1,720,959 research outputs found
Numerical Modeling of the Ultrafast Plasmonic Response of Titanium Nitride Nanostructures
Full text linkTitanium nitride (TiN) is a promising plasmonic material that has come into the spotlight as a valid alternative to noble metals. As for other plasmonic materials, great attention has been given to the development of TiN nanostructures so as to efficiently tune the plasmon resonance, also combining them in lattices and with other metals or semiconductors. In addition, TiN boasts carrier cooling dynamics more than one order of magnitude faster than that of gold, which gives it a clear advantage in many applications. However, the knowledge of the ultrafast optical response of TiN nanostructures is narrowed down to experimental evidence, without a complete modeling. In this work, we numerically model the nonequilibrium hot-carrier-mediated mechanisms and ultrafast nonlinear changes in the optical response of TiN nanostructures photoinduced by femtosecond laser pulses. Specifically, we focus on nanodisks and nanospheres and compare our simulations with experimental pump-probe measurements. Our approach enables us to disentangle the interband and intraband contributions to the permittivity modulation, pointing out the critical role of the interplay between the TiN interband transitions and the nanostructure optical resonances in the early stages of the photoinduced ultrafast dynamics
Real‐Time Detection of Coherent Vibrational Dynamics in TiN Films
Full text linkTitanium nitride (TiN) has recently gained considerable interest because of its remarkable plasmonic properties and for its strong electron-phonon (e-ph) coupling, leading to extremely fast (<100 fs) electron-lattice cooling. Here, the generation of coherent phonons in TiN films is reported, along with their real-time detection by means of broadband transient reflection spectroscopy with sub-15-fs temporal resolution. The measurements show damped oscillations, superimposed to excited state electronic decay. A coherent vibrational mode is revealed, with approximate to 10 THz frequency ascribed to defect-activated normal modes, consistent with spontaneous Raman scattering data, and a dephasing time of approximate to 250 fs. Two pi-phase flips are also observed located at photon energies corresponding to interband optical transitions (at 3.2 and 2.5 eV), ascribed to selective coupling of the vibrational mode to these transitions; the energy modulation induced by the vibrational coherence is evaluated. It is shown that the displacive excitation of coherent phonons model describes the coherent response in terms of temporal behavior and of spectral amplitude profile. Overall, a comprehensive and detailed analysis of coherent phonons in TiN films, so far undected, is provided and relevant information on TiN photo-physical properties, potentially useful for its applications, is given
Spiral plasmonic lenses for tunable nanoscale ultrafast electron emitters
No full textLAUREA MAGISTRALEUn’importante limitazione che caratterizza, al giorno d’oggi, i fasci elettronici generati all’interno di cannoni elettronici a radiofrequenza, nonostante gli elevati valori di corrente ottenuti, è la bassa brillanza. Un modo per aumentare la brillanza del fascio è diminuirne l’emittanza, estraendo elettroni da fotocatodi con aree di piccole dimensioni. Non è tuttavia possibile utilizzare fotocatodi a punta in ambienti di campo così elevato, poiché essi sono caratterizzati da vita limitata. L’alternativa è utilizzare fotocatodi metallici piatti; il problema con i catodi utilizzati al presente stato dell’arte è connesso con la loro area di emissione, ancora elevata, limitata dalle dimensioni dello spot laser sul catodo. Quest’ area è solitamente non inferiore ai 10 μm. Tutto ciò limita le applicazioni in cui questi fasci elettronici vengono utilizzati; una di queste è la Ultrafast Electron Diffraction (UED), la quale richiede contemporaneamente elevata risoluzione temporale e spaziale per indagare la dinamica strutturale della materia alla nanoscala. L’idea è quindi quella di manipolare le oscillazioni di elettroni liberi sulla superficie di un metallo, usando campi ottici. L’accoppiamento della luce in Surface Plasmon Polaritons (SPPs) utilizzando un grating, nelle cosiddette lenti plasmoniche, permette di focalizzare l’energia in una piccola area centrale di dimensioni ben inferiori alla lunghezza d’onda della luce. Gli elevati valori di intensità luminosa così ottenuti, uniti al fatto che i metalli sono caratterizzati da un’elevata funzione lavoro, portano all’estrazione di elettroni tramite un processo non lineare, chiamato fotoemissione a più fotoni, usando un laser nell’infrarosso per eccitare le strutture. In questo modo si superano alcune limitazioni che caratterizzano i metalli quando stimolati con luce UV, impiegata per ottenere fotoemissione a singolo fotone; i metalli infatti mostrano bassa efficienza quantica in questo range di frequenze; inoltre sono necessarie elevate potenze per convertire l’armonica fondamentale in armoniche superiori. Un esempio di lente plasmonica è il bull’s eye, il quale tuttavia richiede un preciso allineamento del laser al centro della struttura per eccitare i SPPs tramite polarizzazione radiale. Nel presente lavoro, sviluppato presso Lawrence Berkeley National Laboratory (LBNL), la spirale archimedea è analizzata come valida alternativa di lente plasmonica, superando il problema dell’allineamento con l’utilizzo di una polarizzazione circolare, come già emerso nel primo lavoro incentrato su