Active Plasmonics Research Articles

Exploring the synergy between hot-electron dynamics and active plasmonics: A perspective

Exploring the synergy between hot-electron dynamics and active plasmonics: A perspective

Room-temperature-grown tungsten oxide hybridized dipole cavities to realize thermally tunable terahertz surface plasmons

Recent developments in the field of terahertz (THz) subwavelength structures have inspired novel discoveries and innovations involving the use of multidimensional materials in fields such as spectroscopy, photonics, biomedical imaging, nonlinearity, and communication technology. In this context, metal oxide materials with adaptable electronic and optical properties have emerged as promising candidates to realize dynamically reconfigurable THz subwavelength devices. Therefore, we experimentally demonstrated temperature-tunable surface plasmon resonances (SPRs) in an array of tungsten oxide (WO3) hybridized subwavelength cavities. By using THz time-domain spectroscopy, we examined the thermally tunable dielectric properties of a WO3 thin film over a temperature range of 25 °C–110 °C. On the basis of the tunable optoelectronic properties of WO3, we implement a THz subwavelength dipole cavity that can exhibit a temperature-dependent dynamic modification in SPR characteristics. Such tunability could be used to fabricate novel active SPR-based plasmonic devices like temperature sensors, spatial modulators, filters, and integrated THz optoelectronic elements.

A Review on Metamaterial Sensors Based on Active Plasmonic Materials

A Review on Metamaterial Sensors Based on Active Plasmonic Materials

Quantum Optical Effective-Medium Theory for Layered Metamaterials at Any Angle of Incidence

The quantum optics of metamaterials starts with the question of whether the same effective-medium theories apply as in classical optics. In general, the answer is negative. For active plasmonics but also for some passive metamaterials, we show that an additional effective-medium parameter is indispensable besides the effective index, namely, the effective noise-photon distribution. Only with the extra parameter can one predict how well the quantumness of states of light is preserved in the metamaterial. The fact that the effective index alone is not always sufficient and that one additional effective parameter suffices in the quantum optics of metamaterials is both of fundamental and practical interest. Here, from a Lagrangian description of the quantum electrodynamics of media with both linear gain and loss, we compute the effective noise-photon distribution for quantum light propagation in arbitrary directions in layered metamaterials, thereby detailing and generalizing our previous work. The effective index with its direction and polarization dependence is the same as in classical effective-medium theories. As our main result, we derive both for passive and for active media how the value of the effective noise-photon distribution too depends on the polarization and propagation directions of the light. Interestingly, for s-polarized light incident on passive metamaterials, the noise-photon distribution reduces to a thermal distribution, but for p-polarized light it does not. We illustrate the robustness of our quantum optical effective-medium theory by accurate predictions both for power spectra and for balanced homodyne detection of output quantum states of the metamaterial.

Moisture‐Driven Switching of Plasmonic Bound States in the Continuum in the Visible Region

AbstractFast and full switching of plasmonic resonances would provide a building block for integrated electro‐optically active plasmonics. To date, limited by the material properties, achieving a plasmonic resonance that can be turned fully ON and OFF in the visible region remains a formidable challenge. In this study, a nearly full optical switching based on a moisture‐driven metal‐hydrogel‐metal (MHM) metasurface at visible frequency is experimentally realized. As a result of the bound state in the continuum (BIC)‐to‐quasi‐BIC transition in the MHM, a sharp Fano‐type quasi‐BIC resonance can be switched off and back on with an ultrahigh reflectance modulation depth up to −14.6 dB within 1 s by moisture loading. Such a BIC‐to‐quasi‐BIC transition can be well mediated by engineering the coupling between the magnetic mode and surface plasmon polariton via an active control of the gap distance between the two metallic layers. Using this concept, the MHM is demonstrated as a fast‐response breathing sensor with a maximum detectable respiratory rate of up to 30 breaths per minute (bpm). These results suggest that our approach will help to realize plasmonic‐based integrated active optical devices in optical sensing and modulation.

Smart Modulators Based on Electric Field-Triggering of Surface Plasmon-Polariton for Active Plasmonics.

