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In-Sensor Passive Speech Classification with Phononic Metamaterials
Full text linkMitigating the energy requirements of artificial intelligence requires novel physical substrates for computation. Phononic metamaterials have vanishingly low power dissipation and hence are a prime candidate for green, always-on computers. However, their use in machine learning applications has not been explored due to the complexity of their design process. Current phononic metamaterials are restricted to simple geometries (e.g., periodic and tapered) and hence do not possess sufficient expressivity to encode machine learning tasks. A non-periodic phononic metamaterial, directly from data samples, that can distinguish between pairs of spoken words in the presence of a simple readout nonlinearity is designed and fabricated, hence demonstrating that phononic metamaterials are a viable avenue towards zero-power smart devices
Observation of Electrostatically Driven Surface Adsorption in Mixed Surfactant Systems
Full text linkWe employed heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy to obtain a molecular-level understanding of the interaction between the anionic surfactant sodium dodecyl ammonium sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). We observed that these surfactants show a strong cooperative effect on their adsorption to the water-air interface. Even at bulk concentrations 1000 times lower than the critical micelle concentrations of SDS and DTAB, a nearly complete surface surfactant layer is observed when both surfactants are present. This strong enhancement of the surface concentrations of DS- and DTA+ can be quantitatively explained from the favorable Coulomb interaction of the oppositely charged headgroups of DS- and DTA+ and the electrostatic interactions with their counterions. The HD-VSFG results are complemented by a modified Langmuir adsorption model in which we include the free energy associated with the electrostatic interactions of the surfactant ions and their counterions
Compose and Convert: Controlling Shape and Chemical Composition of Self-Organizing Nanocomposites
Full text linkOrganizing the right material at the right place has the potential to revolutionize bottom-up assembly of functional architectures. Despite tremendous progress, this is still difficult in particular because control over chemical composition and morphology are typically inherently entangled. Here a two-step strategy is introduced based on self-organization and conversion reactions to shape a wide selection of chemical compositions into user-defined designs. First, photogeneration of CO2 induces precipitation of nanocomposites of barium carbonate nanocrystals and amorphous silica (BaCO3/SiO2) with control over shape and location following arbitrary illumination patterns. Second, the resulting nanocrystals are converted by sequential ion exchange into a palette of chemical compositions, while the original shape is preserved. By considering thermodynamic stability and chemical reactivity, orthogonal conversion reactions are designed for sequentially positioning nanocomposites of different metal chalcogenide semiconductors next to each other. Based on these strategies, different compositions are integrated into the same hybrid architecture, and the functionality potential is demonstrated by forming a light-emitting perovskite semiconductor that is embedded into an optical waveguide. Combining light-controlled self-organization and shape-preserving ion-exchange reactions offers exciting opportunities for shaping up materials
Nanometer Interlaced Displacement Metrology Using Diffractive Pancharatnam-Berry and Detour Phase Metasurfaces
Full text linkResolving structural misalignments on the nanoscale is of utmost importance in areas such as semiconductor device manufacturing. Metaphotonics provides a powerful toolbox to efficiently transduce information on the nanoscale into measurable far-field observables. In this work, we propose and demonstrate a novel interlaced displacement sensing platform based on diffractive anisotropic metasurfaces combined with polarimetric Fourier microscopy capable of resolving a few nanometer displacements within a device layer. We show that the sensing mechanism relies on an interplay of Pancharatnam-Berry and detour phase shifts and argue how nanoscale displacements are transduced into specific polarization signatures in the diffraction orders. We discuss efficient measurement protocols suitable for high-speed metrology applications and lay out optimization strategies for maximal sensing responsivity. Finally, we show that the proposed platform is capable of resolving arbitrary two-dimensional displacements on a device
Weak ergodicity breaking in optical sensing
Full text linkThe time-integrated intensity transmitted by a laser driven resonator obeys L'evy’s arcsine laws [Ramesh {et al.}, Phys. Rev. Lett. 132, 133801 (2024)]. Here we demonstrate the implications of these laws for optical sensing. We consider the standard goal of resonant optical sensors, namely to report a perturbation to their resonance frequency. In this context, we quantify the sensing precision attained using a finite energy budget combined with time or ensemble averaging of the time-integrated intensity. We find that ensemble averaging outperforms time averaging for short observation times, but the advantage disappears as the observation time increases. We explain this behavior in terms of weak ergodicity breaking, arising when the time for the time-integrated intensity to explore the entire phase space diverges but the observation time remains finite. Evidence that the former time diverges is presented in first passage and return time distributions. Our results are relevant to all types of sensors, in optics and beyond, where stochastic time-integrated fields or intensities are measured to detect an event. In particular, choosing the right averaging strategy can improve sensing precision by orders of magnitude with zero energy cost
Scalable Microscale Artificial Synapses of Lead Halide Perovskite with Femtojoule Energy Consumption
