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This broad review summarizes recent advances and "hot"research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25-27, 2022 in Benicássim, Spain. The key goal of the colloquium was to highlight important developments in these areas, particularly new results that emerged during the last two years. This work thus presents a "snapshot"of the state-of-the-art of different nanophononics- and metamaterial-related topics rather than a historical view on these subjects, in contrast to a conventional review article. The introduction of basic definitions for each topic is followed by an outline of design strategies for the media under consideration, recently developed analysis and implementation techniques, and discussions of current challenges and promising applications. This review, while not comprehensive, will be helpful especially for early-career researchers, among others, as it offers a broad view of the current state-of-the-art and highlights some unique and flourishing research in the mentioned fields, providing insight into multiple exciting research directions., Nanophotonics, 12 (4), ISSN:2192-8614

The hopping mechanism plays a key role in collective phenomena emerging in many-body physics. The ability to create and control systems that display this feature is important for next generation quantum technologies. Here we study two cavities separated by a vibrating two-sided perfect mirror and show that, within currently available experimental parameters, this system displays photon-pair hopping between the two electromagnetic resonators. In particular, the two-photon hopping is not due to tunneling, but rather to higher order resonant processes. Starting from the classical problem, where the vibrating mirror perfectly separates the two sides of the cavity, we quantize the system and then the two sides can interact. This opens the possibility to investigate a new mechanism of photon-pair propagation in optomechanical lattices., Comment: 9 pages, 4 figures

Artificial photonic materials, nanofabricated through wavelength-scale engineering, have shown astounding and promising results in harnessing, tuning, and shaping photonic beams. Metamaterials have proven to be often outperforming the natural materials they take inspiration from. In particular, metallic chiral metasurfaces have demonstrated large circular and linear dichroism of light which can be used, for example, for probing different enantiomers of biological molecules. Moreover, the precise control, through designs on demand, of the output polarization state of light impinging on a metasurface, makes this kind of structures particularly relevant for polarization-based telecommunication protocols. The reduced scale of the metasurfaces makes them also appealing for integration with nanomechanical elements, adding new dynamical features to their otherwise static or quasi-static polarization properties. To this end we designed, fabricated and characterized an all-dielectric metasurface on a suspended nanomembrane. Actuating the membrane mechanical motion, we show how the metasurface reflectance response can be modified, according to the spectral region of operation, with a corresponding intensity modulation or polarization conversion. The broad mechanical resonance at atmospheric pressure, centered at about 400 kHz, makes the metasurfaces structure suitable for high-frequency operation, mainly limited by the piezo-actuator controlling the mechanical displacement, which in our experiment reached modulation frequencies exceeding 1.3 MHz.

High performance accelerometers are required in many different domains as sophisticated navigation control systems, research or consumer electronics.A variety of transduction mechanisms has been used to sense the acceleration: capacitive, piezoresistive, thermal... Optomechanical transduction is a promising avenue to realize accelerometers with extremely sensitive readout of mechanical motion with high bandwidth. This also has the advantage of being immune to electromagnetic interferences contrary to the traditional transduction methods.In this work, an optomechanical accelerometer is presented which employs Whispering Gallery Modes disk or ring resonator as displacement sensor. The motion of an inertial mass detunes the resonant cavity and thus modulates the optical power at the output of the sensor.The designs and technological developments of three optomechanical accelerometers are described. We present also the optical and mechanical sensor characterisations. The aim of the thesis is to evaluate the potential of an optomechanical approach for high performance accelerometers., De nombreux domaines requièrent des accéléromètres à très hautes performances, que ce soit pour des systèmes de navigation ou le marché de l'électronique ou encore dans le domaine de la recherche.Beaucoup de méthodes différentes ont été utilisées pour mesurer une accélération comme les accéléromètres capacitifs, piezorésitifs ou encore thermiques. Cependant la transduction optomécanique semble pouvoir apporter de nouveaux avantages en comparaison des méthodes traditionnelles. En effet, l'optomécanique permet de détecter des déplacements mécaniques extrêmement petits et donc d'obtenir des résolutions très importantes tout en gardant une bande passante intéressante. Les capteurs optomécaniques contrairement aux capteurs basés sur d'autres méthodes, peuvent être utilisés dans des environnements non protégés des ondes électromagnétiques.Dans le cadre de ce travail, nous présentons un accéléromètre optomécanique avec une cavité optique résonante à Whispering Gallery Modes pour détecter le mouvement d'une masse inertielle. Le mouvement de cette dernière modifie les conditions de résonance de la cavité ce qui conduit à une modulation de la puissance optique en sortie du capteur. Trois différents modèles d'accéléromètres optomécaniques seront détaillés ainsi que leur développement technologique. Nous présenterons également leurs caractérisations optiques et mécaniques.Finalement ces travaux de recherche permettent d'évaluer le potentiel d'une approche optomécanique pour la conception d'accéléromètres à hautes performances.

The continuous spontaneous localization (CSL) model strives to describe the quantum-to-classical transition from the viewpoint of collapse models. However, its original formulation suffers from a fundamental inconsistency in that it is explicitly energy nonconserving. Fortunately, a dissipative extension to CSL has been recently formulated that solves such an energy-divergence problem. We compare the predictions of the dissipative and nondissipative CSL models when various optomechanical settings are used and contrast such predictions with available experimental data, thus building the corresponding exclusion plots.

