Epsilon-near-zero materials are an emerging class of nanophotonic materials which engender electromagnetic field enhancement and small phase variation due to their approximate zero permittivity. These quasi-static fields facilitate a number of unique optical properties such as supercoupling, subwavelength confinement, and enhanced light-matter interactions, which has made epsilon-near-zero media a rapidly expanding field of optical physics. Contemporary methods of realizing a system with zero permittivity rely on microwave cavities/waveguides or complex metal-dielectric metamaterials; however, both techniques require advanced fabrication and their operational wavelength is fixed relative to their geometric and optical parameters. It remains an open and substantial challenge to realize an epsilon-near-zero material at pertinent wavelengths, particularly near- and mid-infrared, with tunable/dynamic properties. The focus of this thesis is the exploration of transparent conducting oxides for the development of epsilon-near-zero nanophotonic phenomena and applications. Transparent conducting oxides have an inherent low permittivity, in addition to simple fabrication and tunable optical properties, making them exceptionally promising. Application of transparent conducting oxide films for highly confined modes, nonlinear/ultrafast optics, and strongly coupled systems are discussed.

The past decades have witnessed the development of new materials with large nonlinear optical properties, which have made them attractive candidats for a broad spectrum of breakthrough applications in the electro-optic and photonic fields (e.g., telecommunication and computing). A deeper understanding of the relationship between, on the one hand, the chemical structure and, on the other hand, the electronic and (linear and nonlinear) optical properties has proven useful for the rational design of new efficient materials. Reaching such an understanding has attracted major interest in the scientific community worldwide in both academia and industry. Therefore, the development of new efficient NLO chromophores and materials along with commercial devices of high quality is helped via the establishment of multidisciplinary research teams combining: (i) the theoretical modeling using quantum-chemical computational calculations; (ii) the organic synthesis; (iii) the optical characterization; and (iv) the device fabrication. In this dissertation, quantum-chemistry is used to evaluate the second- and third-order NLO properties of series of new chromophores and take advantage of a feedback loop with the experimental team to understand the structure-property relationships.

The author explores absorptive and refractive optical nonlinearities at 1.06 [mu]m in bulk, semi-insulating, undoped GaAs with a particular emphasis on the influence of the native deep-level defect known as EL2. Picosecond pump-probe experimental technique is used to study the speed, magnitude, and origin of the absorptive and refractive optical nonlinearities and to characterize the dynamics of the optical excitation of EL2 in three distinctly different undoped, semi-insulating GaAs samples. Intense optical excitation of these materials leads to the redistribution of charge among the EL2 states resulting in an absorptive nonlinearity due to different cross sections for electron and hole generation through this level. This absorptive nonlinearity is used in conjunction with the linear optical properties of the material and independent information regarding the EL2 concentration to extract the cross section ratio [sigma][sub p]/[sigma][sub e] [approx equal]0.8, where [sigma][sub p](e) is the absorption cross section for hole (electron) generation from EL2[sup +] (EL2[sup 0]). The picosecond pump-probe technique can be used to determine that EL2/EL2[sup +]density ratio in an arbitrary undoped, semi-insulating GaAs sample. The author describes the use of complementary picosecond pump-probe techniques that are designed to isolate and quantify cumulative and instantaneous absorptive and refractive nonlinear processes. Numerical simulations of the measurements are achieved by solving Maxwell equations with the material equations in a self-consistent manner. The numerical analysis together with the experimental data allows extraction of a set of macroscopic nonlinear optical parameters in undoped GaAs. The nonlinearities in this material have been used to construct three proof-of-principle nonlinear optical devices for use at 1.06 [mu]m: (1) a weak beam amplifier, (2) a polarization rotation optical switch, and (3) optical limiters.

