Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6602. Adviser: Shun Lien Chuang. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

The terehertz radiation potentially has many interesting applications. From air port security, non-destructive evaluations of electronics and space shuttle panels, to non-ionizing photon energies with the potential to detect cancer growths and quality control of pharmaceutical tables, the list of potential applications is vast as shown in chapter 1. However, there is a lack of cheap, robust and efficient THz sources, detectors and modulators. Further, the long wavelengths render micron sized details unseeable with far-field imaging techniques. This has rendered most imaging applications unusable in the real world. This thesis is based around demonstrating an imaging technique that uses a near-field THz modulator to obtain sub-wavelength images. There are five distinct experimental demonstrations that show the full capacity of the imaging technique developed here. Chapter 2 gives an outline of the background physics knowledge needed to understand the entirety of the thesis. An outline of the mathematics used for modellingis given in the latter part of the chapter as well. Chapter 3 gives a background on the THz generation and detection techniques used in our THz-TDS system, optical rectification and electro-optic sampling in ZnTe. Further more, our system is capable of photoexciting a sample in conjunction to it being probed with a THz pulse. For the most part, we photoexcite a silicon wafer in order to use its photoconductive properties to modulate our THz pulse. Our photoexcitation pulse is spatially modulated, via a digital micromirror device, which in turn spatially modulates our THz pulse. This patterned THz pulse can then be used with a single-element detector to perform imaging. How to do this and the type of patterns needed is described in the latter part of chapter 3. Chapter 4 is the first demonstration that photo-induced conductivity in silicon can be used to manipulate evanescent THz fields for sub-wavelength imaging. For this, we imaged a 1D sub-wavelength slit and were able to obtain the slit profile with 65μm (λ/6 at 0.75T Hz) resolution. Chapter 5 demonstrates what limits the resolution in our imaging system. Namely, the distance which the patterned THz pulse propagates to the object from where itwas spatially modulated. We demonstrate 9μm (λ/45 at 0.75T Hz) resolution using an ultra-thin (6μm) silicon wafer. At such sub-wavelength scales polarization becomes an important factor. We show how one can use polarization in order to detect 8μm breaks in a circuit board hidden by 115μm of silicon. Chapter 6 concerns itself with showing how noise affects our images. Further more, our imaging system is compatible with compressed sensing where one can obtain an image using fewer measurements than the number of pixels. We investigate how different under-sampling techniques perform in our system. Note under-sampling at sub-wavelength resolutions, as is done here, is rather unusual and is of yet to be demonstrated for other part of the electro-magnetic spectrum. Chapter 7 shows that one does not need to photoexcite silicon. One can in principle illuminate any material, hence we photoexcite graphene with our spatially modulated optical pulses. This allows us to obtain the THz photoconductive response of our graphene sample with sub-wavelength resolution (75μm ≈ λ/5 at 0.75T Hz). We compare our results with Raman spectra maps. We find a clear correlation between THz photoconductivity and carrier concentration (extracted from Raman). Chapter 8 exploits the full capacity of our imaging system by performing hyper-spectral near-field THz imaging on a biological sample. For this, in our imaged field of view, we measured the full temporal trace of our THz pulse at a sub-wavelength spatial resolution. This has allowed us to extract the frequency dependent permittivity of our biological sample, articular cartilage, over our spectral range (0.2-2T Hz). We find the permittivity to change on a sub-wavelength scale in correlation with changes in the structure of our sample. However, the permittivity extraction procedures that have been developed make a far-field approximation. We mathematically show the presence of the THz near-fields to render the long wavelength spectral parts of our extracted permittivity to be wrong. Chapter 9 is where we conclude and point out the main problem that needs to be addressed in order to make the measurements presented here more accessible to others. Namely, the cost of the laser system powering the THz-TDS and how to further reduce the acquisition time.

Here a temperature stable optical fiber current sensor based on the Sagnac loop interferometer and a cavity formed from two Faraday rotation mirrors is developed and tested. To the best of the author's knowledge a cavity composed of two Faraday rotation mirrors has never been used for the measurement of alternating currents. For the first time, it is shown that the maximum Faraday rotation angle for a long, static optical fiber is input polarization insensitive. Also, linear birefringence is shown to quench this angle in long optical fiber. The polarization dynamics in an optical ground wire network, for a summer period and a fall period, are reported for the first time. The highest-speed polarization changes are attributed to the high-voltage power line, i.e., the electrical current. A novel spectral analysis polarization optical time domain reflectometry method, that uses an induced birefringent event, is shown to work in long optical fiber.

