Localized Plasmon Modes Research Articles

Plasmonic modulators based on enhanced interaction between graphene and localized transverse-electric plasmonic mode

Active plasmonic modulators with high modulation depth, low energy consumption, ultra-fast speed, and small footprint are of interest and particular significance for nanophotonics and integrated optics. Here by constructing a transverse-electric (TE) plasmonic mode and maximizing the in-plane component localized on the graphene surface, we propose a high-performing plasmonic modulator based on a graphene/split ring-like plasmonic waveguide (SRPW) system with a record high modulation depth (20.46 dB/µm) and suppressed insertion loss (0.248 dB/µm) at telecom wavelength 1310 nm, simultaneously possessing pronounced advantage in broadband ability (800-1650 nm) and superior electrical performance with energy consumption of 0.43 fJ/bit and modulation speed of 200 GHz. This innovative design provides a novel approach and idea for enhancing the interaction between light and matter in the waveguide system and will certainly inspire new schemes for the development of on-chip integrated optoelectronic devices.

Analogical atomic effects of graphene nanostructure induced by nonlinearity.

A scheme to construct analogical atoms using graphene nanostructures via quantum nonlinearity is proposed. Due to the strong field localization capability of graphene plasmons and the significant intrinsic nonlinearity of graphene, a strong nonlinear optical response can be realized even with single-photon excitation. In this process, the quantum vacuum localized plasmonic mode plays a crucial role in achieving the third-order multi-photon nonlinear effect. The eigenfrequency of the nanostructure can shift by an amount tens of times larger than the decay rate, causing the nanostructure cavity to exhibit atomic-like behaviors. Furthermore, multilevel atoms can be constructed through the interaction of composite graphene nanostructures. The parameters of these atoms can be manipulated by adjusting the nanostructure size, Fermi energy, and other parameters. This research holds significant potential for applications in highly integrated, controllable nanophotonics.

Quantum-plasmonic engineering to improve the 1.53 µm radiative emission in Er3+-doped tellurite glasses under controlled temperature

The plasmonic applications at low temperatures of metallic nanoparticles in glasses doped with rare-earth ions are not yet covered in-depth analysis in the literature. This paper insights into the coupling between gold nanoparticles and Er3+ embedded in tellurite glasses via luminescence enhancement of the emission centred at 1.53 µm under controlled temperature. This enhancement is obtained for the sample with 24 h of heat treatment (TErAu24) concerning the Er3+ single doped (TEr) at room temperature under 980 nm excitation. The enhancement remains under cooling conditions, consequence of the strong coupling between the plasmon and the Er3+, attributed to an increment of the localized plasmon mode volume at low temperatures. Further, the band area increment of the TErAu24 sample comparing the spectra at 98 and 348 K is 414 %, whereas for the TEr is about 348 %. These findings provide advanced understanding of the plasmonics on quantum emitters engineering.

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.

Enhanced circular dichroism in twisted double layer nanoparticle arrays with the excitation of hybridized surface lattice resonances

The realization of enhanced circular dichroism using chiral nanostructures is crucial for the development of functional nanophotonic devices. This study proposes to construct chiral structures by adjusting the relative orientation between two vertically shifted rectangular nanodisk arrays. In comparison to localized plasmon modes, the twisted array allows selective excitation of hybridized surface lattice resonances by left- or right-handed circularly polarized incidence, and the strong interactions among individual nanoparticles result in enhanced circular dichroism. The calculation results demonstrate a significant differential transmittance of up to 0.61, which is more than one order of magnitude stronger than that of localized modes. Moreover, the chiral responses can be effectively controlled by manipulating geometry parameters such as inter-layer separation, twisted angle, and lattice spacing, as these factors govern the coupling efficiency with the external field and the radiative damping of the hybridized modes. The proposed twisted array structure offers a versatile platform for manipulating chiral light fields.

Quantifying Efficiency of Remote Excitation for Surface-Enhanced Raman Spectroscopy in Molecular Junctions.

Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of the lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare the SERS intensity and the OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when the photon count rates are comparable. By statistical analysis of 33 devices, the coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow.

Femtosecond Laser-Printed Gold Nanoantennas for Electrically Driven and Bias-Tuned Nanoscale Light Sources Operating in Visible and Infrared Spectral Ranges.

Nanoscale electrically driven light-emitting sources with tunable wavelength represent a milestone for implementation of integrated optoelectronic chips. Plasmonic nanoantennas exhibiting an enhanced local density of optical states (LDOS) and strong Purcell effect hold promise for fabrication of bright nanoscale light emitters. Here, we justify gold parabola-shaped nanobumps and their ordered arrays produced by direct ablation-free femtosecond laser printing as broadband plasmonic light sources electrically excited by a probe of scanning tunneling microscope (STM). I-V curves of the probe-nanoantenna tunnel junction reveal characteristic bias voltages correlating with visible-range localized (0.55 and 0.85 μm) and near-IR (1.65 and 1.87 μm) collective plasmonic modes of these nanoantennas. These multiband resonances confirmed by optical spectroscopy and full-wave simulations provide enhanced LDOS for efficient electrically driven and bias-tuned light emission. Additionally, our studies confirm remarkable applicability of STM for accurate study of optical modes supported by the plasmonic nanoantennas at nanoscale spatial resolution.

