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Reconfigurable devices offer the ability to program electronic circuits on demand. In this work, we demonstrated on-demand creation of artificial neurons, synapses, and memory capacitors in post-fabricated perovskite NdNiO3devices that can be simply reconfigured for a specific purpose by single-shot electric pulses. The sensitivity of electronic properties of perovskite nickelates to the local distribution of hydrogen ions enabled these results. With experimental data from our memory capacitors, simulation results of a reservoir computing framework showed excellent performance for tasks such as digit recognition and classification of electrocardiogram heartbeat activity. Using our reconfigurable artificial neurons and synapses, simulated dynamic networks outperformed static networks for incremental learning scenarios. The ability to fashion the building blocks of brain-inspired computers on demand opens up new directions in adaptive networks.

The topological features of optical vortices have been opening opportunities for free-space and on-chip photonic technologies, e.g., for multiplexed optical communications and robust information transport. In a parallel but disjoint effort, polar anisotropic van der Waals nanomaterials supporting hyperbolic phonon polaritons (HP2s) have been leveraged to drastically boost light-matter interactions. So far HP2 studies have been mainly focusing on the control of their amplitude and scale features. Here we report the generation and observation of mid-infrared hyperbolic polariton vortices (HP2Vs) associated with reconfigurable topological charges. Spiral-shaped gold disks coated with a flake of hexagonal boron nitride are exploited to tailor spin–orbit interactions and realise deeply subwavelength HP2Vs. The complex interplay between excitation spin, spiral geometry and HP2 dispersion enables robust reconfigurability of the associated topological charges. Our results reveal unique opportunities to extend the application of HP2s into topological photonics, quantum information processing by integrating these phenomena with single-photon emitters, robust on-chip optical applications, sensing and nanoparticle manipulation.

Infrared dielectric properties of muscovite mica, one of the first van der Waals crystals, exfoliated on silicon and SiO2 substrates is studied using near-field nano-FTIR spectroscopy. The spectra ...

Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO3), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide SmNiO3 (SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale real-time reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with unmatched resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering as a new paradigm for reconfigurable nanophotonic applications facilitated by a hybrid material platform exploiting extreme light-matter interactions in polaritonic systems.

Habituation and sensitization (nonassociative learning) are among the most fundamental forms of learning and memory behavior present in organisms that enable adaptation and learning in dynamic environments. Emulating such features of intelligence found in nature in the solid state can serve as inspiration for algorithmic simulations in artificial neural networks and potential use in neuromorphic computing. Here, we demonstrate nonassociative learning with a prototypical Mott insulator, nickel oxide (NiO), under a variety of external stimuli at and above room temperature. Similar to biological species such as Aplysia, habituation and sensitization of NiO possess time-dependent plasticity relying on both strength and time interval between stimuli. A combination of experimental approaches and first-principles calculations reveals that such learning behavior of NiO results from dynamic modulation of its defect and electronic structure. An artificial neural network model inspired by such nonassociative learning is simulated to show advantages for an unsupervised clustering task in accuracy and reducing catastrophic interference, which could help mitigate the stability–plasticity dilemma. Mott insulators can therefore serve as building blocks to examine learning behavior noted in biology and inspire new learning algorithms for artificial intelligence.

Single-layer heterostructures exhibit striking quasiparticle properties and many-body interaction effects that hold promise for a range of applications. However, their properties can be altered by intrinsic and extrinsic defects, thus diminishing their applicability. Therefore, it is of paramount importance to identify defects and understand 2D materials' degradation over time using advanced multimodal imaging techniques. Here we implemented a liquid-phase precursor approach to synthesize 2D in-plane MoS

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality., Here, the authors demonstrate that the dispersion of hyperbolic phonon polaritons can be controlled using the permittivity changes inherent in the different phases of phase change materials. This enables direct launching, reflection, transmission and refraction at the phase change material domain boundaries.

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO 3 (Li-SNO) contains a large amount of mobile Li + located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li + conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na + . The results highlight the potential of quantum materials and emergent physics in design of ion conductors.

