Auswahl der wissenschaftlichen Literatur zum Thema „Optomechanical sensing“

Abstract Cavity optomechanical systems enable interactions between light and mechanical resonators, providing a platform both for fundamental physics of macroscopic quantum systems and for practical applications of precision sensing. The resonant enhancement of both mechanical and optical response in the cavity optomechanical systems has enabled precision sensing of multiple physical quantities, including displacements, masses, forces, accelerations, magnetic fields, and ultrasounds. In this article, we review the progress of precision sensing applications using cavity optomechanical systems. The review is organized in the following way: first we will introduce the physical principles of optomechanical sensing, including a discussion of the noises and sensitivity of the systems, and then review the progress in displacement sensing, mass sensing, force sensing, atomic force microscope (AFM) and magnetic resonance force microscope (MRFM), accelerometry, magnetometry, and ultrasound sensing, and introduce the progress of using quantum techniques especially squeezed light to enhance the performance of the optomechanical sensors. Finally, we give a summary and outlook.

Cavity optomechanics with picometer displacement measurement resolution has shown vital applications in high-precision sensing areas. In this paper, an optomechanical micro hemispherical shell resonator gyroscope (MHSRG) is proposed, for the first time. The MHSRG is driven by the strong opto-mechanical coupling effect based on the established whispering gallery mode (WGM). And the angular rate is characterized by measuring the transmission amplitude changing of laser coupled in and out from the optomechanical MHSRG based on the dispersive resonance wavelength shift and/or dissipative losses varying. The detailed operating principle of high-precision angular rate detection is theoretically explored and the fully characteristic parameters are numerically investigated. Simulation results show that the optomechanical MHSRG can achieve scale factor of 414.8 mV/ (°/ s) and angular random walk of 0.0555 °/ h1/2 when the input laser power is 3 mW and resonator mass is just 98 ng. Such proposed optomechanical MHSRG can be widely used for chip-scale inertial navigation, attitude measurement, and stabilization.

Abstract We propose how to achieve synthetic PT $\mathcal{PT}$ symmetry in optomechanics without using any active medium. We find that harnessing the Stokes process in such a system can lead to the emergence of exceptional point (EP), i.e., the coalescing of both the eigenvalues and the eigenvectors of the system. By encircling the EP, both nonreciprocal optical amplification and chiral mode switching can be achieved. As a result, our synthetic PT $\mathcal{PT}$ -symmetric optomechanics works as a topological optomechanical amplifier. This provides a surprisingly simplified route to realize PT $\mathcal{PT}$ -symmetric optomechanics, indicating that a wide range of EP devices can be created and utilized for various applications such as topological optical engineering and nanomechanical processing or sensing.

In this paper, we review the linear and non-linear dynamics of an optomechanical system made of a two-membrane etalon in a high-finesse Fabry–Pérot cavity. This two-membrane setup has the capacity to modify on demand the single-photon optomechanical coupling, and in the linearized interaction regime to cool simultaneously two mechanical oscillators. It is a promising platform for realizing cavity optomechanics with multiple resonators. In the non-linear regime, an analytical approach based on slowly varying amplitude equations allows us to derive a consistent and full characterization of the non-linear displacement detection, enabling a truthful detection of membrane displacements much above the usual linear sensing limited by the cavity linewidth. Such a high quality system also shows a pre-synchronization regime.

Optomechanical nanocavities open a new hybrid platform such that the interaction between an optical cavity and mechanical oscillator can be achieved on a nanophotonic scale. Owing to attractive advantages such as ultrasmall mass, high optical quality, small mode volume and flexible mechanics, a pair of coupled photonic crystal nanobeam (PCN) cavities are utilized in this paper to establish an optomechanical nanosystem, thus enabling strong optomechanical coupling effects. In coupled PCN cavities, one nanobeam with a mass meff~3 pg works as an in-plane movable mechanical oscillator at a fundamental frequency of πΩm/2π=4.148 MHz. The other nanobeam couples light to excite optical fundamental supermodes at 1542.858 and 1554.464 nm with a Qo larger than 4 × 104. Because of the optomechanical backaction arising from an optical force, abundant optomechanical phenomena in the unresolved sideband are observed in the movable nanobeam. Moreover, benefiting from the in-plane movement of the flexible nanobeam, we achieved a maximum displacement of the movable nanobeam as 1468 fm/Hz1/2. These characteristics indicate that this optomechanical nanocavity is capable of ultrasensitive motion measurements.

L'optomécanique est la science des interactions entre la lumière et les mouvements mécaniques. Ce rapport de thèse décrit des expériences réalisées avec des microdisques fabriqué dans différents résonateurs semi-conducteurs III-V: l'Arséniure de Gallium (GaAs), l'Arséniure d'Aluminium Gallium (AlGaAs) et l'Arséniure d'Indium Phosphide (InGaP). Ces matériaux sont compatibles avec les fonctionnalités de l’optoélectronique et procurent un couplage optomécanique géant. Pour améliorer les performances des résonateurs en GaAs, nous avons développé des méthodes de traitement de surface permettant de réduire la dissipation optique par un facteur dix et ainsi d'atteindre un facteur de qualité de six millions. En plus de ces études sur le GaAs, nous avons réalisés une étude comparative des interactions optomecaniques dans des microdisques d'InGaP et d'AlGaAs, et nous avons mis en évidences leurs résonances optomécaniques. Finalement, nous avons réalisé des mesures de force avec des résonateurs en GaAs, démontrant un nouveau principe de détection basé sur notre étude de leur la trajectoire dans l'espace de phase et leur bruit de phase
Optomechanics studies the interaction between light and mechanical motion. This PhD thesis reports on optomechanical experiments carried with miniature disk resonators fabricated out of distinct III-V semiconductors: Gallium Arsenide (GaAs), Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Phosphide (InGaP). These materials are compliant with optoelectronics functionalities and provide giant optomechanical coupling. In order to boost performances of GaAs resonators, we implemented surface control techniques and obtained a ten-fold reduction of optical dissipation, attaining a Q of six million. On top of GaAs, we performed a comparative investigation of optomechanical interactions in InGaP and AlGaAs disk resonators, and demonstrated their operation as optomechanical oscillators. Finally, we carried out optomechanical force sensing experiments with GaAs resonators, analyzing a new sensing principle in light of the phase space trajectory and phase noise of the corresponding oscillators

