Ultra-broadband and strongly enhanced diffraction with metasurfaces

Near-unity uniformity and efficiency broadband meta-beam-splitter/combiner

Subwavelength planar structured interfaces, also known as metasurfaces, are ultra-thin optical elements modulating the amplitude, phase, and polarization of incident light using nanostructures called meta-atoms. The optical properties of such metasurfaces can be controlled across wavelengths by selecting geometries and materials of the meta-atoms. Given recent technological developments in optical device miniaturization, components for beam splitting and beam combining are sought for use within these devices as two quintessential components of every optical setup. However, realizing such devices using metasurfaces typically leads to poor uniformity of diffraction orders and narrow-band operation. Using a modified version of particle swarm optimization, we propose and numerically demonstrate a broadband, reciprocal metasurface beam combiner/splitter with uniformity > 97% and diffraction efficiency > 90% in the continuous band from λ=1525 nm to λ=1575 nm. The proposed approach significantly extends the current state of the art of metasurfaces design in terms of uniformity, bandwidth, and efficiency, and opens the door for devices requiring high power or near-unit uniformity.

Polarization-insensitive broadband visible-light steering with tunable direction enabled by scalable plasmonics meta-gratings

As an emerging field in the discipline of optics, plasmonics and metasurfaces have been demonstrated to enable a new degree of freedom to manipulate light for arbitrary beam steering, spectral splitting as well as precise wavefront shaping. However, it has been mostly studied in parallel with the field of diffractive optics, and awaits the unveiling of how the hybridizations between plasmonic effect and diffraction effect interact and impact. Here, we have theoretically proposed a new type of polarization-insensitive meta-grating structure across the broadband visible regime. The structure design combines the width gradient (critical resonant length) from a trapezoid-nanoantenna with the height gradient from a blazed grating profile. The hybridized meta-grating creates both plasmonic effect and grating effect, which enables all the optical incident photons to be directed to the same orientation regardless of the light polarization. As we know, both metasurfaces and diffractive optical elements (such as gratings) are, more often than not, quite sensitive to the incident light polarization. Moreover, if placing our meta-grating on a flexible/stretchable substrate (such as polydimethylsiloxane), the outgoing angle can be effectively adjusted by tuning the period or density of meta-grating arrays. Such meta-grating architectures can be potentially manufactured by existing photolithography and nanoimprint techniques, and can easily find a wide range of practical polarization-insensitive applications, including broadband deflector and emitter, tunable display and imaging device, high signal-to-noise ratio spectrometer, polarization-insensitive plasmonic coupler, etc.

Modeling of microsphere photolithography

Microsphere photolithography (MPL) is a fabrication technique that combines the ability to self-assemble arrays of microspheres with the ability of a microsphere to focus light to a photonic jet, in order to create highly ordered nanoscale features in photoresist. This paper presents a model of photoresist exposure with the photonic jet, combining a full-wave electromagnetic model of the microsphere/photoresist interaction with the sequential removal of exposed photoresist by the developer. The model is used to predict the dose curves for the MPL process based on the photoresist thickness, illumination conditions, and development time. After experimental validation, the model provides insight into the process including the resolution, sensitivity, and effects of off-normal illumination. This guides the fabrication of sub-100 nm hole/disk arrays using lift-off, and superposition is shown to predict the geometry for split-ring resonators created using multiple exposures. This model will assist synthesizing fabrication parameters to create large area scalable metasurfaces with sensing and energy management applications.

Visible-frequency meta-gratings for light steering, beam splitting and absorption tunable functionality

Diffractive grating and plasmonic metasurface have always been developing as two parallel optical domains, which have not met for studying their hybridization to discover new applications and potentials. Here, we proposed a novel meta-grating design, which hybridizes the metasurface interfacial gradient with the blazed grating profile. The unique architecture takes advantage of both grating effect and plasmonic resonances with minimum cross-coupling, thus leading to the polarization-selective behaviors to steer different polarized light to drastically inverse directions (> 90°). Furthermore, the hybridized surface also exhibits angle-dependent broadband absorptive tunability (∼ 5% - 86%) by migrating the strong blazed order and plasmonic order at the far field. We believe that the integrated meta-grating device would suggest various potential applications including polarization beam splitters, high signal-to-noise ratio (SNR) optical spectrometer, high-efficiency plasmonic couplers and filter, etc.

Measurements of Radiation Pressure Owing to the Grating Momentum

The radiation pressure force on a nearly single-order diffraction grating was measured for a transmission grating near the Littrow angles at wavelengths of 808 and 447 nm. The component of force parallel to the grating agreed well with our prediction, being proportional to the product of the grating order and the ratio of the wavelength and grating period. The normal component of force varied with the incident angle, vanishing near the Littrow angle as expected. The measurements verify a correspondence between the Fourier grating momentum and the mechanical momentum. This Letter provides opportunities for in-space fly-by-light sailcraft as well as terrestrial applications.

