Nanostructured Metasurface Research Articles

Bi-functional and tri-channel image displays based on a single-size nanostructured metasurface enabled by dual-orientation-degeneracy

Benefiting from the extraordinary ability of manipulating lightwaves at the subwavelength scale, nanostructured metasurfaces are expected to achieve multifunctional and multichannel integration to expand functionality and increase information capacity. However, multifunctional and multichannel metasurfaces always consist of various anisotropic nanostructures, inevitably bringing challenges to design and fabrication. In this study, we propose a concept of dual-orientation-degeneracy containing twofold orientation degeneracy. The first-level degeneracy is a one-to-four mapping scheme between the intensity of Channel 1 and orientation angle and the second-level degeneracy refers to a one-to-two mapping between the intensity of Channel 2 and orientation angle. Additionally, we provide a minimalist design of bi-functional and tri-channel image displays based on a single-size nanostructured metasurface. The designed metasurface integrates two functionalities of nanoprinting and holography, which can generate a continuous grayscale meta-image, a binary meta-image and a phase-only holographic image. Three channel displays can be readily switched by polarization controls. More importantly, the metasurface is achieved merely by reconfiguring the orientation angles of the nanostructures with fixed geometries, relieving the structure design and fabrication burden. The presented minimalist design strategy is universal and applicable, which can contribute to advanced research and applications in ultra-compact image displays, high-dense optical storage, multi-folded optical anti-counterfeiting, etc.

Subwavelength confinement of a quantum emitter in ladder configuration adjacent to a nanostructured plasmonic metasurface

Subwavelength confinement of a quantum emitter in ladder configuration adjacent to a nanostructured plasmonic metasurface

Resonant Enhancement of Polymer-Cell Optostimulation by a Plasmonic Metasurface.

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders.

Stokes polarimetry with Poincaré–Hopf index beams

Polarization structured beams have immense potential in various applications. Poincaré–Hopf (PH) index beam contains structured polarization distribution represented by a ring corresponding to a constant latitude on the standard Poincaré sphere. These structured beams can be represented by points on a recently introduced higher order Poincaré sphere (HOPS). For every PH index, HOPS can be constructed. Beams exhibit diverse properties depending on the polarity and magnitude of the PH index. Two examples of positive PH index beams are the most commonly encountered beams with radial and azimuthal polarization distributions. Negative PH index beams are polarization structured optical beams in which the symmetry in the polarization distribution is not readily discernible. There can be a set of holonomically constrained polarization transformations that can be represented as trajectories on the surface of HOPS. With the advent of nanostructured metasurface optics, it is possible to fabricate elements that perform this set of holonomically constrained transformations. This mapping is useful in the study of the acquisition of topological phases of these beams. Further identifying the coordinates of the input and output beams can be useful in the study of chirality and dichroism. Chirality is due to differential phase shifts, whereas circular dichroism is due to differential absorption between the right and left handed circular polarization components. We present here a method called Stokes polarimetry on HOPS beams that can precisely locate the HOPS latitude and longitude coordinates for a given beam. This method is based on single intensity measurement and is envisaged to play a paramount role in the practical use of HOPS and unveiling its potential.

Dual-channel anticounterfeiting color-nanoprinting with a single-size nanostructured metasurface.

Metasurface-based structural-colors are usually implemented by changing the dimensions of nanostructures to produce different spectral responses. Therefore, a single-size nanostructured metasurface usually cannot display structural-colors since it has only one design degree of freedom (DOF), i.e., the orientation angles of nanostructures. Here, we show structural-color nanoprinting images can be generated with a single-size nanostructured metasurface, enabled by designing the anisotropic nanostructure with different spectral responses along its long- and short-axis directions, respectively. More interestingly, the concept of orientation degeneracy of nanostructures can be applied in the metasurface design, which shows two spectral modulations can be implemented under different polarization directions of output light, thus extending the color-nanoprinting from single-channel to dual-channel. The proposed dual-channel metasurface used for anticounterfeiting color-nanoprinting has presented the advantages of ultra-compactness, high information capacity, and vivid colors, which can develop broad applications in fields such as high-end anticounterfeiting, high-density information storage, optical encryption, etc.

Terahertz Pulse Generation from GaAs Metasurfaces.

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

Asymmetric hologram with a single-size nanostructured metasurface.

