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Stanford University Department of Applied Physics

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Works: 140 works in 140 publications in 1 language and 146 library holdings
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Publications about Stanford University
Publications by Stanford University
Most widely held works about Stanford University
 
Most widely held works by Stanford University
Bose-einstein condensation in spin dimer compounds by Suchitra E Sebastian( Archival Material )
1 edition published in 2006 in English and held by 2 libraries worldwide
Laser-tissue interactions in retinal photo-thermal therapy mechanisms and applications by Christopher Koerner Sramek( Computer File )
1 edition published in 2010 in English and held by 2 libraries worldwide
Since its introduction nearly 40 years ago, laser photocoagulation has been the standard of care for treatment of numerous retinal pathologies. Recently, new approaches have been introduced for more selective targeting of various retinal layers, including patterned scanning photocoagulation, selective retinal therapy, and sub-lethal treatment. Despite its broad use in clinical practice, there remains a need for the quantitative description of laser-tissue interactions involved in retinal phototherapy. A unified description of various treatment regimes and associated mechanisms of tissue damage would allow for optimization of laser parameters to improve selectivity and safety of retinal photocoagulation, and for avoidance of undesirable collateral damage. The presented work describes an investigation into the dynamics of the retinal response to hyperthermia and vaporization. A finite-element computational model of photocoagulation and rupture was constructed based on experimental measurements of laser interactions with tissue, and verified in vivo in the millisecond time domain. Two approaches towards improvement of tissue heating uniformity were studied: spatial and temporal modulation of the treatment beam. After optimization using the computational model, beam shaping and pulse modulation systems were constructed. Experimental studies in vivo confirmed improvements in safety of the retinal treatment, potentially allowing for reductions in treatment time, thermal damage extent, and perceived pain. In addition, tissue response to sub-lethal thermal stress in the retina was explored using expression of heat shock protein in an animal model. Computational modeling of the corresponding treatment regime demonstrated that a similar response is likely to occur in clinical application of sub-lethal exposures. Photo-mechanical interactions in the retina were investigated in model systems and in vivo with microsecond-range exposures. The dominant mechanisms of tissue damage were identified and the corresponding limits of the safe therapeutic window were computed over a broad range of pulse durations - from microseconds to seconds. An understanding of the thermal and mechanical interactions involved in laser heating of the retina allows for the realization of safer and more selective treatment regimes. All three mechanisms investigated in the current study -- photocoagulation, photomechanical interactions and sub-lethal hyperthermia -- play a role in clinical treatment. The developed quantitative models of these interactions have immediate applicability to clinical practice, providing guidance towards optimization of retinal phototherapy, evaluation of retinal safety, and development of new clinical applications
Design, fabrication and applications of mems tunable blazed gratings by Xiang Li( Archival Material )
1 edition published in 2006 in English and held by 2 libraries worldwide
Multiphoton interactions with transparent tissues applications to imaging and surgery by Ilya Toytman( Computer File )
1 edition published in 2010 in English and held by 2 libraries worldwide
Ultrafast lasers offer an advantage of highly localized interactions with transparent materials due to non-linearity of multiphoton processes with laser intensity. Biological and medical applications of these interactions can be nominally divided into two classes: (1) diagnostic imaging and spectroscopy and (2) plasma-mediated surgery. Imaging techniques, such as multiphoton fluorescence, harmonic generation, and stimulated Raman scattering typically employ relatively low power laser sources to avoid damage to the specimens. Surgical applications, on the other hand, rely on formation of plasma at the focus of high peak power laser beam. Current paradigm in applications of multiphoton interactions is based on scanning of a focused beam within the sample in order to image extended areas or produce long cuts point-by-point. We demonstrate several optical systems based on a new paradigm -- distributed multiphoton interactions, where the scanning is reduced or even not required at all. In the area of diagnostic imaging, we have developed and successfully tested a wide-field Coherent Anti-Stokes Raman Scattering (CARS) microscopy technique, which is based on simultaneous imaging of the extended area of the sample. The signal generation relies on the non-phase-matching illumination, and the image acquisition is performed from the entire illuminated area without scanning, using an array detector. We have characterized the spatial and spectral resolution of the method, and demonstrated its chemical selectivity. Optimization of the illumination geometry and proper selection of the wavelengths of the pump and Stokes beams allowed acquisition of the myelin-specific images of nerve tissue with diffraction-limited spatial resolution of 0.5[mu]m. Single-shot imaging capability has been demonstrated on a test sample of polystyrene beads. In surgical applications of ultrafast lasers the extent of the rupture zone in tissue is often determined by dynamics of cavitation bubble resulting from the optical breakdown. Typically tissue cutting is performed point-by-point using a scanning laser. We have studied the possibilities of enhancing the cutting efficiency using two methods. First approach is based on hydrodynamic interactions between two simultaneously created bubbles. A theoretical model of the flow induced by the cavitation bubbles was developed and experimentally verified. Based on experimentally measured rupture threshold strain of a material we derived the shape of the rupture zone for a given distance between the focal spots. We have found that for the threshold strain of 0.7, a continuous cut is 1.35 longer than the one produced by two bubbles applied sequentially. This ratio increases up to 1.7 if a linear series of multiple bubbles is applied simultaneously. Counter-propagating liquid jets forming during collapse of two bubbles in inviscid liquid can increase the rupture zone up to a factor of 2.5. Alternative approach to extending the cutting zone in transparent tissue is based on generation of optical breakdown in a highly elongated zone. By focusing a picosecond laser pulse with a combination of a lens and an axicon we have obtained breakdown zone with aspect ratio of 250:1. The axial intensity distribution was analyzed based on the shape of the resulting cavitation bubble, and was further confirmed by numerical evaluation of a Fresnel diffraction integral. We have optimized the incident laser beam profile to obtain uniform intensity along the breakdown region and to minimize the amount of energy deposited into the sample. We also demonstrate dielectric breakdown and associated cavitation with adjustable length and axial position controlled by modulation of the laser beam profile using an amplitude mask
Probing RNA folding through electrostatic and coarse-grained simulations by Vincent Bangping Chu( Computer File )
1 edition published in 2009 in English and held by 2 libraries worldwide
The discovery by Cech and coworkers that structured RNA molecules could catalyze specific reactions has revolutionized our understanding of RNA's role and place in the biological machinery of life. The notion of understanding RNA folding from a biophysical perspective means understanding the formation of RNA structure in terms of the basic physical forces at play. This thesis describes the the use of electrostatic and coarse grain simulations and associated experiments to investigate different features of RNA folding. Chapter 1 gives an brief introduction to RNA folding, the primary physical forces that influence its formation, and a review of recent advances in our understanding of structure formation in RNA. Chapters 2 and 3 comprise the next section of the thesis and detail advances in our understanding of electrostatic effects around nucleic acids, a topic of great importance in RNA folding. Specifically, chapter 2 presents the development of a size-modified Poisson-Boltzmann theory to help account for the effects of ionic size while chapter 3 presents a critical assessment of the Poisson-Boltzmann description of electrostatic relaxation in tethered duplex model systems. Chapter 4 highlights a general theoretical framework for understanding the combined effects of electrostatics and junction topology on RNA folding stability and specificity. The last section focuses on the use of coarse grained simulation to understand the role of junction topology in shaping the allowed conformational space of the Transactivation Response (TAR) element from the genome of the Human Immunodeficiency Virus (HIV). Though the last section is not, strictly speaking, a study of RNA folding, understanding RNA conformational motion is of critical importance to the question of structure acquisition in RNAs
Critical temperature oscillations in superconductor-ferromagnet trilayers by Leonid Litvak( Archival Material )
1 edition published in 2006 in English and held by 2 libraries worldwide
Nonreciprocal photonic crystal circuits by Zheng Wang( Archival Material )
1 edition published in 2006 in English and held by 2 libraries worldwide
Optical sensor design for advanced drag-free satellites by Graham Scott Allen( Book )
