Excitation Of Plasmon Modes Research Articles

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon‐Optical and Plasmon‐Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Flexible nanofiber-coupled hybrid plasmonic Bragg grating.

We report a hybrid plasmonic Bragg grating composed of a nanofiber coupled with orthogonally oriented metal strips. Numerical simulations are performed to study the transmission and reflection spectra of the grating. It shows that the TM polarization has much stronger Bragg reflection due to the excitation of hybrid plasmonic modes. The dependence of reflection peaks on several key device parameters is analyzed. Light propagation simulation further reveals that both fundamental and first-order TM modes are excited upon Bragg reflection, leading to two separate peaks in the spectrum. We implement the prototype device by attaching a nanofiber onto the surface of an array of sub-micrometer-wide metal strips. The main reflection peak is measured to have a 3-dB bandwidth of 15 nm and out-of-band rejection of more than 30 dB. The effects of nanofiber radius, alignment angle and coupling length on the device performance are also experimentally investigated.

Magnetic circular dichroism of non-local surface lattice resonances in magnetic nanoparticle arrays.

Subwavelength metallic particles support plasmon resonances that allow extreme confinement of light down to the nanoscale. Irradiation with left- and right hand circularly polarized light results in the excitation of circular plasmon modes with opposite helicity. The Lorenz force lifts the degeneracy of the two modes in magnetic nanoparticles. Consequently, the confinement and frequency of localized surface plasmon resonances can be tuned by an external magnetic field. In this paper, we experimentally demonstrate this effect for nickel nanoparticles using magnetic circular dichroism (MCD). Besides, we show that non-local surface lattice resonances in periodic arrays of the same nanoparticles significantly enhance the MCD signal. A numerical model based on the modified long wavelength approximation is used to reproduce the main features in the experimental spectra and provide design rules for large MCD effects in sensing applications.

Study of TiN nanodisks with regard to application for Heat-Assisted Magnetic Recording

ABSTRACTIn recent years titanium nitride is being considered as a very promising plasmonic material for data storage applications as it exhibits a pronounced plasmonic dipolar resonance and has high thermal stability. However, there is a lack of research where higher order resonance modes are examined. We address this here by performing angle dependent spectral transmission measurements nanodisks arrays made from titanium nitride. The measurements show strong polarization dependence with s-polarized light causing excitation of the quadrupole and higher order resonance plasmonic modes. These higher order modes are required for the state-of-the-art designs of near-field transducers. This, together with its outstanding thermal properties, makes TiN a favourable material for data storage applications.

Photon-induced generation and spatial control of extreme pressure at the nanoscale with a gold bowtie nano-antenna platform.

Precise spatial and temporal control of pressure stimulation at the nanometer scale is essential for the fabrication and manipulation of nano-objects, and for exploring single-molecule behaviour of matter under extreme conditions. However, state-of-the-art nano-mechanical transducers require sophisticated driving hardware and are currently limited to moderate pressure regimes. Here we report a gold plasmonic bowtie (AuBT) nano-antennas array that can generate extreme pressure stimulus of ∼100 GPa in the ps (10-12 s) time scale with sub-wavelength resolution upon irradiation with ultra-short laser pulses. Our method leverages the non-linear interaction of photons with water molecules to excite a nano-plasma in the plasmon-enhanced near-field and induce extreme thermodynamic states. The proposed method utilizes laser pulses, which in contrast to micro- and nano-mechanical actuators offers simplicity and versatility. We present time-resolved shadowgraphic imaging, electron microscopy and simulation data that suggest that our platform can efficiently create cavitation nano-bubbles and generate intense pressure in specific patterns, which can be controlled by the selective excitation of plasmon modes of distinct polarizations. This novel platform should enable probing non-invasively the mechanical response of cells and single-molecules at time and pressure regimes that are currently difficult to reach with other methods.

