Spin and orbital angular momenta are two of the most fundamental physical quantities that describe the complex dynamic behaviors of optical fields. A strong coupling between these two quantities leads to many intriguing spatial topological phenomena, where one remarkable example is the generation of a helicity-dependent optical vortex that converts spin to orbital degrees of freedom. The spin-to-orbit conversion occurs inherently in lots of optical processes and has attracted increasing attention due to its crucial applications in spin–orbit photonics. However, current researches in this area are mainly focused on the monochromatic optical fields whose temporal properties are naturally neglected. In this work, we demonstrate an intriguing temporal evolution of the spin-to-orbit conversion induced by tightly-focused femtosecond optical fields. The results indicate that the conversion in such a polychromatic focused field obviously depends on time. This temporal effect originates from the superposition of local fields at the focus with different frequencies and is sensitive to the settings of pulse width and central wavelength. This work can provide fundamental insights into the spin–orbit dynamics within ultrafast wave packets, and possesses the potential for applications in spin-controlled manipulations of light.

Multilayer hyperbolic metamaterials consisting of alternating metal and dielectric layers have important applications in spontaneous emission enhancement. In contrast to the conventional choice of at least dozens of layers in multilayer structures to achieve tunable Purcell effect on quantum emitters, our numerical calculations reveal that multilayers with fewer layers and thinner layers would outperform in the Purcell effect. These discoveries are attributed to the negative contributions by an increasing layer number to the imaginary part of the reflection coefficient and the stronger coupling between surface plasmon polariton modes on a thinner metal layer. This work could provide fundamental insights and a practical guide for optimizing the local density of optical states enhancement functionality of layered metamaterials.

Spin-orbit interactions are inherent in many basic optical processes in anisotropic and inhomogeneous materials, under tight focusing or strong scattering, and have attracted enormous attention and research efforts. Since the spin-orbit interactions depend on the materials where they occur, the study of the effects of materials on the spin-orbit interactions could play an important role in understanding and utilizing many novel optical phenomena. Here, we investigate the effect of negative-index material on the spin-orbit interactions in a plasmonic lens structure in the form of a circular slot in silver film. Numerical simulations are employed to study the influence of the negative-index material on the plasmonic vortex formation and the plasmonic focusing in the structure under circularly polarized excitations bearing different orbital angular momentum. We reveal that the presence of negative-index material leaves the plasmonic vortex field distribution and the corresponding topological charge unaltered during the spin-to-orbital angular momentum conversion, whereas reverses the rotation direction of in-plane energy flux of the plasmonic vortex and shifts the surface plasmon polariton focus position to the opposite direction compared to the case without negative-index material. This work will help further the understanding of the regulation of optical spin-orbital interactions by material properties and design optical devices with novel functions.

Metasurfaces can offer unprecedented superiority in manipulating the wavefront of electromagnetic waves and have attracted much attention around the world. However, to date, most of the metasurfaces reported only operate in either transmission or reflection space, leaving half of the space unexplored. Here we propose a general scheme for designing full-space polarization-regulated wavefront steering via single-layer metasurfaces. Specifically, the designed metasurface can change its functionality and working space (from transmission space to reflection space and vice versa) by varying the incident polarization. For a proof of concept, we demonstrate numerically two full-space polarization-regulated metasurfaces. As incident x-polarized light changes to y-polarized light, the functionality of two devices is switched from a reflected metalens and an Airy bean generator to a transmitted focusing vortex generator and a metalens, respectively. Here the design strategy is generalized and can be adapted to design other polarization-regulated meta-devices at other wavelengths. In regard to wavefront control, these results significantly expand the scope of metasurfaces, providing new possibilities to develop full-space multifunctional meta-devices.

The orbital angular momentum (OAM) of light has important applications in a variety of fields, including optical communication, quantum information, super-resolution microscopic imaging, particle trapping, and others. However, the temporal properties of OAM in ultrafast pulses and in the evolution process of spin-orbit coupling has yet to be revealed. In this work, we theoretically studied the spatiotemporal property of time-varying OAM in the tightly focused field of ultrafast light pulses. The focusing of an incident light pulse composed of two time-delayed femtosecond sub-pulses with the same OAM but orthogonal spin states is investigated, and the ultrafast dynamics of OAM variation during the focusing process driven by the spin-orbit coupling is visualized. Temporal properties of three typical examples, including formation, increase, and transformation of topological charge are investigated to reveal the non-uniform evolutions of phase singularities, local topological charges, self-torques, and time-varying OAM per photon. This work could deepen the understanding of spin-orbit coupling in time domain and promote many promising applications such as ultrafast OAM modulation, laser micromachining, high harmonic generation, and manipulation of molecules and nanostructures.