1 edition published in 2009 in English and held by 2 libraries worldwide
Applied physics at Stanford by Stanford University( serial )
in English and held by 1 library worldwide
Exciton recombination in the fullerene phase of bulk heterojunction organic solar cells by George Frederick Burkhard( Computer File )
1 edition published in 2011 in English and held by 1 library worldwide
Finding alternatives to fossil fuel energy sources is necessary to stem global warming, to provide economic and political independence, and to keep up with increasing energy demand. Because of their low cost, flexibility, and because the material resources needed to make them are abundant, organic polymer solar cells are an attractive alternative to conventional solar technology. Organic solar technology has been developing rapidly; however, with the best power conversion efficiencies at ~8%, much improvement is needed before it can be competitive with established solar technologies. Poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells are the most studied type of organic solar cell. Nevertheless, their loss mechanisms are still not fully understood. In this work, we study excitonic losses in the PCBM phase of the blend. We develop a way to accurately measure internal quantum efficiencies (IQEs) and use this technique to characterize P3HT:PCBM devices. We observe spectral dependence of the IQE and conclude that a majority of excitons generated in the PCBM are lost to Auger recombination with polarons that are trapped in that phase. We also provide evidence that this process may happen in other materials and may be a critical factor in limiting exciton diffusion in organic semiconductors
Towards high efficiency and low cost nano-structured III-V solar cells by Gu Anjia( Computer File )
1 edition published in 2011 in English and held by 1 library worldwide
State-of-the-art III-V multijunction solar cells have achieved a record efficiency of 42%, the highest solar-electric conversion efficiency achieved by any technology. This has fueled great interest in the utility sector for large-scale deployment of solar cells. However, III-V solar cells have thus far proven too expensive for widespread terrestrial applications due to the combined cost of substrates, growth processes and materials. Here, we propose a novel III-V solar cell design based on the epitaxial growth of AlGaAs/GaAs on pre-patterned low-cost substrates to provide a path to cost-effective, large-scale deployment. This approach is based on our discovery that the surface kinetics of epitaxial growth by MBE is significantly altered when growing on three dimensional nanostructures instead of planar surfaces. Based on our exploratory results, we present the device design, electrical and optical simulation, and materials growth and device fabrication and characterization of core-shell nanostructured III-V solar cells. We use both bottom-up and top-down approaches to prepare the nanostructured templates in shape of nanowires and nanopyramids. Finite-difference time-domain (FDTD) and Rigorous Coupled Wave Analysis (RCWA) simulation show that the nanostructures have enhanced absorption and much wider incident acceptance angles than their planar counterpart, and outperform planar three-layer anti-reflective coatings. We first demonstrated high quality, single crystal III-V (GaAs and AlGaAs) polar material conformally epi grown on group IV (nanostructured Ge on Si substrate) nonpolar material via MBE and MOVPE (also known as MOCVD) with largely reduced anti-phase domains. We developed complete and mature routines to fabricate a working, single crystalline III-V solar cell on a nanostructured template. The I-V characterization of the fabricated nanostructured GaAs solar cell proves the concept and shows the great potential of making high-efficiency nano-structured III-V solar cells on low-cost substrates
The large hadron collider and models of new physics by Anson Zhao Yu Hook( Computer File )
1 edition published in 2012 in English and held by 1 library worldwide
The Large Hadron Collider (LHC) is pushing the boundaries of the Standard Model of particle physics. Quantum corrections push the mass of the Higgs to be very heavy; If the hints of the Higgs boson now seen at the LHC are in fact the Higgs, the lightness of the Higgs boson must be explained. Supersymmetry provides a natural explanation for the Higgs mass. This thesis discusses two aspects in Supersymmetric theories, how to build them and how to see them
Learning networks in biological systems by Bokyung Choi( Computer File )
1 edition published in 2013 in English and held by 1 library worldwide