Optical microfiber-loaded surface plasmonic TE-pass polarizer

We propose a novel optical microfiber-loaded plasmonic TE-pass polarizer consisting of an optical microfiber placed on top of a silver substrate and demonstrate its performance both numerically by using the finite element method (FEM) and experimentally. The simulation results show that the loss in the fundamental TE mode is relatively low while at the same time the fundamental TM mode suffers from a large metal dissipation loss induced by excitation of the microfiber-loaded surface plasmonic mode. The microfiber was fabricated using the standard microheater brushing–tapering technique. The measured extinction ratio over the range of the C-band wavelengths is greater than 20dB for the polarizer with a microfiber diameter of 4µm, which agrees well with the simulation results.

Semitransparent inverted organic solar cell with improved absorption and reasonable transparency perception based on the nanopatterned MoO 3 / Ag / MoO 3 anode

We demonstrate an inverted low bandgap semitransparent organic solar cell with improved absorption as well as reasonable transparency perception based on a nanopatterned MoO 3 /Ag/MoO 3 (MAM) multilayer film as the transparent anode under illumination from the MAM side. The integrated absorption efficiency of the active layer at normal hybrid-polarized incidence considering an AM 1.5G solar spectrum is up to 51.69%, increased by 18.53% as compared to that of the equivalent planar device (43.61%) and reaching 77.3% of that of the corresponding opaque nanopatterned device (66.90%). Detailed investigations reveal that the excitation of plasmonic waveguide modes (at transverse magnetic polarization) and photonic modes (at transverse electric polarization) are responsible for the observed enhancement in absorption. Importantly, the proposed device exhibits an average transmittance of up to 28.4% and an average transparency perception of 26.3% for the human eyes under hybrid-polarized light illumination along with a good color rendering property. Additionally, our proposal works very well over a fairly wide angular range.

Generalized analytical model based on harmonic coupling for hybrid plasmonic modes: comparison with numerical and experimental results.

Metal nanoparticle arrays have proved useful for different applications due to their ability to enhance electromagnetic fields within a few tens of nanometers. This field enhancement results from the excitation of various plasmonic modes at certain resonance frequencies. In this article, we have studied an array of metallic nanocylinders placed on a thin metallic film. A simple analytical model is proposed to explain the existence of the different types of modes that can be excited in such a structure. Owing to the cylinder array, the structure can support localized surface plasmon (LSP) modes. The LSP mode couples to the propagating surface plasmon (PSP) mode of the thin film to give rise to the hybrid lattice plasmon (HLP) mode and anti-crossing phenomenon. Due to the periodicity of the array, the Bragg modes (BM) are also excited in the structure. We have calculated analytically the resonance frequencies of the BM, LSP and the corresponding HLP, and have verified the calculations by rigorous numerical methods. Experimental results obtained in the Kretschmann configuration also validate the proposed analytical model. The dependency of the resonance frequencies of these modes on the structural parameters such as cylinder diameter, height and the periodicity of the array is shown. Such a detailed study can offer insights on the physical phenomenon that governs the excitation of various plasmonic modes in the system. It is also useful to optimize the structure as per required for the different applications, where such types of structures are used.

Subradiant, Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.

Two plasmonic mode excitation using a double-step rectangle grating

Excitation of plasmonic modes is investigated on a grating having a double-step rectangle profile. A single-step grating excites one side of a plasmonic mode, whereas the double-step profile can induce odd and even plasmonic modes simultaneously. The mode wavelength of the double-step grating can be individually controlled by selecting the rule density and the depths of the steps. In addition, the field intensity on the double-step grating has wider and stronger distributions than that appearing on the single-step grating.

Second harmonic generation spectroscopy on second harmonic resonant plasmonic metamaterials

The field enhancement resulting from the resonant excitation of plasmonic modes in metallic nanostructures allows to amplify nonlinear processes such as second harmonic generation in plasmonic metamaterials. Thus far, nonlinear experiments with metallic nanostructures relied on the resonant enhancement of the pump light by a plasmonic mode. Here, we report on second harmonic generation spectroscopy on plasmonic metamaterials that only exhibit a plasmonic resonance at twice the pump frequency. We also observe in this case a strong enhancement of the second harmonic generation signal, even though the driving pump field is not resonantly enhanced. The experimental data can be explained in terms of the anharmonic oscillator model and classical symmetry selection rules applied to nonlinear metamaterials. Our findings pave the way for further optimizations of the nonlinear response of plasmonic metamaterials.