Geometric metasurfaces, governed by PB phase, have shown their strong polarization sensitivity and can generate opposite phase delay when the handedness of incident circularly-polarized (CP) light is opposite. Here, we show this interesting characteristic can be employed to generate asymmetric forward and backward propagation with the same incident left- or right-handed CP light, which is hard to achieve with conventional optical elements and devices. Specifically, with the modified holographic design algorithm to consider both forward and backward CP light, an asymmetric meta-hologram is designed, which can project two different holographic images in the forward and backward directions, respectively. We demonstrate this concept by fabricating an asymmetric hologram with a single-size nanostructured metasurface, and the experimentally obtained holographic images in both directions have shown their advantages of high fidelity, broadband response and low crosstalk. The proposed asymmetric metasurface can play an important role in data storages, anti-counterfeitings, optical communications, displays and many other related fields.

Enhancement of Optical Chirality Using Metasurfaces for Enantiomer-Selective Molecular Sensing

Circular dichroism (CD) is a physical property observed in chiral molecules by inducing the difference of absorption between left- and right-handed circularly polarized light (CPL). Circular dichroism spectroscopy is widely used in the field of chemistry and biology to distinguish the enantiomers, which typically show either positive or severe side effects in biological applications depending on the molecular structures’ chirality. To effectively detect the chirality of molecules, diverse designs of nanostructured platforms are proposed based on optical resonances that can enhance the optical chirality and amplify the signal of circular dichroism. However, the underlying physics between the optical chirality and the resonance in a nanostructure is largely unexplored, and thus designing rules for optimal chiral detection is still elusive. Here, we carry out an in-depth analysis of chiral enhancement (C enhancement) in nanostructured surfaces to find the relationship between optical resonances and chirality. Based on the relations, we optimize the nanostructured metasurface to induce effective chiral detection of enantiomers for diverse conditions of molecule distribution. We believe that the proposed designing rules and physics pave the important pathway to enhance the optical chirality for effective circular dichroism spectroscopy.

Endless Single-Mode Photonics Crystal Fiber Metalens for Broadband and Efficient Focusing in Near-Infrared Range.

The advent of the ‘lab-on-fiber’ concept has boosted the prosperity of optical fiber-based platforms integrated with nanostructured metasurface technology which are capable of controlling the light at the nanoscale for multifunctional applications. Here, we propose an endless single-mode large-mode-area photonic crystal fiber (LMA-PCF) integrated metalens for broadband and efficient focusing from 800 to 1550 nm. In the present work, the optical properties of the substrate LMA-PCF were investigated, and the metalens, consisting of dielectric TiO2 nanorods with varying radii, was elaborately designed in the fiber core region with a diameter of 48 μm to cover the required phase profile for efficient focusing with a high transmission. The focusing characteristics of the designed metalens were also investigated in detail over a wide wavelength range. It is shown that the in-fiber metalens is capable of converging the incident beams into the bright, symmetric, and legible focal spots with a large focal length of 315–380 μm depending on the operating wavelength. A high and average focusing efficiency of 70% was also obtained with varying wavelengths. It is believed the proposed fiber metalens may show great potential in applications including fiber laser configuration, machining, and fiber communication.

Full-space metasurface holograms in the visible range.

Conventional metasurface holography is usually implemented in either transmission space or reflection space. Herein, we show a dielectric metasurface that can simultaneously project two independent holographic images in the transmission and reflection spaces, respectively, merely with a single-layer design approach. Specifically, two types of dielectric nanobricks in a nanostructured metasurface are employed to act as half-wave plates for geometric phase modulation. One type of nanobrick is designed to reflect most of incident circularly-polarized light into reflection space, enabled with magnetic resonance, while another type of nanobrick transmits it into transmission space, without resonance involved. By controlling the orientation angles and randomly interleaving the two types of nanobricks to form a metasurface, a full-space metasurface hologram can be established. We experimentally demonstrate this trans-reflective meta-holography by encoding the geometric phase information of two independent images into a single metasurface, and all observed holographic images agree well with our predictions. Our research expands the field-of-view of metasurface holography from half- to full-space, which can find its markets in optical sensing, image displays, optical storages and many other potential applications.

LSPR immuno-sensing based on iso-Y nanopillars for highly sensitive and specific imidacloprid detection.

