Nondispersive optical activity of meshed helical metamaterials

Chiro-optical response of a wafer scale metamaterial with ellipsoidal metal nanoparticles

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Accelerating polarization structures in vectorial fields

We generate optical fields whose polarization structures not only rotate about their propagation axis but also can be controlled to accelerate independently from their spatial profile. We show that by combining accelerated intensity transport with orthogonal polarization states, we can produce a vector beam that displays optical activity with periodical acceleration and deceleration of the Stokes vector during propagation. We achieve this with orthogonal, scalar fields, represented by weighted superpositions of oppositely charged Bessel beams. In addition to their creation, we show that the Stokes vector can be made to accelerate or decelerate at specific locations along the Poincaré sphere by tailoring the generating basis. We also witness an optical current, or intensity transport, between local positions in the field that corresponds with the occurrence of the state-of-polarization accelerating or decelerating.

Phase-Transition Optical Activity in Chiral Metamaterials

Optical activity from chiral metamaterials is both fundamental in electrodynamics and useful for polarization control applications. It is normally expected that due to infinitesimally small thickness, two-dimensional (2D) planar metamaterials cannot introduce large optical rotations. Here, we present a new mechanism to achieve strong optical rotation up to 90° by evoking phase transition in the 2D metamaterials through tuning coupling strength between meta-atoms. We analytically elucidate such phenomenon by developing a model of phase-transition coupled-oscillator array. And we further corroborate our ideas with both numerical simulations and experiments. Our findings would pave a new way for applying the concept of phase transition in photonics for designing novel optical devices for strong polarization controls and other novel applications.

Constructing Metastructures with Broadband Electromagnetic Functionality

Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade-off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology.

Single-Layer Metasurfaces as Spectrally Tunable Terahertz Half- and Quarter-Waveplates

We propose a single-layer terahertz metasurface that acts as an efficient terahertz waveplate, providing phase retardation of up to 180° with a tunable operation frequency. Designed with the tight coupling of elementary resonators, our metasurface provides extraordinarily strong hyperbolicity that is closely associated with the distance between resonators, enabling both significant phase retardation and spectral tunability through mechanical deformation. The proposed concept of terahertz waveplates based on relatively simple metastructures fabricated on stretchable polydimethylsiloxane is experimentally confirmed using terahertz spectroscopy. It is believed that the proposed design will pave the way for a diverse range of terahertz applications.

Multiband circular dichroism from bilayer rotational F4 nanostructure arrays

A chiral nanostructure array is designed, which is composed of a bilayer rotational F4-shaped nanoarray configuration. The surface plasmon resonance and circular dichroism are studied by changing the parameters of the structure. The results show that the structure has strong multiband circular dichroism, which is attributed to the coupling of the layers. In theory, based on the Born-Kuhn model, the upper and lower nanostructures are equivalent to electric dipoles. By analyzing the coupling mode of electric dipoles in the upper and lower layer, the mechanism of circular dichroism and the shift of the circular dichroism resonance are revealed. Besides, there are several specific modes that are fault tolerant of fabrication issues. This feature unveils the bright prospect of spectral anti-interference. So, the suggested chiral nanostructure can be used in biologically targeted molecular detection and spectral sensing.

Near-infrared chiral plasmonic metasurface absorbers

Chirality plays an essential role in the fields of biology, medicine and physics. However, natural materials exhibit very weak chiroptical response. In this paper, near-infrared chiral plasmonic metasurface absorbers are demonstrated to selectively absorb either the left-handed or right-handed circularly polarized light for achieving large circular dichroism (CD) across the wavelength range from 1.3 µm to 1.8 µm. It is shown that the maximum chiral absorption can reach to 0.87 and that the maximum CD in absorption is around 0.70. The current chiral metasurface design is able to achieve strong chiroptical response, which also leads to high thermal CD for the local temperature increase. The high-contrast reflective chiral images are also realized with the designed metasurface absorbers. The demonstrated chiral metasurface absorbers can be applied in many areas, such as optical filters, thermal energy harvesting, optical communication, and chiral imaging.

