DNA-mediated assembly of Au bipyramids into anisotropic light emitting kagome superlattices.

Colloidal crystal engineering with DNA allows one to design diverse superlattices with tunable lattice symmetry, composition, and spacing. Most of these structures follow the complementary contact model, maximizing DNA hybridization on building blocks and producing relatively close-packed lattices. Here, low-symmetry kagome superlattices are assembled from DNA-modified gold bipyramids that can engage only in partial DNA surface matching. The bipyramid dimensions and DNA length can be engineered for two different superlattices with rhombohedral unit cells, including one composed of a periodic stacking of kagome lattices. Enabled by the partial facet alignment, the kagome lattices exhibit lattice distortion, bipyramid twisting, and planar chirality. When conjugated with Cy-5 dyes, the kagome lattices serve as cavities with high-density optical states and large Purcell factors along lateral directions, leading to strong dipole radiation along the z axis and facet-dependent light emission. Such complex optical properties make these materials attractive for lasers, displays, and quantum sensing constructs.

Metasurface enabled broadband all optical edge detection in visible frequencies.

Image processing is of fundamental importance for numerous modern technologies. In recent years, due to increasing demand for real-time and continuous data processing, metamaterial and metasurface based all-optical computation techniques emerged as a promising alternative to digital computation. Most of the pioneer research focused on all-optical edge detection as a fundamental step of image processing. Metasurfaces have been shown to enable real time edge detection with low to no power consumption. However, the previous demonstrations were subjected to the several limitations such as need for oblique-incidence, polarization dependence, need for additional polarizers, narrow operation bandwidth, being limited with processing in 1D, operation with coherent light only, and requiring digital post-processing. Here, we propose and experimentally demonstrate 2D isotropic, polarization-independent, broadband edge detection with high transmission efficiency under both coherent and incoherent illumination along the visible frequency range using a metasurface based on Fourier optics principles.

Shape memory in self-adapting colloidal crystals.

Reconfigurable, mechanically responsive crystalline materials are central components in many sensing, soft robotic, and energy conversion and storage devices1-4. Crystalline materials can readily deform under various stimuli and the extent of recoverable deformation is highly dependent upon bond type1,2,5-10. Indeed, for structures held together via simple electrostatic interactions, minimal deformations are tolerated. By contrast, structures held together by molecular bonds can, in principle, sustain much larger deformations and more easily recover their original configurations. Here we study the deformation properties of well-faceted colloidal crystals engineered with DNA. These crystals are large in size (greater than 100 µm) and have a body-centred cubic (bcc) structure with a high viscoelastic volume fraction (of more than 97%). Therefore, they can be compressed into irregular shapes with wrinkles and creases, and, notably, these deformed crystals, upon rehydration, assume their initial well-formed crystalline morphology and internal nanoscale order within seconds. For most crystals, such compression and deformation would lead to permanent, irreversible damage. The substantial structural changes to the colloidal crystals are accompanied by notable and reversible optical property changes. For example, whereas the original and structurally recovered crystals exhibit near-perfect (over 98%) broadband absorption in the ultraviolet-visible region, the deformed crystals exhibit significantly increased reflection (up to 50% of incident light at certain wavelengths), mainly because of increases in their refractive index and inhomogeneity.

Open-channel metal particle superlattices.

Although tremendous advances have been made in preparing porous crystals from molecular precursors1,2, there are no general ways of designing and making topologically diversified porous colloidal crystals over the 10-1,000 nm length scale. Control over porosity in this size range would enable the tailoring of molecular absorption and storage, separation, chemical sensing, catalytic and optical properties of such materials. Here, a universal approach for synthesizing metallic open-channel superlattices with pores of 10 to 1,000 nm from DNA-modified hollow colloidal nanoparticles (NPs) is reported. By tuning hollow NP geometry and DNA design, one can adjust crystal pore geometry (pore size and shape) and channel topology (the way in which pores are interconnected). The assembly of hollow NPs is driven by edge-to-edge rather than face-to-face DNA-DNA interactions. Two new design rules describing this assembly regime emerge from these studies and are then used to synthesize 12 open-channel superlattices with control over crystal symmetry, channel geometry and topology. The open channels can be selectively occupied by guests of the appropriate size and that are modified with complementary DNA (for example, Au NPs).

