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Optical Active Thin Films for Micro-Cavity Lasers
Full text linkOptical microcavities can find broad application areas, like tunable and compact sources, dynamic filters for optical communications, biological and chemical sensors, etc.
Optical microcavities are key component allowing one to obtain compact laser devices exhibiting small cavity volume and low threshold. Among the different resonators architectures for microcavities laser, photonic crystal (PC) structures are one of the most promising. These structures feature the periodicity in one or more dimensions, and are resonant for light waves of a specific wavelength.
Two-dimensional PCs are planar dielectric waveguide where photons are vertically confined by the vertical profile of the optical index, while the crystal periodicity acts only in the slab plane.
In photonic crystals the refractive index contrast of the periodic structure is high enough to open a full band gap, and thus to fully confine light at very small scale.
Strong coupling is theoretically feasible, and quality factor of more than 106 have been experimentally achieved for small modal volumes.
Substantial additional gains are possible with confinement improvement in microfabrication techniques and with implementation of low loss design.
Photonic crystal whose index contrast is lower can be used as DFB gratings. In a DFB device, the laser modes receive feedback at one specific wavelength, determined by the grating period of the structure. The feedback strength is related to the coupling constant, ?, which in turn depends on the grating index contrast, and to the grating length, L. The ?L product must be high enough to ensure the feedback required for lasing.
In an optically pumped laser, an external source supplies the excitation energy necessary to get the population inversion. To do that, it must be on resonance with one of the absorption transitions of the active medium. When the external source provides enough energy, the active material exhibits gain: more photons are generated than lost.
For intense incoming beams, i.e. intense laser sources, multiple-photon absorption processes become appreciable.
It is thus possible to have absorption also by pumping with sources having photon energies lower than the resonance energy of the active medium.
The two-photon absorption (TPA), described from the 3rd order susceptibility, involves the simultaneous absorption of two photons with energy:
E_exc-E_ground=2??
The absorption of the first photon causes the promotion of the electron to the virtual level. Here the simultaneous absorption of the second photon promotes the electron to the real excited state.
The system can then decay to the ground state emitting an up-converted photon, i.e., a photon having energy higher than that of the exciting ones.
A large part of this work is devoted to the realization and characterization of active microresonators behaving as laser sources.
Two main research subjects will be pursued:
An integrated InP semiconductors photonic crystal microcavity laser, operating in the telecommunication wavelength.
A distributed feedback laser for two-photon induced IR-to-visible up-conversion lasing.
Within the second subject, particular attention will be devoted to the characterization of the two-photon induced emission properties of organic push-pull dyes and II-VI semiconductors quantum dots (QDs) to evaluate their potentiality as candidates for all-solid-state up-converted laser devices.
The third reported research subject is the exploitation of hybrid silica-titania sol-gel films for UV lithography application, finalized to the production of surface relief gratings.
MICROLASER BASED ON EFFECTIVE INDEX CONFINED, SLOW-LIGHT MODES IN PC WAVEGUIDES
This research regards the study of photonic crystal microcavity having small mode volume V and high quality factor Q, for the production of low threshold integrated laser devices. The light propagation inside the PC is modified because of its periodicity. In this study we exploit the low-light guided modes at the high symmetry point of the dispersion curve of a PC-W1 waveguide. The PC-W1 waveguide is a PC having triangular symmetry with a missing row of hole along the ?K direction. The linear defect entails the appearance of defect modes with frequencies localized inside the unperturbed PC band gap, and thus modes that exponentially decay inside the PC. The band associated with the defect mode becomes flat at the K point of the band diagram, leading to slow-modes whose group velocity goes to zero. The lateral confinement of low-group velocity modes is controlled by locally changing the refractive index of the two dimensional photonic crystal waveguides. The index modulation is induced by post-processing a dielectric strip on top of the two-dimensional PC waveguide. This results in a photonic heterostructure whose confinement properties are the result of the effective index shift and the local curvature of the band associated with the waveguide mode.
In this thesis the results of the device simulation, experimental realization and characterization will be reported.
Computational tools, such as MPB and 3D-FDTD software have been used for the device design and for the study the electromagnetic field behavior inside the cavity.
The realization of the PC structure has been accomplished through lithography techniques like e-beam lithography and reactive ion etching. Intense clean-room activities have been necessary to reach optimized structure quality.
