Plasmon Excitation Research Articles

Reversibly Alterable Hot-Electron Photodetection Without Altering Working Wavelengths Through Phase-Change Material Sb2S3

Generally, the responsivities of hot-electron photodetectors (HE PDs) are mainly dependent on the device working wavelengths. Therefore, a common approach to altering device responsivities is to change the working wavelengths. Another strategy for manipulating electrical performances of HE PDs is to harness electric bias that can be used to regulate hot-electron harvesting at specified working wavelengths. However, the reliance on bias hampers the flexibility in device operations. In this study, we propose a purely planar design of HE PDs that contains the phase-change material Sb2S3, realizing reversibly alterable hot-electron photodetection without altering the working wavelengths. Optical simulations show that the designed device exhibits strong absorptance (>0.95) at the identical resonance wavelengths due to the excitations of Tamm plasmons (TPs), regardless of Sb2S3 phases. Detailed electrical calculations demonstrate that, by inducing Sb2S3 transitions between crystalline and amorphous phases back and forth, the device responsivities at TP wavelengths can be reversibly altered between 59.9 nA/mW to 128.7 nA/mW. Moreover, when device structural parameters are variable and biases are involved, the reversibly alterable hot-electron photodetection at specified TP wavelengths is maintained.

Transverse-Electric Surface Plasmon in Graphene Under Uniform Strain

Abstract Compared to surface plasmon polariton in metals, graphene can support transverse electric (TE) surface modes when the imaginary part of its conductivity is negative. TE graphene plasmons are generally weakly confined in direction perpendicular to the graphene plane, and they cannot be resonantly excited by an external incident wave because their dispersion curve spectrally lies just below the light line. In this work, we investigate theoretically the light reflectance of a graphene layer under uniform strain on the top of a one-dimensional photonic crystal consisting of high and low-index dielectric materials and a material film layer on the graphene sheet. The strain not only changes the electronic band structure but also can be employed to influence the electronic collective excitations and thus the optical reflectance of graphene monolayers. We demonstrate that TE plasmon excitation is based on the observation of a pronounced sharp minimum in the reflection coefficient of the suggested photonic structure upon the incident angle, the wavelength, and refractive index. Therefore, the graphene under uniform strain on the photonic structure is found to be promising in the fabrication of optical sensors devices with TE plasmons.

Slotted gap-surface plasmon resonator as an efficient platform for sensing.

Film-coupled plasmonic resonators offer efficient platforms for light enhancement due to the excitation of gap surface plasmons (GSPs) at metal-insulator-metal interfaces, where electromagnetic energy is stored within the spacer. In applications like biosensing and spontaneous emission control, spatial overlap between the target molecule and plasmonic hotspots is essential. Here, we propose utilizing the controllable, efficient light enhancement capabilities of a specifically designed GSP disk resonator for biosensing and spontaneous emission enhancement. To create an external plasmonic hotspot and make the strong field stored in the spacer accessible to nearby molecules, we introduce a nanoslot in the top metallic disk with its long axis oriented perpendicular to the incident field polarization. This orientation ensures significant electric field enhancement due to boundary conditions, while the resonant modes of the GSP and nanoslot are further tailored to optimize the field distribution. Finite element method-based simulations reveal the simultaneous excitation of electric-dipole modes due to the nanoslot alongside GSP modes, resulting in a more than two-order magnitude increase in total electromagnetic energy. Additionally, varying the slot length allows precise control over resonances, revealing different modes of the system. The external hotspot in the nanoslot ensures direct interaction with nearby molecules, enhancing the radiative decay rate by nearly three orders of magnitude. The suggested configuration of a plasmonic disk combined with a rectangular nanoslot extends the degree of freedom for designing external electromagnetic hot spots.

Direct and Indirect Interfacial Electron Transfer at a Plasmonic p-Cu7S4/CdS Heterojunction.

