33 Unresolved Questions in Nanoscience and Nanotechnology.

Significant advances in science and engineering often emerge at the intersections of disciplines. Nanoscience and nanotechnology are inherently interdisciplinary, uniting researchers from chemistry, physics, biology, medicine, materials science, and engineering. This convergence has fostered novel ways of thinking and enabled the development of materials, tools, and technologies that have transformed both basic and applied research, as well as how we address critical societal challenges.

Harnessing Phonon Polaritons for Dynamic and Sensitive Hydrogen Detection in the Mid-Infrared.

Phonon polaritons─quasiparticles formed by coupling infrared (IR) photons with optical phonons in polar materials─enable highly confined light-matter interactions with lower losses than those of plasmonic systems. Although they have been successfully exploited for enhanced mid-IR chemical sensing in solid- and liquid-phase environments, their application in gas-phase detection remains largely underexplored. Here, we introduce a low-loss phonon polariton platform based on planar Pd/SiC heterostructures and nanostructured Pd/SiC metasurfaces for enhanced mid-IR gas detection.

Programming Defects and Cavities into Colloidal Crystals Engineered With DNA.

Taking inspiration from seed-mediated crystal growth in atomic and molecular systems, a strategy is developed for incorporating particle and volume defects into the interior of colloidal crystals consisting of programmable atom equivalents (PAEs, oligonucleotide-functionalized nanoparticles) assembled with DNA. Discrete PAEs spanning a range of shapes, sizes, and compositions serve as nucleation sites for seed-mediated colloidal crystal growth and are incorporated into the centers of colloidal crystal lattices as cavities. Importantly, seed PAE shapes or sizes that are geometrically mismatched with the colloidal crystal lattice symmetry introduce defects such as local lattice disorder and long-range grain boundaries that arise through geometric frustration.

Nanofabrication for Nanophotonics.

Nanofabrication, a pivotal technology at the intersection of nanoscale engineering and high-resolution patterning, has substantially advanced over recent decades. This technology enables the creation of nanopatterns on substrates crucial for developing nanophotonic devices and other applications in diverse fields including electronics and biosciences. Here, this mega-review comprehensively explores various facets of nanofabrication focusing on its application in nanophotonics.

Mixed 1D/0D Structures of Chiral Hybrid Lead Halides with an l-Nicotinium Spacer Exhibit Broadband Photoluminescence and Polarized Emission.

Hybrid lead halide materials have been highly studied for optoelectronics, but their use with chiral organics in chiro-optoelectronics remains limited. Understanding the ability to impart chirality into bulk materials by incorporating a chiral precursor is of great interest due to unique optical properties directly arising from chirality such as circular dichroism, optical rotatory dispersion, and circularly polarized luminescence. Herein, we used protonated l-nicotine (L-n) (nicotinium cations) as a spacer and templated a variety of structures including a series of (L-n)MX (M = Sn, Pb; X = Cl, Br, I) that take space group 6 in which the lead halide regions template two distinct moieties, both a 1D chain motif and a 0D cluster motif.

High-Q Emission from Colloidal Quantum Dots Embedded in Polymer Quasi-BIC Metasurfaces.

Metasurfaces supporting narrowband resonances are of significant interest in photonics for molecular sensing, quantum light source engineering, and nonlinear photonics. However, many device architectures rely on large refractive index dielectric materials and lengthy fabrication processes. In this work, we demonstrate quasi-bound states in the continuum (quasi-BICs) using a polymer metasurface exhibiting experimental quality factors of 305 at visible wavelengths.

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.

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.

Open-channel metal particle superlattices.

Although tremendous advances have been made in preparing porous crystals from molecular precursors, 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.

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 devices. Crystalline materials can readily deform under various stimuli and the extent of recoverable deformation is highly dependent upon bond type. Indeed, for structures held together via simple electrostatic interactions, minimal deformations are tolerated.

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.

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

Heterostructures of optical cavities and quantum emitters have been highlighted for enhanced light-matter interactions. A silicon nanosphere, , and MoS, , 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.

Tuning and hybridization of surface phonon polaritons in α-MoO 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-MoO 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.

Resonance Couplings in Si@MoS 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.

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.

Tuning of Optical Phonons in α-MoO-VO Multilayers.

Merging the properties of VO 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 VO 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-MoO in a multilayer structure with VO.

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.

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.

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.

Effect of heating/cooling dynamics in the hysteresis loop and tunable IR emissivity of VO 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, VO 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.

Lithography-free IR polarization converters via orthogonal in-plane phonons in α-MoO 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-MoO 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-MoO to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material.

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.

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.