Recently emerged layered transition metal dichalcogenides have attracted great interest due to their intriguing fundamental physical properties and potential applications in optoelectronics. Using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical modeling, we study propagating surface waves in the visible spectral range that are excited at sharp edges of layered transition metal dichalcogenides (TMDC) such as molybdenum disulfide and tungsten diselenide. These surface waves form fringes in s-SNOM measurements. By measuring how the fringes change when the sample is rotated with respect to the incident beam, we obtain evidence that exfoliated MoS2 on a silicon substrate supports two types of Zenneck surface waves that are predicted to exist in materials with large real and imaginary parts of the permittivity. We have compared MoS2 interference fringes with those formed on layered insulator such as hexagonal boron nitride where only leaky modes are possible due to its small permittivity. Interpretation of experimental data is supported by theoretical models. Our results could pave the way to the investigation of surface waves on TMDCs and other van der Waals materials and their novel photonics applications.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO(3) (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO(3) can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

Terahertz (THz) near-field microscopy has wide and unprecedented application potential for nanoscale materials and photonic-device characterization. Here, we introduce hyperspectral THz nano-imaging by combining scattering-type scanning near-field optical microscopy (s-SNOM) with THz time-domain spectroscopy (TDS). We describe the technical implementations that enabled this achievement and demonstrate its performance with a heterogeneously doped Si semiconductor sample. Specifically, we recorded a hyperspectral image of 40 by 20 pixels in 180 minutes and with a spatial resolution of about ~170 nm by measuring at each pixel with a time domain spectrum covering the range from 0.4 to 1.8 THz. Fitting the spectra with a Drude model allows for measuring-noninvasively and without the need for Ohmic contacts-the local mobile carrier concentration of the differently doped Si areas. We envision wide application potential for THz hyperspectral nano-imaging, including nanoscale carrier profiling of industrial semiconductor structures or characterizing complex and correlated electron matter, as well as low dimensional (1D or 2D) materials.

We employ nano-scale imaging and spectroscopy techniques to elucidate polariton characteristics as a function of the complex refractive index of the substrate. Although higher-order polariton modes exhibit wavelengths weakly sensitive to the environment, the principal mode strongly depends on the substrate dielectric constant.

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, all recent designs have exhibited concepts using geometrically fixed structures, or used materials with excessive propagation losses, thereby limiting potential applications. Here we show how to overcome these limitations using a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with a phase-change material (PCM), single crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment to tune the wavelength of hyperbolic phonon polaritons (HPhPs) supported in hBN by a factor of 1.6. This contrasts with earlier work using surface phonon polaritons, where propagation could only be observed above a low-loss dielectric phase. We demonstrate the first realization of in-plane HPhP refraction, which obeys Snell’s law and the means for launching, reflecting and transmitting HPhPs at the PCM domain boundaries. To demonstrate practical applications of this platform, we show how hBN could be combined with either VO2 or GeSbTe glasses to make refractive nanophotonic waveguides and lenses. This approach offers control of in-plane HPhP propagation at the nanoscale and exemplifies a reconfigurable framework combining hyperbolic media and PCMs to design new optical functionalities including resonant cavities, beam steering and waveguiding.

Hyperbolic phonon polaritons (HPhPs) are generated when infrared photons couple to polar optic phonons in anisotropic media, confining long-wavelength light to nanoscale volumes. However, to realize the full potential of HPhPs for infrared optics, it is crucial to understand propagation and loss mechanisms on substrates suitable for applications from waveguiding to infrared sensing. In this paper, we employ scattering-type scanning near-field optical microscopy (s-SNOM) and nano-Fourier transform infrared (FTIR) spectroscopy, in concert with analytical and numerical calculations, to elucidate HPhP characteristics as a function of the complex substrate dielectric function. We consider propagation on suspended, dielectric and metallic substrates to demonstrate that the thickness-normalized wavevector can be reduced by a factor of 25 simply by changing the substrate from dielectric to metallic behavior. Moreover, by incorporating the imaginary contribution to the dielectric function in lossy materials, the wavevector can be dynamically controlled by small local variations in loss or carrier density. Such effects may therefore be used to spatially separate hyperbolic modes of different orders, and indicates that for index-based sensing schemes that HPhPs can be more sensitive than surface polaritons in the thin film limit. Our results advance our understanding of fundamental polariton excitations and their potential for on-chip photonics and planar metasurface optics.

