Browsing by Subject "Optics"

Now showing 1 - 18 of 18

Controlling infrared radiation with subwavelength metamaterials and silicon carbide
(2011-05) Neuner, Burton Hamilton; Shvets, G.; Fink, Manfred; Florin, Ernst-Ludwig; Yao, Zhen; Zhang, Xiaojing
The control and manipulation of infrared (IR) radiation beyond the capabilities of natural materials using silicon carbide (SiC), metamaterials, or a combination thereof, is presented. Control is first demonstrated using SiC, a polar crystal that exhibits a dielectric permittivity less than zero in the mid-IR range, through the excitation of tightly confined surface phonon-polaritons (SPPs), thus enabling a multitude of applications not possible with conventional dielectrics. Optimal, or critical coupling to SPPs is explored in SiC films through Otto-configuration attenuated total reflection. One practical application based on Otto-coupled SPPs is presented: IR refractive index sensing is shown for three pL-scale fluid analytes. It is then demonstrated that when two SiC films are brought to a few-micron separation, IR radiation can excite surface modes that possess phase velocities near the speed of light, a property required for efficient table-top particle accelerators. Metamaterials are engineered with subwavelength structure and possess optical properties not found in nature. Two such metamaterials will be introduced: metal films perforated with arrays of rectangular holes display the ability to control IR light polarization through spoof surface plasmon excitation, and metal/dielectric multilayers patterned with subwavelength-pitch corrugations display frequency-tunable, wide-angle, perfect IR absorption. Two experiments, which have implications in polarization control and thermal emission, combine the benefits of SiC with those of metamaterials: extraordinary optical transmission and absorption are observed in SiC hole arrays, and the design of individual SiC antennas permits the control of the bulk metamaterial responses of impedance and absorption/emission. Finally, a new optical beamline based on Fourier transform IR spectroscopy was designed, built, characterized, and implemented, serving as the major experimental objective of this dissertation. The novel beamline, which confines radiation to a 200-micron diameter and enables angle-dependent IR spectroscopy, was verified using multiple metamaterial structures.
Experimental demonstration of new optical properties in hybrid nanostructures
(2015-12) Hartsfield, Thomas Murray; Li, Elaine; Bengtson, Roger; Shih, Chih-Kang; Sitz, Greg; Wang, Zheng
In this dissertation, I present experimental investigation of the optical properties of nanoscale systems composed of both metallic and semiconductor components. Metallic nanostructures may act as resonant cavities for conduction electrons, allowing drastic electromagnetic field enhancement and the concentration of these surface plasmon field modes into tiny volumes. Semiconductor quantum dot emitters demonstrate desirable and broadly tunable optical properties due to the quantized nature of their internal electron states. When paired together, the absorption, emission, optical gain, and internal energy decay pathways of the quantum dot as well as the scattering of the cavity may be strongly modified. This work focuses on the optical properties of two such model hybrid nanostructure systems. Of the many studies of plasmonic cavities, relatively few investigate the influence of a quantum dot on the scattering of the plasmonic cavity itself. The main experimental challenge lies in the difficulty of placing an absorber or emitter at the desired position: the very virtue of the small mode volume of a plasmonic cavity demands precise spatial emitter placement. We will study the simplest plasmonic cavity, a single metal nanoparticle and a single quantum dot. We assembled a hybrid nanostructure using a nanomanipulation “nano-golfing” technique and demonstrated for the first time that the state of a single quantum dot can resonantly control the scattering of a vastly larger plasmonic cavity, manifested as a Fano resonance. A device of this design could potentially be used as a photon source capable of outputting photons of classical or quantum statistics on demand. We then turn to the optical properties of the emitter element of a hybrid nanostructure. We measured the ability of an atomically smooth Ag film to influence the optical properties of a quantum dot. This novel system has been shown to produce more uniform emitter-plasmon coupling and a greater product of excitation and radiative decay rates than possible with traditional systems relying upon rough metal films. Applications utilizing coupling between metallic films and quantum emitters could see benefit from high quality atomically smooth films as demonstrated by our studies.
