New mono-chelate complexes with ferrocenyl (Fc), octamethylferrocenyl (Me8Fc) and diaminophenyl (Dap) donor groups, connected through a conjugated bridge, to ZnCl2, Zn(OAc)2 or ReCl(CO)3 acceptor groups, are described. A thorough characterisation of the complexes is provided, including single crystal structures for one pro-ligand and three complexes. Visible d(FeII)→π* metal-to-ligand charge-transfer (MLCT) bands accompany π→π* intraligand charge-transfer absorptions in the UV region. TD-DFT calculations confirm the nature of these absorptions and indicate transitions at higher energies also contain some d(FeII)→π* character. Fc and Me8Fc containing chromophores display a fully reversible oxidation process when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined using hyper-Rayleigh scattering (HRS) and Stark spectroscopy. Larger β values are determined for complexes with Dap donors and ReICl(CO)3 acceptors. A family of novel fac-ReI(Lax)(CO)3(N-N) complexes, where N-N is 4,4′-dicyano-2,2′-bipyridyl (dcnbpy) or 4,4′-bis-(p-cyanophenyl)-2,2′-bipyridine (bbnbpy), with various axial ligands (Lax) are reported. The ReI complexes are useful precursors for metalation with electron-rich RuII ammine donor groups, to prepare novel tri-metallic V-shaped molecules. Single crystal X-ray structures are reported for five ReI complexes, confirming the fac geometry of the carbonyl ligands; the ReI complexes also display stretching frequencies typical of fac-ReI(Lax)(CO)3(N-N) complexes. The UV-visible spectra contain a low intensity band due to a d(ReI)→π*(bpy) transition, along with a more intense π→π* band. 1H NMR studies reveal the formation of trimetallic complexes, upon treatment of ReI complexes with a molar excess of [RuII(NH3)5(H2O)][PF6]2.The synthesis of octupolar heptametallic complexes containing RuII ammine donor groups has been investigated. The ligand 4,4′-bis-[(E)-2-(4-cyanophenyl)ethenyl]-2,2′-bipyridine (bbnpe) was used to prepare tris-chelate complexes of various transition metals, in order to understand its complexation behaviour. The ZnII tris-chelate BPh4– salt was treated with a 10 molar excess of a RuII ammine aquo complex, to produce the heptametallic complex as a mixed anion salt. HRS and Stark spectroscopy have been used to determine the quadratic NLO response for the heptametallic mixed anion complex salts, the latter gives large β0 values, approximately 10–27 esu. Density functional theory (DFT) calculations have been carried out on twelve cationic, 2D NLO chromophores with pyrazinylbis(pyridinium) electron acceptors with either 4-(methoxy/dimethylamino)-phenyl or pyridyl coordinated {RuII(NH3)5}2+/trans-{RuII(NH3)4(py)}2+ electron donor groups and the results compared with data previously obtained experimentally. The B3LYP/6-311G(d) level of theory was used to model the absorption spectra and to calculate static hyperpolarisability (β0) values, whilst the B3LYP/LANL2DZ/6-311G(d) level was used for the complexes. The extent of prediction of trends in ICT bands and β0 is partial, with the main discrepancies relating to the progression from one to two electron donor groups. The quantitative accuracy of predictions diminishes as the systems depart from a relatively simple one-dimensional (1D) dipolar motif.

The organic-metal interface is ubiquitous in a wide variety of natural environments and industrial applications. As a result, the interfacial chemistry has been studied for many decades. Specifically, the surfactant-metal interfaces play an important role in the prevention of metallic corrosion where surface active corrosion inhibitors are often used as a prevention method. Development of a spectroscopic method based on vibrational sum frequency generation, specifically for metal interfaces, is carried out with the goal of elucidating the surface structure of these molecules. The contribution to the signalarising from the metal substrate often plays a crucial role in the quantitative analysis of spectra. By adopting a phase-resolved detection scheme, the polar orientation of the organic molecule adsorbed on metal surfaces is experimentally obtained. Furthermore,the development of a novel acquisition scheme is demonstrated where the incident angle is scanned while simultaneously measuring the magnitude and phase of the nonlinear response. This enables the separation of all contributions to the nonlinear susceptibility tensor governing the response. Such an approach is especially useful when the conventional nonlinear vibrational technique is inaccessible in beam polarizations where the infrared field is perpendicular to the plane of incidence, due to the infrared surface selection rule of metals. Finally, this approach is used to examine the structure of a surfactant on iron surfaces. / Graduate / 2022-09-08

