Plasmonic Field Research Articles

Plasmon localization and field enhancement in flexible metasurfaces

Plasmon localization and field enhancement in flexible metasurfaces

Quantum Wire Coupled to Light

Experimental advances in cavity QED are raising the prospect of using light to probe quantum materials beyond the linear response regime. The capability to access quantum coherent phenomena would significantly advance the field. However, theoretical work on many-body systems coupled to light in the quantum coherent regime has been select. Here, we investigate the radiative properties of a finite-sized quantum wire in a microwave cavity. Examples of quantum wires include single-walled carbon nanotubes, a key experimental system in the field of nano-optics and plasmonics. We find that, for a variety of excited states, the repeated emission of photons results in the generation of many-body quantum entanglement. This leads to an increase in the rate at which subsequent photons are emitted, an example of Dicke superradiance. On the other hand, Pauli blocking tends to reduce this effect. Bosonization, the description of the excitations of a one-dimensional electron system as a gas of bosons, is found to be a powerful theoretical tool in this context. Its application means that many of our results generalize to wires with strong electron-electron interactions. The quantum wire thus represents a new platform to realize Dicke-model physics that does not rely on the various fine tunings necessary in traditional realizations involving many spatially isolated emitters. More broadly, this work demonstrates how quantum entanglement can be generated and measured in a many-body system. Published by the American Physical Society 2024

Ultrasensitive NIR-II Surface-Enhanced Resonance Raman Scattering Nanoprobes with Nonlinear Photothermal Effect for Optimized Phototheranostics.

Surface-enhanced resonance Raman scattering (SERRS) in the second near-infrared (NIR-II) window has great potential for improved phototheranostics, but lacks nonfluorescent, resonant and high-affinity Raman dyes. Herein, it is designed and synthesize a multi-sulfur Raman reporter, NF1064, whose maximum absorption of 1064nm rigidly resonates with NIR-II excitation laser while possessing absolutely nonfluorescent backgrounds. Ultrafast spectroscopy suggests that the fluorescence quenching mechanism of NF1064 originates from twisted intramolecular charge transfer (TICT) in the excited state. Gold nanorods (AuNRs) decorated with such nonfluorescent NF1064 (AuNR@NF1064) show remarkable SERRS performances, including zero-fluorescence background, femtomolar-level sensitivity as well as superb photostability without fluorescence photobleaching. More importantly, AuNR@NF1064 exhibits a nonlinear photothermal effect upon plasmonic fields of AuNRs by amplifying the non-radiative decay of nonfluorescent NF1064, thus achieving a high photothermal conversion of 68.5% in NIR-II window with potential for further augmentation. With remarkable SERRS and photothermal properties, the NIR-II nanoprobes allow for high-precision intraoperative guided tumor resection within 8min, and high-efficient hyperthermia combating of drug-resistant bacterial infection within living mouse body. This work not only unlocks the potential of nonfluorescent resonant dyes for NIR-II Raman imaging, but also opens up a new method for boosting photothermal conversion efficiency of nanomaterials.

Magnetic field sensor based on multiple Fano resonance in metal-insulator-metal waveguide coupled with cross-shape resonator

Abstract In this paper, an innovative plasmonic magnetic field sensor (MFS) that exploits a Metal-insulator-Metal (MIM) waveguide configuration with an asymmetric cross-shaped resonator is constructed. The design enables the observation of double Fano resonance in the transmission spectrum, resulting from the coupling between continuous spectrum and discrete spectrum. By incorporating magnetic fluid in the asymmetric cross-shape resonator, the Fano line shapes can be tuned by the external magnetic field. Finite-Difference Time-Domain (FDTD) is used to simulate the spectra response to external magnetic fields and geometrical sizes. The proposed structure exhibits remarkable magnetic field sensitivity of 12.1 p m / O e within the detection range of 15 O e to 199 O e . The resolution of MIM magnetic field sensors can reach 0.0826 O e . The optimal figure of merit (FOM) and maximum Q factor of the Fano dip are about 6.9 × 10 − 4 / O e and 66.74, respectively. Meanwhile, The proposed Fano resonance MFS has some advantages, including compact size, high sensitivity, and cost-effectiveness. A strong linear relationship between the magnetic field and resonance wavelength indicates that the proposed structure can find potential applications in diverse fields such as magnetic field sensors, aircraft navigation, and even nuclear energy generation.

