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High Aspect Ratio Silver Nanogrids by Bottom-Up Electrochemical Growth as Transparent Electrode
Full text linkA scalable selective-area electrochemical method is reported for the fabrication of interconnected metal nanostructures. In this work, the fabrication of silver nanowire grids for the application of transparent electrodes is explored. The presented method is based on a through-the-mask electrodeposition method, where the mask is made by using substrate conformal imprint lithography. We find that the nucleation density of the silver nanoparticles is the key parameter for successful homogeneous void-free filling of the template. We independently controlled the density of the silver nuclei and their growth by using a double potential pulse. The silver nanowire grids show high transmission (95.9%) and low sheet resistance (as low as 3.7 Ω/sq), resulting in a superior figure of merit (FoM). Due to the bottom-up nature of this technique, arbitrarily high aspect ratio nanowires can be achieved, therefore decreasing the sheet resistance without affecting transmittance and carrier collection. The presented method can be generalized to the large-area nanofabrication of any well-defined nanostructure design of any metal transparent electrode for multiple applications
Near-Unity All-Optical Modulation of Third-Harmonic Generation with a Fano-Resonant Dielectric Metasurface
Full text linkWe demonstrate all-optical modulation with a near-unity contrast of nonlinear light generation in a dielectric metasurface. We study third-harmonic generation from silicon Fano-resonant metasurfaces excited by femtosecond pulses at 1480 nm wavelength. We modulate the metasurface resonance by free carrier excitation induced by absorption of an 800 nm pump pulse, leading to up to 93% suppression of third-harmonic generation. Modulation and recovery occur on (sub)picosecond time scales. According to the Drude model, the pump-induced refractive index change blue-shifts the metasurface resonance away from the generation pulse, causing a strong modulation of third-harmonic conversion efficiency. The principle holds great promise for spatiotemporal programmability of nonlinear light generation
Temporal Dynamics of Collective Resonances in Periodic Metasurfaces
Full text linkTemporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems
Toward Optimum Coupling between Free Electrons and Confined Optical Modes
Full text linkFree electrons are excellent tools to probe and manipulate nanoscale optical fields with emerging applications in ultrafast spectromicroscopy and quantum metrology. However, advances in this field are hindered by the small probability associated with the excitation of single optical modes by individual free electrons. Here, we theoretically investigate the scaling properties of the electron-driven excitation probability for a wide variety of optical modes including plasmons in metallic nanostructures and Mie resonances in dielectric cavities, spanning a broad spectral range that extends from the ultraviolet to the infrared region. The highest probabilities for the direct generation of three-dimensionally confined modes are observed at low electron and mode energies in small structures, with order-unity (∼100%) coupling demanding the use of <100 eV electrons interacting with eV polaritons confined down to tens of nanometers in space. Electronic transitions in artificial atoms also emerge as practical systems to realize strong coupling to few-eV free electrons. In contrast, conventional dielectric cavities reach a maximum probability in the few-percent range. In addition, we show that waveguide modes can be generated with higher-than-unity efficiency by phase-matched interaction with grazing electrons, suggesting a practical method to create multiple excitations of a localized optical mode by an individual electron through funneling the so-generated propagating photons into a confining cavity─an alternative approach to direct electron–cavity interaction. Our work provides a roadmap to optimize electron–photon coupling with potential applications in electron spectromicroscopy as well as nonlinear and quantum optics at the nanoscale
Memory-induced excitability in optical cavities
Full text linkNeurons and other excitable systems can release energy suddenly given a small stimulus. Excitability has recently drawn increasing interest in optics, as it is key to realize all-optical artificial neurons enabling speed-of-light information processing. However, the realization of all-optical excitable units and networks remains challenging. Here we demonstrate how laser-driven optical cavities with memory in their nonlinear response can sustain excitability beyond the constraints of memoryless systems. First we demonstrate different classes of excitability and spiking, and their control in a single cavity with memory. This single-cavity excitability is limited to a narrow range of memory times commensurate with the linear dissipation time. To overcome this limitation, we explore coupled cavities with memory. We demonstrate that this system can exhibit excitability for arbitrarily long memory times, even when the intercavity coupling rate is smaller than the dissipation rate. Our coupled-cavity system also sustains spike trains - a hallmark of neurons - that spontaneously break mirror symmetry. Our predictions can be readily tested in thermo-optical cavities, where thermal dynamics effectively give memory to the nonlinear optical response. The huge separation between thermal and optical timescales in such cavities is promising for the realization of artificial neurons that can self-organize to the edge of a phase transition, like many biological systems do