Plasmonic semiconductors exhibit significant potential for harvesting near-IR solar energy, although their mechanisms of plasmon-induced hot electron transfer (HET) are poorly understood. We report a transient absorption study of plasmon-induced HET in p-Cu7S4/CdS type II heterojunctions. Near-IR excitation of the p-Cu7S4 plasmon band at ∼1400 nm leads to ultrafast HET into the CdS conduction band with a time constant of <150 fs and a quantum efficiency of ∼0.054%. The injected hot electrons remain in CdS with an amplitude-weighted average lifetime of 1.9 ± 0.5 ns, significantly longer than that in Au/CdS heterostructures, suggesting that plasmonic semiconductors can slow down charge recombination due to the presence of a bandgap. The excited near-IR plasmon does not decay by coupling to the interfacial charge transfer transition, likely due to its energy mismatch. This study provides a detailed mechanistic understanding and possible directions for improving plasmonic HET in plasmonic semiconductor heterojunctions.

Effect of the incorporation of gold nanoparticles on the current‒voltage characteristics of Ag/Si diodes in the dark and under light

The use of nanoparticles in electronic devices has become common practice for controlling and improving device performance. In this research, gold nanoparticles (AuNPs) were introduced into silver‒silicon (Ag/Si) Schottky diodes to investigate their impact on the current voltage (I‒V) behavior in both the dark and light surroundings. Spin coating was used to deposit a film of AuNPs on the Si substrate, followed by thermal evaporation of Ag metal to create the concluding Ag/AuNP/Si diode. Study of the I‒V curves revealed a reduction in the barrier height after the addition of AuNPs in both the dark and illuminated settings. Furthermore, the presence of AuNPs led to a decrease in both the series (Rs) and shunt (Rsh) resistances when the diode was exposed to light, indicating an improvement in the photosensitivity of the Ag/Si structures. This enhancement was attributed to the ability of AuNPs to increase light absorption through local interface plasmon excitations, resulting from the shared oscillation of free electrons on metallic surfaces

In Ukrainian

Due to its excellent electrical, mechanical, thermal and optical properties, graphene has attracted much interest since it was discovered in 2004. Its two-dimensional nature and other remarkable properties meet the needs of surface plasmons and have greatly enriched the field of plasmonics. The paper will review recent advances and applications of graphene in plasmonic, including theoretical mechanisms, experimental observations, and meaningful applications. Due to its flexibility and good tunability, graphene can be a promising plasmonic material as an alternative to noble metals. Optical conversion, plasmonic metamaterials, light harvesting, etc. have already been realized in graphene-based devices, which are useful for applications in electronics, optics, energy storage, THz technology, etc. In addition, the excellent biocompatibility of graphene makes it a very good candidate for applications in biotechnology and medical science. Surface plasmons in graphene offer a compelling route to many useful photonic technologies. As a plasmonic material, graphene offers several intriguing properties, such as excellent electro-optic tunability, crystal stability, large optical nonlinearity, and extremely high electromagnetic field concentration. Thus, recent demonstrations of surface plasmon excitation in graphene using near-infrared light scattering] have attracted great interest. Here we present an all-optical plasmonic coupling scheme that takes advantage of the intrinsic nonlinear optical response of graphene. To generate plasmons, pulses of visible light in a free in-plane graphene sheet are used using difference frequency mixing of the waves to match both the wave vector and the energy of the surface wave. By carefully controlling the phase with matching conditions, we show that it is possible to excite surface plasmons with a defined wave vector and direction in a wide frequency range with high photon efficiency. Prospects for the practical use of graphene in plasmonics are discussed.

Analysis of plasmon modes in Bi2Se3/graphene heterostructures via electron energy loss spectroscopy

Topological Insulators (TIs) are promising platforms for Quantum Technology due to their topologically protected surface states (TSS). Plasmonic excitations in TIs are especially interesting both as a method of characterisation for TI heterostructures, and as potential routes to couple optical and spin signals in low-loss devices. Since the electrical properties of the TI surface are critical, tuning TI surfaces is a vital step in developing TI structures that can be applied in real world plasmonic devices. Here, we present a study of Bi2Se3/graphene heterostructures, prepared using a low-cost transfer method that reliably produces mono-layer graphene coatings on Bi2Se3 flakes. Using both Raman spectroscopy and electron energy loss spectroscopy (EELS), we show that the graphene layer redshifts the energy of the plasmon mode in Bi2Se3, creating a distinct surface plasmon that differs significantly from the behaviour of a TI-trivial insulator boundary. We demonstrate that this is likely due to band-bending and electron transfer between the TI surface and the graphene layer. Based on these results, we outline how graphene overlayers can be used to create tuneable, stable plasmonic materials based on topological insulators.