The characterization of the microcavity laser has been pursued with a proper optical set-up, in such a way to determine its performance.
UP-CONVERTED LASING
The up-converted lasing is an alternative method to convert the emission of a cheap, easily available IR laser into that of a more technological valuable visible laser.
It involves the two photon pumping (TPP) mechanism, i.e., the NLO system is excited through the simultaneously absorption of two photons in the near-IR range.
In this work we report our effort towards the realization of a solid-state visible laser device based on a TPP induced emission.
The starting point of this technology is to find a system able to efficiently convert the IR incoming radiation to a visible one.
We have studied the up-conversion process both in push-pull organic dyes embedded in sol-gel hybrid films and in semiconductor core-shell CdSe-CdS-ZnS quantum dots embedded in zirconia films. The excitation source is an amplified Ti:Sapphire laser at 800 nm.
Concerning the organic compounds, it has been possible to characterize the emission properties only in solution, because of their poor photostability when they are embedded in sol-gel matrices.
On the opposite, quantum dots embedded in zirconia films show promising amplified emission properties, with interesting gain value and extremely long time stability.
We have investigated the possibility to implement this material with a distributed feedback optical resonator for obtaining compact and integrated laser devices.
The grating parameters have been determined with MPB software, and first attempts of e-beam lithography of the pattern have been done.
We have also prepared a devoted optical set-up for the optical characterization of the laser devices.
SILICA-TITANIA SOL-GEL FILM FOR DIRECT PHOTOPATTERNIG APLICATIONS
The possibility to exploit the photocatalytic action of hybrid silica-titania sol-gel film towards the decomposition of their organic component, for the direct patterning of surface structure has been investigated. These films have been characterized to study their microstructural properties, and to confirm the presence of crystalline titanium oxo-clusters. Their photocatalytic efficiency has been measured using stearic acid as reference material.
To test the potentiality of this system for UV-lithography, it has been exposed to a UV-lamp.
The organic component decomposition leads to a film shrinkage of about 60%, accompanied by a refractive index increases of about 0.1
By irradiating the spin-coated films through an UV-mask, structures of different shapes and micrometer dimension have been achieved.Lo studio delle microcavità ottiche riveste un grande interesse per applicazioni in svariati campi, quali la ricerca di sorgenti laser tunabili e compatte, filtri per le telecomunicazioni, sensori chimici e biologici, etc.
Le microcavità ottiche sono fondamentali per l’ottenimento di dispositivi laser compatti, aventi bassa soglia di emissione laser, ove il campo elettromagnetico è confinato in volumi estremamente ridotti, con conseguente aumento dell’interazione radiazione-materia,.
Tra le possibili architetture della cavità risonante, per dispositivi pompati otticamente, i cristalli fotonici rappresentano una delle soluzioni più promettenti.
Questi ultimi sfruttano la periodicità in una o più direzioni e sono risonanti con determinate lunghezze d’oda della radiazione elettromagnetica.
In un cristallo fotonico bidimensionale il confinamento verticale è garantito dal profilo verticale dell’indice di rifrazione, mentre il confinamento nel piano del cristallo è opera della strutture periodica.
Nei cristalli fotonici il contrasto di indice di rifrazione della struttura periodica è tale da aprire un intervallo completo di energie proibite per la propagazione della radiazione nel mezzo. Essa può quindi essere confinata in volumi molto piccoli, dell’ordine del cubo della lunghezza d’onda, con fattori di qualità sperimentali superiori a 106. Inoltre i valori ottenuti sperimentalmente sono inferiori a quelli previsti teoricamente, e ulteriori passi in avanti saranno possibili con lo sviluppo delle tecniche litografiche e di produzione del materiale attivo.
I cristalli fotonici nei quali il contrasto di indice di rifrazione è insufficiente per aprire un band-gap completo si comportano come reticoli distributed feedback, DFB. In un dispositivo DFB, i modi risonanti ricevono il feedback a lunghezze d’onda specifiche, determinate dal periodo del reticolo. La forza dell’accoppiamento è legata alla costante di accoppiamento ?, la quale, a sua volta, dipende dal contrasto di indice nel reticolo e all’estensione totale del reticolo. Il prodotto ?L deve essere sufficiente per garantire il feedback richiesto per l’emissione laser.
