33,814 research outputs found
High-power supercontinuum generation with picosecond pulses
No full textWe investigate both theoretically and experimentally the properties of supercontinuum generation in the nearinfrared and visible regimes using high-power picosecond pulses propagating in a photonic crystal optical fiber. We discuss the effect of the pump wavelength relative to the fiber zero-dispersion wavelength on the conversion efficiency of pump power to shorter wavelengths. Finally, we consider picosecond supercontinuum generation in the presence of a mode cutoff when the pump is launched into a higher order fiber mode
Thin-film photonic crystal LEDs with enhanced directionality
No full textThe use of photonic crystals for light extraction from light-emitting diodes (LEDs)
gives the possibility to shape the farfield emission pattern. This is of particular interest
for étendue-limited LED applications that require a more directional farfield than state-
of-the-art Lambertian emitters. However, the application of a photonic crystal in a LED
results in directional emission only if the photonic crystal and the distribution of guided
modes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-film
PhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed,
fabricated and characterized. The first experimental results show that light extraction
with photonic crystals from AlGaInP thin-film LEDs several microns thick is neither
directional nor more efficient than state-of-the-art LEDs with a rough surface structure.
Directional light extraction for AlGaInP PhC-LEDs is for the first time demonstrated in
much thinner devices where the photonic crystal light extraction of guided modes is
combined with the resonant-cavity effect. In an attempt to approach the ideal PhC-LED,
strong photonic crystal farfield shaping is demonstrated in InGaN thin-film LEDs of
sub-micron thickness. Analysis of their spectral farfields unexpectedly shows that high
order diffraction contributes significantly to the light extraction efficiency if the mode
absorption is sufficiently low. It is also demonstrated that directional photonic crystal
light extraction is possible in InGaN thin-film LEDs several microns thick. The
directionality stems from the modulation of the spontaneous emission caused by the
proximity of the active region to the bottom mirror. Two new concepts for enhanced
light extraction and high directionality are presented: Photonic crystals with two
dominating lattice constants are found to outperform conventional photonic crystal
LEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations show
that the high extraction efficiency of LEDs with surface roughness is combined with the
higher directionality of photonic crystal light extraction
Disorder in Extra-Large Pore Zeolite ITQ-33 Revealed by Single Crystal XRD
No full textThe single crystal of the extra-large pore zeolite, ITQ-33, was obtained and used to explore its crystal structure details. The ITQ-33 structure was found to be disordered with the columnar periodic building unit, explaining the morphology changes upon the different Si/Ge ratio, and the formation of the hierarchical structure from assembling of ITQ-33 nanofibers.Chemistry, MultidisciplinaryCrystallographyMaterials Science, MultidisciplinarySCI(E)EI0ARTICLE104168-41711
Photonic crystal waveguides in chalcogenide glasses
No full textThe growing speed and bandwidth requirements of telecommunication systems
demand all-optical on-chip solutions. Microphotonic devices can deliver
low power nonlinear signal processing solutions. This thesis looks at the slow
light photonic crystals in chalcogenide glasses to enhance low power nonlinear
operation.
I demonstrate the development of new fabrication techniques for this delicate
class of materials. Both, reactive ion etching and chemically assisted ion
beam etching are investigated for high quality photonic crystal fabrication.
A new resist-removal technique was developed for the chemical, mechanical
and light sensitive thin films.
I have developed a membraning method based on vapor phase etching
in combination with the development of a save and economical etching tool
that can be used for a variety of vapour phase processes.
Dispersion engineered slow light photonic crystals in Ge₃₃As₁₂Se₅₅ are designed
and fabricated. The demonstration of low losses down to 21±8dB/cm
is a prerequisite for the successful demonstration of dispersion engineered
slow light waveguides up to a group index of around n[subscript(g)] ≈ 40.
The slow light waveguides are used to demonstrate highly efficient third
harmonic generation and the first advantages of a pure chalcogenide system
over the commonly used silicon. Ge₁₁.₅As₂₄24Se₆₄.₅ is used for the fabrication
of photonic crystal cavities. Quality factors of up to 13000 are demonstrated.
The low nonlinear losses have enabled the demonstration of second and third
harmonic generation in those cavities with powers up to twice as high as
possible in silicon.
A computationally efficient model for designing coupled resonator bandpass
filters is used to design bandpass filters. Single ring resonators are
fabricated using a novel method to define the circular shape of the rings to
improve the fabrication quality. The spectral responses of the ring resonators
are used to determine the coupling coefficient needed for the design and fabrication
of the bandpass filters. A flat top bandpass filter is fabricated and
characterized as demonstration of this method.
A passive all-optical regenerator is proposed, by integrating a slow-light
photonic crystal waveguide with a band-pass filter based on coupled ring
resonators. A route of designing the regenerator is proposed by first using
the dispersion engineering results for nonlinear pulse propagation and then
using the filter responses to calculate the nonlinear transfer function
Propagation loss in slow light photonic crystal waveguides
No full textThe field of nanophotonics is a major research topic, as it offers potential solutions to
important challenges, such as the creation of low power, high bandwidth interconnects or
optical sensors. Within this field, resonant structures and slow light waveguides are used
to improve device performance further. Photonic crystals are of particular interest, as they
allow the fabrication of a wide variety of structures, including high Q-factor cavities and
slow light waveguides.