Exploring Light: the Optics of Diffraction
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
Fluorinated Polymethacrylates as Highly Sensitive Non-chemically Amplified E-beam Resists
(2009-04) Strahan, Jeff R.; Adams, Jacob R.; Jen, Wei-Lun; Vanleenhove, Anja; Neikirk, Colin C.; Rochelle, Timothy; Gronheid, Roel; Willson, C. Grant; Strahan, Jeff R.; Adams, Jacob R.; Jen, Wei-Lun; Neikirk, Colin C.; Rochelle, Timothy; Willson, C. Grant
In an effort to improve upon the sensitivity of commercial non-chemically amplified e-beam resists, four polyacrylates functionalized with alpha-CF3 and/or CH2CF3 alkoxy substituents were studied. The alpha-CF3 substituent is known to increase backbone-scission efficiency while simultaneously eliminating acidic out-gassing and cross-linking known to occur in alpha-halogen substituted polyacrylates. Contrast curves for the polymeric alpha-CF3 acrylates, generated through e-beam exposure, showed the resists required an order of magnitude less dose than the current industry-standards, PMMA and ZEP. The fundamental sensitivity of these materials to backbone scissioning was determined via Co-60 gamma-ray irradiation. The chain scissioning, G(s), and cross-linking, G(x), values calculated from the resulting change in molecular weight demonstrated that all fluorinated resists possess higher G(s) values than either PMMA or ZEP and have no detectable G(x) values. Utilizing e-beam and EUV interference lithographies, the photospeed of PMTFMA was found to be 2.8x and 4.0x faster, respectively, than PMMA.
An investigation of the physical parameters of young stellar objects
(2011-12) Deen, Casey Patrick; Jaffe, D. T.; Lacy, John; Sneden, Chris; Scalo, John; Johns-Krull, Christopher; Evans, Neal J.
Studies of the temporal evolution of young stars and their associated properties rely upon the ability of astronomers to determine ages and masses of objects in different evolutionary states. The best method for determining the age and mass of a young stellar object is to place the object on the Hertzsprung-Russell (HR) diagram and to compare to theoretical evolutionary tracks. Accurate ages allow the investigation of the temporal evolution of properties associated with stellar youth (accretion rates, X-ray activity, circumstellar excess, etc...). One property intimately linked with stellar youth is the presence (or absence) of an optically thick primordial circumstellar disk. Objects in "young" star forming regions are more likely to show evidence for a disk than objects in "older" clusters. Within a single cluster, the picture is not as clear. There exist objects in very young clusters (~1 Myr) which show no evidence for circumstellar disks, and there exist objects in very old clusters (~10 Myr), which show evidence for robust disks, suggesting a variable other than stellar age is driving the evolution of the disks. To investigate whether these outliers are due to age spreads, initial conditions, or simply appear anomalous due to erroneous age determinations, we must determine better placements in the HR diagram by carefully transforming observable quantities (spectral type and apparent magnitude) into the quantities necessary for comparison evolutionary models (effective temperature and luminosity). In the Ophiuchus star forming region, I investigate whether or not objects with disks are younger than disk-less objects. I find no difference in the ages of the two populations, but the systematic and random uncertainties are large enough to mask all but the largest age differences. In the hope of better determining the physical parameters of young stellar objects, I embark on a spectral synthesis campaign to produce comparison synthetic spectra which account for the effects of magnetic fields. This requires the modification of the MOOG spectral synthesis program to handle the full Stokes vector treatment for polarized radiation through a magnetized medium. I create a grid of synthetic spectra covering ranges in effective temperature, surface gravity, and average magnetic field strength relevant for studies of young stellar objects, and develop a Chi-squared minimization routine to determine the best fit synthetic spectrum for a given observed spectrum at an arbitrary resolving power. This grid of synthetic spectra will be an invaluable complement to future near infrared, large band-pass, high-resolving power spectrographs (i.e. IGRINS). In addition to these observational and theoretical attempts to reduce systematic errors, I also helped to develop a suite of silicon and KRS-5 grisms for use in the FORCAST instrument, a mid infrared camera on the SOFIA telescope. These grisms will afford the imaging instrument a mid infrared spectroscopic capability at wavelengths normally inaccessible from the ground. I also report on my work to help write FG Widget, the quick-look reduction software package developed to support grism observations.