In un laser a pompaggio ottico, una sorgente esterna fornisce al mezzo attivo l’energia di eccitazione richiesta per raggiungere l’inversione di popolazione, requisito necessario per ottenere il guadagno all’interno del mezzo e quindi l’amplificazione.
Affinché si verifichi assorbimento, l’energia del fascio di pompa deve essere in risonanza con una delle transizioni del mezzo attivo. Per campi incidenti molto intensi, come possono essere quelli legati a fasci laser focalizzati, diventano tuttavia apprezzabili anche fenomeni di assorbimento multi fotonici.
Si può quindi avere assorbimento anche utilizzando sorgenti di pompa aventi energie inferiori all’energia di risonanza del mezzo attivo.
L’assorbimento a due fotoni (TPA), legato alla suscettibilità non lineare al terzo ordine, comporta l’assorbimento simultaneo di due fotoni, con energia:
E_exc-E_ground=2??
L’assorbimento del primo fotone promuove l’elettrone dallo stato fondamentale a uno stato virtuale, dal quale esso passa immediatamente allo stato eccitato attraverso l’assorbimento simultaneo di un secondo fotone incidente. Infine il sistema può tornare allo stato fondamentale, attraverso l’emissione di un fotone a energia superiore rispetto ala pompa.
Gran parte del lavoro di dottorato è incentrato sulla realizzazione e caratterizzazione di microcavità attive per l’ottenimento di sorgenti laser.
All’interno di tale attività sono stati studiati due sistemi differenti:
Una microcavità laser a semiconduttore, realizzata sfruttando le proprietà dei cristalli fotonici bi-dimensionali, che emette alla lunghezza d’onda delle telecomunicazioni.
Un dispositivo laser DFB, pompato oticamente a due fotoni, per la conversione di emissione laser dall’infrarosso al visibile.
All’interno della seconda tematica, particolare attenzione è stata rivolta alla caratterizzazione delle proprietà di emissione indotta a due fotoni di un cromoforo organico e di quantum dots di un semiconduttore II-VI, il CdSe, entrambi inglobati in matrice sol-gel.
Un terzo soggetto è costituito dallo studio delle proprietà foto catalitiche di film sol-gel ibridi a base di silica e titania, in vista di possibili applicazioni per il patterning diretto tramite radiazione UV.
CONFINAMENTO DI MODI LENTI IN GUIDA D’ONDA A CRISTALLO FOTONICO PER L’OTTENIMENTO DI MICROCAVITA’ LASER
Questa ricerca riguarda lo studio di cavità, ottenute sfruttando cristalli fotonici bidimensionali, a basso volume modale e alto fattore di qualità Q, finalizzate all’ottenimento di dispositivi laser integrati a bassa soglia. Questo lavoro si basa sull’utilizzo dei modi guidati lenti corrispondenti al punto ad elevata simmetria K della curva di dispersione di una guida d’onda W1-PC.
Una guida d’onda W1-PC si ottiene da un cristallo fotonico a simmetria triangolare, attraverso la rimozione di una fila di buche lungo la direzione ?K. In questo modo si introduce un difetto lineare, il quale si riflette nella comparse di modi del difetto, aventi frequenze localizzate all’interno del band-gap del cristallo fotonico, che pertanto decadono esponenzialmente all’interno del cristallo. Le bande associate ai modi del difetto hanno curvatura nulla in corrispondenza dei punti a elevata simmetria, e ciò implica una velocità di gruppo del modo nulla in corrispondenza di tali punti.
L’estensione laterale dei modi lenti viene controllata agendo sull’indice di rifrazione del cristallo fotonico, in modo da creare una etero struttura in grado di confinarli efficacemente. L’indice effettivo della guida viene modificato localmente depositando un film di polimero all’interfaccia superiore della guida. La forza del confinamento dipende dall’entità della variazione dell’indice e dalla curvatura della banda associata al modo lento.
L’attività svolta all’interno di questo progetto consiste nel design della struttura, nella sua realizzazione sperimentale e infine nella caratterizzazione ottica del dispositivo.
Per ottimizzare i parametri del dispositivo e comprendere il comportamento della radiazione elettromagnetica all’interno della cavità, sono stati impiegati strumenti di calcolo computazionale, quali i software MPB e TESSA 3D-FDTD.
