Optical polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications, such as information delivery, 3D imaging, and quantum computation. Metasurfaces, sort of artificially designed planar structure, have attracted immense attention due to their ability to control the amplitude and phase of electromagnetic waves at a subwavelength scale. Metasurfaces hold promise for the fields of nonlinear dynamics, light beam shaping, quantum computation, etc. Beside these promising applications, metasurfaces can also be used for versatile polarization generation in a compact device dimension. Therefore, metasurfaces can be used for creation of flat optical devices with novel functionalities. In this talk, I will discuss how we can generate versatile polarization states by using metasurfaces. Firstly, I will describe geometric phase metasurfaces, which can be used for passive polarization control. We demonstrate six metasurface chips integrated on a single sample, in which each chip is responsible for generating one specific polarization state thus generating versatile polarization sates. Subsequently, I will discuss a scheme of active polarization modulation by using indium tin oxide (ITO)-based tunable metasurfaces. By suitably biasing the metasurface, the linearly-polarized incident light can be actively converted to a cross-polarized, circularly-polarized or elliptically-polarized light.
The ability to control all the important constitutive properties of light via interaction with nanoscale elements is a central concept in nanophotonics. In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces has typically been encoded at the time of fabrication. However, active control of the properties of metasurfaces would enable dynamic holograms, focusing lenses with reconfigurable focal lengths, and beam steering, a key requirement for future chip-based light detection and ranging (LIDAR) systems. Here, we report the design and experimental demonstration of a continuous beam steering at telecommunication wavelengths using field-effect-tunable metasurfaces. The proposed beam steering device is actively controlled by incorporating indium tin oxide (ITO), as a material with voltage-tunable optical properties, into a metasurface. Using ITO as an active material and a composite hafnium-aluminum-oxide nanolaminate as the gate dielectric, we demonstrate a prototype tunable metasurface with a continuous phase shift from 0 to 300°. Our design enables independent control of each metasurface element via an individual application of DC voltage. This enables electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continuous beam steering. By careful application of bias voltages to 96 biasing channels, we achieve a quasi-continuous beam steering with the steering angles of up to 75°.
The functionalities of traditional optical component are mainly based on the phase accumulation through the propagation length, leading to a bulky optical component such as converging lens and waveplate. Metasurfaces composed of planar structures with artificial design have attracted a huge number of interests due to their ability on controlling the electromagnetic phase as well as amplitude at a subwavelength scale. The feasible applications based on metasurfaces include nonlinear dynamics, light beam shaping, quantum interference etc. Beside those promising characteristics, people now intend to discover the field of meta-devices, where we can attain optical properties and functionalities through changing the feature characteristics of metasurfaces in demand. They therefore pave a potential way for the development of flat optical devices and integrated optoelectronic systems and toward the far-reaching applications which are impossible previously. In this talk, four research topics for photonic applications with metasurfaces and meta-devices will be performed and discussed: high efficiency anomalous beam deflector, highly dimensional holographic imaging, versatile polarization control and metadevices with active property.
Metasurfaces, the two-dimensional (2D) sub-wavelength artificial structures, where light is not required to have a deep penetration, have shown the ability to tailor the amplitude, phase and polarization of light. The functionalities of various optical components can be realized by metasurface-based design, such as beam splitters, filters, waveplates, deflector, lens and holograms. Here, we propose a new type of metasurface based on the concept of ultra-thin film interference and experimentally demonstrate its feasibilities in beam deflector, light focusing and broadband meta-hologram in visible spectrum. Considering an ultra-thin thin film interference system, a sandwich structure, composed of air, a lossy material layer and a metallic mirror, the reflection of this system can be regarded as the linear superposition of the partial reflections from first interface and from the cavity after several roundtrips. First, we calculate the phases and reflections of various thicknesses of amorphous silicon (a-Si) on top of aluminum layer under normal illumination of an unpolarized light in the wavelength region from 400 to 850nm. 2 π phase coverage can be achieved by changing the film thickness of a-Si within 50 nanometers. We select two thicknesses (2-level phase modulation) for the demonstration of meta-devices. The ultra-flat grating metasurface for beam steering are designed. The reflection angles of grating metasurface can be modulated by changing its period, while the specular reflection is inhibited. We further demonstrate computer-generated holograms (CGH) based on ultra-thin interference metasurface. The holographic images are reconstructed by the combinations of phase- and amplitude- modulation. These devices show the great potential and CMOS-compatibility in the application of optics, display, security printing, and metasurface-based optical storage system.
Natural toroidal molecules, such as biomolecules and proteins, possess toroidal dipole moments that are hard to be
detected, which leads to extensive studies of artificial toroidal materials. Recently, toroidal metamaterials have been
widely investigated to enhance toroidal dipole moments while the other multipoles are eliminated due to the spacial
symmetry. In this talk, we will show several cases on the plasmonic toroidal excitation by engineering the near-field
coupling between metamaterials, including their promising applications. In addition, a novel design for a toroidal
metamaterial with engineering anapole mode will also be discussed.
Metamaterials, with the ability of tailoring optical properties of materials, have been applied to holograms recently, which has shown the priorities of switchable polarization and multicolor image comparing with the conventional holograms. However, the current metasurface based multicolor holograms have suffered the problems of narrow band and low efficiency in phase modulation for gold and silver when their feature dimensions are in few tens of nanometers. Interestingly, aluminum with higher plasma frequency could yield surface plasmon resonance across a broader range of the spectrum ranging from visible to UV. Metasurfaces incorporating with the aluminum offer the unique opportunity to extend the working wavelength to cover the entire visible spectrum for the generation of full color meta-holograms.
