Influence of oxide glass modifiers on the structural and spectroscopic properties of phosphate glasses for visible and near-infrared photonic applications

The e ect of oxide modifiers on multiple properties (structural and spectroscopic) of phosphate glasses with molar composition 60P2O5-(10x)Ga2O3-30MO-xEu2O3 and 60P2O5- (10y)Ga2O3-30MO-yEr2O3 (where M = Ca, Sr, Ba; x = 0, 0.5; y = 0, 1) were systematically examined and discussed. The local structure of systems was evidenced by the infrared (IR-ATR) and Raman spectroscopic techniques. The spectroscopic behaviors of the studied glass systems were determined based on analysis of recorded spectra (excitation and emission) as well as luminescence decay curves. Intense red and near-infrared emissions (1.5 m) were observed for samples doped with Eu3+ and Er3+ ions, respectively. It was found that the value of fluorescence intensity ratio R/O related to 5D0!7F2 (red) and 5D0!7F1 (orange) transition of Eu3+ ions depends on the oxide modifiers MO in the glass host. However, no clear influence of glass modifiers on the luminescence linewidth (FWHM) was observed for phosphate systems doped with Er3+ ions. Moreover, the 5D0 and 4I13/2 luminescence lifetimes of Eu3+ and Er3+ ions increase with the increasing ionic radius of M2+ (M = Ca, Sr, Ba) in the host matrix. The obtained results suggest the applicability of the phosphate glasses with oxide modifiers as potential red and near-infrared photoluminescent materials in photonic devices

NUCLEATION AND GROWTH BEHAVIOR OF TELLURITE-BASED GLASSES SUITABLE FOR MID-INFRARED APPLICATIONS

Optical fibers transmitting in the 2-5 μm mid-infrared (MIR) spectral region are highly desirable for a variety of military and civilian applications including super-continuum generation, infrared countermeasures (IRCM), and MIR laser sources. These new applications in the mid-infrared require novel optical materials that transmit in this window and can be fabricated into fiber. As tellurite glasses are known to have good transparency in the (NIR) region, tellurite-based glasses are the material of choice for this study due to their high linear and nonlinear refractive index, their low glass transition temperature and the ability to form them into optical fiber. This dissertation summarizes findings on tellurite-based glasses with the composition (90-x)TeO2-10Bi2O3-xZnO with x = 15, 17.5, 20 and 25 that were processed and characterized for their potential application as novel optical fibers. Different techniques were deployed for characterization purposes, which include primarily linear refractive index measurements, structural characterization using Raman spectroscopy, and nucleation and growth behaviors, among others. The viscosity of the glasses was measured using a beam bending and parallel plate viscometers. The kinetics of crystallization of the bulk glasses and fiber with x =20 were studied using a differential scanning analyzer (DTA), a hot stage XRD and an optical microscope. The influence of compositional variation on the physical, thermal and optical properties of the glasses in the TeO2-Bi2O3-ZnO family was established. The parameters such as the thermal properties, activation energy for crystallization, Johnson-Mehl-Avrami exponent, or nucleation and growth domains and rates were determined and were found to depend on the glass composition. We correlated the composition-dependent variation of these parameters to the structure of the glasses via Raman spectroscopy. Key physical, thermal, structural and optical differences were observed and quantified between bulk glasses and their corresponding core and core-clad fibers. Also reported are the processing and characterization of modified tellurite-based glass in the TeO2-Bi2O3-ZnO glass family and efforts to reduce their absorption loss due to residual hydroxyl (OH) content. We discuss the impact of this OH reduction in the tellurite network on the physical, thermal and structural properties as well as nucleation and growth behavior of bulk glass and fiber

Structural and optical properties of silver and titanium nanoparticles co-embedded magnesium-zinc-sulfophosphate glass with neodymium doping

Demand of high emission cross-section laser amplifier is ever-increasing. The rare earth ions (REIs) doped magnesium zinc sulfophosphate (PMZ) glasses with noble metal nanoparticles (NPs) inclusion were shown potential as laser host. This study reported that the emission/absorption traits of these REIs inside the glass matrix could be improved using the NPs which enabled localized surface plasmon resonance (LSPR). The LSPR produces strong local electric field (LEF) in the vicinity of the REIs. The melt-quenching method was used to synthesize glass composition 58.5P2O5-20.0MgO-20.0ZnSO4-1.5 Nd2O3-yAg NPs-zTi NPs mol% where y and z could be in the range of 0.0-1.5 mol%. The co-embedment of two types of metal NPs inside the glass could provide flexibility to customize the generated LEF strength in the proximity of the REIs. The X-ray diffraction (XRD) analyses confirmed the amorphous nature of the as-quenched samples. The high-resolution transmission electron microscopy (HRTEM) images showed the nucleation of both Ag and Ti3O5 NPs inside the glass matrix. The nucleation of Ti into Ti3O5 NPs occurred via two-step Finke-Watzky mechanism. Both the Fourier transform infrared (FTIR) and Raman spectral analyses confirmed an insignificant structural change from the incorporation of the NPs into the glass matrix. The observed LSPR band of the Ag (?442 nm) and Ti3O5 (?588 and 774 nm) NPs verified their successful embedment into the host matrix. The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra of the glasses exhibited twelve characteristic absorption bands of Nd3+. The photoluminescence (PL) emission spectra of the prepared glasses disclosed a prominent yellow band around 585 nm, corresponding to the transition of 2G7/2+4G5/2?4I9/2 in Nd3+. The coupling of Ag and Ti NPs into the glass at a certain concentration could lead to PL intensity quenching. This ascribed to the local heating effect of Ti3O5 NPs. The Judd-Ofelt intensity and radiative parameters were calculated from the absorption and PL spectral data. The PMZ1.5Nd04Ti0.3Ag glass sample showed the highest stimulated emission cross-section of 4.78×10–24 cm2. The results suggested that the co-embedment of metal NPs into glass could be an innovative strategy to tailor the spectroscopic characteristics of the glasses which is potential for photonic and solid-state laser applications

