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Full-wave analysis and design of opto-mechanical systems
No full textLe micro-cavità optomeccaniche (OM) sono oggetto di intenso studio, come possibile fonte di nuove funzionalità, concetti e opportunità che vanno oltre la tecnologia standardin termini di propagazione, generazione ed elaborazione dei fononi a microonde al micro e nanoscala. In questo framework, l'interazione OM non lineare è oggetto di indagini approfondite, Il tipo di ricerca sopra riportato è altamente interdisciplinare, e si sposta lungo la frontiera di molti campi, come ad esempio i filtri a microonde per onde acustiche di superficie (SAW), fotonica, elettronica ed, inoltre, nanofisica e ottica quantistica. In particolare, alcuni recenti risultati sul raffreddamento dello stato fondamentale OM promettono un controllo coerente del movimento quantico di risonatori meccanici. Un esempio pertinente è fornito dal recente accoppiamento di un qubit superconduttore alle vibrazioni meccaniche di un risonatore piezoelettrico, ovvero risuonatori meccanici a microonde operati al limite quantico. Meccanismi fondamentali dei sistemi OM, come la pressione di radiazione e la forza elettrostrittiva esercitata da campi elettromagnetici sulla materia e, al contrario, modulazione fotoelastica dei campi elettromagnetici, sono responsabili delle forti interazioni che si verificano nelle cavità risonanti. In effetti, pressioni enormi possono agire nella regione della cavità a causa del fattori Q elevati raggiungibili sia per le modalità ottiche che meccaniche che coesistono nella cavità. Il meccanismo sopra descritto produce utili trasduzioni e attuazioni vibrazionali ai livelli di potenza da μ- a m-watt. Inoltre porta potenzialmente allo sviluppo di fonon laser e rivelatori. È probabile che tale sviluppo consenta l'elaborazione del segnale fononico a temperatura ambiente e l'integrazione di elementi fonetici su una piattaforma di silicio, in grado di generare nuovi modi di elaborare le informazioni. L'obiettivo finale è l'utilizzo di fononi, invece di elettroni, come vettori di informazione e la costruzione di un chip che possa lavorare a temperatura ambiente, analogamente al funzionamento di dispositivi basati su SAW, per applicazioni di filtraggio e rilevamento. I vantaggi di questa nuova tecnologia sono dati, ad esempio, dal possibile funzionamento ad alta frequenza e dal basso consumo energetico. Inoltre, rispetto all'azionamento nanoelettromeccanico puro, l'approccio ottico è meno sensibile ai problemi di discrepanza capacitiva e di impedenza. Alla luce di quanto sopra, la prima parte di questa tesi tratta dell'interpretazione teorica del fenomeno OM che si verifica nelle μ-cavità ad alto Q, con l'obiettivo di fornire un approccio affidabile e rigoroso per massimizzare le prestazioni di un generatore coerente di fononi. Se si considerara il futuro sviluppo della circuiteria OM, avere un metodo in grado di fornire sia l'esatta popolazione fononica generata in una data cavità e la valutazione delle forze ottiche e meccaniche che giocano il loro ruolo nell'interazione tra le due fisiche sarà di fondamentale importanza. Insieme al modello numerico sopra descritto, è stata realizzata anche un'applicazione intuitiva. La seconda parte di questa tesi si concentra sull'eccitazione ottica del generatore di fononi, che propone un design fatto di una lastra di Si che viene prima miscelata e poi inciso selettivamente, in modo che la coda evanescente del campo elettrico possa essere migliorata e, di conseguenza, il trasferimento di energia EM tra tale lastra e la cavità OM può essere massimizzato. Questa soluzione non solo garantisce migliori prestazioni rispetto al approccio standard in fibra ottica, ma è anche adatto per l'integrazione su chip del generatore fononico, dato l'incremento della sua stabilità e affidabilità. Inoltre, sono stati compiuti sforzi per indagare i) l'estrazione del coerente fononi generati, con particolare attenzione all'equilibrio tra l'avere la necessità di un elevato fattore Q (essenziale per consentire il comportamento auto-pompato) insieme con la necessità di generare una perdita di energia meccanica e ii) la progettazione di una guida d'onda a bassa perdita, in grado di trasportare le informazioni fononiche lungo il chip. La terza parte della tesi introduce il SAW come soluzione valida per la lavorazione del fonone. Inizialmente, l'integrazione tra un launcher SAW generico (generalmente effettuata di una combinazione di InterDigitated Transciever (IDT), un materiale piezoelettrico sottile e un Si-substrato) e la cavitá OM é investigata. Un nuovo design in cui i terminali elettrici, ovvero dove viene generato il SAW, sono stati curvati radialmente per facilitare, dal punto di vista geometrico, il focus dell'onda meccanica verso una determinata regione è discusso. Successivamente, il problema di come modellare tale IDT curvo é affrontato. Addizionalmente, al fine di garantire una corretta calibrazione del SAW-launcher, un metodo numerico cha passa dalla sia rappresentazione di tipo Scattering Matrix (SM) sia dell'approccio TRL è stato sviluppato.Optomechanical (OM) micro-cavities are object of intense tudy, as a possible source of new functionalities, concepts, and opportunities beyond standard technology, with regard to microwave phonon propagation, generation, and processing at micro and nanoscale. In this framework, nonlinear OM interaction is being intensively investigated, with a view to advanced device modulation. The above kind of research is highly interdisciplinary, moving along the frontier of many fields, i.e. microwave Surface Acoustic Wave (SAW) filters, photonics, and electronics, and at the boundary between nanophysics and quantum optics. In particular, some recent achievements of OM ground state cooling promise coherent control of the quantum motion of mechanical resonators. A relevant example is provided by the recent coupling of a superconducting qubit to the mechanical vibrations of a piezoelectric resonator, i.e. microwave mechanical resonators operated at the quantum limit. Fundamental mechanisms of OM systems, such as the radiation pressure and the electrostrictive force exerted by electromagnetic fields on the matter, and, reversely, photoelastic modulation of electromagnetic fields, are responsible for strong