Plasmon Modes Research Articles

Statistical and Correlative Characterization of Individual Nanoparticle in Gap Plasmon Resonator Sensors

AbstractPlasmonic nanocavities are receving increased attention from the nanophotonics, nanoelectronics, and quantum optics community due to their ability to confine light in extremely small volumes. Coupling colloidal plasmonic nanocrystals to metal films is an inexpensive approach to fabricate virtually unlimited individual resonators that can be tuned by adjusting nanoparticle size, gap thickness, and refractive index. The focus is on silver nanocubes separated from a gold mirror by thin amorphous dielectric layers (Al2O3, TiO2). The optical response is measured and correlative electron microscopy is performed on a large number (>800) of individual resonators to unveil the statistical distribution of gap plasmon modes and assess systematically their sensitivity. A sensitivity as large as 8 nm/nm to nanocube size variation, 50 nm/nm to Al2O3 thickness variation, and 130 nm/RIU for index change of the spacer layer is found. With the help of numerical simulations, this approach enables to infer a quantitative statistical distribution of molecular coatings present on the nanocubes surface, such as polyvinylpyrrolidone, which can affect the performance of nanoelectronic devices and assess the strategy to remove it. Finally, polarization‐resolved measurements enable to unveil a birefringent behavior when nanocubes are supported on annealed TiO2 layers (2–4 nm).

Theoretical and Experimental Analysis of Single-Arm Bimodal Plasmo-Photonic Refractive Index Sensors.

In this paper, we study both theoretically and experimentally the sensitivity of bimodal interferometric sensors where interference occurs between two plasmonic modes with different properties propagating in the same physical waveguide. In contrast to the well-known Mach-Zehnder interferometric (MZI) sensor, we show for the first time that the sensitivity of the bimodal sensor is independent of the sensing area length. This is validated by applying the theory to an integrated plasmo-photonic bimodal sensor that comprises an aluminum (Al) plasmonic stripe waveguide co-integrated between two accessible SU-8 photonic waveguides. A series of such bimodal sensors utilizing plasmonic stripes of different lengths were numerically simulated, demonstrating bulk refractive index (RI) sensitivities around 5700 nm/RIU for all sensor variants, confirming the theoretical results. The theoretical and numerical results were also validated experimentally through chip-level RI sensing experiments on three fabricated SU-8/Al bimodal sensors with plasmonic sensing lengths of 50, 75, and 100 μm. The obtained experimental RI sensitivities were found to be very close and equal to 4464, 4386, and 4362 nm/RIU, respectively, confirming that the sensing length has no effect on the bimodal sensor sensitivity. The above outcome alleviates the design and optical loss constraints, paving the way for more compact and powerful sensors that can achieve high sensitivity values at ultra-short sensing lengths.

Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions.

Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport. Such combined optical and electrical probes have proven useful for the fundamental understanding of metal-molecule contacts and contribute to the development of nanoscale optoelectronic devices including ultrafast electronics and nanosensors. Here, we employ a self-assembled metal-molecule-metal junction with a nanoparticle bridge to investigate correlated fluctuations in conductance and tunneling-induced light emission at room temperature. Despite the presence of hundreds of molecules in the junction, the electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations-a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photoexcited plasmonic junctions. Discrete steps in conductance associated with fluctuating emission intensities through the multiple plasmonic modes of the junction are consistent with a finite number of randomly localized, point-like sources dominating the optoelectronic response. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature long-term survival at room temperature and under an electrical bias of a few volts, allowing for measurements over several months.

Hyperbolic response and low-frequency ultra-flat plasmons in inhomogeneous charge-distributed transition-metal monohalides

Plasmon, the collective oscillations of free electron gas in materials, determines the long-wavelength excitation spectrum and optical response, are pivotal in the realm of nanophotonics and optoelectronics. In this study, using the first-principles calculations, we systematically investigated the dielectric response and plasmon properties of bulk transition-metal monohalides MXs (M = Zr, Mo; X = Cl, F). Due to the strong electronic anisotropy, MXs exhibit a broadband type-II hyperbolic response and direction-dependent plasmon modes. Particularly, local field effect (LFE) driven by the charge distribution inhomogeneity, significantly modifies the optical response and excitation spectra in MX along the out-of-plane direction. Taking into account LFE, the energy dissipation along the out-of-plane direction is almost completely suppressed, and an ultra-flat and long-lived plasmon mode with a slow group velocity is introduced. This finding reveals the role of charge density in modifying the optical response and excitation behavior, shedding light on potential applications in plasmonics.

