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Flow and extraction of energy and charge carriers in hybrid plasmonic nanostructures
Strong interactions of electromagnetic fields with plasmonic nanomaterials have been exploited in various applications. These applications have centred on plasmon-enhanced scattering rates in nearby molecules or plasmon-induced heating. A question that has emerged recently is whether it is possible to use plasmonic nanostructures in a range of hot electron (hole) applications, including photocatalysis, photovoltaics and photodetection. These applications require coupling of a plasmonic component, which amplifies the interaction of light with the material, to an attached non-plasmonic component that extracts this energy in the form of electronic excitations to perform a function. Here, in this Perspective, we discuss recent work in the emerging field of hybrid plasmonics. We focus on fundamental questions related to the nanoscopic flow of energy and excited charge carriers in these multicomponent materials. We also address critical misconceptions, challenges and opportunities that require more attention.
Transmission and reflection of terahertz plasmons in two-dimensional plasmonic devices
We found that plasmons in two-dimensional semiconductor devices will be reflected by discontinuities, notably, junctions between gated and non-gated electron channels. The transmitted and reflected plasmons can form spatially- and frequency-varying signals, and their understanding is important for the design of terahertz detectors, oscillators, and plasmonic crystals. Using mode decomposition, we studied terahertz plasmons incident on a junction between a gated and a nongated channel. The plasmon reflection and transmission coefficients were found numerically and analytically and studied between 0.3 and 1 THz for a range of electron densities. At higher frequencies, we could describe the plasmons by a simplified model of channels in homogeneous dielectrics, for which the analytical approximations were accurate. At low frequencies, however, the full geometry and mode spectrum had to be taken into account. Moreover, the results agreed with simulations by the finite-element method. As a result, mode decomposition thus proved to be a powerful method for plasmonic devices, combining the rigor of complete solutions of Maxwell's equations with the convenience of analytical expressions.
Giant enhancement of nanoscale thermal radiation based on hyperbolic graphene plasmons
Excitation of surface plasmons enables super-Planckian thermal radiation far beyond the blackbody limit. By patterning a single layer of graphene sheet into ribbons, the closed circular dispersion of graphene plasmons is opened to become hyperbolic, leading to broadband singularities of density of states. Extremely high- k evanescent waves can now couple with hyperbolic graphene plasmons. Consequently, a giant enhancement of the near-field radiative heat flux, by more than one order of magnitude, is demonstrated here using rigorous numerical simulations. The findings may open promising pathways for highly efficient thermal management, energy harvesting, and sub-wavelength thermal imaging.
Localized to Propagating Surface Plasmon Resonance Transition in Ni-Au Magneto-Plasmonic Gratings
Magneto-plasmonic structures, which are the best candidates for different applications, have been the subject of intense research in recent years. In this paper, we proposed new magneto-plasmonic structures based on different uni-dimensional gratings to investigate the localized to propagative surface plasmon resonance transition. For this purpose, simulation of reflectance and also surface plasmon resonance coupling were used in three different fabricated samples compared with nickel thin film under different azimuthaly excitations. The fabricated samples show an enhanced magneto-optical rotation due to the transition of two types of plasmon in the sample with intermediate dimensions. The prospects of magneto-plasmonic response dependency on the azimuthaly excitation are discussed.
