Electrical tuning of a quantum plasmonic resonance

Confinement-induced nonlocality and casimir force in transdimensional systems

We study within the framework of the Lifshitz theory the long-range Casimir force for in-plane isotropic and anisotropic free-standing transdimensional material slabs. In the former case, we show that the confinement-induced nonlocality not only weakens the attraction of ultrathin slabs but also changes the distance dependence of the material-dependent correction to the Casimir force to go as contrary to the ∼1/l dependence of that of the local Lifshitz force. In the latter case, we use closely packed array of parallel aligned single-wall carbon nanotubes in a dielectric layer of finite thickness to demonstrate strong orientational anisotropy and crossover behavior for the inter-slab attractive force in addition to its reduction with decreasing slab thickness. We give physical insight as to why such a pair of ultrathin slabs prefers to stick together in the perpendicularly oriented manner, rather than in the parallel relative orientation as one would customarily expect.

Ultranarrow Mid-infrared Quantum Plasmon Resonance of Self-Doped Silver Selenide Nanocrystal

The infrared quantum plasmon resonance (IR QPR) of nanocrystals (NCs) exhibits the combined properties of classical and quantum mechanics, potentially overcoming the limitations of conventional optical features. However, research on the development of localized surface plasmon resonance (LSPR) from colloidal quantum dots has stagnated, owing to the challenge of increasing the carrier density of semiconductor NCs. Herein, we present the mid-IR QPR of a self-doped Ag2Se NC with an exceptionally narrow bandwidth. Chemical modification of the NC surface with chloride realizes this narrow QPR bandwidth by achieving a high free-carrier density in the NC. The mid-IR QPR feature was thoroughly analyzed by using various experimental methods such as Fourier transform (FT) IR spectroscopy, X-ray photoelectron spectroscopy, and current-voltage measurements. In addition, the optical properties were theoretically analyzed using the plamon-in-a-box model and a modified hydrodynamic model that revealed the effect of coupling with the intraband transition and the limited nature of electron density in semiconductor NCs. Integrating the quantum effect into the plasmonic resonance reduces the peak bandwidth to 19.7 meV, which is an extremely narrow bandwidth compared with that of the LSPR of conventional metal oxide or metal chalcogenide NCs. Our results demonstrate that self-doped silver selenide quantum dots are excellent systems for studying mid-IR QPR.

Nanoscale modeling of dynamically tunable planar optical absorbers utilizing InAs and InSb in metal-oxide-semiconductor-metal configurations

The attainment of dynamic tunability in spectrally selective optical absorption has been a longstanding objective in modern optics. Typically, Fabry-Perot resonators comprising metal and semiconductor thin films have been employed for spectrally selective light absorption. In such resonators, the resonance wavelength can be altered via structural modifications. The research has progressed further with the advent of specialized patterning of thin films and the utilization of metasurfaces. Nonetheless, achieving dynamic tuning of the absorption wavelength without altering the geometry of the thin film or without resorting to lithographic fabrication still poses a challenge. In this study, the incorporation of a metal-oxide-semiconductor (MOS) architecture into the Fabry-Perot nanocavity is shown to yield dynamic spectral tuning in a perfect narrowband light absorber within the visible range. Such spectral tuning is achieved using n-type-doped indium antimonide and n-type-doped indium arsenide as semiconductors in a MOS-type structure. These semiconductors offer significant tuning of their optical properties via electrically induced carrier accumulation. The planar structure of the absorber models presented facilitates simple thin-film fabrication. With judicious material selection and appropriate bias voltage, a spectral shift of 47 nm can be achieved within the visible range, thus producing a discernible color change.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm^-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (V_G) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied V_G without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which V_G is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 10¹³ cm^-2 for electrons and 1.4 × 10¹³ cm^-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO₄, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Thickness-Dependent Drude Plasma Frequency in Transdimensional Plasmonic TiN

Plasmonic transdimensional materials (TDMs), which are atomically thin metals of precisely controlled thickness, are expected to exhibit large tailorability and dynamic tunability of their optical response as well as strong light confinement and nonlocal effects. Using spectroscopic ellipsometry, we characterize the complex permittivity of ultrathin films of passivated plasmonic titanium nitride with thicknesses ranging from 1 to 10 nm. By measuring passivated TiN, we experimentally distinguish between the contributions of an oxide layer and thickness to the optical properties. A decrease in the Drude plasma frequency and increase in the damping in thinner films is observed due to spatial confinement. We explain the experimental trends using a nonlocal Drude dielectric response theory based on the Keldysh-Rytova (KR) potential that predicts the thickness-dependent optical properties caused by electron confinement in plasmonic TDMs. Our experimental findings are consistent with the KR model and demonstrate quantum-confinement-induced optical properties in plasmonic transdimensional TiN.

