Chiral Metalens of Circular Polarization Dichroism with Helical Surface Arrays in Mid-Infrared Region

A chiral metalens of circular polarization dichroism (CPD) in the mid-infrared region of 3-5 mu m is demonstrated both theoretically and experimentally, in which one of the circularly polarized light beams (either left-handed or right-handed) is transmissively focused at a designed focal length, while the other beam is reflected. The metalens consists of subwavelength helical surface arrays covered by a gold thin film that gives rise to CPD with the handedness of the metallic helical surface, while the focusing is realized with a phase distribution that is found to be determined by the rotation angle of each individual helical surface along the radial direction of the lens. A chiral metalens of circular dichroism (CMCD) operating in the 3-5 mu m region is designed and experimentally fabricated by the 3D laser direct writing technique followed by electron beam evaporation. Experimental focusing of the circularly polarized lights is demonstrated successfully with the lock-in thermography technique to eliminate the severe background infrared radiation in the 3-5 mu m wavelength region, and it is found that the results are consistent with the theoretical simulation. The demonstrated CMCD provides a new idea of implementing integrated functions of both imaging and circular dichroism with a single element.