Session B62: Nanostructures and Metamaterials II

Nonlinear Optical Metasurfaces

Nonlinear optics is a decades-old, well-established discipline that normally relies on macroscopic media and propagation lengths that are much longer than the wavelength. Recent progress in artificially structured materials has enabled a resurgence of this field into new directions and phenomena. Examples are increased efficiencies using materials that were not useful for bulk nonlinear optics and complete relaxation of phase matching conditions. In this talk I will cover some of these new developments in nonlinear optical metasurfaces and hybrid nonlinear metasurfaces that include semiconductor quantum wells.   

Microwave hybrid resonance with an electromagnetic metasurface

Hybrid resonances were discovered in acoustics a few years ago. Here we demonstrate through full waveform simulations the realization of microwave hybrid resonance by using a simple H-fractal metallic metasurface, with unit cell’s lateral dimension much subwavelength in size. With an extremely thin back cavity, the resonances of the metallic structure at different frequencies can be hybridized to generate a new mode near the anti-resonance frequency. The oscillator strength and dissipation power of the hybrid resonance can be easily tuned, with total absorption occurring when the surface impedance matches that of vacuum. Similar to the acoustic case, the local fields are found to be much larger than the incident wave amplitude while the surface averaged fields are comparable to the incident fields. The total thickness of the surface is less than the peak absorption wavelength by two orders of magnitude. And we also found this mircowave hybrid resonance by using the metallic metasurface and its complementary structure, which is hard to achieve in acoustics.   

Regular sloshing modes in irregular cavity using metabathymetry

We demonstrate experimentally and numerically that metamaterials can be used to control water wave propagation and resonance properties of a closed cavity. The anisotropic medium, designed using coordinate transformation theory [1] and the homogenization of fully three-dimensional linear water wave problem [2], consists of a bathymetry with a layered structure at a subwavelength scale. Three cavities with bending angles of 15°, 30° and 45° are tested and compared to a reference case with flat bathymetry. Fourier Transform Profilometry [3] as well as Confocal Displacement Sensors are used for space-time resolved measurements of a water surface deformation. Experimental data show the capability of water-wave metamaterials to provide a robust anisotropic medium for wave propagation.
References
[1] C.P. Berraquero, A. Maurel, P. Petitjeans and V. Pagneux. Experimental realization of a water-wave metamaterial shifter. Phys. Rev. E 88, 051002(R) (2013)
[2] A. Maurel, J.-J. Marigo, P. Cobelli, P. Petitjeans and V. Pagneux. Revisiting the anisotropy of metamaterials for water waves. Phys. Rev. B 96, 134310 (2017)
[3] P. Cobelli, A. Maurel, V. Pagneux and P. Petitjeans. Global measurement of water waves by Fourier transform profilometry. Exp Fluids 46, 1037-1047 (2009)   

Measurement of asymmetrical water waves wake due to an anisotropic bathymetry

Metamaterials have the surprising property of modifying the wave propagation, and more precisely in our investigation, the waves forming the wakes. One example of wakes in metamaterials is the study of Luo et al [1] describing different behavior for the Cerenkov radiation in a photonic crystal. In our case, we consider ship wakes. This domain have received considerable attention over the last decade, even though this specific field of water waves has been studied for almost a century and a half, if we refered to the Lord Kelvin studies.
We show, experimentaly, that ship wakes can become highly asymmetrical when they propagate on an anisotropic metamaterial. In our study, the metamaterial is obtained with a periodic stratified bathymetry which varies at a subwavelength order. More precisely, vertical plates are placed in parallel on the bottom of a water tank, as in [2,3]. The experimental results can be numerically reproduced using a theory based on an anisotropic dispersion relation [3].
[1] C. Luo, M. Ibanescu, S. G. Johnson, and J. D. Joannopoulos, Science 299, 5605, (2003)
[2] C. P. Berraquero, A. Maurel, P. Petitjeans, and V. Pagneux, Physical Review E 88, 051002 (2013)
[3] A. Maurel, J.-J. Marigo, P. Cobelli, P. Petitjeans, and V. Pagneux, Physical Review B 96 (2017)   

