Investigation of novel functional glasses for photonic applications

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Investigation of novel functional glasses for photonic applications

 

Author: Tao, Lili
Title: Investigation of novel functional glasses for photonic applications
Degree: Ph.D.
Year: 2013
Subject: Optical glass.
Photonics -- Materials.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xxiii, 150 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2681809
URI: http://theses.lib.polyu.edu.hk/handle/200/7368
Abstract: Functional optical glasses have received substantial attention in recent years due to their great potential in diverse optical and photonic applications, specific functions of which can be realized by selecting the appropriate dopant and thus the glass provides a platform for further engineering. A representative example is rare earth ions doped glass fibers having already been utilized as fiber amplifiers in the telecommunication systems because of their high optical gain property in the near-infrared (NIR) emissions. Solid state lasers and light sources operating in rarely explored mid-infrared wavelength region (~2-3 μm) have also aroused remarkable attention due to their versatile applications in medical surgery, remote sensing, and eye-safe laser radar etc. Energy up- and down-conversion based on rare earth couples have been proposed to satisfy specific applications such as down-conversion from ultraviolet(UV)-visible to NIR through rare earth couples, establishing a promising way to improve the efficiency of silicon solar cell. Another part of the study will focus on the synthesis and investigations of graphene oxide contained so-gel glass materials, which exhibit strong nonlinear optical properties, and this solid state glass would be more suitable for practical applications. In this thesis, light sources and devices operating in the expanded low-loss communication window (~1.2-1.7 μm) have been systematically studied based on rare earth co-dopants. Superbroad optical emission band extending from 1.3 to 1.7 μm in Neodymium (Nd³⁺)/ Thulium (Tm³⁺)/ Erbium (Er³⁺) tri-doped fluorotellurite glasses was realized under 793 nm excitation. This superbroad luminescence is composed of the Nd³⁺: 4F3/2→4I13/2, Tm³⁺: 3H4→3F4, and Er³⁺: 4I13/2→4I15/2 transitions leading to broad emission bands with peaks at 1.34, 1.47, and 1.53 μm, respectively. Broadband emission spectrum is also observed in the ion-exchanged planar waveguide fabricated on such glass, allowing the broadband waveguide amplifiers to be formed based on such glass system. Moreover, broadband photoluminescence covering 1.30-1.67 μm wavelength range was observed in Praseodymium (Pr³⁺) singly-doped fluorotellurite glasses under 488 or 590 nm wavelength excitations. Such broad emission band originates from the radiative transition from the manifolds Pr³⁺: 1D2 to 1G4. The line-shape, band width, and lifetime were modified by changing the doping concentration of Pr³⁺, and a quantum efficiency as high as 73.7% was achieved with Pr³⁺ dopant in a low concentration of 0.05 mom%, which offers an alternative for the broadband luminescence sources covering the expanded low-loss window of the optical communication system. Approaches to improve mid-infrared emissions (~2-3 μm in wavelength) have been explored. The Holmium (Ho³⁺) 2 μm emission (5I7→5I8 transition) in Ho³⁺/Ytterbium (Yb³⁺) co-doped oxyfluoride silicate glass has been enhanced by precipitating fluoride nanocrystals Ba2YbF7 through heat treatment. The nanocrystals were characterized using the X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy. In addition, the Ho³⁺ 2.0 μm emission has also been enhanced by incorporation of Cerium (Ce³⁺) in pure glass host, providing an easier approach for the development of ~2.0 μm laser. The photoluminescence characteristics and the energy transfer processes involved were investigated, and the spontaneous transition properties of Ho3+ ion were studied based on the Judd-Ofelt theory. With regard to the Er³⁺ 2.7 μm emission (4I11/2→4I13/2 transition) in fluorotellurite glass, a large enhancement has been reached by codoping Pr³⁺ ion, due to the energy transfer from Er³⁺ (4I13/2) to Pr³⁺ (3F4,3) with efficiency as high as 85% which leads to the heavy depleting of the lower lasing level. Forming nanocrystals in glass host and codoping strategy have been proved to be effective methods in improving the mid-infrared luminescence benefiting the development of mid-infrared lasers.
Regarding the radiation energy conversion, Tm3+ exhibits relative large energy spaces compared with other rare earth ions, making it less dependent on the phonon energy of host matrix. Intense ultraviolet upconverted emission has been observed in Tm³⁺/Yb³⁺ co-doped gallogermanate oxide glass upon a low cost 980 nm diode laser excitation, initiating innovation ways for the development of UV light sources in oxide host. On the other hand, in order to make use of the broad UV high-energy radiation of solar spectrum, we propose Gadolinium (Gd³⁺)/Yb³⁺ co-doped germanate glass as a potential spectral modifier to enhance silicon photovoltaic efficiency. To the best of our knowledge, the visible and NIR emission under UV excitation in Gd³⁺/Yb³⁺ co-doped glass has been observed for the first time. The luminescent mechanism is investigated according to the excitation and emission spectra, lifetime measurements, in addition to the discussion of its potential application for silicon photovoltaic improvement. Solid-state graphene oxide (GO) homogeneously incorporated silica gel glasses were fabricated and the good nonlinear optical (NLO) response proves its high potential as the optical limiting material for protecting detectors and human eyes from being damaged by high power lasers. The homogeneous dispersion of GO in silica gel glass is confirmed by digital images and element mapping analysis result of carbon. For the sample doped with 5.1 mol% GO it has linear transmittance of 56% and 65% at 532 nm and 1064 nm, respectively, starting to exhibit nonlinear optical response at a input influence of as low as 1.3 J cm-2 and 2.3 J cm-2 at 532 nm and 1064 nm, respectively. Moreover, the stronger NLO performance of GO in solid-state silica gel glass than in deionized water has been first demonstrated, and the probable reasons behind are discussed. In summary, functional optical glasses doped with rare earth ions and graphene oxide have been investigated systematically, and a series of journal publications have been obtained through these studies. These results would advance and promote the further development of functional glasses and their practical applications.

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