Design and properties of a plasmonic modulator in situ tunable by electric field are presented. Our design comprises the creation of periodic surface pattern on the surface of an elastic polymer supported by a piezo–substrate by excimer laser irradiation and subsequent selective coverage by silver by tilted angle vacuum evaporation. The structure creation was confirmed by AFM and FIB-SEM techniques. An external electric field is used for fine control of the polymer pattern amplitude, which tends to decrease with increasing voltage. As a result, surface plasmon–polariton excitation is quenched, leading to the less pronounced structure of plasmon response. This quenching was checked using UV–Vis spectroscopy and SERS measurements, and confirmed by numerical simulation. All methods prove the proposed functionality of the structures enabling the creation smart plasmonic materials for a very broad range of advanced optical applications.

All-State Switching of the Mie Resonance of Conductive Polyaniline Nanospheres.

Polyaniline (PANI), a conductive polymer, is a promising active material for optical switching. In most studies, active switching has so far been realized only between two states, whereas PANI has a total of six states. The optical properties of nanoscale PANI in all six states have remained unclear. Herein we report on all-state switching of the Mie resonance on PANI nanospheres (NSs) and active plasmon switching on PANI-coated Au nanodisks (NDs). All-state switching of differently sized PANI NSs is achieved by proton doping/dedoping and electrochemical methods. Theoretical studies show that the scattering peaks of the individual PANI NSs originate from Mie resonances. All-state switching is further demonstrated on PANI-coated circular Au NDs, where an unprecedentedly large plasmon peak shift of ∼200 nm is realized. Our study not only provides a fundamental understanding of the optical properties of PANI but also opens the probability for developing high-performance dynamic media for active plasmonics.

Active Graphene Plasmonic Tweezers: Size Based Nanoparticle Trapping and Sorting

In this article, a novel graphene based Optical tweezer (OT) and size based particle sorter is proposed and numerically investigated by finite difference time domain (FDTD) method and Maxwells stress tensor (MST) analysis. Each unit cell of the structure is composed of a graphene layer on top a metallic nano-ring which is embedded in SiO2. Strong hotspots with field enhancements reaching values as high as 140 are obtained inside the rings with no need to sharp edges conventional in plasmonic structures. Size based trapping can be achieved by controlling the fermi energy of the graphene through a gate. By cascading several unit cells, each operating at a different gate voltage, a particle size based sorter is designed which can effectively sort Polystyrene particles with their diameter in the range of smaller than 10 nm to 100 nm. The structure is illuminated by a laser beam in the mid-IR frequency range of 4-8 m and its wavelength and intensity are kept constant even for sorting functionality. Instead, tunable plasmonic characteristics of graphene is utilized for active particle sorting. The fermi level-particle size-gate voltage relationship of the proposed OT is fully investigated at the article. Moreover, it is shown that the proposed structure is capable of sorting particles according to their refractive indices as well.

High Transmission Efficiency of Opto-electronic Devices Using Active Hybrid Plasmonic Coupler

High Transmission Efficiency of Opto-electronic Devices Using Active Hybrid Plasmonic Coupler

Active Plasmonics with Responsive, Binary Assemblies of Gold Nanorods and Nanospheres.

Self-assembly of metal nanoparticles has applications in the fabrication of optically active materials. Here, we introduce a facile strategy for the fabrication of films of binary nanoparticle assemblies. Dynamic control over the configuration of gold nanorods and nanospheres is achieved via the melting of bound and unbound fractions of liquid-crystal-like nanoparticle ligands. This approach provides a route for the preparation of hierarchical nanoparticle superstructures with applications in reversibly switchable, visible-range plasmonic technologies.