Learning networks from experimental evidence is an important problem to understand many biological systems. With the help of recent technological developments, such as high dimensional flow cytometry and gene expression arrays, a large amount of quality data are available for the task. Many different approaches exist, to model and learn biological networks from such data. Among those, we have developed methods for probabilistic models and differential equation models. The first part of this paper will cover our research on the Gaussian network model learning and its application to recover signal transduction networks. We develop a fast algorithm to learn Gaussian networks from data, based on a novel heuristic. We show that this algorithm can be extended to handle difficult situations such as non-Gaussian noise as well as limited number of simultaneous measurements. The performance of the algorithm against the standard alarm network is showed. Finally we apply the method to learn signal transduction networks from single cell level multi- channel flow cytometry data. We show that the method can recover networks with limited number of channels and non-Gaussian measurement noise. The second part will cover our research on the dynamical model of gene regulation. In this model, gene regulation is represented with differential equations in rational forms. We show that this type of model is able to reflect complex behaviors of gene regulation, compared to other existing models. Reconstucting the structure and parameters of such dynamical model is not triv- ial, given that limited measurability of gene expression. We develop a novel method to recover networks from measurements at so-called perturbed equilibrium. We show that this method can reconstruct gene regulatory networks with well-established ex- perimental designs. Model benchmark based on simulation data will be presented
The dynamics of translation initiation and elongation by Albert Tsai( file )
1 edition published in 2013 in English and held by 1 library worldwide
Translation is the final step that converts genetic information into proteins composed of amino acids using the ribosome-- a complex molecular machine that coordinates the kinetics, chemistry, and mechanics necessary to synthesize a polypeptide. Translation is highly dynamic with processes taking place on the microseconds to minutes timescales. Among the different techniques employed to probe the dynamics of translation, single-molecule fluorescence occupies a unique position, being able to track heterogeneous compositional and conformational changes occurring within tens of milliseconds to minutes. Leveraging several fluorescent signals, we probed the dynamics of translation initiation. Instead of a linear pathway, we observed several different pathways all leading to the formation of an elongation competent ribosome complex, with the flux through each modulated by initiation factors, tRNAs, and mRNA sequence. Next, we tracked how the aminoglycoside family of antibiotics inhibits ribosome and tRNA dynamics during peptide synthesis (elongation), concluding that their potency as bactericides stems from kinetically inhibiting elongation. Finally, we transitioned from model systems into observing elongation dynamics on a biologically functional protein stall sequence, SecM. Our results suggest that stalling the ribosome requires several precisely-timed peptide-ribosome interactions and that ribosomes stall in a distribution of locations on the SecM sequence. These results demonstrate the power of single-molecule fluorescence to monitor global translation dynamics, as well as highlighting the importance of the temporal dimension in forming a coherent and global mechanism for translation
Advanced single molecule fluorescence analysis and force spectroscopy to understand myosin motors by Jongmin Sung( file )
1 edition published in 2013 in English and held by 1 library worldwide
Myosins are actin-based molecular motor proteins that are associated with numerous cellular functions both in muscle and non-muscle cells. In the past decades, single molecule biophysics has provided many useful in-vitro techniques that enable one to understand detailed molecular mechanisms of myosin-actin interaction. I will describe our results and continuing efforts in (i) single-molecule fluorescence localization analysis with TIRF microscopy, and (ii) single-molecule force spectroscopy with optical traps to understand myosin motors. Our theoretical point-spread function combined with maximum-likelihood-estimation precisely extracts both positions and orientations of fixed fluorescent molecules simultaneously. This approach is now mature as a structural tool, and I have applied it to two examples: myosin V labeled with a fixed dye walking on actin filaments, and dsDNA with two differently colored probes doubly attached to its backbone. We experimentally show that our method is not only precise but also accurate. Familial hypertrophic cardiomyopathies (HCM) are common genetic heart diseases that are often caused by single point mutations, especially in human beta cardiac myosin II. Using an optical trap assay, we found that the human beta cardiac myosin with the R453C HCM-causing mutation produces significantly elevated intrinsic force. We further developed a new method called harmonic force spectroscopy that can extract a force-velocity curve from a single cardiac myosin molecule. We found that a strong-to-weak transition of a cardiac myosin bound to an actin filament is modulated by an external load, which can be explained by simple Arrhenius transition theory