Machine-learning assisted scatterometry metrology on nanosheet transistors
(2024-05) Wang, James Yuanfang; Djurdjanovic, Dragan
This research presents an innovative approach in scatterometry metrology for accurately determining the critical dimensions of GAA nanosheet transistor structures, a key aspect in advancing nanoelectronics. We employed Finite-Difference Time-Domain (FDTD) simulation to construct a comprehensive library that correlates diverse nanostructure dimensions with their specific reflectance spectra across visible light wavelengths. This library serves as the foundation for our analytical model. To address the inverse problem of optics, a 4th order polynomial regression with L2 ridge regularization was applied to characterize transistor dimensions from light spectra. This approach allowed for the efficient decoding of the reflectance spectra to extract precise dimensional information of the nanosheet transistors. The results showcase high accuracy in measuring all critical dimensions, indicating the method's effectiveness and potential for broad application in nanotechnology fabrication. In addition, a simulated Gage R&R study was developed to demonstrate the preciseness of this model as a measuring tool. The methodology discussed in the paper is shown to be a promising part of high throughput manufacturing for future microelectronic designs.
Micro/nano fabrication of polymeric materials by DMD-based micro-stereolithography and photothermal imprinting
(2006) Lu, Yi; Chen, Shaochen
The revolutionary advancement in semiconductor device manufacturing promoted micro/nano fabrication technologies viable for research and applications in broader fields such as biology and optics. This dissertation is aimed at developing parallel fabrication technologies for polymeric micro/nano structures that can potentially be used in biomedical or optical devices. The objective of the dissertation is three told: a) develop and characterize a digital micro-mirror device (DMD)-based micro-stereolithographic system and explore the fabrication of hydrogel tissue engineering scaffolds, b) use the micro-stereolithographic system to fabricate microlens arrays, c) develop a photothermal imprinting technique to pattern nanostructures on the surface of polymer composites. In the first part of the dissertation, we demonstrated a simple and fast, layer-by-layer micro-stereolithographic system based on DMD dynamic photomask that allows fabrication of complex internal features along the precise spatial distribution of biological factors inside a single scaffold. Photo-crosslinkable poly(ethylene glycol) diacrylate and diamethacrylate were used as the scaffold material. In situ encapsulation of fluorescently-labeled micro-particles and cells was demonstrated. We investigated the photopolymerization process and its effects on the properties of the scaffolds. This technique could provide a powerful tool in studying progenitor cell behavior and differentiation under biomimetic, three-dimensional (3D) culture conditions. In the second part, we developed a novel fabrication technique for microlens arrays using a modified DMD-based micro-stereolithographic system. The DMD can generate high resolution images with quasi-continuous intensity gradient, thanks to its high density mirror elements with a bandwidth of 10 KHz. The projected UV patterns were simply drawn in a computer software. Topographic patterns were created in photocurable resin by spatially controlling the curing depth. Spherical microlens arrays were fabricated and their optical performance was characterized. This technique is capable of fabricating optical elements with any surface topography. In the third part, we discussed the photo-induced radical polymerization. A numerical model was established to correlate the geometry of the resulting gels and system parameters. In the fourth part, we reported a laser-assisted photothermal imprinting method for directly patterning carbon nanofiber reinforced polyethylene nanocomposite. A single laser pulse was used to melt/soften a thin skin layer of polymer nanocomposite. Meanwhile, high resolution patterns were transferred from a quartz mold to the surface of the composite.
Polymer-based integrated photonic devices for interconnects
(2018-06-19) Pan, Zeyu; Chen, Ray T.; Pan, Zhigang; Ho, Paul S.; Wang, Yaguo; Tao, Hu
Integrated photonic devices based on optical waveguides have been extensively studied for various applications, especially the high-speed intra- and inter-chip interconnects. Usually, a waveguide contains a core with high refractive index and cladding with lower refractive index. Among various waveguides, silicon, polymer, and silicon-polymer hybrid devices are the most promising candidates for low cost, small size, light weight, and low power consumption (CSWaP) optical interconnect. Firstly, silicon-based optical devices can be fabricated using CMOS compatible nanofabrication technology, which is already widely used to manufacture integrated circuits. Silicon photonic devices can have very small footprint and enable high density photonic circuits, due to high