questa struttura e sviluppato sempre a LBNL [1]. Le spirali sono simulate e ottimizzate in due differenti configurazioni di utilizzo, riflessione e trasmissione, utilizzando Lumerical FDTD Solutions. Si mostra la possibilità di estrarre impulsi elettronici ultraveloci, compatibili per l’utilizzo in UED. Le strutture sono quindi fabbricate in una cleanroom classe 100 nella Nanofabrication division presso la Molecular Foundry (LBNL). Le tecniche di Electron Beam Lithography e Template stripping, già sperimentate in [1], sono impiegate per fabbricare le spirali in riflessione. Le spirali in trasmissione sono fabbricate qui per la prima volta; la tecnica scelta è quella di Focused Ion Beam. Un’ultima parte del lavoro è focalizzata nel tentativo di estrarre elettroni tramite fotoemissione a tre fotoni da un catodo di rame posto all’interno di un cannone elettronico (Direct Current gun) nel Building 2 di LBNL, usando un laser Titanio Zaffiro in mode locking. Quest’ultima parte del lavoro può essere considerata preliminare al futuro testing dei nanocatodi plasmonici prodotti.Nowadays, electron beam sources obtained in radiofrequency guns suffer from low brightness, in spite of the high values of current that can be obtained. To increase the brightness, the idea is to reduce the emittance of the electron beam, going towards photocathodes with a small emitting area. Tips cannot be used in high field environments such the ones created inside the guns, for their limited lifetimes; the idea is to use flat metal photocathodes. The problem with the photocathodes in the state of the art is that the emitting area is limited by the laser spot size on the cathode, usually around 10 μm or more. This limits the applications in which these electron beams are used, such as Ultrafast Electron Diffraction (UED), that requires both high temporal and spatial resolution to obtain maps of structural dynamics at the nanoscale. The idea then, is to manipulate the oscillations of free electrons in a metal using optical fields. The coupling of light into Surface Plasmon Polaritons (SPPs) using a grating in the so called plasmonic lenses is exploited to focus the electromagnetic energy in a flat central spot with subwavelength dimensions. The high values of the intensity reached in this way, and the fact that metals usually have high work functions, lead to the extraction of electrons through a nonlinear process, called multiphoton photoemission, using an infrared laser to excite the structure. In this way we can overcome some limitations linked to the exploitation of UV light to obtain single photon photoemission, mainly consisting in low quantum efficiency of metals and in high power needed to upconvert the fundamental harmonic. An example of plasmonic lens is the bull’s eye, but it presents the problem of requiring a very accurate laser alignment in the very center of the structure to excite the SPPs through a radial polarization. In this work, developed at Lawrence Berkeley National Laboratory (LBNL), the Archimedean spiral is studied as a valid and interesting alternative, overcoming the problem of the alignment by exploiting a circular polarization of the laser light, following a first study on this structure conducted at LBNL in [1]. The spirals are simulated and optimized using Lumerical FDTD Solutions both in reflection and transmission configuration, showing the possibility of obtaining ultrafast electron pulses. The spirals are then fabricated in a cleanroom class 100 in the Nanofabrication division at Molecular Foundry (LBNL). Electron Beam Lithography and Template stripping are used to fabricate the spirals in reflection configuration, as already tested in [1]. For the first time also the spirals in transmission configuration are fabricated; the technique chosen is Focused Ion Beam. In the end, a careful experimental activity is made at Building 2 of LBNL, trying to obtain a current through a 3-photon process from a copper cathode placed inside a DC gun and using a mode locked Titanium Sapphire laser. This last experimental work can be considered preliminary to the future testing of the fabricated plasmonic nanocathodes
Ultrafast optical phenomena in advanced nanostructured materials for light harvesting and all-optical modulation
No full textDOTTORATODi recente, le Nazioni Unite hanno delineato 17 Obiettivi di Sviluppo Sostenibile, con l'obiettivo di "pace e prosperità per le persone e il pianeta, ora e in futuro". Infatti, la ricerca di fonti di energia affidabili, sostenibili e moderne (obiettivo 7) e per l’ innovazione nell'industria e nelle infrastrutture (obiettivo 9) è una priorità. In questo contesto, la ricerca di nuovi materiali può fornire miglioramenti significativi: oggi grandi sforzi sono dedicati, in tutto il mondo, allo sviluppo di materiali innovativi per la conversione efficiente dell'energia solare e per l'elaborazione dati ad alta velocità, fondamentale per la gestione di grandi quantità di dati e per le tecnologie dell'informazione e della comunicazione (ICT). Nell'ultimo decennio, nel campo delle applicazioni di raccolta di luce, i nanocristalli semiconduttori sono stati i principali candidati, grazie ai loro gap regolabili, agli efficienti processi di separazione e ricombinazione di carica e a buone caratteristiche di trasporto degli elettroni. D'altra parte, la strada verso l'elaborazione ultraveloce dei dati si basa su un controllo tutto ottico, che potrebbe fornire uno switching veloce attraverso le nonlinearità ottiche; i materiali plasmonici e, in particolare, le nanostrutture di metalli