178 research outputs found
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Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates
No full textIn this work, we study the use of a spatial light modulator (SLM) for local manipulation of phase in interfering laser beams to fabricate photonic crystal templates with embedded, engineered defects. A SLM displaying geometric phase patterns was used as a digitally programmable phase mask to fabricate 4-fold and 6-fold symmetric photonic crystal templates. Through pixel-by-pixel phase engineering, digital control of the phases of one or more of the interfering beams was demonstrated, thus allowing change in the interference pattern. The phases of the generated beams were programmed at specific locations, resulting in defect structures in the fabricated photonic lattices such as missing lattice line defects, and single-motif lattice defects in dual-motif lattice background. The diffraction efficiency from the phase pattern was used to locally modify the filling fraction in holographically fabricated structures, resulting in defects with a different fill fraction than the bulk lattice. Through two steps of phase engineering, a spatially variant lattice defect with a 90° bend in a periodic bulk lattice was fabricated. Finally, by reducing the relative phase shift of the defect line and utilizing the different diffraction efficiency between the defect line and the background phase pattern, desired and functional defect lattices can be registered into the background lattice through direct imaging of the designed phase patterns
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Nanophotonics of Plasmonic and Two-Dimensional Metamaterials
No full textVarious nanostructured materials display unique and interesting optical properties. Specific nanoscale objects discussed in an experimental perspective in this dissertation include optical metamaterials, surface plasmon sensors, and two-dimensional materials. These nanoscale objects were fabricated, investigated optically, and their applications are assessed. First, one-dimensional magnetic gratings were studied, followed by their two-dimensional analog, the so-called "fishnet." Both were fabricated, characterized, and their properties, such as waveguiding modes, are examined. Interestingly, these devices can exhibit optical magnetism and even negative refraction; however, their general characterization at oblique incidence is challenging due to diffraction. Here, a new method of optical characterization of metamaterials which takes into account diffraction is presented. Next, surface plasmon resonance (SPR) was experimentally used in two schemes, for the first time, to determine the transition layer characteristics between a metal and dielectric. The physics of interfaces, namely the singularity of electric permittivity and how it can be electrically shifted, becomes clearer owing to the extreme sensitivity of SPR detection mechanisms. Finally, ultra-thin two-dimensional semiconducting materials had their radiative lifetime analyzed. Their lifetimes are tuned both by number of atomic layers and applied voltage biasing across the surface, and the changes in lifetime are suspected to be due to quenching or enhancement of non-radiative process rates
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Fabrication and Study of the Optical Properties of 3D Photonic Crystals and 2D Graded Photonic Super-Crystals
No full textIn this dissertation, I am presenting my research on the fabrication and simulation of the optical properties of 3D photonic crystals and 2D graded photonic super-crystals. The 3D photonic crystals were fabricated using holographic lithography with a single, custom-built reflective optical element (ROE) and single exposure from a visible light laser. Fully 3D photonic crystals with 4-fold, 5- fold, and 6-fold symmetries were fabricated using the flexible, 3D printed ROE. In addition, novel 2D graded photonic super-crystals were fabricated using a spatial light modulator (SLM) in a 4f setup for pixel-by-pixel phase engineering. The SLM was used to control the phase and intensity of sets of beams to fabricate the 2D photonic crystals in a single exposure. The 2D photonic crystals integrate super-cell periodicities with 4-fold, 5-fold, and 6-fold symmetries and a graded fill fraction. The simulations of the 2D graded photonic super-crystals show extraordinary properties such as full photonic band gaps and cavity modes with Q-factors of ~106. This research could help in the development of organic light emitting diodes, high-efficiency solar cells, and other devices
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Fabrication of Photonic Crystal Templates through Holographic Lithography and Study of their Optical and Plasmonic Properties in Aluminium Doped Zinc Oxide
No full textThis dissertation focuses on two aspects of integrating near-infrared plasmonics with electronics with the intent of developing the platform for future photonics. The first aspect focuses on fabrication by introducing and developing a simple, single reflective optical element capable of high–throughput, large scale fabrication of micro- and nano-sized structure templates using holographic lithography. This reflective optical element is then utilized to show proof of concept in fabricating three dimensional structures in negative photoresists as well as tuning subwavelength features in two dimensional compound lattices for the fabrication of dimer and trimer antenna templates. The second aspect focuses on the study of aluminum zinc oxide (AZO), which belongs to recently popularized material class of transparent conducting oxides, capable of tunable plasmonic capabilities in the near-IR regime. Holographic lithography is used to pattern an AZO film with a square lattice array that are shown to form standing wave resonances at the interface of the AZO and the substrate. To demonstrate device level integration the final experiment utilizes AZO patterned gratings and measures the variation of diffraction efficiency as a negative bias is applied to change the AZO optical properties. Additionally efforts to understand the behavior of these structures through optical measurements is complemented with finite difference time domain simulations