Dans cette thèse, nous présentons une étude approfondie des résonateurs à disques optomécaniques ultra-haute fréquence, en fonctionnement dans divers environnements liquides. L'objectif du travail était de développer des techniques expérimentales optiques et des modèles théoriques pour étudier les interactions fluide-structure dans des dispositifs micro ou nanométriques vibrants, ayant des applications potentielles en fluidique, en détection pour le biomédical et en science des matériaux. Nous avons appliqué des techniques de transduction optomécaniques à des résonateurs à disque en silicium pour mesurer diverses propriétés des liquides. En s'appuyant sur des modèles analytiques et numériques, nos mesures permettent de remonter à l'indice de réfraction, la conductivité thermique, la viscosité, la densité et la compressibilité du liquide. Nous avons notamment obtenu des expressions explicites pour le décalage en fréquence et le facteur de qualité mécanique d'un disque immergé dans un liquide, le transformant en un rhéomètre calibré. Puisque ce rhéomètre couvre la gamme de fréquences de 200 MHz à 3 GHz, nous avons pu observer d'importants effets de compressibilité dans l'eau, et confirmé que ce liquide reste pour autant newtonien dans cette gamme. En revanche, le 1-décanol liquide présente un comportement non newtonien, avec une viscosité dépendant de la fréquence, et des temps de relaxation associés proche de la nanoseconde que nous avons pu mettre en évidence expérimentalement. Le travail de thèse apporte un éclairage sur le comportement des résonateurs à disque optomécanique immergés, et démontre leur potentiel pour sonder les propriétés multiphysiques d'un liquide à l'échelle micronique
In this thesis, we present an in-depth study of ultra-high frequency optomechanical disk resonators operating in various liquid environments. The goal of the work was to develop optical experimental techniques and theoretical models to study fluid-structure interactions in micro- and nanoscale vibrating devices, with potential applications in fluidics, biomedical sensing, and materials science. We employed optomechanical transduction techniques on silicon disk resonators to measure various properties of liquids. Backed by analytical and numerical models, our measurements give access to the liquid's refractive index, thermal conductivity, viscosity, density, and compressibility. We notably derived closed-formed expressions for the mechanical frequency shift and quality factor of a disk immersed in liquid, transforming it into a calibrated rheometer. As this rheometer covers the frequency range from 200 MHz to 3 GHz, we observed pronounced compressibility effects in liquid water, and confirmed that this liquid remains Newtonian in this range. In contrast, 1-decanol liquid exhibits a non-Newtonian behavior, with a frequency-dependent viscosity associated with relaxation times that we could reveal experimentally. The thesis work provides insights into the behavior of immersed optomechanical disk resonators and demonstrates their potential to probe the multiphysics properties of a liquid at the micron scale

