Plasmonic States Research Articles

Oxygen defect-induced localized surface plasmon resonance at the WO3-x quantum dot/silver nanowire interface: SERS and photocatalysis.

Oxygen defects play a crucial role in a variety of functional transition metal oxides, ranging from photocatalytic materials to photoelectric devices. Tungsten oxide (WO3-x) is a type of transition metal oxide that has rich substoichiometric compositions and possesses oxygen defects. These oxygen defects determine the photon-electron interactions in the WO3-x structures. Therein, WO3-x quantum dots (QDs) exhibit fast carrier-transport for photon-electron interactions due to their strong quantum-size effects. Here, we report the use of non-stoichiometric WO3-x QDs, as a model material, in combination with silver nanowires (Ag NWs) to study photon-electron interactions on the nanoscale. We demonstrate that the incident photon-to-electron conversion efficiency can be increased by 8.5% and that the dye photodegradation performance was improved by 40% in a WO2.72 QD@Ag NW (WO2.72 QDs supported on AgNWs) composite compared to those of individual WO2.72 QDs under simulated AM 1.5G light. Furthermore, the WO3-x QD@Ag NW composite exhibits both photocatalytic activity and surface-enhanced Raman scattering (SERS) features, and the WO3-x QDs can be switched between a "photocatalytic state" and a "SERS state" by changing the stoichiometric ratio. The synergistic effects are ascribed to the "plasmonic state" of WO2.72 QDs upon light irradiation. This work provides new insight into the design of highly efficient transition metal oxide/plasmonic metal nanocomposites for photoelectric devices.

Communication—Epsilon-Negative Metacomposite Realized by Titanium Carbide Alumina Binary Ceramics in Radio Frequency

Titanium carbide alumina (TiC-Al2O3) binary ceramics were fabricated and the distributions of TiC within the ceramic composites mattered most the dielectric performances and electrical conductivity. There have taken on a low frequency plasmonic state when TiC particles connected with each other and formed a three-dimensional conductive pathway within Al2O3 matrix, thus plasma-like negative permittivity appeared. The conduction mechanism changed from hopping conduction to metal-like conduction. The reactance spectrum suggested negative permittivity media showed inductive character. This work could contribute to the integration of structure and function for TiC-Al2O3 binary ceramics and the obtained ceramics have promising applications for electromagnetic metamaterials.

Communication—Tunable and Weakly Negative Permittivity in CNTs-CBs/Polystyrene Metacomposites

The CNTs-CBs nano-units composed of carbon nanotubes (CNTs) and carbon black (CBs) have been prepared by electrostatic self-assembly. Herein, the CNTs-CBs/PS composites with different CNTs-CBs contents were prepared. The negative permittivity behavior was observed when three-dimensional conductive network was constructed by CNTs-CBs. Tunable and weakly negative permittivity originated from the low frequency plasmonic state which was collective electrons oscillation in CNTs-CBs network. The Jonscher's power law and Drude model were used to analyze the dielectric behavior and conductive mechanism changing. This work has broadened the applications of carbonaceous materials and polymers in the field of metamaterials.

Observation of terahertz plasmon and plasmon-polariton splitting in a grating-coupled AlGaN/GaN heterostructure.

Plasmon in two-dimensional electron gas (2DEG) has long been considered as a promising active medium for terahertz emitters and detectors. However, the efficiency of terahertz plasmonic devices is severely limited by the high damping rate of plasma wave in solid state. In addition to the enhancement of plasmon lifetime by using 2DEGs with higher carrier mobility, engineering on the boundary condition and electromagnetic environment of plasmon cavity helps to preserve the plasmon states. Here we report on terahertz reflection spectroscopy of plasmon states in a grating-coupled AlGaN/GaN-2DEG plasmonic device at 7 K in equilibrium with ambient blackbody irradiation. Localized plasmon states and plasmon-polariton states were observed when the core plasmonic device is integrated with a silicon lens and when it is embedded in a terahertz Fabry-Pérot cavity, respectively. Simulation results including the reflection spectra and total reflection power agree well with the measured results. The Rabi splitting is found to be inversely proportional to the resonance frequency, and follows a linear relation with the square root of the sheet electron density. A normalized coupling ratio, ΩRω0≈0.13, is achieved between the Rabi splitting ΩR and the resonance frequency ω0. The coupling ratio could be further increased to allow for ultrastrong coupling between terahertz photons and plasmons.

