92 research outputs found
Photoluminescence Characterization of Patterned Quantum dots and Inverse Quantum dots
The ever increasing demand for oil and its limited supply have forced us to look for alternate sources of energy. Solar energy offers a cheap, alternate form of energy. The efficiency of a solar cell is set by the Shockley-Queisser limit and is currently very low. New techniques to increase the efficiency of solar cells are being explored. Quantum dots and inverse quantum dots are promising future ways to increase the efficiency of solar cells through multiple exciton generation. In this thesis, the fabrication and characterization of defect-free quantum dots and anti-dots are discussed
Rehearsals
This artifact will be an article that discusses a major aspect of Kazemi’s and Ghousseini’s research, rehearsals. Rehearsals are a form of simulation where novice teachers learn complex teaching methods through practice, learning by doing. It is a form of simulation that allows teachers to share and develop teaching methods. This article will relate the concept of rehearsals back to the concept of “teaching teachers to teach” by exploring what rehearsals are and how they assist teachers. It will discuss how through rehearsals, teachers, especially math teachers, gain insight into the child\u27s perspective while being taught. This allows the teacher to better communicate the material. Rehearsals are a very important concept in Kazemi’s and Ghousseini’s research, and will be discussed here
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Interactive, Computation Assisted Design Tools
Realistic modeling, rendering, and animation of physical and virtual shapes have matured significantly over the last few decades. Yet, the creation and subsequent modeling of three-dimensional shapes remains a tedious task which requires not only artistic and creative talent, but also significant technical skill. The perfection witnessed in computer-generated feature films requires extensive manual processing and touch-ups. Every researcher working in graphics and related fields has likely experienced the difficulty of creating even a moderate-quality 3D model, whether based on a mental concept, a hand sketch, or inspirations from one or more photographs or existing 3D designs. This situation, frequently referred to as the content creation bottleneck, is arguably the major obstacle to making computer graphics as ubiquitous as it could be. Classical modeling techniques have primarily dealt with local or low-level geometric entities (e.g., points or triangles) and criteria (e.g., smoothness or detail preservation), lacking the freedom necessary to produce novel and creative content.
A major unresolved challenge towards a new unhindered design paradigm is how to support the design process to create visually pleasing and yet functional objects by users who lack specialized skills and training. Most of the existing geometric modeling tools are intended either for use by experts (e.g., computer-aided design [CAD] systems) or for modeling objects whose visual aspects are the only consideration (e.g., computer graphics modeling systems). Furthermore, rapid prototyping, brought on by technological advances 3D printing has drastically altered production and consumption practices. These technologies empower individuals to design and produce original objects, customized according to their own needs. Thus, a new generation of design tools is needed to support both the creation of designs within the domain's constraints, that not only allows capturing the novice user's design intent but also meets the fabrication constraints such that the designs can be realized with minimal tweaking by experts.
To fill this void, the premise of this thesis relies on the following two tenets:
1. users benefit from an interactive design environment that allows novice users to continuously explore a design space and immediately see the tradeoffs of their design choices.
2. the machine's processing power is used to assist and guide the user to maintain constraints imposed by the problem domain (e.g., fabrication/material constraints) as well as help the user in exploring feasible solutions close to their design intent.
Finding the appropriate balance between interactive design tools and the computation needed for productive workflows is the problem addressed by this thesis. This thesis makes the following contributions:
1. We take a close look at thin shells--materials that have a thickness significantly smaller than other dimensions. Towards the goal of achieving interactive and controllable simulations we realize a particular geometric insight to develop an efficient bending model for the simulation of thin shells. Under isometric deformations (deformations that undergo little to no stretching), we can reduce the nonlinear bending energy into a cubic polynomial that has a linear Hessian. This linear Hessian can be further approximated with a constant one, providing significant speedups during simulation. We also build upon this simple bending model and show how orthotropic materials can be modeled and simulated efficiently.
2. We study the theory of Chebyshev nets--a geometric model of woven materials using a two-dimensional net composed of inextensible yarns. The theory of Chebyshev nets sheds some light on their limitations in globally covering a target surface. As it turns out, Chebyshev nets are a good geometric model for wire meshes, free-form surfaces composed of woven wires arranged in a regular grid. In the context of designing sculptures with wire mesh, we rely on the mathematical theory laid out by Hazzidakis~\cite{Hazzidakis1879} to determine an artistically driven workflow for approximately covering a target surface with a wire mesh, while globally maintaining material and fabrication constraints. This alleviates the user from worrying about feasibility and allows focus on design.