Potassium doping of vertically aligned multi-walled carbon nanotubes.

Alkali metal doping of multi-walled carbon nanotubes is of great interest, both fundamentally to explore the effect of dopants on quasi-one-dimensional electrical systems and for energy applications such as alkali metal storage. We present an investigation with complementary photoemission and Raman spectroscopies, fully carried out in an ultra-high vacuum, to unveil the electronic and vibrational response of a forest of highly aligned multi-walled carbon nanotubes by in situ potassium doping. The charge donation by the alkali adatoms induces a plasmon mode, and the density of states undergoes an energy shift consistent with electron donation and band filling of the multi-walled carbon nanotube band structure. The π-states in the valence band and the Raman peaks unveil an evolution that can be ascribed to charge donation and partially to a tensile strain exerted by the K adatoms on the carbon lattice. All these effects are thermally reversible, fostering these materials as a potential system for electronic charge harvesting.

Surface plasmon resonance biosensor with anti-crossing modulation readout

A novel approach to surface plasmon resonance (SPR) biosensors providing simplified label-free monitoring of biomolecular affinity binding events is reported. It is based on the interrogation of anti-crossing surface plasmon modes traveling along opposite interfaces of a thin metal film on the top of a tailored multi-periodic grating structure. It allows for diffraction-based backside excitation of surface plasmons without the need of optical matching of the sensor chip to a prism and it allows avoiding of optical probing through the analyzed liquid sample. In conjunction with low angular dispersion of resonantly excited surface plasmon modes, it provides sensitive and versatile optical interrogation of SPR changes associated with biomolecular binding-induced refractive index variations. Direct readout with a fiber optic probe, as well as multi-channel configuration compatible with regular SPR readers, is implemented with the use of sensor chips prepared by mass production-compatible UV-nanoimprint lithography. The potential of the reported SPR sensor chips is illustrated by its ability to characterize affinity interaction of antibodies specific to cancer biomarker CSPG4 on antifouling mixed thiolated self-assembled monolayer with zwitterionic carboxybetaine and sulfobetaine headgroups.

Plasmon Excitations across the Charge-Density-Wave Transition in Single-Layer TiSe2.

1T-TiSe2 is believed to possess a soft electronic mode, i.e., plasmon or exciton, that might be responsible for the exciton condensation and charge-density-wave (CDW) transition. Here, we explore collective electronic excitations in single-layer 1T-TiSe2 by using the ab initio electromagnetic linear response and unveil intricate scattering pathways of the two-dimensional (2D) plasmon mode near the CDW phase. We found the dominant role of plasmon-phonon scattering, which in combination with the CDW gap excitations leads to the anomalous temperature dependence of the plasmon line width across the CDW transition. Below the transition temperature TCDW a strong hybridization between the 2D plasmon and CDW excitations is obtained. These optical features are highly tunable due to temperature-dependent CDW-related modifications of electronic structure and electron-phonon coupling and make CDW-bearing systems potentially interesting for applications in optoelectronics and low-loss plasmonics.

Compact unidirectional waveguide grating emitter with enhanced wavelength sensitivity based on the hybrid plasmonic mode

Waveguide grating antennas are widely adopted in beam-steering devices, typically enabling the beam steering in longitudinal direction within a two-dimensional scanning optical array by changing the input wavelength. However, traditional waveguide grating antennas suffer from limited tuning range due to low dispersion of the gratings. In this paper, a compact silicon grating waveguide antenna array is proposed with enhanced wavelength sensitivity by introducing a periodically modulated hybrid plasmonic mode. The hybrid plasmonic mode is supported by the hybrid plasmonic waveguides (HPWs) composed of silicon waveguides and periodic subwavelength silver strips. In order to convert the guided waves to the radiated waves, a series of silicon emitting segments are deposited above the HPWs. Additionally, the horizontally arranged array of HPWs also acts as a reflector of the downward radiation, resulting in an effective unidirectional emission. Through the optimization of physical parameters, the proposed antenna array achieves a wavelength-length tuning efficiency up to 0.3°/nm within the wavelength range of 1500∼1600 nm, exhibiting a significant improvement compared with traditional ones. Moreover, an average upward emissivity exceeding 80% with a maximum value of 89% within the 100 nm bandwidth is demonstrated through the numerical simulations. The proposed compact antenna array provides an alternative solution in realizing large-scale integrated high-tuning-efficiency optical beam-steering devices.