refractive index contrast. However, one of the intrinsic obstacles is the absence of χ⁽²⁾-nonlinearity in unstrained silicon due to its centrosymmetric crystal structure, making modulating photons on silicon platform a great challenge. Secondly, polymer-based devices have been found very attractive, owing to the advantages of high thermo-optic (TO) or electro-optic (EO) coefficient, high transparency in the telecommunication wavelength windows, and fabrication feasibility over large areas on printed circuit board (PCB) or other kinds of substrates. The roll-to-roll (R2R) compatible imprinting and ink-jet printing for developing polymer-based devices on flexible or rigid substrates enable large-area, light-weight, low-cost optical interconnects. However, due to the low refractive index contrast, the polymer photonic devices always require large footprint. Finally, the silicon-organic hybrid (SOH) platform enables the marriage of the best of these two materials and thus has been receiving substantial attention. In this dissertation, integrated photonic devices based on silicon, polymer, or hybrid platform will be presented. First, high-efficiency quasi-vertical tapers for polymer waveguide based inter-board optical interconnects will be demonstrated. A triangular-shape tapered structure is adopted above the waveguide core to transform a fiber mode into a single mode polymer rib waveguide mode as an optical mode transformer. A coupling loss of 1.79±0.30 dB and 2.23±0.31 dB per coupler for the quasi-TM and quasi-TE mode respectively have been experimentally demonstrated, across the C and L bands (1535 nm – 1610 nm). Then, a reconfigurable thermo-optic polymer switch based true-time-delay network will be analyzed and demonstrated. Thirdly, I will show a novel subwavelength-grating waveguide ring resonator based high-speed modulators, which is the largest bandwidth and the most compact footprint that has been demonstrated for the ring resonators on the silicon-organic hybrid (SOH) platform. Finally, the on-chip time-division multiplexing and de-multiplexing system will be designed and analyzed.
Precise blaze angle measurements of lithographically fabricated Silicon immersion gratings and the design of simple prototype instruments for grating deployment
(2022-09-12) Lubar, Emily; Jaffe, D. T.; Mace, Gregory N.
Silicon immersion gratings and grisms enable compact, high-throughput, near-infrared spectrographs. These instruments are used in ground-based efforts to characterize stellar and exoplanet atmospheres and in space-based observatories. Our grating fabrication technique uses x-ray crystallography to orient the crystal structure of silicon, followed by a specialized lithography and wet chemical etching to produce a blazed grating. The etching process takes advantage of the Silicon crystal structure and relative difference in etch rates between the [100] and [111] crystal planes. This allows us to produce parts that have phase uniformities of <λ/4 at the operating wavelengths of each grating (J- through M-band). Previous measurements indicate that chemical etching may yield a final etched blaze that slightly differs from the orientation of the [111] plane. The presence of a discrepancy between expected blaze and actual blaze changes the optical performance of the grating and therefore jeopardizes instrument performance. Understanding the magnitude of the discrepancy is the first step toward controlling the process that produces it, so measuring the discrepancy resulting from fabrication is paramount. Knowing the fabricated blaze very precisely is especially critical for silicon gratings operating in immersion because discrepancies are inflated by Snell’s law at the interface between silicon and air. I overview the measurement method I developed and report on measurement results for the blaze of our in-house fabricated GMTNIRS silicon immersion gratings to ~0.05° precision. In addition to my work on blaze characterization, I have developed an instrument concept with science goals related to stellar characterization. Star spots introduce uncertainty in derived ages of young active stars. In recent years, stellar variability of exoplanet host stars has proven to be a bottleneck for progress in atmospheric characterization. JWST was successfully launched in December 2021 and will improve our ability to study exoplanetary atmospheres even in the presence of stellar variability, but is in high demand and not optimized for this science case alone. I propose an inexpensive, compact, and simple instrument concept to spectroscopically and photometrically observe and characterize star spots. This is a ground-based instrument concept with high resolving powers R~10,000 in the infrared H-band. Early instrument design is always motivated by science goals, and the process of converging on final instrument specifications is inherently iterative. Deen et al. 2017 outline a streamlined process of choosing the various specifications, significantly reducing the number of iterations required to converge on a preliminary set of parameters. I showcase this method in my design process.