Vertically integrated visible and near-infrared metasurfaces enabling an ultra-broadband and highly angle-resolved anomalous reflection

An optical device with minimized dimensions, which is capable of efficiently resolving an ultra-broad spectrum into a wide splitting angle but incurring no spectrum overlap, is of importance in advancing the development of spectroscopy. Unfortunately, this challenging task cannot be easily addressed through conventional geometrical or diffractive optical elements. Herein, we propose and demonstrate vertically integrated visible and near-infrared metasurfaces which render an ultra-broadband and highly angle-resolved anomalous reflection. The proposed metasurface capitalizes on a supercell that comprises two vertically concatenated trapezoid-shaped aluminum antennae, which are paired with a metallic ground plane via a dielectric layer. Under normal incidence, reflected light within a spectral bandwidth of 1000 nm ranging from λ = 456 nm to 1456 nm is efficiently angle-resolved to a single diffraction order with no spectrum overlap via the anomalous reflection, exhibiting an average reflection efficiency over 70% and a substantial angular splitting of 58°. In light of a supercell pitch of 1500 nm, to the best of our knowledge, the micron-scale bandwidth is the largest ever reported. It is noted that the substantially wide bandwidth has been accomplished by taking advantage of spectral selective vertical coupling effects between antennae and ground plane. In the visible regime, the upper antenna primarily renders an anomalous reflection by cooperating with the lower antenna, which in turn cooperates with the ground plane and produces phase variations leading to an anomalous reflection in the near-infrared regime. Misalignments between the two antennae have been particularly inspected to not adversely affect the anomalous reflection, thus guaranteeing enhanced structural tolerance of the proposed metasurface.

97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold

In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO₂) nanowires. It is found that the randomly oriented dense TiO₂ nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO₂ random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.

Covalent bonding-assisted nanotransfer lithography for the fabrication of plasmonic nano-optical elements

Many high-resolution patterning techniques have been developed to realize nano- and microscale applications of electric devices, sensors, and transistors. However, conventional patterning methods based on photo or e-beam lithography are not employed to fabricate optical elements of high aspect ratio and a sub-100 nm scale due to the limit of resolution, high costs and low throughput. In this study, covalent bonding-assisted nanotransfer lithography (CBNL) was proposed to fabricate various structures of high resolution and high aspect ratio at low cost by a robust and fast chemical reaction. The proposed process is based on the formation of covalent bonds between silicon of adhesive layers on a substrate and oxygen of the deposited material on the polymer stamp. The covalent bond is strong enough to detach multiple layers from the stamp for a large area without defects. The obtained nanostructures can be used for direct application or as a hard mask for etching. Two nano-optical applications were demonstrated in this study, i.e., a meta-surface and a wire-grid polarizer. A perfect absorption meta-surface was generated by transferring subwavelength hole arrays onto a substrate without any post-processing procedures. In addition, a wire-grid polarizer with high aspect ratio (1 : 3) and 50 nm line width was prepared by the nano-transfer of materials, which were used as a hard mask for etching. Therefore, CBNL provides a means of achieving large-area nano-optical elements with a simple roll-to-plate process at low cost.

Ultra-broadband, wide angle absorber utilizing metal insulator multilayers stack with a multi-thickness metal surface texture

In this paper, we propose a facile route to fabricate a metal insulator multilayer stack to obtain ultra-broadband, wide angle behavior from the structure. The absorber, which covers near infrared (NIR) and visible (Vis) ranges, consists of a metal-insulator-metal-insulator (MIMI) multilayer where the middle metal layer has a variant thickness. It is found that this non-uniform thickness of the metal provides us with an absorption that is much broader compared to planar architecture. In the non-uniform case, each thickness is responsible for a specific wavelength range where the overall absorption is the superposition of these resonant responses and consequently a broad, perfect light absorption is attained. We first numerically examine the impact of different geometries on the overall light absorption property of the multilayer design. Afterward, we fabricate the designs and characterize them to experimentally verify our numerical findings. Characterizations show a good agreement with numerical results where the optimum absorption bandwidth for planar design is found to be 620 nm (380 nm-1000 nm) and it is significantly boosted to an amount of 1060 nm (350 nm-1410 nm) for multi-thickness case.

Infrared metasurfaces created with off-normal incidence microsphere photolithography

Fabricating metasurfaces over large areas at low costs remains a critical challenge to their practical implementation. This paper reports on the use of microsphere photolithography (MPL) to create infrared metasurfaces by changing the angle-of-incidence of the illumination to steer the photonic jet. The displacement of the photonic jet is shown to scale with the diameter of the microsphere while the exposure dose scales with the square of the microsphere diameter. This process is robust in the presence of local defects in the microsphere lattice. The paper demonstrates patterning split ring resonators and tripole based metasurfaces using MPL, which are fabricated and characterized with FTIR. The combination of bottom-up and top-down approaches in off-normal incidence microsphere photolithography technique provides cost-effective, flexible, and high-throughput fabrication of infrared metasurfaces.

Polycrystalline metasurface perfect absorbers fabricated using microsphere photolithography

Microsphere photolithography (MPL) is a practical, cost-effective nanofabrication technique. It uses self-assembled microspheres in contact with the photoresist as microlenses. The microspheres focus incident light to a sub-diffraction limited array of photonic jets in the photoresist. This Letter explores the MPL technique to pattern metal-insulator-metal metasurfaces with near-perfect absorption at mid-wave infrared (MWIR) frequencies. Experimental results are compared to electromagnetic simulations of both the exposure process and the metasurface response. The microsphere self-assembly technique results in a polycrystalline metasurface; however, the metal-insulator-metal structure is shown to be defect tolerant. While the MPL approach imposes geometric constraints on the metasurface design, once understood, the technique can be used to create functional devices. In particular, the ability to tune the resonant wavelength with the exposure dose raises the potential of hierarchical structures.