Switching plasmonic nanogaps between classical and quantum regimes with supramolecular interactions

In the realm of extreme nanophotonics, nanogap plasmons support reliable field enhancements up to 1000, which provide unique opportunities to access a single molecule for strong coupling and a single atom for quantum catalysis. The quantum plasmonics are intriguing but difficult to modulate largely because of the lack of proper spacers that can reversibly actuate the sub-1-nm gaps. Here, we demonstrate that supramolecular systems made of oligoamide sequences can reversibly switch the gap plasmons of Au nanoparticles on mirror between classical and quantum tunneling regimes via supramolecular interactions. The results reveal detailed plasmon shift near the quantum tunneling limit, which fits well with both classical- and quantum-corrected models. In the quantum tunneling regime, we demonstrate that plasmonic hot electron tunneling can further blue shift the quantum plasmons because of the increased conductance in the nanogaps, making it a promising prototype of optical tunable quantum plasmonic devices.

General Framework of Canonical Quasinormal Mode Analysis for Extreme Nano-optics

Optical phenomena associated with an extremely localized field should be understood with considerations of nonlocal and quantum effects, which pose a hurdle to conceptualize the physics with a picture of eigenmodes. Here we first propose a generalized Lorentz model to describe general nonlocal media under linear mean-field approximation and formulate source-free Maxwell's equations as a linear eigenvalue problem to define the quasinormal modes. Then we introduce an orthonormalization scheme for the modes and establish a canonical quasinormal mode framework for general nonlocal media. Explicit formalisms for metals described by a quantum hydrodynamic model and polar dielectrics with nonlocal response are exemplified. The framework enables for the first time a direct modal analysis of mode transition in the quantum tunneling regime and provides physical insights beyond usual far-field spectroscopic analysis. Applied to nonlocal polar dielectrics, the framework also unveils the important roles of longitudinal phonon polaritons in optical response.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Surface plasmon induced spot and line formation at interfaces of ITO coated LiNbO3 slabs and gigantic nonlinearity

Remarkable spots and lines were clearly observed at the two interfaces of indium-tin-oxide coated Z-cut Fe-doped lithium noibate plates under illumination by milliwatt continuous-wave laser light; this occurred because of the visible surface plasmons (SPs) supported by the promising non-metal plasmonic system. The intriguing observations are here explained via the SP-strengthened nonlinear effect, through consideration of the electrostatic field (which is comparable to the atomic field) and its large gradient; this hints at a promising, highly sensitive plasmonic system. The gigantic nonlinear effect discussed in this paper should be ubiquitously existed in many oxide ferroelectric/semiconductor combinations and is promising for visible plasmonic applications.

Modulating Mechanism of the LSPR and SERS in Ag/ITO Film: Carrier Density Effect

Herein, we fabricated a uniform and dispersible Ag/indium tin oxide (ITO) cosputtered film on a two-dimensional ordered polystyrene template and observed distinct localized surface plasmon resonance (LSPR) properties that can be tuned by changing the doping level. The increase in the optical band gap is due to the variation in the metallic Ag content, which can effectively change the accumulation of free electrons in the conduction band, in addition to the near-IR absorbance. Surface-enhanced Raman scattering (SERS) was used to monitor the variations in the band gap and transfer of electrons, which causes variations in the SERS intensity. The presented research provides new insights into the relationships between the carrier density and maximum absorption wavelength, band gap distribution, and charge transfer process. This is the first study on the influence of the carrier density on the properties of Ag/ITO cosputtered films and suggests practical applications of these films.