Chiral Metamaterials of Plasmonic Slanted Nanoapertures with Symmetry Breaking

We propose a universal design scheme for a new type of chiral metamaterials based on plasmonic slanted nanoapertures simply milled in a single metal layer. Strong optical chirality is introduced by tilting nanoapertures with almost arbitrary shape along a certain direction to break all the mirror symmetries. As a typical example, chiral metamaterial based on slanted split-ring apertures is demonstrated with giant circular dichroism in transmission (CDT) over 78% at 760 nm. We reveal that the high CDT originates from the circularly dichroic mode coupling process in the slanted nanoapertures induced by spin-dependent field overlap conditions. Furthermore, tunable CDT is presented through the in-plane rotation of nanoapertures to form chiral images with controllable image contrast. Besides, chiral metamaterials with slanted nanoapertures of two other shapes including L-shaped aperture and rectangular aperture are also presented with large circular dichroism. We envision that our demonstrated chiral metamaterials enable promising platforms for a variety of applications in nonlinear optics, chiral imaging and sensing, spectroscopy, and polarization manipulation.

Resonant cavity enhanced waveguide transmission for broadband and high efficiency quarter-wave plate

Existing transmission type optical quarter-wave plates based on metallic sub-wavelength structures can hardly realize transmission efficiency above 60%. And their working bandwidths are still very narrow. In this paper, we demonstrate a transmission type quarter-wave plate design with efficiency above 92% over a broad wavelength range (from 1260 nm to 1560 nm). The device proposed is based on a one-dimensional metal-insulator-metal waveguide array buried in silica. Phase difference between transmitted TE and TM components can be tuned continuously. At the same time, transmission efficiency can be kept above 90% in the same spectral range for both the TE and TM incidences. The broad bandwidth and remarkable efficiency are explained with the combination of low dispersion of waveguide modes and the resonant cavity enhanced transmission effect. To give a better understanding of the structure, we also propose a modified effective medium model. The optical response of the structure can be well reproduced with the semi-analytic effective medium model.

Dynamic three-dimensional multifocal spots in high numerical-aperture objectives

Multifocal spots in high numerical-aperture (NA) objectives has emerged as a rapid, parallel, and multi-location method in a multitude of applications. However, the typical method used for forming three-dimensional (3D) multifocal spots based on iterative algorithms limits the potential applications. We demonstrate a non-iterative method using annular subzone phases (ASPs) that are composed of many annular subareas in which phase-only distributions with different 3D displacements are filled. The dynamic 3D multifocal spots with controllable position of each focal spot in the focal volume of the objective are created using the ASPs. The experimental results of such dynamic tunable 3D multifocal spots offer the possibility of versatile process in laser 3D fabrication, optical trapping, and fast focusing scanned microscopic imaging.

Broadband angle- and permittivity-insensitive nondispersive optical activity based on planar chiral metamaterials

Because of the strong inherent resonances, the giant optical activity obtained via chiral metamaterials generally suffers from high dispersion, which has been a big stumbling block to broadband applications. In this paper, we propose a type of planar chiral metamaterial consisting of interconnected metal helix slat structures with four-fold symmetry, which exhibits nonresonant Drude-like response and can therefore avoid the highly dispersive optical activity resulting from resonances. It shows that the well-designed chiral metamaterial can achieve nondispersive and pure optical activity with high transmittance in a broadband frequency range. And the optical activity of multi-layer chiral metamaterials is proportional to the layer numbers of single-layer chiral metamaterial. Most remarkably, the broadband behaviors of nondispersive optical activity and high transmission are insensitive to the incident angles of electromagnetic waves and permittivity of dielectric substrate, thereby enabling more flexibility in polarization manipulation.

High efficiency broadband -90° to 90° arbitrary optical rotation realized with meta reflectarray

We theoretically demonstrate high efficiency broadband -90° to 90° arbitrary optical rotation realized with meta reflectarray composed of a L-shaped silver antenna array, a silica spacer, and a silver ground plane. Co-polarized and cross-polarized components of reflected wave can be manipulated efficiently by adjusting arm length of the L-shaped antenna, and 0° to 90° arbitrary optical rotation with high degree of linear polarization (DoLP) over a broadband can be achieved readily. The phase of cross-polarized field component can be reversed by turning the L-shaped antennas upside down, and 0° to 90° optical rotation can be turned into 0° to -90° rotation. Reflected phase can be shift by π after a 90° rotation of the L-shaped or Γ-shaped antennas, while optical rotation angle remains the same. Thus, rotation angle θ is changed to 180° + θ after the rotation, and we realized 0° to 360° polarization rotation with a step of 60° with the combination of six discrete structure units. In addition, we proposed metamaterial structures for highly efficient generation of vector beams with these units. The high efficiency broadband arbitrary angle optical rotation will profoundly affect a wide range of applications involving optical polarization.

