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Plasmonic metal nanostructures have been widely used to enhance the upconversion efficiency of the near‐infrared (NIR) photons into the visible region via the localized surface plasmon resonance (LSPR) effect. However, the direct utilization of low‐cost nonmetallic semiconductors to both concentrate and transfer the NIR‐plasmonic energy in the upconversion system remains a significant challenge. Here, a fascinating process of NIR‐plasmonic energy upconversion in Yb3+/Er3+‐doped NaYF4 nanoparticles (NaYF4:Yb‐Er NPs)/W18O49 nanowires (NWs) heterostructures, which can selectively enhance the upconversion luminescence by two orders of magnitude, is demonstrated. Combined with theoretical calculations, it is proposed that the NIR‐excited LSPR of W18O49 NWs is the primary reason for the enhanced upconversion luminescence of NaYF4:Yb‐Er NPs. Meanwhile, this plasmon‐enhanced upconversion luminescence can be partly absorbed by the W18O49 NWs to re‐excite its higher energy LSPR, thus leading to the selective enhancement of upconversion luminescence for the NaYF4:Yb‐Er/W18O49 heterostructures. More importantly, based on this process of plasmonic energy transfer, an NIR‐driven catalyst of NaYF4:Yb‐Er NPs@W18O49 NWs quasi‐core/shell heterostructure, which exhibits a ≈35‐fold increase in the catalytic H2 evolution from ammonia borane (BH3NH3) is designed and synthesized. This work provides insight on the development of nonmetallic plasmon‐sensitized optical materials that can potentially be applied in photocatalysis, optoelectronic, and photovoltaic devices. Nonmetallic plasmon‐induced selective enhancement of upconversion luminescence is observed in a layer‐structured NaYF4:Yb‐Er/W18O49 film due to the near‐infrared‐plasmonic energy upconversion. Based on this photonics process, an infrared‐driven plasmonic catalyst of NaYF4:Yb‐Er@W18O49 heterostructures is designed and synthesized, which exhibits a ≈35‐fold increase in catalytic H2 evolution upon IR excitation.

Using localized surface plasmon resonance (LSPR) as an optical probe we demonstrate the presence of free carriers in phosphorus doped silicon nanocrystals (SiNCs) embedded in a silica matrix. In small SiNCs, with radius ranging from 2.6 to 5.5  nm, the infrared spectroscopy study coupled to numerical simulations allows us to determine the number of electrically active phosphorus atoms with a precision of a few atoms. We demonstrate that LSP resonances can be supported with only about 10 free electrons per nanocrystal, confirming theoretical predictions and probing the limit of the collective nature of plasmons. We reveal the appearance of an avoided crossing behavior linked to the hybridization between the localized surface plasmon in the doped nanocrystals and the silica matrix phonon modes. Finally, a careful analysis of the scattering time dependence versus carrier density in the small size regime allows us to detect the appearance of a new scattering process at high dopant concentration, which can be explained by P clustering inside the SiNCs.

Plasmonic semiconductors have attracted extensive interest in optoelectronics and photocatalysis due to their broadened absorption spectral range, hot‐electron injection, and near‐field enhancement. Among various plasmonic semiconductors, WO3‐x allows high concentrations of oxygen vacancies (OV) and pronounced localized surface plasmon resonance (LSPR) effects, enabling continuous tuning of optical and electronic properties. However, the LSPR effect in WO3‐x depends critically on OV concentration and their stability. Herein, a surface reconstruction approach (structural rearrangement forming a dense surface layer with altered stoichiometry) was employed to synthesize WO3‐x nanosheets consisting of an inner layer with rich OV concentration and a dense WO3 surface passivation layer, which suppresses OV healing and thereby allows stable OV concentration during exposure in ambient atmosphere conditions or photocatalytic reactions. The as‐synthesized plasmonic WO3‐x semiconductor exhibits enhanced LSPR effect due to the formation of a dense WO3 passivation layer on the surface, which significantly improves the efficiency and stability of photocatalytic degradation of methyl orange under visible‐near‐infrared light illumination. This study provides a novel approach to improve the OV stability in plasmonic WO3‐x semiconductors, offering important insights for stabilizing OV concentrations in various plasmonic semiconductors. This advancement facilitates the application of plasmonic semiconductors in fields such as photocatalysis and nano‐optoelectronics. This study reports a surface reconstruction strategy to stabilize oxygen vacancies in plasmonic WO3‐x nanosheets. By engineering a dense WO3 passivation layer, the authors effectively trap interior vacancies against environmental reoxidation. This architecture preserves strong localized surface plasmon resonance effects, resulting in significantly enhanced photocatalytic degradation of methyl orange performance and superior stability during light‐driven reactions.