Charge accumulation resulting in metallization of II-VI semiconductor (ZnX X = O, S, Se) films neighboring polar liquid crystal molecules and their surface plasmonic response in the visible region

The surfaces of some IIB-VI semiconductors (ZnX, X = O, S, Se) are metallized by neighboring highly polar and atomically vertically aligned (VA) liquid crystal (LC) molecules. Owing to polar catastrophe, the charge carriers swarm in an extremely thin layer and the density can achieve 4.86 × 1028 m-3 close to the LC layer, which can be regarded as a 2-dimensional electron gas (2DEG). Using density functional theory (DFT), it was found that the dielectric functions of the modified layer become negative in the visible region. This indicates the semiconductor/LC platform is an ideal active plasmonic candidate, apart from the lossy metal constituents. Experimentally, after mediation with phase gratings written in the LC system, surface plasmon polaritons (SPPs) can be excited at the semiconductor surface and localized charges are gathered in an adjacent LC layer. With the help of the enhanced static electric field from the metallic surface, significantly more 2D diffraction orders in many rows and columns and a huge energy transfer between the laser beams and SPPs was observed, which is consistent with the metallization results and the bidirectional coupling between the SPPs and incident lights. The generalization of the II-VI semiconductors means the system has great promise for use in practical applications owing to the ultra-low loss. The novel insights regarding this combination with liquid crystals will be beneficial for real-time holographic displays and the study of tunable epsilon near zero points.

Ultra-high sensitivity sensing based on ultraviolet plasmonic enhancements in semiconductor triangular prism meta-antenna systems

Silicon (Si), germanium (Ge), and gallium arsenide (GaAs) are familiar semiconductors that always act in the role of optical dielectrics. However, these semiconductors also have plasmonic behaviors in ultraviolet (UV) ranges due to the strong interband transitions or valence electrons. And few studies are aimed at investigating plasmonic properties in the semiconductor at the nanoscale. In this work, we discuss UV plasmonics and sensing properties in single and dimer Si, Ge, and GaAs triangular prism meta-antenna systems. The results show that obvious local surface plasmon resonances (LSPRs) can be realized in the proposed triangular prism meta-antennas, and the resonant wavelength, electromagnetic field distribution, surface charge distribution, and surface current density can be effectively tuned by structural and material parameters. In addition, we also find that the Si triangular prism meta-antenna shows more intense plasmonic responses in UV ranges than that in the Ge or GaAs triangular prism nanostructures. Especially, the phase difference between the triangular prism nanostructure and light source can effectively regulate the symbol and value of the surface charge. Moreover, the great enhancement of electric field can be seen in the dimer triangular prism meta-antennas when the distance of the gap is g<5 nm, especially g=1 nm. The most interesting result is that the maximum of refractive index sensitivity s and figure of merit (FoM) are greatly enlarged in dimer triangular prism meta-antennas. Particularly, the sensitivity can reach up to 215 nm/RIU in the dimer GaAs triangular prism meta-antennas, which is improved more than one order of magnitude. These research results may play important roles in applications of the photo detecting, plasmonic sensing and disinfecting in UV ranges.

Hybrid Plasmonic Fiber-Optic Sensors

With the increasing demand of achieving comprehensive perception in every aspect of life, optical fibers have shown great potential in various applications due to their highly-sensitive, highly-integrated, flexible and real-time sensing capabilities. Among various sensing mechanisms, plasmonics based fiber-optic sensors provide remarkable sensitivity benefiting from their outstanding plasmon-matter interaction. Therefore, surface plasmon resonance (SPR) and localized SPR (LSPR)-based hybrid fiber-optic sensors have captured intensive research attention. Conventionally, SPR- or LSPR-based hybrid fiber-optic sensors rely on the resonant electron oscillations of thin metallic films or metallic nanoparticles functionalized on fiber surfaces. Coupled with the new advances in functional nanomaterials as well as fiber structure design and fabrication in recent years, new solutions continue to emerge to further improve the fiber-optic plasmonic sensors' performances in terms of sensitivity, specificity and biocompatibility. For instance, 2D materials like graphene can enhance the surface plasmon intensity at the metallic film surface due to the plasmon-matter interaction. Two-dimensional (2D) morphology of transition metal oxides can be doped with abundant free electrons to facilitate intrinsic plasmonics in visible or near-infrared frequencies, realizing exceptional field confinement and high sensitivity detection of analyte molecules. Gold nanoparticles capped with macrocyclic supramolecules show excellent selectivity to target biomolecules and ultralow limits of detection. Moreover, specially designed microstructured optical fibers are able to achieve high birefringence that can suppress the output inaccuracy induced by polarization crosstalk and meanwhile deliver promising sensitivity. This review aims to reveal and explore the frontiers of such hybrid plasmonic fiber-optic platforms in various sensing applications.

