Nanostructured Metasurface Research Articles

Optical absorbing origin of chiroptical activity in planar plasmonic metasurfaces

As a significant characteristic of many biomolecules, chemical substances, and artificial nanostructures, chirality conduce different types of optical interactions with the spin angular momentum of the impinging light field. Although, chiral arrangement and spatial phase retardation are the key factors for obtaining chirality in three-dimensional (3D) structures, the origin of chirality in the feasible planar structures has not been thoroughly addressed. Here using an intuitive and simple analytical approach, called cross-hybridization model, the essence and properties of the optical chirality of individual planar nanostructures are unveiled. In order to fundamentally address this chirality in terms of circular dichroism (CD), the chiroptical response of a simple dimer composed of the lossy nanoblocks in L-shape arrangement are investigated based on the provided optical interaction and loss effects. The theoretical findings, adequately supported by the numerical calculations, reveal that chiroptical activity occurs predominantly due to handedness-dependent absorption or heating loss in a nanostructured metasurface.

Shaping a far-field optical needle by a regular nanostructured metasurface

A new method is proposed for shaping a far-field uniform optical needle based on a regular nanostructured metasurface, viz. high-NA (numerical aperture) micro-Fresnel zone plate (FZP). The designed microstructure is comprised of two planar FZP-fragments with different focal lengths. Delicate interference of diffracted beams results in an optical needle at a required working distance in the post-evanescent field. For a 44.88µm-diameter microstructure illuminated with a linearly x-polarized beam, a 5.77λ-long uniform optical needle is produced at a distance of 12.88λ away from the mask surface. The transverse beam sizes are 0.97λ and 0.39λ in x and y directions, respectively. The designed result calculated by the vectorial angular spectrum theory is in good agreement with the rigorous electromagnetic calculation using the three-dimensional finite-difference time-domain (3D FDTD) method. The designed microstructure is easy-to-fabricated with required NA and working distance. The proposed method can be readily modified for other polarized beams. These far-field uniform optical needles potentially suit the fields of nanolithography, optical trapping, and microscopy.

Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials.

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

Atomic Response in the Near-Field of Nanostructured Plasmonic Metamaterial.

We report on reflection spectra of cesium atoms in close vicinity of a nanostructured metallic meta-surface. We show that the hyperfine sub-Doppler spectrum of the 6(2)S1/2-6(2)P3/2 resonance transition at 852 nm is strongly affected by the coupling to the plasmonic resonance of the nanostructure. Fine tuning of dispersion and positions of the atomic lines in the near-field of plasmonic metamaterials could have uses and implications for atom-based metrology, sensing, and the development of atom-on-a-chip devices.

Electromagnetic Cloaking of a Finite Conducting Wedge With a Nanostructured Graphene Metasurface

Here, we present the analytical formulation for the analysis of electromagnetic interaction with a finite conducting wedge covered with a cylindrically shaped nanostructured graphene metasurface, resulting in the scattering cancellation of the dominant scattering mode for all the incident and all the observation angles. By properly tuning the bias of graphene, it is shown that the surface reactance of graphene nanopatches can be adjusted for cloaking of wedges with an arbitrary angle of opening. It is also shown that the tunable cylindrical graphene metasurface can be effectively used for cloaking of several concentric finite conducting wedges. In addition, a wedge shaped metasurface is proposed as an alternative approach for cloaking of finite wedges. The analytical results are verified with the full-wave electromagnetic solver CST Microwave Studio, showing good agreement for different wedge scenarios.

Cloaking of single and multiple elliptical cylinders and strips with confocal elliptical nanostructured graphene metasurface

In this paper, we present a novel analytical approach for cloaking of dielectric and metallic elliptical cylinders with a graphene monolayer and a nanostructured graphene metasurface at low-terahertz frequencies. The analytical approach is based on the solution of the electromagnetic scattering problem in terms of elliptical waves represented by the radial and angular even and odd Mathieu functions, with the use of sheet impedance boundary conditions at the metasurface. It is shown that scattering cancellation occurs for all incident and observation angles. A special case concerns cloaking of a 2D metallic strip represented by a degenerated ellipse, wherein the focal points of the cloak metasurface correspond to the edges of the strip. The analytical approach has been extended in order to cloak a cluster of elliptical objects for different cases of closely spaced, merging, and overlapping configurations. The results obtained by our analytical approach are validated with full-wave numerical simulations.

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale.

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.