High-index nanostructures and layered materials for light scattering control

Planar optical elements with efficient light control at the nanoscale can be designed based on transdimensional photonic lattices that operate in the translational regime between two and three dimensions. Such transdimensional lattices include 3D-engineered nanoantennas supporting multipole Mie resonances and arranged in the 2D arrays to harness collective effects in the nanostructure [1]. Optical antennas made out of van der Waals material with naturally-occurring hyperbolic dispersion is a promising alternative to plasmonic and high-refractive-index dielectric structures in the practical realization of nanoscale photonic elements. The antenna made out of hexagonal boron nitride (hBN) possesses different multipole resonances enabled by the supporting high-k modes and their reflection from the antenna boundaries. The full range of the resonances is demonstrated for the hBN cuboid antenna, a decrease of reflection from the array, and highly directional resonant scattering from antennas pairs. We show that transdimensional lattices consisting of resonant hBN antennas in the engineered periodic arrays have great potential to serve as functional elements in ultra-thin optical components and photonic devices. [1] V.E. Babicheva, MRS Advances 4, 713 (2019).   

Towards perfect absorption via block copolymer designed metasurfaces

Engineering light-matter nanoscale interactions has diverse applications for advancing optical nanodevices, sensors, and energy harvesting. An emerging route to realise perfect absorption is based on artificial ultrathin nanosurfaces, known as “metasurfaces”.1 We describe a versatile metasurface fabrication strategy based upon gold (Au) selective deposition in block copolymer (BCP) templates towards perfect absorption. Since BCPs can be patterned over large wafer scale areas (i.e. 300 mm) and are industry compatible, they offer a viable path towards realising nanophotonic devices.2 Here, we describe a Au−Al₂O₃−Au stack layer for perfect absorption at visible frequencies using BCP templating. Our approach opens up a flexible methodology to precisely tune Au nanostructure height, e.g. 5-30 nm and resulting absorption wavelength range. The ability to tailor Au features precisely is extremely appealing and surpasses various wet chemical approaches that cannot be processed in thin film form. We will present key experimental parameters guided by numerical simulations to show the effect of Au height and stack architecture on absorption properties.

1. A. Alvarez-Fernandez, et. al., Nanoscale Adv., 1, 849–857 (2019).
2. C. Cummins et. al. Advanced Materials 28 (27), 5586-5618 (2016).
  

Femtosecond Polarization Pulse Shaping by Dielectric Metasurfaces

Metasurfaces are ultra-thin, planar optical elements that have been successfully employed for a variety of spatial-domain wavefront manipulations[1]. Recently, the time-domain shaping of a large bandwidth, near-infrared femtosecond pulse was demonstrated using dielectric metasurfaces. Simultaneous and independent control of the phase and amplitude of frequency components of a pulse enables finely tailored pulse-shaping operations, including splitting, compression, chirping and higher-order distortion[2]. Here, we further exploit dielectric metasurfaces to control the temporal polarization state within a single pulse. Such an approach expands the versatility of the already fruitful metasurfaces, revealing new possibilities in the field of ultrafast science and technology.
[1] N. Yu and F. Capasso, Nature Materials 13, 139 (2014).
[2] S. Divitt, W. Zhu, C. Zhang, H. J. Lezec, and A. Agrawal, Science 364, 890 (2019).   

Broadband Linear-to-Circular Polarization Conversion Enabled by Birefringent Off-Resonance Reflective Metasurfaces

Circularly polarized light in the terahertz (THz) regime is essential for a variety of scientific research and technological applications. However, it is challenging to achieve broadband circular polarization at these long wavelengths due to the dispersion of common birefringent materials. Here we propose and demonstrate the broadband linear-to-circular polarization conversion with up to 80% fractional bandwidth and near-unity power conversion efficiency based on the coupled-mode theory using reflective birefringent metasurfaces [1]. The arbitrary rotation of a linear polarization can also be achieved by leveraging the design parameters of the unit cell. This novel design approach can be further expanded to the mid-infrared and visible regimes by simply scaling the size of the structure. The realization of the polarization converter and rotator may significantly enrich the polarization-based optical probe of matter, including circular dichroism spectroscopy at THz regime and studies of the rising quantum materials.
[1] C.C. Chang*, Z. Zhao*, D. Li*, A.J. Taylor, S. Fan, and H.T. Chen, “Broadband Linear-to-Circular Polarization Conversion Enabled by Birefringent Off-Resonance Reflective Metasurfaces”. Accepted by Phys. Rev. Lett. (2019)   

Constructing broadband multifunctional wave plates with stereo-metastructures

Polarization is one of the most important properties of light. Over the past few years, the development of metamaterials offers different approaches to manipulate the polarization state. Most designs are based on resonantly coupling. Due to the high losses and strong dispersion, their working frequency ranges are usually narrow. The multifunction design is another approach to integrating the element. Realizing different functionalities with one element can miniaturize the element. However, due to the uncontrollable electro-magnetic coupling between different parts, the multifunctional metastructures is difficult. Here we demonstrated two 3D broadband multifunctional wave plates. Simulated and experimental results indicate that half wave plate and quarter wave plate functions can be realized separately at lower and higher frequency with one metastructure. Moreover, coding and display depending on polarization were also proposed. Our work provides a new idea for design of highly compact metastructure and shows prospects for applications in display technology.   