Nanoantenna Structure with Mid-Infrared Plasmonic Niobium-Doped Titanium Oxide

Among conductive oxide materials, niobium doped titanium dioxide has recently emerged as a stimulating and promising contestant for numerous applications. With carrier concentration tunability, high thermal stability, mechanical and environmental robustness, this is a material-of-choice for infrared plasmonics, which can substitute indium tin oxide (ITO). In this report, to illustrate great advantages of this material, we describe successful fabrication and characterization of niobium doped titanium oxide nanoantenna arrays aiming at surface-enhanced infrared absorption spectroscopy. The niobium doped titanium oxide film was deposited with co-sputtering method. Then the nanopatterned arrays were prepared by electron beam lithography combined with plasma etching and oxygen plasma ashing processes. The relative transmittance of the nanostrip and nanodisk antenna arrays was evaluated with Fourier transform infrared spectroscopy. Polarization dependence of surface plasmon resonances on incident light was examined confirming good agreements with calculations. Simulated spectra also present red-shift as length, width or diameter of the nanostructures increase, as predicted by classical antenna theory.

Interfacial Defect-Mediated Near-Infrared Silicon Photodetection with Metal Oxides

Due to recent breakthroughs in silicon photonics, sub-band-gap photodetection in silicon (Si) has become vital to the development of next-generation integrated photonic devices for telecommunication systems. In particular, photodetection in Si using complementary metal-oxide semiconductor (CMOS) compatible materials is in high demand for cost-effective integration. Here, we achieve broad-band near-infrared photodetection in Si/metal-oxide Schottky junctions where the photocurrent is generated from interface defects induced by aluminum-doped zinc oxide (AZO) films deposited on a Si substrate. The combination of photoexcited carrier generation from both interface defect states and intrinsic Si bulk defect states contributes to a photoresponse of 1 mA/W at 1325 nm and 0.22 mA/W at 1550 nm with zero-biasing. From a fit to the Fowler equation for photoemission, we quantitatively determine the individual contributions from these effects. Finally, using this analysis, we demonstrate a gold-nanoparticle-coated photodiode that has three distinct photocurrent responses resulting from hot carriers in the gold, interface defects from the AZO, and bulk defects within the Si. The hot carrier response is found to dominate near the band gap of Si, while the interface defects dominate for longer wavelengths.

Ultraviolet Interband Plasmonics With Si Nanostructures

Although Si acts as an electrical semiconductor, it has properties of an optical dielectric. Here, we revisit the behavior of Si as a plasmonic metal. This behavior was previously shown to arise from strong interband transitions that lead to negative permittivity of Si across the ultraviolet spectral range. However, few have studied the plasmonic characteristics of Si, particularly in its nanostructures. In this paper, we report localized plasmon resonances of Si nanostructures and the observation of plasmon hybridization in the UV (∼250 nm wavelength). In addition, simulation results show that Si nanodisk dimers can achieve a local intensity enhancement greater than ∼500-fold in a 1 nm gap. Lastly, we investigate hybrid Si-Al nanostructures to achieve sharp resonances in the UV, due to the coupling between plasmon resonances supported by Si and Al nanostructures. These results will have potential applications in the UV range, such as nanostructured devices for spectral filtering, plasmon-enhanced Si photodetectors, interrogation of molecular chirality, and catalysis. It could have significant impact on UV photolithography on patterned Si structures.

Dynamic Tuning of Gap Plasmon Resonances Using a Solid-State Electrochromic Device

Plasmonic antennas and metasurfaces can effectively control light-matter interactions, and this facilitates a deterministic design of optical materials properties, including structural color. However, these optical properties are generally fixed after synthesis and fabrication, while many modern-day optics applications require active, low-power, and nonvolatile tuning. These needs have spurred broad research activities aimed at identifying materials and resonant structures capable of achieving large, dynamic changes in optical properties, especially in the challenging visible spectral range. In this work, we demonstrate dynamic tuning of polarization-dependent gap plasmon resonators that contain the electrochromic oxide WO₃. Its refractive index in the visible changes continuously from n = 2.1 to 1.9 upon electrochemical lithium insertion and removal in a solid-state device. By incorporating WO₃ into a gap plasmon resonator, the resonant wavelength can be shifted continuously and reversibly by up to 58 nm with less than 2 V electrochemical bias voltage. The resonator can remain in a tuned state for tens of minutes under open circuit conditions.