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobility thereby damping the plasmonic response. We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices. Terahertz optics: plasmonic metamaterials Nonlinear plasmonic metamaterials that are resonant in the terahertz spectral range have been made from disks of indium arsenide. Huseyin Seren and co-workers in the USA fabricated these materials and note that nonlinear semiconductor plasmonic disks may be useful as optical limiters or saturable absorbers. The team used dry etching to fabricate hexagonal arrays of indium arsenide disks that are 70 micrometres in diameter. The samples were doped to a level of 10 17 cm −3 resulting in a strong plasmonic response at 0.8 terahertz. A key advantage of using semiconductors as the plasmonic medium is that the doping level can be used to control the resonance frequency and other properties of the metamaterial.

A 3D flexible domestic waste styrofoam is reported as a surface enhanced Raman scattering (SERS) substrate loaded with BiOCl-BiOBr@Pt/Au semiconductor-plasmonic composites. The hydrothermally prepared BiOCl-BiOBr nanocomposite is thoroughly characterized for its crystal structure using X-Ray diffraction, morphology through scanning electron microscopy, and electronic states of the elements using X-ray photoelectron spectroscopy. The alpha cypermethrin (ACM) is chosen as a model pesticide analyte for SERS investigation. The BiOCl-BiOBr@Pt/Au loaded foam substrate exhibited a high enhancement factor (10 6 ) and low limit of detection (10 −10  M) upon SERS investigation. The unique architecture of the semiconductor-plasmonic composite enables an efficient charge transfer capability and plasmonic hotspots which aids in the enhancement of target analytes. In order to better demonstrate the versatility towards other pesticides, SERS detection of glyphosate and paraquat pesticides are also performed using the fabricated SERS substrate. The stability of the substrate has been investigated in detail for 30 days and the substrate was highly stable. The BiOCl-BiOBr@Pt/Au-based foam substrate also performed well in rapid real-time sensing of alpha cypermethrin on the kiwi fruit exocarp at lower level concentrations. Graphical abstract

Scalable, high‐speed, small‐footprint photonic switching platforms are essential for advancing optical communication. An effective optical switch must operate at high duty cycles with fast recovery times, while maintaining substantial modulation depth and full reversibility. Colloidal nanocrystals, such as indium tin oxide (ITO), offer a scalable platform to meet these requirements. In this work, the transmission of ITO nanocrystals near their epsilon‐near‐zero wavelength is modulated by two‐cycle optical pulses at a repetition rate of one megahertz. The modulator exhibits a broad bandwidth spanning from 2 to 2.5 µm. Sensitive fieldoscopy measurements resolve the transient electric‐field response of the ITO for the first time, showing that the modulation remains reversible for excitation fluences up to 1.2 mJ cm−2 with a modulation depth of 10%, and becomes fully irreversible beyond 3.3 mJ cm−2, while reaching modulation depth of up to 20%. Field sampling further indicates that at higher excitation fluences, the relative contribution from the first cycle of the optical pulses is reduced. These findings are crucial for the development of all‐optical switching, telecommunications, and sensing technologies capable of operating at terahertz switching frequencies. The ultrafast nonlinear dynamics of indium tin oxide nanocrystals excited with 1 MHz, 10 fs pulses, at their localized surface plasmon frequency, and near their epsilon‐near‐zero wavelength is probed via fieldoscopy. The study demonstrates fluence‐dependent switching behavior, advancing the understanding of high‐speed optical modulation in plasmonic semiconductor nanocrystals.