In this thesis we report on two matters, (i) time-resolved single particle bio-sensing using a cavity enhanced refractive index sensor with unmatched sensitivity, and (ii) the theoretical analysis of parametric normal mode splitting in cavity optomechanics, as well as the quantum limit of a displacement transducer that relies on multiple cavity modes. It is the unifying element of these studies that they rely on a high-Q optical cavity transducer and amount to a precision measurement of an optical frequency. In the first part, we describe an experiment where a high-Q toroidal microcavity is used as a refractive index sensor for single particle studies. The resonator supports whispering gallery modes (WGM) that feature an evanescent fraction, probing the environment close to the toroid's surface. When a particle with a refractive index, different from its environment, enters the evanescent field of the WGM, the resonance frequency shifts. Here, we monitor the shift with a frequency resolution of df/f=7.7e-11 at a time resolution of 100µs , which constitutes a x10 improvement of the sensitivity and a x100 improvement in time resolution, compared to the state of the art. This unprecedented sensitivity is the key to real-time resolution of single lipid vesicles with 25nm radius adsorbing onto the surface. Moreover -- for the first time within one distinct measurement -- a record number of up to 200 identifiable events was recorded, which provides the foundation for a meaningful statistical analysis. Strikingly, the large number of recorded events and the high precision revealed a disagreement with the theoretical model for the single particle frequency shift. A correction factor that fully accounts for the polarizability of the particle, and thus corrects the deviation, was introduced and establishes a quantitative understanding of the binding events. Directed towards biological application, we introduce an elegant method to cover the resonator surface with a single lipid bilayer, which creates a universal, biomimetic interface for specific functionalization with lipid bound receptors or membrane proteins. Quantitative binding of streptavidin to biotinylated lipids is demonstrated. Moving beyond the detection limit, we provide evidence that the presence of single IgG proteins (that cannot be resolved individually) manifests in the frequency noise spectrum. The theoretical analysis of the thermo-refractive noise floor yields a fundamental limit of the sensors resolution. The second part of the thesis deals with the theoretical analysis of the coupling between an optical cavity mode and a mechanical mode of much lower frequency. Despite the vastly different resonance frequencies, a regime of strong coupling between the mechanics and the light field can be achieved, which manifests as a hybridization of the modes and as a mode splitting in the spectrum of the quadrature fluctuations. The regime is a precondition for coherent energy exchange between the mechanical oscillator and the light field. Experimental observation of optomechanical mode splitting was reported shortly after publication of our results [cf. Gröblacher et al., Nature 460, 724--727]. Dynamical backaction cooling of the mechanical mode can be achieved, when the optical mode is driven red-detuned from resonance. We use a perturbation and a covariance approach to calculate both, the power dependence of the mechanical occupation number and the influence of excess noise in the optical drive that is used for cooling. The result was one to one applied for data analysis in a seminal article on ground state cooling of a mechanical oscillator [cf. Teufel et al., Nature 475, 359--363]. In addition we investigate a setting, where multiple optical cavity modes are coupled to a single mechanical degree of freedom. Resonant build-up of the motional sidebands amplifies the mechanical displacement signal, such that the standard quantum limit for linear position detection can be reached at significantly lower input power.
In dieser Dissertation werden zwei Themen behandelt. Im ersten Teil widmen wir uns experimentell der zeitaufgelösten Messung von Liposomen mit Hilfe eines Nahfeld-Brechungsindex-Sensors. Der zweite Teil handelt von der theoretischen Beschreibung des Regimes der starken Kopplung zwischen einem mechanischen Oszillator und dem Feld eines optischen Resonators. Des Weiteren erörtern wir ein Messschema, das es erlaubt eine mechanische Bewegung, mit Hilfe von mehreren optischen Resonatormoden genauer auszulesen. Die Gemeinsamkeit beider Arbeiten besteht darin, dass es sich jeweils um eine Präzisionsmessung einer optischen Frequenz handelt. Im experimentellen Teil benutzen wir Toroid-Mikroresonatoren mit extrem hoher optischer Güte als Biosensoren. Dabei handelt es sich um eine ringförmige Glasstruktur, entlang welcher Licht im Kreis geleitet wird. Dazu muss eine Resonanzbedingung erfüllt sein, die besagt, dass der (effektive) Umfang des Rings einem ganzzahligen Vielfachen der optischen Wellenlänge entspricht. Ein Teil des zirkulierenden Lichts ist als evaneszente Welle empfänglich für Brechungsindexänderungen nahe der Oberfläche des Resonators. Ein Partikel, dessen Brechungsindex sich von dem der Umgebung unterscheidet, induziert beim Eintritt in das evaneszente Feld eine Frequenzverschiebung der optischen Resonanz. Im Rahmen dieser Arbeit lösen wir relative Frequenzverschiebungen mit einer Genauigkeit von df/f=7.7e-11 und einer Zeitkonstante von 100µs auf. Dies stellt eine Verbesserung des derzeitigen Stands der Technik um einen Faktor x10 in der Frequenz und einen Faktor x100 in der Zeit dar. Diese bisher unerreichte Empfindlichkeit der Messmethode ist der Schlüssel zur Echtzeitdetektion einzelner Lipidvesikel mit einem Radius von 25nm . Zudem gelingt es uns innerhalb einer Messung, bis zu 200 Einzelteilchenereignisse aufzunehmen, welche die Basis für eine aussagekräftige Statistik liefern. Bemerkenswerterweise konnten wir Dank der außerordentlichen Präzision und der Vielzahl der Ereignisse eine Abweichung zur bis dato akzeptierten und angewandten Theorie feststellen. Wir ergänzen das Model um einen Korrekturfaktor, der die Polarisierbarkeit des Teilchens vollständig berücksichtigt und erlangen dadurch ein umfassendes und quantitatives Verständnis der Messergebnisse. Im Hinblick auf biologisch relevante Fragestellungen zeigen wir eine elegante Methode auf, die es erlaubt, den Resonator mit einer einzelnen Lipidmembran zu beschichten. Wir kreieren somit eine biomimetische Schnittstelle, welche das Grundgerüst für eine spezifische Funktionalisierung mit lipidgebundenen Rezeptoren, Antikörpern oder Membranproteinen darstellt. Des Weiteren zeigen wir, dass der Empfindlichkeit eine fundamentale Grenze durch thermische Brechungsindexfluktuationen gesetzt ist. Hierzu wird ein theoretisches Modell speziell für den relevanten niederfrequenten Bereich errechnet. Im zweiten Teil der Arbeit beschäftigen wir uns mit der theoretischen Beschreibung eines optischen Resonators, dessen Lichtfeld an eine mechanische Schwingung gekoppelt ist. Obwohl sich die Resonanzfrequenzen der Optik und der Mechanik typischerweise um mehrere Größenordnungen unterscheiden, existiert ein Regime der starken Kopplung, in dem die Fluktuationen des Lichts und die mechanischen Vibrationen hybridisieren. Dies offenbart sich zum Beispiel im Phasenspektrum, wo sich das ursprüngliche Maximum der Resonanz in zwei Maxima aufspaltet. Die starke Kopplung stellt die Grundlage für kohärenten Energie- und Informationsaustausch zwischen Licht und Mechanik dar und ist daher von besonderem technischen und wissenschaftlichen Interesse. Es ist anzumerken, dass die starke Kopplung und die einhergehende Aufspaltung der Resonanz bereits kurz nach Veröffentlichung unserer theoretischen Beschreibung im Experiment beobachtet wurde [vgl. Gröblacher et al., Nature 460, 724--727]. Wenn der optische Resonator (zur längeren Wellenlänge hin) verstimmt von der Resonanz angeregt wird, kann über dynamische Rückkopplung eine effektive Kühlung der mechanischen Schwingung erreicht werden. Wir berechnen die thermische Besetzungszahl der mechanischen Mode (und somit die Temperatur) mit Hilfe eines störungstheoretischen und eines Kovarianzansatzes. Dabei berücksichtigen wir sowohl ein klassisches Rauschen des optischen Feldes als auch den Einfluss der optomechanischen Kopplung auf die Grenztemperatur. Der hergeleitete Ausdruck für die finale Besetzungszahl wurde eins zu eins für die Datenanalyse in dem wegweisenden Artikel über das Kühlen eines mechanischen Oszillators in den Quantengrundzustand verwendet [vgl. Teufel et al., Nature 475, 359--363]. Abschließend betrachten wir ein Schema, bei dem die Lichtfelder mehrerer optischer Resonanzen an eine mechanischen Schwingung gekoppelt sind. Die resonante Verstärkung der Information über die mechanische Bewegung in den optischen Seitenbändern ermöglicht es, eine durch das Standard Quantenlimit begrenzte Empfindlichkeit bei signifikant niedriger Eingangsleistung zu erreichen.