Controlled synthesis of highly-branched plasmonic gold nanoparticles through peptoid engineering
Inspired by peptide- and protein-controlled formation of hierarchical inorganic nanostructures in nature, herein we report the design of sequence-defined peptoids for biomimetic synthesis of flower-like gold nanosuperstruc-tures. We demonstrate that the peptoid-induced morphosyn-thesis of gold nanoflowers is sequence-dependent. By varying the hydrophobicity, number of carboxylate and amino groups, and side-chain positions of peptoids, we develop a rule of thumb for designing peptoids that yield gold nanoflowers. Through a combination of hyperspectral UV-Vis absorption micro-spectroscopy (H-UVVIS-AMS) and three-photon photoemission electron microscopy (TP-PEEM), we measured the plasmonic properties of individual nanoflowers. Whereas H-UVVIS-AMS experiments revealed well-defined plasmon resonances centered at ~528 nm, TP-PEEM measurements showed an unprecedented plasmonic enhancement as high as 105 for a single nanoflower. A com-bination of the unique plasmonic property and the high sta-bility endows the peptoid-engineered nanoflowers with po-tential applications in chemical and biological imaging. FY, YC, HJ and C-LC acknowledge support by the Materials Synthesis and Simulation Across Scales (MS3) Initiative, through the Laboratory Directed Research & Development (LDRD) fund at Pacific Northwest National Laboratory (PNNL). FY was also partially supported by fellowships from Shandong Provincial Education Association for International Exchanges. YG was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. PZE acknowledges support from the Laboratory Directed Research and Development Program through a Linus Pauling Fellowship at PNNL. PNNL is multi-program national laboratory operated for the Department of Energy by Battelle under Contracts No. DE-AC05-76RL01830. The authors are most grateful to Alan Joly and Wayne Hess (PNNL) for their support with the PEEM measurements and several useful discussions.
Diffraction limited focusing and routing of gap plasmons by a metal-dielectric-metal lens
Passive optical elements can play key roles in photonic applications such as plasmonic integrated circuits. Here we experimentally demonstrate passive gap-plasmon focusing and routing in two-dimensions. This is accomplished using a high numerical-aperture metal-dielectric-metal lens incorporated into a planar-waveguide device. Fabrication via metal sputtering, oxide deposition, electron- and focused-ion- beam lithography, and argon ion-milling is reported on in detail. Diffraction-limited focusing is optically characterized by sampling out-coupled light with a microscope. The measured focal distance and full-width-half-maximum spot size agree well with the calculated lens performance. The surface plasmon polariton propagation length is measured by sampling light from multiple out-coupler slits.
Terahertz plasmonic laser radiating in an ultra-narrow beam
Plasmonic lasers (spasers) generate coherent surface plasmon polaritons (SPPs) and could be realized at subwavelength dimensions in metallic cavities for applications in nanoscale optics. Plasmonic cavities are also utilized for terahertz quantum-cascade lasers (QCLs), which are the brightest available solid-state sources of terahertz radiation. A long standing challenge for spasers that are utilized as nanoscale sources of radiation, is their poor coupling to the far-field radiation. Unlike conventional lasers that could produce directional beams, spasers have highly divergent radiation patterns due to their subwavelength apertures. Here, we theoretically and experimentally demonstrate a new technique for implementing distributed feedback (DFB) that is distinct from any other previously utilized DFB schemes for semiconductor lasers. The so-termed antenna-feedback scheme leads to single-mode operation in plasmonic lasers, couples the resonant SPP mode to a highly directional far-field radiation pattern, and integrates hybrid SPPs in surrounding medium into the operation of the DFB lasers. Experimentally, the antenna-feedback method, which does not require the phase matching to a well-defined effective index, is implemented for terahertz QCLs, and single-mode terahertz QCLs with a beam divergence as small as 4°×4° are demonstrated, which is the narrowest beam reported for any terahertz QCL to date. Moreover, in contrast to a negligible radiative field in conventional photonic band-edge lasers, in which the periodicity follows the integer multiple of half-wavelengths inside the active medium, antenna-feedback breaks this integer limit for the first time and enhances the radiative field of the lasing mode. Terahertz lasers with narrow-beam emission will find applications for integrated as well as standoff terahertz spectroscopy and sensing. Furthermore, the antenna-feedback scheme is generally applicable to any plasmonic laser with a Fabry–Perot cavity irrespective of its operating wavelength and could bring plasmonic lasers closer to practical applications.
Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function
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Plasmon-Mediated Coherent Superposition of Discrete Excitons under Strong Exciton–Plasmon Coupling in Few-Layer MoS 2 at Room Temperature
In this paper, we demonstrate room temperature coherent hybridization of the A- and B-excitons in few-layer MoS 2 , mediated by simultaneous strong coupling to surface plasmon polaritons. Few-layer MoS 2 was placed on a tunable plasmonic structure and the system’s dispersion was measured by tuning the plasmon energy across the exciton energies. Strong coupling was observed as double Rabi splitting at the A- and B-excitons of 81 and 93 meV, respectively. A coupled harmonic oscillator model sheds light on the nature of the interaction, revealing a quantum superposition of the A- and B-excitons, mediated by the plasmon interaction. This observation suggests the possibility of room temperature intra- or intervalley quantum information transport and/or spin entanglement. The experiment confirms a previous theoretical prediction of room temperature exciton–exciton hybridization in two-dimensional MoS 2 . Further, through modeling we find that room temperature strong coupling is a general phenomenon among two-dimensional transition metal dichalcogenide exciton–plasmon systems.
Optical Rotation Reversal in the Optical Response of Chiral Plasmonic Nanosystems: The Role of Plasmon Hybridization
Chirality is an important molecular property for structural analysis. Similarly, it has been shown that plasmonic chiral systems exhibit strong circular dichroism (CD) responses that can be used to determine the relative positions of their constituent plasmonic elements. Here we show that the sign of the circular dichroism spectrum in a plasmonic system can be controllably changed through small geometric perturbations that change the energetic ordering of the hybridized modes. This mechanism is distinct from geometrical changes that explicitly change the handedness of the system. In a simple system composed of two stacked L-shaped resonators we observe a reversal of the optical rotation spectral signature for small relative shifts, and we show through electromagnetic modeling and experiments on lithographically patterned samples that this is due to a rearrangement of the relative energies between modes. The plasmonic system allows for geometric perturbation along controlled directions and therefore offers more control than corresponding molecular examples. Interestingly, this strong sensitivity in the optical response encodes more spatial information into the optical spectrum, emphasizing the importance of chiral plasmonic assemblies for structural investigations on the nanoscale.
Integrating collapsible plasmonic gaps on near-field probes for polarization-resolved mapping of plasmon-enhanced emission in 2D material
Scanning near-field optical microscopy (SNOM) is an important technique used to study the optical properties of material systems at the nanoscale. In previous work, we reported on the use of nanoimprinting to improve the reproducibility and throughput of near-field probes including complicated optical antenna structures such as the ‘campanile’ probe. However, precise control over the plasmonic gap size, which determines the near-field enhancement and spatial resolution, remains a challenge. Here, we present a novel approach to fabricating a sub-20 nm plasmonic gap in a near-field plasmonic probe through the controlled collapse of imprinted nanostructures using atomic layer deposition (ALD) coatings to define the gap width. The resulting ultranarrow gap at the apex of the probe provides a strong polarization-sensitive near-field optical response, which results in an enhancement of the optical transmission in a broad wavelength range from 620 to 820 nm, enabling tip-enhanced photoluminescence (TEPL) mapping of 2-dimensional (2D) materials. We demonstrate the potential of this near-field probe by mapping a 2D exciton coupled to a linearly polarized plasmonic resonance with below 30 nm spatial resolution. This work proposes a novel approach for integrating a plasmonic antenna at the apex of the near-field probe, paving the way for the fundamental study of light-matter interactions at the nanoscale.