All-optical control of ultrafast plasmon resonances in the pulse-driven extraordinary optical transmission

Understanding ultrafast processes in their natural timescale is crucial for controlling and manipulating nanoscale optoelectronic devices under light–matter interaction. Here, we demonstrate that ultrafast plasmon resonances, attributed to the phenomenon of extraordinary optical transmission (EOT), can be significantly modified by tuning the spectral and temporal properties of the ultrashort light pulse. In this scheme, all-optical active tuning governs the spatial and temporal enhancement of plasmon oscillations in the EOT system without device customization. We analyze the spectral and temporal evolution of the system using two approaches. First, we develop a theoretical framework based on the coupled harmonic oscillator model, which analytically describes the dynamics of plasmon modes in the coupled and uncoupled states. Later, we compare the evolution of the system under continuous-wave and pulsed illumination. Further, we discuss the time-resolved spectral and spatial dynamics of plasmon modes using a 3D finite difference time-domain simulation method and wavelet transform. Our results show that optical tuning of the oscillation time, intensity, and spectral properties of propagating and localized plasmon modes yields a three-fold enhancement in the EOT signal. The active tuning of the EOT sensor through ultrashort light pulses paves the way for the development of on-chip photonic devices employing high-resolution imaging and sensing of abundant atomic and molecular systems.

Highly Efficient Single-Exciton Strong Coupling with Plasmons by Lowering Critical Interaction Strength at an Exceptional Point.

The single-exciton strong coupling with the localized plasmon mode (LPM) at room temperature is highly desirable for exploiting quantum technology. However, its realization has been a very low probability event due to the harsh critical conditions, severely compromising its application. Here, we present a highly efficient approach for achieving such a strong coupling by reducing the critical interaction strength at the exceptional point based upon the damping inhibition and matching of the coupled system, instead of enhancing the coupling strength to overcome the system's large damping. Experimentally, we compress the LPM's damping linewidth from about 45nm to about 14nm using a leaky Fabry-Perot cavity, a good match to the excitonic linewidth of about 10nm. This method dramatically relaxes the harsh requirement in mode volume by more than an order of magnitude and allows a maximum direction angle of the exciton dipole relative to the mode field of up to around 71.9°, significantly improving the success rate of achieving the single-exciton strong coupling with LPMs from about 1% to about 80%.

High-rectification near-field radiative thermal diode using Weyl semimetals

Thermal diodes, which allow heat transfer in a preferential direction while being blocked in a reverse direction, have numerous applications in thermal management, information processing, energy harvesting, etc. Typical materials of thermal diodes in previous works include phase-change and magneto-optical materials. However, such thermal diodes depend highly on specific working temperatures or external magnetic fields. In this work, we propose a near-field radiative thermal diode (NFRTD) based on two Weyl semimetal (WSM) nanoparticles (NPs) mediated by a WSM planar substrate, which works without an external magnetic field and with flexible temperatures. Numerical results show that the maximum rectification ratio of NFRTD can be up to 2673 when the emitter is 200 K and receiver is 180 K, which exceeds the maximum value reported in some previous works by more than 10 times. The underlying physical mechanism is the strong coupling of the localized plasmon modes in the NPs and nonreciprocal surface plasmon polaritons in the substrate. In addition, we calculate the distribution of the Green's function and reflection coefficient to investigate nonreciprocal energy transfer in NFRTDs. Finally, we discuss the effects of momentum separation on the rectification performance of the NFRTD. This work demonstrates the great potential of WSMs in thermal rectification and paves a path for designing high-performance NFRTDs.

Strongly Coupled Plasmon and Phonon Polaritons as Seen by Photon and Electron Probes

The ability to control and modify infrared excitations in condensed matter is of both fundamental and applied interest. Here we explore a system supporting low-energy excitations, in particular, mid-infrared localized plasmon modes and phonon polaritons that are tuned to be strongly coupled. We study the coupled modes by using far-field infrared spectroscopy, state-of-the-art monochromated electron energy-loss spectroscopy, numerical simulations, and analytical modeling. We demonstrate that the electron probe facilitates a precise characterization of polaritons constituting the coupled system, and enables an active control over the coupling and the resulting sample response both in frequency and space. Although far-field optical spectra can be substantially different from near-field electron energy-loss spectra, we show that a direct comparison is possible via postprocessing and right positioning of the electron beam. The resulting spectra allow us to evaluate the key parameters of the coupled system, such as the coupling strength, which we demonstrate to be probe independent. Our work establishes a rigorous description of the spectral features observed in light- and localized electron-based spectroscopies, which can be extended to the analysis of analogous optical systems with applications in heat management and electromagnetic field concentration or nanofocusing.