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.

Durant l’última dècada, els materials intel·ligents han emergit com a una tendència fascinant en la ciència de materials. En aquest àmbit, els materials optomecànics tous són especialment interessants per desenvolupar dispositius de sensat i actuació innovadors gràcies a la naturalesa inalàmbrica dels sistemes òptics i la possibilitat de ser combinada amb altres tipus d’estimulació. En particular, la inclusió de nanopartícules o nanoestructures plasmòniques en substrats polimèrics tous comporta possibilitats interessants, com les característiques òptiques fàcils de modificar dels materials plasmonics i la gran elasticitat i robustesa dels materials tous. Aquesta nova classe de materials és referida en aquesta tesi com a metamaterials plasmomecànics tous. Tot i així, aquest particular camp d’estudi es relativament recent. Per aquest motiu, aquesta tesi està dedicada al desenvolupament de nous metamaterials plasmomecànics tous, portant a terme l’estudi detallat de les seves propietats òptiques i mecàniques i el seu disseny per a l’ús en aplicacions pràctiques en l’àmbit del sensat i l’actuació. Específicament, les dificultats d’implementar absorbents de llum en ampla de banda eficients en substrats flexibles o elàstics són abordades amb el desenvolupament d’un nou metamaterial basat en una capa de ferro nanoestructurat sobre una capa fina elastomèrica. Aquest nou metamaterial combina les ressonàncies plasmòniques amortides del ferro nanoestructurat amb l’absorció infraroja del PDMS per aconseguir una absorció independent de l’angle i amb un gran ample de banda. Aquest excepcional comportament òptic és explotat per a desenvolupar diversos dispositius foto-termo-mecànics inalàmbrics i innovadors. A través de l’explotació de les propietats magnètiques del ferro, el mateix metamaterial és utilitzat també per a desenvolupar un actuador inalàmbric i multi-funcional. Específicament, el control de la força i direcció de l’actuació magnètica és combinada amb la actuació lumínica, permetent condicions d’operació remotes i versàtils. A més a més, s’ha aconseguit la incorporació de la funcionalitat d’auto-sensat a través d’incloure una estructura de reixa fotònica a la part posterior de l’actuador. La resposta mecànica de l’actuador a qualsevol estímul extern es mostra com a un canvi de coloració i és quantificada en temps real a través de les imatges preses a través d’una càmera convencional. L’actuació remota i multi-estímul del dispositiu, juntament amb les seves capacitats d’auto-sensat estableixen les bases per al desenvolupament de mecanismes per a operacions en robòtica tova en ambients inaccessibles o perillosos. Finalment, s’ha demostrat el desenvolupament de la primera cavitat Fabry-Perot estirable i amplificada plasmònicament per al sensat òptic d’esforç. Aquest nou material consisteix en una matriu de “mitja-closques” d’or plasmonic auto-organitzades, les quals són auto-incrustades dins un substrat elastomèric arrugat. Aquesta morfologia dóna lloc a un comportament òptic poc convencional que pot ser afinat a través de les condicions de fabricació. El material presenta una resposta òptica intensa a l’esforç mecànic, amb sensibilitat similar a altres aproximacions basades en processos de fabricació més complexes. A més a més, presenta gran robustesa i deformabilitat, les quals permet la seva aplicació com a sensor inalàmbric d’esforç en superfícies corbades. En resum, aquesta tesi aborda diferents reptes en el desenvolupament de materials intel·ligents optomecànics tous per a diverses plataformes de sensat i actuació.
Durante la ultima década, los materiales inteligentes han emergido como una tendencia fascinante en la ciencia de materiales. En éste ámbito, los materiales optomecánicos blandos son especialmente interesantes para desarrollar dispositivos de sensado y actuación innovadores gracias a la naturaleza inalámbrica de los sistemas ópticos y la posibilidad de ser combinada con otros tipos de estimulación. En particular, la inclusión de nanopartículas o nanoestructuras plasmónicas en sustratos poliméricos blandos conlleva posibilidades interesantes, como las características ópticas fáciles de modificar de los materiales plasmónicos y la gran elasticidad y robustez de los materiales blandos. Ésta nueva clase de materiales es referida en esta tesis como a metamateriales plasmomecánicos blandos. Aún así, éste particular campo de estudio es relativamente reciente. Por éste motivo, ésta tesis está dedicada al desarrollo de nuevos metamateriales plasmomecánicos blandos, llevando a cabo el estudio detallado de sus propiedades ópticas y mecánicas y su diseño para el uso en aplicaciones prácticas en el ámbito del sensado y la actuación. Específicamente, las dificultades de implementar absorbentes lumínicos de ancho de banda amplio eficientes en sustratos flexibles o elásticos son abordadas con el desarrollo de un nuevo metamaterial basado en una capa de hierro nanoestructurado sobre una capa fina elastomèrica. Éste nuevo metamaterial combina las resonancias plasmónicas amortiguadas del hierro nanoestructurado con la absorción infrarroja del PDMS para conseguir una absorción independiente del ángulo y con un gran ancho de anda. Ése excepcional comportamiento óptico es explotado para desarrollar diferentes dispositivos foto-termo-mecánicos inalámbricos y innovadores. A través de la explotación de las propiedades magnéticas del hierro, el mismo metamaterial es utilizado para desarrollar un actuador inalámbrico y multi-funcional. Específicamente, el control de la fuerza y dirección de la actuación magnética es combinada con la actuación lumínica, permitiendo condiciones de operación remotas y versátiles. Además, se ha conseguido la incorporación de la funcionalidad de auto-sensado a través de incluir una estructura de malla fotónica en la parte posterior del actuador. La respuesta mecánica del actuador a cualquier estímulo externo se muestra como un cambio de coloración y es cuantificada en tiempo real a través de las imágenes tomadas a través de una cámara convencional. La actuación remota y multi-estímulo del dispositivo, juntamente con las capacidades de auto-sensado establecen las bases para el desarrollo de mecanismos para operaciones en robótica blanda en ambientes inaccesibles o peligrosos. Finalmente, se ha demostrado el desarrollo de la primera cavidad Fabry-Perot estirable y amplificada plasmónicamente para el sensado óptico de esfuerzo. Éste nuevo material consiste en una matriz de “media-cáscara” de oro plasmónico auto-organizadas, las cuales son auto-incrustadas dentro de un sustrato elastomérico arrugado. Ésta morfología da lugar a un comportamiento óptico poco convencional que puede ser ajustado a través delas condiciones de fabricación. El material presenta una respuesta óptica intensa al esfuerzo mecánico, con sensibilidad similar a otras aproximaciones basadas en procesos de fabricación más complejas. Además, presenta gran robustez y deformabilidad, las cuales permiten su aplicación como sensor inalámbrico de esfuerzo en superficies curvas. En resumen, ésta tesis aborda diferentes retos en el desarrollo de materiales inteligentes optomecánicos blandos para diversas plataformas de sensado y actuación.
During the last decade, smart materials have emerged as an exciting trend in materials science. Within this scope, soft optomechanical materials are especially appealing for developing innovative sensing and actuation devices due to the wireless nature of optics and the possibility to be combined with other types of stimuli. In particular, the inclusion of plasmonic nanoparticles or nanostructures into soft polymer substrates entail interesting possibilities, such as the easily-tunable optical features of plasmonic materials and large elasticity and robustness of soft materials. This new class of materials are referred as soft plasmomechanical metamaterials. However, this particular field of study is relatively recent. To that end, this thesis is dedicated to the development of new soft plasmomechanical metamaterials, bringing together the detailed study of their optical and mechanical properties with the design for their use into practical applications within the scope of sensing and actuation. Specifically, the difficulties of implementing efficient broadband light absorbers into flexible or stretchable substrates are tackled by the development of a novel metamaterial based on a nanostructured iron layer on a thin elastomer film. This new metamaterial combines the damped plasmonic resonances of the nanostructured iron with the infrared absorption of PDMS to achieve an unprecedented broadband and angle-independent light absorption in flexible materials. This exceptional optical behaviour, together with a large mismatch on the mechanical properties of both materials are exploited to develop diverse innovative untethered photo-thermo-mechanical devices. By exploiting the magnetic properties of iron, the same metamaterial is then used to develop an untethered, multi-functional actuator. Specifically, the control of the magnetic actuation strength and direction is combined with the broadband light actuation, enabling remote and versatile work operation conditions for soft-robotics applications. In addition, the incorporation of a self-sensing functionality is achieved by including a photonic grating structure at the actuator back-side, which provides structural coloration to the actuator. The mechanical response of the actuator to any external stimuli is displayed as a coloration shift and quantified in real-time by the images taken by a conventional camera. The remote and multi-stimuli actuation of the device, together with its self-sensing capabilities set the foundations for soft robotics operations in inaccessible or hazardous environments. Finally, the development of the first stretchable plasmonic-enhanced Fabry-Perot cavity is demonstrated for optical strain sensing. This new material consists on an array of self-assembled plasmonic gold semi-shells which are self-embedded into a wrinkled elastomer matrix. This peculiar morphology gives rise to unconventional optical behaviour that can be tuned by the manufacturing conditions. The material shows strong optical to mechanical strain, with similar sensitivity to other sensing approaches based in more complex fabrication processes. Furthermore, it shows large robustness and deformability, that enables its application as wireless pressure/strain sensing into curved surfaces. Overall, this thesis tackles different challenges in the development of soft smart optomechanical materials for diverse sensing and actuation platforms.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