Hybrid mode for absorption enhancement in the Ga2O3 nanocavity photodetector with grating electrodes

Gallium oxide (Ga2O3) photodetectors have drawn increased interest for their widespread applications ranging from military to civil. Due to the inherent oxygen vacancy defects, they seriously suffer from trade-offs that make them incompetent for high-responsivity, quick-response detection. Herein, a Ga2O3 nanocavity photodetector assisted with grating electrodes is designed to break the constraint. The proposed structure supports both the plasmonic mode and the Fabry–Perot (F-P) mode. Numerical calculations show that the absorption of 99.8% is realized for ultra-thin Ga2O3 (30 nm), corresponding to a responsivity of 12.35 A/W. Benefiting from optical mechanisms, the external quantum efficiency (EQE) reaches 6040%, which is 466 times higher than that of bare Ga2O3 film. Furthermore, the proposed photodetector achieves a polarization-dependent dichroism ratio of 9.1, enabling polarization photodetection. The grating electrodes also effectively reduce the transit time of the photo-generated carriers. Our work provides a sophisticated platform for developing high-performance Ga2O3 photodetectors with the advantages of simplified fabrication processes and multidimensional detection.

Strong enhancement of third harmonic generation from a Tamm plasmon multilayer structure with WS2.

Improving the conversion efficiency is particularly important for the generation and applications of harmonic waves in optical microstructures. Herein, we propose to enhance the efficiency of third harmonic generation by integrating a monolayer WS2 with the metal/dielectric/photonic crystal multilayer structure. The numerical simulations show that the multilayer structure enables to generate the Tamm plasmon mode between the metal film and photonic crystal around the telecommunication wavelength, which is consistent with the experimental result. By measuring with a self-built nonlinear optical micro-spectroscopy system, we find that the third harmonic signal can be reinforced by 16-fold through inserting the monolayer WS2 in the dielectric spacer. This work will provide a new way for improving nonlinear optical response, especially THG in multilayer photonic microstructures.

Reduced-Dimensionality Al Nanocrystals: Nanowires, Nanobars, and Nanomoustaches.

Aluminum nanocrystals created by catalyst-driven colloidal synthesis support excellent plasmonic properties, due to their high level of elemental purity, monocrystallinity, and controlled size and shape. Reduction in the rate of nanocrystal growth enables the synthesis of highly anisotropic Al nanowires, nanobars, and singly twinned "nanomoustaches". Electron energy loss spectroscopy was used to study the plasmonic properties of these nanocrystals, spanning the broad energy range needed to map their plasmonic modes. The coupling between these nanocrystals and other plasmonic metal nanostructures, specifically Ag nanocubes and Au films of controlled nanoscale thickness, was investigated. Al nanocrystals show excellent long-term stability under atmospheric conditions, providing a practical alternative to coinage metal-based nanowires in assembled nanoscale devices.

Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers.

Narrow gaps between plasmon-supporting materials can confine infrared electromagnetic energy at the nanoscale, thus enabling applications in areas such as optical sensing. However, in nanoparticle dimers, the nature of the transition between touching (zero gap) and nearly nontouching (nonzero gap ≲15 nm) regimes is still a subject of debate. Here, we observe both singular and nonsingular transitions in infrared plasmons confined to dimers of fluorine-doped indium oxide nanocubes when moving from touching to nontouching configurations depending on the dimensionality of the contact region. Through spatially resolved electron energy-loss spectroscopy, we find a continuous spectral evolution of the lowest-order plasmon mode across the transition for finite touching areas, in excellent agreement with the simulations. This behavior challenges the widely accepted idea that a singular transition always emerges in the near-touching regime of plasmonic particle dimers. The apparent contradiction is resolved by theoretically examining different types of gap morphologies, revealing that the presence of a finite touching area renders the transition nonsingular, while one-dimensional and point-like contacts produce a singular behavior in which the lowest-order dipolar mode in the touching configuration, characterized by a net induced charge in each of the particles, becomes unphysical as soon as they are separated. Our results provide valuable insights into the nature of dimer plasmons in highly doped semiconductors.

