Browsing by Subject "Optics"

Now showing 1 - 7 of 7
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    All-optical ultrasound transducers for high resolution imaging
    (2014-12) Sheaff, Clay
    High frequency ultrasound (HFUS) has increasingly been used within the past few decades to provide high resolution (< 200 µm) imaging in medical applications such as endoluminal imaging, intravascular imaging, ophthalmology, and dermatology. The optical detection and generation of HFUS using thin films offers numerous advantages over traditional piezoelectric technology. Circumvention of an electronic interface with the device head is one of the most significant given the RF noise, crosstalk, and reduced capacitance that encumbers small-scale electronic transducers. Thin film Fabry-Perot interferometers - also known as etalons - are well suited for HFUS receivers on account of their high sensitivity, wide bandwidth, and ease of fabrication. In addition, thin films can be used to generate HFUS when irradiated with optical pulses - a method referred to as Thermoelastic Ultrasound Generation (TUG). By integrating a polyimide (PI) film for TUG into an etalon receiver, we have created for the first time an all-optical ultrasound transducer that is both thermally stable and capable of forming fully sampled 2-D imaging arrays of arbitrary configuration. Here we report (1) the design and fabrication of PI-etalon transducers; (2) an evaluation of their optical and acoustic performance parameters; (3) the ability to conduct high-resolution imaging with synthetic 2-D arrays of PI-etalon elements; and (4) work towards a fiber optic PI-etalon for in vivo use. Successful development of a fiber optic imager would provide a unique field-of-view thereby exposing an abundance of prospects for minimally-invasive analysis, diagnosis, and treatment of disease.
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    Engineering metallic nanogap apertures for enhanced optical transmission
    (2016-10) Yoo, Daehan
    Physics and technology of metallic nanoapertures have been of great interest in nanophotonics. In particular, enhanced optical transmission mediated by surface plasmon waves in metallic nanoapertures has been widely studied and utilized in biochemical sensing, imaging, optical trapping, nonlinear optics, metamaterials, and optoelectronics. State-of-the-art nanotechnology enables researchers to explore optical physics in complex nanostructures. However, the high cost and tedium of conventional fabrication approaches such as photolithography, electron-beam lithography, or focused-ion-beam milling have limited the utilization of metallic nanoapertures for practical applications. This dissertation explores new approaches to enable high-throughput fabrication of sub-10-nm nanogaps and apertures in metal films. In particular, we focus on a new technique called atomic layer lithography, which turns atomic layer deposition into a lithographic patterning technique and can create ultra-small coaxial nanoapertures. The resulting nanostructures allowed us to observe extraordinary optical transmission in mid-infrared regime that originates from an intriguing physical phenomenon called the epsilon-near-zero (ENZ) condition. Subsequently, we turn this nanogap structure into a high-Q-factor plasmonic resonator, called a trench nanogap resonator, by combining a nanogap and sidewall mirrors. This structure is optimized for electrical trapping of biomolecules and concurrent optical detection, which is demonstrated experimentally via dielectrophoresis-enhanced plasmonic sensing. The fabrication technique and resulting structures demonstrated in this thesis work can facilitate practical engineering of metallic nanoapertures towards harnessing the potential of plasmonics.