Thulium-doped fibre laser in the 2 μm wavelength region for gas sensing

The transition 3F4->3H6 of trivalent Thulium is widely studied for generating lasers at wavelength near 2 μm. For decades, tuneable continuous wave narrow line-width sources in this wavelength region have been proved to be very useful as spectroscopic tools for trace gas detection. Semiconductor lasers are often not readily available at a reasonable cost with the specific wavelengths required to provide a close ‘match’ to the key absorption features of the gases of interest. Well-designed fibre laser-based systems, however, can overcome this limitation by offering potentially much wider wavelength ranges, coupled with their distinctive and valuable features such as stability, narrow linewidth and high tuneability at room temperature. In this work, a compact ‘all-fibre’ laser system has been specifically designed, developed and evaluated, as this type of laser systems is highly desirable for ‘in-the-field’ applications. This takes full advantages of the active fibres based on silica glass host compared to other non-oxide glass hosts in terms of their chemical durability, stability and crucial structural compatibility with readily available telecommunication optical fibres. Ideal host composition for Thulium and efficient pumping scheme posses major challenges restricting the production of commercially deployable efficient ‘all-fibre’ lasers in the 2 μm wavelength region. The aim of the thesis work is to address these challenges. The work presented in this thesis demonstrates a modulated Thulium-doped ‘all-fibre’ tuneable laser in the 2 μm wavelength region suitable for detection of a number of gases of interest. The scope of work includes the fabrication and optimization of the active fibre with the core composition suitable for the creation of an effective Thulium-doped fibre laser. Codoping of Ytterbium is explored to investigate the energy-transfer mechanism from Ytterbium to Thulium and thereby opening up the opportunity of using economic pump laser diodes emitting at around 0.98 μm. In this respect, both Thulium- and Thulium/Ytterbium-doped single-mode single-clad silica optical fibres are designed and fabricated for a systematic analysis before being used as laser gain media. The optical preforms having different host compositions, Thulium-ion concentrations and proportions of Ytterbium to Thulium are fabricated by using the Modified Chemical Vapour Deposition technique coupled with solution doping to enable the incorporation of rareearth ions into the preforms. A thorough investigation of the basic absorption and emission properties of Thulium-doped silica fibres has been performed. The step-wise energy-transfer parameters in Thulium/Ytterbium-doped silica fibre have been determined quantitatively from spectroscopic measurements along with migrationassisted energy-transfer model. A set of tuneable Thulium-doped ‘all-fibre’ lasers, offering a narrow line-width in the 2 μm wavelength region, is created by using fabricated Thulium-and Thulium/Ytterbium-doped fibres as gain media and fibre Bragg grating pairs under in-band pumping at 1.6 μm and/or pumping by an economical laser diode at 0.98 μm, utilizing Ytterbium to Thulium energy- transfer. The host composition and the dopnat concentration in the single-mode single-clad fibre configuration are optimized to achieve maximum lasing efficiency. The tuning of laser wavelength has been achieved by using relaxation/compression mechanism of the fibre Bragg grating pair used to confine the laser cavity. A new set of laser resonators has also been formed by using a combination of a high reflective fibre Bragg grating with a low reflective broadband mirror, fabricated at the end of the fibre through silver film deposition, to enable only one fibre Bragg grating to be tuned. The stability of the laser output power, line-width and shape have been monitored throughout the tuning range. This is followed by the design of a compact, high-Q, narrow line-width and low threshold microsphere laser resonator, operating in the 2 μm wavelength region, by coupling a Thulium-doped silica microsphere to a tapered fibre. In the microsphere, laser emission occurred at wavelengths over the range from 1.9 to 2.0 μm under excitation at a wavelength of around 1.6 μm. The designed modulated tuneable Thulium-doped ‘all-fibre’ laser, operating at a wavelength range centred at a wavelength of 1.995 μm, has been tested for CO2 gas detection. Both the modulation of the fibre laser, through pump source modulation and the ‘locking’ detection mechanism have been utilized to eliminate laser intensity noise and therefore to obtain a compact gas sensor with high sensitivity. The absorption spectrum, the line-strength and the concentration level of CO2, have been monitored using the absorption spectroscopic technique. The measured minimum detectable concentration of CO2 obtained using the system confirms the claim that it is capable of detecting trace gases at the ppm level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact yet sensitive fibre laser based gas sensor systems.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Upconverting luminescent materials for solar energy conversion