Monolayer Plasmonic Nanoframes as Large-Area, Broadband Metasurface Absorbers.

Broadband absorbers are useful ultraviolet protection, energy harvesting, sensing, and thermal imaging. The thinner these structures are, the more device-relevant they become. However, it is difficult to synthesize ultrathin absorbers in a scalable and straightforward manner. A general and straightforward synthetic strategy for preparing ultrathin, broadband metasurface absorbers that do not rely on cumbersome lithographic steps is reported. These materials are prepared through the surface-assembly of plasmonic octahedral nanoframes (NFs) into large-area ordered monolayers via drop-casting with subsequent air-drying at room temperature. This strategy is used to produce three types of ultrathin broadband absorbers with thicknesses of ≈200 nm and different lattice symmetries (loose hexagonal, twisted hexagonal, dense hexagonal), all of which exhibit efficient light absorption (≈90%) across wavelengths ranging from 400-800 nm. Their broadband absorption is attributed to the hollow morphologies of the NFs, the incorporation of a high-loss material (i.e., Pt), and the strong field enhancement resulting from surface assembly. The broadband absorption is found to be polarization-independent and maintained for a wide range of incidence angles (±45°). The ability to design and fabricate broadband metasurface absorbers using this high-throughput surface-based assembly strategy is a significant step toward the large-scale, rapid manufacturing of nanophotonic structures and devices.

Probing the Optical Response and Local Dielectric Function of an Unconventional Si@MoS2 Core-Shell Architecture.

Heterostructures of optical cavities and quantum emitters have been highlighted for enhanced light-matter interactions. A silicon nanosphere, core, and MoS2, shell, structure is one such heterostructure referred to as the core@shell architecture. However, the complexity of the synthesis and inherent difficulties to locally probe this architecture have resulted in a lack of information about its localized features limiting its advances. Here, we utilize valence electron energy loss spectroscopy (VEELS) to extract spatially resolved dielectric functions of Si@MoS2 with nanoscale spatial resolution corroborated with simulations. A hybrid electronic critical point is identified ∼3.8 eV for Si@MoS2. The dielectric functions at the Si/MoS2 interface is further probed with a cross-sectioned core-shell to assess the contribution of each component. Various optical parameters can be defined via the dielectric function. Hence, the methodology and evolution of the dielectric function herein reported provide a platform for exploring other complex photonic nanostructures.

Tuning and hybridization of surface phonon polaritons in α-MoO3 based metamaterials.

We propose an effective medium approach to tune and control surface phonon polariton dispersion relations along the three main crystallographic directions of α-phase molybdenum trioxide. We show that a metamaterial consisting of subwavelength air inclusions into the α-MoO3 matrix displays new absorption modes producing a split of the Reststrahlen bands of the crystal and creating new branches of phonon polaritons. In particular, we report hybridization of bulk and surface polariton modes by tailoring metamaterials' structural parameters. Theoretical predictions obtained with the effective medium approach are validated by full-field electromagnetic simulations using finite difference time domain method. Our study sheds light on the use of effective medium theory for modeling and predicting wavefront polaritons. Our simple yet effective approach could potentially enable different functionalities for hyperbolic infrared metasurface devices and circuits on a single compact platform for on-chip infrared photonics.

Resonance Couplings in Si@MoS2 Core-Shell Architectures.

Heterostructures of transition metal dichalcogenides and optical cavities that can couple to each other are rising candidates for advanced quantum optics and electronics. This is due to their enhanced light-matter interactions in the visible to near-infrared range. Core-shell structures are particularly valuable for their maximized interfacial area. Here, the chemical vapor deposition synthesis of Si@MoS2 core-shells and extensive structural characterization are presented. Compared with traditional plasmonic cores, the silicon dielectric Mie resonator core offers low Ohmic losses and a wider spectrum of optical modes. The magnetic dipole (MD) mode of the silicon core efficiently couples with MoS2 through its large tangential component at the core surface. Using transmission electron microscopy and correlative single-particle scattering spectroscopy, MD mode splitting is experimentally demonstrated in this unique Si@MoS2 core-shell structure. This is evidence for resonance coupling, which is limited to theoretical proposals in this particular system. A coupling constant of 39 meV is achieved, which is ≈1.5-fold higher than previous reports of particle-on-film geometries with a smaller interfacial area. Finally, higher-order systems with the potential to tune properties are demonstrated through a dimer system of Si@MoS2 , forming the basis for emerging architectures for optoelectronic and nanophotonic applications.

