Controlling light with subwavelength-designed metasurfaces (MSs) has allowed for the arbitrary creation of structured light by precisely engineered matter. We report on the purity and conversion efficiency of hybrid orbital angular momentum (OAM)-generating MSs. We use a recently reported method to design and fabricate meta-surfaces that exploit generalized spin-orbit coupling to produce vector OAM states with asymmetric OAM superpositions, e.g., 1 and 5, coupled to linear and circular polarization states, fractional vector OAM states with OAM values of 3.5 and 6.5, and also the common conjugate spin and OAM of ±1 as reported in previous spin-orbit coupling devices. The OAM and radial modes in the resulting beams are quantitatively studied by implementing a modal decomposition approach, establishing both purity and conversion efficiency. We find conversion efficiencies exceeding 75% and purities in excess of 95%. A phase-flattening approach reveals that the OAM purity can be very low due to the presence of undesired radial components. We characterize the effect and illustrate how to suppress the undesired radial modes.

Synchronization is of importance in both fundamental and applied physics, but its demonstration at the micro/nanoscale is mainly limited to low-frequency oscillations such as mechanical resonators. We report the synchronization of two coupled optical microresonators, in which the high-frequency resonances in the optical domain are aligned with reduced noise. It is found that two types of synchronization regimes emerge with either the first- or second-order transition, both presenting a process of spontaneous symmetry breaking. In the second-order regime, the synchronization happens with an invariant topological character number and a larger detuning than that of the first-order case. Furthermore, an unconventional hysteresis behavior is revealed for a time-dependent coupling strength, breaking the static limitation and the temporal reciprocity. The synchronization of optical microresonators offers great potential in reconfigurable simulations of many-body physics and scalable photonic devices on a chip.