I parametri delle simulazioni sono stati poi utilizzati per la realizzazione del cristallo fotonico, effettuata tramite tecniche litografiche, quali la litografia con fascio elettronico e l’etching ionico.
La caratterizzazione ottica del dispositivo è stata effettuata con un apposito set-up, al fine di determinarne le prestazioni.
EMISSIONE LASER CON CONVERSIONE DI FREQUENZA
La conversione di frequenza laser fornisce l’interessante possibilità di convertire una sorgente laser economica e di facile reperibilità nell’infrarosso, in una sorgente laser nel visibile di enorme interesse tecnologico.
Essa si basa sull’emissione indotta a seguito di processi di assorbimento a due fotoni nel vicino IR.
In questo lavoro verranno presentati gli sforzi profusi e i risultati preliminari ottenuti nella ricerca di un dispositivo laser allo stato solido per la conversione di frequenza.
A tal fine sono state investigate le proprietà di conversione di un cromoforo push-pull organico disperso in matrici sol-gel ibride, e di quantum dots di semiconduttore II-VI, CdSe-CdS-ZnS, dispersi in una matrice inorganica a base di zirconia.
Il composto organico presenta interessanti proprietà di emissione indotta a due fotoni in soluzione. Tuttavia la sua scarsa resistenza al pompaggio ottico in matrice solida preclude un suo possibile impiego e rende estremamente problematica la stessa caratterizzazione ottica.
Al contrario i film di QDs-ZrO2 mostrano una buona efficienza di conversione di frequenza, con valori di guadagno per l’emissione spontanea amplificata interessanti, e elevata stabilità del segnale emesso nel tempo.
E’ stata pertanto studiata la possibilità di implementare i film di QDs-ZrO2 all’interno di una cavità risonante di tipo distributed feedback per ottenere un dispositivo laser compatto e integrabile.
I parametri del reticolo sono stati determinati con il software MPB e sono stati fissati in modo da avere amplificazione in corrispondenza del massimo di emissione dei QDs.
Sono tutt’ora in corso delle prove di realizzazione del reticolo DFB tramite litografia elettronica su film sol-gel appositamente sviluppati per il patterning diretto.
Infine è stato messo appunto un set-up dedicato per la caratterizzazione ottica dei dispositivi prodotti.
FILM SOL-GEL IBRIDI A BASE DI SILICA-TITANIA PER IL PATTERNING DIRETTO CON LUCE UV
E’ stata studiata l’attività fotocatalitica di film sol-gel ibridi a base di silica-titania, promossa dalla radiazione UV.
I film sono stati caratterizzati a livello micro strutturale tramite spettroscopia infrarossa, e sono stati osservati al microscopio elettronico per confermare la presenza di cluster di titanio cristallino al loro interno.
L’efficienza del processo di fotocatalisi è stata determinata mediante test standard che si avvalgono dell’acido stearico come materiale di riferimento. Quest’ultimo infatti è in grado di simulare efficacemente i comuni inquinanti organici, è può essere depositato facilmente per spin-coating.
Successivamente è stata valutata la possibilità di sfruttare l’attività foto catalitica per il patterning diretto dei film. Tale studio parte dall’osservazione che la fotocatalisi si manifesta anche nei confronti della componente organica dei film sol gel ibridi.. Questo fenomeno è accompagnato da una diminuzione dello spessore del film, fino al 60% sullo spessore iniziale, e può pertanto essere sfruttato per la realizzazione di strutture a rilievo.
