Volume phase Holographic Optical Elements (VHOE) are innovative diffractive optical devices that find application in different industrial fields. Volume Phase Holographic Gratings (VPHGs) are of particular interest as dispersing elements for astronomical instrumentation thanks to the very high diffraction efficiency that they can achieve. Light dispersion in VPHGs is due to a periodical change of the refractive index in the volume of the material. The efficiency of such elements depends on their thickness and on the refractive index modulation (∆n). The optimization of the latter property is crucial to obtain reliable and effective devices. Among the different existing classes of materials for holography, photopolymers probably represent the most promising one, but the ∆n achievable should be increased to extend their applicability range and cover the different spectroscopic needs. In this framework, the goal of this thesis work is the design of new photopolymer materials based on innovative monomers, the application of available photopolymer systems in the manufacturing of VPHGs for astronomical instruments and finally the study of new reactions/molecules that can be the base for innovative photosensitive systems for volume holography. The photopolymer system presented in this thesis is based on cyclic allylic sulfide (CAS) monomers, which polymerize through a free-radical ring-opening mechanism that does not suffer from large polymerization-induced shrinkage contrary to common free-radical polymerizations. This allows for a very precise pattern transfer with a limited material distortion. The addition of multifunctional thiols to the formulation changes the reaction to a thiol-ene polymerization, which turns into a high monomer conversion and allows exploiting almost all the potential dynamic range of the mixture. Moreover, a thermal treatment after the holographic writing promotes a further rearrangement of the formed polymer chains in the grating, leading to an important increase of ∆n that reaches values above 0.03. In order to maximize the index contrast and improve the performances of the photopolymer system, new high-index CAS monomers showing high polarizable groups are designed and synthetized. The use of known photopolymer systems (Bayfol® HX) for the manufacturing of astronomical dispersing elements is also reported, covering different case studies and showing the versatility of these materials. As for the new materials and reactions for volume holography, materials undergoing the photo-Fries reaction and triazenes were selected, synthetized and characterized. The first system consists in the photoinduced rearrangement of an aromatic ester into a hydroxy ketone, while the second one leads to the release of volatile nitrogen during light irradiation. Both systems are characterized in terms of reaction in solution and the behavior of triazenes in thin films is studied. The results are interesting and pave the way for further development of such holographic systems.
Gli elementi ottici olografici di volume (VHOE) rappresentano degli innovativi dispositivi ottici diffrattivi che trovano applicazione in diversi ambiti industriali. I reticoli olografici di volume (VPHG) sono di particolare interesse nell’ambito della strumentazione astronomica grazie alle elevate efficienza ottenibili. La dispersione della luce nei VPHG è dovuta ad una modulazione periodica dell’incide di rifrazione nel volume del materiale. L’efficienza di questi elementi dipende dallo spessore del materiale e dalla modulazione dell’indice di rifrazione (∆n). L’ ottimizzazione di questa ultima proprietà è cruciale per la realizzazione di dispositivi affidabili e efficienti. Tra le diverse classi esistenti di materiali per olografia, i fotopolimeri probabilmente rappresentano la più promettente, ma è necessario aumentare i valori di Δn ottenibili in modo da estendere il range di applicabilità in base alle diverse esigenze spettroscopiche. In tale contesto, lo scopo di questa tesi di dottorato è lo sviluppo di nuovi materiali fotopolimerici basati su monomeri innovativi, l’applicazione di sistemi fotopolimerici esistenti per la produzione di VPHG per strumentazioni astronomiche e, in fine, lo studio di nuove molecole/reazioni che possano essere la base per nuovi sistemi fotosensibili per l’olografia di volume. Il sistema fotopolimerico presentato in questa tesi è basato su un solfuro allilico ciclico (CAS) che polimerizza con un meccanismo radicalico di apertura dell’anello, caratterizzato da una bassa variazione di volume, diversamente rispetto alle classiche polimerizzazioni radicaliche. Ciò permette di trasferire il pattern olografico