Collective excitations and low-energy ionization signatures of relativistic particles in silicon detectors

Solid-state detectors with a low energy threshold have several applications, including searches of non-relativistic halo dark-matter particles with sub-GeV masses. When searching for relativistic, beyond-the-Standard-Model particles with enhanced cross sections for small energy transfers, a small detector with a low energy threshold may have better sensitivity than a larger detector with a higher energy threshold. In this paper, we calculate the low-energy ionization spectrum from high-velocity particles scattering in a dielectric material. We consider the full material response including the excitation of bulk plasmons. We generalize the energy-loss function to relativistic kinematics, and benchmark existing tools used for halo dark-matter scattering against electron energy-loss spectroscopy data. Compared to calculations commonly used in the literature, such as the Photo-Absorption-Ionization model or the free-electron model, including collective effects shifts the recoil ionization spectrum towards higher energies, typically peaking around 4–6 electron-hole pairs. We apply our results to the three benchmark examples: millicharged particles produced in a beam, neutrinos with a magnetic dipole moment produced in a reactor, and upscattered dark-matter particles. Our results show that the proper inclusion of collective effects typically enhances a detector’s sensitivity to these particles, since detector backgrounds, such as dark counts, peak at lower energies.

Time-dependent quantum/continuum modeling of plasmon-enhanced electronic circular dichroism.

In this work, we present a multiscale real-time approach to study the plasmonic effects of a metal nanoparticle (NP) on the electronic circular-dichroism (ECD) spectrum of a chiral molecule interacting with it. The method is based on the time-evolution of the molecule's time-dependent wavefunction, expanded in the eigenstates of a perturbed Hamiltonian. A quantum description of the molecular system is coupled to a classical representation of the NP via a continuum model. The method is applied to methyloxirane and peridinin at various distances (1, 3, and 5nm) with respect to a gold NP surface. While no remarkable effect is observed for methyloxirane at any studied distance, an enhancement appears when the peridinin lies at 1nm and the pulse is linearly polarized perpendicularly to the molecular axis, with the ECD signal centered at 4.1eV increased by a factor of around 20. These results are rationalized looking at the gap between the plasmonic peak of the NP at around 2.5eV and the molecular excitations: the smaller the gap between molecular and plasmonic excitations, the larger the plasmonic enhancement of the ECD signal. Moreover, ECD peaks are selectively enhanced due to the favorable coupling between the pulse polarization and the combined effect of electric and magnetic dipole moments. This approach allows one to go through the electronic structure and dynamics of chiral molecules for obtaining a realistic description of plasmon-mediated ECD spectra, e.g., paving the way to applications to molecules of biological relevance interacting with nanostructures of experimental interest.

Two-dimensional optical binding based on graphene surface plasmon excitation.

In this work, we present a detailed analysis of the optical binding force resulting from the near field scattering between dielectric nanoparticles and a graphene substrate. We pay special attention to the two-dimensional array formation for which the stability is raised owing to the multiple surface plasmon (SP) scattering. Because of the small values of the SP wavelength on graphene in comparison with those of the photon, the size of these particle arrangements falls below the diffraction limit. By using a rigorous formalism based on the electromagnetic Green theory, we calculate the binding energy potential whose depth quantifies the stability of such configurations. We find multiple configurations for a given number of nanoparticles, from which only a subset of them remains stable under thermal fluctuations. Our contribution can be valuable for understanding the formation of new optically reconfigurable polaritonic materials.

