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The recent advent of digital coding metasurfaces notably simplifies the design of functional devices, enabling one to manipulate electromagnetic waves in exotic ways using specifically arranged coding elements representing discrete phase states. Here, we propose a design of dielectric coding metasurface with diverse functionalities, which are achieved by encoding the metasurface unit cells with different coding sequences. As proofs of concept, we demonstrate the possibility of realizing five distinct functionalities for the normal incidence of plane microwaves: anomalous reflection, multibeam generation, diffuse scattering, beam focusing, and vortex beam generation. The near-field distributions and far-field scattering patterns are obtained by full-wave numerical simulations to analyze the behavior of the waves in each of the cases and illustrate our general theoretical predictions. [ABSTRACT FROM AUTHOR]

The unique properties of incommensurate two-dimensional bilayer structures, a wide class of optical materials, are largely determined by the relative rotation angle between the layers. Here, we present a comprehensive theoretical analysis of the optical properties of a dodecagonal quasicrystal based on twisted bilayer material with hexagonal layers rotated by 30°. By assuming that the quasicrystal is tuned to the Bragg condition, we calculate its reflectance spectra and reveal the Wood anomalies therein, which are characteristics of structures with a long-range order. It is also shown that the reflectance spectra can be dynamically controlled by the external electric field applied perpendicular to the quasicrystal's surface. This feature can be used to create tunable photonic devices such as optical switches and sharp M-shape bandpass filters. [ABSTRACT FROM AUTHOR]

Various methods for controlling the in-plane propagation direction and topological transitions of phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials rely on using twisted vdW bi-layers or the hybridization of anisotropic vdW materials with other functional materials such as graphene, hBN, and SiC. At the same time, visibly transparent SiC- and SiO2-based metastructures have potential to solve the problem of daytime radiative cooling. Here, as a unique method, we design a bifunctional α-MoO3-integrated SiC metasurface for effectively controlling the in-plane propagation direction of α-MoO3 PhPs. The control is enabled by a topological transition in the PhP dispersion, which is achieved by proper arrangements of the SiC meta-atoms. The proposed functionality of the designed metasurface is beneficial for the mid-infrared in-plane coupling between quantum emitters and heat management. Moreover, the α-MoO3–SiC metasurface functions as an efficient visibly transparent daytime radiative cooler. [ABSTRACT FROM AUTHOR]

Black phosphorous (BP) is a well-known two-dimensional van der Waals (vdW) material with in-plane anisotropy and remarkable electronic and optical properties. Here, we comprehensively analyze the near-field radiative heat transfer (NFRHT) between a pair of parallel non-rotated BP flakes that occurs due to the tunneling of the coupled anisotropic surface plasmon polaritons (SPPs) supported by the flakes. It is demonstrated that the covering of the BP flakes with hexagonal boron nitride (hBN) films leads to the hybridization of the BP's SPPs with the hBN's hyperbolic phonon polaritons and to the significant enhancement of the NFRHT at the hBN's epsilon-near-zero frequencies. It is also shown that the NFRHT in the BP/hBN parallel-plate structure can be actively switched between the ON and OFF states by changing the chemical potential of the BPs and that the NFRHT can be modified by altering the number of the BP layers. Finally, we replace hBN with α-MoO3 and explore how the NFRHT is spectrally and strongly modified in the BP/α-MoO3 parallel-plate structure. We believe that the proposed BP/polar-vdW-material parallel-plate structures can prove useful in the thermal management of optoelectronic devices. [ABSTRACT FROM AUTHOR]

We develop a general theory of optical activity of semiconductor nanocrystals whose chirality is induced by a small perturbation of their otherwise achiral electronic subsystems. The optical activity is described using the quantum-mechanical expressions for the rotatory strengths and dissymmetry factors introduced by Rosenfeld. We show that the rotatory strengths of optically active transitions are decomposed on electric dipole and magnetic dipole contributions, which correspond to the electric dipole and magnetic dipole transitions between the unperturbed quantum states. Remarkably, while the two kinds of rotatory strengths are of the same order of magnitude, the corresponding dissymmetry factors can differ by a factor of 105. By maximizing the dissymmetry of magnetic dipole absorption one can significantly enhance the enantioselectivity in the interaction of semiconductor nanocrystals with circularly polarized light. This feature may advance chiral and analytical methods, which will benefit biophysics, chemistry, and pharmaceutical science. The developed theory is illustrated by an example of intraband transitions inside a semiconductor nanocuboid, whose rotatory strengths and dissymmetry factors are calculated analytically. [ABSTRACT FROM AUTHOR]

