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.

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.

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.

Tunable Fluorescence from Dye-Modified DNA-Assembled Plasmonic Nanocube Arrays.

Colloidal crystal engineering with DNA on template-confined surfaces is used to prepare arrays of nanocube-based plasmonic antennas and deliberately place dyes with sub-nm precision into their hotspots, on the DNA bonds that confine the cubes to the underlying gold substrate. This combined top-down and bottom-up approach provides independent control over both the plasmonic gap and photonic lattice modes of the surface-confined particle assemblies and allows for the tuning of the interactions between the excited dyes and plasmonically active antennas. Furthermore, the gap mode of the antennas can be modified in situ by utilizing the solvent-dependent structure of the DNA bonds. This is studied by placing two dyes, with different emission wavelengths, under the nanocubes and recording their solvent-dependent emission. It is shown that dye emission not only depends upon the in-plane structure of the antennas but also the size of the gap, which is regulated with solvent. Importantly, this approach allows for the systematic understanding of the relationship between nanoscale architecture and plasmonically coupled dye emission, and points toward the use of colloidal crystal engineering with DNA to create stimuli responsive architectures, which can find use in chemical sensing and tunable light sources.