the radial resolution and side lobe suppression ratio of hyperlens simultaneously,a spherical hyperlens based on the recombination of two kind metal-dielectric units is proposed in this paper.Secondly,starting from directly regulating the dispersion curve of hyperlens,we propose a scheme of cascading a graded structure outside the conventional spherical hyperlens to further improve the radial resolution.Specifically,using the finite element method combined with the finite difference time domain method,two-dimensional full-wave simulation of the designed spherical hyperlenses is carried out,and the two schemes have both shown good improvement effect.The main work and results of this research are as follows:(1)Using 9 nm thick silver(Ag)and 22 nm thick titanium dioxide(TiO2)to form the basic unit(also known as period),44-layer(22-period)conventional spherical hyperlens with an inner radius of 200 nm and an outer radius of 882 nm is constructed by stacking multi-unit.Simulation results show that the spherical hyperlens can separate two imaging objects with lateral distance of 140 nm under the incident light of 365 nm,thus breaking the diffraction limit.The far-field focus is 216 nm away from the outer wall of the hyperlens,and the radial resolution is 14 nm while the side lobe suppression ratio of the structure is 5.26 dB.(2)The 17th to 22nd periods of the 22-period Ag-TiO2 conventional spherical hyperlens are replaced by the basic unit composed of Ag and yttrium trioxide(Y2O3)with the same thickness of 10 nm,and the spherical hyperlens based on the recombination of two kind metaldielectric units is constructed.It is verified by simulation that due to the reduced losses caused by the dielectric material via replacing the unit,the radial resolution of the structure is improved to 11 nm,and the side lobe suppression ratio is increased to 7.3 dB.However,compared with the conventional hyperlens,the far-field focus position is closer to the outer wall of the hyperlens,which has a certain degree of negative influence on the detectability of imaging.(3)A further improvement in radial resolution is achieved by cascading a multi-unit graded structure outside the 22-period Ag-TiO2 conventional spherical hyperlens.The graded structure needs to be composed of two subparts,the subpart connected to the conventional spherical hyperlens makes the dispersion curve quite flat to improve the radial resolution while the other subpart makes the dispersion curve steep again to ensure that the focus is still formed outside the whole structure.In the graded structure,the thickness of the metal layer and the dielectric layer are equal,and the product of the thickness and refractive index of the dielectric layer in each unit of the graded structure is fixed to 19.8,while refractive index of the dielectric layers increases linearly along the radial direction from the initial value of 1.38.After systematic optimization,it is determined that the optimal graded structure contains 10 units,and the linearly-increased gradient of refractive index of dielectric layers in graded structure is 0.24.At this time,it can be seen from the simulation results that the radial resolution of the hyperlens is improved to 5 nm,and the distance between the focus and the outer wall of the whole structure is 484 nm.(4)Further,an attempt is made to improve the cascaded multi-unit graded structure.The refractive index of the outermost dielectric layer of the graded structure is changed from 3.54 to 3.9,and the radial resolution of the structure is verified to be 9 nm by simulation,which is still a 36%improvement compared with the 22-period conventional hyperlens;When the graded structure is changed to a constant gradient in thickness and the product of the thickness and refractive index of the dielectric layer is still fixed at 19.8,the simulation results confirm that the radial resolution can still be improved to 7 nm.We change the thickness of the graded structure to decrease isogradiently while the product of the thickness and refractive index of the dielectric layer is still fixed to 19.8,the simulation result confirm that the radial resolution can still be improved to 7 nm.