nel volume del materiale con estrema precisione senza significanti deformazioni. L’aggiunta di un tiolo multifunzionale alla formulazione fotopolimerica permette poi di cambiare il meccanismo di reazione in una polimerizzazione ‘thiol-ene’, che consente di ottenere una maggiore conversione del monomero e permettendo in questo modo di sfruttare quasi interamente il potenziale del materiale. Inoltre, l’applicazione di un trattamento termico dopo la scrittura olografica permette alle catene polimeriche formatesi di riarrangiarsi, portando ad un importante aumento del Δn, il quale raggiunge valori superiori a 0.03. Al fine di migliorare ulteriormente le performance di questo materiale, nuovi monomeri CAS ad alto indice di rifrazione sono stati progettati e sintetizzati. L’uso dei fotopolimeri Bayfol® HX per la produzione di elementi disperdenti per strumentazione astronomica è inoltre riportato, mostrando diversi casi e dimostrando la versatilità di questi materiali. Per quanto riguarda i nuovi materiali e le nuove reazioni per l’olografia di volume, la reazione foto-Fries e i triazeni sono stati scelti, sintetizzati e caratterizzati. La reazione foto-Fries consiste nel riarrangiamento di esteri aromatici in idrossichetoni, mentre la fotoreazione dei triazeni causa il rilascio di azoto gassoso. Entrambe le reazioni sono state studiate in soluzione, e i triazeni anche in matrice polimerica. I risultati ottenuti sono molto promettenti e mostrano la possibilità di utilizzare questi sistemi in olografia.
Innovative polymeric systems for volume holography
Galli, Paola
2020/2021
Abstract
Volume phase Holographic Optical Elements (VHOE) are innovative diffractive optical devices that find application in different industrial fields. Volume Phase Holographic Gratings (VPHGs) are of particular interest as dispersing elements for astronomical instrumentation thanks to the very high diffraction efficiency that they can achieve. Light dispersion in VPHGs is due to a periodical change of the refractive index in the volume of the material. The efficiency of such elements depends on their thickness and on the refractive index modulation (∆n). The optimization of the latter property is crucial to obtain reliable and effective devices. Among the different existing classes of materials for holography, photopolymers probably represent the most promising one, but the ∆n achievable should be increased to extend their applicability range and cover the different spectroscopic needs. In this framework, the goal of this thesis work is the design of new photopolymer materials based on innovative monomers, the application of available photopolymer systems in the manufacturing of VPHGs for astronomical instruments and finally the study of new reactions/molecules that can be the base for innovative photosensitive systems for volume holography. The photopolymer system presented in this thesis is based on cyclic allylic sulfide (CAS) monomers, which polymerize through a free-radical ring-opening mechanism that does not suffer from large polymerization-induced shrinkage contrary to common free-radical polymerizations. This allows for a very precise pattern transfer with a limited material distortion. The addition of multifunctional thiols to the formulation changes the reaction to a thiol-ene polymerization, which turns into a high monomer conversion and allows exploiting almost all the potential dynamic range of the mixture. Moreover, a thermal treatment after the holographic writing promotes a further rearrangement of the formed polymer chains in the grating, leading to an important increase of ∆n that reaches values above 0.03. In order to maximize the index contrast and improve the performances of the photopolymer system, new high-index CAS monomers showing high polarizable groups are designed and synthetized. The use of known photopolymer systems (Bayfol® HX) for the manufacturing of astronomical dispersing elements is also reported, covering different case studies and showing the versatility of these materials. As for the new materials and reactions for volume holography, materials undergoing the photo-Fries reaction and triazenes were selected, synthetized and characterized. The first system consists in the photoinduced rearrangement of an aromatic ester into a hydroxy ketone, while the second one leads to the release of volatile nitrogen during light irradiation. Both systems are characterized in terms of reaction in solution and the behavior of triazenes in thin films is studied. The results are interesting and pave the way for further development of such holographic systems.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/170243