Bulk growth methods have made it possible to synthesize several allotropes of phosphorus such as black, white, red, and violet phosphorus. However, unlike exfoliated black phosphorus (BP), which has been extensively studied, much of the optoelectronic properties and stability of the other allotropes have yet to be comprehensively investigated. Here, we study the chemical degradation of exfoliated violet phosphorus (VP) in comparison to BP under ambient conditions using nanoscale infrared spectroscopy and imaging. We identify oxidized phosphorus species that result from chemical reaction processes on the surfaces of these phosphorus allotropes. We have found that VP exhibits a noticeably different and slower degradation process when compared to BP, establishing it as the more stable of the two allotropes. A better understanding of the stability of VP could lead to the further fundamental study of its monolayer form for potential future applications.

Phase separations in ternary/multinary semiconductor alloys is a major challenge that limits optical and electronic internal device efficiency. We have found ubiquitous local phase separation in In1-xGaxN alloys that persists to nanoscale spatial extent by employing high-resolution nanoimaging technique. We lithographically patterned InN/sapphire substrates with nanolayers of In1-xGaxN down to few atomic layers thick that enabled us to calibrate the near-field infrared response of the semiconductor nanolayers as a function of composition and thickness. We also developed an advanced theoretical approach that considers the full geometry of the probe tip and all the sample and substrate layers. Combining experiment and theory, we identified and quantified phase separation in epitaxially grown individual nanoalloys. We found that the scale of the phase separation varies widely from particle to particle ranging from all Ga- to all In-rich regions and covering everything in between. We have found that between 20 and 25% of particles show some level of Ga-rich phase separation over the entire sample region, which is in qualitative agreement with the known phase diagram of In1-xGaxN system.

We demonstrate hyperspectral imaging of the charge carrier profiles of doped precharacterized SRAM arrays in the THz frequency range 13 cm−1 –60 cm−1 using a s-SNOM. We are able to map free carrier concentration in range of 1016 cm−3 –1020 cm−3 with broadband THz source.

Utilizing scattering-type scanning near-field optical microscopy (s-SNOM) with electrical read-out, we have demonstrated the ability of mapping optical and electronic properties of MoS 2 -based devices at length scales of tens of nanometers.

Muscovite type mica is an inorganic material most commonly used in various electronic devices. We use the near-field imaging and nano-FTIR techniques to investigate the properties of mica exfoliated on different substrates. In order to support experimental results, we use a theoretical model and simulation.

Phonon polaritons in hexagonal boron nitride (hBN) reflect and refract at phase domains in underlying vanadium dioxide (VO2) crystals, a potential reconfigurable metasurface. Also, hBN slows the VO2 phase transition via thermal or strain effects.

Enveloped viruses, such as HIV, Ebola and Influenza, are among the most deadly known viruses. Cellular membrane penetration of enveloped viruses is a critical step in the cascade of events that lead to entry into the host cell. Conventional ensemble fusion assays rely on collective responses to membrane fusion events, and do not allow direct and quantitative studies of the subtle and intricate fusion details. Such details are accessible via single particle investigation techniques, however. Here, we implement nano-infrared spectroscopic imaging to investigate the chemical and structural modifications that occur prior to membrane fusion in the single archetypal enveloped virus, influenza X31. We traced in real-space structural and spectroscopic alterations that occur during environmental pH variations in single virus particles. In addition, using nanospectroscopic imaging we quantified the effectiveness of an antiviral compound in stopping viral membrane disruption (a novel mechanism for inhibiting viral entry into cells) during environmental pH variations.