nobili, sono stati ampiamente esplorati a questo scopo. Infatti, essi consentono di comprimere la luce oltre il limite di diffrazione e vantano una grande nonlinearità ottica, importante nel meccanismo di modulazione tutta ottica della luce. In entrambi i contesti, lo studio delle interazioni luce-materia e in particolare dei processi di rilassamento che governano la dinamica degli elettroni caldi fotogenerati, su scale temporali del femto- e pico-secondo, gioca un ruolo chiave. Essa consente infatti una comprensione chiara dei processi fisici coinvolti, e dunque una scelta consapevole dei materiali e un solido design per dispositivi volti a future applicazioni tecnologiche. In questo lavoro, forniamo uno studio approfondito delle proprietà ottiche ultraveloci dei nanomateriali avanzati per la raccolta della luce e la modulazione tutta ottica della luce, combinando un approccio sia sperimentale che teorico. La ricerca parte dalla i) indagine sperimentale, attraverso la spettroscopia pump-probe ultraveloce, di due sistemi nanostrutturati semiconduttori per la raccolta della luce, ossia i nanocristalli di seleniuro-solfuro di cadmio (CdSeS) ingegnerizzati con vacanze di zolfo e le nanoparticelle di titanio (TiO2)/allumina (Al2O3) con 3-idrossiflavone (3HF). Successivamente, forniamo uno studio completo della non linearità ottica del nitruro di titanio (TiN) come nuovo materiale ultraveloce per la modulazione tutta ottica della luce. In particolare, ii) sviluppiamo un nuovo modello numerico per la non linearità ottica del TiN e iii) lo validiamo su misure di pump-probe ultraveloce sia su film che su nanostrutture di TiN. Infine, forniamo ulteriori sviluppi della ricerca, in termini di confronto tra TiN e oro (Au), perfezionamenti del modello che coinvolgono molteplici modulazioni delle transizioni interbanda, tuning sperimentale della risposta ottica transiente del TiN e uno studio preliminare sui fononi coerenti nei film di TiN. Questa ricerca fornisce utili risultati in vista dello studio e della predizione della modulazione delle proprietà ottiche ultraveloci in nuovi sistemi di materiali avanzati, aprendo la strada al loro impiego in dispositivi per la raccolta e la modulazione tutta ottica della luce.Recently, the United Nations pointed out 17 Sustainable Development Goals (SDGs), with the aim of "peace and prosperity for people and the planet, now and into the future". Indeed, the quest for reliable, sustainable and modern energy sources (goal 7) and for innovation in the industry and infrastructure (goal 9) is a priority. In this context, the research on new materials can provide significant improvements: nowadays great efforts are devoted, all over the world, to the development of innovative materials for efficient solar energy conversion and for high speed data processing, pivotal for the big-data management and for information and communication technology (ICT). In the last decade, for light-harvesting applications, semiconductor nanocrystals have been the main candidates, due to their tunable gaps, efficient charge separation and recombination processes, and good electron transport features.
On the other hand, the route to ultrafast data processing relies on all-optical control, that could provide fast switching through optical nonlinearities; plasmonic materials, and specifically nanostructures of noble metals, have been widely explored for this purpose. Indeed, they enable to squeeze light beyond the diffraction limit and they boost a huge optical nonlinearity, important in the all-optical modulation mechanism.
In both frameworks, the study of light-matter interactions and in particular of the relaxation processes governing the dynamics of photogenerated hot electrons, on femto- and pico-second timescales, plays a key role.
It enables a clearcut understanding of the physical processes involved, thus a conscious choice of the materials and a solid design of devices for future technological applications.
In this work, we provide a deep study of the ultrafast optical properties of advanced nanomaterials for light harvesting and all-optical modulation, combining both an experimental and a theoretical approach. The research starts from i) the experimental investigation, through ultrafast pump-probe spectroscopy, of two semiconductor nanostructured systems for light-harvesting, namely cadmium-selenide-sulfur (CdSeS) nanocrystals engineered with sulfur vacancies and titania (TiO2)/alumina (Al2O3) nanoparticles with chemiadsorbed 3-hydroxyflavone (3HF). Subsequently, we provide a complete study of the optical nonlinearity of titanium nitride (TiN) as a new, ultrafast material for all-optical modulation. Specifically, ii) we develop an original numerical model for TiN optical nonlinearity, and iii) validate it on ultrafast pump-probe measurements on both TiN films and nanostructures. Finally, we provide some further developments of the research, in terms of comparison between TiN and gold (Au), model refinements involving multiple interband transition modulations, experimental tuning of TiN transient optical response, and a preliminary study on coherent phonons in TiN films.
This research provides useful insights in view of the study and the prediction of the modulation of the ultrafast optical properties of new advanced material systems, paving the way to their exploitation in light-harvesting and all-optical modulation devices.DIPARTIMENTO DI FISICA36DELLA VALLE, GIUSEPPEFINAZZI, MARC
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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