Direct Three-Dimensional Observation of the Plasmonic Near-Fields of a Nanoparticle with Circular Dichroism.

Characterizing the spatial distribution of the electromagnetic fields of a plasmonic nanoparticle is crucial for exploiting its strong light-matter interaction for optoelectronic and catalytic applications. However, observing the near-fields in three dimensions with a high spatial resolution is still challenging. To realize efficient three-dimensional (3D) nanoscale mapping of the plasmonic fields of nanoparticles with complex shapes, this work established autoencoder-embedded electron energy loss spectroscopy (EELS) tomography. A 432-symmetric chiral gold nanoparticle, a nanoparticle with a high optical dissymmetry factor, was analyzed to relate its geometrical features to its exotic optical properties. Our deep-learning-based feature extraction method discriminated plasmons with different energies in the EEL spectra of the nanoparticle in which signals from multiple plasmons were intermixed; this component was key for acceptable 3D visualization of each plasmonic field separately using EELS tomography. With this methodology, the electric field of the plasmon that induces far-field circular dichroism was observed in 3D. The field linked to this chiroptical property was strong along the swirling edges of the particle, as predicted by a numerical calculation. This study provides insight into the correlation between structural and optical chiralities through direct 3D observation of the plasmonic fields. Furthermore, the strategy of implementing an autoencoder for EELS tomography can be generalized to achieve competent 3D analysis of other features, including the optical properties of the dielectrics and chemical states.

Time-resolved photoelectron spectroscopy at surfaces

Light is still an important 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 last 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.

Investigation of long-range plasmonic field effects in Au@SiO2 nanostructures using p-nitrophenol reduction

As an emerging field, plasmonic photocatalysis has gradually garnered attention from researchers. It is generally accepted that localized surface plasmonic resonance (LSPR) in noble metallic nanostructures enhances photocatalysis through the induction of a long-range plasmonic field. Furthermore, nanostructures in close proximity can enhance the electromagnetic (EM) field via plasmonic coupling, which also contributes to improved photocatalytic activity. In this study, core-shell nanostructures composed of Au nanorods (AuNRs) with pinhole-free SiO2 shells (pf-AuNRs@SiO2) are synthesized as non-contact, long-range photocatalysts. By tuning the aspect ratios (ARs) of the AuNRs to match the LSPR peaks with the wavelength of the irradiation light source, an optimized photocatalytic effect was achieved in the reduction of p-nitrophenol (p-NP). Additionally, by comparing shell thicknesses of 5 nm and 10 nm, it was observed that the 5 nm thickness exhibited superior catalytic efficiency. This enhancement is attributed to the thinner shell allowing reactants to be placed within a stronger plasmonic field.

Phonon-Induced Geometric Chirality.

Chiral properties have seen increasing use in recent years, leading to the emerging fields of chiral quantum optics, plasmonics, and phononics. While these fields have achieved manipulation of the chirality of light and lattice vibrations, controlling the chirality of materials on demand has yet remained elusive. Here, we demonstrate that linearly polarized phonons can be used to induce geometric chirality in achiral crystals when excited with an ultrashort laser pulse. We show that nonlinear phonon coupling quasistatically displaces the crystal structure along phonon modes that reduce the symmetry of the lattice to that of a chiral point group corresponding to a chiral crystal. By reorienting the polarization of the laser pulse, the two enantiomers can be induced selectively. Therefore, geometric chiral phonons enable the light-induced creation of chiral crystal structures and therefore the engineering of chiral electronic states and optical properties.

