Quasi-3D plasmonic coupling scheme for near-field optical lithography and imaging

Near-field optical imaging and lithography rely on achieving both high resolution and efficient coupling. Particularly conventional near-field scanning optical microscopy (NSOM) suffers from the trade-off between resolution and efficiency. Planar plasmonic lens schemes can partially solve this issue utilizing plasmonic resonances, but the performance is not robust over a large range of sample materials. In this work we show a novel quasi-3D plasmonic scheme to focus light into the extreme subwavelength region in the near field with an efficiency orders higher than NSOM. The superb performance comes from the strong coupling between the localized mode with an off-plane E-field component and the sample being processed. Our scheme can efficiently focus light to a spot with a diameter down to 1/20 of its wavelength, and the coupling efficiency can be as high as 10%. Theoretically, we demonstrate that the FWHM of the focus spot can be 7 nm with an enhancement of about 800 at the UV region. The focusing performance is constantly good over a large variety of materials and the illumination and collection imaging scheme has been demonstrated by simulation. An example design of this quasi-3D coupling scheme is fabricated and its imaging performance is characterized by the apertureless optical near-field measurement. The high coupling efficiency at extreme subwavelength resolution of this quasi-3D coupling scheme opens the door to many applications, such as optical lithography, nanoscale imaging, heat-assisted magnetic recording, plasmon-enhanced Raman spectroscopy, etc.