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Emergent Functionality and Controllability in Beamforming System
No full textThis dissertation presents beamforming designs. Using novel techniques and methods, the performance of the beamforming is improved on dual-band, tri-band, flexible function, tunable function in THz, and dynamic controllability on incident wave
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Electrically Tunable Absorption and Perfect Absorption Using Aluminum-Doped Zinc Oxide and Graphene Sandwiched in Oxides
No full textUnderstanding the fundamental physics in light absorption and perfect light absorption is vital for device applications in detector, sensor, solar energy harvesting and imaging. In this research study, a large area fabrication of Al-doped ZnO/Al2O3/graphene/Al2O3/gold/silicon device was enabled by a spin-processable hydrophilic mono-layer graphene oxide. In contrast to the optical properties of noble metals, which cannot be tuned or changed, the permittivity of transparent metal oxides, such as Al-doped ZnO and indium tin oxide, are tunable. Their optical properties can be adjusted via doping or tuned electrically through carrier accumulation and depletion, providing great advantages for designing tunable photonic devices or realizing perfect absorption. A significant shift of Raman frequency up to 360 cm-1 was observed from graphene in the fabricated device reported in this work. The absorption from the device was tunable with a negative voltage applied on the Al-doped ZnO side. The generated absorption change was sustainable when the voltage was off and erasable when a positive voltage was applied. The reflection change was explained by the Fermi level change in graphene. The sustainability of tuned optical property in graphene can lead to a design of device with less power consumption
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Hydrogenation and Modification of Graphene using Electron Irradiation in the Range of 20-25 KeV
No full textThe presence of defects in graphene alters its atomic configuration thereby resulting in the modification of the properties it possesses. One of the ways of producing defects in a controlled way in graphene is by exposing it to electron irradiation. In this dissertation, the hydrogenation and modification of graphene by electron irradiation in the range of 20 – 25 KeV was studied. The investigations carried out in this dissertation were conducted on graphene supported on SiO2 – Si substrates, which are referred to as supported graphene, and suspended graphene samples which are referred to as graphene drums. Graphene drum is a structure that consists of graphene mechanically exfoliated over micro-etched holes created on a supporting substrate. The study on supported graphene and graphene drums irradiated at an electron energy between 20 – 25 KeV and dosages between 1015 to 1018 e/cm2 suggests that the defects produced in graphene subjected to these conditions don’t cause significant damage to its lattice and are adsorbates that could be hydrogen. It was also observed that these adsorbates are produced at high density in electron-irradiated graphene drums. This implies that irradiating graphene drums with electrons could be an alternative way of modifying graphene with defects at high density without creating any damage to its lattice. In addition, the effects of electron irradiation on graphene drum and supported graphene were modeled using the local activation model. The model shows that the disordered areas produced due to the impact of electrons on graphene are larger in graphene drums as compared to supported graphene. The results presented in this dissertation could be helpful in providing information on how graphene could be modified for potential applications in hydrogen storage and micro-electronics
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Optical Properties of Twisted Single-layer Photonic Crystals and Halide Perovskite and Their Potential Integration
No full textThis research explores the incorporation of perovskite materials into nanotube structures to achieve enhanced optical properties. While perovskites have been integrated into various photonic devices, their potential within nanotubes remains less explored. This work aims to improve the understanding on how perovskites, used as a lasing medium, can enhance the performance of nanotube-based photonic crystals leading to improved broadband absorption and light-matter interaction
Raman studies of low-dimensional conductors and superconductors
No full textUsing a Fourier Raman spectrometer equipped with an infrared laser, together with cryogenics,
three types of materials have been investigated as a function of temperature in this
thesis. The first is the investigation of organic materials including K-(BEDT-TTF)2Cu(NCS)2
(Tc=10.4 K), K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K), αt-(BEDT-TTF)2I3 (Tc=8 K)
and β-(BEDT-TTF)2AuI2 (Tc=5 K) which become superconductors at low temperature.
