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Spin-preserving chiral photonic crystal mirror

5 Jul | By Biophotonics.World
Spin-preserving chiral photonic crystal mirror
An illustrative description of the performance of the photonic crystal mirror. The chiral photonic crystal structure can be printed on banknotes and official documents against counterfeiting. Upon applying purely circularly polarized lights of a certain handedness, the patters would be revealed.
By: by Semnani, Behrooz, Jeremy Flannery, Rubayet Al Maruf, and Michal Bajcsy

Researchers from Institute for Quantum Computing (IQC), University of Waterloo have demonstrated a new planar chiral mirror that exhibits near unity reflectivity contrast for the opposite spin states of light. Conventional mirrors, e.g. metallic and dielectric mirrors, flip the spin of light upon reflection. However, the reported chiral structure is spin-preserving i.e. the selected helicity remains unchanged upon reflection. The spin-preserving mirrors can find potential applications in quantum information processing, quantum optics, atomic and molecular physics as well as fundamental studies of light-matter interactions. 

Light waves are composed of electric and magnetic fields. If the light is circularly polarized, the fields would follow a helical path along the propagation direction. Depending on the rotational direction, clockwise or counter-clockwise, the helicity is defined. When a light beam is circularly polarized, each of its photons carries a definite Spin Angular Momentum (SAM). In general, chirality simply means that an object is not identical in all respects with its mirror image, such as the left and right hands. Thus, when one looks at a chiral object, a handedness or sense of twist stands out. At the same time, in optics and photonics, chirality also refers to a structure having a different effect on left- versus right-handed circularly polarized light. According to a long held notion, creating chiral structures for light requires complex 3D, pillar-shaped features that are delicate and tricky to fabricate. Additionally, the chiral structures that have been reported so far seemed to offer photon-spin selectivity at levels that make the structures of limited use for precise control of interactions between photons and quantum emitters. 

Behrooz Semnani and his colleagues, decided to explore in a different direction and rely on holes instead of pillars. The result was a single-layer, 2D structure formed by punching a repeating pattern of holes into a thin semiconductor membrane resulting in a type of structure that is sometimes called a photonic crystal slab. Amazingly, with the specific pattern of holes that Behrooz and his colleagues proposed, the perforated membrane outperformed in multiple ways the complicated structures that had been reported previously.

Our results prove that strong chirality can be created in a simple structure which is easy to fabricate and gives the best performance, Behrooz Semnani said. Most chiral structures are really complex relying on features such as 3D helical geometries. In fact, researchers in the past have believed that a 3D structure is needed for optical chirality but we've demonstrated you can do it in 2D.

As an added bonus, the structure has another unusual quality. When circularly polarized light reflects from traditional mirrors and reflective structures, the spin of the photons is flipped and the handedness of the circular polarization gets switched. However, with the mirror based on the perforated membrane that Behrooz Semnani and his colleagues created, the spin quite unconventionally remains unchanged upon reflection. While this may sound simple, it can be used to build gas lasers with special properties not available with ordinary mirrors.

Because the 2D structure is relatively easy to fabricate in a single-layer material, it has the potential to be utilized by different industries. For example, one can envision printing the structure directly on banknotes and identity documents as a hidden security feature against counterfeiting. If you look with naked eyes, you won't see anything notable. On the other hand, when illuminated with circularly polarized light at the design frequency, the structure will reflect the light and reveal its presence.

The chirality is strong enough that you can have an imperfect structure that still works, Prof. Michal Bajcsy said. As a result, the structure can be mass produced, which is really important for practical applications.

Source: Changchun  Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences

Related journal article: http://dx.doi.org/10.1038/s41377-020-0256-5

Methods and Techniques: Other Methods and Techniques

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