Impact of hydrogen on the forming and switching behaviors of Pr(0.7)Ca(0.3)MnO(3) thin films for resistance change random access memory by Mihir Prakash Tendulkar( Computer File )
1 edition published in 2011 in English and held by 1 library worldwide
The continued scaling of NAND Flash memory technology is facing significant physical, electrical, and reliability challenges. Beyond the 16 nm technology node, the issues associated with these challenges may offset or even counteract the benefits of increased density. An increased appetite for high-capacity memory devices motivates the need to investigate new functional devices and materials for next-generation memory technology. One promising solution is Resistance-change Random Access Memory (RRAM), which offers the advantages of low cost, simple device structure, low power write and erase, high-speed switching, and integration into monolithic memory. Despite these advantages, some barriers must be overcome. Resistance-change films typically require "electroforming"--A one-time voltage application that induces a change in the film conductivity - before resistance switching can be accessed. Moreover, RRAM devices often display great variation, which partly arises from the lack of thorough understanding of the resistance switching mechanism. Filament formation through oxygen vacancies is typically cited as the underlying mechanism; however, the finer details remain hotly contested. Understanding these details may provide insight into overcoming the aforementioned hurdles. In this work, hydrogen contamination of RF-sputtered Pr(0.7)Ca(0.3)MnO(3) (PCMO) thin films is investigated as a reason for large device-to-device variation. Significant hydrogen is shown to enter the films during standard deposition and processing steps. Its effects on electroforming, switching, dielectric loss, and optical absorption are presented. These measurements are considered together to devise a comprehensive model for hydrogen-assisted electroforming and switching in PCMO
Search for large extra dimensions based on observations of neutron stars with the Fermi-LAT by Bijan Berenji( Computer File )
1 edition published in 2011 in English and held by 1 library worldwide
According to the Large Extra Dimensions (LED) model of Arkani-Hamed, Dimopoulos, and Dvali (ADD), in addition to the (3+1) observed space-time dimensions, there exist n gravity-only spatial dimensions. Due to the presence of the additional dimensions, the Planck scale of gravity should be brought down from 1E16 TeV to the TeV scale, near the electroweak scale, and thus solve the hierarchy problem. Based on the ADD theory, Kaluza-Klein (KK) gravitons, having masses of the order 100 MeV and lifetimes of the order of billions of years, are expected to be produced within supernova cores by nucleon-nucleon gravi-bremsstrahlung in the LED model. Once produced, they are predicted to be trapped by the gravitational potential of subsequently formed neutron stars (NS), and their decay is predicted to contribute to a measurable gamma-ray flux from NS. In this dissertation, refinements to past theoretical models are made, including modifications for the expected spectral energy distribution based on orbital motion of the gravitons, and NS surface magnetic field and age. n = 2,3 ..., 7 extra dimensions are considered. A sample of 6 gamma-ray faint NS sources not reported in the first Fermi gamma-ray source catalog that are good candidates are selected for this analysis, based on age, surface magnetic field, distance, and galactic latitude. Based on 11 months of data from Fermi -LAT, 95% CL upper limits on the size of extra dimensions R from each source are obtained, as well as 95% CL lower limits on the (n+4)-dimensional Planck scale M_D. In addition, the limits from all of the analyzed NSs have been combined statistically using two likelihood-based methods. The results indicate more stringent limits on LED than quoted previously from individual neutron star sources in gamma-rays. In addition, the results are more stringent than current collider limits, from the LHC, for n <4. If the Planck scale is around a TeV, then for n = 2,3, the compactification topology of LED must be more complicated than a torus
High harmonic generation from multiple orbitals in molecular nitrogen by Brian Keith McFarland( Computer File )
1 edition published in 2010 in English and held by 1 library worldwide