Heavily doped semiconductors have emerged as low-loss and tunable materials for plasmonics at mid-infrared frequencies. We analyze the nonlinear optical response of free electrons and show how nonlinear optical phenomena associated with high electron concentration are influenced by the intrinsic properties of semiconductors, namely background permittivity and effective mass. We apply our recently developed hydrodynamic description that takes into account nonlinear contributions up to the third order, usually negligible for noble metals, to compare third-harmonic generation from InP, Ge, GaAs, Si, ITO and InSb. We show how free electron nonlinearities may be enhanced with a proper choice of the semiconductor.

We present a metal–semiconductor (M–S) based electro-optic modulator designed for functional plasmonic circuits, utilizing the active control of surface plasmon polaritons (SPPs) at M–S junction interfaces. Through self-consistent multiphysics simulations, including electromagnetic, thermal, and current–voltage ( IV ) characteristics, we estimate bias- and doping concentration-dependent SPP modulation and switching times. This study focuses on germanium-based Schottky contacts and can be extended to other semiconducting materials. We performed parametric analysis using the developed thermo-electro-optic model to identify device parameters and dimensions for enhanced optical confinement and faster operation. The studied device exhibits signal modulation exceeding −28 dB, responsivity greater than −1800 dB V −1 , and switching rates of 8 GHz, suggesting potential data rates above 16 Gbit s −1 . Additionally, frequency response analysis using the numerical model confirms the device’s electrical tunability and predicts a 3 dB bandwidth of up to 4 GHz. These findings highlight the significant potential of Schottky junctions as active components in the development of plasmonic-based integrated circuits.

The volume plasmon modes of a confined electron gas are engineered in a step-like semiconductor potential, which induces the formation of adjacent regions of different charge density. Each region supports spatially localized collective modes. Adjacent modes are theoretically demonstrated to couple, forming delocalized modes, which are well-described with a hybridization picture. Exploiting the thin-film Berreman effect, the engineered plasmon modes are directly observed in optical measurements. Using a quantum microscopic theory, the asymmetry of the single-particle electronic states is shown to be directly imprinted on the nonuniform polarization of the collective modes.

Semiconductor heterostructures are suitable for the design and fabrication of terahertz (THz) plasmonic devices, due to their matching carrier densities. The classical dispersion relations in the current literature are derived for metal plasmonic materials, such as gold and silver, for which a homogeneous dielectric function is valid. Penetration of the electric fields into semiconductors induces locally varying charge densities and a spatially varying dielectric function is expected. While such an occurrence renders tunable THz plasmonics a possibility, it is crucial to understand the conditions under which propagating resonant conditions for the carriers occur, upon incidence of an electromagnetic radiation. In this manuscript, we derive a dispersion relation for a p–n heterojunction and apply the methodology to a GaAs p–n junction, a material of interest for optoelectronic devices. Considering symmetrically doped p- and n-type regions with equal width, the effect of certain parameters (such as doping and voltage bias) on the dispersion curve of the p–n heterojunction were investigated. Keeping in sight the different effective masses and mobilities of the carriers, we were able to obtain the conditions that yield identical dielectric functions for the p- and n-regions. Our results indicated that the p–n GaAs system can sustain propagating resonances and can be used as a layered plasmonic waveguide. The conditions under which this is feasible fall in the frequency region between the transverse optical phonon resonance of GaAs and the traditional cut-off frequency of the diode waveguide. In addition, our results indicated when the excitation was slightly above the phonon resonance frequency, the plasmon propagation attained low-loss characteristics. We also showed that the existence or nonexistence of the depletion zone between the p- and n- interfaces allowed certain plasmon modes to propagate, while others decayed rapidly, pointing out the possibility for a design of selective filters.