Localized Surface Plasmon Modes Research Articles

A comparative study of coherent and incoherent drives in a four-level quantum dot–based spaser

Abstract In this article, we theoretically investigate a spaser (surface plasmon amplification by stimulated emission of radiation) comprising a spherical silver nanoparticle surrounded by a four-level gain medium of quantum dots. The spaser system is pumped coherently and incoherently with the same excitation rate, and the characteristics of the resultant coherent localized surface plasmon (LSP) mode are compared for the two pumping scenarios. We provide a detailed analytical expression for the steady state and demonstrate that the incoherent pump is more suitable for the continuous spaser mode. The reason is better understood by studying the temporal evolution of the number of LSPs N n , where the LSP oscillation starts earlier for an incoherent drive and relaxes to a steady state with a large value of N n . At a large pump rate, the spaser curve shows saturation. In addition, we have found that the resonance peak of the spaser field is independent of coherent and incoherent pumping, whereas the peak amplitude of the field depends on the pump rate.

Ultrasmall and tunable TeraHertz surface plasmon cavities at the ultimate plasmonic limit

The ability to confine THz photons inside deep-subwavelength cavities promises a transformative impact for THz light engineering with metamaterials and for realizing ultrastrong light-matter coupling at the single emitter level. To that end, the most successful approach taken so far has relied on cavity architectures based on metals, for their ability to constrain the spread of electromagnetic fields and tailor geometrically their resonant behavior. Here, we experimentally demonstrate a comparatively high level of confinement by exploiting a plasmonic mechanism based on localized THz surface plasmon modes in bulk semiconductors. We achieve plasmonic confinement at around 1 THz into record breaking small footprint THz cavities exhibiting mode volumes as low as {V}_{cav}/{lambda }_{0}^{3} sim 1{0}^{-7}-1{0}^{-8}, excellent coupling efficiencies and a large frequency tunability with temperature. Notably, we find that plasmonic-based THz cavities can operate until the emergence of electromagnetic nonlocality and Landau damping, which together constitute a fundamental limit to plasmonic confinement. This work discloses nonlocal plasmonic phenomena at unprecedentedly low frequencies and large spatial scales and opens the door to novel types of ultrastrong light-matter interaction experiments thanks to the plasmonic tunability.

Strong Coupling in Two‐Phase Metamaterials Fabricated by Sequential Self‐Assembly

AbstractSelf‐assembly processes provide the means to achieve scalable and versatile metamaterials by “bottom‐up” fabrication. Despite their enormous potential, especially as a platform for energy materials, self‐assembled metamaterials are often limited to single phase systems, and complex multi‐phase metamaterials have scarcely been explored. A new approach based on sequential self‐assembly (SSA) that enables the formation of a two‐phase metamaterial (TPM) composed of a disordered network metamaterial with embedded nanoparticles (NPs) is proposed. Taking advantage of both the high‐spatial and high‐energy resolution of electron energy loss spectroscopy (EELS), inhomogeneous localization of light in the network is observed, concurrent with dipolar and higher‐order localized surface plasmon modes in the nanoparticles. Moreover, it is demonstrated that the coupling strength deviates from the interaction of two classical dipoles when entering the strong coupling regime. The observed energy exchange between two phases in this complex metamaterial, realized solely through self‐assembly, implies the possibility to exploit these disordered systems for plasmon‐enhanced catalysis.

Characteristics of eco-friendly perovskite solar cell with moth-eye nanostructure array

A novel design of tin perovskite (CH3NH3SnI3) solar cell (PSC) is proposed and analyzed for energy harvesting application. The suggested PSC is lead free where moth-eye nanostructures are implemented in the active material to improve the light trapping and hence the light absorption. The suggested SC is numerically studied using finite difference time domain (FDTD) via Lumerical software package. The geometrical parameters and position of the nanostructures are studied to maximize the absorption and hence the optical efficiency. The reported PSC covered by the moth-eye nanostructures exhibits marked light trapping compared to the conventional planar structure with photocurrent density of 46.0082 (mA/cm2), an optical generation rate of 3.38 e28 (m−3. s−1) and an ultimate efficiency of 31.76%. Therefore, an enhancement of 14.496% is obtained compared to the traditional PSC due to the localized surface plasmons (LSP) modes around the moth eye nanostructures. The suggested design is an efficient replacement to lead—perovskite owing to excellent photovoltaic properties, cheap fabrication cost, suitable band gap of 1.02 eV, eco-friendly and great performance in converting sunlight to electrical energy.