The high harmonic amplitude and phase is modulated by the electronic structure in atoms and molecules. Past studies of high harmonic generation (HHG) attribute features in the high harmonic spectrum to solely the highest occupied molecular orbital (HOMO). Molecular electrons that are energetically below the HOMO should contribute to laser-driven high harmonic generation. We present the first evidence of HHG from multiple orbitals in molecular nitrogen. We describe measurements of the amplitude and phase of the HHG from aligned nitrogen molecules. The HHG phase from aligned nitrogen is measured interferometrically by beating the nitrogen harmonics with those of an argon reference oscillator in a gas mixture. A rapid phase shift of 0.2 pi is observed in the vicinity of the HHG spectral minimum. We compare the phase measurements to a simulation of the HHG recombination step in nitrogen that is based upon a simple interference model. The results of the simulation suggest that modifications beyond the simple interference model are needed to explain HHG spectra in molecules. Possible modifications are the inclusion of the molecular potential and multiple orbital effects. The importance of multiple orbital contributions to HHG is seen in measurements of the alignment dependent HHG amplitude from aligned nitrogen molecules. Measurements of the HHG spectrum in nitrogen molecules aligned perpendicular and parallel to the laser polarization show new features that indicate the influence of the electrons that occupy the orbital just below the molecular nitrogen HOMO, referred to as the HOMO-1. Evidence of the HOMO-1 is seen in the HHG spectrum obtained at the half revival of a rotational wave packet. Such observations of lower lying orbitals are essential to understand the influence of electron motion on atomic dynamics in laser-excited molecules
Quantum imaging and spectroscopy of molecular diamondoids and topological nanostructures by Jason Christopher Randel( Computer File )
1 edition published in 2011 in English and held by 1 library worldwide
Identification and characterization of new and novel materials is one of the major challenges facing the continued advancement of digital electronics. Traditionally, the electronics industry has been dominated by silicon. However, as devices begin to approach atomic scales, state-of-the-art electronics will have to increasingly embrace new and complementary materials, some of which bear little resemblance to their silicon brethren. Molecular systems and crystals that contain novel fermionic states have the potential to rapidly and greatly impact the electronics industry, given the right advances in fabrication and performance. This thesis reports the use of scanning tunneling microscopy to explore a variety of new materials that exhibit novel--and potentially marketable--electronic properties, yet that can also be synthesized using relatively low-cost and straightforward techniques. The first material is the family of diamondoid molecules, which represent an exciting new direction in the field of nanoscale carbon. These molecules are carbon cages consisting of the smallest caged subunits of the diamond lattice, with surface bonds saturated by hydrogen. While theoretically known to be stable, diamondoids have been experimentally inaccessible due to synthesis roadblocks and lack of natural sources, until recently purified from crude oil. This advancement allows for potential access to the unique and extreme properties of diamond (rigidity, thermal conductivity, wide band-gap, and doping behavior, among others) in nanoscale and molecular devices. Of particular interest is the cross-over regime between the molecule-like behavior expected of the smaller diamondoids and the properties of macroscopic diamond. The first part of this dissertation explores the hierarchical nature of these molecules, investigated at the single-molecule level with scanning tunneling microscopy (STM). I will present structural data showing the quality of self-assembled monolayers (SAMs) composed of a series of thiolated diamondoids, and the variation that emerges as the number of diamondoid cages increases. I-V spectroscopy (combined with density functional calculations) allows us to determine the energy band line-ups of the molecular orbitals. The robustness of the SAMs and the insulating behavior implied by spectroscopy suggest that--at the few-eV energy scale typical of STM--diamondoid thiol SAMs may be useful as rigid decoupling layers, tunable by appropriate choice of cage structure. Moving beyond thiolated molecules (which have well-documented uses in the field of molecular electronics), we have begun exploring more exotic diamondoid-based derivates as novel nanoelectronic elements. Over the past few decades, new fields of research have emerged based on the sp2 molecular forms of carbon such as graphene, fullerenes, and nanotubes. Materials that sit at the intersection of the sp2 and sp3 bonding structures are an exciting new area for nanoscale science, combining the unique electronic properties of these two very different hybridizations. I will introduce hybrid molecules that fuse the sp2 and sp3 allotropes-in the form of C60 fullerenes and diamondoids-into one well-defined molecular system. These molecules were synthesized with the intention of creating diode-like elements for single- or few-molecule electronic devices. STM measurements on SAMs of these molecules indeed show evidence of unconventional rectifying behavior. These measurements represent (to our knowledge) the first