The high scattering loss of photonic crystal waveguides, caused by fabrication disorder,
however, has so far proven to be the limiting factor for device applications. In this thesis, I
present a detailed study of propagation loss in slow light photonic crystal waveguides.
I examine the dependence of propagation loss on the group index, and on disorder, in
more depth than previous work by other authors. I present a detailed study of the relative
importance of different components of the propagation loss, as well as a calculation method
for the average device properties.
A new calculation method is introduced to study different device designs and to show that
photonic crystal waveguide propagation loss can be reduced by device design alone. These
“loss engineered” waveguides have been used to demonstrate the lowest loss photonic crystal
based delay line (35 dB/ns) with further improvements being predicted (< 20 dB/ns).
Novel fabrication techniques were investigated, with the aim of reducing fabrication
disorder. Initial results showed the feasibility of a silicon anneal in a nitrogen atmosphere,
however poor process control led to repeatability issues.
The use of a slow-fast-slow light interface allowed for the fabrication of waveguides spanning multiple writefields of the electron-beam lithography tool, overcoming the problem of
stitching errors.
The slow-fast-slow light interfaces were combined with loss engineering waveguide designs, to achieve an order of magnitude reduction in the propagation loss compared to a W1
waveguide, with values as low as 130 dB/cm being achieved for a group index around 60
Photonic crystal interfaces: a design-driven approach
No full textPhotonic Crystal structures have been heralded as a disruptive technology
for the miniaturization of opto-electronic devices, offering as they do the
possibility of guiding and manipulating light in sub-micron scale waveguides.
Applications of photonic crystal guiding - the ability to send light around sharp
bends or compactly split signals into two or more channels have attracted a
great deal of attention. Other effects of this waveguiding mechanism have
become apparent, and attracted much interest - the novel dispersion surfaces
of photonic crystal structures allow the possibility of “slow light” in a dielectric
medium, which as well as the possibility of compact optical delay lines may
allow enhanced light-matter interaction, and hence miniaturisation of active
optical devices. I also consider a third, more traditional type of photonic
crystal, in the form of a grating for surface coupling.
In this thesis, I address many of the aspects of passive photonic crystals,
from the underlying theory through applied device modelling, fabrication
concerns and experimental results and analysis. Further, for the devices
studied, I consider both the relative merits of the photonic crystal approach
and of my work compared to that of others in the field. Thus, the complete
spectrum of photonic crystal devices is covered.
With regard to specific results, the highlights of the work contained in this
thesis are as follows:
Realisation of surface grating couplers in a novel material system
demonstrating some of the highest reported fibre coupling efficiencies.
Development of a short “injecting” taper for coupling into photonic
crystal devices.
Optimisation and experimental validation of photonic crystal routing
elements (Y-splitter and bend).
Exploration of interfaces and coupling for “slow light” photonic crystals
Subduing power : Indigenous sovereignty matters
No full textThe concept of ‘power’ can refer to the institutionalised and\ud
embodied capacity and right to dominate through a variety\ud
of means including ideology, politics, science, religion, class,\ud
race, gender and sexuality. Early feminist theorising within\ud
the West, for example, conceptualised the structure and\ud
nature of power as being connected to male domination and\ud
authority within society. Marxists, alternately, argue it is the\ud
ruling class that holds power and exercises it as owners of the\ud
means of production. In a general sense, we can say that as\ud
feminists have tied power to patriarchy and Marxists’ definitions\ud
of power have been connected to capitalism. The essays\ud
in this section, though, are less concerned with such totalising\ud
conceptualisations of power than they are with processes of\ud
interpellation or subject creation within dominant or dominating\ud
discursive spaces.1 Not power as such, but its many\ud
workings and apparatuses
Low insertion loss modulator based on a vertically coupled photonic crystal resonator
No full textWe experimentally demonstrate a simple but more efficient technique to modulate and multiplex multiple WDM channels. Our design is based on a bus waveguide vertically coupled to multiple Photonic Crystal (PhC) resonator, each of which modulates an individual channel in place. The Photonic crystal resonator modulator provide very low switching energies (similar to fJ) while the bus waveguide can be made from a material with a low refractive index thereby allowing very efficient coupling with an optical fiber
Photonic crystal and photonic wire nano-photonics based on silicon on insulator
No full textSilicon-on-insulator (SOI) is a strong candidate for application in future planar waveguide integration technology, whether or not luminescence is extracted from the silicon. We review recent research on photonic devices based on silicon-on-insulator. These devices exploit either photonic crystal or photonic wire concepts—or combinations of both. Aspects of the technologies used that are particularly critical for successful implementation of SOI-based photonics are addressed
Hanging the harp on the willow tree: music and national identity in postcolonial Ireland
No full textAn inquiry into how music served as a nation building tool in the early decades of the Irish Free State.M.A.Includes bibliographical referencesby Crystal N. Galyea
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