Generally, nonlinear optics studies investigate optically-induced changes in refraction or absorption, and their application to spectroscopy or device fabrication. The photorefractive effect is a nonlinear optical effect that occurs in solids, where transport of an optically-induced free-carrier population results in an internal space-charge field, which produces an index change via the linear electrooptic effect. The photorefractive effect has been widely studied for a variety of materials and device applications, mainly because it allows large index changes to be generated with laser beams having only a few milliwatts of average power.Compound semiconductors are important photorefractive materials because they offer a near-infrared optical response, and because their carrier transport properties allow the index change to be generated quickly and efficiently. While many researchers have attempted to measure the fundamental temporal dynamics of the photorefractive effect in semiconductors using continuous-wave, nanosecond- and picosecond-pulsed laser beams, these investigations have been unsuccessful. However, studies with this goal are of clear relevance because they provide information about the fundamental physical processes that produce this effect, as well as the material's speed and efficiency limitations for device applications.In this dissertation, for the first time, we time-resolve the temporal dynamics of the photorefractive nonlinearities in two zincblende semiconductors, semi-insulating GaAs and undoped CdTe. While CdTe offers a lattice-match to the infrared material HgxCd1-xTe, semi-insulating GaAs has been widely used in optoelectronic and high-speed electronic applications. We use a novel transient-grating experimental method that allows picosecond temporal resolution and high sensitivity. Our results provide a clear and detailed picture of the picosecond photorefractive response of both materials, showing nonlinearities due to hot-carrier transport and the Dember space-charge field, and a long-lived nonlinearity that is due to the EL2 midgap species in GaAs. We numerically model our experimental results using a general set of equations that describe nonlinear diffraction and carrier transport, and obtain excellent agreement with the experimental results in both materials, for a wide variety of experimental conditions.

<p dir="ltr">Some of the simplest molecules that are found in abundance in nature, like oxygen, nitrogen, carbon dioxide and water can be playgrounds for complex quantum mechanical phenomenon. Although we can calculate their static properties, like binding energies, equilibrium geometries and ionization/decay rates with extraordinary precision, their dynamics offer new avenues for exploration. Although analytical techniques have been successfully applied in studying single-particle and many-particle systems, few-particle systems like simple molecules are still best understood through a combination of numerical calculations and experimental work. However, the small size of these molecules endows them with dynamics that occur on timescales of a few picoseconds to a few attoseconds, making their experimental study challenging. The overarching goal of this work is the study of such ‘ultrafast’ dynamics in excited state molecules/atoms, by developing and demonstrating novel optical probes of quantum dynamics.</p><p dir="ltr">One way to probe ultrafast dynamics in molecules is by measuring their nonlinear optical response. Such a measurement can potentially track the evolution of the symmetries of excited molecules, shedding light on their transient dynamics. We start chapter 1 with a brief discussion of the formalism behind nonlinear optical spectroscopy. Direct measurement of ultrafast (and ultraweak) optical pulses is discussed as a useful probe of nonlinear processes. After presenting preliminary results on direct electric field reconstruction, experimental work on measuring emitted nonlinear electric fields from impulsively aligned molecules is discussed. In such an experiment, however, contributions from both aligned and unaligned molecules are present, and new experimental capabilities had to be developed to disentangle and measure the ultraweak signal from aligned molecules. Following a detailed discussion of the developed measurement capabilities, results from experiments done on aligned carbon dioxide and nitrogen molecules are discussed.</p><p dir="ltr">Unlike solids, where electronic states can be excited with visible/UV light, binding energies in isolated atoms/molecules are on the order of electron-volts (eVs), and they need vacuum-ultraviolet (VUV) extreme-ultraviolet (EUV) light to excite electronically. Polyatomic molecules, like ethylene, when excited to an electronic state with VUV light, often relax back to the ground state by redistributing energy to their internal degrees of freedom non-adiabatically. These relaxation pathways are important in many chemical and biological systems, and control the yield of chemical reactions ranging from elementary reactions involving few atoms to large biomolecules such as DNA and proteins. For instance, in the photochemical reaction of the protein Rhodopsin, considered to be the primary event in human vision. In chapter 2 we discuss progress made towards extending nonlinear response measurements to study ultrafast dynamics in electronically excited molecules, using a high-harmonic VUV source. Details about the design of the high-harmonic generation beamline, and preliminary experimental data are presented. In chapter 3 we discuss preliminary theoretical work on the development of an EUV entangled-photon source, using two-photon emission from the metastable 2s state in neutral Helium. Such a source, if demonstrated, can possibly even extended to the zeptosecond regime in the future.</p>

