Dilute noble metal films for infrared optics and plasmonics [Invited

Mid-infrared deep subwavelength confinement in graphene plasmonic waveguides

Here we present the results of our theoretical analysis for guided modes in parallel-plate waveguides filled with pairs of parallel layers made of any two of the following materials: (1) a material with negative real permittivity, but positive real permeability (epsilon-negative); (2) a material with negative real permeability, but positive real permittivity (mu-negative); (3) a material with both negative real permittivity and permeability (double-negative); and (4) a conventional material with both positive real permittivity and permeability (double-positive) in a given range of frequency. Salient properties of these guided modes are studied in terms of how these materials and their parameters are chosen to be paired, and are then compared and contrasted with those of the guided modes in conventional waveguides. Special features such as monomodality in thick waveguides and presence of TE modes with no-cutoff thickness in thin parallel-plate waveguides are highlighted and discussed. Physical insights and intuitive justifications for the mathematical findings are also presented.

Negative real permittivity in (Bi0.3Eu0.7)Sr2CaCu2O6.5 ceramic

Hyperbolic polaritons (HPs) supported by van der Waals (vdW) materials enable exceptionally strong light-matter interactions through deep subwavelength confinement. This confinement can be further enhanced when a polaritonic mode couples to its mirror image in a metallic substrate, giving rise to acoustic hyperbolic polaritons (AHPs). While most previous studies have focused on volume-confined AHPs (v-AHPs), edge-confined AHPs (e-AHPs) remain experimentally elusive. Here, we provide the first near-field observation of e-AHPs by launching and imaging them in the prototypical sample of hexagonal boron nitride (hBN) on a gold substrate. Unlike v-AHPs propagating inside hBN, these e-AHPs are guided along the hBN edges and exhibit shorter polariton wavelengths, as revealed by both near-field imaging and extracted dispersion relations. Enhanced vibrational strong coupling in e-AHPs, experimentally verified by monitoring molecular-induced near-field responses, demonstrates their suitability for high-sensitivity molecule detection. These distinctive properties establish e-AHPs as a promising platform for optical sensing and on-chip nanophotonic devices.

We demonstrate strong coupling between light in a dielectric nanocavity with deep subwavelength confinement and excitons in a monolayer of molybdenum ditelluride. Avoided crossing is demonstrated by both photoluminescence and reflection measurements, from which we extract a light-matter interaction strength of g PL = 5.3 ± 0.3 m eV and g R = 4.7 ± 0.7 m eV , respectively. The associated Rabi splitting is twice as large as the system's losses. These values are in good agreement with values obtained by an exciton reaction coordinate formalism, yielding g theory = 5.2 ± 0.7 m eV . The strong light-matter interaction, combined with low losses and subwavelength confinement of light, demonstrates a regime of light-matter interactions where strong nonlinearities at the single-photon level are expected.

Bimetallic nanoalloys combining magnetic and noble metals are promising for applications in magnetic sensors, catalysis, optical detection, and biomedical imaging. Their development relies on understanding morphology, electronic structure, and crystallography. This study explores iron-based magnetic nanoalloys using efficient synthesis and advanced characterization. Molecular dynamics (MD) simulations examined atomic-scale morphology and structural features, linking them to magnetic behavior. A spin-lattice dynamics algorithm simulated iron-copper (FeCu) nanoalloys of varying sizes and compositions. FeCu nanoalloys were synthesized via a one-step reduction reaction and analyzed using multiple techniques, yielding nanoparticles with high saturation magnetization and an 11 nm average size. Simulations and experiments confirmed core-shell (CS) and Janus morphologies, where copper shells an iron core. Findings suggest that composition, rather than morphology alone, predominantly influences magnetic properties, while the core-shell morphology enhances oxidation resistance due to the noble copper metal employed. This study is the first to integrate the spin-lattice algorithm with experimental analysis, providing consistent insights into design and accurate characterization. Thus, it confirms the practical and novel synthesis of low-size FeCu nanoparticles with exact ideal superparamagnetic properties—exhibiting no hysteresis—suitable for various research and industrial applications.

Germanium phosphide (GeP), a rising star of novel two-dimensional (2D) material composed of Group IV–V elements, has been extensively studied and applied in photonics thanks to its broadband optical absorption, strong light–matter interaction and flexible bandgap structure. Here, we show the strong nonlinear optical (NLO) properties of 2D GeP nanoflakes in the broadband range with open-aperture Z-scan technique to explore the performance of 2D GeP microfiber photonic devices (GMPDs) in near-infrared (near-IR) and mid-infrared (mid-IR) ultrafast photonics. Our results suggest that employing the GMPD as an optical device in an erbium-doped fiber laser (EDFL) system results in ultrashort pulses and rogue waves (RWs) at 1.55 μm. Likewise, by the incorporation of GMPD into a thulium-doped fiber laser (TDFL) system, stable ultrashort pulse operation is also achieved at 2.0 μm. We expect these findings to be an excellent GMPD that can be applied in mode-locked fiber lasers to open up new avenues for its development and application in ultrafast photonics.

Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.

In the present study, the negative real permittivity behavior of a copolyester of bisphenol-A with terephthalic acid and isophthalic acid (PAr) containing 1.5 to 7.5 wt% multi-walled carbon nanotubes (MWCNTs) have been investigated in detail. The structural and morphological analysis of the melt-mixed composites was performed by Fourier transform infrared spectroscopy using attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), x-ray diffraction (XRD), and light microscopy. The influences of the MWCNT filler on the AC impedance, complex permittivity, and AC conductivity of the PAr polymer matrix were investigated at different operating temperatures varied between 296 K and 373 K. The transition from a negative to positive real permittivity was observed at different crossover frequencies depending on the MWCNT content of the composites whereas pure PAr showed positive values at all frequencies. The negative real permittivity characteristic of the composites was discussed in the context of Drude model.

