Photoactive materials for holographic optical elements

The progressive increase in telescope size and in complexity of the astronomical instrumentation has highlighted how the current technologies and traditional materials do not completely meet the present and future astronomical requirements. Therefore, new materials and solutions have to be developed not only to realise future astronomical facilities, but also to improve the performances of already available instrumentations. In this context, this research project deals mainly with the study of photoactive materials for the production of either refractive or diffractive holographic optical elements. In particular, attention has focused to Volume Phase Holographic Gratings (VPHGs) as reference diffractive optical elements, since they are considered nowadays the baseline for dispersing elements in modern astronomical spectrographs thanks to their high diffraction efficiency. Moreover, VPHGs can be used as a tool to determine the performances of holographic materials. Their working principle is based on the periodic modulation of the refractive index (Delta n) in a thin film of photosensitive material having a uniform thickness. This modulation is usually induced by means of a holographic process. The light diffraction takes place through the thickness and the diffraction efficiency, which is one of the key properties in the astronomical field, directly depends on Delta n and film thickness. The dichromated gelatin (DCG) is the common used holographic material, which provides high performances. However, it requires a complex developing process, its chemical composition is variable and it is highly sensitive to humidity. It turns out that there is a limited number of manufacturers of VPHGs for astronomy, which are located only in the US. Therefore, alternatives in term of holographic materials that overcome the drawbacks of DCGs while providing equal performances, are highly desired and the achievement provides a spin-off for economy in Europe. We have found a good candidate in photopolymers, which is an important class of holographic materials that are becoming popular in visual art, anti-counterfeit and for displays. In the framework of a scientific collaboration for diffractive holographic elements for astronomy, Bayer MaterialScience AG and Polygrama Lynx provided solid and liquid acrylic-based photopolymers both green sensitive and panchromatic. Materials were characterised in terms of refractive index modulation as function of chemical composition, grating line density, and holographic writing conditions, i.e., light power density and exposure time. Moreover, transparency of the photopolymers before and after the exposure has been measured in the UV-Vis-NIR in order to determine the wavelengths range of use. The gratings based on photopolymers were tested at normal conditions and in cryogenic environment, and ageing resistance was also evaluated. Interestingly, the VPHGs based on the Bayer's materials showed constant performances at room and cryogenic temperatures for long time. VPHG dispersing elements for the AFOSC camera of the Asiago Telescope (1.82 m) and for the ALFOSC camera at the Nordic Optical Telescope (2.56 m) in La Palma (Canary Islands) were successfully designed and manufactured. All the elements were tested both at the laboratory level and on sky providing excellent results. Along with the main project on photopolymers, another important research activity concerned the study of a new class of materials that show a high refractive modulation upon exposure to UV light. Specifically, we focused our attention on the Photo-Fries rearrangement that occurs in aromatic esters. In literature, many polymers have been reported showing high Delta n values. Nevertheless, a complete understanding of the mechanism leading to such a great variation still missed. By means of DFT calculations on reference molecules and applying a Lorentz-Lorenz model, polymers that undergo a photo-Fries rearrangement were studied to predict the refractive index modulation, which accompanies this light-induced process. The results demonstrated that a change in material density has to be considered the main source of the modulation of the refractive index. The change in material density was experimentally confirmed by measuring the spectral reflectance of thin films of polystyrene derivatives undergoing photo-Fries rearrangement. Such results provide useful guidelines to design polymers with enhanced refractive index modulation. Indeed, we recently designed new thiophene-based molecules that could be interesting candidates for a next generation of photo-Fries polymers.

Il progressivo aumento delle dimensioni dei telescopi e della loro complessità ha sottolineato la necessità di disporre di nuove tecnologie per la realizzazione degli elementi disperdenti da utilizzare in ambito spettroscopico. Nuovi materiali sono stati studiati per offrire rinnovate prestazioni alle strumentazioni esistenti. Questo lavoro è focalizzato sullo studio di materiali fotoattivi per la realizzazione di elementi ottici olografici, in particolare di reticoli olografici di volume (VPHG) utilizzati anche come tool per la caratterizzazione del materiale impiegato. Il principio di funzionamento dei VPHG si basa sulla modulazione dell’indice di rifrazione all’interno del volume del film attivo con determinato spessore. La diffrazione avviene infatti all’interno del volume dell’elemento e ha una efficienza che dipende sia dal delta n che dallo spessore. Per ovviare ai limiti dei materiali comunemente utilizzati, abbiamo iniziato lo studio di una nuova classe di materiali fotoattivi: i fotopolimeri. Attraverso una collaborazione con Bayer MaterialScience AG and Polygrama Lynx abbiamo caratterizzato questa famiglia di materiali indagando proprietà ottiche, chimico-fisiche e performance in funzione dei parametri chiave dei reicoli. Grazie all’esperienza acquisita con questi materiali abbiamo potuto realizzare elementi disperdenti astronomici per due importanti telescopi (Copernico ad Asiago e NOT alle Isole Canarie) che grazie ad uno specifico design hanno reso possibile il raggiungimento di importanti risultati scientifici. Assieme a questi materiali abbiamo anche portato avanti un filone di ricerca atto a studiare un nuovo meccanismo per instaurare la variazione di indice di rifrazione: il riarrangiamento photo-Fries. Sono state studiate alcune molecole e polimeri capaci di mostrare questa variazione per poter, in futuro, sviluppare un nuovo materiale da utilizzare in campo olografico astronomico. Grazie a questi studi abbiamo inoltre dato un nuovo contributo nella comprensione del meccanismo che realizza il cambiameto di indice nel riarrangiamento photo-Fries.