Vector structured beams (VSBs) offer infinite eigenstates and open up new possibilities for high-capacity optical and quantum communications by the multiplexing of the states. Therefore, the sorting and measuring of VSBs are extremely important. However, the efficient manipulations of a large number of VSBs have simultaneously remained challenging up to now, especially in integrated optical systems. Here, we propose a compact spin-multiplexed diffractive metasurface capable of continuously sorting and detecting arbitrary VSBs through spatial intensity separation. By introducing a diffractive optical neural network with cascaded metasurface systems, we demonstrate arbitrary VSBs sorters that can simultaneously identify Laguerre–Gaussian modes (l=−4 to 4, p=1 to 4), Hermitian–Gaussian modes (m=1 to 4, n=1 to 3), and Bessel–Gaussian modes (l=1 to 12). Such a sorter for arbitrary VSBs could revolutionize applications in integrated and high-dimensional optical communication systems.
Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object’s optical parameters. The third one is designing the structured material background, in which the light–matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.
Transmission properties of bus waveguides coupled with two nested ring resonators are investigated using the finite
difference in time domain (FDTD) method. Both the symmetrical and asymmetrical nested coupling structures are
considered and different functionalities are realized in the single structure. Fano resonant transmission spectrums are
obtained, of which the parameters can be tuned precisely by changing the position of the inner ring resonator. More
importantly, near the Fano resonant wavelength, sharply asymmetrical field localizations in the two ring resonators are
obtained when the same signals are incident from different ports. These properties are used to realize optical diodes with
an ultra high contrast ratio larger than 2000. We also find that the frequencies of the resonant signals localized in the
rings may be shifted due to the dynamic modulation of the coupling coefficients. These structures and properties
presented here may find potential applications in nano-photonic systems.
Second harmonic generation of coupled cavity structures (CCSs) fabricated in one-dimensional photonic crystals, which are composed of dispersive materials, is investigated. The fundamental wave is a localized mode, and the second harmonic wave is a traveling mode of the CCS. Using transfer matrix and effective refractive index methods, we analyze the localization and phase matching of the two modes. Using a nonlinear finite-difference time-domain method, we simulate the nonlinear process with a Gaussian pumping pulse, and both transmitted and reflected second harmonic signals are obtained. We investigate the effect of cavity number on the conversion efficiency and find the material dispersion can be compensated by the structural dispersion. For this reason, the conversion efficiency in a CCS is three orders larger than that in a bulk material of the coherence length.
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