purely hydrocarbon rectifier, and demonstrate the emerging diversity of electronic phenomena observed in diamondoid-based molecules. The second half of this thesis turns from molecular systems to a particular set of crystalline systems that exhibit electrons that behave as Dirac fermions. These particles are very different from the standard electrons in metals or semiconductors in that they propagate relativistically, despite being confined to a solid state crystal. STM has proven to be an indispensable tool in characterizing the signatures of such particles. These materials have a host of technological applications, so lowering the cost of synthesis is an important research direction. I therefore survey a growth techniques by using STM to look for Dirac fermions in graphene and topological insulator systems. As in the molecular systems introduced above, STM studies are important in these Dirac systems because their unique transport behavior depends critically on their nanoscale properties
Plasmonic devices employing extreme light concentration by Ragip Pala( Computer File )
1 edition published in 2010 in English and held by 1 library worldwide
The development of integrated electronic and photonic circuits has led to remarkable data processing and transport capabilities that permeate almost every facet of our daily lives. Scaling these devices to smaller and smaller dimensions has enabled faster, more power efficient and inexpensive components but has also brought about a myriad of new challenges. One very important challenge is the growing size mismatch between electronic and photonic components. To overcome this challenge, we will need to develop radically new device technologies that can facilitate information transport between nanoscale components at optical frequencies and form a bridge between the world of nano-electronic and micro-photonics. Plasmonics is an exciting new field of science and technology that aims to exploit the unique optical properties of metallic nanostructures to gain a new level of control over light-matter interactions. The use of nanometallic (plasmonic) structures may help bridge the size gap between the two technologies and enable an increased synergy between chip-scale electronics and photonics. In the first part of this dissertation we analyze the performance of a surface plasmon-polariton all-optical switch that combines the unique physical properties of small molecules and metallic (plasmonic) nanostructures. The switch consists of a pair of gratings defined on an aluminum film coated with a thin layer of photochromic (PC) molecules. The first grating couples a signal beam consisting of free space photons to SPPs that interact effectively with the PC molecules. These molecules can reversibly be switched between transparent and absorbing states using a free space optical pump. In the transparent (signal "on") state, the SPPs freely propagate through the molecular layer, and in the absorbing (signal "off") state, the SPPs are strongly attenuated. The second grating serves to decouple the SPPs back into a free space optical beam, enabling measurement of the modulated signal with a far-field detector. We confirm and quantify the switching behavior of the PC molecules by using a surface plasmon resonance spectroscopy. The quantitative experimental and theoretical analysis of the nonvolatile switching behavior guides the design of future nanoscale optically or electrically pumped optical switches. In the second part of the dissertation we provide a critical assessment of the opportunities for use of plasmonic nanostructures in thin film solar cell technology. Thin-film solar cells have attracted significant attention as they provide a viable pathway towards reduced materials and processing costs. Unfortunately, the materials quality and resulting energy conversion efficiencies of such cells is still limiting their rapid large-scale implementation. The low efficiencies are a direct result of the large mismatch between electronic and photonic length scales in these devices; the absorption depth of light in popular PV semiconductors tends to be longer than the electronic (minority carrier) diffusion length in deposited thin-film materials. As a result, charge extraction from optically thick cells is challenging due to carrier recombination in the bulk of the semiconductor. We discuss how light absorption could be improved in ultra-thin layers of active material making use of large scattering cross sections of plasmonic structures. We present a combined computational-experimental study aimed at optimizing plasmon-enhanced absorption using periodic and non-periodic metal nanostructure arrays
 
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controlled identity Stanford University. School of Humanities and Sciences

Stanford University. Dept. of Applied Physics
Stanford University. School of Humanities and Sciences. Department of Applied Physics
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English (24)
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