As attractive energy converters, solar cells are considered to play a vital role in covering the constantly increasing energy demands in the future. Despite the strong success in the field, a variety of solar cells including the commercially available ones are utilizing mainly the visible region of the solar spectrum, and thus are lacking the capacity to exploit effectively the infrared region. This issue gives a possibility to improve the solar cell efficiencies by using the unutilized energy of the infrared radiation. One solution to overcome this issue is to use upconverting luminescent materials which are capable of converting infrared radiation to visible by stacking photons. However, despite the potential, the rather weak conversion efficiency still limits the use of these materials. On the other hand, even small portions of this converted radiation would have significant effect on the solar cell efficiency which is why the development of these materials is desirable. The main aims of this thesis work were to investigate and develop different fabrication methods and properties of the upconverting materials that could be used in solar cells. These methods included the co-precipitation method to prepare crystalline inorganic upconverting fluoride materials, the atomic and the molecular layer deposition techniques to fabricate upconverting oxide and hybrid thin films, and the direct particles doping method for preparation of upconverting luminescent glasses. In addition, another goal was to study the possibilities to enhance the upconversion luminescence which was done by adding transition metal ions (Cr3+ or Mnn+) into the Yb3+ and Er3+ doped fluoride material. Improvement in the upconversion luminescence intensity was obtained by using Cr3+ ions. Another enhancement possibility is to use efficient NIR absorbers together with a strong upconverting lanthanide ion. The combined ALD/MLD technique was shown to enable the combination of NIR harvesting organic moiety and upconverting lanthanide ions to form a hybrid thin film. The ALD and the combined ALD/MLD techniques were demonstrated to be well suitable for upconverting thin film fabrication. Moreover, the direct particles doping method was shown to offer a promising way to introduce a variety of crystalline luminescent materials into different glass matrices.Käänteisviritteiset luminoivat materiaalit aurinkoenergian hyödyntämisessä Aurinkoenergian tarjoamia mahdollisuuksia pidetään yhtenä lupaavimmista vaihtoehdoista kasvavien energiatarpeiden täyttämisessä tulevaisuudessa. Aurinkokennot ovat kehittyneet paljon viime aikoina, mutta silti suurin osa tutkituista ja kaupallisesti saatavilla olevista aurinkokennoista pystyy hyödyntämään auringon spektristä vain näkyvän valon alueen jättäen infrapuna-alueen lähes kokonaan hyödyntämättä. Tämä ongelma tarjoaa mahdollisuuden parantaa aurinkokennojen tehokkuutta. Yksi tapa hyödyntää infrapunasäteilyn energiaa on käyttää käänteisviritteisiä luminoivia materiaaleja, jotka pystyvät muuntamaan infrapunasäteilyä näkyväksi valoksi pinoamalla fotoneja. Tämän prosessin heikko teho rajoittaa toistaiseksi materiaalien käyttöä. Toisaalta, jopa pienellä IR-säteilyn muuntomäärällä on mahdollisuus vaikuttaa merkittävästi aurinkokennon tehokkuuteen, minkä vuoksi materiaalien kehittäminen on herättänyt kiinnostusta. Väitöskirjatyön tavoitteena oli tutkia ja kehittää aurinkokennoissa mahdollisesti käytettävien käänteisviritteisten materiaalien ominaisuuksia ja valmistusmenetelmiä. Menetelmiin kuului kerasaostusmenetelmä, jolla valmistettiin kiteisiä fluoridimateriaaleja; atomi- ja molekyylikerroskasvatusmenetelmät, joilla valmistettiin kiteisiä oksidi ja amorfisia hybridiohutkalvoja; sekä seostusmetelemä luminoivien lasien valmistukseen. Lisäksi tavoitteena oli tutkia mahdollisuuksia parantaa käänteisviritteistä luminesenssia käyttämällä siirtymämetalli-ioneja (Cr3+ ja Mnn+). Käänteisviritteisen luminesenssin intensiteettiä kasvatettiin lisäämällä Cr3+ - ioneja fluoridimateriaaliin, joka oli seostettu Yb3+ ja Er3+ -ioneilla. Käänteisviritteisen luminesenssin tehostamisessa voidaan hyödyntää myös tehokkaasti NIR-säteilyä absorboivan orgaanisen osan yhdistämistä käänteisviritteisesti luminoivaan lantanidi-ioniin. Näiden osien yhdistäminen todettiin olevan mahdollista yhdistetyllä ALD/MLD -tekniikalla, jolla valmistettiin hybridikalvoja. Tämän yhdistetyn tekniikan ja ALD-tekniikan osoitettiin olevan hyvin käyttökelpoisia käänteisviritteisten kalvojen valmistuksessa. Lasimateriaaleilla tutkitun seostusmenetelmän osoitettiin olevan lupaava menetelmä erilaisten luminoivien kiteisten materiaalien lisäämisessä lasiin