Features of Light Transmission and Stark Effect in a Plasmonic Nanostructure Comprising Organic Semiconductor and Subwavelength Aluminum Grating

Specific features of light transmission and electroabsorption in a plasmonic nanostructure with organic semiconductor (zinc phthalocyanine, ZnPc) and subwavelength aluminum grating (AlGr) have been experimentally investigated. The glass–AlGr–ZnPc–Al nanostructure was prepared using layer-by-layer deposition. First, only a part of the structure has been analyzed to check the grating quality. After deposition of an organic semiconductor (ZnPc) layer on the grating, the transmission of TE and TM polarized light through AlGr and ZnPc layers upon excitation of plasmon resonances has been studied. Afterwards, a semitransparent aluminum layer has been deposited on the ZnPc layer (an additional electrode for measuring the electroabsorption effect). For TM polarized light, a multiple increase in the electroabsorption effect has been found (in comparison with a structure without subwavelength grating). This result can be explained by the Stark effect on exciton transitions and the presence of two cavities in the structure, which are related to the excitation of plasmonic states and the Fabry–Perot effect between aluminum electrodes. The presence of the cavities leads to a decrease in the group velocity of light and, accordingly, increases the density of exciton states, which are characterized by a large difference between polarizabilities in the excited and ground states.

Zak phase and topological plasmonic Tamm states in one-dimensional plasmonic crystals.

The Zak phase and topological plasmonic Tamm states in plasmonic crystals based on periodic metal-insulator-metal waveguides are systematically investigated. We reveal that robust topological interfacial states against structural defects exist when the Zak phase between two adjoining plasmonic lattices are different in a common band gap. A kind of efficient admittance-based transfer matrix method is proposed to calculate and optimize the configuration with inverse symmetry. The topologically protected states are favorable for the spatial confinement and enhancement of electromagnetic fields, which open a new avenue for topological photonic applications.

Plasmonic valley chiral states in graphene based plasmonic crystals

A new degree of freedom, valley, has recently been introduced into the fields of acoustic and photonic crystals. This provides a promising platform for classical waves to mimic the valley-contrasting properties of condensed matter physics. Here, we study the plasmonic version of valley states in graphene plasmonic crystals of inversion symmetry broken honeycomb lattices. The intrinsic valley vortex feature is unambiguously revealed. The evolution of the valley chiral states involving topological transition is theoretically explored. Furthermore, numerical simulations for the selective excitations of the valley chiral states are carried out in our designed valley plasmonic crystals. Our work has potential applications in the orbital angular momentum-assisted surface plasmon polariton manipulation and the applications of plasmonic valleytronics in nanophotonics and on-chip integration.

Temperature-mediated invocation of the vacuum state for switchable ultrawide-angle and broadband deflection

Temperature-mediated appearance and disappearance of a deflection grating in a diffracting structure is possible by employing InSb as the grating material. InSb transits from the dielectric state to the plasmonic state in the terahertz regime as the temperature increases, this transition being reversible. An intermediate state is the vacuum state in which the real part of the relative permittivity of InSb equals unity while the imaginary part is much smaller. Then the grating virtually disappears, deflection being impossible as only specular reflection can occur. This ON/OFF switching of deflection and relevant angular filtering are realizable over wide ranges of frequency and incidence angle by a temperature change of as low as 20 K. The vacuum state of InSb invoked for ON/OFF switching of deflection and relevant angular filtering can also be obtained for thermally tunable materials other than InSb as well as by using non-thermal mechanisms.