L'optomeccanica è un campo di ricerca in via di sviluppo che esplora l'interazione tra la luce e la meccanica. Le moderne tecniche di nanofabbricazione per dispositivi meccanici e strutture ottiche a bassissima dissipazione hanno dato permesso un importante progresso sperimentale all'optomeccanica, sia per applicazioni che per la ricerca fondamentale. Ci sono diversi modi in cui luce e meccanica interagiscono tra loro. In questa tesi sono state sviluppate tre diverse macroaree dell'optomeccanica: giroscopi ottici, forze optomeccaniche e spettroscopia fotoacustica. L'interazione tra luce e movimento meccanico è stata studiata a partire dal concetto di giroscopi ottici. I giroscopi ottici sono sensori di velocità angolare. Allo stato dell’arte, i principi fisici e le configurazioni utilizzate per la realizzazione di giroscopi ottici non sono adatti per miniaturizzarli alla microscala. In questa tesi sono state presentate e indagate alcune nuove configurazioni che sfruttano il concetto di "punti eccezionali". Secondo l'effetto relativistico chiamato effetto Sagnac, le frequenze di risonanza di due modi contropropaganti in un risonatore ad anello sono separate da una quantità proporzionale alla velocità angolare del “frame”. Tuttavia, la possibilità di miniaturizzare il giroscopio ottico è limitata dal fatto che la sperazione tra le risonanze è proporzionale al raggio del risonatore ad anello. Nel primo capitolo è stato introdotto il concetto di simmetria “parity-time” (PT) come soluzione per l'integrazione di sensori di velocità angolare. Predisponendo due risonatori ottici accoppiati progettati per essere al cosiddetto "punto eccezionale", si è potuto dimostrare che la separazione tra le autofrequenze è proporzionale alla velocità angolare del dispositivo, con una sensibilità che è di diversi ordini di grandezza superiore a quella classica Giroscopio Sagnac. In questa tesi è stato dimostrato che un problema del giroscopio a simmetria PT è l'instabilità dei modi ottici quando il sistema è in rotazione. Ecco perché è stata proposta l'idea del giroscopio a simmetria anti-PT, utilizzando una guida d'onda ausiliaria a forma di U per accoppiare indirettamente due risonatori ottici. La soluzione proposta si è dimostrata un'alternativa interessante per il rilevamento della velocità angolare, grazie allo schema di facile lettura e all'assenza di modi instabili. Una semplice sorgente a banda larga e un fotorilevatore sarebbero sufficienti per leggere l'uscita del sensore. Infine, è stata proposta una nuova configurazione per un giroscopio a simmetria anti-PT. È diverso dalla configurazione a forma di U e utilizza solo una guida d'onda diritta ausiliaria per accoppiare indirettamente due risonatori ottici. Questa architettura si è dimostrata molto più robusta, insensibile ad alcuni errori di fabbricazione, rispetto a quella ad U. La seconda area dell'optomeccanica che è stata studiata in questa tesi include le forze optomeccaniche. In particolare, è stato sviluppato un modello generalizzato in grado di calcolare lo spostamento meccanico di un solo grado di libertà di un setup optomeccanico generale. Il modello inizialmente proposto da Rakich è stato esteso a sistemi in cui si considerano guadagni o perdite. Quindi, il modello è stato utilizzato per valutare l'effetto delle forze ottiche in un sistema a simmetria PT con guide d'onda sospese nella regione di accoppiamento. È stato dimostrato che è possibile incrementare le forze ottiche grazie alla condizione di simmetria PT. In secondo luogo, è stata proposta una modellazione analitica della dinamica meccanica di guide d'onda ottiche sospese accoppiate soggette a forze optomeccaniche, inclusa una modellazione dello smorzamento, con effetto squeezing. Tale modello analitico, insieme all'algoritmo numerico proposto, può essere utilizzato per trovare l'evoluzione del sistema nel dominio del tempo di complesse strutture optomeccaniche, come gli interruttori optomeccanici. Inoltre, è stato mostrato un lavoro sperimentale su un interruttore optomeccanico. Sono state spiegate tutte le fasi di fabbricazione per realizzare il dispositivo optomeccanico integrato. Infine, è stata analizzata la spettroscopia fotoacustica. Il sensore allo stato dell'arte della spettroscopia fotoacustica al quarzo (QEPAS) è stato modellato e simulato ed è stato proposto un nuovo sensore semi-integrato. Un problema dei sensori QEPAS attuali è la necessità di allineamento per i componenti ottici. Inoltre, la dimensione di tutti i dispositivi coinvolti nel setup rende difficile realizzare sensori portatili e compatti. L'idea proposta in questa tesi è quella di integrare tutti i componenti ottici necessari a guidare la luce in prossimità del diapason al quarzo per ridurre drasticamente le dimensioni del setup complessivo ed evitare il problema dell'allineamento ottico. La possibilità di utilizzare guide d'onda ottiche integrate per guidare la luce rende possibile utilizzare risonatori ottici per migliorare il segnale fotoacustico che viene letto attraverso il diapason al quarzo. La configurazione proposta è pensata per utilizzare un laser integrato legato a un chip di silicio, dove vengono realizzate tutte le guide d'onda. In questo caso un risuonatore meccanico molto piccolo può essere collegato al chip di silicio, al fine di aumentare l'ampiezza del segnale di pressione. In tal modo, è possibile ottenere prestazioni paragonabili al sensore QEPAS all'avanguardia. Un risultato del genere potrebbe aprire la strada a una nuova generazione di sensori QEPAS compatti, in grado di superare il problema delle dimensioni dei setup e dell'allineamento dei componenti ottici.