Scattering engineering at the extreme with metamaterials, metasurfaces and nanostructures
(2016-12-15) Monticone, Francesco; Alù, Andrea; Li, Xiaoqin; Bank, Seth; Wang, Zheng; Hall, Neal
The interaction of waves with materials is at the basis of a large variety of phenomena of scientific and technological interest. Interestingly, with the advent of metamaterials in the past decade and half, several anomalous wave-matter interactions have been demonstrated, which are not commonly observed in nature. Metamaterials have indeed opened a new landscape of material properties, which can now be designed by suitably engineering subwavelength meta-atoms and meta-molecules. This has enabled new possibilities for manipulating the scattering and propagation of various types of waves (electromagnetic, acoustic, elastic, etc.) in different frequency regimes, challenging well-established limits for wave-matter interactions. Within this broad context, in this work I discuss to what extent we can engineer the scattering response of individual polarizable bodies, and collections of them, using metamaterials and plasmonic nanomaterials, enabling extreme scattering effects, such as invisibility, resonant superscattering and light trapping. New functionalities and emergent phenomena can then be achieved when multiple subwavelength meta-atoms are assembled in small clusters. In particular, I will demonstrate the possibility of systematically modularizing the optical response at the nanoscale by taking inspiration from circuit and antenna theory, realizing optical lumped nanocircuits and nanofilters, as well as meta-molecules that support giant optical magnetism. Furthermore, by suitably arranging meta-atoms in large thin arrays, or metasurfaces, one can achieve a new degree of control of wave propagation and scattering/radiation over a surface, which may lead to a new paradigm of flatland optics. In this context, I will present our work on the design of passive and active metasurfaces for a variety of applications, from wavefront manipulation and cloaking, to wave-based analog computing and volumetric imaging. In this work, particular emphasis will also be placed on the derivation of fundamental bounds on the wave interaction with passive scattering systems, of large importance to assess the practical applicability of metamaterial/metasurface devices, such as invisibility cloaks, in different scenarios. These advances may pave the way to a new generation of electromagnetic and optical devices that are smaller, thinner, faster, and more energy efficient, even by orders of magnitude compared to today’s available technology.
Single-stage large-angle beam steering optical phased array on silicon nanomembrane
(2010-05) Kwong, David Nien; Chen, Ray T.; Bank, Seth R.
In this paper, we present the results of the design and fabrication of a 12 channel nano-membrane-based optical phased array that allows for large angle beam steering operating at wavelength=1.55µm. Our device is fabricated on silicon-on-insulator using standard CMOS process. By implementing unequally spaced waveguide array elements, we can relax the half-wavelength spacing requirement for large angle beam steering, thereby avoiding the optical coupling between adjacent waveguides and reducing the side-lobe-level of the array radiation pattern. 1D beam steering of tranverse-electric polarized single mode light is designed to be achieved thermo-optically through the use of thin film metal phase shifters.
Study of applications of second harmonic generation
(2011-05) Prem, Adrienne Marie; Downer, Michael Coffin; Sitz, Greg O.
Two applications of second harmonic generation (SHG), a nonlinear optical technique, are studied. First, Fresnel factors are used with a bond model to describe SHG from vicinal silicon at five incidence angles: 7.5°, 22°, 30°, 45°, and 52°. Second, a prototype apparatus for applying SHG to enhance imaging capabilities of optical coherence tomography, a microscopy technique used in many biological fields, is briefly described.
Subwavelength and nonreciprocal optical and electromagnetic systems for sensing and communications
(2017-06-07) Williamson, Ian Alexander Durant; Wang, Zheng, Ph. D.; Alù, Andrea; Bank, Seth R; Wang, Yaguo; Yu, Edward T
This dissertation is organized into three parts. First, the design for a radio frequency fiber transmission line built out of a grid of micrometer-scale conductors embedded in an insulating polymer cladding is presented to mitigate the skin and proximity effects. By adopting a checkerboard out-of-phase current phasing scheme, the internal inductance of the line is significantly lower than in two-conductor lines and results in an LC bandwidth of approximately 2 GHz, with flat attenuation and linear phase dispersion. The device performance is characterized in terms of its geometric degrees of freedom and a fabricated prototype is presented. Second, the kinetic inductive and plasmonic response of monolayer graphene in the terahertz spectrum is examined in the context of several important applications. The dispersive responses of two-dimensional graphene and three-dimensional copper transmission lines are compared to map the dispersive signaling performance in terms of transmission line cross-sectional size. This demonstrates a surprisingly broadband LC response with flat attenuation in nano-scale lines. This kinetic inductive response of graphene is demonstrated to miniaturize the photonic band structure of a photonic