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Topological semimetals, representing a new topological phase that lacks a full band gap in bulk states and exhibiting nontrivial topological orders, recently have been extended to photonic systems, predominantly in photonic crystals and to a lesser extent metamaterials. Photonic crystal realizations of Dirac degeneracies are protected by various space symmetries, where Bloch modes span the spin and orbital subspaces. Here, we theoretically show that Dirac points can also be realized in effective media through the intrinsic degrees of freedom in electromagnetism under electromagnetic duality. A pair of spin-polarized Fermi-arc-like surface states is observed at the interface between air and the Dirac metamaterials. Furthermore, eigenreflection fields show the decoupling process from a Dirac point to two Weyl points. We also find the topological correlation between a Dirac point and vortex or vector beams in classical photonics. The experimental feasibility of our scheme is demonstrated by designing a realistic metamaterial structure. The theoretical proposal of the photonic Dirac point lays the foundation for unveiling the connection between intrinsic physics and global topology in electromagnetism.

Yellow corn meal was extruded in a ZSK30 Werner and Pfleiderer twin screw extruder. Experiments used a surface response method which included variations in screw speed (100-300 rpm), heating temperature (lOO-200°C) and moisture (20-30%) in the feed. Descriptive sensory analysis characterized appearance, aroma, flavor and texture of extrudates. Temperature was the most significant factor affecting Munsell value, airiness, toasted corn aroma, and flavor, denseness, crispiness, chewiness and hardness of extrudates. Temperature and feed moisture had significant effects on Munsell value, surface texture, raw flour aroma, toasted corn aroma and flavor, chewiness and hardness. Interaction between temperature and screw speed had significant effects on airiness and denseness of extrudates.

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We show that polarization-independent coherent perfect absorption can be realized in a simple dipole-like metasurface by precisely engineering the ratio between the scattering loss γ(s) and the dissipation loss γ(l). This effect can be traced to a critical condition on the scattering matrix in a dipolar picture, which requires that the scattering and dissipation losses are equivalent, i.e., γ(s) = γ(l), at the resonant frequency f(0). This work expands the capability of metasurface in getting extreme optical properties, allowing for many potential applications.

In our article [B. Gu, Y. X. Fan, J. Wang, J. Chen, J. P. Ding, H. T. Wang, and B. Guo, Phys. Rev. A 73, 065803 (2006)] we have presented the theory of open-aperture Gaussian-beam Z-scan, based on the Adomian decomposition method, which is available only for the saturable absorption caused by single-photon absorption transition. In the present article, using the same technique (Adomian decomposition method, as a common technique), we develop an open-aperture Z-scan theory for evaluating the property of the saturable absorption originating from two-photon absorption (2PA) transition, when a spatial Gaussian beam is used as the excitation source. We find analytic polynomial expressions of the Z-scan traces for a continuous wave laser or a temporal Gaussian pulsed laser. As the experimental evidence, we investigate the saturable 2PA behaviors caused by the interband two-photon transition in the direct-gap II–VI semiconductors CdS, CdSe, ZnSe, and ZnTe, under the excitation condition of a femtosecond laser with a 140 fs pulse duration, a 1 kHz low repetition rate, and a 1.6 eV photon energy.

We propose a metamaterial design for realizing inhomogeneous and anisotropic effective media based on the localized waveguide resonance mechanism. Such a design can be easily achieved in experiment and enables us to simultaneously manipulate the wavefront and the state of polarization of the transmitted electromagnetic field by the polarization-sensitive extraordinary optical transmission. Numerical simulations, including the generation of the hybridized vector fields (especially twisted vector fields that are azimuthally polarized carrying a helical phase), prove the feasibility of our proposal. It could be expected as a good candidate of the specially designed subwavelength element for creating the exotic vector fields beyond the functionality of the existing vector fields in a wide spectral regime, especially the terahertz and radio regimes.

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