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Nonreciprocal nonequilibrium process are attracting growing interest in sociology, animal behaviour, chemistry, and nanotechnology, and may have played a role in the origin of life. It is less widely recognized, however, that in open systems light can induce nonreciprocal predator-prey like forces between nanoparticles. Such forces provide access to the continuous time crystal state of matter, which has been demonstrated in a plasmonic metamaterial array of nanowires wherein light triggers a spontaneous mobilization transition to the robust oscillatory state, breaking time translation symmetry. Here, we report on the first experimental study of the transient dynamics of light-induced mobilization and demobilization in a time crystal. By analysing time resolved phase trajectories of the system of nanowires, we show that the mobilization transition is accompanied by breaking of continuous time translation symmetry and ergodicity, and a decrease in the entropy of motion. This insight into the transient dynamics of a nonreciprocity-driven time crystal is relevant to optical timetronics, an information and communications technology paradigm relying on the unique functionalities of time crystals, and applications of the interacting nanowire oscillator platform to modelling a wide range of nonreciprocal processes from many-body dynamics to the early stages of matter-to-life transitions., Comment: 9 pages, 4 figures

Continuous time crystals (CTCs) - media with broken continuous time translation symmetry - are an eagerly sought state of matter that spontaneously transition from a time-independent state to one of periodic motion in response to a small perturbation. The state has been realized recently in an array of nanowires decorated with plasmonic metamolecules illuminated with light. Here we show that this as-yet-unexplained CTC state can be understood as arising from a nonreciprocal phase transition induced by nonconservative radiation pressure forces among plasmonic metamolecules: above a certain intensity threshold, light drives the inhomogeneously broadened array of thermally-driven noisy nanowire oscillators to a synchronized coherent space-time crystal state and ergodicity of the system is broken. At the onset of synchronization, this mechanism does not require nonlinearity in the oscillators but depends instead on nonreciprocal forces. As such it is fundamentally different from the regimes of synchronization that depend on nonlinearity., Comment: 7 pages, 5 figures

Time crystals are an eagerly sought phase of matter with broken time-translation symmetry. Quantum time crystals with discretely broken time-translation symmetry have been demonstrated in trapped ions, atoms and spins while continuously broken time-translation symmetry has been observed in an atomic condensate inside an optical cavity. Here we report that a classical metamaterial nanostructure, a two-dimensional array of plasmonic metamolecules supported on flexible nanowires, can be driven to a state possessing all of the key features of a continuous time crystal: continuous coherent illumination by light resonant with the metamolecules' plasmonic mode triggers a spontaneous phase transition to a superradiant-like state of transmissivity oscillations, resulting from many-body interactions among the metamolecules, characterized by long-range order in space and time. The phenomenon is of interest to the study of dynamic classical many-body states in the strongly correlated regime and applications in all-optical modulation, frequency conversion and timing., Comment: 10 pages, 6 figures

Flexural oscillations of free-standing films, nano-membranes and nano-wires are attracting growing attention for their importance to the thermal, electrical and mechanical properties of 2D materials. Here we report on the observation of short-timescale ballistic motion in the flexural mode of a nano-membrane cantilever, driven by thermal fluctuation of flexural phonons, including measurements of ballistic velocities and displacements performed with sub-atomic resolution, using a new free electron edge-scattering technique. Within intervals <10 {\mu}s, the membrane moves ballistically at a constant velocity, typically ~300 {\mu}m/s, while Brownian-like dynamics emerge for longer observation periods. Access to the ballistic regime provides verification of the equipartition theorem and Maxwell-Boltzmann statistics for flexural modes, and can be used in fast thermometry and mass sensing during atomic absorption/desorption processes on the membrane. We argue that the ballistic regime should be accounted for in understanding the electrical, optical, thermal and mechanical properties of 2D materials., Comment: 7 pages, 4 figures