Tuning of the Cut-Off Frequency in Effective Medium Approximation and Long Range Surface Plasmon Excitation in Graphene and Black Phosphorus

Tuning of the Cut-Off Frequency in Effective Medium Approximation and Long Range Surface Plasmon Excitation in Graphene and Black Phosphorus

Plasmon-Assisted Electrochemical CO2 Reduction on Copper Nanocubes

Optical excitation of plasmons on metal nanoparticles presents a promising opportunity to enable electrochemical CO2 conversion into hydrocarbons at low overpotentials on stable electrodes. Electrochemical CO2 conversion typically requires high overpotentials, resulting in low energy efficiency and catalyst reshaping. By using light to overcome reaction barriers, plasmon excitation has been shown to lower the required overpotential for electrochemical reactions. Plasmon excitation also facilitates C-C coupling on gold and silver, metals that usually produce no multicarbon products when used as electrocatalysts. While previous studies indicate clear potential that plasmon-based photo-electrocatalysts could enable new electrochemical pathways for CO2 reduction at low overpotentials and stable catalysts, yields and catalytic turnover remained low. To develop industrially viable photo-electrochemical CO2 reduction catalysts, the effect of plasmon excitation at high current density and high mass flux needs to be understood and optimized.Here we investigate the photo-electrochemical CO2 reduction on gold and copper nanoparticles on a gas diffusion electrode. We choose copper nanoparticles because they are the most promising electrocatalysts for C-C coupling and exhibit a strong plasmon resonance around 600 nm, yet have not been explored for plasmonic CO2 reduction. Gold nanoparticles are chosen due to their exceptional stability and because gold has been shown to perform C-C coupling under plasmon excitation, which does not happen electrochemically. Using a gas diffusion electrode allows efficient gas phase mass transport to and from the electrode. We synthesize 50 nm (100) faceted copper nanocubes using copper bromide and oleylamine, and gold nanorods using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). In both cases, the nanoparticles have a plasmon resonance of 600 nm. We spray coat these nanoparticles onto a carbon paper gas diffusion membrane and incorporate them into our home-built gas reactor. First, we use real-time GC to investigate electrocatalysis in the dark and find high selectivity for CO on gold and high selectivity for ethylene and CH4 on copper. Then, we vary the optical illumination wavelength from 480 nm to 700 nm and the applied potential, investigating how these affect Faradaic efficiencies for the production of ethylene, CO, H2, and CH4 as well as photothermal contributions using an infrared camera. Finally, we use in-situ liquid cell transmission electron microscopy (TEM) to monitor nanoparticle stability and correlate morphological evolution with CO2 reaction rate. Our work provides the fundamental understanding necessary for building photo-electrocatalysts that can convert CO2 into value-added products under industrially relevant conditions.Figure caption: Schematic of our photo-electrochemical plasmonic gas diffusion electrode converting CO2 into hydrocarbons. Electrochemical CO2 conversion on gold typically leads to CO, under plasmon excitation gold can form hydrocarbons. Figure 1

Electron confinement-induced plasmonic breakdown in metals.

Plasmon resonance represents the collective oscillation of free electron gas density and enables enhanced light-matter interactions in nanoscale dimensions. Traditionally, the classical Drude model describes plasmonic excitation, wherein plasma frequency exhibits no spatial dispersion. Here, we show conclusive experimental evidence of the breakdown of plasmon resonance and a consequent metal-insulator transition in an ultrathin refractory plasmonic material, hafnium nitride (HfN). Epitaxial HfN thick films exhibit a low-loss and high-quality Drude-like plasmon resonance in the visible spectral range. However, as the film thickness is reduced to nanoscale dimensions, Coulomb interaction among electrons increases because of electron confinement, leading to the spatial dispersion of plasma frequency. With a further decrease in thickness, electrons lose their ability to shield the incident electric field, turning the medium into a dielectric. The observed metal-insulator transition might carry some signatures of Wigner crystallization and indicates that such transdimensional, between 2D and 3D, films can serve as a promising playground to study strongly correlated electron systems.

Observation of orbital angular momentum from an ultrathin topological insulator metasurface.

