Design, fabrication and test of a cascaded plamonic superlens

• Applicant: Professor Dr. Wolfgang Osten
• Subject Area: Measurement Systems ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2013 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 234120627

• Final Report Year: 2018

Final Report Abstract

Our goal for this project is to realize a far-field plasmonic superlens for direct observation under a conventional microscope at visible wavelengths. To achieve this, we suggested a cascaded plasmonic, which is composed of two plasmonic sub-components. One is a plasmonic cavity structure used to couple and transfer near-field waves from objects with sub-wavelength features. The other one is a planar plasmonic lens (PPL) used to compensate the phase of the waves so that the near-field waves can be projected into the far field with magnification and to enable microscope observation. Two lens designs were developed and fabricated within the frame of this project. With the first design, the plasmonic cavity structure is implemented by a double-layer (metallic) meander cavity (DLMC) structure. With the second design, a modified DLMC structure (a flat lens center) is used instead. To explore the imaging properties of the lenses for both numerical simulations and experimental verifications, two slits in an optical opaque Cr film was used as an object. Numerical calculations were performed to optimize the structural parameters for achieving high resolution and for feasible nano-fabrication. Then the lenses were fabricated. Before fabricating the whole lens, we fabricated each sub-components in the imaging system separately. This includes the double-slit object, the PPL, the single- and double-layer meander structures, as well as the modified meander cavity structure. The fabricated elements were evaluated by SEM images and the measured optical properties. When the fabrication of each element is satisfactory, we then proceed with the fabrication of the cascaded plasmonic superlenses by stacking the elements together above the objects. The fabricated superlens with the first design exhibits a lateral resolving power of 180 nm at λ = 640 nm, a power better than 1/3λ. However, there is an alignment problem with this design, which comes from near-field interactions between the grating in the DLMC structure and the object. To solve this problem, in the second design we modified the center of the DLMC structure into a flat cavity. The fabricated superlens with the second design exhibits a lateral resolving power of 200 nm at λ = 640 nm, which is still better than 1/3λ. In further, it was also confirmed by experiments that a relative shift between the meander cavity structure and the object with different amplitudes induces no notable influence on imaging. In this project, we have achieved the first experimental demonstration of a far-field superlens at middle visible wavelength ranges. This can mainly be attributed to the employment of a periodically corrugated metallic cavity structure (or the modified structure), with which the excitation of surface plasmon polaritons can be shifted in a large visible wavelength range. Moreover, we have also achieved the first experimental demonstration to combine a planar plasmonic lens with other metamaterials for phase compensation in near-field imaging, although the concept has been suggested many years before. The PPL structure alone cannot be used as a near field lens due to strong near-field interactions with objects. Nevertheless, when it is combined with a DLMC structure or a modified DLMC structure, near-field interactions are reduced greatly and a far-field superlens can be formed. Although the aperture size of the PPL is very limited, with its compact size very interesting applications can be found, e.g., for local imaging with single lens or lens array. The design can also be extended to 2D sub-wavelength imaging by combining a circular flat cavity at the center with circular gratings outside when the modified design is considered. Based on the results obtained for this project, we expect that a far-field superlens with a large field of view could be developed through replacing the PPL by a planar dielectric metalens, which has a much larger aperture size.

Publications

• (2018) Cascaded DBR plasmonic cavity lens for far-field subwavelength imaging at a visible wavelength. Optics express 26 (15) 19574–19582
  Li, Huiyu; Fu, Liwei; Frenner, Karsten; Osten, Wolfgang
  
• “A cascaded plasmonic superlens for near field imaging with magnification,” Proc. SPIE 9526, 95260Z (2015)
  L. Fu, P. Schau, K. Frenner, and W. Osten
  
• “A silicon superlens with a simple design working at visible wavelengths”, Proc. SPIE 9890, 98900l (2016)
  L. Fu, K. Frenner, H. Li, and W. Osten
  
• “Nanofabrication results of a novel cascaded plasmonic superlens: lessons learned,” Proc. SPIE 10330, 103300Y (2017)
  H. Li, L. Fu, K. Frenner, and W. Osten
  
• “Cascaded plasmonic superlens for far-field imaging with magnification at visible wavelength,” Opt. Express 26(8), 10888 (2018)
  H. Li, L. Fu, K. Frenner, and W. Osten
  
• “Design, fabrication and characterization of a cascaded plasmonic superlens for the visible spectrums”, Stuttgart : Institut für Technische Optik, Universität Stuttgart, 2019. 128 S.
  H. Li