We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at sub-nanometer scales. Our measurement of the resolved lines of 30nm half-pitch shows a full-width at half-maximum better than 37nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens lead promise to parallel imaging and nanofabrication in a single snapshot, a feat that are not yet available with other nanoscale imaging techniques such as atomic force microscope or scanning electron microscope.
It has been demonstrated experimentally that a silver superlens allows to resolve features well below the working wavelength. Resolution as high as 60nm half-pitch or 1/6th of wavelength has been achieved.
Theoretically, it was predicted that a resolution as high as λ/20 (where λ is the
illumination wavelength) is feasible with careful design of silver superlens. However, challenges remain to realize such a high resolution imaging system, such as minimizing the information loss due to evanescent decay, absorption or scattering. Our numerical simulations have indicated that the thickness of spacer layer (separating the object and the lens) and that of silver film are the two major governing factors that determine subwavelength information loss due to evanescent decay and material absorption.
In conclusion, we have demonstrated a new approach to realize ultra-smooth Ag superlenses with an unprecedented λ/12 resolution capability. Incorporating few monolayers of Ge drastically improves Ag film quality and minimizes the subwavelength information loss due to scattering. Our theoretical and experimental results clearly indicate subdiffraction imaging down to 30nm half-pitch resolution with 380nm illumination. This ultra-high image resolution capability can also be extended to far-field by incorporating a corrugated silver surface on top of Ag-Ge superlens.
Fabrication of sub-20nm thick smooth Ag films will also enable development of novelmultilayer (Ag-dielectric-Ag) superlenses operating at visible wavelengths. The development of visible superlenses will allow the use of white light sources instead of specialized UV sources and facilitate possible integration of superlens with optical microscopes. The development of such novel superlenses would enable real-time, dynamic imaging at the molecular level.