Tailoring femtosecond LSP resonance and lifetime in a nanoresonator via phase retardation.

The manipulation of femtosecond plasmon resonance and lifetime in a nanoantenna is crucial for the realization of integrated and miniaturized plasmonic circuits. Here, we have used FDTD simulations to study the plasmonic resonance and lifetime variation of the far-end (PA) and near-end (PB) hotspots of size-gradient nanoresonators. We found that the near-field spectrum of PA is red-shifted compared to PB due to the phase retardation effect. By capturing the ultrafast dynamics of both hotspots, we confirm that these phenomena are governed by the transient evolution of the plasmonic field in the forced oscillation regime. Furthermore, the lifetimes of plasmonic hotspots scale directly with their near-field intensities. Meanwhile, the lifetime τA is always larger than that of τB in the same nanoresonator because of the smaller non-radiative damping of hotspot PA. Our results provide a basis for the miniaturization of plasmonic nanoresonators.

Quantifying the ultimate limit of plasmonic near-field enhancement

Quantitatively probing the ultimate limit of near-field enhancement around plasmonic nanostructures remains elusive, despite more than five decades since the discovery of surface-enhanced Raman scattering. Theoretical calculations have predicted an ultimate near-field enhancement exceeding 1000 using the best plasmonic material silver, but experimental estimations disperse by orders of magnitude. Here, we design a high-quality silver plasmonic nanocavity with atomic precision and precisely quantify the upper limit of near-field enhancement in ~1 nm junctions. A hot-spot averaged Raman enhancement of 4.27 × 1010 is recorded with a small fluctuation, corresponding to an averaged electric field enhancement larger than 1000 times. This result quantitatively delineates the ultimate limit of plasmonic field enhancement around plasmonic nanostructures, establishing a foundation for diverse plasmon-enhanced processes and strong light-matter interactions at the atomic scale.

Abruptly autofocused electron beam generation through electron-plasmon interactions.

Electron Airy beams and electron vortex beams are commonly generated using phase masks that imprint a transverse modulation on the particle wave function. Plasmons sustained by nanostructured conductors facilitate substantial interactions with free electrons, enabling considerable transverse modulation of the electron wave function. Consequently, electron Airy and vortex beams can also be produced through interactions between electrons and structured plasmonic fields. Here we illustrate the generation mechanism of the electron ring Airy vortex beams by allowing electrons to traverse an Airy plasmon field with phase singularities and calculate the excitation intensity probability. Subsequently, we numerically investigate the autofocus behavior of the generated Airy vortex beams. Our findings indicate that electrons in the ℓ = 1 channel exhibit the highest excitation probability and produce optimized autofocused ring-shaped Airy vortex beams in our proposed scheme. Furthermore, the number of exchanging plasmons does not significantly influence the position of the primary Airy ring in the initial plane, yet it markedly affects the focal distance and spot size in the focal planes. Our study supports the utilization of chiral plasmons sustained by externally illuminated thin films as a method for generating autofocused chiral electrons, resulting in remarkably large diffracted beam fractions in attainable conditions.

Extracting Thermodynamic Properties of Carbyne from Tip-Enhanced Raman Scattering Images.

The measurement of thermodynamic properties for nanosystems is essential to comprehend the inherent characteristics of nanomaterials. Traditional spectroscopy measurements, such as Raman or ultraviolet-visible spectroscopies, are limited to offering insights near the Γ point in the Brillouin zone and thus cannot precisely determine the system's thermodynamic properties, for example, heat capacity. Utilizing the intrinsic broad momentum distribution in highly confined plasmonic fields, here we take sp-hybridized carbyne as a proof-of-the-principle example to show that ultrahigh-resolution tip-enhanced Raman scattering (TERS) images have the ability to access all k-points in the phonon Brillouin zone of one-dimensional nanosystems, allowing the comprehensive determination of vibrational features and heat capacity for finite carbon chains. Comparing phonon dispersion spectra and heat capacities under different boundary conditions, i.e., linear carbon chains and cyclic carbon molecules, we find that the heat capacities of linear structures converge more rapidly than the counterparts of cyclic structures to the benchmark of ideal carbyne. We also study the effects of different terminal groups in linear structures as well as the aromaticity in cyclic structures on heat capacity. This study provides a practical method for characterizing the thermodynamic properties of nanosystems, demonstrating the potential applications of TERS imaging in nanomaterial science.

