Effects Of Loss On Subwavelength Imaging In Near-field Superlens

Metamaterials refer to artificial structures,which are composed of natural materials,and they have exotic properties that natural materials do not have,such as negative refraction.Negative refraction is essential to perfect lens for enhancing the evanescent waves and overcoming the diffraction limit.In the electrostatic near-field limit,the electric and magnetic responses of materials are decoupled.Therefore,the surface plasmon resonance(SPPs)can be excited by transverse magnetic(TM)waves in the materials with negative permittivity,enhancing the evanescent waves and realizing the subwavelength imaging.Besides,the loss must be taken into account,which governs the resolution limit.The loss of various materials is different at the different electromagnetic spectral regimes.For instance,at the visible and mid-infrared frequencies,the near-field subwavelength imaging have been proposed,while it is not the case at the nearinfrared(NIR)frequencies because of no appropriate low-loss building blocks.It was found that TCOs have metal-like optical properties and lower losses at the NIR range,possibly benefiting the excitation and propagation of SPPs.In this thesis,we investigated the imaging characteristics and analyzed the effects of loss on the imaging characteristics of TCO-based near-field superlens at the near-infrared frequencies.In addition,the imaging characteristics of low-loss ultra-thin silver superlens with a sub-skin depth were studied.The main results of this thesis are as follows:1.Based on AZO(Al-doped ZnO)in TCO,two kinds of AZO structures of superlens were designed.The optical transfer function(OTF)of two AZO superlenses in NIR was calculated by a transfer matrix method.The results showed that the AZO superlens could effectively enhance the evanescent waves.The distribution of imaging electromagnetic field of the two superlenses was simulated using a finite element method.AZO was considered in both stratified ZnO-AZO-ZnO and single-layered AZO structures,which had the subwavelength resolution of better than ?/25 at a working wavelength of 2.57?m and ?/20 at 2.01?m,respectively.2.In this thesis,two sandwich superlens,S1 and S2,which had the different loss(i.e.,different imaginary part of complex permittivity),were studied.The results showed that the image FWHM(full width at half maximum)of S1(??=3.74)and S2(??=7.24)superlens were 0.07 and 0.09,respectively,in the case of single-slit imaging with silt width of 100 nm.In the case of double-slits imaging with a silt width of 100 nm,their image contrasts were 0.53 and 0.2,respectively,at a working wavelength of 2.57.Compared with AZO(,the image quality was significantly affected by the loss.3.A single-period superlens and a multi-periods superlens were designed,in which one period consisted of a 7-nm silver and a7-nm PMMA.Their imaging characteristics were investigated by the OTF and the finite element calculations.The results showed that the ultra-thin silver-based single-period and 5-periods superlens had the subwavelength resolution of better than ?/36 and ?/18,respectively,at a working wavelength of 365 nm.Finally,the influences of different filling factors of metal on the imaging characteristics of multi-periods superlens was studied at a working wavelength of 490 nm,in which one period consisted of ultrathin silver and Ga P.When the filling factors of silver were 0.5,0.41,and 0.35,their FWHM were 0.07?,0.17?,and 0.2?,respectively,at an image plane of L=?.At a farther image plane of L=3?,when the silver filling factors were 0.41 and 0.35,their FWHM were 0.25? and 0.43?,respectively,and the FWHM for a filling factor of 0.5 was not available due to the trivial energy density.It implied that the higher the filling factor of metal was,the higher the obtained imaging resolution was at a close image plane of multiperiods superlens.