Volume 13, Issue 26

Plasmon-driven photocatalysis offers a unique means of leveraging nanoscale light–matter interactions to convert photon energy into chemical energy in a chemoselective and regioselective manner under mild reaction conditions. Plasmon-driven bond cleavage in molecular adsorbates represents a critical step in virtually all plasmon-mediated photocatalytic reactions and has been identified as the rate-determining step in many cases. This review article summarizes critical insights concerning plasmon-triggered bond-cleaving mechanisms gained through combined experimental and computational efforts over the past decade or so, elaborating on how the plasmon-derived physiochemical effects, metal–adsorbate interactions, and local chemical environments profoundly influence chemoselective bond-cleaving processes in a diverse set of molecular adsorbates ranging from small diatomic molecules to aliphatic and aromatic organic compounds. As demonstrated by several noteworthy examples, insights gained from fundamental mechanistic studies lay a critical knowledge foundation guiding rational design of nanoparticle–adsorbate systems with desired plasmonic molecule-scissoring functions for targeted applications, such as controlled release of molecular cargos, surface coating of solid-state materials, and selective bond activation for polymerization reactions.

Thin-film-lithium-niobate (TFLN) photonics has attracted intensive attention and become very popular in recent years. Here, an ultra-compact TFLN on-chip dispersion compensator is proposed and realized to provide a promising solution for dispersion control. The proposed dispersion compensator is composed of chirped multimode waveguide gratings (CMWGs) arranged in zigzag-cascade, enabling high footprint compactness and scalability. Particularly, these CMWGs are circulator-free and very convenient for cascading, owing to the TE 0 –TE 1 mode conversion and the assistance of the TE 0 –TE 1 mode (de)multiplexer. The present configuration with CMWGs in zigzag-cascade also overcomes the drawback of being unable to use waveguide spirals for large-range time delay and dispersion control due to the TFLN’s anisotropy. In addition, positive/negative dispersion control is realized by appropriately choosing the input port of the CMWGs. In the experiment, 2-mm-long CMWGs are used to provide a dispersion value of about +1.5 ps/nm and −1.2 ps/nm over a 21-nm-wide bandwidth, and there are up to 32 CMWGs in cascade demonstrated experimentally, showing a maximal dispersion of 49.2 ps/nm and −39.3 ps/nm. The corresponding average propagation loss is as low as 0.47 dB/cm, and the fabricated chip with 32 CMWGs in zigzag-cascade has a footprint as compact as 0.16 × 4.65 mm 2 . Finally, the present on-chip dispersion compensator is used successfully to compensate for the dispersion originating from a 5-km-long singlemode fiber (SMF) and high-quality eye-diagrams are achieved for the recovered 40 Gbps OOK signals, showing great potential for optical systems such as high-speed interconnects in datacenters.

In this work, negative-capacitance (NC) and local surface plasmon resonance (LSPR) coupled MoS 2 phototransistors with a gate stack of HZO/AuNPs/Al 2 O 3 /MoS 2 are fabricated, and the impacts of Al 2 O 3 interlayer-thickness ( T AlO ) on the LSPR effect, the tensile strain on MoS 2 from the Au nanoparticles (AuNPs), the capacitance matching of the NC effect from Hf 0.5 Zr 0.5 O 2 (HZO) ferroelectric layer and the optoelectrical properties of the relevant devices are investigated. Through optimizing T AlO , excellent optoelectrical properties of phototransistors with a T AlO of 3 nm are achieved: a subthreshold swing ( SS ) of 25.76 mV/dec and ultrahigh detectivity of over 10 14 Jones under 740 nm illumination. This is primarily because the NC-LSPR coupled structure can achieve an ultra-low SS through capacitance matching and a good interface passivation through optimizing Al 2 O 3 interlayer to maintain effective LSPR and strain effects cross the MoS 2 to enhance optical absorption and detection range. This work provides a comprehensive analysis on effective distance range of the non-direct-contacted LSPR effect and its combination with capacitance matching of NC effect, culminating in an optimized NC-LSPR coupled MoS 2 phototransistor with a good consistency across an array of 30 devices, and offering a viable solution for the preparation of large-area, high-performance and broad-spectrum response 2D phototransistor array.

Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe 2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark ( X D ) and grey ( X G ) excitons and the negative dark trion ( T D ) differ substantially at particular temperature. More importantly, they weaken considerably by about 3–4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i . e . neutral bright exciton ( X B ) and spin-singlet ( T S ) and spin-triplet ( T T ) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X D and T D complexes and the corresponding bright X B , T S , and T T ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton–phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties.

Rotational motion of charges in plasmonic nanostructures plays an important role in transferring angular momentum between light and matter on the nanometer scale. Although sophisticated control of rotational charge motion has been achieved using spatially structured light, its extension to simultaneous excitation of the same charge motion in multiple nanostructures is not straightforward. In this study, we perform model calculations to show that spatially homogeneous circularly polarized (CP) light can excite rotational charge motions with a high degrees of freedom by exploiting the rotational symmetry of the plasmonic structure and that of the plasmon mode. Finite-difference time-domain simulations demonstrate selective excitation of rotational charge motion for both isolated nanoplates and periodic array structures, showing that complex charge rotations can be manipulated by plane CP waves in a wide range of plasmonic structures.