Despite recent tremendous progress in optical imaging and metrology, the resolution gap between atomic scale transmission electron microscopy and optical techniques has not been closed. Is optical imaging and metrology of nanostructures exhibiting Brownian motion possible with resolution beyond thermal fluctuations? Here we report on an experiment in which the average position of a nanowire with a thermal oscillation amplitude of ~150 pm is resolved in single-shot measurements with precision of 92 pm using light at a wavelength of {\lambda} = 488 nm, providing the first example of such sub-Brownian metrology with ~{\lambda}/5,300 precision. To localize the nanowire, we employ a deep learning analysis of the scattering of topologically structured light, which is highly sensitive to the nanowire's position. As a non-invasive optical metrology with sub-Brownian absolute errors, down to a fraction of the typical size of an atom (Si: 220 pm diameter), it opens the exciting field of picophotonics., Comment: 11 pages, 4 figures

Nanomechanical photonic metamaterials provide a wealth of active switching, nonlinear and enhanced light-matter interaction functionalities by coupling optically and mechanically resonant subsystems. Thermal (Brownian) motion of the nanostructural components of such metamaterials leads to fluctuations in optical properties, which may manifest as noise, but which also present opportunity to characterize performance and thereby optimize design at the level of individual nanomechanical elements. We show that Brownian motion in an all-dielectric metamaterial ensemble of silicon-on-silicon-nitride nanowires can be controlled by light at sub-{\mu}W/{\mu}m2 intensities. Induced changes in nanowire temperature of just a few Kelvin, dependent upon nanowire dimensions, material composition, and the direction of light propagation, yield proportional changes of several percent in the few-MHz Eigenfrequencies and picometric displacement amplitudes of Brownian motion. The tuning mechanism can provide active control of frequency response in photonic metadevices and may serve as a basis for bolometric, mass and micro/nanostructural stress sensing., Comment: 11 pages, 5 figures

This paper evaluates and compares two randomized interventions in Tonga, one targeting the home environment of children up to age 5 and one targeting the school environment for first and second grade students. The first intervention supports communities to set up and run playgroups that aim to improve caregiver-child interaction at home and ultimately improve children's readiness for school. Among children of mothers without high school education, being in a treatment community positively affects the school readiness literacy domains and overall score by 0.19 and 0.2 standard deviation, respectively, for girls and the literacy domains for boys by 0.17 standard deviation. The second intervention provides teachers with training, materials, and coaching to improve reading instruction practices and students' reading ability. It increased average early grade reading scores by approximately 0.18 standard deviation per year of exposure. Two cohorts of children were potentially exposed to both interventions, providing an opportunity to compare, for the same population of children, the effects of an early childhood intervention with a school-based intervention using a common measure of learning achievement. The school readiness intervention is found to have positive effects on early grade reading scores only among children in the reading intervention's control group; effect sizes of approximately 0.29 standard deviation were found at the end of second grade for children exposed to one year of the school readiness intervention and at the end of first grade for girls exposed to two years of the intervention. [This paper is a product of the Education Global Practice.]

We have developed a technique that extends static scanning electron microscopic imaging to include hyperspectral mapping of fast thermal and externally-driven movements at up to Megahertz frequencies. It is based on spectral analysis of the secondary electron flux generated by a focused electron beam incident on the moving object. We demonstrate detection of nanowire Brownian motion and hyperspectral mapping of stimulated oscillations of flea setae with deep sub-nanometer displacement sensitivity.

It has recently been shown that holographically nanostructured surfaces can be employed to control the wavefront of (predominantly plasmonic) optical-frequency light emission generated by the injection of medium-energy electrons into a gold surface. Here we apply the concept to manipulation of the spatial distribution of transition radiation emission from high-refractive-index dielectric/semiconductor target materials, finding that concomitant incoherent luminescent emission at the same wavelength is unperturbed by holographic surface-relief structures, and thereby deriving a means of discriminating between the two emission components., Comment: 5 pages, 3 figures

Free electrons travelling in vacuum carry evanescent electric field, which can only be coupled to free space light via interaction with a surrounding or nearby medium: as Cherenkov radiation when travelling faster than the local velocity of light or as "Smith-Purcell" (SP) radiation when passing over the surface of a grating. The SP emission characteristics of simple gratings such as a regular array of wires or slits are a well-understood phenomenon. Here we show that for a compound grating made up of several closely-spaced slits repeated for each period, the characteristic angular dispersion of Smith-Purcell radiation can be selectively attenuated or enhanced. We analyze, in particular, the change in intensity as slit elements are added to the structure for two slit sizes and how this affects emission enhancement., Comment: 4 pages, 3 figures