3. Finally, we present a practical design tool for the design and exploration of reconfigurables, defined as an object or collection of objects whose transformation between various states defines its functionality or aesthetic appeal (e.g., a mechanical assembly composed of interlocking pieces, a transforming folding bicycle, or a space-saving arrangement of apartment furniture). A novel space-time collision detection and response technique is presented that can be used to create an interactive workflow for managing and designing objects with various states. This work also considers a graph-based timeline during the design process instead of the traditional linear timeline and shows its many benefits as well as challenges for the design of reconfigurables
Unveiling the X-ray polarimetric properties of LMC X-3 with IXPE, NICER, and Swift/XRT
The incoming Imaging X-ray Polarimetry Explorer (IXPE) observations of X-ray
binaries provide a new tool to investigate the underlying accretion geometry.
Here we report the first measurements of X-ray polarization of the
extra-galactic black-hole X-ray binary LMC X3. We find a polarization
fraction of 3 % at a polarization angle of in the
keV energy band with statistical significance at the 7 level. This
polarization measurement significantly exceeds the minimum detectable
polarization threshold of 1.2 % for the source, ascertained at a 99 %
confidence level within the keV energy band. The simultaneous
spectro-polarimetric fitting of NICER, Swift/XRT, and IXPE revealed the
presence of a disc with a temperature of 1 keV and a Comptonized component with
a power-law index of 2.4, confirming the soft nature of the source. The
polarization degree increases with energy from 3 % in the keV band
to 8 % in the keV band, while the polarization angle is energy
independent. The observed energy dependence and the sudden jump of polarization
fraction at 5 keV supports the idea of a static slab coronal geometry
for the comptonizing medium of LMC X3. We further observed no change in the
polarization properties with time over the period of the IXPE observations.Comment: 6 pages, 5 figures, 3 tables, submitted to MNRA
Spectropolarimetric study of 4U 163047 in Steep Power law state with IXPE and NICER
We probe the spectropolarimetric properties of the black-hole binary source
4U 163047 in the steep power law state. We detect a significant polarization
fraction of 7 % at a polarization angle of 21 . The
keV NICER spectrum can be fitted with a combination of a thermal and a
Comptonization component, the latter characterized by a spectral index, 2.1, along with a reflection feature at 7.0 keV. In the keV
band, the degree of polarization of 4U 163047 in the steep power law state
is 4.4 different from the value previously measured in the high soft
state. In the steep power law state, the polarization fraction increases as a
function of energy but exhibits an overall drop in each energy band compared to
that of the high soft state. We propose that the decrease in the polarization
fraction in the steep power law state could be attributed to the presence of a
corona. The observed polarization properties in both states can be explained by
the self-irradiation of the disk around a Kerr black hole, likely influenced by
the frame-dragging effect.Comment: 6 pages, 7 figures, 3 tables, accepted for publication in MNRA
Investigating the Energy-Dependent Temporal Nature of Black Hole Binary System H 1743-322
Black hole X-ray binaries routinely exhibit Quasi Periodic Oscillations
(QPOs) in their Power density spectrum. Studies of QPOs have demonstrated
immense ability to understand these dynamical systems although their
unambiguous origin still remains a challenge. We investigate the
energy-dependent properties of the Type-C QPOs detected for H 1743-322 as
observed with AstroSat in its two X-ray outbursts of 2016 and 2017. The
combined broadband LAXPC and SXT spectrum is well modelled with a soft thermal
and a hard Comptonization component. The QPO exhibits soft/negative lags i.e.
variation in soft band lags the variation in hard band, although the upper
harmonic shows opposite behaviour i.e. hard/positive lags. Here, we model
energy-dependent properties (fractional root mean square and time-lag variation
with energy) of the QPO and its upper harmonic individually with a general
scheme that fits these properties by utilizing the spectral information and
consequently allows to identify the radiative component responsible for
producing the variability. Considering the truncated disk picture of accretion
flow, a simple model with variation in inner disk temperature, heating rate and
fractional scattering with time delays is able to describe the fractional RMS
and time-lag spectra. In this work, we show that this technique can
successfully describe the energy-dependent features and identify the spectral
parameters generating the variability.Comment: 6 Figures, 3 Tables, Accepted for publication in MNRA
Wire mesh design
We present a computational approach for designing wire meshes, i.e., freeform surfaces composed of woven wires arranged in a regular grid. To facilitate shape exploration, we map material properties of wire meshes to the geometric model of Chebyshev nets. This abstraction is exploited to build an efficient optimization scheme. While the theory of Chebyshev nets suggests a highly constrained design space, we show that allowing controlled deviations from the underlying surface provides a rich shape space for design exploration. Our algorithm balances globally coupled material constraints with aesthetic and geometric design objectives that can be specified by the user in an interactive design session. In addition to sculptural art, wire meshes represent an innovative medium for industrial applications including composite materials and architectural façades. We demonstrate the effectiveness of our approach using a variety of digital and physical prototypes with a level of shape complexity unobtainable using previous methods
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