Quantum mechanical effects in plasmonic sub-nanometer cavities formed by a tip and substrate structure.

Enhancing local field intensity through light field compression is one of the core issues in surface plasmon-enhanced spectroscopy. The theoretical framework for the nanostructure composed of a tip and a substrate has predominantly relied on classical electromagnetic models, ignoring the electron tunneling effect. In this paper, we investigate the plasmonic near-field characteristics in the sub-nanometer cavity formed by the tip and the substrate using a quantum-corrected model. Additionally, we analyze the local electric field and Raman enhancement when hexagonal boron nitride (h-BN) monolayer is used as a decoupling layer for the nanocavity. The results indicate that classical electromagnetic theory fails to accurately describe the plasmonic electric field in smaller sub-nanometer gaps. When the gap is reduced to 0.32 nm, the quantum-corrected model shows that the local electric field in the sub-nanometer cavity is significantly reduced due to the tunneling current, aligning more closely with experimental results. Moreover, adding a high-barrier h-BN layer effectively prevents the occurrence of tunneling current, allowing for a strong local electric field even when the gap is less than 0.32 nm. The calculated maximum Raman enhancement reaches up to 15 orders of magnitude. Our research results provide a deep understanding of quantum mechanical effects in tip-enhanced spectroscopy systems, enabling the potential applications based on quantum plasmons in nanocavity.

Laser Construction TiOx-Ag heterostructure interface for high sensitive coupling resonance enhanced fluorescence in bacterial nucleic acid amplification sensor chip

Coupled resonance at the noble metals and metal oxides heterogeneous interface can enhance the fluorescence of molecules. This fascinating direction provides high value for the preparation and engineering design of fluorescence sensing platforms in nucleic acid sensing, pathogen detection and biological imaging. Here, an integrated nucleic acid amplification sensor chip consists of an array of silver nanoparticles/oxygen vacancy titanium oxide (TiOx-Ag) heterostructure substrates. The femA gene of Staphylococcus aureus was amplified and detected by enhancement fluorescence on the integrated chip. The TiOx-Ag heterostructure array was prepared by a laser method, the enhanced fluorescence performance was investigated in detail using calcein as a probe. The optical, electronic structure and plasmonic local field analysis indicate the strong coupling between the fluorescence molecule and the TiOx-Ag crystal plane, which inspired the excellent fluorescence signal of calcein, with an enhancement factor of up to 5. Staphylococcus aureus was detected by the above chip with a detection limit as low as 157 cfu/mL. These methods were applied to the detection of spiked serum samples with good detected performances, providing a powerful tool for rapid and reliable diagnosis of pathogens in medical institutions.

Unveiling Spatial and Temporal Dynamics of Plasmon-Enhanced Localized Fields in Metallic Nanoframes through Ultrafast Electron Microscopy.

Plasmonic nanomaterials, particularly noble metal nanoframes (NFs), are important for applications such as catalysis, biosensing, and energy harvesting due to their ability to enhance localized electric fields and atomic efficiency via localized surface plasmon resonance (LSPR). Yet the fundamental structure-function relationships and plasmonic dynamics of the NFS are difficult to study experimentally and thus far rely predominately on computational methodologies, limiting their utilization. This study leverages the capabilities of ultrafast electron microscopy (UEM), specifically photon-induced near-field electron microscopy (PINEM), to probe the light-matter interactions within plasmonic NF structures. The effects of shape, size, and plasmonic coupling of Pt@Au core-shell NFs on spatial and temporal characteristics of plasmon-enhanced localized electric fields are explored. Importantly, time-resolved PINEM analysis reveals that the plasmonic fields around hexagonal NF prisms exhibit a spatially dependent excitation and decay rate, indicating a nuanced interplay between the spatial geometry of the NF and the temporal evolution of the localized electric field. These results and observations uncover nanophotonic energy transfer dynamics in NFs and highlight their potential for applications in biosensing and photocatalysis.