Dual color infrared photodetector with superconducting metamaterials.

Superconducting photodetection offers a wide spectral coverage ranging from the microwave to X-ray, and in the short wavelength range, single photon sensitivity can be achieved. However, in the longer wavelength infrared region, the system detection efficiency is low due to the lower internal quantum efficiency and weak optical absorption. Here, we utilized the superconducting metamatieral to enhance the light coupling efficiency and reach nearly perfect absorption at dual color infrared wavelengths. Dual color resonances arise from hybridization of local surface plasmon mode of the metamaterial structure and the Fabry-Perot-like cavity mode of metal (Nb)-dielectric (Si)-metamatieral (NbN) tri-layer structure. We demonstrated that, at the working temperature of 8 K slightly below TC ∼8.8 K, this infrared detector exhibits the peak responsivity of 1.2 × 106V/W and 3.2 × 106V/W at two resonant frequencies 36.6 THz and 104 THz, respectively. The peak responsivity is enhanced about ∼8 and ∼22 times, respectively, compared to that of non-resonant frequency (67 THz). Our work provides a way to harvest infrared light efficiently and hence improve the sensitivity of superconducting photodetectors in multispectral infrared range, which may find promising applications in thermal image and gas sensing etc.

Tuning magneto-electric coherent resonance with a deep-subwavelength localized spoof surface plasmonic structure.

It has been recently shown that an ultrathin corrugated spiral metal strip can simultaneously support electric and magnetic localized spoof plasmonic modes at lower frequencies. In this Letter, we report a mirror quasi-symmetrical corrugated spiral metal disk which can support coherent resonance of an orthogonal electric dipole and a magnetic dipole to achieve azimuthally symmetric unidirectional scattering. By tuning the geometric dimensions, reconfigurable magneto-electric (ME) coherent resonance enhancement is realized. Excellent agreement between numerical simulations and experimental results verifies the tunable ME coherent resonance phenomenon. Our finding could anticipate future sensitive and versatile functional devices based on high-Q coherent resonance from the microwave to the terahertz bands.

Optical Anomalies due to Volume Collective Modes of Plasmonic Metamaterials

AbstractThe emergence of optical anomalies in 3D plasmonic metamaterials upon excitation of volume collective modes is studied. These modes engage a collective response of all the meta‐atoms across the volume of the structure and arise due to coupling of localized plasmonic modes with Bloch modes of the 3D lattice. Two types of volume collective modes are introduced; a reflective mode resilient to the plasmonic absorption, exhibiting reflection that approaches unity with extremely low loss, through a distinct high‐Q spectral locking of the response of all the constituent plasmonic resonators in the volume; in addition, a transmissive mode that supports the emergence of lattice matched scattered waves within the volume, leading to full transmission through a spectral transparency window. These attractive optical properties of the volume collective modes may lead to a breakthrough in the design of low‐loss and efficient 3D plasmonic metamaterials for novel linear and nonlinear photonic applications.

Negative differential thermal conductance between Weyl semimetals nanoparticles through vacuum

In this work, the near-field radiative heat transfer (NFRHT) between two Weyl semimetal (WSM) nanoparticles (NPs) is investigated. The numerical results show that negative differential thermal conductance (NDTC) effect can be obtained in this system, i.e., when the temperature of the emitter is fixed, the heat flux does not decrease monotonically with the increase of the temperature of the receiver. Specifically, when the temperature of the emitter is 300 K, the heat flux is identical when the temperature of the receiver is 50 K or 280 K. The NDTC effect is attributed to the fact that the permittivity of the WSMs changes with the temperature. The coupling effects of polarizability of two WSM NPs have been further identified at different temperature to reveal the physical mechanism of the NDTC effect. In addition, the NFRHT between two WSM NPs can be greatly enhanced by exciting the localized plasmon and circular modes. This work indicates that the WSMs maybe promising candidate materials for manipulating NFRHT.

Direct Electro Plasmonic and Optic Modulation via a Nanoscopic Electron Reservoir.

Direct electrical tuning of localized plasmons at optical frequencies boasts the fascinating prospects of being ultrafast and energy efficient and having an ultrasmall footprint. However, the prospects are obscured by the grand challenge of effectively modulating the very large number of conduction electrons in three-dimensional metallic structures. Here we propose the concept of nanoscopic electron reservoir (NER) for direct electro plasmonic and electro-optic modulation. A NER is a few-to-ten-nanometer size metal feature on a metal host and supports a localized plasmon mode. We provide a general guideline to construct highly electrically susceptible NERs and theoretically demonstrate pronounced direct electrical tuning of the plasmon mode by exploiting the nonclassical effects of conduction electrons. Moreover, we show the electro-plasmonic tuning can be efficiently translated into modulation of optical scattering by utilizing the antenna effect of the metal host for the NER. Our work extends the landscape of electro plasmonic modulation and opens appealing new opportunities for quantum plasmonics.