We have studied the formation of near-field fringes when sharp edges of materials are imaged using scattering-type scanning near-field optical microscope (s-SNOM). The materials we have investigated include dielectrics, metals, a near-perfect conductor, and those that possess anisotropic permittivity and hyperbolic dispersion. For our theoretical analysis, we use a technique that combines full-wave numerical simulations of tip-sample near-field interaction and signal demodulation at higher orders akin to what is done in typical s-SNOM experiments. Unlike previous tip-sample interaction near-field models, our advanced technique allows simulation of the realistic tip and sample structure. Our analysis clarifies edge imaging of recently emerged layered materials such as hexagonal boron nitride and transition metal dichalcogenides (in particular, molybdenum disulfide), as well as traditional plasmonic materials such as gold. Hexagonal boron nitride is studied at several wavelengths, including the wavelength where it possesses excitation of phonon-polaritons and hyperbolic dispersion. Based on our results of s-SNOM imaging in different demodulation orders, we specify resonant and non-resonant types of edges and describe the edge fringes for each case. We clarify near-field edge-fringe formation at material sharp boundaries, both outside bright fringes and the low-contrast region at the edge, and elaborate on the necessity of separating them from propagating waves on the surface of polaritonic materials.

Black phosphorus (BP) possesses several extraordinary physical properties, which include in-plane anisotropy, thickness dependent direct bandgap and high carrier mobility. These physical properties make BP highly desirable from the point of view of fundamental science and modern optoelectronics applications. The excitement about this material has always been accompanied by unreserved skepticism due to its extraordinary degradation under ambient conditions. Here we show ambient degradation of exfoliated BP can be effectively suppressed using thin layer of hybrid metal organic chemical vapor deposition coating of boron nitride (BN) followed by atomic layer deposition coating of Al2O3. We have extensively studied the time dependent surface, optical and electrical properties of BP encapsulated by BN and/or Al2O3 using nanoscale infrared imaging and I–V characterizations. Our results show hybrid thin layer (~5 nm) BN/Al2O3 coated BP exfoliated on SiO2 substrate is protected from degradation in ambient for over 6 months, much longer than those coated only by BN or Al2O3 layers. Our theoretical modeling of the experimental degradation growth pattern shows that the influence of neighboring elements on the degradation of a given element is minimal for BP flakes with hybrid coating. Electrical characterization further confirms the effectiveness of BN/Al2O3 as encapsulation layer and gate dielectrics with minor changes after several weeks.

The appearance of stripe phases is a characteristic signature of strongly correlated quantum materials, and its origin in phase-changing materials has only recently been recognized as the result of the delicate balance between atomic and mesoscopic materials properties. A vanadium dioxide (VO2) single crystal is one such strongly correlated material with stripe phases. Infrared nano-imaging on low-aspect-ratio, single-crystal VO2 microbeams decorated with resonant plasmonic nanoantennas reveals a novel herringbone pattern of coexisting metallic and insulating domains intercepted and altered by ferroelastic domains, unlike previous reports on high-aspect-ratio VO2 crystals where the coexisting metal/insulator domains appear as alternating stripe phases perpendicular to the growth axis. The metallic domains nucleate below the crystal surface and grow towards the surface with increasing temperature as suggested by the near-field plasmonic response of the gold nanorod antennas.

Group III-V semiconductor nanostructures have been at the forefront of numerous applications in high-power, high frequency optical and optoelectronic devices. Although, significant progress has been made in fabrication and characterization of these materials, there are still challenges in the formation of compositional uniform indium-rich ternary epilayers, embedded in wide bandgap III-N’s. For example, nanoscale lateral compositional inhomogeneities at the growth surface lead to bulk phase segregations will reduce the structural quality of the semiconductor heterostructures both in macro and nanometer scales if not controlled through the process parameter space at the surface. Studying and understanding the fundamental physical and structural properties at the nanoscale level and correlating the findings with processing parameters is essential to mitigate compositional fluctuations in multinary III-N compounds. In this work we introduce infrared scattering type scanning near-field microscopy (s-SNOM) for spectroscopic study of nanoscale optical properties of InGaN epilayers on GaN- or InN templates. S-SNOM possesses spatial resolution of few nanometers (~15 nm) far below the diffraction limit and allows spectroscopic imaging of simultaneous chemical and structural information correlated with morphology. We correlate s-SNOM near-field amplitude and phase optical contrasts at infrared frequencies to the dielectric constants and growth parameters of InN/InGaN heterostructures and/or single nanoparticles. We observed that both the real and imaginary dielectric function values of mono-/bi-layers of InN/InGaN can be extracted from s-SNOM data. By performing nano-spectroscopy on lithographically patterned samples, we also show that self-assembled InGaN nanoparticles have similar dielectric function values as that of thin film InGaN.