The second is the study of the first organic conductor TTF-TCNQ which behaves in exactly
the opposite way by becoming an insulator at low temperature. The third is the study of
the strontium-doped lanthanum copper oxide superconductors with higher transition temperature.
For BEDT-TTF based organic superconductors, the electron-phonon coupling is very
strong. The frequencies and intensities of three strongest features ( v3 (Ag), vg (A9 ) and
v60 (B3g) modes) in the Raman spectra have been analyzed as a function of temperature.
The frequencies of some modes are observed to soften in the temperature range where antiferromagnetic
spin fluctuations have been observed, providing evidence of interactions between
the phonons and the magnetism. The v60 (B3g) mode is observed to be very unusual in
many ways, such as having an inverse isotope frequency shift. Below Tc, this mode exhibits
an increase of 2.2 cm-1 in K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K) and a decrease of 1.7
cm-1 in αt-(BEDT-TTF)2l3 (Tc=8 K). This is the highest frequency phonon in any material
to be affected by superconductivity.
For TTF-TCNQ, many new lines are observed at temperatures below 150 K as the
fluctuating charge-density-wave occurs. The intensity of these lines increases with decreasing
temperature. These new lines are assigned according to the deuterium-isotope frequency
shifts. In the fluctuating charge-density-wave phase the Frohlich electron-phonon interaction
is the probable cause of the appearance of Raman-forbidden scattering originating from the
infrared-active-only modes. The strong out-of-plane vibrational Raman modes correspond
to the large out-of-plane distortion of the TCNQ molecule, which is in agreement with the
x-ray results.
For lanthanum copper oxide materials, firstly we observe seven Raman-forbidden longitudinal
optical phonons, which appear to be activated by the Frohlich mechanism, in the
single-phonon Raman scattering of La2CuO4. Good agreement is obtained between the peak
frequencies and those of the longitudinal optical modes measured by infrared reflectivity and
inelastic neutron scattering. Secondly, strong magnetic Raman scattering is observed in one
crystal of La1.9Sr0.1CuO4, which has a suppressed Tc of 12 K, due to an ordered spin phase
below 40 K. A weak second peak indicates the possible existence of phase separation in the
sample. In agreement with neutron scattering results, the Raman intensity of the intense
peak increases with decreasing temperature below 40 K. The line shape and the temperature
dependence of the magnetic scattering intensity are totally different from those observed in
the parent compound La2CuO4. The temperature dependences of the peak frequency and
damping, however, are similar to those of other two-dimensional antiferromagnets. The line
shape is fitted within the traditional Loudon-Fleury Raman theory of two-magnon scattering.
The resulting super-exchange constant is found to be 1519 K, in accordance with
EPR-measurements on the same compound. [Scientific formulae used in this abstract could not be reproduced.]Science, Faculty ofPhysics and Astronomy, Department ofGraduat
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Twisted Moire Photonic Crystals: Their Nano-Fabrications, Optical Properties, and Applications in Light Extraction
No full textIn this dissertation, I report the results of my research on twisted moiré photonic crystals which can be formed through multi-beam holographic interference without a physical rotation and later fabricated by electron-beam lithography. Their optical properties, such as photonic bandgaps, multiple resonance modes, and quality factor are presented. Randomized moire photonic crystals in lattice are also studied. The applications of moire photonic crystals in improving light extraction efficiency are simulated and verified in light emitting devices. Furthermore, I simulated the light extraction efficiency in OLED when the Al layer is patterned with a triangular GPSC, square moiré PhC with defects in the uniform area, and random locations of the photonic lattice, and obtain light extraction efficiency of 78.9%, 79.9%, 81.7%, respectively. Also, the ratios of photoluminescence intensity of LED integrated with twisted moiré PhCs and random moiré PhCs over that without moiré PhCs are measured to be (1.3-1.9) and 1.74, respectively, in a good agreement with simulated ratios of 1.69 and 1.8
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