Spatio-spectral metrics in electron energy loss spectroscopy as a tool to resolve nearly degenerate plasmon modes in dimer plasmonic antennas

Abstract Electron energy loss spectroscopy (EELS) is often utilized to characterize localized surface plasmon modes supported by plasmonic antennas. However, the spectral resolution of this technique is only mediocre, and it can be rather difficult to resolve modes close in the energy, such as coupled modes of dimer antennas. Here, we address this issue for a case study of the dimer plasmonic antenna composed of two gold discs. We analyze four nearly degenerate coupled plasmon modes of the dimer: longitudinal and transverse bonding and antibonding dipole modes. With a traditional approach, which takes into account the spectral response of the antennas recorded at specific points, the modes cannot be experimentally identified with EELS. Therefore, we employ the spectral and spatial sensitivity of EELS simultaneously. We propose several metrics that can be utilized to resolve the modes. First, we utilize electrodynamic simulations to verify that the metrics indeed represent the spectral positions of the plasmon modes. Next, we apply the metrics to experimental data, demonstrating their ability to resolve three of the above-mentioned modes (with transverse bonding and antibonding modes still unresolved), identify them unequivocally, and determine their energies. In this respect, the spatio-spectral metrics increase the information extracted from electron energy loss spectroscopy applied to plasmonic antennas.

Photoconductive control of high-order localized surface plasmon modes in Au-Si-Au nanodisk stacking

Photoconductive control of high-order localized surface plasmon modes in Au-Si-Au nanodisk stacking

Electromagnetically induced transparency via localized surface plasmon mode-assisted hybrid cavity QED

In recent years, most studies have focused on the perfect absorption and high-efficiency quantum memory of the one-sided system, ignoring the characteristics of its optical switching contrast. Thus, the performance of all-optical switching and optical transistors is limited. Herein, we propose a localized surface plasmon (LSP) mode-assisted cavity QED system which consists of a Λ-shaped three-level quantum emitter (QE), a metal nanoparticle and a one-sided optical cavity with a fully reflected mirror. In this system, the QE coherently couples to the cavity and LSP mode respectively, which is manipulated by the control field. As a result, considerably high and stable switch contrast of 90% can be achievable due to the strong confined field of the LSP mode and perfect absorption of the optical medium. In addition, we obtain a power dependent effect between the control field and the transmitted frequency as a result of the converted dark state. We employ the Heisenberg–Langevin equation and numerical master equation formalisms to explain high switching, controllable output light and the dark state. Our system introduces an effective method to improve the performance of optical switches based on the one-sided system in quantum information storage and quantum communication.

Hybridization of surface lattice modes: towards plasmonic metasurfaces with high flexible tunability

Abstract When assembled in periodic arrangements, metallic nanostructures (NSs) support plasmonic surface lattice (SL) resonances resulting from long-range interactions these surface lattice resonances differ radically from localized surface plasmon (LSP). Similarly to the hybridization of LSP resonances, observed in short-range interactions, we demonstrate the possibility to generate a hybridization of surface lattice (SL) plasmon resonances, by the excitation of grazing order diffraction within the metasurface. This hybridization leads to the emergence of bonding and anti-bonding modes. If hybridization of LSP modes has been widely described in recent literature, there is still no experimental proof-of-concept reporting such hybridization with SL plasmon resonances. We fill this gap in the present paper by considering surfaces made of binary arrays with unit cells made of two gold disks of distinct diameters. We demonstrate the possibility to maximize or cancel the interaction between the hybridized SL resonances by simply controlling the distance between particles. All our experimental results are supported by FDTD calculations. The hybridization of SL modes results in much richer hybridization scenario in terms of wavelength and quality factor control, compared to a hybridization of LSP in a short-range configuration. It offers unprecedented opportunities to generate innovative optical devices, with high flexible tunability.

Wavefront engineering based on hybrid plasmonic mode

Plasmonic lens provided great optical performance through the interaction between light and metallic nanostructure in the past years. However, the wavefront control on the metallic lens was still limited by the fabrication and geometrical design. Here, an unconventional plasmonic lens with sufficient phase modulation was developed as a focusing device; it was constructed using both circular and annular apertures in a silver film. The circular and annular apertures induced localized and cylindrical surface plasmon modes with the incident light, respectively. The propagation phase in these apertures was related to their geometric parameters. These apertures with lower aspect ratios were designed to have the artificial phase from zero to Pi and arranged in concentric arrays to manipulate the wavefront of the lens. The focal length and spot size were determined by the phase distribution on the plasmonic lens. The focal spot with a subwavelength size was obtained by the appropriate manipulation of the phase. We also demonstrated the focusing properties of this plasmonic lens experimentally to validate our design. Owing to the circular symmetry of the circular and annular apertures, this type of plasmonic lens has considerable potential for use as a polarization-insensitive device.