This contribution presents results on the structural and optoelectronic properties of InN layers grown on AlN/sapphire (0001) templates by Migration-Enhanced Plasma Assisted Metal Organic Chemical Vapor Deposition (MEPAMOCVD). The AlN nucleation layer (NL) was varied to assess the physical properties of the InN layers. For ex-situ analysis of the deposited structures, Raman spectroscopy, Atomic Force Microscopy (AFM), and Fourier Transform Infrared (FTIR) reflectance spectroscopy have been utilized. The structural and optoelectronic properties are assessed by Raman-E2 high FWHM values, surface roughness, free carrier concentrations, mobility of the free carriers, and high frequency dielectric function. This study focus on optimizing the AlN nucleation layer (e.g. temporal precursor exposure, nitrogen plasma exposure, plasma power and AlN buffer growth temperature) and its effect on the InN layer properties.

The performed project was centered around two separate, albeit related topics. The high-resolution infrared near-field imaging experiments were performed to better understand the origin of emergence of metal insulator transitions (MIT) in known transition metal oxides and to utilize the phase transition states of VO2 to control and manipulate local plasmon resonaces. First, we imaged the charge dynamics in (MIT)of both amorphous VO2 films and stress free suspended VO2 crystals and probed the existence of mixed phases in tapered suspended crystalVO2 beams. Then we studied the interaction of VO2 with plasmons. Both the film VO2 and the crystal VO2 were explored for local unprecedented control of local plasmon light concentrations. We have demonstrated for the first time dynamic reversible switching of VO2metal-insulator transition and control of plasmonic nanoantennas dipolar modes. The other component of the work was an exploratory synthesis of novel transition metal oxides and chalcogenides, which resulted in discovery of novel phases such as BaYFeO4, CaGaYO4, Li4MgOsO6, Li4NiOsO6, Li3Ni2OsO6, La3Si1-xAlxCuQ7 (Q = S, Se) and Ca2ScOsO6. We successfully synthesized the single-phase samples of the above-mentioned phases and investigated their crystal structures and physical properties.

This article reports the preparation of gold plasmonic transducers using a nanoparticle self-assembly/heating method and the characterization of the films using scattering-type scanning near-field optical microscopy (s-SNOM). Nanoparticle-polymer multilayer films were prepared by the layer-by-layer assembly on glass slides by alternating exposures to monodisperse Au25 nanoparticles and ionic polymer linkers. Thermal evaporation of organic matters from the nanoparticle-polymer multilayer films at 600 °C allowed the nanoparticles to coalescence and form nanostructured films. Characterization of the nanostructured films generated from Au25 nanoparticles using atomic force microscopy (AFM) showed that the films have rounded, small, island-like morphologies (d: 30–50 nm) with a pit in the center of many islands. However, further characterizations with s-SNOM revealed that the produced nanoislands contain a single gold cluster in a pit surrounded by donut-shaped dielectric species. Formation of such a structure is thought to be resulted from the embedding of gold clusters under the reorganized polysiloxane binder coatings and glass surfaces during heat treatment of the Au25 nanoparticle multilayer films. The nanostructured films displayed strong surface plasmon resonance bands in UV–vis spectra with a peak absorbance occurring at ∼545–550 nm. The optical sensing capability of the films was examined using d -glucose-functionalized gold island films with the interaction of Concanavalin A (ConA). The result showed that the adsorption of ConA on island films causes a large change in the LSPR band intensity.

Infrared absorption spectroscopy is a powerful and widely used tool for analyzing the chemical composition and structure of materials. Because of the diffraction limit, however, it cannot be applied for studying individual nanostructures. Here we demonstrate that the phase contrast in substrate-enhanced scattering-type scanning near-field optical microscopy (s-SNOM) provides a map of the infrared absorption spectrum of individual nanoparticles with nanometer-scale spatial resolution. We succeeded in the chemical identification of silicon nitride nanoislands with heights well below 10 nm, by infrared near-field fingerprint spectroscopy of the Si-N stretching bond. Employing a novel theoretical model, we show that the near-field phase spectra of small particles correlate well with their far-field absorption spectra. On the other hand, the spectral near-field contrast does not scale with the volume of the particles. We find a nearly linear scaling law, which we can attribute to the near-field coupling between the near-field probe and the substrate. Our results provide fundamental insights into the spectral near-field contrast of nanoparticles and clearly demonstrate the capability of s-SNOM for nanoscale chemical mapping based on local infrared absorption.

