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Researchers Propose Unit Optimization Method for Super-broadband Geometric Phase Devices
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Geometric phase devices (GPDs) are a new type of planar, ultra-thin, and arbitrarily constructible optical element, which have a wide application prospect in lidar, augmented reality, spectral imaging, and other fields. In order to apply it to these scenarios, the GPD needs to be designed and optimized to increase its working waveband.
In general, the design of high-efficiency wideband GPDs is achieved by implementing multiple twisted layers in the thickness direction. However, nonlinear unconstrained numerical optimization is needed for multiple variables, which becomes more and more difficult with the increase of the number of twisted layers. Therefore, it is necessary to develop a simple and effective design method for wideband GPDs.
In a study published in Applied Physics Letters, a research group led by Prof. MU Quanquan from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed a simplified design method for wideband GPDs as unit optimization method.
In this method, GPDs are reduced to achromatic circular polarization converters (CPCs) considering the connection between geometric phase and the optical axis of birefringent materials and the connection between high diffraction efficiency and circular polarization state conversion. The CPC is considered as the structural unit of the GPD, which reduces the design dimension.
Under the premise of the same number of twisted layers, the achromatic bandwidth of the mirror symmetric structure is always wider than that of the non-mirror symmetric structure. As a result, the mirror symmetric structure reduces the design parameters by half.
Based on the above design principles, the researchers designed a four-layer super-broadband liquid crystal polarization grating. Considering the birefringence dispersion, the diffraction efficiency of the super-broadband polarization grating is over 99% in the range of 420-945 nm. This design provides a satisfactory angular response, and it provides a better means for the design of GPDs with wideband and large view angle.
Contact
MU Quanquan
Changchun Institute of Optics, Fine Mechanics and Physics
E-mail: muquanquan@ciomp.ac.cn
