In recent years, rare-earth doped materials with its unique luminescence and
energy conversion properties have played a vital role in the development of advanced
materials for a wide variety of applications in telecommunications, solar energy and life
sciences. Over the decades, rare-earth doped optical materials have a noticeable impact
on telecommunications. Due to the successful development of Erbium doped fiber
amplifier for long distance application and predictions to have fiber to home connection
in future; it is attractive to develop interconnections/components for the integration of
optical amplifiers in photonic integrated circuits (PIC). Optical waveguide devices
based on inorganic hosts materials; such as glass, crystal and semiconducting materials,
have the advantages of good transparency, low transmission loss, stable performance
and high power operation. However, the fabrication involves complicated processing
steps and expensive equipments such as reactive ion etching (RIE), plasma-enhanced
chemical vapor deposition (PECVD), radio frequency (RF) sputtering and molecular
beam epitaxy (MBE). On the other hand, the usage of polymeric hosts for the
fabrication of optical waveguide amplifiers have many advantages including low
fabrication costs, simple processing steps and compatible to many processing
techniques.
Unfortunately, compatibility is a long existed problem between the
combination of rare-earth dopants and organic host materials. One solution is to apply
nanotechnology to address this problem. This is a new area because the properties of
many conventional bulk materials are different when they are in nano-scale particles. In
fact, nano-materials research has drawn much attention in recent years due to a wide
variety of potential applications in life science and electronics engineering. Nanocrystals in small uniform particle size have a monodisperse property to solve the problem of organic/inorganic hybridization and reduce the scattering loss as well. The development of efficient luminescence nanocrystals has attracted attention and NaYF4 is
chosen since it is one of the suitable host materials for Er3+ doping for photoluminescent
applications. Fluoride-based host materials have advantages of low phonon energy that
could minimize non-radiative transitions. In bulk materials, fluoride is highly toxic and
can easily release hydrofluoric gas during the synthesis process. However, nanocrystals
growth by wet chemical synthesis method employed in this project can help to ease this
problem. Ytterbium(III) ion, acting as a co-dopant with Er3+, is proposed since it can
absorb and transfer the pump radiation at 980 nm efficiently from its 2F5/2 level to the
4I13/2 level of the Er3+. The nanocrystals are also encapsulated with a layer of long
aliphatic chain of oleic acid to enhance the mixing with the polymer materials. A
detailed study on the materials and spectroscopic characterizations of the Er3+ -Yb3+
codoped nanocrystals were carried out.
In fact, polymer materials exhibit low absorption losses in the visible
wavelength region; therefore, optical polymer waveguide devices in this region have
attracted considerable interests in the development of photonic integrated circuits and
applications in local area networks. Among all the polymeric materials, biomaterials are
of particular interest over the conventional organic materials since they are renewable
and eco friendly materials. Deoxyribonucleic acid sodium salt from salmon fish was
used as polymer matrix materials. It has advantages as optical materials since it is
reported to have excellent optical properties with nearly 100% optical transmission over
300-1600 nm wavelength region. Therefore, rare-earth doped biomaterials for optical
amplifiers are very attractive. Europium thenoyltrifluoroacetonate [Eu(TTFA)3] was
used as dopant since it has a strong luminescence at ~612 nm wavelength, matching well with the low loss visible wavelength windows of the polymer materials. In addition,
using TTFA as ligands can solve the incompatibility problems between rare-earth and
organic materials. A detailed study on the materials and spectroscopic characterizations
of the Eu3+ -doped biopolymers were carried out.
Furthermore, another application of rare-earth doped materials is solar
spectral conversion. Solar energy is very important nowadays due to the imminent
energy crisis. In the commercial crystalline Si (c-Si) solar cells, the energy efficiency is
only about 15% and over 70% of the energy is lost due to the spectral energy mismatch
with the 1.1eV energy gap of the solar cells. Most of the photon energy exceeding the
bandgap cannot be totally absorbed and lost as heat dissipation. Therefore, the
maximum energy efficiency is only about 30% and is known as the Shockley-Queisser
limit. One approach is to use rare-earth materials to convert the solar spectrum to match
with the Si solar cell. Since the high energy photons incurred in ultraviolet to visible
(UV-VIS) spectrum are more attractive to harvest whilst the conversion efficiency in
this region is the lowest for the Si solar cell. The approach of down-shifting to convert a
high energy photon into lower energy photon matching the solar cell is attractive. Solar
spectral converting material is studied in order to convert the ultraviolet to near infrared
(NIR) radiations for the absorption of silicon photovoltaic cells. Additions of different
ligands to enhance the NIR fluorescence for solar cell were studied. Ytterbium
Hexafluoro-2,4-Pentanedionate in PMMA polymer was used as photoluminescence
materials due to its infra-red wavelength emission at ~1μm. Three different ligands,
(Triphenylphosphine (TPP), 2,2'-Bipyridine (Bipy) and Tris(1-pyrazoly)methane (TPM)
in PMMA were used as the tuning materials and a detailed study on the materials and
spectroscopic characterizations on the Yb3+ -doped PMMAs was carried out. An obvious
enhancement in the fluorescence was observed and Bipy demonstrated to be the most efficient tuning materials, with 4.8 times longer in lifetime than the Yb complex without
the ligands.