Electro-plasmonic nanoantenna: A nonfluorescent optical probe for ultrasensitive label-free detection of electrophysiological signals

Harnessing the unprecedented spatiotemporal resolution capability of light to detect electrophysiological signals has been the goal of scientists for nearly 50 years. Yet, progress toward that goal remains elusive due to lack of electro-optic translators that can efficiently convert electrical activity to high photon count optical signals. Here, we introduce an ultrasensitive and extremely bright nanoscale electric-field probe overcoming the low photon count limitations of existing optical field reporters. Our electro-plasmonic nanoantennas with drastically enhanced cross sections (~10⁴ nm² compared to typical values of ~10^-2 nm² for voltage-sensitive fluorescence dyes and ~1 nm² for quantum dots) offer reliable detection of local electric-field dynamics with remarkably high sensitivities and signal-to-shot noise ratios (~60 to 220) from diffraction-limited spots. In our electro-optics experiments, we demonstrate high-temporal resolution electric-field measurements at kilohertz frequencies and achieved label-free optical recording of network-level electrogenic activity of cardiomyocyte cells with low-intensity light (11 mW/mm²).

High-contrast optical switching using an epsilon-near-zero material coupled to a Bragg microcavity

Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constructing all-optical and electro-optical switches in the infrared. The performance of previously reported ENZ material-based optical switches, however, has been greatly hampered by the low quality- (Q-) factor of the ENZ cavity, resulting in a large required optical pump fluence or applied voltage, a large insertion loss, or a small modulation depth. Here, we propose a solution by integrating the ENZ material into a Bragg microcavity, such that the Q-factor of the coupled cavity can be dramatically enhanced. Using high-mobility Dysprosium-doped cadmium oxide (CdO) as the prototype ENZ material, we numerically show an infrared all-optical switch with its reflectance modulated from near-zero to 94% under a pump fluence of only 7 μJ cm^-2, about a 59-time-reduction compared with a state-of-the-art Berreman-type cavity. Moreover, the high-Q coupled cavity can also be adopted to realize a reflective electro-optical switch. Its reflectance can be switched from near-zero to 89%, with a bias electric field well below the breakdown field of conventional gate dielectrics. The switching operation can further be extended to the transmission mode with a slightly modified cavity geometry, with its absolute transmittance modulated by 40%.

Gate-tunable optical filter based on conducting oxide metasurface heterostructure

A gate-tunable plasmonic optical filter incorporating a subwavelength patterned metal-insulator-metal metasurface heterostructure is proposed. An additional thin transparent conducting oxide (TCO) layer is embedded in the insulator layer to form a double metal-oxide-semiconductor configuration. Heavily n-doped indium tin oxide (ITO) is employed as the TCO material, whose optical property can be electrically tuned by the formation of a thin active epsilon-near-zero layer at the ITO-oxide interfaces. Full-wave electromagnetic simulations show that amplitude modulation and shift of transmission peak are achievable with 3-5 V applied bias, depending on the application. Moreover, the modulation strength and transmission peak shift increase with a thinner ITO layer. This work is an essential step toward a realization of next-generation compact photonic/plasmonic integrated devices.

Extreme nanophotonics from ultrathin metallic gaps

Ultrathin dielectric gaps between metals can trap plasmonic optical modes with surprisingly low loss and with volumes below 1 nm³. We review the origin and subtle properties of these modes, and show how they can be well accounted for by simple models. Particularly important is the mixing between radiating antennas and confined nanogap modes, which is extremely sensitive to precise nanogeometry, right down to the single-atom level. Coupling nanogap plasmons to electronic and vibronic transitions yields a host of phenomena including single-molecule strong coupling and molecular optomechanics, opening access to atomic-scale chemistry and materials science, as well as quantum metamaterials. Ultimate low-energy devices such as robust bottom-up assembled single-atom switches are thus in prospect.