Orbital angular momentum (OAM) existing in the vortex light beam with isolated singularities and spiral phase distribution presents significant applications in optical communications and light-field manipulation. The generation of OAM based on plasmonic metasurfaces is generally limited by the large optical loss and weak tunability of metal materials. Three-dimensional (3D) topological insulators (TIs) with insulating bulk states and topologically protected surface states allow the excitation of surface plasmons with low loss in the high-frequency region. Herein, we designed and fabricated an ultrathin Sb2Te3 TI plasmonic metasurface using the magnetron sputtering deposition and focused ion beam lithography. The results show that the 18 nm thick TI metasurface can efficiently generate surface plasmon resonances (SPRs) in the visible spectrum, which can effectively modulate the spatial phase of incident light for the generation of OAM. We find that the OAM conversion efficiency of the TI-based metasurface is remarkable compared with that of the gold-based metasurface. The experimental results obtained by a self-built OAM testing system demonstrate that the ultrathin TI metasurface can generate a distinct vortex beam with a first-order topological charge. This work will provide a new approach for generating OAM in ultrathin structures and exploring the applications of TIs in light-field manipulation.

Near-Infrared Light-Driven CO2 Reduction on Cup-Stacked Carbon Nanotubes.

Carbon nanotubes feature one-dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup-stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge-plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near-infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo-induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs-C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near-unity NIR light-driven CO2 reduction to CO with a rate of 1.35 μmol g-1 h-1. This work sheds light on the exploitation of metal-free carbon-based plasmonic nanostructures for photocatalytic applications.

Near‐Infrared Light‐Driven CO2 Reduction on Cup‐Stacked Carbon Nanotubes

AbstractCarbon nanotubes feature one‐dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup‐stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge‐plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near‐infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo‐induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs‐C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near‐unity NIR light‐driven CO2 reduction to CO with a rate of 1.35 μmol g−1 h−1. This work sheds light on the exploitation of metal‐free carbon‐based plasmonic nanostructures for photocatalytic applications.

Dynamically Switchable Mid-Infrared Perfect Absorber Based on Grating-Assisted Graphene Plasmon Excitation

Dynamically Switchable Mid-Infrared Perfect Absorber Based on Grating-Assisted Graphene Plasmon Excitation

Hot carrier transfer from plasmon decay in Ag20at H-Si(111) surface: real-time TDDFT simulation in Wannier gauge.

Plasmon decay is believed to play an essential role in inducing hot carrier transfer at the interfaces between plasmonic nanoparticles and semiconductor surfaces. In this work, we employ real-time time-dependent density functional theory (RT-TDDFT) simulation in the Wannier gauge to gain quantum-mechanical insights into the nonlinear dynamics of the plasmon decay in the Ag20nanoparticle at a semiconductor surface. The first-principles simulations show that the plasmon decay is more than two times faster when the Ag20nanoparticle is adsorbed on a hydrogen-terminated Si(111) surface, taking place within 100 femtoseconds of the plasmon excitation. Hot carrier transfer across the interface is observed as the plasmon decay takes place, and nearly 30% of holes are generated deep in the valence band of the semiconductor surface. The use of Wannier gauge in RT-TDDFT simulation is particularly convenient for gaining quantum-mechanical insights into non-equilibrium electron dynamics in complex heterogeneous systems.

Plasmon enhanced oxygen evolution reaction on Au decorated Ni(OH)2 nanostructures: The role of alkaline cations solvation

Water electrolysis is widely acknowledged to be kinetically hindered by the oxygen evolution reaction (OER), which involves multiple electron transfer steps. A critical yet often overlooked factor ruling the OER rate is the intricate interplay between alkaline cations of electrolyte, reaction intermediates, and the electrocatalyst surface. This study investigates the impact of plasmonic excitation on the OER using hexagonal hybrid Ni(OH)2-Au nanoplates in different alkaline electrolytes. While CsOH electrolyte exhibited the highest overall OER electroactivity, LiOH presented a remarkable enhancement under plasmonic excitation. Furthermore, FTIR-RAS and calculations analysis revealed that plasmonic relaxation mechanisms accelerate the Li+ environment modification process, mimicking the electrostatic configuration of Cs+, which facilitates a stabilized interaction between the active site (Ni3+) and OER intermediates, promoting rapid O−O bond formation. Our findings suggest that manipulating the cation’s vicinity through plasmonic heating is a promising strategy for enhancing OER efficiency.

Time-resolved photoelectron spectroscopy at surfaces

Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the issue on “The first thirty years” of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron–magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. Following the introduction, the interested reader will find a list of contributions and a brief summary in Section 1.3.