Test di patterning diretto sono stati effettuati irradiando il film con una lampada UV attraverso una maschera in quarzo, ottenendo strutture di dimensione micrometrica ben definite
Photocatalytic Performance of Hybrid SiO2-TiO2 Films
No full textIn this paper, we have investigated the properties of a composite SiO2-TiO2/TiO2 nanoparticle system. Hybrid films have been prepared using a sol-gel process, starting from tetraisopropoxy titanate and 3-glycidoxipropyltrimethoxysilane as precursors and performing the synthesis at room temperature. The spin-coated films have been thoroughly characterized with UV-vis and FT-IR spectroscopies, TEM analysis, profilometry, and ellipsometry to evaluate their physical and structural properties. Particular attention has been devoted to the study of their photocatalytic action
Settlement, space organization and land-use of a small Middle Bronze Age community of central Italy. The case study of Gorgo del Ciliegio (Arezzo-Tuscany)
No full textThe Middle Bronze Age site of Gorgo del Ciliegio is located in the Afra valley (Upper Tiber basin) in the proximity of strategic trans-Apennine passes connecting the Tyrrhenian regions of Central Italy to the Po plain and the Umbrian-Marches territories. At Gorgo del Ciliegio a Middle Bronze Age small settlement composed of a single house and various kinds of ancillary features was discovered. Through a multi-disciplinary approach it was possible to gather a large amount of information about settlement dynamics implemented by people living at Gorgo del Ciliegio and about the ways in which this community interacted with and took advantage of the surrounding environment/landscape, while adapting the site's economy to natural features. This paper focuses on the evidence provided by material culture, environment and landscape considered as interrelating indicators of social identity and economic strategies. In order to contextualize these issues, broad space was given to the spatial organization and the architectural features as key factors of socio-economic complexity. The authors conclude that Gorgo del Ciliegio was a farm settlement being probably part of a more extensive settlement system, composed of several farms dispersed along the Afra valley. This specific context proved to be particularly suitable for interweaving relations and exchanges with the Adriatic regions, and thus promoting (and being promoted by) the development of an economy mainly based on mobile pastoralism
Synthesis of 3-Glycidoxypropyltrimethoxysilane-TiO2 UV-sensitive waveguides
No full textA hybrid organic-inorganic sol-gel material based on 3-(glycidoxypropyl)-trimethoxysilane (GPTMS), an organically
modified alkoxide containing an epoxy ring in the organic functional group, and titanium isopropoxide Ti(OPri)4 has
been synthesised. The presence of Ti alkoxide is important in order to increase the refractive index and to improve the
mechanical and physical properties of the film.
A basic catalysis has been used to perform the hydrolysis/condensation reactions of the inorganic network and to
preserve the presence of unreacted epoxy rings. The epoxy groups are thus available in the final solid state films for the
UV-photopolymerization process. The photopolymerization allows one to increase the refractive index and to pattern the
film directly.
The modification of the hybrid sol–gel TiO2 matrix microstructure during the thermal treatment and the interaction with
the UV light have been investigated by FT-IR and UV-Vis spectroscopies, DTA and TGA thermal analysis. The sol-gel
film thickness and roughness have been analyzed by a profilometer: the films possess high optical quality and well
controlled thickness, ranging from 500 nm to 2 μm. The refractive index of sol-gel thin films was measured by m-line
spectroscopy and ellipsometry. An increase of the refractive index in the range 0,02÷0,1 was achieved, depending on the
UV irradiation dose
Highly Efficient Amplified Stimulated Emission from CdSe-CdS-ZnS Quantum Dot Doped Waveguides with Two-Photon Infrared Optical Pumping
No full textIn principle, a population inversion in semiconductor quantum
dots can be achieved through electrical, chemical or optical
pumping. To date however, it has only been successfully
demonstrated with optical pumping in the 1-photon absorption
range (i.e., above the semiconductor bandgap). Under
these conditions amplified stimulated emission (ASE) in 1-D
waveguides and lasing within microsphere cavities and distributed
feedback structures has been observed. In these studies,
it was demonstrated that for the archetypal CdSe system,
a given nanocrystal must encompass more than one electronhole
(e–h) pair for a population inversion to be achieved. This
value reflects the twofold degeneracy of the lowest electronic
state in the wurtzite crystal structure
Microlasers based on effective index confined slow light modes in photonic crystal waveguides
Full text linkWe present the design, theory and experimental implementation of a low modal volume microlaser based on a line-defect 2D-photonic crystal waveguide. The lateral confinement of low-group velocity modes is controlled by the post-processing of 1 to 3μm wide PMMA strips on top of two dimensional photonic crystal waveguides. Modal volume around 1.3 (λ/n)3can be achieved using this scheme. We use this concept to fabricate microlaser devices from an InP-based heterostructure including InAs0.65P0.35quantum wells emitting around 1550nm and bonded onto a fused silica wafer. We observe stable, room-temperature laser operation with an effective lasing threshold around 0.5mW. © 2008 Optical Society of America
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
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