Apertureless near-field scanning optical microscopy (ANSOM) is used to image optical near-field light scattering from uncapped gold nanoparticles and gold nanoparticles capped with the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). The measurements investigate the gold-particle size-dependent signals and the modification of those signals by the spacer layer of commonly used CTAB in the visible at λ = 633 nm. Imaging of capped nanoparticles by apertureless near-field microscopy opens the possibility to predict quantitative layer thicknesses of capping agents on the surface of nanoparticles, as well as the effect of capping layers on the optical scattering properties of nanoparticles.

This review focuses on the spectroscopy and chemical dynamics of bimolecular complexes of group II metal ions with formaldehyde using mass resolved photodissociation spectroscopy techniques. Photodissociation spectroscopy of isolated metal ion-molecule complexes gives quantitative information about the structure and bonding of the complex, and valuable insight into the electronic nonadiabatic interactions that determine the reaction pathways and energy partitioning. The experimental techniques of photodissociation spectroscopy are briefly reviewed. Results from several experiments are then discussed including the photodissociation of Mg + -, Ca + -, and Zn + -based complexes with formaldehyde. Despite the similarities in valence electronic structure for these metal ions, the spectroscopy and quenching dynamics show remarkable differences. We also compare results with the photodissociation spectroscopy of Mg + -acetaldehyde where additional chemical reaction channels are accessible.

Black phosphorus (BP) is an emerging two-dimensional material with intriguing physical properties. It is highly anisotropic and highly tunable by means of both the number of monolayers and surface doping. Here, we experimentally investigate and theoretically interpret the near-field properties of a-few-atomic-monolayer nanoflakes of BP. We discover near-field patterns of bright outside fringes and a high surface polarizability of nanofilm BP consistent with its surface-metallic, plasmonic behavior at mid-infrared frequencies

This paper presents optoelectronic and structural layer properties of InN and InGaN epilayers grown on sapphire templates by Migration-Enhanced Plasma Assisted Metal Organic Chemical Vapor Deposition (MEPA-MOCVD). Real-time characterization techniques have been applied during the growth process to gain insight of the plasma-assisted decomposition of the nitrogen precursor and associated growth surface processes. Analyzed Plasma Emission Spectroscopy (PES) and UV Absorption Spectroscopy (UVAS) provide detection and concentrations of plasma generated active species (N*/NH*/NHx*). Various precursors have been used to assess the nitrogen-active fragments that are directed from the hollow cathode plasma tube to the growth surface. The in-situ diagnostics results are supplemented with ex-situ materials structures investigation results of nanoscale structures using Scanning Near-field Optical Microscopy (SNOM). The structural properties have been analyzed by Raman spectroscopy and Fourier transform infrared (FTIR) reflectance. The Optoelectronic and optical properties were extracted by modeling the FTIR reflectance (e.g. free carrier concentration, high frequency dielectric constant, mobility) and optical absorption spectroscopy. The correlation and comparison between the in-situ metrology results with the ex-situ nano-structural and optoelectronic layer properties provides insides into the growth mechanism on how plasma-activated nitrogen-fragments can be utilized as nitrogen precursor for group III-nitride growth. The here assessed growth process parameter focus on the temporal precursor exposure of the growth surface, the reactor pressure, substrate temperature and their effects of the properties of the InN and InGaN epilayers.

Nanophotonic (nanoplasmonic) structures confine, guide, and concentrate light on the nanoscale. Advancement of nanophotonics critically depends on active nanoscale control of these phenomena. Localized control of the insulator and metallic phases of vanadium dioxide (VO2) would open up a universe of applications in nanophotonics via modulation of the local dielectric environment of nanophotonic structures allowing them to function as active devices. Here we show dynamic reversible control of VO2 insulator-to-metal transition (IMT) locally on the scale of 15 nm or less and control of nanoantennas, observed in the near-field for the first time. Using polarization-selective near-field imaging techniques, we monitor simultaneously the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipolar antennas to monopole antennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique capabilities open up exciting novel applications in active nanophotonics.

