Spin-preserving chiral photonic crystal mirror

Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image. Structures made of nanoscale chiral elements can exhibit chiroptical effects, such as dichroism for left- and right-handed circularly polarized light, which makes these structures highly suitable for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition. At the same time, strong chiroptical effects have been challenging to achieve even in synthetic optical media, and chiroptical effects for light with normal incidence have been speculated to be prohibited in thin, lossless quasi-two-dimensional structures. Here, we report an experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror. Unlike conventional mirrors, our mirror selectively reflects only one spin state of light while preserving its handedness, with a near-unity level of circular dichroism. The operational principle of the photonic crystal mirror relies on guided-mode resonance (GMR) with a simultaneous excitation of leaky transverse electric (TE-like) and transverse magnetic (TM-like) Bloch modes in the photonic crystal slab. Such modes are not reliant on the suppression of radiative losses through long-range destructive interference, and even small areas of the photonic crystal exhibit robust circular dichroism. Despite its simplicity, the mirror strongly outperforms earlier reported structures and, contrary to a prevailing notion, demonstrates that near-unity reflectivity contrast for opposite helicities is achievable in a quasi-two-dimensional structure. Chiral mirrors that selectively reflect either left- or right-handed circularly polarized light are potentially useful for applications in quantum optics where the spin state (helicity) of light needs to be controlled. Now, Behrooz Semnani and coworkers from the University of Waterloo in Canada have built such mirrors from a thin (thickness of 309nm) silicon nitride photonic crystal membrane featuring an array of circular and ellipitical holes arranged in a carefully-designed pattern. Operating at 870nm in the infrared, the mirrors make use of a guided-mode resonance of transverse electric and transverse magnetic modes. Upon illumination, light with the chosen spin is almost perfectly reflected from the mirror with its state of polarization preserved while light with the opposite spin is completely transmitted. Polarization-resolved imaging experiments demonstrate the behaviour of the chiral mirrors.