A terahertz in-line polarization converter based on through-via connected double layer slot structures

A terahertz (THz) in-line polarization converter that yields a polarization conversion ratio as high as 99.9% is demonstrated at 1 THz. It has double-layer slot structures oriented in orthogonal directions that are electrically connected by 1/8-wavelngth-long through-via holes beside the slot structures. The slots on the front metal-plane respond to the incident THz wave with polarization orthogonal to the slots and generates a circulating surface current around the slots. The surface current propagates along a pair of through-via holes that function as a two-wire transmission line. The propagating current generates a surface current around the backside slot structures oriented orthogonal to the slot structures on the front metal layer. The circulating current generates a terahertz wave polarized orthogonal to the backside slot structures and the 90° polarization conversion is completed. The re-radiating THz wave with 90° converted polarization propagates in the same direction as the incident THz wave.

Controllable optical activity with non-chiral plasmonic metasurfaces

Optical activity is the rotation of the plane of linearly polarized light along the propagation direction as the light travels through optically active materials. In existing methods, the strength of the optical activity is determined by the chirality of the materials, which is difficult to control quantitatively. Here we numerically and experimentally investigated an alternative approach to realize and control the optical activity with non-chiral plasmonic metasurfaces. Through judicious design of the structural units of the metasurfaces, the right and left circular polarization components of the linearly polarized light have different phase retardations after transmitting through the metasurfaces, leading to large optical activity. Moreover, the strength of the optical activity can be easily and accurately tuned by directly adjusting the phase difference. The proposed approach based on non-chiral plasmonic metasurfaces exhibits large optical activity with a high controllable degree of freedom, which may provide more possibilities for applications in photonics.

Chiral metamaterials: enhancement and control of optical activity and circular dichroism

The control of the optical activity and ellipticity of a medium has drawn considerable attention due to the recent developments in metamaterial design techniques and a deeper understanding of the light matter interaction in composite metallic structures. Indeed, recently proposed designs of metaatoms have enabled the realisation of materials with unprecedented chiral optical properties e.g. strong optical activity, broadband optical activity, and nondispersive zero ellipticity. Combining chiral metamaterials with nonlinear materials has opened up new possibilities in the field of nonlinear chirality as well as provided the foundation for switchable chiral devices. Furthermore, chirality together with hyperbolicity can be used to realise new exciting materials such as photonic topological insulators. In this review, we will outline the fundamental principles of chiral metamaterials and report on recent progress in providing the foundations for promising applications of switchable chiral metamaterials.

Dynamic manipulation of particles via transformative optofluidic waveguides

Optofluidics is one of the most remarkable areas in the field of microfluidic research. Particle manipulation with optofluidic platforms has become central to optical chromatography, biotechnology, and μ-total analysis systems. Optical manipulation of particles depends on their sizes and refractive indices (n), which occasionally leads to undesirable separation consequences when their optical mobilities are identical. Here, we demonstrate rapid and dynamic particle manipulation according to n, regardless of size. Integrated liquid-core/solid-cladding (LS) and liquid-core/liquid-cladding (L²) waveguides were fabricated and their characteristics were experimentally and theoretically determined. The high and low n particles showed the opposite behaviors by controlling the contrast of their n values to those of the working fluids. The LS waveguide was found to successfully manipulate particles according to n, and the L² waveguide was found to provide additional system stability and flexibility, compared to the LS system.

Dispersion-free broadband optical polarization rotation based on helix photonic metamaterials

We present a helix photonic metamaterial that exhibits nondispersive optical rotation in a broad passband at optical frequencies. Several features, including zero dispersion, zero ellipticity, and high transmission, can be simultaneously achieved in the presented structure. Pure optical rotation with extremely low dispersion is exhibited in a broad band covering the optical telecommunication wavelengths along with high transmission above 95%. We show that the chiral responses as well as the wavelength-dependent properties of the passband are governed by the behaviors of adjacent resonances. A systematic study of the optical properties with various geometrical parameters is performed, where the dependence of passband properties on resonance behaviors is examined and discussed. Such broadband dispersion-free optical rotation at optical frequencies may be of great interest for high-performance polarization manipulation and relevant applications.