Stable and tunable plasmon resonance of molybdenum oxide nanosheets from the ultraviolet to the near-infrared region for ultrasensitive surface-enhanced Raman analysis

Preparation of color-tunable and stable plasmonic MoO₃ nanomaterials remains challenging, due to the lack of an effective preparation strategy and surface protection in heavily doped MoO₃. Herein, we report a facile and reliable method for synthesis of oxygen-deficient MoO₃ (MoO_3-x ) nanosheets using dopamine as the reducing agent and precursor for the formation of a polydopamine (PDA) surface coating. The PDA-coated MoO_3-x nanosheets show stable and tunable localized surface plasmon resonance (LSPR) from the ultraviolet to the near-infrared region (361-809 nm) via altering the pH value of the medium, accompanying the generation of multicolor nanosheet dispersions, such as deep blue, faint bluish, orange, yellow and black. Importantly, the resulting PDA-coated MoO_3-x nanosheets are quite stable even in the presence of oxidants, and they can be used as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate. The limit of detection for rhodamine 6G (R6G) dye is down to 0.3 fM concentration, and the corresponding Raman enhancement factor reaches 1 × 10¹⁰. The coupling of charge transfer between R6G and PDA-coated MoO_3-x nanosheets and molecular resonances may be responsible for the strong SERS effect.

Sb2Te3 topological insulator: surface plasmon resonance and application in refractive index monitoring

Topological insulators as new emerging building blocks in electronics and photonics present promising prospects for exciting surface plasmons and enhancing light-matter interaction. Thus, exploring the visible-range plasmonic response of topological insulators is significant to reveal their optical characteristics and broaden their applications at high frequencies. Herein, we report the experimental demonstration of a visible-range surface plasmon resonance (SPR) effect on an antimony telluride (Sb2Te3) topological insulator film. The results show that the SPR can be excited with a relatively small incident angle in the Kretschmann configuration based on the Sb2Te3 film. Especially, we develop an impactful digital holographic imaging system based on the topological insulator SPR and realize the dynamic monitoring of refractive index variation. Compared with the traditional SPR, the Sb2Te3-based SPR possesses a broader measurement range. Our findings open a new avenue for exploring the optical physics and practical applications of topological insulators, such as environmental and biochemical sensing.

Active control of plasmonic colors: emerging display technologies

In recent years there has been a growing interest in the use of plasmonic nanostructures for color generation, a technology that dates back to ancient times. Plasmonic structural colors have several attractive features but once the structures are prepared the colors are normally fixed. Lately, several concepts have emerged for actively tuning the colors, which opens up for many new potential applications, the most obvious being novel color displays. In this review we summarize recent progress in active control of plasmonic colors and evaluate them with respect to performance criteria for color displays. It is suggested that actively controlled plasmonic colors are generally less interesting for emissive displays but could be useful for new types of electrochromic devices relying on ambient light (electronic paper). Furthermore, there are several other potential applications such as images to be revealed on demand and colorimetric sensors.

Facile fabrication of configuration controllable self-assembled Al nanostructures as UV SERS substrates

The resonantly enhanced near-surface electromagnetic field of Al nanostructures (NSs) excited with UV light offers an effective and precise approach to obtain vibration information in complex mixtures up to a single-molecule magnitude. In this work, we propose a facile, cost-effective and large-scale way to fabricate self-assembled Al NSs with systematically controllable configuration and size for the preparation of SERS substrates. The stalagmite-shaped Al nanoparticles (NPs) are directly utilized based on the Volmer-Weber model and drastically develop as a function of the annealing temperature with tunable optical properties, resulting in a maximum enhancement factor of the vibrational Raman signal from adenine molecules up to 3.49 × 10⁷. With a variation of the deposition thickness at an identical temperature, the self-assembled Al NSs sensitively evolve from stalagmite-shaped NPs to worm-like nano-mounds with increased light consumption induced by scattering, which inherently leads to a noticeable reduction in the EF due to the reduced plasmon coupling between NSs with the sparse distribution. The enhancement effect of the SERS substrates is theoretically discussed with the FDTD simulation, providing an instructive reference for promising applications in the bio-sensing technique.

Dynamic thermal emission control with InAs-based plasmonic metasurfaces

Thermal emission from objects tends to be spectrally broadband, unpolarized, and temporally invariant. These common notions are now challenged with the emergence of new nanophotonic structures and concepts that afford on-demand, active manipulation of the thermal emission process. This opens a myriad of new applications in chemistry, health care, thermal management, imaging, sensing, and spectroscopy. Here, we theoretically propose and experimentally demonstrate a new approach to actively tailor thermal emission with a reflective, plasmonic metasurface in which the active material and reflector element are epitaxially grown, high-carrier-mobility InAs layers. Electrical gating induces changes in the charge carrier density of the active InAs layer that are translated into large changes in the optical absorption and thermal emission from metasurface. We demonstrate polarization-dependent and electrically controlled emissivity changes of 3.6%P (6.5% in relative scale) in the mid-infrared spectral range.