Quantum plasmonic N00N state in a silver nanowire and its use for quantum sensing

The control of quantum states of light at the nanoscale has become possible in recent years with the use of plasmonics. Here, many types of nanophotonic devices and applications have been suggested that take advantage of quantum optical effects, despite the inherent presence of loss. A key example is quantum plasmonic sensing, which provides sensitivity beyond the classical limit using entangled N00N states and their generalizations in a compact system operating below the diffraction limit. In this work, we experimentally demonstrate the excitation and propagation of a two-plasmon entangled N00N state (N=2) in a silver nanowire, and assess the performance of our system for carrying out quantum sensing. Polarization entangled photon pairs are converted into plasmons in the silver nanowire, which propagate over a distance of 5 um and re-convert back into photons. A full analysis of the plasmonic system finds that the high-quality entanglement is preserved throughout. We measure the characteristic super-resolution phase oscillations of the entangled state via coincidence measurements. We also identify various sources of loss in our setup and show how they can be mitigated, in principle, in order to reach super-sensitivity that goes beyond the classical sensing limit. Our results show that polarization entanglement can be preserved in a plasmonic nanowire and that sensing with a quantum advantage is possible with moderate loss present.

Plasmonic topological edge states in ring-structure gate graphene.

Topological photonic states exhibit unique robustness against defects, facilitating fault-tolerant photonic device applications. However, existing proposals either involve a sophisticated and bulky structure or can only operate in the microwave regime. We show a theoretical demonstration for highly confined topologically protected plasmonic states to be realized at infrared frequencies in monolayer graphene with a ring-structure gate. With a suitable bias voltage, the combined gate-graphene structure is shown to produce sufficiently strong Bragg scattering of graphene surface plasmons and to impart them with nontrivial topological properties. Our design is compact and could pave the way for dynamically reconfigurable, robust, nanoscale, integrated photonic devices.

Non-classical plasmon states generation in nonlinear spaser systems

The article is focused on the investigation of features of quantum dynamics for localized plasmons in spaser systems consisting of metal nanoparticles and semiconductor quantum dots. The non-classical plasmon states generation in a three-particle spaser system with nonlinear plasmon-exciton interaction is predicted.

Dielectric study of the interplay between charge carriers and electron energy losses in reduced titanium dioxide

The transport mechanism in titanium dioxide through polarons is an open issue. High-resolution electron energy-loss spectroscopy (HREELS) is in principle of great relevance in such context, provided the fingerprints on the loss spectrum of the charge carriers involved in the material are disclosed. This paper aims at evidencing those fingerprints. Through a suitable parametrization of the dielectric function, a theoretical analysis of EELS excitations in defective ${\mathrm{TiO}}_{2}$ rutile is developed in the framework of the semiclassical dielectric theory. The focus is put on the interplay between phonons, interband transitions, and defect-related excitations, namely, plasmon and band-gap states. Transport properties are demonstrated to be more efficiently grasped through the screening they induce on phonons than through the existence of a defined surface plasmon peak. While the corresponding imaginary part of the dielectric function only yields a slight broadening and temperature dependence of the quasielastic peak due to the large static dielectric function and electron effective mass, a sizable upward shift in energy and a decrease in intensity of phonons due to the real part are predicted. Band-gap states also screen phonons but with downward shift in energy loss. Due to its large oscillator strength, the high-energy-lying surface phonon at 95 meV is a very sensitive reporter of the combined effects of transport behavior and band-gap states. Finally, it is highlighted that extracting quantitative information out of EELS experiments requires an accurate modeling of the depth profile.

Local-symmetry distortion, optical properties, and plasmonic states of monoclinic Hf0.5Zr0.5O2 system: a density-functional study