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    Fire and Ice: Thermoluminescent Temperature Sensing in High-Explosive Detonations and Optical Characterization Methods for Glacier Ice Boreholes
    (2017-07) Mah, Merlin
    The environment around a detonating high explosive is incredibly energetic and dynamic, generating shock waves, turbulent mixing, chemical reactions, and temperature excursions of thousands of Kelvin. Probing this violent but short-lived phenomena requires durable sensors with fast response times. By contrast, the glacier ice sheets of Antarctica and Greenland change on geologic time scales; the accumulation and compression of snow into ice preserves samples of atmospheric gas, dust, and volcanic ash, while the crystal orientations of the ice reflect its conditions and movement over hundreds of thousands of years. Here, difficulty of characterization stems primarily from the location, scale, and depth of the ice sheet. This work describes new sensing technologies for both of these environments. Microparticles of thermoluminescent materials are proposed as high-survivability, bulk-deployable temperature sensors for applications such as assessing bioagent inactivation. A technique to reconstruct thermal history from subsequent thermoluminescence observations is described. MEMS devices were designed and fabricated to assist in non-detonation testing: large-area electrostatic membrane actuators were used to apply mechanical stress to thermoluminescent Y2O3:Tb thin film, and microheaters impose rapid temperature excursions upon particles of Mg2SiO4:Tb,Co to demonstrate predictable thermoluminescent response. Closed- and open-chamber explosive detonation tests using dosimetric LiF:Mg,Ti and two experimental thermometry materials were performed to test survivability and attempt thermal event reconstruction. Two borehole logging devices are described for optical characterization of glacier ice. For detecting and recording layers of volcanic ash in glacier ice, we developed a lightweight, compact probe which uses optical fibers and purely passive downhole components to detect single-scattered long-wavelength light. To characterize ice fabric orientation, we propose a technique which uses reflection measurements from a small, fixed set of geometries. The design and construction of a borehole logger implementing these techniques is described, and its testing discussed.
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    I'm Not Yelling
    (2024-04-06) Polikoff, Whalen
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    Light-matter interactions in optical nanostructures based on organic semiconductors
    (2012-12) Lodden, Grant H.
    The confinement of a semiconductor material to an optical microcavity leads to an inherent coupling between light and matter. Depending on the lifetime of the excited state of the semiconductor (the exciton) and the cavity photon, two distinct regimes of interaction are possible. The system is said to be weakly coupled if either the exciton or the cavity photon decay before the two species interact. Weak exciton-photon coupling results in a modification of the exciton lifetime, the spectral shape, and the angular dispersion of emission from the microcavity. Conversely, when the lifetimes of the exciton and cavity photon are long enough so that an interaction occurs prior to either state decaying, the regime of strong exciton-photon coupling is realized. The timescale for coupling is the Rabi period, which depends on exciton and cavity parameters including the exciton oscillator strength and transition linewidths. The eigenstates of the strongly coupled system are known as microcavity polaritons. Microcavity polaritons have unique properties arising from their mixed exciton-photon character, permitting the realization of novel optoelectronic devices. Organic semiconductors are attractive for application in strongly coupled systems due to their large exciton binding energy (~1 eV), which permits a robust coupled state that is stable at room temperature and under electrical excitation. In addition, organic semiconductors exhibit large exciton oscillator strengths (~1015 cm-2) resulting in a strong interaction between the cavity photon and the exciton. We aim to better the understanding of polaritons in organic semiconductor microcavities to push the field towards novel optoelectronic devices.
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    MEMS Actuators for Tuning Nanometer-scale Airgaps in Heterostructures and Optical Instrumentation for Glacier Ice Studies
    (2016-01) Chan, Wing Shan
    MEMS Actuators for Tuning Nanometer-scale Airgaps in Heterostructures We developed a new actuator microstructure to control the spacing between closely spaced surfaces. Creating and controlling nanometer gaps is of interest in areas such as plasmonics and quantum electronics. For example, energy states in quantum well heterostructures can be tuned by adjusting the physical coupling distance between wells. Unfortunately, such an application calls for active control of a nano-scale air gap between surfaces which are orders of magnitude larger, which is difficult due to stiction forces. A vertical electrostatic wedge actuator was designed to control the air gap between two closely spaced quantum wells in a collapsed cantilever structure. A six-mask fab- rication process was developed and carried out on an InGaAs/InP quantum well het- erostructure on an InP substrate. Upon actuation, the gap spacing between the surfaces was tuned over a maximum range of 55 nm from contact with an applied voltage of 60 V. Challenges in designing and fabricating the device are discussed. Optical Instrumentation for Glacier Ice Studies We explored new optical instrumentation for glacier ice studies. Glacier ice, such as that of the Greenland and Antarctic ice sheets, is formed by the accumulation of snowfall over hundreds of thousands of years. Not all snowfalls are the same. Their isotopic compositions vary according to the planet’s climate at the time, and may contain part of the past atmosphere. The physical properties and chemical content of the ice are therefore proxies of Earth’s climate history. In this work, new optical methods and instrumentation based on light scattering and polarization were developed to more efficiently study glacier ice. Field deployments in Antarctica of said instrumentation and results acquired are presented.