Large-Area, Highly Crystalline DNA-Assembled Metasurfaces Exhibiting Widely Tunable Epsilon-Near-Zero Behavior.

Metasurfaces prepared via bottom-up nanoparticle assembly enable the deliberate manipulation of light in the optical regime, resulting in media with various engineered optical responses. Here, we report a scalable method to grow highly crystalline 2D metasurfaces composed of colloidal gold nanocubes, over macroscopic areas, using DNA-mediated assembly under equilibrium conditions. Using an effective medium description, we predict that these plasmonic metasurfaces behave as dielectric media with high refractive indices that can be dynamically tuned by tuning DNA length. Furthermore, we predict that, when coupled with an underlying thin gold film, the real permittivity of these metasurfaces exhibits a crossover region between positive and negative values, known as the epsilon-near-zero (ENZ) condition, which can be tuned between 1.5 and 2.6 µm by changing DNA length. Optical characterization performed on the DNA-assembled metasurfaces reveals that the predicted optical properties agree well with the measured response. Overall, we propose an efficient method for realizing large-area plasmonic metasurfaces that enable dynamic control over optical characteristics. High-index and ENZ metasurfaces operating in the telecommunications regime could have significant implications in high-speed optical computing, optical communications, optical imaging, and other areas.

Neural networks enabled forward and inverse design of reconfigurable metasurfaces.

Nanophotonics has joined the application areas of deep neural networks (DNNs) in recent years. Various network architectures and learning approaches have been employed to design and simulate nanophotonic structures and devices. Design and simulation of reconfigurable metasurfaces is another promising application area for neural network enabled nanophotonic design. The tunable optical response of these metasurfaces rely on the phase transitions of phase-change materials, which correspond to significant changes in their dielectric permittivity. Consequently, simulation and design of these metasurfaces requires the ability to model a diverse span of optical properties. In this work, to realize forward and inverse design of reconfigurable metasurfaces, we construct forward and inverse networks to model a wide range of optical characteristics covering from lossless dielectric to lossy plasmonic materials. As proof-of-concept demonstrations, we design a Ge2Sb2Te5 (GST) tunable resonator and a VO2 tunable absorber using our forward and inverse networks, respectively.

Tuning of Optical Phonons in α-MoO3-VO2 Multilayers.

Merging the properties of VO2 and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO2 on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO3 in a multilayer structure with VO2. Our experiments show the frequency and intensity tuning of 2 cm-1 and 11% for OPhs in the [100] direction and 2 cm-1 and 28% for OPhs in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices.

Enhanced Interaction of Optical Phonons in h-BN with Plasmonic Lattice and Cavity Modes.

Hexagonal boron nitride (h-BN) is regarded as a milestone in the investigation of light interaction with phonon polaritons in two-dimensional van der Waals materials, showing significant potential in novel and high-efficient photonics devices in the mid-infrared region. Here, we investigate a structure composed of Au-grating arrays fabricated onto a Fabry-Perot (FP) cavity composed of h-BN, Ge, and Au back-reflector layers. The plasmonic FP cavity reduces the required device thickness by enhancing modal interactions and introduces in-plane polarization sensitivity based on the Au array lattice. Our experiments show multiple absorption peaks of over 90% in the mid-infrared region and the band stop filters with 80% efficiency using only a 15 nm h-BN slab. Moreover, mode interaction with experimental coupling strengths as high as 10.8 meV in the mid-infrared region is investigated. In particular, the interaction and hybridization of optical phonon modes with plasmonic modes including the lattice and cavity modes are studied. Anticrossing splitting ascribed to the coupling of optical phonons to plasmonic modes can be tuned by the designed geometry which can be tailored to efficient response band engineering for infrared photonics. We also show that in practical applications involving wet transfer of h-BN thin films, the contribution of minor optical phonon modes to resonant peaks should not be ignored, which originate from defects and multicrystallinity in the h-BN slab. Our findings provide a favorable complement to manipulation of light-phonon interaction, inspiring a promising design of phonon-based nanophotonic devices in the infrared range.