We study structural, electronic, and optical properties of the monoclinic Hf0.5Zr0.5O2 (space group: P21/c). Monoclinic ZrO2 and HfO2 systems are used as the references. This study employs the plane-wave density-functional method within the generalized gradient approximation. The structural properties emphasize the strongest local-symmetry distortion in Hf0.5Zr0.5O2 system. The bandgap (3.67 eV) and valence bandwidth (∼4.94 eV) of the system are between that of ZrO2 and HfO2 systems. Furthermore, the negative average defect formation energy confirms the stability of Hf0.5Z0.5O2 system. In all the systems, the significant differences between n0x(n0y) and n0z imply the strong optical dichroism between xy plane and z axis. On the other hand, the small differences between n0x and n0y imply the weak optical dichroism along xy plane. In Hf0.5Zr0.5O2 system, the optical properties show the zero-frequency refractive indices of (n0x, n0y, n0z) = (2.039, 2.086, 1.851). The system also shows the highest intensity and weakest dichroism of plasmonic states along y axis and xy plane, respectively. Moreover, we find the lowest energy level of maximum transmittance (T ≈ 1), strong-reflectance width, absorption width, and saturation threshold energy (∼9.6 eV) of the effective number of valence electrons. Our result emphasizes the importance of the doping treatment to the significant tuning of optical properties of Hf1−xZrxO2 system. This study presents the essential properties to guide future experiments to develop novel optical devices.

Unveiling and Imaging Degenerate States in Plasmonic Nanoparticles with Nanometer Resolution.

Metal nanoparticles host localized plasmon excitations that allow the manipulation of optical fields at the nanoscale. Despite the availability of several techniques for imaging plasmons, direct access into the symmetries of these excitations remains elusive, thus hindering progress in the development of applications. Here, we present a combination of angle-, polarization-, and space-resolved cathodoluminescence spectroscopy methods to selectively access the symmetry and degeneracy of plasmonic states in lithographically fabricated gold nanoprisms. We experimentally reveal and spatially map degenerate states of multipole plasmon modes with nanometer spatial resolution and further provide recipes for resolving optically dark and out-of-plane modes. Full-wave simulations in conjunction with a simple tight-binding model explain the complex plasmon structure in these particles and reveal intriguing mode-symmetry phenomena. Our approach introduces systematics for a comprehensive symmetry characterization of plasmonic states in high-symmetry nanostructures.

The Role of Metal Nanostructures in CO2 Photoreduction

Process of CO2 reduction requires substantial decrease of the energy barriers. Consequently, one of the promising way to approach this problem is using use of metal nanoparticles to accelerate electrons and make them more electronegative at the end of this process. Better understanding of the photo-induced phenomena enables to improve design & construction of semiconductor architectures governing the efficiency of CO2 reduction and its selectivity. The objective of this presentation will rely onto use of semiconductor photocathode with co-assembly of plasmonic and catalytic metal nanoparticles, such as silver or gold with other metal electrocatalysts which decrease the energy barriers of the CO2 reduction process through interaction of photo-excited surface plasmon states with adsorbed reaction intermediates. The significance to investigate reduction of CO2 with gold, silver and copper nanoparticles, enhanced by the extra activation of the process provided by the illuminated, incorporated plasmonic metallic nanostructures is supported by the occurrence of the photo-emission process associated with the decay of photo-excited surface plasmons. The occurrence, and contribution of this process to the overall efficiency of the CO2 reduction process as well as its impact on the distribution of the products will be investigated and discussed in detail. Moreover, the tradeoff between CO2 and water reduction will be continuously investigated and discussed in view of choice of the best metal-based catalyst designed for high efficiency and best selectivity CO2 reduction process.

Percolation-induced low frequency plasmonic state in metal granular composite materials

Percolation-induced low frequency plasmonic state in metal granular composite materials

Bonding state energy of metal nanoparticle dimer and its dependence on nanosphere size and interparticle separation

A study is conducted regarding the effects of particle size [Formula: see text] and interparticle separation [Formula: see text] on the electromagnetic (plasmon) coupling in a dimer of two identical metal nanospheres. The dimer states are modeled as the hybridized bonding and antibonding states of two isolated plasmon states, with the associated energies given in terms of the isolated plasmon energy ([Formula: see text], the coupling energy ([Formula: see text] and the overlap integral ([Formula: see text] of the constituent plasmonic fields. The resonance absorption energies of the isolated plasmon and the dimer in certain dielectric medium are calculated according to the Mie theory for incident light of parallel polarization along the dimer axis. The results are fitted with the bonding state energies of both Au and Ag nanosphere dimers for [Formula: see text] ranging within 10–20[Formula: see text]nm and x varied within [Formula: see text]–200[Formula: see text]nm in compliance with the restricted consideration of dipole absorption spectra. The excellent fits of the bonding state energies [Formula: see text] for the ranges of [Formula: see text] and [Formula: see text] variations are consistently achieved with [Formula: see text] around 0.99 by a single function of the form [Formula: see text] where [Formula: see text] and [Formula: see text] vary with the nanosphere materials and the surrounding media considered. This result suggests the possible relation of the best fitted functional form [Formula: see text] with the underlying physical mechanism.

