Liquid-crystal adaptive equivalent q-plate device with tunable topological charge

In this work we present the proof-of-concept of what we believe to be a novel liquid-crystal compact and transmissive device useful to generate cylindrical vector beams (CVBs) of tunable topological charge. The device combines two adaptive spiral phase plates (ASPP) in orthogonal directions and a quarter-wave plate (QWP). Each ASPP relies on a specially designed circular transmission electrode with radial lines to generate a spiral phase upon addressing two voltages. The combination of two orthogonal ASPP allows imparting two independent spiral phases onto two orthogonal linear polarization states that are converted into circular states by the final QWP, thus efficiently realizing arbitrary CVBs. By adjusting the four addressed voltages, the topological charges can be tuned to provide pure or hybrid CVBs with arbitrary topological charges. The device performance is analyzed in terms of the mapping relations between the Poincaré sphere and the higher-order Poincaré sphere. We find that it performs as a q-plate, but with the remarkable advantage of providing dynamic control of the topological charge in vortices and CVBs. Hence, we name it adaptive q-plate (AQ-plate). Experimental results demonstrate the versatility of the device.