crystal slab where an in-plane periodicity of 300 nm has its photonic band gap in the terahertz spectrum. The sub-diffraction photonic band structure resembles that of the two-dimensional photonic crystal, supporting a wide photonic band gap in extremely thin slabs. The viability of graphene for cavity optomechanics is analyzed from near infrared to terahertz wavelengths, demonstrating a large optomechanical coupling, on the order of 3D optomechanical materials. Third, a class of nonreciprocal devices is proposed based on coupling to the sideband states, called Floquet resonances, that arise in temporally modulated optical resonators. The degrees of freedom in the modulating waveform tailor the energy exchange and phase of the Floquet resonances to realize unique nonreciprocal spectral responses in compact devices. We examine optical scattering from Floquet resonators coupled to narrowband waveguides using temporal coupled-mode theory. A three-port circulator is built out of a cascade of Floquet resonators to demonstrate broadband forward transmission and ideal isolation for dual-carrier waves. Full-wave numerical simulations in the coupled frequency domain demonstrate the circulator in an on-chip photonic crystal platform
Temperature dependent refractive index of lipid tissue by optical coherence tomography imaging
(2011-05) Lim, Hyunji; Milner, Thomas E.; Tunnell, James
Temperature dependent optical properties of lipid tissue verify critical information of tissue dynamics which can be applied to tissue treatment and diagnosis of various pathological features. Current methods of treating lipid rich tissues via heating are associated with post operation complications. Recent studies shows potential of lipid rich tissue removal by cooling. For monitoring cooling procedure and physical and chemical changes in lipid tissue, temperature dependent optical properties in subzero cooling need to be verified. This study designed heat transfer system estimating heat flux by cooling and programmed codes for image and data processing to obtain refractive indices of rodent subcutaneous lipid tissue. Phase transition of lipid tissue was observed and finally verified temperature dependent refractive index coefficient of lipid tissue from 24°C to -10°C.
Terawatt Raman laser system for two-color laser plasma interactions
(2014-08) Sanders, James Christopher; Downer, Michael Coffin
In some high-field laser-plasma experiments, it is advantageous to accompany the main high-energy (~1 J) laser with a second high-energy pulse (~0.1 J) which has been frequency-shifted by ~10-20%. Such a pulse-pair would have a low walk-off velocity while remaining spectrally distinct for use in two-color pump-probe experiments. Moreover, by shifting the second pulse by ~plasma frequency, it is theoretically possible to exercise some amount of control over a variety of laser-plasma instabilities, including forward Raman scattering, electromagnetic cascading, and relativistic self-focusing. Alternatively, the two pulses may be counter-propagated so that the collide in the plasma and create a slowly-propagating beatwave which can be used to inject electrons into a laser wakefield accelerator. The design, characeterization, and performance of a hybrid chirped-pulse Raman amplifier (CPRA)/Ti-Sapphire amplifier are reported and discussed. This hybrid system allows for the generation of a high-energy (>200 mJ), broadband (15-20 nm bandwidth FWHM), short duration (>100 fs duration) laser sideband. When amplified and compressed, the Raman beam's power exceeds 1 TW. This sideband is combined with the primary laser system to create a bi-color terawatt laser system which is capable of performing two-color high-field experiments. This two-color capability can be added to any commercial terawatt laser system without compromising the energy, duration or beam quality of the primary system. Preliminary two-color laser-plasma experiments are also discussed.
The origins of strong Pockels responses
(2019-12) Hamze, Ali Kassem; Demkov, Alexander A.; Chelikowsky, James R; Ekerdt, John G; Niu, Qian; Downer, Michael C
The linear electro-optic (Pockels) effect, which relates the change in the index of refraction of a crystal to an applied electric field, has been subject to increasing study in recent years due to its potential applications in integrated photonics, which include interchip optical interconnects, neuromorphic computing, and photonic chips for quantum computing. The current “gold standard” Pockels-active material is LiNbO₃, which sees wide use as an optical modulator in the telecommunications industry. However, LiNbO₃ has a small Pockels response (~30 pm/V) and does not integrate well with Si. Therefore, finding other Pockels-active materials is of great importance for their potential use in future devices. Most current studies are focused on BaTiO₃, which has an enormous response (~1600 pm/V) in bulk, and which can be epitaxially integrated on silicon (001). However, it of great technological importance to find other strong Pockels materials and to understand the underlying physical principles which drive strong Pockels responses. In this work, we calculate the Pockels response of a wide variety of materials from first principles. We show that SrTiO₃, a centrosymmetric crystal which ordinarily cannot exhibit a Pockels response, can be made to have a strong response through epitaxial strain. The phonon modes driving the large response in SrTiO₃ are very