7 - Noble metals and nonnoble metal oxides based electrochemical sensors

Strong light–matter interaction in 2D materials at the few-exciton level is important for both fundamental studies and quantum optical applications. Characterized by a fast coherent energy exchange between photons and excitons, strongly coupled plasmon–exciton systems in 2D materials have been reported with large Rabi splitting. However, large Rabi splitting at the few-exciton level generally requires large optical fields in a highly confined mode volume, which are difficult to achieve for in-plane excitons in 2D materials. In this work, we present a study of a strongly coupled gold dimer antenna with a sub-10 nm gap on a monolayer tungsten disulphide ( W S 2 ), with an estimated number of excitons of 4.67 ± 0.99 . We demonstrate that varying the spatial mode overlap between the plasmonic field and the 2D material can result in up to a ∼ t e n f o l d increase in the number of excitons, a value that can be further actively tuned via plasmon-induced heating effects. The demonstrated results would represent a key step toward quantum optical applications operating at room temperatures.

In this theoretical work, we report on voltage-controllable hybridization of electromagnetic modes arising from strong interaction between graphene plasmons and molecular vibrations. Compared with the strong light-matter interaction platforms based on noble metals, graphene offers much tighter plasmonic field confinement thus smaller effective mode volume and higher quality-factor due to longer carrier relaxation time in midinfrared regime, leading to Rabi splitting and hybridized polaritonic modes at 3 orders of magnitude lower molecular densities. Electrostatically tunable carrier density in graphene allows for dynamic control over the interaction strength. In addition, the flat dispersion band arising from the deep confinement of the polaritonic modes gives rise to the omni-directional excitation. Our approach is promising for practical implementations in infrared sensing and detection.

ConspectusPlasmonic nanostructures have garnered widescale scientific interest because of their strong light-matter interactions and the tunability of their absorption across the solar spectrum. At the heart of their superlative interaction with light is the resonant excitation of a collective oscillation of electrons in the nanostructure by the incident electromagnetic field. These resonant oscillations are known as localized surface plasmon resonances (LSPRs). In recent years, the community has uncovered intriguing photochemical attributes of noble metal nanostructures arising from their LSPRs. Chemical reactions that are otherwise unfavorable or sluggish in the dark are induced on the nanostructure surface upon photoexcitation of LSPRs. This phenomenon has led to the birth of plasmonic catalysis. The rates of a variety of kinetically challenging reactions are enhanced by plasmon-excited nanostructures. While the potential utility for solar energy harvesting and chemical production is clear, there is a natural curiosity about the precise origin(s) of plasmonic catalysis. One explanation is that the reactions are facilitated by the action of the intensely concentrated and confined electric fields generated on the nanostructure upon LSPR excitation. Another mechanism of activation involves hot carriers transiently produced in the metal nanostructure by damping of LSPRs.In this Account, we visit a phenomenon that has received less attention but has a key role to play in plasmonic catalysis and chemistry. Under common chemical scenarios, plasmonic excitation induces a potential or a voltage on a nanoparticle. This photopotential modifies the energetics of a chemical reaction on noble metal nanoparticles. In a range of cases studied by our laboratory and others, light-induced potentials underlie the plasmonic enhancement of reaction kinetics. The photopotential model does not replace other known mechanisms, but it complements them. There are multiple ways in which an electrostatic photopotential is produced by LSPR excitation, such as optical rectification, but one that is most relevant in chemical media is asymmetric charge transfer to solution-phase acceptors. Electrons and holes produced in a nanostructure by damping of LSPRs are not removed at the same rate. As a result, the slower carrier accumulates on the nanostructure, and a steady-state charge is built up on the nanostructure, leading to a photopotential. Potentials of up to a few hundred millivolts have been measured by our laboratory and others. A photocharged nanoparticle is a source of carriers of a higher potential than an uncharged one. As a result, redox chemical reactions on noble metal nanoparticles exhibit lower activation barriers under photoexcitation. In electrochemical reactions on noble metal nanoparticles, the photopotential supplements the applied potential. In a diverse set of reactions, the photopotential model explains the photoenhancement of rates as well as the trends as a function of light intensity and photon energy. With further gains, light-induced potentials may be used as a knob for controlling the activities and selectivities of noble metal nanoparticle catalysts.

Angstrom-scale-porous plasmonic molybdenum oxide for ultrasensitive optical chemical sensing

While plasmons in noble metal nanostructures enable strong light-matter interactions on nanometer length scales, the overabundance of free electrons in these systems inhibits their sensitivity to weak external stimuli. Countering this limitation, doped graphene has recently arisen as an actively-tunable material platform for plasmonics, offering extreme electromagnetic field concentration for the price of significantly fewer electrons [1,2]. Here we investigate transient modulation in the optical response of nanostructured graphene associated with the absorption of individual plasmons. We base our analysis on complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, lightly-doped graphene nanoisland, can sufficiently modify the temperature of its electrons and chemical potential to produce substantial changes in the optical response within sub-picosecond timescales, effectively shifting or damping the original plasmon absorption resonance peak and thereby blockading subsequent excitation of a second plasmon. The thero-optical single-plasmon blockade consist in a viable ultra-low power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics. [1] F. J. Garcia de Abajo, ACS Photon. 1, 135 (2014). [2] J. D. Cox and F. J. Garcia de Abajo, Optica 5, 429 (2018).