Optomechanics is a developing field of research exploring the interaction between light and mechanical motion. The modern nanofabrication techniques for mechanical devices and ultralow dissipation optical structures have provided a way for giving an important experimental progress to optomechanics, both for applications and for fundamental investigations. In this thesis optomechanics will be investigated in different aspects, in its general meaning, both theoretically and experimentally. There are different ways in which light and mechanics interact with each other. In this thesis three different macro areas of optomechanics have been developed: optical gyroscopes, optomechanical forces and photoacoustic spectroscopy. The interaction between light and mechanical motion has been investigated starting from the concept of optical gyroscopes. Optical gyroscopes are sensors of angular velocity. In the present state of the art, the physical principles and the configurations used for realizing optical gyroscopes are not suitable for miniaturizing them to the microscale. In this thesis some new configurations exploiting the concept of "exceptional points" have been presented and investigated. According to the relativistic effect called Sagnac effect, the resonance frequencies of two counterpropagating modes in a ring resonator are separated by a quantity proportional to the angular velocity of the frame. However, the possibility of miniaturizing the optical gyroscope is limited by the fact that the resonance splitting is proportional to the radius of the ring resonator. In the first chapter the concept of parity-time symmetry has been introduced as a solution for the integration of angular velocity sensors. By setting up two coupled optical resonators designed to be at the so called "exceptional point", it could be demonstrated that the eigenfrequency splitting is proportional to the angular velocity of the device, with a sensitivity that is several orders of magnitude higher than the classical Sagnac gyroscope. In this thesis it has been demonstrated that one problem of the parity-time symmetric gyroscope is the instability of the optical eigenmodes when the system is in rotation. That is why the idea of the anti-parity-time-symmetric gyroscope was proposed, using a U-shaped auxiliary waveguide to indirectly couple two optical resonators. The proposed solution has been shown to be an interesting alternative for angular velocity sensing, thanks to the easy readout scheme and the absence of modes instability. A simple broadband source, together with a photodetector could be used to read the output of the sensor. Finally, a new configuration for an anti-parity-time-symmetric gyroscope has been proposed. It is different from the U-shaped configuration and uses only an auxiliary straight waveguide to indirectly couple two optical resonators. This architecture has been shown to be much more robust, insensitive to some fabrication errors, with respect to the U-shaped one. The second area of optomechanics that has been investigated in this thesis includes optomechanical forces. In particular, a generalized model able to calculate the mechanical displacement of only one degree of freedom of a general optomechanical setup has been developed. The model initially proposed by Rakich has been extended to systems where gain or loss are considered. Then, the model has been used to evaluate the effect of optical forces in parity-time symmetric system with suspended waveguides in the coupling region. It has been demonstrated that it is possible to enhance the optical forces thanks to condition of parity-time symmetry. Secondly, an analytical modelling of the dynamics of optomechanically coupled suspended optical waveguides has been proposed, including a modelling of the damping, with the squeezing effect. Such an analytical model, together with the numerical proposed algorithm can be used to find the evolution of the system in the time domain of complex optomechanical structures, such as optomechanical switches. Also, an experimental work on an optomechanical switch has been shown. All the fabrication steps to fabricate the integrated optomechanical device has been explained. The most critical part during the fabrication has been the underetching of suspended waveguides. In fact, using a wet HF etching process caused the suspended waveguides to get stuck. Using a ZEP mask and a vapor HF etching, unexpected HF bubbles appeared on the surface. So, a hard mask has been used to guarantee the successful underetching of the device. Finally, the experimental measurement on the chip showed the expected behaviour of the device. Finally, Photoacoustic Spectroscopy has been analysed. The state-of-art Quartz-Enhanced PhotoAcoustic Spectroscopy (QEPAS) sensor has been modelled and simulated and a new semi-integrated sensor has been proposed. One problem of the state-of-art QEPAS sensors is the necessity of alignment for optical components. Moreover, the dimension of all the devices involved in the setup makes it difficult to realize portable and compact sensors. The idea proposed in this thesis is to integrate all the optical components needed to guide the light in the proximity of the Quartz Tuning Fork to drastically reduce the dimension of the overall setup and to avoid the problem of optical alignment. The possibility of using integrated optical waveguides to guide light makes it possible to use optical resonators to enhance the photoacoustic signal that is read through a Quartz Tuning Fork. The proposed setup is meant to use an integrated laser bonded to a Silicon chip, where all the waveguides are realized. In this case a very small mechanical resonator can be bonded over the Silicon chip, in order to enhance the amplitude of the pressure signal. In such a way, performance comparable with the state-of-art QEPAS sensor can been achieved. Such a result could pave the way to a new generation of compact QEPAS sensor, that could overcome the problem of the size of the setups and of the alignment of optical components.