anharmonic. Noticing that other strong Pockels materials are also strongly anharmonic, we investigate whether crystal anharmonicity in non-centrosymmetric crystals is a predictor of strong Pockels responses. We do this by through an in-depth study of LiB₃O₅, which has thermal anharmonicity an order of magnitude larger than that of BaTiO₃ or SrTiO₃. We find that crystal anharmonicity (or rather, soft phonon modes) is a necessary, but not sufficient requirement for strong Pockels responses. Large Raman susceptibilities, which we associate with strong electron-phonon interactions and large deformation potentials, are also required. Finally, we summarize unpublished calculations of the Pockels response of a variety of crystals, many of which have not been considered for the electro-optical applications, and we provide suggestions for future first-principles studies of the Pockels effect
Ultra-precise manipulation and assembly of nanoparticles using three fundamental optical forces
(2012-12) Demergis, Vassili; Florin, Ernst-Ludwig; Shubeita, George T; Fink, Manfred; Makarov, Dmitrii E; Korgel, Brian A
The invention of the laser in 1960 opened the door for a myriad of studies on the interactions between light and matter. Eventually it was shown that highly focused laser beams could be used to con fine and manipulate matter in a controlled way, and these instruments were known as optical traps. However, challenges remain as there is a delicate balance between object size, precision of control, laser power, and temperature that must be satisfied. In Part I of this dissertation, I describe the development of two optical trapping instruments which substantially extend the allowed parameter ranges. Both instruments utilize a standing wave optical field to generate strong optical gradient forces while minimizing the optical scattering forces, thus dramatically improving trapping efficiency. One instrument uses a cylinder lens to extend the trapping region into a line focus, rather than a point focus, thereby confining objects to 1D motion. By translation of the cylinder lens, lateral scattering forces can be generated to transport objects along the 1D trapping volume, and these scattering forces can be controlled independently of the optical gradient forces. The second instrument uses a collimated beam to generate wide, planar trapping regions which can con fine nanoparticles to 2D motion. In Part II, I use these instruments to provide the first quantitative measurements of the optical binding interaction between nanoparticles. I show that the optical binding force can be over 20 times stronger than the optical gradient force generated in typical optical traps, and I map out the 2D optical binding energy landscape between a pair of gold nanoparticles. I show how this ultra-strong optical binding leads to the self-assembly of multiple nanoparticles into larger contactless clusters of well de ned geometry. I nally show that these clusters have a geometry dependent coupling to the external optical field.
Ultrafast third-harmonic generation from nanostructured optical thin films and interfaces
(2006) Stoker, David Stevens; Keto, John W.
Optical harmonic generation from nanostructured thin films and interfaces was investigated experimentally and theoretically. Sample materials were large band gap optical semiconductors (AlN/GaN), rare earth oxides (NdAlO3), and noble metals (Ag). They were examined as solids, nanoparticles, and as hybrid nanocomposites. The goal of the project was to create and characterize high susceptibility, third-order (third-harmonic) materials that relied on nanostructure for an enhanced response. Laser ablation of a microparticle aerosol (LAMA) was used to produce these materials. Two routes to nanostructured materials were investigated. In the first method, a microparticle aerosol, composed of a small concentration of metal or semiconductor, and a larger amount of glass microparticles, was ablated by a focused excimer laser, and the resultant nanoparticle aerosol was supersonically deposited and sintered. In the second method, a monolayer of silver nanoparticles was deposited by LAMA, and this film was further processed by pulsed laser deposition (PLD) of either a passive glass or active matrix material. Better optical quality was found in the hybrid LAMA/PLD materials. Many optical properties were required for characterization: linear transmission and absorption spectroscopy of plasmon resonances, second-harmonic generation (SHG) for field-enhancement analysis, and fluorescence spectroscopy and fluorescence lifetime experiments provided preliminary data for third-harmonic generation studies. The third-harmonic generation experiments were performed using an ultrafast laser system, and modeling the ultrafast dynamics of harmonic generation showed that pulse breakup occurs in the third-harmonic field. Interfaces were found to produce the harmonics, through cooperative group-velocity and phase mismatching. This uniquely ultrafast effect allowed for z-scan measurements to be simplified and for focusing effects to be eliminated. Using frequency-domain interferometry allowed for the measurement of the absolute phase of a third-harmonic pulse, and for an accurate determination of the third-order susceptibility of AlN. Finally, enhancement of second- and third- harmonic generation in PLD-coated Ag nanoparticle films was found to depend both on the material microstructure and the fundamental laser intensity.