Plasmon-Induced Hot-Carrier Excited-State Dynamics in Plasmonic Semiconductor Nanocrystals
The variance of carrier relaxation pathways for WO 3–x plasmonic semiconductor nanocrystals (PSNCs) is monitored by transient absorption spectroscopy following excitation of the localized surface plasmon resonance (LSPR) versus the optical band gap (E g,opt ). Excitation of the LSPR leads to efficient hot carrier population above the Fermi level in WO 3–x via Landau damping, in analogy to noble metal LSPR relaxation mechanisms. Hot carrier depopulation occurs on the femtosecond timescale, observed as the concomitant recovery of an LSPR bleach with the appearance of discrete interband and intraband photoinduced absorption features. Further, by comparison, the direct excitation of E g,opt results in trion recombination at donor–acceptor sites within the WO 3–x NC, consistent with exciton decay dynamics observed for typical wide-band-gap semiconductor NCs. From the analysis of pump power dependency data, a hot-carrier electron–phonon coupling constant of 1.47 × 10 11 J K –1 s –1 cm –3 is extracted. The direct comparison of the decay dynamics following E g,opt versus LSPR excitation confirms that the observed plasmon in trioctylphosphine oxide passivated, spherical WO 3–x is a resonance state in which hot carriers are generated only from excitation on resonance with the LSPR frequency. This study on WO 3–x PSNCs provides a toolset that can be used to evaluate the role of hot carriers following LSPR excitation of n-type, plasmonic transparent conducting oxide NCs, where enhancement of photocatalysis, photovoltaic performance, and optical enhancement has been reported.
Plasmon 3D Electron Tomography and Local Electric-Field Enhancement of Engineered Plasmonic Nanoantennas
Plasmonic nano-antennas are pushing the limits of optical imaging resolution capabilities in near-field scanning optical microscopy (NSOM). Accordingly, these techniques are driving the basic understanding of photonic and optoelectronic nanoscale devices with applications in sensing, energy conversion, solid-state lighting and information technology. Imaging the localized surface plasmon resonance (LSPR) at the nanoscale is a key to understanding the optical responses of a given tip geometry in order to engineer better plasmonic nano-antennas for near-field experiments. In recent years the advancement of focused ion beam technology provides the ability to directly modify plasmonic structures with nanometer resolution. Also, scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is an established technique allowing imaging of LSPR. Specifically, the combination of these two techniques provides spectrally sensitive two-dimensional (2D) imaging information to better visualize and understand LSPR on the nanometer scale. This can be combined with electron tomography to provide the three-dimensional LSPR distribution. Here in this paper we demonstrate the fabrication of Au nano-pyramids using helium ion microscopy, and analyze the LSPR in 3D reconstructions produced by total variation (TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally, a boundary element simulation method was used to verify the experimentally observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions (PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field enhancement under optical excitation making direct comparison to NSOM experimental resolution difficult. Furthermore, the STEM-EELS results show how LSPR modes are influenced by the tip characteristics, which can inform the development of new nano-antenna designs.
Plasmon-mediated energy relaxation in graphene
Energy relaxation of hot carriers in graphene is studied at low temperatures, where the loss rate may differ significantly from that predicted for electron-phonon interactions. We show here that plasmons, important in the relaxation of energetic carriers in bulk semiconductors, can also provide a pathway for energy relaxation in transport experiments in graphene. We obtain a total loss rate to plasmons that results in energy relaxation times whose dependence on temperature and density closely matches that found experimentally.
On the Amplification of Terahertz Radiation by High-Q Resonant Plasmons in a Periodic Graphene Bilayer under Plasmon-Mode Anticrossing
The spectrum of amplification of terahertz radiation in a structured active graphene bilayer consisting of two identical periodic graphene microribbon arrays separated by a thin dielectric barrier layer is theoretically investigated. The system supports optical and acoustic plasmon modes. The resonant frequencies of the optical and acoustic modes change oppositely with the dielectric-layer thickness, which allows plasmon-mode anticrossing. It is shown that the investigated graphene structure is characterized by a strong plasmon response and giant terahertz-radiation amplification at plasma resonance frequencies in the vicinity of the anticrossing between the optical and acoustic plasmon modes at room temperature.
Aluminum plasmonic metamaterials for structural color printing
We report a structural color printing platform based on aluminum plasmonic metamaterials supporting near perfect light absorption and narrow-band spectral response tunable across the visible spectrum to realize high-resolution, angle-insensitive color printing with high color purity and saturation. Additionally, the fabricated metamaterials can be protected by a transparent polymer thin layer for ambient use with further improved color performance. The demonstrated structural color printing with aluminum plasmonic metamaterials offers great potential for relevant applications such as security marking and information storage.