The material requirements for optoelectronic devices can vary dramatically depending on the application. Often disparate material systems need to be combined to allow for full device functionality. At the nanometer scale, this can often be challenging because of the inherent chemical and structural incompatibilities of nanofabrication. This dissertation concerns the integration of seemingly dissimilar materials into hybrid optoelectronic devices for photovoltaic, plasmonic, and photonic applications. First, we show that combining a single strip of conjugated polymer and inorganic nanowire can yield a nanoscale solar cell, and modeling of optical absorption and exciton diffusion in this device can provide insight into the efficiency of charge separation. Second, we use an on-chip nanowire light emitting diode to pump a colloidal quantum dot coupled to a silver waveguide. The resulting device is an electro-optic single plasmon source. Finally, we transfer diamond waveguides onto near-field avalanche photodiodes fabricated from GaAs. Embedded in the diamond waveguides are nitrogen vacancy color centers, and the mapping of emission from these single-photon sources is demonstrated using our on-chip detectors, eliminating the need for external photodetectors on an optical table. These studies show the promise of hybrid optoelectronic devices at the nanoscale with applications in alternative energy, optical communication, and quantum optics. / Engineering and Applied Sciences - Applied Physics

Typically a top-down approach is used in the fabrication of functional nanodevices beginning with the bulk material and imposing a two or three-dimensional structure on the material through a combination of lithography and etching. Pre-patterning of a substrate, resulting in the selective growth of a material, has potential for forming three-dimensional device structures in ways that can be more efficient and which can avoid process complexity and process induced damage. In this thesis, the low temperature (90°C) aqueous growth of complex, single crystalline zinc oxide (ZnO) three-dimensional devices through pre-patterned micron and nanometer sized molds is presented. This work focuses on the quality of the single crystalline ZnO material, the constrained growth of ZnO through various sizes and shapes of molds, and the fabrication of several device structures including pillars, rings, and photonic crystals. Due to their single crystalline nature and crystallographically smooth sidewalls, photonic devices created using this growth method have the potential to outperform traditionally fabricated structures in a range of optoelectronic applications. In addition, metal-oxide interfaces are the critical components of many electrical and optical devices, and it is rare to find epitaxial metal-oxide structures. In this work, the first demonstration of low temperature, epitaxial growth of ZnO on single crystalline gold plates is presented. The quality and structure of the ZnO on the gold plates is investigated using scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. The epitaxial growth is confirmed using electron backscatter diffraction and transmission electron microscopy. The metal-oxide interfaces fabricated have the potential to be used in a number of technologically important applications. Possible examples include creating high quality electrical contacts on high bandgap materials and improving light extraction from planar LED structures. / Engineering and Applied Sciences - Applied Physics

Charge carriers in graphene mimic two-dimensional massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range. Interband transitions in graphene give rise to pronounced optical absorption in the mid-infrared to visible spectral range, where the optical conductivity is close to a universal value $\sigma_0 = \pi e^2/2h$. Free-carrier intraband transitions, on the other hand, cause low-frequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density. These properties together suggest a rich variety of possible optoelectronic applications for graphene. In this thesis, we investigate the optoelectronic properties of graphene by measuring transient photoconductivity with optical pump-terahertz probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations are accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Our measurements also reveal the non-monotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. / Physics

Transition metal nitrides have recently garnered much interest as alternative materials for robust plasmonic device architecture including potential applications in solar absorbers, photothermal medical therapy, and heat-assisted magnetic recording. Titanium nitride (TiN) is one such potential candidate. One advantage of the transition metal nitrides is that their optical properties are tunable according to the deposition conditions. The controlled achievement of tunability, however, is also a challenge. Although the formation of TiN has been the subject of numerous previous studies, a thorough analysis of the deposition parameters necessary to form metallic TiN films optimized for plasmonic applications had not been demonstrated. Similarly, such TiN films had not been subjected to detailed optical measurements which could be used in FDTD device simulations to optimize plasmonic device designs. To be able to design, simulate and build robust and optimal device structures, in this work a systematic and thorough examination of the effect of varied substrates, temperatures, and reactive gas compositions on magnetron sputtered TiN was conducted. In addition, the effects of application of an additional substrate bias were studied. The resulting optical properties at visible to near-infrared frequencies were the focus of this thesis. The optical properties of each film were measured via spectroscopic ellipsometry with more "metallic” films demonstrating a larger negative value of the real part of the permittivity. These optical measurements were correlated with both the films’ deposition conditions and microstructural measurements including x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and transmission electron microscopy (TEM) measurements; the different deposition conditions resulted in TiN and TiOxNy films with widely tunable optical responses. By sputtering under different conditions, the value of the real part of the permittivity was tuned from small positive values, through small and moderate negative values, and finally all of the way to large negative values which are comparable to those measured in gold. It was determined that both the chemical composition as well as the film crystallinity had a significant effect on the resulting properties with the most metallic films in general exhibiting a Ti:N ratio close to 1:1, low oxygen incorporation, more N bound as TiN rather than in oxynitride form, and better crystallinity. Increased substrate temperature in general increased the metallic character while application of a substrate bias reduced crystalline order, however also reduced oxygen incorporation and allowed for deposition of metallic TiN at room temperature. The close lattice match of TiN and MgO allowed for heteroepitaxial growth on this substrate under carefully controlled conditions. Finally, to demonstrate the viability of the optimized TiN thin films for plasmonic applications, three benchmark plasmonic structures were simulated using the measured, optimized optical properties including a plasmonic grating coupler, infrared nanoantennas, and a nanopyramidal array. The devices were successfully fabricated and preliminary measurements show promise for plasmonic applications for example in solar conversion and photothermal medical therapy. / Engineering and Applied Sciences - Applied Physics

