Miniature metallic spiral lens for polarimetric imaging
Optical imaging has become indispensable in many aspects of life. Most optical imagers rely on a scene's intensity distribution to provide contrast. Spectral (color) information has been exploited to reach better contrast that helps distinguish objects in the field of view. The polarization of light is another sensitive information resource. Polarimetric imaging techniques have been developed for remote-sensing and surveillance applications. Polarization contrast enables differentiation between natural and man-made objects, and surface orientations.
Polarimetric imaging can be achieved with combinations of traditional polarizers and wave plates. However, this setup is generally bulky and slow. A trend has emerged to integrate microstructural components directly into focal-plane array detectors to remove the need for any moving parts. Recent advances in micro- and nanofabrication techniques have made this possible.1,2 Researchers have demonstrated IR polarimetry with on-chip wiregrid arrays.2 However, the micropolarization component has been limited to wiregrid micropolarizers (linear metal gratings). These are only sensitive to linear polarization states, and thus do not recover the full information contained in the Stokes parameters3 (s0, s1, s2, and s3): only the first three components can be measured. Detection of s3, which characterizes an object's circular polarization, could provide improved sensitivity, especially for objects with chiral structures or optical activity.
We have developed a new type of miniature device that can detect s3. It uses the latest developments in plasmonic lenses. By designing a spiral-shaped slot with the appropriate pitch etched into a metal film, we can make its response dependent on the handedness of incident circular polarization.4 Arrays of such devices can be fabricated with existing micro- and nanofabrication techniques, making them suitable for integration with current focal-plane array formats.
Through an analytical derivation, we demonstrated that an Archimedean spiral slot etched into metal film can focus circular polarization of a given handedness while simultaneously defocusing the circular polarization of the opposite chirality.4 By integrating a detector at the center of the spiral lens, we can distinguish between the two circular polarization states. The analytical model also shows that a larger spiral size or multiple turns of the spiral can be used to create a stronger focus at the lens center.
Through finite-element-method modeling, we numerically confirmed the spin dependence of the spiral-shaped plasmonic lens. A lens with a radius of only twice the wavelength of interest can be designed to achieve the desired focusing effects. In the example shown in Figure 1, a left-handed spiral lens focuses right-handed circular polarization illumination into a solid spot, while a right-handed spiral lens causes the same illumination to display a doughnut with a dark center.
We have studied the performance of these devices for circular-polarization distinction both analytically and numerically. The results showed that a circular-polarization extinction ratio of better than 100 is achievable. This is sufficient for polarimetric imaging applications. The fabrication techniques of these spiral lenses are compatible with those for wiregrid micropolarizers.
We expect this approach to enable faster, more compact, and sensitive polarimetric imaging that may find wide use in surveillance, national security, agriculture, environmental studies, and many other remote-sensing-related areas. We have constructed a prototype of such a device (see Figure 2). Preliminary near-field scanning-optical-microscope testing has confirmed the anticipated effect. We will report the full characterization results in the near future. In the longer term, we plan to integrate pixelated patterns of left- and right-handed spiral lenses along with wiregrid micropolarizers into focal-plane array detectors to demonstrate that this setup allows full Stokes-parameter polarimetric imaging.
We thank the Air Force Office of Scientific Research and the Air Force Research Laboratory's (Materials and Manufacturing Directorate) metamaterials program for their support.
Qiwen Zhan is an associate professor. He directs the research activities of the Nano-Electro-Optics Laboratories. He obtained his PhD in electrical and computer engineering from the University of Minnesota (Twin Cities) in 2002.
