However, the nonlinear behavior of chiral metamaterials with intensity-sensitive optical properties or wave mixing characteristics, has thus far received rather scant attention. Of the previous nonlinear, chiral studies, the topics have focused on single meta-atom imaging and refinement of minor structural deformations that contribute to extreme hot spots.[25–28] In addition, these studies apply SHG imaging to further understand metamaterial resonances and field enhancements in the chiral structures. The limited but essential research in this field demonstrates that nonlinear, chiral metamaterials offer significant potential for three major reasons. First, just as SHG requires the disruption of symmetry in order to produce a non-centrosymmetric χ (2) electromagnetic tensor, a unit cell must not have a point of inversion symmetry in order to produce chirality. By disrupting the typical symmetry for the susceptibility tensor, the harmonic response is forced to rely on the handedness of the structure, thereby allowing for disambiguation of the two enantiomers in the nonlinear regime. Secondly, metamaterials consisting of metallic particles of subwavelength dimensions have pronounced optical resonances which magnify their SHG response. These optical responses spawn from the properties of the metallic nanoparticles, including localized surface plasmon resonances, lightning rod effects, and their high surface to volume ratio. A third reason to explore nonlinear responses for chiral metamaterials is that metals offer the perfect platform by which to induce surface second-harmonic generation. This type of SHG is most strongly witnessed at interfaces where the symmetry of a material’s electromagnetic tensor χ (2) is disrupted due to the juxtaposition of another material at that interface. For these reasons, nonlinear responses in chiral metamaterials show enhanced contrast between oppositely circularly polarized waves. The study of nonlinear phenomena in chiral metamaterials is also in-line with the emerging field of nonlinear metamaterials, which promises exciting opportunities to create customized nonlinear media with artificial, molecular arrangements and tailored geometries.[29–32] In this Communication, we investigate the nonlinear spectroscopic properties of a chiral metamaterial, and exploit the resonance-enhanced and chiral-selective nonlinear signals for highcontrast second-harmonic optical imaging. This effort presents a number of noticeable features that favor a comprehensive study of chirality-selective nonlinear processes in metamaterials. Our sample possesses engineered structural variation along the propagation direction, which enables strong chiral response for transmitted light at normal incidence. The size of the unit cell is substantially smaller than the wavelength of light under consideration, which allows us to treat the chiral periodic structure as a metamaterial and describe its properties using homogenized parameters. Spectral-resolved circular dichroism of both linear and harmonic signals are analyzed, compared, and utilized for imaging of a chiral pattern consisting of both enantiomers. The chiral metamaterial in this research is based on a twisted-arc

Chirality describes a material in which its mirror image can never be superposed onto itself. Just like our hands are chiral structures, organic compounds on the molecular level often exhibit this property of chirality if their structure lacks a point of inversion symmetry. The original unit structure and its nonsuperposable are said to be enantiomers of each other, which can rotate interacting beams in either a dextro-or levo-rotating manner. Specifically, when a linearly polarized beam impinges upon this …