Recently, coherent control of the optical response of thin films of matter in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to demonstrations of all-optical image processing. Here we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, that controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fully fiberized network at wavelengths between 1530 nm and 1565 nm. The metadevice performance has been tested with optical signals equivalent to a bitrate of up to 40 Gbit/s and sub-milliwatt power levels. Since coherent absorption can operate at the single photon level and also with 100 THz bandwidth, we argue that the demonstrated all-optical switch concept has potential applications in coherent and quantum information networks., Comment: 9 pages, 6 figures

We harness non-volatile, amorphous-crystalline transitions in the chalcogenide phase-change medium germanium antimony telluride (GST) to realize optically-switchable, all-dielectric metamaterials. Nanostructured, subwavelength-thickness films of GST present high-quality resonances that are spectrally shifted by laser-induced structural transitions, providing reflectivity and transmission switching contrast ratios of up to 5:1 (7 dB) at visible/near-infrared wavelengths selected by design., Comment: 8 pages, 8 figures

By harnessing the resonant nature of localized electromagnetic modes in a nanostructured silicon membrane, an all-dielectric metamaterial can act as nonlinear medium at optical telecommunications wavelengths. We show that such metamaterials provide extremely large optomechanical nonlinearities, operating at intensities of only a few {\mu}W per unit cell and modulation frequencies as high as 152 MHz, thereby offering a path to fast, compact and energy efficient all-optical metadevices., Comment: 5 pages, 3 figures

Although vast amounts of information are conveyed by photons in optical fibers, the majority of data processing is performed electronically, creating the infamous 'information bottleneck' and consuming energy at an increasingly unsustainable rate. The potential for photonic devices to directly manipulate light remains unfulfilled due largely to a lack of materials with strong, fast optical nonlinearities. In this paper, we show that small-signal amplifier, summator and invertor functions for optical signals may be realized using a four-port device that exploits the coherent interaction of beams on a planar plasmonic metamaterial, assuming no intrinsic nonlinearity. The redistribution of energy among ports can be coherently controlled at the single photon level, with THz bandwidth and without introducing signal distortion, thereby presenting powerful opportunities for novel optical data processing architectures, complexity oracles and the locally coherent networks that are becoming part of the mainstream telecommunications agenda.

We report on the demonstration of a femtosecond all-optical modulator providing, without nonlinearity and therefore at arbitrarily low intensity, ultrafast light-by-light control. The device engages the coherent interaction of optical waves on a metamaterial nanostructure only 30 nm thick to efficiently control absorption of near-infrared (750-1040 nm) femtosecond pulses, providing switching contrast ratios approaching 3:1 with a modulation bandwidth in excess of 2 THz. The functional paradigm illustrated here opens the path to a family of novel meta-devices for ultra-fast optical data processing in coherent networks., Comment: 5 pages, 4 figures

According to Huygens' superposition principle, light beams traveling in a linear medium will pass though one another without mutual disturbance. Indeed, it is widely held that controlling light signals with light requires intense laser fields to facilitate beam interactions in nonlinear media, where the superposition principle can be broken. We demonstrate here that two coherent beams of light of arbitrarily low intensity can interact on a metamaterial layer of nanoscale thickness in such a way that one beam modulates the intensity of the other. We show that the interference of beams can eliminate the plasmonic Joule losses of light energy in the metamaterial or, in contrast, can lead to almost total absorbtion of light. Applications of this phenomenon may lie in ultrafast all-optical pulse-recovery devices, coherence filters and THz-bandwidth light-by-light modulators.

The fabrication of indented ('intaglio') or raised ('bas-relief') sub-wavelength metamaterial patterns on a metal surface provides a mechanism for changing and controlling the colour of the metal without employing any form of chemical surface modification, thin-film coating or diffraction effects. We show that a broad range of colours can be achieved by varying the structural parameters of metamaterial designs to tune absorption resonances. This novel approach to the 'structural colouring' of pure metals offers great versatility and scalability for both aesthetic (e.g. jewellery design) and functional (e.g. sensors, optical modulators) applications. We focus here on visible colour but the concept can equally be applied to the engineering of metallic spectral response in other electromagnetic domains.