Strong coupling in an Au plasmonic antenna– SiO2 layer system: A hybrid-mode analysis

A detailed analysis of the optical response of a system accommodating several coupled modes is needed for the complete understanding of the strong coupling effect. In this paper, we report on the analysis of scattering cross-section spectra of Au antennas on a ${\mathrm{SiO}}_{2}$ layer on a Si substrate in the IR region. A classical model of coupled oscillators is used for determining the resonant energies, damping rates, and coupling strengths of four phonon polariton modes in the ${\mathrm{SiO}}_{2}$ layer coupled to a localized surface plasmon mode in a Au antenna. The calculated Hopfield mixing coefficients then show the contribution of the individual uncoupled modes to the hybrid modes of the coupled system.

Coaction effect of radiative and non-radiative damping on the lifetime of localized surface plasmon modes in individual gold nanorods.

Revealing the coaction effect of radiative and non-radiative damping on the lifetime of the localized surface plasmon resonance (LSPR) mode is a prerequisite for the applications of LSPR. Here, we systematically investigated the coaction effect of radiative and non-radiative damping on the lifetime of the super-radiant and sub-radiant LSPR modes of gold nanorods using time-resolved photoemission electron microscopy (TR-PEEM). The results show that the lifetime of the LSPR mode depends on the length of the gold nanorod, and the different variation behavior of an LSPR mode lifetime exists between the super-radiative mode and the sub-radiative one with the increase of nanorod length (volume). Surprisingly, it is found that the lifetime of the super-radiant LSPR mode can be comparable to or even longer than that of the sub-radiant LSPR mode, instead of the usual claim that a sub-radiant LSPR mode has a longer life than the super-radiant mode. Those TR-PEEM experimental results are supported by finite-difference time-domain simulations and are well explained by the coaction effect with the calculation of the radiative and non-radiative damping rate with the increase of the nanorod volume. We believe that this study is beneficial to build a low-threshold nano-laser and ultrasensitive molecular spectroscopy system.

Strong coupling between longitudinal and U-shaped localized surface plasmon modes in rectangular grating-shaped gold nanostructures

Strong couplings between localized surface plasmon resonance (LSPR) modes and single quantum emitters have been intensively investigated recently, and meanwhile the coupling between different LSPR modes inside individual metallic nanoparticle is still rarely researched. Herein, the strong coupling is investigated for different LSPR modes inside individual rectangular-grating-shaped gold nanostructure composed of one main-cuboid and two side-attached subcuboids. Original uncoupled LSPR modes are revealed to be longitudinal and U-shaped LSPRs. For the nanostructure with increased main-cuboid length, the dispersion curves of simulated dual-original LSPR scattering wavelengths and dual-coupled LSPR scattering wavelengths show typical strong coupling patterns featuring anti-crossing and large Rabi splitting of 598.1 meV. The strong coupling is considered to be caused by the overlap of longitudinal and U-shaped LSPR oscillation modes in the gold nanostructure. The extracted coupling strength is found in order of 1013 Hz and it increases with the LSPR overlap length. The spatial mode and time evolution of the coupled LSPR modes are also numerically investigated. The simulated results are well comprehended with the classical strong coupling model of oscillators, further confirming the coupling between the longitudinal and the U-shaped LSPRs. The experimental dark-field scattering spectrum shows the existence of U-shaped LSPR mode in the gold nanostructure.

Coherent coupling between surface plasmons and localized surface plasmons in black phosphorus metamaterials

We propose a hybrid structure supporting electromagnetic coupling between black phosphorus surface plasmons (BPSP) mode and localized surface plasmons (BPLSP) mode theoretically. By dynamically controlling the carrier concentration of the black phosphorus strip, we show the coherent BPLSP-BPSP mode coupling. The coherent coupling can be described by using the coupled oscillator model, which shows a distinct Rabi splitting phenomena with frequency of 8.7 THz. The coherent coupling between BPLSP mode and BPSP mode also leads to the formation of multiple hybrid modes. In addition, we also explore the influence of structural parameters on coherent coupling. Our work may provide a reference for the interaction between different mode coupling.

Analysis of dephasing time of plasmonic hybridization modes using a quasi-normal mode method

Measuring and regulating the localized surface plasmon (LSP) dephasing time (lifetime) of bonding and antibonding modes in nanosystems is the basis for the application of LSP devices. Here, we investigate the modulation of the dephasing time of hybridization modes generated by the coupling between dipole and quadrupole LSP modes in an asymmetric nanorod dimer by combining finite-difference time-domain numerical simulations with the quasi-normal mode (QNM) method. We surprisingly find that the dephasing time of the antibonding mode can be as long as 10.55 fs, which is 30% longer than that of the quadrupole mode supported by an isolated long rod (7.94 fs). Moreover, it is found that the dephasing times of the antibonding and bonding modes can be selectively modulated by changing the gap size or rod length in the dimer. A 6 THz difference between peak frequencies obtained from directly reading the near-field spectra and from the QNM fitting of near-field spectra is observed due to mode interactions existing with a gap size of 60 nm. This work demonstrates that QNM is a superior method in analyzing complex spectra, and it advances our understanding of the dynamical evolution of plasmonic hybrid modes.