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses. By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated. High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam. Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration. The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields. / QC 20100831

This dissertation investigates two long standing issues in nonlinear optics: complete characterization of the ultrafast dynamics of simple molecules, and the potential of a two-photon laser using a bulk semiconductor gain medium. Within the Born-Oppenheimer approximation, nonlinear refraction in molecular liquids and gases can arise from both bound-electronic and nuclear origins. Knowledge of the magnitudes, temporal dynamics, polarization and spectral dependences of each of these mechanisms is important for many applications including filamentation, white-light continuum generation, all-optical switching, and nonlinear spectroscopy. In this work the nonlinear dynamics of molecules are investigated in both liquid and gas phase with the recently developed beam deflection technique which measures nonlinear refraction directly in the time domain. Thanks to the utility of the beam deflection technique we are able to completely determine the third-order response function of one of the most important molecular liquids in nonlinear optics, carbon disulfide. This allows the prediction of essentially any nonlinear refraction or two-photon absorption experiment on CS2. Measurements conducted on air (N2 and O2) and gaseous CS2 reveal coherent rotational revivals in the degree of alignment of the ensemble at a period that depends on its moment of inertia. This allows measurement of the rotational and centrifugal distortion constants of the isolated molecules. Additionally, the rotational contribution to the beam deflection measurement can be eliminated thanks to the particular polarization dependence of the mechanism. At a specific polarization, the dominant remaining contribution is due to the bound-electrons. Thus both the bound-electronic nonlinear refractive index of air, and second hyperpolarizability of isolated CS2 molecules, are measured directly. The later agrees well with liquid CS2 measurements, where local field effects are significant. The second major portion of this dissertation addresses the possibility of using bulk semiconductors as a two-photon gain medium. A two-photon laser has been a goal of nonlinear optics since shortly after the original laser*s development. In this case, two-photons are emitted from a single electronic transition rather than only one. This processes is known as two-photon gain (2PG). Semiconductors have large two-photon absorption coefficients, which are enhanced by ~2 orders of magnitude when using photons of very different energies, e.g., ћωa≈10ћωb. This enhancement should translate into large 2PG coefficients as well, given the inverse relationship between absorption and gain. Here, we experimentally demonstrate both degenerate and nondegenerate 2PG in optically excited bulk GaAs via pump-probe experiments. This constitutes, to my knowledge, the first report of nondegenerate two-photon gain. Competition between 2PG and competing processes, namely intervalence band and nondegenerate three-photon absorption (ND-3PA), in both cases are theoretically analyzed. Experimental measurements of ND-3PA agree with this analysis and show that it is enhanced much more than ND-2PG. It is found for both degenerate and nondegenerate photon pairs that the losses dominate the two-photon gain, preventing the possibility of a two-photon semiconductor laser.

Developing a greater understanding of multiferroic materials, particularly those in which a strong coupling is exhibited between magnetic and electrical orderings, is of great importance if potential applications are to be realised. This thesis reports new experimental findings on several multiferroics using the techniques of X-ray and neutron diffraction together with nonlinear optical experiments. Spherical neutron polarimetry measurements on RbFe(MoO<sub<4</sub>)₂ show how this system's chiral magnetic structure can be controlled by an external electric field. Consideration is given to the axial distortion that the crystal structure makes, and the effect that this has on the stabilised magnetic structures. A ferroaxial coupling is invoked to explain, from a symmetry point of view, the spin driven multiferroicity in this proper screw system. The charge ordering in YbFe₂O₄ is examined by a detailed imaging of reciprocal space measured by elastic X-ray diffraction. Continuous helices of scattering are observed above the three-dimensional ordering transition temperature, whereas the intensity is concentrated onto separated maxima below this. The low temperature data are modelled using a simple oxygen displacement pattern, generalised to an incommensurate structure. The observed incommensurability implies that YbFe₂O₄ cannot be truly ferroelectric. The low field magnetic structures of a Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ are observed in a resonant soft X-ray diffraction study. In zero field the system is helimagnetic, and with small applied fields peaks corresponding to a new phase appear. Energy calculations are used to suggest a suitable magnetic structure for the new phase and to show how this relates to the known commensurate phases that are present in low fields. Finally, an experimental setup designed to measure second harmonic generation from non-centrosymmetric crystals is presented, along with static measurements on the multiferroic system MnWO₄. An optical pump / second harmonic probe study is then undertaken, with the result that a pump induced enhancement in the efficiency of the second harmonic generation is observed.