Imidacloprid is the most widely used insecticide in agriculture and its intensive use over the last 30 years has caused a global concern due to its potentially toxic effects on the ecosystem. Considering the recent scientific interest in novel simple methods for imidacloprid analysis, we propose a label-free sensitive and specific localised surface plasmon resonance system for the detection of the insecticide based on 2D nanostructured metasurfaces with highly performing plasmonic properties. The specificity of the sensor proposed was achieved by covalent bio-functionalization of the metasurface using a smart and easy one-step procedure mediated by carbon disulphide. The biosensor produced was tested using a set of imidacloprid standard solutions showing a competitive limit of detection, lower than 1 ng mL-1. Our novel nanosensing configuration represents a valid and reliable solution to realize low-cost portable POC tests as an alternative to the laborious and expensive methods traditionally used for insecticide detection.

Advances in exploiting the degrees of freedom in nanostructured metasurface design: from 1 to 3 to more

Abstract The unusual electromagnetic responses of nanostructured metasurfaces endow them with an ability to manipulate the four fundamental properties (amplitude, phase, polarization, and frequency) of lightwave at the subwavelength scale. Based on this, in the past several years, a lot of innovative optical elements and devices, such as metagratings, metalens, metaholograms, printings, vortex beam generators, or even their combinations, have been proposed, which have greatly empowered the advanced research and applications of metasurfaces in many fields. Behind these achievements are scientists’ continuous exploration of new physics and degrees of freedom in nanostructured metasurface design. This review will focus on the progress on the design of different nanostructured metasurfaces for lightwave manipulation, including by varying/fixing the dimensions and/or orientations of isotropic/anisotropic nanostructures, which can therefore provide various functionalities for different applications. Exploiting the design degrees of freedom of optical metasurfaces provides great flexibility in the design of multifunctional and multiplexing devices, which can be applied in anticounterfeiting, information encoding and hiding, high-density optical storage, multichannel imaging and displays, sensing, optical communications, and many other related fields.

Optomechanics of Chiral Dielectric Metasurfaces

AbstractThe coupling between electromagnetic fields and mechanical motion in micro‐ and nanostructured materials has recently produced intriguing fundamental physics, such as the observation of mesoscopic optomechanical phenomena in objects operating in the quantum regime. It is also yielding innovative device applications, for instance in the manipulation of the optical response of photonic elements. Following this concept, here it is shown that combining a nanostructured chiral metasurface with a semiconductor suspended micromembrane can open new scenarios where the mechanical motion affects the polarization state of a light beam, and vice versa. Optical characterization of the fabricated samples, assisted by theory and numerical modeling, reveals that the interaction is mediated via moving‐boundary and thermoelastic effects, triggered by intracavity photons. This work represents a first example of “Polarization Optomechanics,” which can give access to new forms of polarization nonlinearities and control. It can also lead to wide applications in fast polarimetric devices, polarization modulators, and dynamically tunable chiral state generators and detectors.

X-shaped chiral plasmonic nanostructured metasurfaces: A numerical study

In this paper, the author reports a numerical investigation of spectroscopic circular dichroism (CD) properties, near-field enhancement, and optical chirality near a chiral plasmonic nanostructured metasurface. The individual constituents of the metasurface consist of periodic X-shaped Au nanostructures on a glass substrate. The nanostructure is made of two overlapping nanorods, including one vertical nanorod and one tilted nanorod. Strong CD of the metasurface was observed and tunable upon rotation of the individual nanorods due to geometrical symmetry breaking. Near-field enhancement of up to 5 times and a large local optical chirality were found at a plane 20 nm above the X-shaped nanostructure. Electromagnetic modeling analysis provides valuable guidelines for designing alternative two-dimensional chiral metamaterials that have potential applications for enhanced molecule and chiral plasmon interaction.

Metasurface interferometry toward quantum sensors

Optical metasurfaces open new avenues for the precise wavefront control of light for integrated quantum technology. Here, we demonstrate a hybrid integrated quantum photonic system that is capable of entangling and disentangling two-photon spin states at a dielectric metasurface. Via the interference of single-photon pairs at a nanostructured dielectric metasurface, a path-entangled two-photon NOON state with circular polarization that exhibits a quantum HOM interference visibility of 86 ± 4% is generated. Furthermore, we demonstrate nonclassicality andphase sensitivity in a metasurface-based interferometer with a fringe visibility of 86.8 ± 1.1% in the coincidence counts. This high visibility proves the metasurface-induced path entanglement inside the interferometer. Our findings provide a promising way to develop hybrid-integrated quantum technology operating in the high-dimensional mode space in various applications, such as imaging, sensing, and computing.