interactions occurring in resonant cavities. In fact, huge pressures can act in the cavity region, owing to the high Q-factors achievable both for the optical and mechanical modes that coexist in the cavity. The above mechanism yields useful vibrational transduction and actuation at the μ- to m-watt power levels. Also it potentially leads to the development of phonon laser and detectors. Such development is likely to enable processing phonon signal at room temperature and integration of phononic elements on a silicon platform, capable of generating novel ways of information processing. The final objective is using phonons, instead of electrons, as vectors of information and building chips that work at room temperature, analogously to the operation of SAW-based devices, for filtering and sensing applications. Important advantages of this new technology are given, for example, by possible high frequency operation and low power consumption. In addition, comparing with pure nano-electromechanical actuation, the optical approach is less sensitive to capacitive and impedance mismatch issues. Given the above, the first part of this thesis deals with the theoretical interpretation of the OM phenomenon taking place in high-Q μ-cavities, having the aim of providing a reliable and rigorous approach for maximize the performances of a coherent phonon generator. If to consider the future development and progress of the OM circuitry, having a method capable of providing both the exact phononic population generated in a given cavity and the evaluation of the optical and mechanical forces that play their role in the interaction between the two physics, will be of fundamental importance. Together with the aforementioned numerical model, that has additionally been ported to a user-friendly application, original cavity designs exhibiting high degrees of confinement of both the optical and the mechanical resonating modes are given. The second part of this thesis focuses on the optical excitation of the coherent phonon generator, proposing a design made of a Si-slab that is first blended and then selectively etched, so that the evanescent tail of the E-field can be enhanced and, consequently, the EM energy transfer between such slab and the OM cavity can be maximized. This solution not only guarantees better performances in respect to the standard fiber-loop approach, but it is also suitable for an on-chip integration of the phononic generator, given its improved stability and reliability. Additionally, efforts are put in the investigation of i) the extraction of the coherent phonons generated in theOMcavity, with special focus on the balance between having the need of a high Q-factor (essential for enabling the self-pumped behavior) together with the necessity of generating some mechanical energy leak, and ii) the design of a low-loss waveguide, capable of carrying the phononic information along the chip. The third part of the thesis introduces the SAW as a valid solution for the phonon processing. At first, the integration between a generic SAW launcher (generally made of a combination of InterDigitated Transciever (IDT), a thin piezoelectric material and a Si-sustrate) and the targeted OM cavity is investigated. A novel design in which the electrical terminals, e.g. where the SAW is generated, have been radially curved for facilitating, from a geometrical point of view, the focus of the mechanical wave towards a certain region is given. Successively, the issue of modeling such curved IDT fingers is addressed. For ensuring a correct calibration of the SAW-launcher, aimed to isolate the latter from the rest of the OM circuitry, a numerical method making use of both the Scattering Matrix (SM) representation and the TRL approach is developed and tested against its analytic counterpart
Electromagnetic amplification of microwave phonons in nonlinear resonant microcavities
No full textIn this paper, we report on the numerical simulation of a multiphysic electromagnetic/mechanical problem, as a further step toward the development of microwave circuits based on phonon propagation at micro and nanoscale. In the future, this kind of circuits is likely to integrate optomechanically pumped phonon sources and detectors, as well as phonon processing components (waveguides, splitters, and memories) to process information by means of phonons. In particular, we propose a rigorous approach for the solution of the electromagnetic/mechanical system of equations governing light behavior in optomechanical cavities, with the help of the transformation optics method. Such a kind of calculation allows the development of an efficient generation of microwave coherent phonon sources, by engineering their propagation or coupling with phonon waveguide
Transformation Optics: Large Multiphysics Simulation of Nonlinear Optomechanical Coupling in Microstructured Resonant Cavities
No full textThe interfacing between optical cavities and mechanical systems has given rise to the rapid development of cavity optomechanics, which aims to confine light into small volumes by means of high-Q resonant recirculation [1], [2]. The purpose of an optomechanical system is to investigate the interaction of light with a mechanical oscillator. Its highly interdisciplinary nature leads to several potential applications in various fields of research, especially in quantum processin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Accurate analysis of plasmon propagation in metal and graphene nanostructures
No full textWe introduce an accurate analytical and numerical characterization of plasmonic propagation in meso- and nanoheterostructure for noble metal- and graphene-based device configurations. The method developes into three steps, i) the Kretschmann configuration technique, ii) the transfer matrices approach and iii) a full-wave multi-scale numerical simulation. The method is first tested by comparing theoretical results to experimental ones from the literature, and then applied to investigate plasmon propagation in graphene-based devices. The final aim is the full-wave design of plasmonic components/devices ranging from the THz to the optical range, involving both noble metals and energing 2D-materials, e.g. graphen
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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