A Circularly Polarized Omnidirectional Rydberg Atomic Sensor Using Characteristic Mode Analysis

Rydberg atomic sensors (RASs) have garnered significant attention as a novel type of electric field sensor with high sensitivity. While the integration of electric field enhancement structure (EFES) and RASs has been proven to improve the sensitivity and the minimal detectable electric field of RAS, there are concerns regarding their omnidirectional and polarization performance. However, most existing RASs or RASs loaded with EFES operate with linear polarization, which reduces the sensitivity to circularly polarized (CP) microwave reception. It is a challenging method to change the property of the lasers to modify the RAS's CP property, while it is simpler to use EFES to change RASs' polarization indirectly. Meanwhile, current EFES research mainly focuses on the response of electric field enhancement at a fixed incident angle, limiting extension to 3-D CP patterns and lacking clear physical insights to guide. To address this, we propose an RAS with CP EFES to achieve CP reception capability. The EFES converts spatial CP electric field into local linearly polarized (LP) electric field, adjusting the RAS characteristics accordingly. We use characteristics mode (CM) methods, dividing the problem into modal current, governing the modal patterns, and external CP excitation sources, determining the coefficients of each modal pattern. This method reduces the complexity of the pattern analysis and improves the efficiency of optimization design. To validate the proposed method's efficiency, we fabricate a prototype to measure reception pattern and further study the CP performance using an atomic heterodyne approach. The results indicate that the RAS with EFES achieves the right-hand CP (RHCP) omnidirectional reception capability, and the sensitivity is improved by 19.2 dB at 2.195 GHz. The cross-polarized level can be further improved to 24.5 dB using an atomic heterodyne approach.