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    Metal structures for photonics and plasmonics
    (2013-07) Park, Jong Hyuk
    The goal of this thesis is to investigate metal structures for photonics and plasmonics and to provide theoretical and experimental bases for their practical applications. Engineered micro- and nanostructures of a metal can efficiently manipulate surface plasmon polaritons (SPPs) - coupled photon-electron waves propagating along a metal-dielectric interface. Since SPPs are able to contain both characteristics of light and charge, exploiting SPPs can lead to novel optical behaviors, for example, concentration of light below the optical diffraction limit, generating large electric-field enhancements in confined regions. This unique characteristic of SPPs has opened up new opportunities for photonic and plasmonic applications such as surface-enhanced spectroscopy, subwavelength waveguides, optical antennas, solar cells, and thermophotovoltaics. However, while many fabrication techniques have been developed and utilized to prepare metal structures, some applications would still benefit from improved methods because SPPs are extremely sensitive to inhomogeneities on a metallic surface arising from roughness, impurities and even grain boundaries of a metal. To minimize the surface inhomogeneities of the metal structures and thus to exploit SPPs effectively, we introduced novel fabrication methods. First, the template-stripping method was employed to obtain high-quality silver films for SPPs in the visible wavelengths. The template-stripped films showed very smooth surfaces, leading to the improved dielectric function with high electrical conductivity and low optical loss. The dielectric function of the template-stripped films was compared with that of conventional films. As a result, the relation between the surface roughness and dielectric function of metal films could be derived. As another approach to reduce the inhomogeneities on a metal surface, we prepared single-crystalline silver films via epitaxial growth. Under controlled deposition conditions, single-crystalline silver films exhibited ultrasmooth surfaces with a root mean square roughness of 0.2 nm. Moreover, we observed that the absence of the grain boundaries can lead to an increase in SPP propagation length as well as precise patterning for metal structures. Beyond noble metals, we then introduced an effective route to obtain smooth patterned structures of refractory metals, semiconductors, and oxides via template stripping. The smooth structures of such materials can be favorable for many applications including thermal emitters, metamaterials, solar absorbers, and photovoltaics. We demonstrated that a variety of desired materials deposited on a thin noble metal layer can be peeled from silicon templates. After removing the noble metal layer, the revealed surfaces had very small roughness. This approach could easily reproduce structures via reuse of templates, leading to a low-cost and high-throughput process in micro- and nanofabrication. Finally, we showed that thermal excitation of SPPs in patterned metallic structures can provide tailored thermal emission. Typically, SPPs on metal structures are generated by using an optical source and then re-radiated as light, of which the emission angle and wavelength are determined by the geometry of the metal structures. However, since thermal energy can be another excitation source to create SPPs, heating of properly designed metal structures can result in tailored thermal emission. We experimentally demonstrated that at high temperatures, tungsten films with bull's-eye patterns exhibit tailored thermal emission with a unidirectional and monochromatic beam. In addition, since the thermal stability of the structures could be enhanced by coating with a protective oxide layer on the metal surface, the bull's-eye structures can be utilized as a novel radiation source. Overall, we pursued efficient engineering of SPPs in metal structures and development of improved fabrication methods for the metal structures. We believe that these results will promote the practical application of SPPs for electronic, photonic and plasmonic devices.