Magneto-optical methods for magnetoplasmonics in noble metal nanostructures

The use of magneto-optical techniques to tune the plasmonic response of nanostructures—magnetoplasmonics—is a hot topic in active plasmonics, with fascinating implications for several plasmon-based applications and devices. In this exciting field, plasmonic nanomaterials with strong optical response to magnetic fields are desired, which is generally challenging to achieve with pure noble metals. To overcome this issue, several efforts have been carried out to design and tailor the magneto-optical response of metal nanostructures, mainly by combining plasmonic and magnetic materials or using ferromagnetic materials able to sustain a plasmonic response. However, despite their weak magneto-optical response, noble metals are a valuable model system allowing an accurate rationalization of magnetoplasmonic effects based on the interaction of magnetic fields with charge carriers. In addition, the emerging class of non-magnetic plasmonic heavily doped semiconductors is showing great potential for high performance magnetoplasmonics in the infrared range. In this Tutorial, the most common magneto-optical experimental methods employed to measure these effects are introduced, followed by a review of the major experimental observations that are discussed within the framework of an analytical model developed for the rationalization of magnetoplasmonic effects. Different materials are discussed, from noble metals to heavily doped semiconductors.

Active Graphene Plasmonics with a Drift-Current Bias

We theoretically demonstrate that a system formed by a drift-current biased graphene sheet on a silicon carbide substrate enables loss compensation and plasmon amplification. The active response of...

Topology-Changing Broadband Metamaterials Enabled by Closable Nanotrenches.

One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible substrates by mechanically closing/opening embedded nanotrenches of various geometries. When an inner bending is applied on the substrate, the nanotrench closes and the accompanying topological change results in abrupt switching of metamaterial functionalities such as resonance, chirality, and polarization selectivity. Closable nanotrenches can be embedded in metamaterials of broadband spectrum, ranging from visible to microwave. The 99.9% extinction performance is robust, enduring more than a thousand bending cycles. Our work provides a wafer-scale platform for active quantum plasmonics and photonic application of subnanometer phenomena.

Surface metallization from ab initio theory and subwavelength coupling via surface plasmon polaritons in Cu-doped lithium niobate/tantalate owing to charge accumulation

Surface metallization in Z-cut Cu-doped lithium niobate and its isomorphous lithium tantalate slabs was achieved by exhibiting metal optical properties in the visible regime using ab initio calculation. The giant opposite surface potentials were measured with sKPFM, indicating large amount of surface charges accumulated. The metallization was firstly evidenced from reflectivity enhancement under single laser beam irradiation on a Cu-doped LT slab, which is compared with the calculated reflectivity from a metallic film, seemingly amplified by energy coupling from visible surface plasmon polaritons supported by the metallic film. Since the very first reflection (VFR) is dictated by what happens in the subwavelength range, it leads us a direct way to monitoring SPPs excitation in the metallic layer. The VFR varies from 17% to 86% under two counterpropagating beams illumination, pointing clearly to a subwavelength energy coupling via SPPs. In addition, several intriguing phenomena were observed and aligned well with the SPPs picture. This work should be universal to all n-type ferroelectric oxides with photorefractive and photovoltaic effects, unveiling a missing important aspect in the past and opening an avenue towards designing novel photonic circuits, devices and active plasmonics applications in a truly nanometric scale.

Performance limitations of active plasmonic devices based on magnetic field detuning of the surface plasmon-polariton resonance in subwavelength semiconductor wires

Subwavelength plasmonic wires provide a very simple, ultracompact, and efficient platform for active plasmonics. We theoretically investigate magnetically switched transparency in subwavelength semiconductor wires. The main difficulty limiting the performance of magnetoplasmonic devices and subwavelength optical isolators is the need to use a strong magnetic field. We reveal that the required magnetic field intensity in subwavelength semiconductor wires is twice higher than that in planar semiconductor interfaces and is determined by the carrier mobility of the constituent semiconductor and does not directly depend on other material parameters such as the coating dielectric permittivity, the semiconductor carrier density, and the electron's effective mass. The azimuthal magnetic field can be excited by a direct electric current or by an optical pump through the semiconductor wires. Our finding points to the important limitations on the performance of active plasmonic devices based on magneto-optical transparency in the plasmonic wires.

Plasmons and magnetoplasmon resonances in nanorings

Plasmonic sensors based on metallic nanorings benefit from resonances covering a wide spectral range and homogeneous cavity fields. Here, we explore the potential for nanorings in active plasmonics by examining the tunability of plasmon resonances due to external magnetic fields. Within the electrostatic approximation, we compute plasmon resonances and their shifts in magnetic fields for both solid and planar nanorings. In particular, solid nanorings of circular, elliptical, and disk-shaped cross sections are critically examined and compared to planar rings. Overall, we find that magnetoplasmon shifts in nanorings are greatly reduced compared to standard nanoparticles. However, flat geometries are found to be preferable and allow for relatively large shifts.