Active modulation of the transmission properties of a far infrared all-dielectric metasurface

All-dielectric metasurfaces with tunable and reconfigurable properties have broad application prospects. Here,an all-dielectric metasurface with a MgF 2 layer and a ZrTe 5 array is measured and simulated (The simulation software is HFSS) A transmission valley V1 (amplitude is 0.21, resonance frequency is 10.18 THz) is achieved based on the localized surface plasmon (LSP) mode coupling between four unit cells without external magnetic field. In the first set of measurements, a magnetic field is applied along the x-axis, the measured transmission valley V1 is reduced and the resonant frequency is stabilized. Moreover, a new transmission valley V2 is excited based on the LSP mode coupling between two unit cells. The amplitude of the transmission valley V2 is dependent on the applied magnetic field. In the second set of measurements, a magnetic field is applied along the 450-axis, the amplitude of the transmission valley V1 is also reduced, and a transmission valley V3 is obtained due to the surface plasmon polarization (SPP) modes coupling. The amplitude of the transmission valley V3 is decreased with the increase of the magnetic field, and the resonance frequency is shifted towards the higher frequencies.

Relative humidity sensor based on a silver nano-grating made from DVD discs.

A relative humidity sensor based on a silver nano-grating was proposed. By stripping and cleaning commercially available CD and DVD discs, polycarbonate plates with different grating periods are obtained. These plates as templates are coated with a layer of sputtered silver film to form silver nano-gratings, which exhibit refractive index sensing sensitivities of 517nm/RIU and 742.9nm/RIU, respectively. The finite-difference time-domain simulation results conform the excited surface plasmon polariton modes and localized surface plasmon modes on the nano-grating. By spin coating a layer of humidity-sensitive porous silica with optimized thickness, the silver nano-grating shows a relative humidity detection sensitivity of 0.23nm/%RH.

Plasmonic Bound States in the Continuum in Compact Nanostructures

AbstractOptical bound states in the continuum (BICs) have become a unique way to produce the extreme localization and enhancement of light waves, however, it remains challenging to realize them in single‐particle systems. This work shows the existence of optical BICs in a compact corrugated metallic cylinder, and quasi‐BICs are observed in multiple channels when adjacent grooves have different depths. It is revealed that such BICs are purely plasmonic, resulting from the destructive interference of two localized surface plasmon modes in the nanostructures. Benefiting from the surface plasmon effects, the findings extend optical BICs and the associated physics to subwavelength optics, providing a route to break the diffraction limit and boost light–matter interactions.

Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

Active control of thermal emission by graphene-nanowire coupled plasmonic metasurfaces

Metasurfaces, together with graphene plasmonics, have become prominent for the emissivity control in thermal engineering, both passively, through changing the geometric parameters and packing density of the metasurfaces, and actively, through graphene gating or doping. We demonstrate a graphene-nanowire coupled plasmonic metasurface utilizing the hybrid localized surface plasmon modes of the nanowire array and graphene. The nanowire array makes the hybrid surface plasmon mode localized, allowing a free-space excitation. The single-layer graphene, via the gating between the underneath mirror and a top electrode, can actively tune the spectral emissivity by almost 90%. In addition, the graphene surface plasmon modes remove the strict polarization dependence of nanowire array emission, resulting in a fivefold enhancement of the P-polarized emissivity, especially for large emission angles.

Localized Surface Plasmons on Textiles for Non-Contact Vital Sign Sensing

Radio frequency technologies are capable of remotely sensing human vital signs for broad applications in healthcare. However, their use during daily life is hindered by challenges such as environmental interference, low sensitivity, and nonideal form factors. Here, we report a radio frequency sensor made from a conductive textile that provides sensitive noncontact monitoring of vital signs. The sensor (13 cm2 in size) comprises a radial pattern of grooves that support spoof localized surface plasmon (LSP) modes and a ground plane with a complementary window that controls the sensor’s interaction with the body. These spoof LSP modes provide a sharp resonant dip and a broadly tunable evanescent field that enables tracking of heartbeat and respiration via the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|S_{11}|$ </tex-math></inline-formula> spectrum measured in a connector-free manner. We establish the optimal operating mode of the sensor and show that it provides a resonant frequency shift of 250 MHz for respiration and 100 MHz for heartbeat motions. Experiments with a healthy human subject show that the measured heart rate (HR) and respiration rate (RR) are in close agreement with gold standard sensors and are robust to daily activities performed while sitting.