This article shows the coarsening behavior of nanoparticle multilayers during heat treatments which produce larger metallic nanostructures with varying shapes and sizes on glass slides. Nanoparticle multilayer films are initially constructed via the layer-by-layer self-assembly of small and monodispersed gold and/or palladium nanoparticles with different compositions (gold only, palladium only, or both gold and palladium) and assembly orders (compounding layers of gold layers over palladium layers or vice versa). Upon heating the slides at 600°C, the surface nanoparticles undergo coalescence becoming larger nanostructured metallic films. UV-Vis results show a clear reliance of the layering sequence on the optical properties of these metal films, which demonstrates an importance of the outmost (top) layers in each nanoparticle multilayer films. Topographic surface features show that the heat treatments of nanoparticle multilayer films result in the nucleation of nanoparticles and the formation of metallic cluster structures. The results confirm that different composition and layering sequence of nanoparticle multilayer films clearly affect the coalescence behavior of nanoparticles during heat treatments.

We report on a combined experimental and theoretical study of the photodissociation spectroscopy and chemical dynamics of Zn+−formaldehyde. We identify two overlapping absorption bands correspondin...

Active heat flow control is essential for broad applications of heating, cooling, and energy conversion. Like electronic devices developed for the control of electric power, it is very desirable to develop advanced all-thermal solid-state devices that actively control heat flow without consuming other forms of energy. Here we demonstrate temperature-gated thermal rectification using vanadium dioxide beams in which the environmental temperature actively modulates asymmetric heat flow. In this three terminal device, there are two switchable states, which can be regulated by global heating. In the "Rectifier" state, we observe up to 28% thermal rectification. In the "Resistor" state, the thermal rectification is significantly suppressed (

Near-field dipolar plasmon interactions of multiple infrared antenna structures in the strong coupling limit are studied using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical finite-difference time-domain (FDTD) calculations. We monitor in real-space the evolution of plasmon dipolar mode of a stationary antenna structure as multiple resonantly matched dipolar plasmon particles are closely approaching it. Interparticle separation, length and polarization dependent studies show that the cross geometry structure favors strong interparticle charge–charge, dipole–dipole and charge–dipole Coulomb interactions in the nanometer scale gap region, which results in strong field enhancement in cross-bowties and further allows these structures to be used as polarization filters. The nanoscale local field amplitude and phase maps show that due to strong interparticle Coulomb coupling, cross-bowtie structures redistribute and highly enhance the out-of-plane (perpendicular to the plane of the sample) plasmon near-field component at the gap region relative to ordinary bowties.

Bi2Se3 thin films are imaged in the near-field using spectroscopic scattering type near-field optical microscopy (s-SNOM) at mid infrared laser wavelength region (9-11μm). Single phases Bi2Se3 thin film structures were prepared by mechanical exfoliation on silicon wafers. We report size and wavelength dependent near-field interaction contrasts in both optical amplitude and phase. We show that near-field optical imaging allows material specific identification and characterization of Bi2Se3 exfoliated samples including the confirmation of residual tape presence or removal in stacked films. We describe an alternative “shear exfoliation” sample preparation method which reliably deposits Bi2Se3 without the possibility of adhesive contaminants.

Spectroscopic near-field imaging of single silica-shell/Au-core and pure silica nanoparticles deposited on a silicon substrate is performed in the infrared wavelength range (λ = 9–11 μm) using scattering-type scanning near-field optical microscopy (s-SNOM). By tuning the wavelength of the incident light, we have acquired information on the spectral phonon–polariton resonant near-field interactions of the silica-shell/Au-core and pure silica nanoparticles with the probing tip. We made use of the enhanced near-field coupling between the high index Au-core and the probing tip to achieve spectral near-field contrast of the thin silica coating (thickness

Tip‐enhance Rayleigh scattering of Ge/Si quantum dots (QDs) has been observed with nanometer‐scale spatial resolution. By using a modulated homodyne technique, real and imaginary parts of a dielectric constant of the QDs is evaluated. The dielectric constant of the QDs is larger than that of bulk Ge.