We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-$Q$ resonances arising from a distortion of symmetry-protected bound states in the continuum. We develop a rigorous theory of such asymmetric periodic structures and demonstrate a link between the bound states in the continuum and Fano resonances. Our results suggest the way for smart engineering of resonances in metasurfaces for many applications in nanophotonics and metaoptics.

/pdf/asymmetric-metasurfaces-with-high-q-resonances-governed-by-2rcm6ptly2.pdf

Asymmetric Metasurfaces with High-Q Resonances Governed by Bound States in the Continuum.

We discuss the recent advances in meta-optics and nanophotonics associated with the physics of bound states in the continuum (BICs). Such resonant states appear due to a strong coupling between leaky modes in optical guiding structures being supported by subwavelength high-index dielectric Mie-resonant nanoantennas or all-dielectric metasurfaces. First, we review briefly very recent developments in the BIC physics in application to isolated subwavelength particles. We pay a special attention to novel opportunities for nonlinear nanophotonics due to the large field enhancement inside the particle volume creating the resonant states with high-quality (high-Q) factors, the so-called quasi-BIC, that can be supported by the subwavelength particles. Second, we discuss novel applications of the BIC physics to all-dielectric optical metasurfaces with broken-symmetry meta-atoms when tuning to the BIC conditions allows to enhance substantially the Q factor of the flat-optics dielectric structures. We also present the original results on nonlinear high-Q metasurfaces and predict that the frequency conversion efficiency can be boosted dramatically by smart engineering of the asymmetry parameter of dielectric metasurfaces in the vicinity of the quasi-BIC regime.

/pdf/meta-optics-and-bound-states-in-the-continuum-1xghm0xooy.pdf

Meta-optics and bound states in the continuum

We demonstrate a photonic metamaterial that shows extraordinary sensitivity to the presence of a single atomic layer of graphene on its surface. Metamaterial's optical transmission increases multi-fold at the resonance frequency linked to the Fano-type plasmonic mode supported by the periodic metallic nanostructure. The experiments were performed with chemical vapor deposited (CVD) graphene covering a number of size-scaled metamaterial samples with plasmonic modes at different frequencies ranging from 167 to 187 Thz.

/pdf/graphene-in-a-photonic-metamaterial-1q7hgu7pia.pdf

Graphene in a photonic metamaterial.

We summarize the progress in the development and application of chiral metamaterials. After a brief review of the salient features of chiral metamaterials, such as giant optical activity, circular dichroism, and negative refractive index, the common method for the retrieval of effective parameters for chiral metamaterials is surveyed. Then, we introduce some typical chiral structures, e.g., chiral metamaterial consisting of split ring resonators, complementary chiral metamaterial, and composite chiral metamaterial, on the basis of the studies of the authors’ group. The coupling effect during the construction of bulk chiral metamaterials is mentioned and discussed. We introduce the application of bianisotropic chiral structures in the field of asymmetric transmission. Finally, we mention a few directions for future research on chiral metamaterials.

/pdf/chiral-metamaterials-from-optical-activity-and-negative-2nxpezki69.pdf

Chiral metamaterials: from optical activity and negative refractive index to asymmetric transmission

In 1950, a quarter of a century after his first ever nonlinear optical experiment when intensity dependent absorption was observed in uranium-doped glass, Sergey Vavilov predicted that birefringence, dichroism and polarization rotatory power should be dependent on light intensity. It required the invention of the laser to observe the barely detectable effect of light intensity on the polarization rotary power of the optically active lithium iodate crystal, the phenomenon now known as the nonlinear optical activity, a high-intensity counterpart of the fundamental optical effect of polarization rotation in chiral media. Here we report that a plasmonic metamaterial exhibits nonlinear optical activity 30 million times stronger than lithium iodate crystals thus transforming this fundamental phenomenon of polarization nonlinear optics from an esoteric phenomenon into a major effect of nonlinear plasmonics with potential for practical applications.

/pdf/giant-nonlinear-optical-activity-in-a-plasmonic-metamaterial-1vkn85a9n8.pdf

Giant nonlinear optical activity in a plasmonic metamaterial

The problem of the near-IR light reflection from and transmittance through a planar 2D periodic metal–dielectric structure with a square periodic cell of two complex-shaped asymmetric metal elements has been solved. Conditions of the light confinement by excitation of the trapped mode resonances in certain structures, both polarization-sensitive and polarization-insensitive, were studied. For the first time, the existence of a high-order trapped mode resonance with the greater quality factor than that of the lowest one has been shown. It was ascertained that the Babinet principle provides a good prediction of the resonance properties of the complementary structures, despite the very high Joule losses in the metal strips in near-IR, a finite thickness of the metal elements and the presence of a dielectric substrate.

Trapping of light by metal arrays

Results of research of resonant phenomena in planar periodic structures consisted of dielectric elements are presented. A problem of plane wave diffraction by a structure which a periodic cell is composed of two dielectric bars with different lengths has been solved. For the first time an existence of high Q-factor trapped mode resonances are revealed in these structures. The main difference of studied structure in contrast to the planar structure with a single dielectric bar in the unit cell is appearing of a great red shift of a trapped mode resonant wavelength. The red shift is caused by strong coupling of electromagnetic fields in neighbor dielectric bar resonators. This property is very attractive to design resonant periodic planar structures by using moderate refractive index materials like semiconductors within their transparency window. In the near infrared band, the trapped mode resonances of periodic planar structure with bars made of germanium have been studied. It is shown that decreasing of structure asymmetry degree results in increasing of both red shift and Q-factor of resonance.

Giant red shift and enhancement of resonant light confinement in planar array of dielectric bars

The results of research into resonant phenomena in double-periodic subwavelength planar structures made up of paired dielectric bars are presented. For the first time the existence of high Q-factor trapped mode resonances is revealed in these low-loss entirely dielectric structures. A large red shift of the trapped mode resonance of the structure is observed as compared with the resonant wavelength of the periodic structure with only one dielectric bar per unit cell. This shift of the resonant wavelength is caused by a strong coupling of the electromagnetic fields in the adjacent dielectric bar resonators.

https://iopscience.iop.org/article/10.1088/2040-8978/14/3/035103/pdf

A giant red shift and enhancement of the light confinement in a planar array of dielectric bars

Trapped-Mode Resonances in Light Diffraction by a Planar Doubly Periodic Structure with Asymmetric Metal Elements

Paper deals with analysis of the ldquotrapped moderdquo resonance properties in optical planar structures for metamaterials applications. The light diffraction solve algorithm for planar periodic metal-dielectric optical structures combines the mapped PSTD method with analytical transfer matrix method. The wavelength dependencies of reflection coefficients for periodic array with broken c-shaped and circle metal elements are presented.

Ekaterina O Iarko

Papers

Trapping of light by metal arrays

Giant red shift and enhancement of resonant light confinement in planar array of dielectric bars

A giant red shift and enhancement of the light confinement in a planar array of dielectric bars

Trapped-Mode Resonances in Light Diffraction by a Planar Doubly Periodic Structure with Asymmetric Metal Elements

Trapped-mode resonances in optical planar metamaterials with metal elements