Light has long been used for the precise measurement of moving bodies, but the burgeoning field of optomechanics is concerned with the interaction of light and matter in a regime where the typically weak radiation pressure force of light is able to push back on the moving object. This field began with the realization in the late 1960's that the momentum imparted by a recoiling photon on a mirror would place fundamental limits on the smallest measurable displacement of that mirror. This coupling between the frequency of light and the motion of a mechanical object does much more than simply add noise, however. It has been used to cool objects to their quantum ground state, demonstrate electromagnetically-induced-transparency, and modify the damping and spring constant of the resonator. Amazingly, these radiation pressure effects have now been demonstrated in systems ranging 18 orders of magnitude in mass (kg to fg).

In this work we will focus on three diverse experiments in three different optomechanical devices which span the fields of inertial sensors, closed-loop feedback, and nonlinear dynamics. The mechanical elements presented cover 6 orders of magnitude in mass (ng to fg), but they all employ nano-scale photonic crystals to trap light and resonantly enhance the light-matter interaction. In the first experiment we take advantage of the sub-femtometer displacement resolution of our photonic crystals to demonstrate a sensitive chip-scale optical accelerometer with a kHz-frequency mechanical resonator. This sensor has a noise density of approximately 10 micro-g/rt-Hz over a useable bandwidth of approximately 20 kHz and we demonstrate at least 50 dB of linear dynamic sensor range. We also discuss methods to further improve performance of this device by a factor of 10.

In the second experiment, we used a closed-loop measurement and feedback system to damp and cool a room-temperature MHz-frequency mechanical oscillator from a phonon occupation of 6.5 million down to just 66. At the time of the experiment, this represented a world-record result for the laser cooling of a macroscopic mechanical element without the aid of cryogenic pre-cooling. Furthermore, this closed-loop damping yields a high-resolution force sensor with a practical bandwidth of 200 kHZ and the method has applications to other optomechanical sensors.

The final experiment contains results from a GHz-frequency mechanical resonator in a regime where the nonlinearity of the radiation-pressure interaction dominates the system dynamics. In this device we show self-oscillations of the mechanical element that are driven by multi-photon-phonon scattering. Control of the system allows us to initialize the mechanical oscillator into a stable high-amplitude attractor which would otherwise be inaccessible. To provide context, we begin this work by first presenting an intuitive overview of optomechanical systems and then providing an extended discussion of the principles underlying the design and fabrication of our optomechanical devices.