Timers, variability, and body-wide coordination: C. elegans as a model system for whole-animal developmental timing
Full text linkSuccessful development requires both precise timing of cellular processes, such as division and differentiation, and tight coordination of timing between tissues and organs. Yet, how time information is encoded with high precision and synchronized between tissues, despite inherent molecular noise, is unsolved. Here, we propose the nematode C. elegans as a unique model system for studying body-wide control of developmental timing. Recent studies combining genetics, quantitative analysis, and simulations have 1) mapped core timers controlling larval development, indicating temporal gradients as an underlying mechanism, and 2) elucidated general principles that make timing insensitive to inherent fluctuations and variation in environmental conditions. As the molecular regulators of C. elegans developmental timing are broadly conserved, these mechanisms likely apply also to higher organisms
The collective snapping of a pair of bumping buckled beams
No full textWhen a pair of parallel buckling beams of unequal thickness make lateral contact under increasing compression, eventually either the thin or the thick beam will snap, leading to collective motion of the beam pair. Using experiments and FEM simulations, we find that the distance D between the beams selects which beam snaps first, and that the critical distance D∗ scales linear with the combined thickness of the two beams. To understand this behavior, we show that the collective motion of the beams is governed by a pitchfork bifurcation that occurs at strains just below snapping. Specifically, we use a model of two coupled Bellini trusses to find a closed form expression for the location of this pitchfork bifurcation that captures the linear scaling of D∗ with beam thickness. Our work uncovers a novel elastic instability that combines buckling, snapping and contact nonlinearities. This instability underlies the packing of parallel confined beams, and can be leveraged in advanced metamaterials
Considerations for electromagnetic simulations for a quantitative correlation of optical spectroscopy and electron tomography of plasmonic nanoparticles
Full text linkThe optical cross sections of plasmonic nanoparticles are intricately linked to their morphologies. Accurately capturing this link could allow determination of particles’ shapes from their optical cross sections alone. Electromagnetic simulations bridge morphology and optical properties, provided they are sufficiently accurate. This study examines key factors affecting simulation precision, comparing common methods and detailing the impacts of meshing accuracy, dielectric function selection, and substrate inclusion within the boundary element method. To support the method’s complex parameterization, we develop a workflow incorporating reconstruction, meshing, and mesh simplification, to enable the use of electron tomography data. We analyze how choices of reconstruction algorithm and image segmentation affect simulated optical cross sections, relating these to shape errors minimized during data processing. Optimal results are obtained using the total variation minimization (TVM) reconstruction method with Otsu thresholding and light smoothing, ensuring reliable, watertight surface meshes through the marching cubes algorithm, even for complex shapes
Sub-ablation-threshold light-induced modification of thin ruthenium layers detected using optical reflectance
Full text linkIn semiconductor device manufacturing, wafer materials may be exposed to intense light sources by optical metrology tools. The desired light fluence often needs to be maximized to levels just below the optical damage threshold of materials deposited on the wafer, such as ruthenium. We, therefore, investigate light-induced permanent structural changes to thin Ru films after exposure to single wavelength femtosecond pulses in the fluence regime before catastrophic damage. For fluences below that where full-ablation occurs, small optical increases in the reflectivity of up to are observed in the aftermath with a weak probe beam. In this fluence regime, dark-field, scanning electron, and atomic force microscopy images reveal morphological changes such as top-level ablation, where only the top part of the ruthenium layer is ablated whereas the lower part still remains on the substrate, and nanovolcano formation. However, neither top-level ablation nor nanovolcano formation is responsible for the reflection increase. Instead, Electron Backscatter Diffraction reveals that in this low fluence regime where reflectivity increases, Ru grains melt and resolidify into larger grains, which is likely responsible for the observed reflectivity increases. This result is reminiscent of our earlier work on aluminum layers and it suggests that there may be more metals that display this behavior
Helical interfacial modulation for perovskite photovoltaics
No full textHybrid metal halide perovskites have demonstrated remarkable performances in modern photovoltaics, although their stabilities remain limited. We assess the capacity to advance their properties by relying on interfacial modulators featuring helical chirality based on P,M-(1-methylene-3-methyl-imidazolium)[6]helicene iodides. We investigate their characteristics, demonstrating comparable charge injection for enantiomers and the racemic mixture. Overall, they maintain the resulting photovoltaic performance while improving operational stability, challenging the role of helical chirality in the interfacial modulation of perovskite solar cells