Tight Focusing Properties of Azimuthally Polarized Pair of Vortex Beams through a Dielectric Interface

Tight focusing properties of an azimuthally polarized Gaussian beam with a pair of vortices through a dielectric interface is theoretically investigated by vector diffraction theory. For the incident beam with a pair of vortices of opposite topological charges, the vortices move toward each other, annihilate and revive in the vicinity of focal plane, which results in the generation of many novel focal patterns. The usable focal structures generated through the tight focusing of the double-vortex beams may find applications in micro-particle trapping, manipulation, and material processing, etc.

Dielectric or plasmonic Mie object at air–liquid interface: The transferred and the traveling momenta of photon

Considering the inhomogeneous or heterogeneous background, we have demonstrated that if the background and the half-immersed object are both non-absorbing, the transferred photon momentum to the pulled object can be considered as the one of Minkowski exactly at the interface. In contrast, the presence of loss inside matter, either in the half-immersed object or in the background, causes optical pushing of the object. Our analysis suggests that for half-immersed plasmonic or lossy dielectric, the transferred momentum of photon can mathematically be modeled as the type of Minkowski and also of Abraham. However, according to a final critical analysis, the idea of Abraham momentum transfer has been rejected. Hence,an obvious question arises: whence the Abraham momentum? It is demonstrated that though the transferred momentum to a half-immersed Mie object(lossy or lossless) can better be considered as the Minkowski momentum, Lorentz force analysis suggests that the momentum of a photon traveling through the continuous background, however, can be modeled as the type of Abraham. Finally, as an interesting sidewalk, a machine learning based system has been developed to predict the time-averaged force within a very short time avoiding time-consuming full wave simulation.

Transmission upper limit of band-pass double-layer FSS and method of transmission performance improvement

The transmission upper limit of a double-layer frequency selective surface （FSS） with two infinitely thin metal arrays is pre- sented based on the study of the general equivalent transmission line model of a double-layer FSS. Results of theoretical analyses, numerical simulations and experiments show that this transmis- sion upper limit is independent of the array and the element, which indicates that it is impossible to achieve a transmission upper limit higher than this one under a given incident and dielectric- supporting condition by the design of the periodic array. Both the applicable condition and the possible application of the transmis- sion upper limit are discussed. The results show that the transmis- sion upper limit not only has a good reachability, but also provides a key to effectively improve the transmission performance of a double-layer FSS or more complex frequency selective structures.

Photon momentum transfer in inhomogeneous dielectric mixtures and induced tractor beams

The determination of optical force as a consequence of momentum transfer is inevitably subject to the use of the proper momentum density and stress tensor.It is imperative and valuable to consider the intrinsic scheme of photon momentum transfer,particularly when a particle is embedded in a complex dielectric environment.Typically,we consider a particle submerged in an inhomogeneous background composed of different dielectric materials,excluding coherent illumination or hydrodynamic effects.A ray-tracing method is adopted to capture the direct process of momentum transfer from the complex background medium,and this approach is validated using the modified Einstein–Laub method,which uses only the interior fields of the particle in the calculation.In this way,debates regarding the calculation of the force with different stress tensors using exterior fields can be avoided.Our suggested interpretation supports only the Minkowski approach for the optical momentum transfer to the embedded scatterer while rejecting Peierls’s and Abraham’s approaches,though the momentum of a stably moving photon in a continuous background medium should be considered to be of the Abraham type.Our interpretation also provides a novel method of realizing a tractor beam for the exertion of negative force that offers an alternative to the use of negative-index materials,optical gain,or highly non-paraxial or multiple-light interference.