This thesis investigated single nanoparticle/molecule detections using a whispering gallery mode (WGM) microcavity, with focuses on sensing with the cavity optomechanical oscillation (OMO). The high quality (Q) factor and small mode volume properties of a WGM microcavity make it possible to establish a strong intracavity power density with a small amount of input optical power. Such a high optical power density exerts a radiation pressure that is sufficient to push the cavity wall moving outward. The dynamic interaction between the optical field and the mechanical motion eventually results in a regenerative mechanical oscillation of the WGM cavity, which is termed as the optomechanical oscillation. With a high Q spherical microcavity, the observation of OMO in heavy water is reported. To the best knowledge of the author, this is the first demonstration of the cavity OMO in an aqueous environment. Furthermore, by utilizing the properties of reactive sensing, cavity OMO, and optical spring effect, we demonstrated a new sensing mechanism that improves the WGM microcavity sensing resolution by several orders of magnitude. Finally, we conducted the demonstration of in-vitro molecule sensing by detecting single bindings of the 66 kDa Bovine Serum Albumin (BSA) protein molecules at a signal-to-noise ratio of 16.8.
Graduate

The penetration of magnetic fields in the form of quantized vortices in type-II superconductors is well known. However, it is not well known that such vortices can be electrically charged. The effect is quite subtle and originates from the particle-hole symmetry in a superconductor. The high Tc superconductors (HTS) are predicted to be better candidates for vortex-charge detection due to their large superconducting gap. Thus far, a direct measurement of charged cores has remained challenging due to their small value and electrostatic screening by the surrounding opposite charges. In recent years, cavity-optomechanical techniques have emerged as an attractive method to improve the sensitivity of various measurements. Such methods have shown exquisite force sensitivities down to the standard quantum limit and control over the quantum states of the motion. Recently, such techniques have also drawn attention to probe the thermodynamic properties of atomically thin two-dimensional (2D) materials. The 2D crystals are particularly attractive for developing mechanical resonators and their integration in optomechanical device due to their low mass, and hence larger coupling with light field. Motivated from these aspects, we develop a device to directly detect the charges in the flux-vortices by measuring the electromechanical response. Here the electrostatic effect of vortex-charge is transduced to the mechanical response. To study the vortex charge, a few UC thick crystals of high-transition temperature superconductor Bi2Sr2CaCu2O8+δ (BSCCO) is used for the mechanical resonator. One important parameter of the mechanical resonator is its resonant frequency. However, estimating the resonant frequency requires elastic modulus like Young's modulus and pre-tension in the flake. While the elastic coefficients of the bulk crystals of BSCCO have been observed with large variations, there is no investigation into the elastic properties of a few UC thick nanoscale samples. Further, the mechanical properties of a few unit cells (UC) thick exfoliated crystals could be significantly different from their bulk counterpart. To begin with, we present systematic measurements of the mechanical properties of a few unit cells (UC) thick exfoliated crystals of a high-Tc cuprate superconductor BSCCO. We determine the elastic properties of these crystals by deformation using an atomic force microscope (AFM) at room temperature. With the spatial measurements of local compliance and their detailed modelling, we determine Young's modulus of rigidity and the pre-stress. Young's modulus of rigidity is found to be in the range of 22 GPa to 30 GPa for flakes with thickness from 5 UC to 18 UC. The pre-stress spreads over the range of 5 MPa - 46 MPa, indicating a run-to-run variation during the exfoliation process. The determination of Young's modulus of rigidity for thin flakes is further verified from the recently reported buckling technique [1]. In the next chapter, we present nanoelectromechanical resonators fabricated with thin exfoliated crystals of BSCCO. The mechanical r= eadout is performed by capacitively coupling their motion to a coplanar waveguide microwave cavity fabricated with a superconducting alloy of molybdenum-rhenium (MoRe). We demonstrate mechanical frequency tunability with external dc-bias voltage and quality factors up to ~36600. Our spectroscopic and time-domain measurements show that mechanical dissipation in these systems is limited by the contact resistance arising from resistive outer layers. The temperature dependence of dissipation indicates the presence of tunnelling states, further suggesting that their intrinsic performance could be as good as other two-dimensional atomic crystals such as grap= hene [2]. Learning from these two experiments, we improve the performance of the device and carry out the mechanical exfoliation in inert atmosphere. We integrate a mechanical resonator made of a thin flake of HTS BSCCO into a microwave circuit to realize a cavity-electromechanical device. In the final chapter, we studied the electromechanical response of the mechanical resonator when a magnetic field perpendicular to the CuO2 plane is applied. As the magnetic field penetrates the surface of a superconductor, it results in the formation of flux-vortices. These flux-vortices will have charged vortex core and create a dipolelike electric field. Due to the exquisite sensitivity of cavity-based devices to the external forces, we directly detect the charges in the flux vortices by measuring the electromechanical response of the mechanical resonator [3]. Our measurements reveal the strength of surface electric dipole moment due to a single vortex core to be approximately 30 |e|aB, where aB is the Bohr radius and e is the electron charge. Further, using the value of surface dipole moment, we have estimated the vortex line charge to be +4.9 × 10-2|e|/nm, which is equivalent to a charge per CuO_2 layer to be +3.7 × 10-2|e|.

These notes discuss electromechanical devices in the quantum regime, a topic closely related to cavity optomechanics. Both cavity optomechanics and quantum electromechanics have their roots in gravitational-wave detection. As such, most of their applications are associated with ultrasensitive sensing. In contrast, these notes deal with an emerging application of quantum electromechanics: signal processing. Such applications are a natural consequence of shrinking the mechanical elements from the metre-scale resonators used in gravitational wave detectors to the micron scale, where quantum effects are more evident. Indeed, MEMS are a crucial technology for classical information processing and modern wireless communication. The advent of quantum information processing, particularly with superconducting circuits, means that there is now a need for analogue signal processing functions operating at microwave frequencies and in the quantum regime. Electromechanical devices have now entered this regime as they can store, amplify, squeeze, entangle, temporally shape, and frequency convert microwave signals.

We experimentally demonstrate entanglement-enhanced optomechanical sensing in which entangled optical probes jointly read out the displacements of two mechanical membranes, enabling enhanced force sensitivities and enlarged measurement bandwidths.

We have developed a nanometer-sized ultrasound sensor based on 1D photonic crystals, capable of detecting ~mPa noise pressures from 10kHz to 300kHz. This sensor can be used for fibre-based optomechanical ultrasound sensing.