Optical force and torque in near-field excitation of C3H6: A first-principles study using RT-TDDFT.

Optical trapping is an effective tool for manipulating micrometer-sized particles, although its application to nanometer-sized particles remains difficult. The field of optical trapping has advanced significantly, incorporating more advanced techniques such as plasmonic structures. However, single-molecule trapping remains a challenge. To achieve a deeper understanding of optical forces acting on molecular systems, a first-principles approach to analyze the optical force on molecules interacting with a plasmonic field is crucial. In our study, the optical force and torque induced by the near-field excitation of C3H6 were investigated using real-time time-dependent density functional theory calculations on real-space grids. The near field from the scanning tunneling probe was adopted as the excitation source for the molecule. The optical force was calculated using the polarization charges induced in the molecule based on Lorentz force. While the optical force and torque calculated as functions of the light energy were in moderate agreement with the oscillator strengths obtained from the far-field excitation of C3H6, a closer correspondence was achieved with the power spectrum of the induced dipole moment using near-field excitation. Time-domain analysis of the optical force suggests that the simultaneous excitation of multiple excited states generally weakens the force because of mismatches between the directions of the induced polarization and the electric field. This study revealed a subtle damping mechanism for the optical force arising from intrinsic electronic states and the influence of beating.

Unveiling the Loss Mode Enabled Tunable Plasmonic Chirality at Flat Metal Surface.

Plasmonic chirality has garnered significant interest in the past decade due to its enhanced chiral light-matter interactions. Current methods for achieving plasmonic chirality often rely on complex nanostructures or metamaterials, which are hampered by intricate fabrication processes. In this work, we present an approach to generate plasmonic chiral structured surface plasmon polariton (s-SPP) fields on a single, flat metal surface, bypassing elaborate fabrication techniques. The plasmonic chiral s-SPP fields are excited by the superposition of multiple differently oriented transverse magnetic polarized plane waves. We demonstrate, both theoretically and experimentally, the flexible tuning of chiral plasmonic patterns by adjusting the symmetry and phase differences of the incident waves. This method provides a facile mean to optically tailor plasmonic chiral properties on a subwavelength scale, offering potential applications in sensing, enantioselective reactions, imaging, and reconfigurable chiral switches.

Single Molecule Thermodynamic Penalties Applied to Enzymes by Whispering Gallery Mode Biosensors.

Optical microcavities, particularly whispering gallery mode (WGM) microcavities enhanced by plasmonic nanorods, are emerging as powerful platforms for single-molecule sensing. However, the impact of optical forces from the plasmonic near field on analyte molecules is inadequately understood. Using a standard optoplasmonic WGM single-molecule sensor to monitor two enzymes, both of which undergo an open-to-closed-to-open conformational transition, the work done on an enzyme by the WGM sensor as atoms of the enzyme movethrough the electric field gradient of the plasmonic hotspot during conformational change has been quantified. As the work done by the sensor on analyte enzymes can be modulated by varying WGM intensity, the WGM microcavity system can be used to apply free energy penalties to regulate enzyme activity at the single-molecule level. The findings advance the understanding of optical forces in WGM single-molecule sensing, potentially leading to the capability to precisely manipulate enzyme activity at the single-molecule level through tailored optical modulation.

Rapid Fingerprinting of Urinary Volatile Metabolites and Point-of-Care Diagnosis of Phenylketonuria on a Patterned Nanorod Sensor Array with Multiplexed Surface-Enhanced Raman Scattering Readouts.