Active Chiral Plasmonics: Flexoelectric Control of Nanoscale Chirality

The ability to electrically control the optical properties of metamaterials is an essential capability required for technological innovation. The creation of dynamic electrically tunable metamaterials in the visible and near infrared regions is important for a range of imaging and fiber optic technologies. However, current approaches require complex nanofabrication processes which are incompatible for low‐cost device production. Herein, a novel simple approach is reported for electrical control of optical properties which uses a flexoelectric dielectric element to electromechanically manipulate the form factor of a chiral nanostructure. By altering the dimensions of the chiral nanostructure, the polarization properties of light are allowed to be electrically controlled. The flexoelectric element is part of a composite metafilm that is templated onto a nanostructured polymer substrate. As the flexoelectric element does not require in situ high temperature annealing, it can be readily combined with polymer‐based substrates produced by high throughput methods. This is not the case for piezoelectric elements, routinely used in microelectromechanical (MEM) devices which require high temperature processing. Consequently, combining amorphous flexoelectric dielectrics and low‐cost polymer‐based materials provides a route to the high throughput production of electrically responsive disposable metadevices.

Electrically tunable graphene metamaterial with strong broadband absorption

The coupling system with dynamic manipulation characteristics is of great importance for the field of active plasmonics and tunable metamaterials. However, the traditional metal-based architectures suffer from a lack of electrical tunability. In this study, a metamaterial composed of perpendicular or parallel graphene-Al2O3-graphene stacks is proposed and demonstrated, which allows for the electric modulation of both graphene layers simultaneously. The resultant absorption of hybridized modes can be modulated to more than 50% by applying an external voltage, and the absorption bandwidth can reach 3.55 μm, which is 1.7 times enhanced than the counterpart of single-layer graphene. The modeling results demonstrate that the small relaxation time of graphene is of great importance to realize the broadband absorption. Moreover, the optical behaviors of the tunable metamaterial can be influenced by the incident polarization, the dielectric thickness, and especially by the Fermi energy of graphene. This work is of a crucial role in the design and fabrication of graphene-based broadband optical and optoelectronic devices.

All-Optical Emission Control and Lasing in Plasmonic Lattices

We report on reversible all-optical emission control and lasing in plasmonic nanoparticle lattices. By incorporating photochromic molecules into the liquid gain medium composed of organic fluorescent molecules, we realize all-optical control over gain and absorption, the two key parameters associated with both conventional and nanoscale lasing. We demonstrate reversible photoswitching between two distinct modes of operation: (1) spontaneous emission to the lattice mode, characterized by broad emission line width, low emission intensity, and large angular distribution; and (2) lasing action, characterized by very narrow (sub-nm) line widths due to the emergence of increased gain and temporal coherence in the system, approximately 3 orders of magnitude increase in emission intensity, and narrow 0.7° angular divergence of the beam. A rate-equation model is employed to describe the operation of the switchable plasmonic laser. Our results provide the first demonstration of optically tunable losses in plasmonic lattice lasers, which is an important milestone for the development of active plasmonics and paves the way for ultrafast all-optical switching of plasmonic nanolasers.

Electrical Dynamic Switching of Magnetic Plasmon Resonance Based on Selective Lithium Deposition.

Active plasmonic nanostructures have garnered considerable interest in physics, chemistry, and material science due to the dynamically switchable capability of plasmonic responses. Here, the first electrically dynamic control of magnetic plasmon resonance (MPR) through structure transformation by selective deposition of lithium on a metal-insulator-metal (MIM) structure is reported. Distinct optical switching between MPR and surface plasmon polariton (SPP) excitations can be enabled by applying a proper electrical current to the electrochemical cell. Furthermore, the structure transformation through lithium metal deposition indicates the reconfigurable MPR excitation in a full cycling of the charging and discharging process. The results may shed light on electrically compatible self-powered active plasmonics as well as nondestructive optical sensing for electrochemical evolution.