Inverse Design and 3D Printing of a Metalens on an Optical Fiber Tip for Direct Laser Lithography.

An inverse-designed metalens is proposed, designed, and fabricated on an optical fiber tip via a 3D direct laser-writing technique through two-photon polymerization. A computational inverse-design method based on an objective-first algorithm was used to design a thin circular grating-like structure to transform the parallel wavefront into a spherical wavefront at the near-infrared range. With a focal length about 8 µm at an operating wavelength of 980 nm and an optimized focal spot at the scale of 100 nm, our proposed metalens platform is suitable for two-photon direct laser lithography. We demonstrate the use of the fabricated metalens in a direct laser lithography system. The proposed platform, which combines the 3D printing technique and the computational inverse-design method, shows great promise for the fabrication and integration of multiscale and multiple photonic devices with complex functionalities.

Highly Efficient Light Absorption of Monolayer Graphene by Quasi-Bound State in the Continuum.

Graphene is an ideal ultrathin material for various optoelectronic devices, but poor light-graphene interaction limits its further applications particularly in the visible (Vis) to near-infrared (NIR) region. Despite tremendous efforts to improve light absorption in graphene, achieving highly efficient light absorption of monolayer graphene within a comparatively simple architecture is still urgently needed. Here, we demonstrate the interesting attribute of bound state in the continuum (BIC) for highly efficient light absorption of graphene by using a simple Si-based photonic crystal slab (PCS) with a slit. Near-perfect absorption of monolayer graphene can be realized due to high confinement of light and near-field enhancement in the Si-based PCS, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Theoretical analysis based on the coupled mode theory (CMT) is proposed to evaluate the absorption performances of monolayer graphene integrated with the symmetry-broken PCS, which indicates that high absorption of graphene is feasible at critical coupling based on the destructive interference of transmission light. Moreover, the absorption spectra of the monolayer graphene are stable to the variations of the structural parameters, and the angular tolerances of classical incidence can be effectively improved via full conical incidence. By using the full conical incidence, the angular bandwidths for the peak absorptivity and for the central wavelength of graphene absorption can be enhanced more than five times and 2.92 times, respectively. When the Si-based PCS with graphene is used in refractive index sensors, excellent sensing performances with sensitivity of 604 nm/RIU and figure of merit (FoM) of 151 can be achieved.

Effect of heating/cooling dynamics in the hysteresis loop and tunable IR emissivity of VO2 thin films.

We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films using an infrared thermographic technique. During the semiconductor to metal phase change process, VO2 optical properties dynamically change and infrared emission undergoes a hysteresis loop due to differences between heating and cooling stages. The shape of the hysteresis loop was accurately monitored under different dynamic heating/cooling rates. In order to quantify and understand the effects of different rates, we used a numerical modelling approach in which a VO2 thin layer was modeled as metamaterial. The main experimental findings are interpreted assuming that both the rate of formation and shape of metallic inclusions are tuned with the heating/cooling rate. The structural transition from monoclinic to tetragonal phases is the main mechanism for controlling the global properties of the phase transition. However, our experimental results reveal that the dynamics of the heating/cooling process can become a useful parameter for further tuning options and lays out a macroscopic optical sensing scheme for the microscopic phase change dynamics of VO2. Our study sheds light on phase-transition dynamics and their effect on the infrared emission spectra of VO2 thin films, therefore enabling the heating/cooling rate to be an additional parameter to control infrared emission characteristics of thermal emitters. The hysteresis loop represents the phase coexistence region, thus being of fundamental importance for several applications, such as the operation of radiative thermal logic elements based on phase transition materials. For such applications, the phase transition region is shifted for heating and cooling processes. We also show that, depending on the way the phase change elements are heated, the temperature operation range will be slightly modified.

Lithography-free IR polarization converters via orthogonal in-plane phonons in α-MoO3 flakes.

Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO3 to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO3 is reported without the need for nanoscale fabrication on the α-MoO3. We observe coupling between the α-MoO3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.