Multiple hybrid Spp-Tamm modes in Ag grating/DBR microcavity

High confinement of surface plasmon polariton (SPP) have important applications in many aspects. However, access to high-Q resonant modes in metal cavity have many difficulties because of high Ohmic losses, large radiative losses and limited cavity designs. The Tamm mode is another surface plasmonic mode which has a high Q value but poor confinement. Here, we present a grating Tamm structure in which both nonradiative and radiative damping are suppressed, enabling excitation of high-Q and high confinement of hybrid SPP-Tamm mode. Theoretical analysis and simulations show that the proposed structure supports six resonance modes. By manipulating the geometric parameters of the metal grating, the multiple hybrid SPP-Tamm resonances could be well-defined and tuned with wavelength tuning sensitivity up to 1 nm. These results are promising for potential applications such as multiplexing, multi-frequency sensing and imaging.

Quantum size luminescence effect in local field enhancement of Ag NP added Er3+ glass system

This research investigates the effect of silver (Ag) addition on the photoluminescence enhancement properties of 20Li2O–5Bi2O3-(74-x)TeO2–1Er2O3-xAg glass samples, aiming to optimize luminescence for applications in optical amplifiers and laser technologies. The enhancement of luminescence properties for both upconversion 4S3/2 → 4I15/2 green band and 4F9/2 → 4I15/2 red peak luminescence observed are analyzed by quantum yield ξ and relative intensity enhancement (I/Io) parameters. The enhancement on green luminescence properties are observed with high ξgreen and I/Iogreen at x < 1 mol% which can be explained within the cavity quantum electrodynamics (CQED) framework due to very low dipolar localized surface plasmon mode volume Veff (∼10–20 × 10−22 m3) allowing stronger quantum size luminescence effect. Meanwhile, the quenching of ξgreen (0.61) at x≥ 1 mol% may indicate the weaker quantum size luminescence effect. An anomalous enhancement on red luminescence properties with high ξred (2.07) and I/Iored (1.79) at x = 1.25 mol% cannot be explained by CQED framework but from phonon-assisted energy transfer (PET) mechanism occurred due to structural changes. The near-field and far-field parameters of AgNP are simulated. Results demonstrated stronger near field electric field distribution enhancement at x = 0.5 mol% and x = 0.75 mol%. This suggests that an optimal AgNP radius exists for promoting the quantum size luminescence effect in our glass samples, falling within the range of 50.00–60.00 nm. Further supporting this conclusion, theoretical calculations using light intensity scattering efficiency (Qsca) revealed the highest values between 50.00 and 70.00 nm of AgNP radius in our glass samples.

Non-thermal emission in gap-mode plasmon photoluminescence

Photoluminescence from spatially inhomogeneous plasmonic nanostructures exhibits fascinating wavelength-dependent nonlinear behaviors due to the intraband recombination of hot electrons excited into the conduction band of the metal. The properties of the excited carrier distribution and the role of localized plasmonic modes are subjects of debate. In this work, we use plasmonic gap-mode resonators with precise nanometer-scale confinement to show that the nonlinear photoluminescence behavior can become dominated by non-thermal contributions produced by the excited carrier population that strongly deviates from the Fermi-Dirac distribution due to the confinement-induced large-momentum free carrier absorption beyond the dipole approximation. These findings open new pathways for controllable light conversion using nonequilibrium electron states at the nanoscale.

Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities.