Here we demonstrated a phase modulated multicolor meta-hologram that is polarization dependent and capable of producing images in red, green and blue colors. The metahologram is made of aluminum nanorods that are arranged in a two-dimensional array of pixels with surface plasmon resonance in the visible to UV range. The aluminum nanorod array is patterned on a 30 nm thick SIO2 spacer layer sputtered on top of a 130nm thick aluminum mirror. With proper design of the structure, we obtain resonances of narrow bandwidths to allow for implementation of multicolor scheme. Taking into account of the wavelength dependence of the diffraction angle, we can project images to specific locations with predetermined size and order. With tuning of aluminum nanorod size, we demonstrate that the image color can be continuously varied across the visible spectrum.
The toroidal dipole moments of natural molecules are hard to be detected so the artificial toroidal materials made by metamaterial attract more attentions. Metamaterial, the sub-wavelength artificial structures, can modulate reflection or transmission of light. The toroidal metamaterial can not only amplify the toroidal moment but also repress the electric and magnetic dipole so it can be used to study the properties of toroidal dipole moment. However, there are many limitations for the experiments, such as the lateral light is necessary to excite the toroidal response. Most of the toroidal dipole moments oscillate perpendicularly to the substrate, therefore it is difficult to couple it with other dipole moments and could be only excited in the microwave region. In this paper, we design a toroidal metamaterial consisting of dumbbell-shaped aperture and vertical split ring resonator (VSRR) vertically. The toroidal dipole moment of our metamaterial is excited in the optical region. The arrangement of our nanostructures is vertical instead of planar annular arrangement to reduce the size of the unit cell and increase the density of the toroidal dipole moment. Moreover, the direction of toroidal dipole moment is parallel to the substrate which can be used for the study of the coupling effect with other kinds of dipolar moments.
Split-ring resonator (SRR), one kind of building block of metamaterials, attracts wide attentions due to the resonance excitation of electric and magnetic dipolar response. The fundamental plasmonic properties and potential applications in novel three dimensional vertical split-ring resonators (VSRRs) are designed and investigated. The resonant properties arose from the electric and magnetic interactions between the VSRR and light are theoretically and experimentally studied. Tuning the configuration of VSRR unit cells is able to generate various novel coupling phenomena in VSRRs, such as plasmon hybridization and Fano resonance. The magnetic resonance plays a key role in plasmon coupling in VSRRs. The VSRR-based refractive-index sensor is demonstrated. Due to the unique structural configuration, the enhanced plasmon fields localized in VSRR gaps can be lifted off from the dielectric substrate, allowing for the increase of sensing volume and enhancing the sensitivity. We perform a VSRR based metasurface for light manipulation in optical communication frequency. By changing the prong heights, the 2π phase modulation can be achieved in VSRR for the design of metasurface which can be used for high areal density integration of metal nanostructures and optoelectronic devices.
Toroidal dipole moments, the third kind of fundamental dipole moment, have unusual electromagnetic properties
different from the electric and magnetic multipoles. We fabricate a new type of 3D plasmonic toroidal metamaterial by
using mutual coupling between dumbbell-shaped gold apertures with vertical split-ring resonators (VSRRs) at optical
frequency. The radiated power of multipole moments are calculated and analyzed to improve the meta-system is
dominated by the toroidal dipole moment. This result paves a way for practical application on metamaterial based
devices, such as biosensor and lasing spaser.
Holograms, the optical devices to reconstruct pre-designed images, have been evolved dramatically since the advances in today’s nanotechnology [1-4]. Metamaterials, the sub-wavelength artificial structures with tailored refraction index, enable us to design the meta-hologram working in arbitrary frequency region. Here we demonstrated the first reflective type, dual image and high efficient meta-hologram with the incident angle as well as the coherence of incident wave insensitivity in visible region at least from λ = 632.8 nm to λ = 850 nm. The meta-hologram is composed of 50-nm-thick gold cross nano-antenna coupled with 130-nm-thick gold mirror with a 50-nm-thick MgF<sub>2</sub> as spacer. It shows different images “RCAS” and “NTU” with high image contract under x- and y-polarized illumination, respectively. Making use of the characteristic of meta-materials, these optical properties of proposed meta-hologram can be transferred to arbitrary electromagnetic region by scale-up the size of the unit cell of meta-hologram, leading to more compact, efficient and promising electromagnetic components.
Split ring resonator (SRR) has attracted wide attentions since the discovery of negative refraction in 2002. Here, we
designed and fabricated vertical SRR (VSRR) arrays and toroidal metamolecule by using double exposure e-beam
lithography with precise alignment technique, and their resonance behaviors are subsequently studied in optical region.
The fundamental resonance properties of VSRR are studied as well as the plasmon coupling in a VSRR dimer structure
by changing the gap distance between SRRs. In addition, we proposed a three-dimensional toroidal structure composed a
VSRR with a dumbbell structure that supported a toroidal resonance under normal incidence with broadband working
frequency. Such toroidal metamaterial confines effectively the electric as well as magnetic energy paving a way for
promising applications in the field of plasmonics, such as integrated 3D plasmonic metamaterials, plasmonic biosensor
and lasing spaser.
The toroidal as well as magnetic spectral responses at optical frequencies by integrating four gold U-shaped split-ring
resonators (SRRs) are numerically studied. We study two kinds of toroidal structures; the first one is consisted of four-up
U-shaped SRRs. The second kind, two of the four U-shaped SRRs is reversed showing two-up-two-down configuration.
By reversing two SRRs of toroidal structure, their toroidal resonance and magnetic resonance are also reversed between
higher and lower ones. The optical properties of toroidal resonance are also investigated in this paper.