Development of VUV tunable laser spectroscopy techniques for characterizing calcium fluoride

Matindi, Tresor (2014-12)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: The large band gap (approximately 11.5-12.1 eV) and high transmission of calcium fluoride (CaF2) crystal in the ultraviolet (UV) and vacuum ultraviolet (VUV) region makes it an important material for optics for laser applications in UV. However, CaF2 degrades during long exposure to UV irradiation due to defect generation. The formation of selftrapped excitons (STE) is considered the first step in defect generation. In this project the possibility of observing STE states in CaF2 using a narrow bandwidth tunable VUV laser source is investigated. This is the first spectroscopy study of an alkaline earth fluoride using VUV tunable laser radiation instead of a fixed wavelength laser. The use of a VUV tunable laser source has potential for determining the energies of the STE states, which are unknown. Our main objective is addressed by developing techniques to measure absorption spectra of pure and doped CaF2 samples, using a VUV scanning monochromator and a tunable VUV laser, and by doing a literature study. The results obtained with the scanning monochromator show absorption features in 126-180 nm range of all our samples. These vary for different samples and correlate with information from the supplier on the samples’ fluorescence spectra. Total absorption of the VUV light by CaF2 in the 115-126 nm range is observed. With the narrow bandwidth tunable laser light, absorption spectra were obtained in the range of 143-146.7 nm of all our CaF2 samples. No significance peaks which can be related to the STE states in CaF2 were observed in the VUV laser absorption spectra, but the results are valuable to improve the technique. The conclusion is that either a different spectral range or fluorescence detection can be investigated in future.

AFRIKAANSE OPSOMMING: Die groot bandgaping (ongeveer 11.5-12.1 eV) en hoë transmissie van kalsiumfluoried (CaF2) kristal in die ultraviolet (UV) en die vakuum ultraviolet (VUV) gebied maak dit ’n belangrike materiaal vir optika vir laser toepassings in die UV. CaF2 degradeer egter gedurende langdurige blootstelling aan UV lig as gevolg van die generering van defekte. Die vorming van ’n elektron-holte paar wat deur die kristalstuktuur gestabiliseer word teen rekombinasie (self-trapped excitons, afgekort STE) word beskou as die eerste stap in defek generering. In hierdie projek word die moontlikheid ondersoek om STE toestande in CaF2 waar te neem deur die gebruik van ’n afstembare VUV laserbron met emissie in ’n smal spektrale band. Dit is die eerste spektroskopiese studie van ’n aardalkali-fluoried deur die gebruik van afstembare VUV laserlig in plaas van ’n vaste golflengte laser. Die gebruik van ’n afstembare VUV laserbron het potensiaal vir die bepaling van die energieë van die STE teostande, wat onbekend is. Ons hoofdoel word aangespreek deur die ontwikkeling van tegnieke vir die meet van absorpsie spektra van suiwer en gedoteerde CaF2 monsters met behulp van ’n VUV skanderende monochromator en ’n afstembare VUV laser, en deur ’n literatuurstudie. Die resultate wat behaal is met die skanderende monochromator toon die absorpsieprofiele van al ons monsters in die 126-180 nm spektrale gebied. Die absopsieprofiele varieer vir die verskillende monsters en korreleer met die inligting van die verskaffer oor die fluoressensie spektra van die monsters. Totale absorpsie van die VUV lig deur CaF2 in die 115-126 nm gebied is waargeneem. Met die smalband afstembare laserlig is absorpsie spektra in die 143-146.7 nm gebied vir al ons CaF2 monsters verkry. Geen beduidende pieke wat verband hou met die STE toestande in CaF2 is waargeneem in die VUV laser absorpsie spektra nie, maar die resultate is waardevol vir die verbetering van die tegniek. Die gevolgtrekking is dat of ’n ander spektraalgebied of fluoressensiedeteksie in die toekoms ondersoek kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/95804
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