Thesis (MSc (Physics))--University of Stellenbosch, 2005. / The extremely high light intensities produced by lasers and the increasing use of lasers highlights the need for measures to prevent damage to materials due to exposure to high intensity laser light. In particular it necessitates the development of systems to protect optical sensors, including the human eye. In this work optical limiters were investigated as a system for protecting sensors. An optical limiter transmits ambient light, but absorbs high intensity light. This makes it ideal for protecting sensors from laser radiation, since it allows the sensor to operate unhindered at design intensities while protecting it from harmful high intensity radiation. There are various mechanisms used for optical limiting, and in this work the nonlinear absorption and the nonlinear index of refraction changes of materials were investigated. A facility was established to measure the nonlinear optical properties of a variety of materials, in order to classify them as possible optical limiters. This entailed creating a so called Zscan setup, which enabled us to measure the nonlinear absorption coefficient and the nonlinear index of refraction of a material. The theory and the design of the setup are discussed and experimental results obtained using this setup are presented. A wide variety of material types were investigated to show the versatility of the experimental setup. These included C60, which was analyzed in solution; ZnO which is a crystal; CdS quantum dots in solution; and poly(dioctyl-fluorene), which is a large polymer molecule, in solution. The materials investigated in this work were chosen based on their known strong nonlinear optical properties. Emphasis was placed on measuring the nonlinear absorption coefficients since it was the dominant optical limiting effect of the materials under investigation. The results obtained displayed the same trends as published results and it shows that the established facility was capable of measuring the nonlinear properties of these samples. The experimental limitations of the setup were determined, and critical experimental parameters were identified for measurements of this nature. Improvements to the experimental facility are suggested to improve the accuracy of future measurements.

Second harmonic generation (SHG) is a nonlinear optical effect sensitive to interfaces between materials with inversion symmetry. It is used as an effective tool for detection of the adsorption of a substance to microscopic particles, cells, liposomes, emulsions and similar structures, surface analysis and characterization of microparticles. The scattered second harmonic (SH) intensity from surfaces of suspended microparticles is characterized by its complex angular distribution dependence on the shape, size, and physical and chemical properties of the molecules making up the outer layer of the particles. In particular, the overall scattered SH intensity has been predicted to have a dramatic and nontrivial dependence on the particle size. Results are reported for aqueous suspensions of polystyrene microspheres with different dye molecules adsorbed on their surfaces. They indicate that the scattered SH power has an oscillatory dependence on the particle size. It is also shown that adsorption of one of the dyes (malachite green) on polystyrene particles is strongly affected when SDS surfactants are added to the solution. For this system a rapid increase of the SH signal with increasing concentration of SDS was observed in the range of low SDS concentration. Three different theoretical models are used to analyze the observed particle size dependence of SHG. The calculated angular and particle size dependences of the SH scattered power show that the models do not agree very well between each other when the size of the particles is of the order of the fundamental light wavelength, as here. One of the models - nonlinear Mie scattering - predicts oscillatory behaviour of the scattered SH power with the particle size, but fails to reproduce the position of the maxima and minima of the experimentally observed oscillations. The obtained results on the size dependence of the SH can be used in all applications to increase the count rate by choosing particles of the size for which the SH efficiency was found to the highest. A new effect of cooperative malachite green and SDS interaction at the polystyrene surface can be employed, for example, in the areas of microbiology or biotechnology, where adsorption macromolecules, surfactants and dyes to polystyrene microparticles is widely used.