van der Waals (vdW) materials supporting phonon polaritons (PhPs) – light coupled to lattice vibrations – have gathered significant interest because of their intrinsic anisotropy and low losses. In particular, α-MoO 3 supports PhPs with in-plane anisotropic propagation, which has been exploited to tune the optical response of twisted bilayers and trilayers. Additionally, various studies have explored the realization of polaritonic crystals (PCs) – lattices with periods comparable to the polariton wavelength. PCs consisting of hole arrays etched in α-MoO 3 slabs exhibit Bragg resonances dependent on the angle between the crystallographic axes and the lattice vectors. However, such PC concept, with a fixed orientation and size of its geometrical parameters, constrains practical applications and introduces additional scattering losses due to invasive fabrication processes. Here, we demonstrate a novel PC concept that overcomes these limitations, enabling low-loss optical tuning. It comprises a rotatable pristine α-MoO 3 layer located on a periodic hole array fabricated in a metallic layer. Our design prevents degradation of the α-MoO 3 optical properties caused by fabrication, preserving its intrinsic low-loss and in-plane anisotropic propagation of PhPs. The resulting PC exhibits rotation of the Bloch modes, which is experimentally visualized by scanning near-field microscopy. In addition, we experimentally determine the polaritons momentum and reconstruct their band structure. These results pave the way for mechanically tunable nano-optical components based on polaritons for potential lasing, sensing, or energy harvesting applications.

In the realm of nanotechnology, the integration of quantum emitters with plasmonic nanostructures has emerged as an innovative pathway for applications in quantum technologies, sensing, and imaging. This research paper provides a comprehensive exploration of the photoluminescence enhancement induced by the interaction between quantum emitters and tailored nanostructure configurations. Four canonical nanoantennas (spheres, rods, disks, and crescents) are systematically investigated theoretically in three distinct configurations (single, gap, and nanoparticle-on-mirror nanoantennas), as a representative selection of the most fundamental and commonly studied structures and arrangements. A detailed analysis reveals that the rod gap nanoantenna configuration achieves the largest photoluminescence enhancement factor, of up to three orders of magnitude. The study presented here provides insights for the strategic design of plasmonic nanoantennas in the visible and near-IR spectral range, offering a roadmap for these structures to meet specific requirements in plasmon-enhanced fluorescence. Key properties such as the excitation rate, the quantum yield, the enhanced emitted power, or the directionality of the emission are thoroughly reviewed. The results of this overview contribute not only to the fundamental understanding of plasmon-enhanced emission of quantum emitters but also set the basis for the development of advanced nanophotonic devices with enhanced functionalities.

We present a solution to a longstanding challenge in nanoplasmonics and colloid chemistry: the anomalous optical absorption of noble metal nanoparticles in the ultrafine size range of 2.5–10 nm, characterized by a rapid long-wavelength shift in plasmon resonance as the particle size increases. Our investigation delves into the impact of alterations in electron density along the radial direction of nanoparticles and the resulting variations in dielectric constants on the spectral positioning of the plasmon resonance. We explore the interplay of the spill-out effect, volumetric compression, and their combined impact in different experimental conditions on electron density variation within the particle volume and its blurring at the particle boundary. The latter effectively forms a surface layer with altered dielectric constants and a size-independent extent. As particle size decreases, the influence of the surface layer becomes more pronounced, especially when its extent is comparable to the particle radius. These findings are specific to ultrafine plasmonic nanoparticles and highlight their unique properties.

Optically controllable delivery of microparticles excites interesting research and applications in various fields because of the noninvasive and noncontact features. However, long-distance delivery with a static low-power light source remains challenging. Here, the constant-force photonic projectile (CFPP) is employed to achieve long-distance delivery of microparticles with a low-power laser beam. The CFPP takes advantage of photon absorption to create a constant optical force within a large range, surpassing traditional tweezers. The concept of CFPP has been experimentally corroborated by remote control over micrometer-sized absorptive particles (APs) using a simple tilted focused beam. At the laser focus, strong photon absorption results in a large constant optical force that ejects the APs along the optical axis. Furthermore, the additional thermal convection field, which attracts particles from a distance into the working range of the CFPP, is utilized to collect the unbound APs for reuse. Finally, we demonstrate the concept of drug delivery by transporting a small microparticle onto a host particle at a remote location. The proposed CFPP provides a new perspective for drug delivery and heat-enhanced photodynamic therapy.

We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle with an active core and a plasmonic metal shell. Using time-dependent equations for the gain medium and metal, we calculate the lasing threshold through an instability analysis. Below threshold, the nanoshell acts as an optical amplifier when excited by an external probe field, while above threshold, it enters a regime of autonomous lasing. At the gain threshold, the lasing starts at one frequency, typically a plasmon resonance of the nanoparticle. With increasing gain, the emission then broadens to additional frequencies. This result contrasts with previous findings reporting only a single emission wavelength above threshold. We also compute the full spectrum and linewidth of the nanolaser, revealing strong frequency shifts and an asymmetrical lineshape. Finally, we demonstrate that the emission line can be tuned across the visible spectrum by modifying the aspect ratio of the nanoshell.

Conventional thermal emitters, such as a blackbody or the incandescent filament of a light bulb, lack the directionality or narrow linewidth required in many applications such as thermophotovoltaics and infrared sensing. Although thermal emission from bulk materials is well understood based on phenomenological heat transfer concepts like emissivity and the framework of classical electrodynamics, there still remains a significant gap in our understanding of thermal emission at the nanoscale. In this work, by leveraging the quasi-normal mode theory, we derive a general and self-consistent formalism to describe the thermal radiation from nanoscale resonant thermal emitters with high-order symmetric geometries, which are the basic building blocks of metasurfaces and metamaterials. The complex symmetrical geometries of the emitters yield degeneracy of quasi-normal modes. The introduction of the degeneracy can strongly mediate far-field thermal emission from nanoscale resonators, which is closely correlated to the number of degenerate modes and the coupling among the degenerate modes. Our formalism from the quasi-normal mode theory serves as a general guideline to design the complex metastructures with high-ordered degeneracy to achieve optimized absorption or emission capabilities.