Parametric down-conversion (PDC) is a nonlinear optical process widely used to generate pairs of photons. It occurs when an intense laser traverses an optical parametric amplifier (OPA). When the gain of the amplifier is increased, the number of downconverted photons increases exponentially: this is the high-gain regime of PDC. High-gain PDC is potentially a versatile tool for metrology. It is a source of highly-entangled states and bright squeezed states for applications in quantum information and interferometry. In addition, the high number of photons in high-gain PDC makes it possible to use diodes and cameras directly, instead of single-photon detectors and coincidence-counting apparatus. However, all the quantum-optical experimental methods need to be generalized or adapted for a high-photon flux. Most of the theoretical and experimental techniques used or developed in this thesis aim to address this transition from low to high-photon flux of PDC. I theoretically and experimentally provide strategies to harness the mode structure of PDC, bringing us steps closer to a usable source of bright squeezed vacuum for interferometry and quantum imaging. I present experimental progress in reducing the number of frequency modes of high-gain PDC, which is naturally broadband, and consequently highly multimode. Our theory for high-gain PDC generated in a nonlinear crystal provides a set of modes containing physically meaningful information, i.e. the pairwise quantum correlations between independant modes. In addition, I provide a thorough discussion on the limit of SU(1,1) interferometry in regards to internal loss and gain unbalancing. Finally, I tie the frequency spectrum of high-gain PDC to the properties of vacuum fluctuations, allowing to predict the number of photons from first principles, making it a powerful tool for spectroradiometry. Those developments are a springboard towards usable high-gain PDC for metrology.

published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

Fluorinated norbornene monomers exhibit the requisite properties for inclusion in 157 nm photoresists, but traditional addition and radical polymerizations with these monomers have failed. Norbornanediols provide an alternate route to these materials via condensation polymerization, and methods have been developed for the efficient synthesis of the exo-2-syn-7- and endo-2-exo-3-dihydroxynorbornanes. Synthesis of the fluorinated analogues is complicated by steric and electronic effects; however, a high-yielding synthesis of endo-2-exo-3-dihydroxynorbornane bearing a 5-endo-[2,2-bis(trifluoromethyl)hydroxyethyl] substituent as well as its corresponding polymer are reported. As an alternative to 157 nm and other optical lithographies, Step and Flash Imprint Lithography, or S-FIL®, was introduced in 1999 by The University of Texas at Austin. It has proven to be a cost effective, high resolution alternative to traditional optical lithography. Often in the S-FIL process, residual resist may become imbedded within the template features resulting in device defects due to the imprint and repeat nature of S-FIL. The high silicon and cross-linking content of the resist formulations are extremely difficult, if not impossible to remove from quartz imprint mold without template degradation. Our approach to this problem was the synthesis of a family of thermally reversible, cross-linkable monomers that will facilitate resist removal while maintaining template integrity. Our monomers utilize classic Diels-Alder chemistry to provide thermal reversibility, while pendant acrylate functionalities facilitate cross-linking. Herein we report the synthesis of several Diels-Alder compounds, incorporate them into resist formulations, and test their efficacy for resist removal. In an effort to develop unique patternable materials, our laboratory is currently engaged in the design and development of photonic crystals comprised of organic elements with highly stable electro-optic activity. Fabrication of these devices requires polymers that can be patterned at high resolution, have large second order nonlinear optical (NLO) coefficients, and that are thermally stable after poling. Our route to these materials involves the synthesis of a prepolymer that can be spin coated, poled, and then fixed by a photochemical cross-linking reaction. We now describe an efficient synthetic route to a new class of biscross-linkable monomers and the characteristics of their corresponding nonlinear optical polymers. / text

No description available.