The coherent coupling of quantum dots is a sensitive indicator of the energy and phase relaxation processes taking place in the nanostructure components. We formulate a theory of low-temperature, stationary photoluminescence from a quantum-dot molecule composed of two spherical quantum dots whose electronic subsystems are resonantly coupled via the Coulomb interaction. We show that the coupling leads to the hybridization of the first excited states of the quantum dots, manifesting itself as a pair of photoluminescence peaks with intensities and spectral positions strongly dependent on the geometric, material, and relaxation parameters of the quantum-dot molecule. These parameters are explicitly contained in the analytical expression for the photoluminescence differential cross section derived in the paper. The developed theory and expression obtained are essential in interpreting and analyzing spectroscopic data on the secondary emission of coherently coupled quantum systems. [ABSTRACT FROM AUTHOR]

Despite chiral metamaterials being widely appreciated for their giant optical activity and negative refractive index with respect to the transmitted electromagnetic waves, little research efforts are devoted to effects occurring upon optical reflection from such metamaterials. Here, we theoretically demonstrate highly energy-efficient cross-polarization conversion for a normally incident wave reflecting from metamaterial with a broken four-fold symmetry of chiral structure. We do this by designing metamaterial that makes the polarization plane of a linearly polarized electromagnetic wave reflected from its surface almost perpendicular to the polarization plane of the incident wave. Using reflection from this metamaterial, one can also freely convert between left-handed and right-handed circular polarizations without significant energy loss. The proposed chiral metamaterial may prove useful in electromagnetic communication systems, polarization controllable antennas, and on-chip biomedical sensors. [ABSTRACT FROM AUTHOR]

Doped semiconductor nanocrystals is a versatile material base for contemporary photonics and optoelectronics devices. Here, for the first time to the best of our knowledge, we theoretically calculate the radiative decay rates of the lowest-energy states of donor impurity in spherical nanocrystals made of four widely used semiconductors: ZnS, CdSe, Ge, and GaAs. The decay rates were shown to vary significantly with the nanocrystal radius, increasing by almost three orders of magnitude when the radius is reduced from 15 to 5 nm. Our results suggest that spontaneous emission may dominate the decay of impurity states at low temperatures, and should be taken into account in the design of advanced materials and devices based on doped semiconductor nanocrystals. [ABSTRACT FROM AUTHOR]

We theoretically demonstrate an advanced optical modulator based on graphene-enabled metamaterial with voltage-controllable reflectance. A significant modulation depth of the reflection coefficient is achieved through a voltage biasing of the graphene's Fermi level, leading to an almost instantaneous change in the effective permittivity of the graphene. We show that even a single graphene layer integrated into the structure of a simple metamaterial absorber enables a relative change in the reflectance as high as 361% at near-infrared frequencies. The designed modulator may be used in optical communication systems and biomedical sensing apparatus. [ABSTRACT FROM AUTHOR]

The article provides response from the author of the article "Graphene metamaterial for optical reflection modulation" regarding the comments of Chinese academicians Liu Jian-Qiang, He Meng-Dong and Wang Ling-Ling on the article.

We present an effective method for actively controlling intrinsic resonances of optical metamaterials using graphene. Exploiting the Fermi level shift and associated variations in optical transitions of graphene due to voltage biasing, we attain the ability to significantly modulate the intrinsic resonance of the fishnet structure. Despite being atomically thin and having a weak optical response, graphene can be strongly coupled with the left-handed resonance of the fishnet metamaterial. We unambiguously demonstrate that the resonant transmission, absorption, and effective constitutive parameters of the graphene-coupled fishnet metamaterial can be precisely controlled by varying the bias voltage. [ABSTRACT FROM AUTHOR]

Although zero-index metamaterials (ZIMs) have been extensively studied in electromagnetics, their applications in plasmonics have not been investigated so far. In this Letter, we propose strategies for efficient manipulation of surface plasmon polariton (SPP) flow using anisotropic ZIMs. Through rigorous theoretical analysis backed up by full-wave simulations, we suggest two applications of anisotropic ZIMs in plasmonics, namely, adjusting the phase delay of SPPs and guiding SPPs about a rounded metal corner. The proposed applications of ZIMs may prove useful for plasmonic interconnects in integrated ultra-high speed circuitry. [ABSTRACT FROM AUTHOR]

The concept of light amplification in a system of zero-index metamaterial with gain inserts is studied. We demonstrate that the radiation from a point source embedded in the system can be effectively amplified and the net gain is independent of the positions of gain inserts and the source. We further investigate how the net gain grows with the imaginary part of the inserts' permittivities and find that it saturates when the imaginary part is sufficiently large. It is also shown that the efficiency of amplification is significantly reduced when lossy inserts are introduced into the system. [ABSTRACT FROM AUTHOR]