Investigating Plasmonic Structure-Dependent Light Amplification and Electronic Dynamics Using Advances in Nonlinear Optical Microscopy

Our group has made several recent advances using single-particle nonlinear optical (NLO) microscopy techniques to examine nanostructure-specific plasmon-mediated interactions with electromagnetic energy. These efforts are directed toward the larger goal of optimizing the structure of photonic nanoparticle assemblies for the use and control of energy at the nanoscale. By combining statistical localization methods with second harmonic generation (SHG) imaging, nonlinear signal hot spots can be located within a plasmonic network with nanometer spatial accuracy. This experimental capability was applied to study electromagnetic lensing effects in plasmonic nanoparticle assemblies, with the efficiency of cascaded energy transfer through the network being determined by the resonantly excited plasmon mode. The polarization and time dependencies of the NLO signals were examined using a sequence of phase-locked broad bandwidth femtosecond laser pulses. The methodologies for generating, characterizing, and incorpora...

Random Plasmonic Nanowire Gratings for Enhanced Light Absorption in Organic Solar Cells

The effects of random nanowire gratings on the light absorption of organic solar cells are studied. The gratings are deposited on the back silver electrode, causing the excitation of surface plasmonic modes. An advantage of such gratings is that they will not cause any blockage to the incident light; hence, they can be as dense as necessary. Using a finite-difference time-domain numerical method, several Monte Carlo experiments are tested, in two- and three-dimensional geometries. It is shown that cells with random gratings clearly outperform flat cells, i.e., cells without any gratings while their average efficiency is slightly below those of the optimized geometries. However, optimized geometries require nanoscale accurate deposition of grating nanowires and require expensive fabrication procedures. This suggests that random gratings can effectively replace nanoscale accurate geometries, with a slight tolerance of absorption efficiency.

Tuning the acoustic frequency of a gold nanodisk through its adhesion layer.

To fabricate robust metallic nanostructures with top-down patterning methods such as electron-beam lithography, an initial nanometer-scale layer of a second metal is deposited to promote adhesion of the metal of interest. However, how this nanoscale layer affects the mechanical properties of the nanostructure and how adhesion layer thickness controls the binding strength to the substrate are still open questions. Here we use ultrafast laser pulses to impulsively launch acoustic phonons in single gold nanodisks with variable titanium layer thicknesses, and observe an increase in phonon frequencies as a thicker adhesion layer facilitates stronger binding to the glass substrate. In addition to an all-optical interrogation of nanoscale mechanical properties, our results show that the adhesion layer can be used to controllably modify the acoustic phonon modes of a gold nanodisk. This direct coupling between optically excited plasmon modes and phonon modes can be exploited for a variety of emerging optomechanical applications.

Scattering of light waves by finite metal nanostrip gratings: Nystrom-Type method and resonance effects

Efficient and rapidly convergent numerical algorithm for the simulation of the scattering of light waves by a finite gratings consisting of thin (thinner than the wavelength in the free space) metal nanostrips is presented. The model is based on the utilization of generalized boundary conditions (GBC), which allow one to exclude from consideration the field inside each strip and to reduce the two-dimensional boundary problem to one-dimensional systems of singular/hypersingular integral equations (IE). The obtained IE are solved numerically using the Nystrom-type method and the quadrature formulas of interpolation type, that provides guarantee convergence and controlled accuracy. The article presents the results of characteristics calculations for optical scattering and absorption by the gratings, which consist of silver nanostrips, as dependences on the width and on the thickness of the strips, and on the grating period. The nature of resonance phenomena has been investigated, namely the article presents the analysis of intensive optical scaterring and absorption in the case of excitation of plasmonic modes (plasmons) and of grating modes, which are induced by the periodicity.

