Spin and orbital angular momenta of electromagnetic waves : generation, conversion and detection

Abstract

As a new attractive degree of freedom in electromagnetic (EM) waves, orbital angular momentum (OAM) has exciting possibilities in both classical and quantum applications. Spin angular momentum (SAM) characterizes the polarization state of an EM wave, while OAM describes structured waves possessing a helical wavefront. OAM can be internally generated along with the change in SAM according to the angular momentum (AM) conservation law. Alternatively, OAM can be externally introduced through the structure with special symmetry. Although extensive research has been undertaken on the manipulation of SAM and OAM, their generation, conversion and detection at radio frequencies still face challenges, such as the complexity in modeling, low efficiency and poor flexibility. These challenging issues inspire us to conduct fundamental research to develop novel structures for SAM and OAM manipulation. In this thesis, novel approaches using artificial materials and photonic crystals are developed for the manipulation of AM. Firstly, we propose a novel chiral metamaterial for the polarization control of EM waves. The high tunability of its geometry enables the control of polarization in a large range. A typical chiral sample that introduces a SAM to EM waves is demonstrated both numerically and experimentally. Secondly, we develop several metasurface prototypes for generating OAM based on the coupling among SAM, OAM and metasurfaces. We use Jones matrix to model the scatterers on the metasurfaces. The scatterers are classified into two types: reflection and transmission. The reflective scatterer design is based on the perfect electric conductor (PEC) and perfect magnetic conductor (PMC), which abandons the complex optimization process. The transmissive scatterers are optimized based on our proposed equivalent circuit. This circuit model reveals the working physics and facilitates the optimization. Moreover, with the assistance of Greens function, the response of the transmissive metasurfaces is fast estimated. Thirdly, by employing the holographic technology, convenient and effective detection of multiple OAM beams is achieved. The required hologram is implemented using a metasurface and it converts the incident OAM wave to a detectable Gaussian wave. In this process, no complex field information is required, which is a common method to analyze OAM. Finally, we make use of photonic crystals (PCs) to convert a guided wave to a leaked wave with OAM. Defect states with proper weights and phase difference are superposed by the PCs and the resultant state carries OAM. It offers a new mechanism for OAM generation. In summary, the design principles and prototypes provide a useful and practical design route for the generation, conversion and detection of SAM and OAM.