The interest in the field of plasmonics is growing steadily in the last two decades; from the ability to transmit and receive information at very high speed with greatly reduced losses to the enhancement of very weak signals for chemical and biological analysis, its range of applicability is boundless. The understanding of plasmonic phenomena is also increasing, and with this comes the ability to tune plasmonic properties to the designer’s will. Among other approaches, the use of MicroElectroMechanical Systems (MEMS) for the modulation of plasmonic properties has been recently reported in literature, but no practical application, such as Raman spectroscopy, has been reported beside the field of Tip-Enhanced Raman Spectroscopy, a powerful approach that demonstrated nanometric chemical spatial resolution, but which remains confined the research laboratory benches due to its intrinsic experimental complexity. In this project, we propose to explore the coupling between mechanical and plasmonic properties of micro and nanosensors in order to realize a mechanical resonator capable of turning on and off a frequency modulated hot spot. Different strategies have been tried to achieve the desired result: the first version of the optomechanical device was based on a vertical resonator (pillar) put in close proximity with a steady structure, 100 nm apart from each other. The devices are fabricated using electron-beam lithography for the high resolution required for the sub-micron gap and ICP-RIE to obtain an inverted tapered structure of the pillar walls; the evaporation of a gold layer on top of the devices ensures a plasmonic activity of the upper surface during the actuation of the devices. Optical lever techniques and Rayleigh scattering mapping have been used for the mechanical characterization and the onset of an impact oscillation condition is discussed. The Raman scattering intensification due to the formation of a plasmonic hot spot in the contact region has been studied functionalizing the devices with pentacene and an enhancing factor for the Raman signal during actuation can be estimated. However, severe drawbacks have been identified in this configuration, since pillars tilt and bend nanometrically during the motion, causing the hot spots to be randomly localized along the gap and reducing their field enhancement capabilities. The problems arisen with the first version of the device have been solved through the careful design of a new geometry: the vertical resonator has been changed into a horizontally-oscillating cantilever tuning the width-to-height ratio, and a tip has been added to the design for a well-defined contact point. This new device has been characterized using sample scanning confocal microscopy, both in its mechanical properties and in the surface distribution of the chosen Raman dye, benzotriazole azo (BT-Azo), after the functionalization. Finally, the plasmonic behaviour has been investigated and the signal coming from the hot spot has been successfully isolated using a combination of polarization-dependent excitation light and lock-in deconvolution of the signal at higher harmonics, thus demonstrating the successful realization of a frequency modulated hot spot for Raman spectroscopy applications. As a side activity, a wire scanner sensor with nanofabricated bridges suspended over a wide window has been fabricated, for the characterization of high-energy electron or light beams. The test of this device has been performed at the BEAR beamline of the Elettra Synchrotron and in the FERMI FEL-1 Free-Electron Laser facility; the performances of this sensor have been proved to be comparable, when not superior, to those of the commercially available devices.

International audience; The continuous spontaneous localization (CSL) model is the best known and studied among collapse models, which modify quantum mechanics and identify the fundamental reasons behind the unobservability of quantum superpositions at the macroscopic scale. Albeit several tests were performed during the last decade, up to date the CSL parameter space still exhibits a vast unexplored region. Here, we study and propose an unattempted non-interferometric test aimed to fill this gap. We show that the angular momentum diffusion predicted by CSL heavily constrains the parametric values of the model when applied to a macroscopic object.

Electromagnetic fields coupled with mechanical degrees of freedom have recently shown exceptional and innovative applications, ultimately leading to mesoscopic optomechanical devices operating in the quantum regime of motion. Simultaneously, micromechanical elements have provided new ways to enhance and manipulate the optical properties of passive photonic elements. Following this concept, in this article we show how combining a chiral metasurface with a GaAs suspended micromembrane can offer new scenarios for controlling the polarization state of near-infrared light beams. Starting from the uncommon properties of chiral metasurface to statically realize target polarization states and circular and linear dichroism, we report mechanically induced, ~300 kHz polarization modulation, which favorably compares, in terms of speed, with liquid-crystals commercial devices. Moreover, we demonstrate how the mechanical resonance can be non-trivially affected by the input light polarization (and chiral state) via a thermoelastic effect triggered by intracavity photons. This work inaugurates the field of Polarization Optomechanics, which could pave the way to fast polarimetric devices, polarization modulators and dynamically tunable chiral state generators and detectors, as well as giving access to new form of polarization nonlinearities and control.

Recently, a non-thermal excess noise, compatible with the theoretical prediction provided by collapse models, was measured in a millikelvin nanomechanical cantilever experiment [Vinante et al., Phys. Rev. Lett. 119, 110401 (2017)]. We propose a feasible implementation of the cantilever experiment able to probe such a noise. The proposed modification, completely within the grasp of current technology and readily implementable also in other type of mechanical non-interferometric experiments, consists in substituting the homogeneous test mass with one composed of different layers of different materials. This will enhance the action of a possible collapse noise above that given by standard noise sources.

Spontaneous collapse models predict that a weak force noise acts on any mechanical system, as a consequence of the collapse of the wave function. Significant upper limits on the collapse rate have been recently inferred from precision mechanical experiments, such as ultracold cantilevers and the space mission LISA Pathfinder. Here, we report new results from an experiment based on a high-Q cantilever cooled to millikelvin temperatures, which is potentially able to improve the current bounds on the continuous spontaneous localization (CSL) model by 1 order of magnitude. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.ler. Several physical mechanisms able to explain the observed noise have been ruled out., 10 pages, 12 figures

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 {\mu}m were used as high quality factor (Q>10^6) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally-activated mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling., Comment: 17 pages, 5 figures

The selfmixer properties of a laser compound cavity are investigated and experimentally exploited to couple the mechanical fluctuations of a silicon nitride membrane to the laser photons and electronical states, producing an active optomechanical system.