Communication: Nickel hydroxide as an exceptional deviation from the quantum size effect

The quantum size effect is a well-known fundamental scientific phenomenon. Due to quantum confinement, downscaling a system to small sizes should increase the bandgap of a solid state material. However, in this work, we present an exception: monolayers of nickel hydroxide have smaller bandgaps than their bulk analogues, due to the surface states appearing at energies within the bandgap region. Our findings are obtained by several state-of-the-art first principles calculations.

Interface-induced nucleation and growth: a new route for fabricating ordered silver nanohole arrays

Metal nanohole arrays exhibit fascinating optical properties originating from the excitation of surface plasmons, and have been demonstrated to be of great potential in many applications. However, the fabrication of large-area ordered metal nanohole arrays with a tunable optical response is still highly desired. Herein, a novel interface-induced vapor phase growth method is developed to achieve hexagonally arranged silver nanohole arrays with a centimeter-scale area, in which an interface is introduced via an ordered template and used to induce Ag selective nucleation and growth. The adhesive force of the template with the substrate is found to be crucial in the determination of the nucleation sites and the resulting nanostructures. The plasmonic responses of the nanohole arrays are regulated by controlling their structural features, which are realized through simply changing the template parameters and the Ag deposition thickness. The Ag nanohole array exhibits more than 20-fold Raman enhancement compared to a rough Ag film when its localized surface plasmon resonance (LSPR) is tuned to an optimized range, which indicates its potential in biochemical sensing applications. The present method for the preparation of large-area metal nanohole arrays may open up a new avenue to fabricate novel metal nanostructures and develop high-performance plasmonic devices.

Atomic Scale Photodetection Enabled by a Memristive Junction

The optical control of atomic relocations in a metallic quantum point contact is of great interest because it addresses the fundamental limit of "CMOS scaling". Here, by developing a platform for combined electronics and photonics on the atomic scale, we demonstrate an optically controlled electronic switch based on the relocation of atoms. It is shown through experiments and simulations how the interplay between electrical, optical, and light-induced thermal forces can reversibly relocate a few atoms and enable atomic photodetection with a digital electronic response, a high resistance extinction ratio (70 dB), and a low OFF-state current (10 pA) at room temperature. Additionally, the device introduced here displays an optically induced pinched hysteretic current (optical memristor). The photodetector has been tested in an experiment with real optical data at 0.5 Gbit/s, from which an eye diagram visualizing millions of detection cycles could be produced. This demonstrates the durability of the realized atomic scale devices and establishes them as alternatives to traditional photodetectors.

Subwavelength-spaced transmissive metallic slits for 360-degree phase control by using transparent conducting oxides

We propose an apparatus that allows for active control of the transmission phase up to 360° through subwavelength-spaced metallic slits partially filled with indium tin oxide (ITO). Incident light is coupled to the guided mode in the metallic slit at one side. After going through the slit with a certain length, light is coupled out to free space at the other side. The transmission phase is governed by the mode index and the slit length. By applying bias to the ITO in the metallic slit, it is possible to control the mode index, which in turn leads to tuning of the transmission phase. The judiciously designed slit configuration facilitates the individual control of the relative phase between the neighboring slit with a subwavelength distance. This phenomenon is different from resonance-based metasurface approaches that suffer from limited range of the phase change. It is believed that the devised configuration may open novel promising future applications such as hologram imaging with phase spatial light modulators, light-field infrared microscopy, and beam forming and steering devices.

Reveal and Control of Chiral Cathodoluminescence at Subnanoscale

Circularly polarized light is crucial for the modern physics research. Highly integrated nanophotonic device further requires the control of circularly polarized light at subnanoscale. Here, we report the tuning of chiral cathodoluminescence (CL) on single Au nanostructure under electron stimulation. The detected CL helicity is found ultrasensitive with the electron impinging position on the structure, and a helicity switch is achieved within a 1.86 nm electron-beam movement, which is applied to construct ternary notation sequence. The proposed configuration provides a delicate platform for the CL helicity control, which opens a way for the future chiral applications at subnanoscale like information coding and quantum communication.