One of the basic functionalities of photonic devices is the ability to manipulate the polarization state of light. Polarization components are usually implemented using the retardation effect in natural birefringent crystals and, thus, have a bulky design. Here, we have demonstrated the polarization manipulation of light by employing a thin subwavelength slab of metamaterial with an extremely anisotropic effective permittivity tensor. Polarization properties of light incident on the metamaterial in the regime of hyperbolic, epsilon-near-zero, and conventional elliptic dispersions were compared. We have shown that both reflection from and transmission through λ/20 thick slab of the metamaterial may provide nearly complete linear-to-circular polarization conversion or 90° linear polarization rotation, not achievable with natural materials. Using ellipsometric measurements, we experimentally studied the polarization conversion properties of the metamaterial slab made of the plasmonic nanorod arrays in different dispersion regimes. We have also suggested all-optical ultrafast control of reflected or transmitted light polarization by employing metal nonlinearities.

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Structured light mechanically interacts with matter via optical forces and torques. The optical torque is traditionally calculated via the flux of total angular momentum (AM) into a volume enclosing an object. In [Phys. Rev. A 92, 043843 (2015)] a powerful method was suggested to calculate optical torque separately from the flux of the spin and the orbital parts of optical AM, rather than the total, providing useful physical insight. However, the method predicted a new type of dipolar torque dependent on the gradient of the helicity density of the optical beam, inconsistent with prior torque calculations. In this work we intend to clarify this discrepancy and clear up the confusion. We re-derive, from first principles and with detailed derivations, both the traditional dipolar total torque using total AM flux, and the spin and orbital torque components based on the corresponding AM contributions, ensuring that their sum agrees with the total torque. We also test our derived analytical expressions against numerical integration, with exact agreement. We find that 'helicity gradient' torque terms indeed exist in the spin and orbital components separately, but we present corrected prefactors, such that upon adding them, they cancel out, and the 'helicity gradient' term vanishes from the total dipolar torque, reconciling literature results. We also derive the analytical expression of the quadrupolar torque, showing that it is proportional to the spin of the EM field gradient, rather than the local EM field spin, as sometimes wrongly assumed in the literature. We provide examples of counter-intuitive situations where the spin of the EM field gradient behaves very differently to the local EM spin. Naively using the local EM field spin leads to wrong predictions of the torque on large particles with strong contributions of quadrupole and higher-order multipoles, especially in a structured incident field.

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