Two-dimensional (2D) transition metal dichalcogenides (TMDCs) and graphene compose a new family of crystalline materials with atomic thicknesses and exotic mechanical, electronic, and optical properties. Due to their inherent exceptional mechanical flexibility and strength, these 2D materials provide an ideal platform for strain engineering, enabling versatile modulation and significant enhancement of their optical properties. For instance, recent theoretical and experimental investigations have demonstrated flexible control over their electronic states via application of external strains, such as uniaxial strain and biaxial strain. Meanwhile, many nondestructive optical measurement methods, typically including absorption, reflectance, photoluminescence, and Raman spectroscopies, can be readily exploited to quantitatively determine strain-engineered optical properties. This review begins with an introduction to the macroscopic theory of crystal elasticity and microscopic effective low-energy Hamiltonians coupled with strain fields, and then summarizes recent advances in strain-induced optical responses of 2D TMDCs and graphene, followed by the strain engineering techniques. It concludes with exciting applications associated with strained 2D materials, discussions on existing open questions, and an outlook on this intriguing emerging field.

Creating highly branched plasmonic superparticles can effectively induce broadband light absorption and convert light to heat regardless of the light wavelength, angle, and polarization. However, their direct synthesis in a controllable manner remains a significant challenge. In this work, we propose a strain modulation strategy to produce branched Au nanostructures that promotes the growth of Au on Au seeds in the Volmer–Weber (island) mode instead of the typical Frank–van der Merwe (layer-by-layer) mode. The key to this strategy is to continuously deposit polydopamine formed in situ on the growing surface of the seeds to increase the chemical potential of the subsequent deposition of Au, thus achieving continuous heterogeneous nucleation and growth. The branched Au superparticles exhibit a photothermal conversion efficiency of 91.0% thanks to their small scattering cross-section and direction-independent absorption. Even at a low light power of 0.5 W/cm2 and a low dosage of 25 ppm, these particles show an excellent efficacy in photothermal cancer therapy. This work provides the fundamental basis for designing branched plasmonic nanostructures and expands the application scope of the plasmonic photothermal effect.

Spintronics employs the spin of electrons to encode information. Akin to spintronics, valleytronics exploits the valley as pseudospin‐carrying controllable binary information. 2D transition metal dichalcogenides (TMDCs) have asymmetric +K and −K valleys. The valley pseudospins of these materials can be manipulated by selective pumping, giving rise to valley‐dependent circularly polarized photoluminescence. The photonic spin–orbit interactions (SOIs) in nanophotonic systems allow the transformation or coupling of optical spin angular momentum to orbital angular momentum of light. Hybridizing 2D TMDC electronic systems with nanophotonic systems can open up an avenue for merging TMDC valley polarization and photonic SOIs to uncover new mechanisms of on‐chip optical information processing and transport. This review focuses on the fundamentals and implications of TMDC valley polarization, photonic SOI effects, and their chiral coupling in hybrid TMDCs–nanophotonic systems. First, the deterministic valley‐dependent optical properties of TMDCs and recent efforts in achieving large valley polarization contrast are reviewed. Then, various SOI effects in nanophotonic systems and their physical mechanisms are summarized. At the end, recent demonstrations of chiral coupling between TMDC valley excitons and light through spin‐direction locking at hybrid interfaces are highlighted.

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Plasmon-mediated chemical reactions have attracted intensive research interest as a means of achieving desirable reaction yields and selectivity. The energetic charge carriers and elevated local temperature induced by the nonradiative decay of surface plasmons are thought to be responsible for improving reaction outcomes. This study reports that the plasmoelectric potential is another key contributor in plasmon-mediated electrochemistry. Additionally, we disclose a convenient and reliable method for quantifying the specific contributions of the plasmoelectric potential, hot electrons, and photothermal heating to the electroreduction of oxygen at the plasmonic Ag electrode, revealing that the plasmoelectric potential is the dominating nonthermal factor under short-wavelength illumination and moderate electrode bias. This work elucidates novel mechanistic understandings of plasmon-mediated electrochemistry, facilitating high-performance plasmonic electrocatalyst design optimization.

Plasmonic nanomaterials with strong absorption at near-infrared frequencies are promising photothermal therapy agents (PTAs). The pursuit of high photothermal conversion efficiency has been the central focus of this research field. Here, we report the development of plasmonic nanoparticle clusters (PNCs) as highly efficient PTAs and provide a semiquantitative approach for calculating their resonant frequency and absorption efficiency by combining the effective medium approximation (EMA) theory and full-wave electrodynamic simulations. Guided by the theoretical prediction, we further develop a universal strategy of space-confined seeded growth to prepare various PNCs. Under optimized growth conditions, we achieve a record photothermal conversion efficiency of up to ∼84% for gold-based PNCs, which is attributed to the collective plasmon-coupling-induced near-unity absorption efficiency. We further demonstrate the extraordinary photothermal therapy performance of the optimized PNCs in in vivo application. Our work demonstrates the high feasibility and efficacy of PNCs as nanoscale PTAs.

The controllable assembly of nanoparticles into nonclosepacked (NCP) arrays exhibits unusual optical properties [e.g., surface lattice resonances (SLRs)] but are challenging to construct, especially for NCP binary arrays composed of diversified nanoparticles. Here, we show the construction of two-dimensional (2D) NCP binary Au nanoparticle arrays by modulating the convective self-assembly of colloidal nanoparticles on a templating NCP nanoparticle array. By dramatically inhibiting the coffee ring effect during convective evaporation, this convective assembly approach allows colloidal nanoparticles to stack around the templating nanoparticles, forming a "core-satellite" basic architecture. Moreover, this approach is generalized for various types, shapes, and periodicities of building nanoparticles. The assembled 2D NCP binary Au arrays attain multiple tunable optical properties, including photonic band gaps, plasmonic SLRs, and dynamic surface-enhanced Raman scattering activity, based on tunable plasmonic coupling. The anticounterfeiting application of these NCP binary arrays is demonstrated by taking advantage of their unique optical properties. This convective assembly approach can pave the way for the development of a new category of NCP binary arrays and help to explore their collective unconventional properties.

Near-field imaging of the propagation of a diffraction-free Bessel-type beam in a guided wave configuration generated by means of a metasurface-based axicon lens integrated on a silicon waveguide is reported. The operation of the axicon lens with a footprint as small as 11µm 2 is based on local engineering of the effective index of the silicon waveguide with plasmonic nanoresonators. This generic approach, which can be adapted to different types of planar lightwave circuit platforms, offers the possibility to design nano-engineered optical devices based on the use of plasmonic resonators to control light at the nanoscale.