Optical nanocavities have revolutionized the manipulation of radiative properties of molecular and semiconductor emitters. Here, we investigate the amplified photoluminescence arising from exciting a dark transition of β-carotene molecules embedded within plasmonic nanocavities. Integrating a molecular monolayer into nanoparticle-on-mirror nanostructures unveils enhancements surpassing 4 orders of magnitude in the initially light-forbidden excitation. Such pronounced enhancements transcend conventional dipolar mechanisms, underscoring the presence of alternative enhancement pathways. Notably, Fourier-plane scattering spectroscopy shows that the photoluminescence excitation resonance aligns with a higher-order plasmonic cavity mode, which supports strong field gradients. Combining quantum chemistry calculations with electromagnetic simulations reveals an important interplay between the Franck-Condon quadrupole and Herzberg-Teller dipole contributions in governing the absorption characteristics of this dark transition. In contrast to free space, the quadrupole moment plays a significant role in photoluminescence enhancement within nanoparticle-on-mirror cavities. These findings provide an approach to access optically inactive transitions, promising advancements in spectroscopy and sensing applications.

TiO2 multi-leg nanotubes for surface-enhanced Raman scattering

Abstract In the recent past, significant research efforts have been put forth to fabricate cost-effective substrates for surface-enhanced Raman scattering (SERS) applications. Here we propose semiconducting TiO2 multi-leg nanotubes and Au nanoparticle-coated TiO2 multi-leg nanotubes (TiO2 MLNTs and Au/TiO2 MLNTs) as SERS substrates. The unique multi-leg architecture of TiO2 nanotubes demonstrated enhanced light-harvesting properties facilitated by an induced photonic absorption edge. Remarkable high SERS sensitivity is observed towards the detection of Methylene blue (MB), up to nM concentration (E.F. ∼104) using TiO2 MLNTs. The same is attributed to the resonantly matched photonic absorption edge of TiO2 MLNTs with the wavelength of incident laser probe light. On the other hand, the Au nanoparticle coating further leveraged the light absorption ability of TiO2 MLNTs with the aid of localized surface plasmon resonance mode. As such, Au/TiO2 MLNTs showed excellent enhancement in SERS sensitivity (E.F. ∼105 , for nM of MB) facilitated by the synergy between the plasmonic modes of Au and the photonic absorption mode of TiO2 MLNTs. UV-Vis diffuse reflectance and Raman spectroscopy measurements are highlighted to elucidate the light absorption and SERS sensitivity of the TiO2 and Au/TiO2 MLNTs.

Возбуждение терагерцовых плазмонных мод в графеновом квадратном микрорезонаторе

We consider a graphene square located in the interface between two half-spaces with different dielectric constants. Using a self-consistent electromagnetic approach based on the integral equation method, we solve the problem of a linearly polarized terahertz electromagnetic wave scattering by the graphene square. The spectra of extinction, absorption, and scattering cross sections reveal the various plasmon modes excitation in graphene square. The properties of the plasmon modes different types are discussed.

High sensitivity terahertz biomedical sensing with graphene metamaterial

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation.

Visible Light-Illuminated Gold Nanohole Arrays With Tunable On-Chip Plasmonic Sensing Properties

Since the discovery of the extraordinary optical transmission phenomenon, nanohole arrays have attracted much attention and been widely applied in sensing. However, their typical fabrication process, utilizing photolithographic top-down manufacturing technologies, has intrinsic drawbacks including the high costs, time consumption, small footprint, and low throughput. This study presented a low-cost, high-throughput, and scalable method for fabricating centimeter-scale (1×2 cm2) nanohole arrays using the improved nanosphere lithography. The large-scale close-packed polystyrene monolayers obtained by the hemispherical-depression-assisted self-assembly method were employed as colloidal masks for the nanosphere lithography, and the nanohole diameter was tuned from 233 nm to 346 nm with a fixed period of 420 nm via plasma etching. The optical properties and sensing performance of the nanohole arrays were investigated, and two transmission dips were observed due to the resonant coupling of plasmonic modes. Both dips were found to be sensitive to the surrounding environment, and the maximum bulk refractive index sensitivity was up to 162.1 nm/RIU with a 233 nm hole diameter. This study offered a promising approach for fabricating large-scale highly ordered nanohole arrays with various periods and nanohole diameters that could be used for the development of low-cost and high-throughput on-chip plasmonic sensors.