Theoretical And Methodological Research On Improving The Resolution Limit Of Sub-diffraction-Limited Optical Imaging

It is widely known that resolution of conventional imaging optics is diffractionlimited. This occurs because evanescent waves that carry the fine features of the objectdecay exponentially in medium and make no contribution to the image plane. Recently,with the development of surface plasmons subwavelength optics, it is capable ofmanipulating the amplitude, phase and transmission behaviors of the evanescentspectrums, which provides the opportunity to achieve sub-diffraction imaging. In thisthesis, we mainly focus on the theoretical and methodological research on improvingthe resolution limit of sub-diffraction-limited optical imaging. This thesis includes thefollowing main work and conclusions:1. We propose to apply phase-shifting masks (PSMs) for superlens lithography toimprove its resolution. The PSM comprises chromium slits filled with Ag and PMMAalternatively, which delivers about π phase shift and nearly equal transmittance ofelectric field intensity for two neighboring slits. The destructive interference of lightthrough superlens delivers improved resolution and image fidelity. Numericalsimulations by FDTD demonstrate that two slits can be well resolved in superlenslithography structure with distance of35nm (~λ/10) for PSM, and only about60nm(~λ/6) for normal mask.2. A truncated hyperlens composed of pairs of metal-dielectric cylindricalmultilayers is proposed to demagnify the diffraction limited spot to achieve deepsubwavelength imaging. The diffraction limited spot is always focused by complexoptics system, which is approximately imitated by far field converging cylindrical wavein our theoretical simulation. Numerical simulations demonstrate that full width at halfmaximum (FWHM) of the image down to32nm (~λ/11) can be achieved from object (~λ/3) by365nm light illumination. It is also discussed that the influence of size andfocusing shift of the spot on those of the demagnification image on photoresist. It is also demonstrated that multi-objects can be demagnified and projected on the photoresist.3. We propose a specially designed plasmonic lens structure to succeed in realizingsub-diffraction phase-contrast imaging of transparent nano objects. The nano objects areembedded inside the insulator layer of the metal-insulator-metal (MIM) plasmonicstructure and bear small refraction index difference from the transparent insulator layer.The excited surface plasmons in the MIM structure help to greatly enhance the finephase features of the nano objects and effectively suppress the transmitted illuminationlight. The spatial resolution down to64nm and minimum distinguishable refractionindex difference down to0.05are numerically demonstrated. For sub-diffractionphase-contrast imaging of irregularly placing three dimensional (3D) nano wires andnano cylinders, the optimized MIM structure shows much better performance than asuperlens.