Phenylketonuria (PKU) is one of the most common genetic metabolic diseases, especially among newborns. Traditional clinical examination of newborn blood samples for PKU is invasive, laborious, and limited to hospitals and healthcare facilities. We reported herein a SERS-based sensor array with three thiophenolic nanoreceptors built on a patterned nanorod vertical array for rapid and inexpensive detection of characteristic volatile biomarkers indicative of PKU in the urine and accurate classification of newborn baby patients all performed on a hand-held SERS spectrophotometer. The well-ordered array was generated from the volatility-driven assembly of gold nanorods (AuNRs) into an upright and closely packed hexagonal configuration. The uniformly distributed nanowells between AuNRs offered an intense and aspect-ratio-dependent plasmonic field for the molecular enhancement of SERS outputs. The SERS-based detector was integrated into a test chip for regular monitoring of volatile phenylketone bodies in the spiked solution or patients' urine within 5 min, allowing the quantification of a wide variety of normal or abnormal metabolites at their physiologically relevant concentration range. The detection limits for common biomarkers of PKU, including phenylpyruvic acid, 4-hydroxyphenylacetic acid, and phenylacetic acid, were at a few μM and well below the diagnostic thresholds. Moreover, the volatile headspace mixtures from a given urine sample could be fingerprinted by the sensor array and discriminated using machine-learning algorithms. Ultimately, the discrimination of baby patients among 26 cases of mild and classic PKU phenotypes and 17 cases of healthy volunteers could be realized with an overall accuracy of 97%. This hand-held SERS platform plays a pivotal role in advancing healthcare applications in quick screening of neonatal PKU through a facile urinary vapor test.

Polarized and Evanescent Guided Wave Surface-Enhanced Raman Spectroscopy of Ligand Interactions on a Plasmonic Nanoparticle Optical Chemical Bench.

This study examined applications of polarized evanescent guided wave surface-enhanced Raman spectroscopy to determine the binding and orientation of small molecules and ligand-modified nanoparticles, and the relevance of this technique to lab-on-a-chip, surface plasmon polariton and other types of field enhancement techniques relevant to Raman biosensing. A simplified tutorial on guided-wave Raman spectroscopy is provided that introduces the notion of plasmonic nanoparticle field enhancements to magnify the otherwise weak TE- and TM-polarized evanescent fields for Raman scattering on a simple plasmonic nanoparticle slab waveguide substrate. The waveguide construct is called an optical chemical bench (OCB) to emphasize its adaptability to different kinds of surface chemistries that can be envisaged to prepare optical biosensors. The OCB forms a complete spectroscopy platform when integrated into a custom-built Raman spectrograph. Plasmonic enhancement of the evanescent field is achieved by attaching porous carpets of Au@Ag core shell nanoparticles to the surface of a multi-mode glass waveguide substrate. We calibrated the OCB by establishing the dependence of SER spectra of adsorbed 4-mercaptopyridine and 4-aminobenzoic acid on the TE/TM polarization state of the evanescent field. We contrasted the OCB construct with more elaborate photonic chip devices that also benefit from enhanced evanescent fields, but without the use of plasmonics. We assemble hierarchies of matter to show that the OCB can resolve the binding of Fe2+ ions from water at the nanoscale interface of the OCB by following the changes in the SER spectra of 4MPy as it coordinates the cation. A brief introduction to magnetoplasmonics sets the stage for a study that resolves the 4ABA ligand interface between guest magnetite nanoparticles adsorbed onto host plasmonic Au@Ag nanoparticles bound to the OCB. In some cases, the evanescent wave TM polarization was strongly attenuated, most likely due to damping by inertial charge carriers that favor optical loss for this polarization state in the presence of dense assemblies of plasmonic nanoparticles. The OCB offers an approach that provides vibrational and orientational information for (bio)sensing at interfaces that may supplement the information content of evanescent wave methods that rely on perturbations in the refractive index in the region of the evanescent wave.