Large-area cost-effective lithography-free infrared metasurface absorbers for molecular detection

The implementation of metasurface absorbers for advanced sensing applications in the infrared spectral range is gaining prominence. Nevertheless, scale-up is challenging because their subwavelength features require complex fabrication techniques. Here, we present centimeter-sized nanostructured metasurface absorbers, prepared using the nanoporous anodic aluminum oxide template method, which exhibit a high and polarization-insensitive absorptivity at near-infrared wavelengths. Their sensing potential as surface-enhanced Raman spectroscopy substrates is demonstrated by analyzing Raman spectra of methyl parathion pesticides at concentrations as low as 100 ppb. Our results offer a stable, cost-effective, scalable, and uniform solution for metasurface-based molecular detection applications with a high sensitivity.

Observation of Second Harmonic Generation in Lightning-Bolt-Like Shaped Nanostructured Metasurface

Second harmonic generation (SHG) was observed on irradiation of a symmetry-broken gold nanostructured surface with an ultrashort pulse of low input power. A metasurface consisting of lightning-bolt-like shaped gold (Au) nanostructures in a periodic arrangement was fabricated on a glass substrate and utilized to excite surface plasmons. The measured extinction spectrum had a dual band maximized at approximately 605 nm and 786 nm under illumination with linearly polarized light. The two-photon induced photoluminescence and SHG intensity were dependent on the polarization states and wavelength of the femtosecond laser pulse irradiation. Furthermore, the SHG intensity peak was linearly dependent on the input power of the incident laser beam, and the spectral property of the emitted second harmonic was correlated to the extinction spectrum of the nanostructured metasurface. The SHG peak over the pumping wavelengths was resonant with a transverse multipolar plasmon mode. The findings provide useful guidelines for efficient production of second harmonic luminescence.

Ultrafast Coherent Absorption in Diamond Metamaterials.

Diamond is introduced as a material platform for visible/near-infrared photonic metamaterials, with a nanostructured polycrystalline diamond metasurface only 170 nm thick providing an experimental demonstration of coherent light-by-light modulation at few-optical-cycle (6 fs) pulse durations. "Coherent control" of absorption in planar (subwavelength-thickness) materials has emerged recently as a mechanism for high-contrast all-optical gating, with a speed of response that is limited only by the spectral width of the absorption line. It is shown here that a free-standing diamond membrane structured by focused ion beam milling can provide strong, spectrally near-flat absorption over a visible to near-infrared wavelength range that is wide enough (wider than is characteristically achievable in plasmonic metal metasurfaces) to facilitate coherent modulation of ultrashort optical pulses comprising only a few oscillations of electromagnetic field.

Dual-blade-like shaped nanostructured metasurface inducing dual-band extinction for dye fluorescence enhancement

The author experimentally reports here strong fluorescence enhancement (FLE) induced by a gold nanostructured surface. The metasurface consisting of periodic arrays of dual-blade-like shaped nanostructures was fabricated on a glass substrate and manipulated to excite surface plasmons. The measured extinction spectrum has a dual band maximized at approximately 645 nm and 795 nm under a linearly polarized light illumination. The author employed the dye molecule IR125 as a fluorophore whose fluorescence was significantly boosted by near-field interactions between the plasmon and the molecule. The resulting FLE induced by the near-field enhancement is attributed to the plasmonic resonances and correlated to the extinction of the metal nanostructure. The calculated Purcell enhancement factor, which simulates a spontaneous emission enhancement of an emitting dipole placed in the vicinity of the nanostructure, is in good agreement with the measured FLE.

All-optical dynamic focusing of light via coherent absorption in a plasmonic metasurface

Vision, microscopy, imaging, optical data projection and storage all depend on focusing of light. Dynamic focusing is conventionally achieved with mechanically reconfigurable lenses, spatial light modulators or microfluidics. Here we demonstrate that dynamic control of focusing can be achieved through coherent interaction of optical waves on a thin beam splitter. We use a nanostructured plasmonic metasurface of subwavelength thickness as the beam splitter, allowing operation in the regimes of coherent absorption and coherent transparency. Focusing of light resulting from illumination of the plasmonic metasurface with a Fresnel zone pattern is controlled by another patterned beam projected on the same metasurface. By altering the control pattern, its phase, or its intensity, we switch the lens function on and off, and alter the focal spot’s depth, diameter and intensity. Switching occurs as fast as the control beam is modulated and therefore tens of gigahertz modulation bandwidth is possible with electro-optical modulators, which is orders of magnitude faster than conventional dynamic focusing technologies.