Electromagnetic Field Enhancement of Nanostructured TiN Electrodes Probed with Surface-Enhanced Raman Spectroscopy.

We present a facile approach for the determination of the electromagnetic field enhancement of nanostructured TiN electrodes. As model system, TiN with partially collapsed nanotube structure obtained from nitridation of TiO2 nanotube arrays was used. Using surface-enhanced Raman scattering (SERS) spectroscopy, the electromagnetic field enhancement factors (EFs) of the substrate across the optical region were determined. The non-surface binding SERS reporter group azidobenzene was chosen, for which contributions from the chemical enhancement effect can be minimized. Derived EFs correlated with the electronic absorption profile and reached 3.9 at 786 nm excitation. Near-field enhancement and far-field absorption simulated with rigorous coupled wave analysis showed good agreement with the experimental observations. The major optical activity of TiN was concluded to originate from collective localized plasmonic modes at ca. 700 nm arising from the specific nanostructure.

Lattice-plasmon-induced asymmetric transmission in two-dimensional chiral arrays

Asymmetric transmission—direction-selective electromagnetic transmission between two ports—is a phenomenon exhibited by two-dimensional chiral systems. The possibility of exploiting this phenomenon in chiral metasurfaces opens exciting possibilities for applications such as optical isolation and routing without external magnetic fields. This work investigates optical asymmetric transmission in chiral plasmonic metasurfaces supporting lattice plasmon modes and unveils its physical origins. We show numerically and experimentally that asymmetric transmission is caused by an unbalanced excitation of such lattice modes by circularly polarized light of opposite handedness. The excitation efficiencies of the lattice modes, and hence, the strength of the asymmetric transmission, are controlled by engineering the in-plane scattering of the individual plasmonic nanoparticles such that the maximum scattering imbalance occurs along one of the in-plane diffraction orders of the metasurface. Furthermore, we show that only the nonzero diffraction orders contribute to this effect. By highlighting the role of the localized plasmon modes supported by the nanoparticle and their radiative coupling to the lattice structure, our study provides a guideline for designing metasurfaces with asymmetric transmission enabled by lattice plasmons.

Enhanced phonon-plasmon interaction in film-coupled dimer nanoridges mediated by surface acoustic waves

The interaction between phonons and localized plasmons in coupled nanoparticles can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we demonstrate by numerical analysis an enhanced optomechanical interaction in a film-coupled gold dimer nanoparticles mediated by surface acoustic waves. Two gold nanoridges are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide substrate. Numerical simulations of the optical properties reveal the existence of three surface localized plasmons in the infrared range with enhanced scattering and narrower linewidths than with a single nanoridge. The physical origin of such modes as well as their tunability as function of key geometrical parameters are successfully captured with a simple model based on effective Metal-Insulator-Metal (MIM)-like plasmonic cavity. We calculate the optomechanic coupling rates between the GHz localized mechanical modes and plasmonic modes of the dimer, finding that the strongest coupling is observed for the in-phase compressional mode followed by the out-of-phase flexural mode. Both such modes can be excited by launching a surface acoustic wave (Sezawa wave) at the inlet in front of the dimer structure. It is also found that the flexural mode which is inactive optomechanically in case of a monomer becomes active due to dimer coupling, with a significant phonon-plasmon coupling rate. The findings in this work may facilitate design of new optomechanical components monitored with fast coherent acoustics, leading to new generation of light acousto-optic modulators where strong optomechanical interactions are required.

Detecting Antibody–Antigen Interactions with Chiral Plasmons: Factors Influencing Chiral Plasmonic Sensing

Chiral near fields possessing enhanced asymmetry (superchirality), created by the interaction of light with (chiral) nanostructures, potentially provide a route to novel sensing and metrology technologies for biophysical applications. However, the mechanisms by which these near fields lead to the detection of chiral media is still poorly understood. Using a combination of numerical modeling and experimental measurements on an antibody–antigen exemplar system, important factors that influence the efficacy of chiral sensing are illustrated. It is demonstrated that localized and lattice chiral resonances display enantiomeric sensitivity. However, only the localized resonances show a strong dependency on the structure of the chiral media detected. This can be attributed to the ability of birefringent chiral layers to strongly modify the properties of near fields by acting as a sink/source of optical chirality, and hence alter inductive coupling between nanostructure elements. In addition, it is highlighted that surface morphology/defects may amplify sensing capabilities of localized chiral plasmonic modes by mediating inductive coupling.