Visualization of polarization sensing scheme: a known anchor polarization is transformed into a vertical polarization with reduced power while any deviations are transformed into the horizontal polarization state with nearly full power, enabling efficient realtime monitoring of input polarization changes.

Polarization is a fundamental property of light and a key aspect of the modern world, spanning applications from quantum to classical realms, laboratory to industrial, and metrology to meteorology. Commonly, polarization is used as both a measurement tool and an information carrier. To extract the information, one needs to characterize the polarization after the interaction of light with the detecting media. However, the full characterization of polarization is less than trivial, given that polarization is a multi-parametric property. As such, characterizing a polarization in full involves its decomposition into the basis states, a process that typically requires several reconfigurable optical elements and computational reconstruction.

However, the complexity of full polarization measurements is not always necessary. Many applications of polarization only require knowledge of the magnitude of the change in polarization from some known states. Although less complicated than full polarization characterization, monitoring small polarization changes is still not straightforward, especially for general elliptical polarizations. As reported in Advanced Photonics Nexus, the Nonlinear and Quantum Photonics Group from ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS) at The Australian National University, together with a collaborator from McGill University in Canada, present here a novel method which can monitor small polarization changes near any known elliptical or circular polarization state, and in limiting the scope of the measurement made, substantially reduce the complexity and thereby facilitate realtime monitoring of polarization variations.

The proposed method focuses on the concept of a known anchor polarization state which is defined as the default, resting state of the system. Any small perturbations or deviations from this anchor state are represented by the appearance of its orthogonal state and are changes to be monitored. To measure the magnitude of these small perturbations, we carefully engineer a transformation of input light such that the anchor state is converted to the vertical polarization and that any perturbations are mapped to the horizontal polarization state at the output. The magnitude of the latter can be directly measured after a single polarizing beam splitter by the power ratio of two outputs. Most importantly, the responsivity of the detection scheme may be enhanced by an order of magnitude via careful design of the transformation realizing a reduced power transmission of the anchor state and a high transmission of the orthogonal state.

We implement this general concept with ultrathin optical metasurfaces. Metasurfaces com- prise arrays of sub-wavelength structures on a substrate, capable of trapping and resonating with incident light to produce highly tailorable polarization transformations, and as such, constitute an extremely flexible platform for next-generation optical components. We develop a highly trans- missive metasurface using etched amorphous silicon on glass, with a simple, two-part structure tiled periodically. Our experimental measurements demonstrate an enhanced responsivity factor of 21.8 for very small perturbations near a highly elliptical anchor state. Additionally, we demon- strate that the anchor state can be tuned simply by rotating the metasurface, allowing a single structure to work effectively for a variety of target anchor states. Unlike previous studies of meta- surfaces for full characterization of polarization, here the polarization deviation is directly obtained through the detected output power ratio without specialized computational processing of measure- ment data. All these properties make the developed metasurface a simple, fast, and accurate device which allows for the monitoring of tiny polarization changes in realtime.

Concept and implementation of polarization monitoring with a metasurface. (a) An arbitrarily chosen elliptical anchor polarization (red cross) on a Poincare´ sphere and deviations |δ|² up to 0.01 are indicated by a crown. (b) A metasurface realising a specially optimised Jones matrix T allowing for real-time monitoring of input polarization deviations using only a polarizing beam splitter and the power measurements P_V and P_H . (c) At the output, the anchor state is converted to the vertical polarization and the horizontal component represents the deviation which is enhanced by a responsivity factor η =|α|². (d) Scanning electron microscope (SEM) image of the fabricated metasurface, where unit cell is indicated using the black dashed box, with the rotation from the horizontal indicated by θ. (e) The experimental power ratios plotted vs. the deviations from the anchor states, demonstrating enhanced responsivity η = 21.8.

The principles of this work are far from limited. Not only are they applicable to any anchor state via an appropriate design process, they are also capable of operating at any wavelength for which metasurfaces can be fabricated, notably including the telecommunications and visible wavelengths. We believe that this work presents a highly flexible and efficient method of solving a common task in polarization optics, and thereby it will contribute to the development of meta-optics devices for polarization sensing.