The effect of vacancies on the optical properties and plasmonic states of zinc oxide: A first-principle study

We study structural and optical properties of ZnO film prepared by pulsed laser deposition. The film was characterized by x-ray diffraction and UV–visible spectroscopies. We find that the as-grown film is mostly amorphous. After hydrogen-annealing treatment, the film is crystallized with lattice parameters of a = 3.241 Å and c = 5.203 Å. From UV–visible spectra, the low absorption edges of 1.44 and 1.43 eV are observed in the as-grown and annealed films, respectively, suggested to be promoted by some vacancies. Then, the generalized gradient approximation method is used to calculate electronic and optical properties of ZnO0.94, and Zn0.94O systems as the possible models of the annealed film. Properties ZnO is also calculated as the reference. Bandgaps of 0.75 and 1.73 eV are obtained for ZnO and ZnO0.94, respectively. The larger bandgap of ZnO0.94 is caused by the increase of Fermi level induced by Zn 4s electrons, leading to the n-type semiconducting behaviour. On the other hand, Zn0.94O exhibits the p-type metallic behaviour caused by the decrease of Fermi level induced by O 2p electrons with a minimum interband transition (ΔE) of 0.95 eV. Then, a shift of ΔE is applied in the optical properties calculation for approaching the experimental results. From the imaginary part of dielectric function (ε2(E)) for xy plane, ΔE of ZnO and ZnO0.94 systems are 3.30 and 4.10 eV, respectively. The optical dichroism of ZnO0.94 is smaller than that of ZnO. On the other hand, ΔE of Zn0.94O is 1.80 eV based on ε2(E) for z axis indicating the optical dichroism flip by the Zn vacancy in ZnO. The low absorption edge of the annealed film is promoted by the Zn vacancy. Furthermore, plasmonic-state energy levels in ZnO can be tuned by the O or Zn vacancies. This study shows the essential properties of ZnO for potential high-energy plasmonic device applications.

Plasmonic Tamm states in insulator–metal–insulator waveguides

Plasmonic Tamm states configuration is proposed for field enhancement in an insulator–metal–insulator (IMI) Bragg reflector by periodic modulation of the dielectrics surrounding the metal core. Finite-difference time-domain simulation is carried out for the structure optimization and the dispersion relation of plasmonic Tamm states. In the metal–Bragg reflector interface, 2 orders of magnitude of electromagnetic field intensity enhancement and 4 orders of magnitude of decay rate enhancement are obtained. Besides the field enhancement, there are two plasmonic Tamm states that are related to the even and odd modes in the IMI waveguide. The two plasmonic Tamm states show large difference in the reflection spectra. The proposed structure would provide promising potential for electromagnetic nanofocusing and nanosensing.

Negative permittivity behavior of titanium nitride/polyphenylene sulfide “metacomposites” under radio frequency

Random composites with negative permittivity or negative permeability have developed rapidly while few were accomplished using ceramic materials as the functional fillers. In this work, titanium nitride/polyphenylene sulfide (TiN/PPS) composites with different TiN content were prepared by hot-pressing method. The electrical percolation threshold phenomenon occurred and the conductive mechanism transferred from hopping conduction to metal-like conduction when TiN’s content increased. Negative permittivity in composite with high TiN content was derived from low frequency plasmonic state of free electrons and the plasm-like negative permittivity spectra was analyzed by Drude model. Besides, it’s indicated that the dielectric loss was dominated by conduction loss and polarization loss in composites with high TiN content. Equivalent circuit model was utilized to investigate the impedance properties and suggested that parallel inductances played critical role in achieving negative permittivity.