This thesis presents novel applications of nonlinear optics in laser fabrication and sources of entangled photons for quantum optics. Femtosecond direct laser writing in transparent media enables mask-less fabrication of sub-micrometer scale features with flexibility in feature shape and position in the x, y, and z-directions. Different applications in optics can be enabled by working in a variety of material platforms. We explore direct laser writing of metal structures in polymer matrices for applications in diffraction optics and modification of hydrogenated amorphous silicon (a-Si:H) for integrated optical devices. These topics explore how nonlinear optical interactions are applied to alter material properties using light. Conversely, nonlinear interactions can be used for wavelength conversion. Nonlinear interactions in nanoscale waveguides can be leveraged to produce efficient sources of entangled photons for applications in quantum optics. We explore using a novel photonic platform, titanium dioxide (TiO2), to realize third-order spontaneous parametric down-conversion (TOSPDC) for direct generation of entangled photon triplets. There is a need for new fabrication techniques that enable true 3D fabrication on the sub-micrometer scale. Diffraction optical elements have many potential applications in imaging, wavelength selection, and dispersion compensation. Multi-layer diffraction optical elements could be used to integrate imaging systems on-chip for lab-on-chip devices, such as microfluidic systems. We explore using 3D laser-written metal structures in polymer matrices for 3D gratings and diffractive elements, such as zone plates and pinholes. We demonstrate diffraction from 3D gratings and imaging using zone plates. 3D fabrication of waveguides has enabled fabrication of complex optical systems within optical fibers and bulk glasses. We explore using femtosecond laser interactions with hydrogenated amorphous silicon to introduce refractive index changes. a-Si:H could be directly integrated with CMOS devices and has the potential for much higher index contrast than bulk glasses, enabling dense, multi-layer optical devices. Efficient sources of three or more entangled photons are necessary for advances in quantum photonics. Current techniques are highly limited because they rely on cascaded second order down-conversion processes to produce entangled photon triplets and often are based in bulk optics. We leverage the high transparency, high linear refractive index, and high chi(3) nonlinearity in TiO2 to develop integrated, on-chip nano-scale waveguide sources of entangled photon triplets via TOSPDC. We present the phase-matching and nonlinear overlap conditions necessary and explore important experimental design considerations. / Engineering and Applied Sciences - Applied Physics

In the last three or so decades, optical scientists have begun to capitalize in earnest on the advances in nanofabrication that is owed to the explosive rise of miniaturized semiconductor electronics. The resulting field, nanophotonics, has opened a vast design space for applied researchers and required revisiting some of the oldest problems and assumptions of optical physics. Polarization, meaning, in the context of light, the direction of oscillation of the electromagnetic field in space, is a particularly malleable property of light that can be used to shape and direct wave fronts, to measure and control light-matter interactions, and to encode information. It remains an underexplored and underutilized feature of nature, though the new methods of nanophotonics can harness its potential to a much greater extent than any previous optical technology platform. This thesis explores some aspects of the role light's polarization plays at the interface of optics and nanotechnology. In particular, it will touch upon the way polarization may be used to control the generation of optical nearfields, how the polarization structure of evanescent waves leads to unusual optical forces, and how nanoscale polarization-transformations enable a new class of polarization-sensitive optical elements. It will also show how nanophotonics may address the problem of measuring polarization based on a new polarimeter architecture. / Engineering and Applied Sciences - Applied Physics

Abstract not available.