This dissertation is an experimental study of a novel type of interferometry based on the generation of Second Harmonic (SH) light. In this work interferometers are described in which an interference pattern arises when two SH waves are superimposed. These waves come from doubling the fundamental frequency of a laser by means of non-linear crystals. Three interferometers are described that have different applications according to their sensitivity to detect wavefront distortions. One interferometer has low sensitivity and is useful in the contouring of refractive objects that produce large wavefront distortions of the order of hundreds of visible wavelengths. The other two interferometers have high sensitivity and one of them is capable of detecting wavefront distortions as small as 1/20 of wavelength. Special emphasis is placed on development of the first interferometer which is a real-time, common-path, self-referencing interferometer that yields interferograms in the visible. The interferometer is based on the fact that a SH wavefront generated under PM conditions is a faithful replica of the laser wavefront. The two interfering SH wavefronts are produced one before and the other after the object under study, and by virtue of its chromatism, they are very slightly different. Consequently, very low density fringes are produced upon their superposition. In this application, noncritically phase matched crystals perform best, and we have found that Y-cut LiNbO₃ crystals configured for surface acousto-optic applications are very convenient. The conversion efficiencies are very low (of the order of 10⁻⁵) consequently optical damage to the LiNbO₃ due to the SH is no problem. The crystals are phase matched by controlling their temperature and are used with a repetitively pulsed Nd:Yag laser operating at 1.06 μm. The chief limitations of this interferometer come from practical considerations in imaging objects with high spatial frequencies that reduce the contrast of the interferograms. The high sensitivity interferometers make use of the changes of phase and amplitude induced in the SH wavefront by the phase mismatch of angle tuned crystals to provide information. The interferometers are directly sensitive to small wavefront tilts and do not require additional reference wavefronts.

Nonlinear phenomena originating from the distributed coupling process were observed when distributed couplers, such as prisms and gratings, were used to couple light into nonlinear ZnS thin film waveguides. The efficiency of the nonlinear distributed coupling process was found to depend on two independent parameters, the angle of the incident beam and the power of the incident beam. Depending on the detuning of the incident angle, from the optimum incident angle at low powers, either optical limiting, power-dependent switching, or power-dependent bistability of the coupling efficiency, and thereby of the in-coupled power, was observed. At zero detuning, a twenty-fold decrease of the coupling efficiency with increasing powers was measured. At a nonzero detuning of the incident angle, power-dependent switching at milliwatt powers was observed. At larger angular detunings, corresponding to the angular width. FWHM, of the coupling peak at low powers, power-dependent bistability was observed, and the width of the bistability loop was found to increase with increasing detunings. All-optical beam scanning via a nonlinear grating coupler was also demonstrated, utilizing a control-signal beam configuration, where the signal beam scanned through an angle of 0.5° when the power of the control beam was varied. The observed nonlinearity in ZnS was positive and of thermal origin. The power-induced change in the refractive index was found to be 0.01 and a relaxation time of 10 μsec was measured. Problems with the long-term stability of the nonlinear distributed coupling process were traced to the occurrence of desorption and adsorption of water vapor in the ZnS films.

A dissertation submitted to the Faculty Of Science in partial fulfilment of the requirements for the degree of Master of Science by research only in the School of Physics, University of the Witwatersrand, Johannesburg. July 11, 2017. / A discrete Q-switched laser that gives a side-lobed single pulse as a laser output was implemented; followed by studying energy extraction e ciencies and pulse characterisation. The aim was to help design a passively Q-switched laser that gives a smooth single pulse of optimum energy as a laser output. The smoothness feature in a single pulse is important in some applications such as range nding. The concepts are demonstrated both experimentally and numerically; the latter using Fox-Li approach to modeling resonator modes with the Fresnel's integral for the system under study. In the rst two chapters, fundamentals of how a laser works and the spatial mode development are studied. In chapter 1, the principles of a laser are discussed: absorption, spontaneous and stimulated emission. Also, di erent types of pumping sources and resonator con gurations that can be used are discussed. In chapter 2, the focus is on developing spatial modes of a laser. The fundamental and high order modes are discussed together with their propagation laws. Then a numerical method is used to nd the eigenmodes of an arbitrary resonator con guration. This numerical method is used to simulate propagation of a fundamental mode and the simulation results are compared to analytical propagation laws. Then, this numerical method is used to simulate a laser resonator. The eigenmode of the lowest loss in the resonator was found. In chapters 3 and 4, experimental work is done on a Q-switched laser where the focus is on the overall laser performance. In chapter 3, Q-switched laser output energies are studied for di erent combinations of Q-switch transmission values and output coupler re ectivities. In addition, the in uence of spatial modes on a Q-switched pulse shape and pulse width are studied, taking into account beam divergence. In chapter 4, conclusions and future work are presented. In future work, the knowledge of spatial mode in uence on pulse shape, pulse width and beam divergence from chapter 3 is exploited. Then particular resonator con guration that gives optimised output results (Q-switched laser output energy, beam divergence, pulse shape and pulse width) is chosen. On that particular resonator, di erent Q-switch transmission values are studied, but now looking only at beam divergence and pulse width. Also, some suggestions on further improving laser performance are given. / LG2018

No description available.