Optical properties of plasmonic metasurface with sub-nm gaps - Refractive index variation originating from electron transports -

Plasmonic metasurfaces consisting of two-dimensionally arrayed metallic nano-objects on a plane have drawn attention in terms of its exotic optical characteristics. Although investigations of metasurfaces conducted to date have focused on structures with sub-wavelength spatial scale, recent experimental studies have demonstrated those with much smaller size in which the gap distances between the nano-objects reach to sub-nm length where quantum mechanical effects becomes important.
In our presentation, we theoretically and numerically investigate the plasmonic metasurface with sub-nm gaps in terms of the refractive index variation due to quantum mechanical effects. To take into account quantum mechanical effects in the analysis, we employ time-dependent density functional theory(TDDFT) treating the constituent nano-particles by a jellium model. Furthermore, to distinguish the importance of the quantum mechanical effects, another simulation based on conventional classical electromagnetism is also employed and compared with the TDDFT. SALMON(https://salmon-tddft.jp/) developed by our group has been used for those numerical calculations. We will show that drastic change of the refractive index starts around gap distances 0.2 nm where electron transports take an important place.   

Dynamic Switching between Surface Wave and Plane Wave with MEMS-based Metasurface

We present polarization-insensitive dynamic surface wave (SW) switching at terahertz frequencies utilizing a MEMS-based metasurface. Our MEMS-based metasurface consists of a micro-cantilever array, enabling dynamic tuning between a plane wave (PW) and a SW for normal incidence of terahertz radiation. Through individual control of the driving voltages on the cantilever unit cells, we achieve nearly 2π phase modulation with minimal reflection amplitude modulation. Coupled mode theory (CMT) is employed to design the metasurface device and agrees with full-wave electromagnetic simulations. Our demonstration paves the road for terahertz multifunctional metasurface devices for spatial light modulation, dynamic beam steering, focusing, and beam coupling.
  

Dynamically-tunable metastructures based on phase transition of vanadium dioxide

Tunable nanophotonic materials and devices are drawing intense attention with great promise for practical applications. In this work, we have experimentally demonstrated several dynamically-tunable metastructured devices based on phase transition of vanadium dioxide, which include dynamic plasmonic color generators, dynamically switchable polarizers, and dynamically tunable bowtie nanoantennas. First, we have fabricated periodic arrays of silver-nanodisks on a vanadium dioxide film to realize different colors, relying on the excitation of localized and propagating surface plasmons. Based on insulator-metal transition of vanadium dioxide, the plasmonic colors can be actively tuned by varying temperature. Second, we have demonstrated a system consisting of anisotropic plasmonic nanostructures with vanadium dioxide that exhibits distinct reflections subjected to different linearly polarized incidence at room temperature and in the heated state. Third, we have made the dynamically tunable bowtie nanoantennas integrated on a vanadium dioxide thin film. The investigations here can be applied in dynamic digital displays, optical data storage, and imaging sensors. References: 1) Adv. Opt. Mater.(2018) 6, 1700939; 2) Phys. Rev. Appl. (2018) 9, 034009; 3) Opt. Lett. (2019) 44, 2752.   

Electrically created metasurfaces

Metasurfaces have enabled unprecedented control of the phase, amplitude, and polarization of optical wavefronts, providing new opportunities in areas such as negative refraction and optical cloaking. The fabrication of optical metasurfaces still requires complex processing techniques that include additive or subtractive methods. Here we electrically create metasurfaces in VO₂ thin film by using a resist layer with periodic boomerang-shaped orifices to strictly confine the metallic VO₂ antennas induced by ionic liquid gating within the insulating VO₂ matrix. The non-volatile insulator-to-metal transitionin VO₂ is due to a tiny reduction of oxygen content of the VO₂ under ionic liquid gating. Such optical plasmonic metasurfaces, with metallic VO₂ feature sizes ranging from 100 to 500 nm, allow the complete phase control (–π to π) and orthorhombic polarization switching of light in the mid infrared range. Our result represents a new paradigm for the fabrication and manipulation of optical metamaterials.