Recently, near-infrared (NIR) absorption and upconversion emission
nanoparticles have important biological applications. Compared with conventional
nanophospors based on downconversion fluorescence, their photoluminescence are
attributable to the high energy excitation, such as ultraviolet or visible wavelength light
source. These kinds of downconversion nanophosphors are usually semiconducting
materials and poisonous to living organism. They also exhibit disadvantages in the low
penetration depth of tissue and high energy damaging effects on the cells due to the
induced cytotoxic on the living organism. Therefore, it is not an attractive option for
applications, such as photodynamic therapy or bio-imaging. Near-infrared NIR emission
wavelength as excitation is more penetrable to the tissue and less damaging effects on
the cell. Besides, lanthanides nanoparticles are more stable in emission. Therefore, NIR
induced upconversion emission of nanoparticles are more attractive.
Synthesis of lanthanide nanoparticles is usually by wet chemical process, the
ligands encapsulated on the nanoparticles are often hydrophobic and not suitable to be
used in biological purpose. Therefore, hydrophilic and biocompatible surfactant
materials on the nanoparticles surfaces are necessary. One of the most successful
biocompatible polymers - polyethylenimine (PEI) has been widely used in gene
delivery and high molecular weight PEI polymer is reported as coating material in
upconversion nanoparticle. However, high molecular PEI has been proved cytotoxic and
could reduce the cell metabolic activity up to 90%. Therefore, the low molecular weight
PEI is more desirable and this is the first demonstration to synthesis low molecular
weight PEI polymer encapsulating the lanthanide nanoparticles. A detailed study on the
materials and spectroscopic characterizations on the PEI coated upconverting Er3+ -Yb3+ co-doped nanocrystals were carried out and strong luminescence at 651nm was
observed.
The Er3+ -Yb3+ nanocrystals-doped KMPR® polymer channel waveguide
with dimension of 11 μm in height, 12 μm in width dimensions was fabricated by using
one-step UV photolithography method. The propagation loss of the waveguide by cutback
method was found to be ~3.42 dB/cm. The performance of the nanocrystals–
polymer hybrid waveguide exhibited strong emission at 1535nm under pumping of 980
nm laser and a maximum optical gain of 7.7 dB was obtained in a 16-mm-long device.
Besides, the DNA-based waveguide amplifiers were fabricated by molding
methods. There is a problem to implement dopants in the DNA-based matrix to
fabricate the optical amplifiers since the dopants would be destroyed during some
conventional processing methods such as plasma etching or high energy electron beam
writing process. Therefore, feasible and efficient methods to fabricate the waveguide
amplifiers are important. In this work, injection molding and soft mold printing are
introduced. Injection molding method is to introduce the Eu3+ -doped biopolymer inside
the micro-scale core layer of quartz capillary tube by injection. Soft molding method is
to transfer the waveguide pattern by PDMS mold pressing on the biopolymer. Eu3+ -doped biopolymer optical waveguide amplifiers were fabricated by mold injection and
soft lithography, and the corresponding propagation losses at 612 nm wavelength were
1.9 dB/cm and 2.52 dB/cm, respectively. Signal enhancement Go of ~24 dB and 15 dB
were obtained for the imprinted waveguide arrays (1.2 cm) and quartz capillary tube
(1.5 cm), respectively.
To conclude, the work described in this thesis provides a study on rare-earth
doped materials and is found potentials for the applications of optical amplifiers, solar
spectral conversion and upconverting nanophosphors.
| Date of Award | 15 Jul 2013 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Yue Bun Edwin PUN (Supervisor) |
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