Rounding corners of nano-square patches for multispectral plasmonic metamaterial absorbers.

Multispectral metamaterial absorbers based on metal-insulator-metal nano-square patch resonators are studied here. For a geometry consisting of perfectly nano-square patches and vertical sidewalls, double resonances in the visible regime are observed due to simultaneous excitation of electric and magnetic plasmon modes. Although slightly modifying the sizes of the square patches makes the resonance wavelengths simply shift, rounding corners of the square patches results in emergence of a third resonance due to excitation of the circular cavity modes. Sidewall angle of the patches are also observed to affect the absorption spectra significantly. Peak absorption values for the triple resonance structures are strongly affected as the sidewall angle varies from 90 to 50 degrees. Rounded corners and slanted sidewalls are typical imperfections for lithographically fabricated metamaterial structures. The presented results suggest that imperfections caused during fabrication of the top nano-structures must be taken into account when designing metamaterial absorbers. Furthermore, it is shown that these fabrication imperfections can be exploited for improving resonance properties and bandwidths of metamaterials for various potential applications such as solar energy harvesting, thermal emitters, surface enhanced spectroscopies and photodetection.

Excitation of multipole plasmons by optical vortex beams.

Localized surface plasmon resonance (LSPR) has been shown to exhibit a strong potential for nanoscale electromagnetic field manipulation beyond the diffraction limit. Particularly dark mode plasmons circumvent radiation loss and store the energy long in time, which raise the prospect of interesting plasmonics applications, for example biochemical sensing and nanoscale lasing. Here we theoretically investigate a method of exciting multipole plasmons, including dark modes, using normally incident light. By performing numerical calculations, we show that multipole plasmons in metal nanodisks can be selectively excited by circularly-polarized optical vortex beams. We study the electromagnetic fields of the beam cross-sections and their correspondence with the excited multipole plasmon modes with respect to spin and orbital angular momenta. The transfer of angular momentum between photons and plasmons is also discussed.

Bright and dark plasmon resonances of nanoplasmonic antennas evanescently coupled with a silicon nitride waveguide

In this work we investigate numerically and experimentally the resonance wavelength tuning of different nanoplasmonic antennas excited through the evanescent field of a single mode silicon nitride waveguide and study their interaction with this excitation field. Experimental interaction efficiencies up to 19% are reported and it is shown that the waveguide geometry can be tuned in order to optimize this interaction. Apart from the excitation of bright plasmon modes, an efficient coupling between the evanescent field and a dark plasmonic resonance is experimentally demonstrated and theoretically explained as a result of the propagation induced phase delay.

Quantum nonlocal effects on optical properties of spherical nanoparticles

To study the scattering of electromagnetic radiation by a spherical metallic nanoparticle with quantum spatial dispersion, we develop the standard nonlocal Mie theory by allowing for the excitation of the quantum longitudinal plasmon modes. To describe the quantum nonlocal effects, we use the quantum longitudinal dielectric function of the system. As in the standard Mie theory, the electromagnetic fields are expanded in terms of spherical vector wavefunctions. Then, the usual Maxwell boundary conditions are imposed plus the appropriate additional boundary conditions. Examples of calculated extinction spectra are presented, and it is found that the frequencies of the subsidiary peaks, due to quantum bulk plasmon excitations exhibit strong dependence on the quantum spatial dispersion.

The effect of symmetry on resonant and nonresonant photoresponses in a field-effect terahertz detector

The effect of the symmetries in the terahertz (THz) field distribution and the field-effect channel on THz photoresponse is examined. Resonant excitation of cavity plasmon modes and nonresonant self-mixing of THz waves are demonstrated in a GaN/AlGaN two-dimensional electron gas with symmetrically designed nanogates, antennas, and filters. We found that the self-mixing signal can be effectively suppressed by the symmetric design and the resonant response benefits from the residual asymmetry. The findings suggest that a single detector may provide both high sensitivity from the self-mixing mechanism and spectral resolution from the resonant response by optimizing the degree of geometrical and/or electronic symmetries.