Mie-Resonant Three-Dimensional Metacrystals.

Optical metamaterials, engineered to exhibit electromagnetic properties not found in natural materials, may enable new light-based applications including cloaking and optical computing. While there have been significant advances in the fabrication of two-dimensional metasurfaces, planar structures create nontrivial angular and polarization sensitivities, making omnidirectional operation impossible. Although three-dimensional (3D) metamaterials have been proposed, their fabrication remains challenging. Here, we use colloidal crystal engineering with DNA to prepare isotropic 3D metacrystals from Au nanocubes. We show that such structures can exhibit refractive indices as large as ∼8 in the mid-infrared, far greater than that of common high-index dielectrics. Additionally, we report the first observation of multipolar Mie resonances in metacrystals with well-formed habits, occurring in the mid-infrared for submicrometer metacrystals, which we measured using synchrotron infrared microspectroscopy. Finally, we predict that arrays of metacrystals could exhibit negative refraction. The results present a promising platform for engineering devices with unnatural optical properties.

Device-quality, reconfigurable metamaterials from shape-directed nanocrystal assembly.

Anchoring nanoscale building blocks, regardless of their shape, into specific arrangements on surfaces presents a significant challenge for the fabrication of next-generation chip-based nanophotonic devices. Current methods to prepare nanocrystal arrays lack the precision, generalizability, and postsynthetic robustness required for the fabrication of device-quality, nanocrystal-based metamaterials [Q. Y. Lin et al. Nano Lett. 15, 4699-4703 (2015); V. Flauraud et al., Nat. Nanotechnol. 12, 73-80 (2017)]. To address this challenge, we have developed a synthetic strategy to precisely arrange any anisotropic colloidal nanoparticle onto a substrate using a shallow-template-assisted, DNA-mediated assembly approach. We show that anisotropic nanoparticles of virtually any shape can be anchored onto surfaces in any desired arrangement, with precise positional and orientational control. Importantly, the technique allows nanoparticles to be patterned over a large surface area, with interparticle distances as small as 4 nm, providing the opportunity to exploit light-matter interactions in an unprecedented manner. As a proof-of-concept, we have synthesized a nanocrystal-based, dynamically tunable metasurface (an anomalous reflector), demonstrating the potential of this nanoparticle-based metamaterial synthesis platform.

Anisotropic localized surface plasmons in borophene.

We present a theoretical study on the plasmonic response of borophene, a monolayer 2D material that is predicted to exhibit metallic response and anisotropic plasmonic behavior in visible wavelengths. We investigate plasmonic properties of borophene thin films as well as borophene nanoribbons and nanopatches where polarization-sensitive absorption values in the order of 50% is obtained with monolayer borophene. It is demonstrated that by adding a metal layer, this absorption can be enhanced to 100%. We also examine giant dichroism in monolayer borophene which can be tuned passively (patterning) and actively (electrostatic gating) and our simulations yield 20% reflected light with significant polarization rotation. These findings reveal the potential of borophene in the manipulation of phase, amplitude and polarization of light at the extreme subwavelength scales.

Unveiling the optical parameters of vanadium dioxide in the phase transition region: a hybrid modeling approach.

The phase change behavior of vanadium dioxide (VO2) has been widely explored in a variety of optical and photonic applications. Commonly, its optical parameters have been studied in two extreme regimes: hot (metallic) and cold (insulating) states. However, in the transition temperatures, VO2 acts like an inherent metamaterial with mixed metallic-insulating character. In this range, the portions of metallic and insulating inclusions are tuned by temperature, and therefore a gradual change of optical parameters can be achieved. In this paper, a universal hybrid modeling approach is developed to model VO2 in the intermediate region. For this aim, the measured reflectivity data, is analyzed and matched through the transfer matrix method (TMM) simulations where an effective medium theory (EMT) is employed. Based on the findings of this approach, not only the relative portions of inclusions are tailored but also their grain shapes are significantly altered in the transition range. Finally, the modeling approach is testified by experimental findings through dynamic device applications operating at short and mid infrared wavelengths. In addition, the hysteretic behaviors on electrical, optical, and structural parameters of the VO2 film along the heating and cooling cycles are demonstrated by the experiments and scrutinized by the simulations.