Spiral plasmonic lens as miniature circular polarization analyzer. (a) Finite element method simulation for (left) left-hand spiral (LHS), and (right) right-hand spiral (RHS), under right-hand circular (RHC) polarization illumination. (b) (left) SEM image of a 10 × 10 spiral plasmonic lens array made of LHS structures, and (right) SEM image of spiral plasmonic lens array with alternating handedness. These spiral plasmonic lenses are 200-nm-wide slots fabricated in 200-nm-thick gold film with FIB milling. (c) Two photon fluorescence microscopy images of (left) LHS array with RHC polarized illumination, and (right) LHS array with LHC polarized illumination. The bright and dark center can be used to differentiate RHC and LHC illumination. (d) NSOM images of the near-field intensity distribution at the air/gold interface for a LHS plasmonic lens under (left) RHC and (right) LHC illumination.
Polarimetric imagers with on-chip wire-grid micropolarizer arrays have been developed for remote sensing and surveillance applications. However, these micropolarizers are only sensitive to linear polarization states and do not fully measure the Stokes parameters (S0, S1, S2, S3). Detection of the last Stokes parameter S3, which characterizes the circular polarization content of the object, requires a miniaturized circular polarization analyzer array that can be integrated with the polarimetric imager.
Here, we demonstrate a miniature spiral plasmonic lens that is suitable for the detection of S3. Both analytical and numerical models have shown that an Archimedean spiral slot etched into metal film can focus circular polarization with one handedness, while the circular polarization is defocused with the opposite handedness.1 By integrating a detector at the center of the spiral lens, we can use such spin dependence to distinguish the two circular polarization states. We designed a spiral lens with a radius as small as only twice the wavelength of interest to realize the desired focusing effects with predicted circular polarization extinction ratio better than 100.
We verified this spin dependence of the focusing behavior of these spiral plasmonic lenses. Due to the evanescent characteristic of the plasmonic focal field, direct optical imaging cannot be used to map the field distribution. Two-photon fluorescent microscopy2 and near-field scanning optical microscopy (NSOM)3 can be employed to visualize the plasmonic fields of individual spiral element and spiral array. Both experiments confirm an extinction ratio better than 50, which is sufficient for practical polarimetric imaging applications.
The fabrication techniques of these spiral lenses are compatible with those for wiregrid micropolarizers. Integration of pixelated patterns of these left-hand and right-hand spiral lenses, along with wiregrid micropolarizers to a focal plane array detector, will enable full Stokes parameter polarimetric imaging. We expect this approach to provide faster, more compact and sensitive polarimetric imaging that may find widespread use in surveillance, national security, agriculture, environmental studies and many other remote-sensing-related areas.
The authors thank the Air Force Office of Scientific Research (AFOSR) and the Air Force Research Laboratory (AFRL) Materials and Manufacturing Directorate metamaterials program for their support.
Weibin Chen, Zhi Wu and Qiwen Zhan are with the Electro-Optics Program, University of Dayton, Ohio, U.S.A. Don C. Abeysinghe is with the department of electrical and computer engineering, University of Cincinnati, and Robert L. Nelson is with the U.S. Air Force Research Laboratory.
References and Resources
1. Y. Yang et al. "Miniature circular polarization analyzer with spiral plasmonic lens," Opt. Lett. 34, 3047-9 (2009).
2. Z. Wu et al. "Two-photon fluorescence characterization of spiral plasmonic lenses as circular polarization analyzers," Opt. Lett. 35, 1755-7 (2010).
3. W. Chen et al. "Experimental confirmation of miniature spiral plasmonic lens as a circular polarization analyzer," Nano Lett. 10, 2075-9 (2010).