Full text linkThe efficient conduction of mobile ions in halide perovskites is highly promising for artificial synapses (or memristive devices), devices with a conductivity that can be varied by applying a bias voltage. Here we address the challenge of downscaling halide perovskite-based artificial synapses to achieve low energy consumption and allow high-density integration. We fabricate halide perovskite artificial synapses in a back-contacted architecture to achieve microscale devices despite the high solubility of halide perovskites in polar solvents that are commonly used in lithography. The energy consumption of a conductance change of the device is as low as 640 fJ, among the lowest reported for two-terminal halide perovskite artificial synapses so far. Moreover, the high resistance of the device up to hundreds of megaohms, low operating voltage of 100 mV and simple two-terminal architecture enable implementation in highly dense crossbar arrays. These arrays could potentially show orders of magnitude lower energy consumption for computation compared to conventional digital computers
Single-Particle Photothermal Circular Dichroism and Photothermal Magnetic Circular Dichroism Microscopy
Full text linkRecent advances in single-particle photothermal circular dichroism (PT CD) and photothermal magnetic circular dichroism (PT MCD) microscopy have shown strong promise for diverse applications in chirality and magnetism. Photothermal circular dichroism microscopy measures direct differential absorption of left- and right-circularly polarized light by a chiral nanoobject and thus can measure a pure circular dichroism signal, which is free from the contribution of circular birefringence and linear dichroism. Photothermal magnetic circular dichroism, which is based on the polar magneto-optical Kerr effect, can probe the magnetic properties of a single nanoparticle (of sizes down to 20 nm) optically. Single-particle measurements enable studies of the spatiotemporal heterogeneity of magnetism at the nanoscale. Both PT CD and PT MCD have already found applications in chiral plasmonics and magnetic nanomaterials. Most importantly, the advent of these microscopic techniques opens possibilities for many novel applications in biology and nanomaterial science
Optical signatures of charge- and energy transfer in TMDC/TMDC and TMDC/perovskite heterostructures
Full text linkHeterostructures (HSs) based on two-dimensional transition metal dichalcogenides (TMDCs) are highly intriguing materials because of the layers’ pronounced excitonic properties and their nontrivial contributions to the HS. These HSs exhibit unique properties that are not observed in either of the constituent components in isolation. Interlayer excitons (IEs), which are electron-hole pairs separated across the HSs, play a central role in determining these HS properties and are of interest both fundamentally and for device applications. In recent years, a major focus has been on understanding and designing HSs composed of two or more TMDC materials. Less attention has been paid to HSs composed of one TMDC layer and a layer of perovskite material. A central challenge in the understanding of HS properties is that basic measurements such as optical spectroscopic analysis can be misinterpreted due to the complexity of the charge transfer dynamics. Addressing these aspects, this review presents an overview of the most common and insightful optical spectroscopic techniques used to study TMDC/TMDC and TMDC/halide perovskite HSs. Emphasis is placed on the interpretation of these measurements in terms of charge transfer and the formation of IEs. Recent advances have started to uncover highly interesting phenomena, and with improved understanding these HSs offer great potential for device applications such as photodetectors and miniaturized optics
Photon superfluidity through dissipation
Full text linkSuperfluidity—frictionless flow—has been observed in various physical systems such as liquid helium, cold atoms, and exciton polaritons. Superfluidity is usually realized by cooling and suppressing all dissipation. Here we challenge this paradigm by demonstrating signatures of superfluidity, enabled by dissipation, in the flow of light within a room-temperature oil-filled cavity. Dissipation in the oil mediates effective photon-photon interactions which are noninstantaneous and nonlocal. Such interactions were expected to severely limit the emergence of superfluidity in conservative photonic systems. Surprisingly, when launching a photon fluid with sufficiently high density and low velocity against an obstacle in our driven-dissipative cavity, we observe a record suppression of backscattering. Our experiments also reveal the reorganization dynamics of photons into a nonscattering steady state and a qualitatively changing behavior of the optical phase as light propagates around the obstacle. The phase is locked between the laser and the obstacle but evolves with the intensity in the wake of the obstacle where the density of the photon fluid and its mean-field interaction energy decrease. Using a generalized Gross–Pitaevskii equation for photons coupled to a thermal field, we model our experiments and elucidate how the noninstantaneous and nonlocal character of interactions influences the suppression of scattering associated with superfluidity. Beyond providing the first signatures of cavity photon superfluidity, and of any superfluid both at room temperature and in steady state, our results pave the way for probing photon hydrodynamics in arbitrary potential landscapes using structured mirrors
Atmospheric Exposure Triggers Light-Induced Degradation in 2D Lead-Halide Perovskites
Full text linkQuasi-2D perovskites have been pivotal in recent efforts to stabilize perovskite solar cells. Despite the stability boost provided when these materials are introduced in perovskite solar cells, little is known about the intrinsic light and environmental stability of quasi-2D perovskites. In this study, we characterize the photostability of exfoliated quasi-2D perovskite single crystals in air using photoluminescence, infrared, X-ray fluorescence, and energy-dispersive X-ray spectroscopy. Photoexcitation leads to severe material loss with oxygen as a prerequisite for material breakdown. The effect can be traced to the formation of reactive oxygen species, as demonstrated by increases in the photostability under oxygen-free conditions. We show the effect of combined passivation steps, showcasing the stability enhancement offered by 2D-capping layers in combination with an oxygen-free atmosphere. Our results reveal that the stability of illuminated quasi-2D perovskites depends critically on oxygen exposure, highlighting the importance of oxygen-blocking passivation strategies for stable 2D perovskite-based devices