published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

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<p>The creation and manipulation of biphotons is important for many applications in quantum optics and quantum information. Topics that benefit from efficient biphoton sources range from the most fundamental quantum science experiments to the highly applied fields of quantum communication and quantum computation. Biphoton sources have long been hailed as one of the leading methods for creating entangled photon pairs for tests of Bell's inequality, creating heralded photon pairs that are used in on-demand single-photon sources and heralded measurement techniques, and for quantum communication protocols to name a few. Specifically for quantum communication, biphoton sources are commonly used for cutting edge quantum key distribution (QKD) protocols. </p><p>In the first part of the thesis, I focus on realizing an efficient biphoton source that produces high yield photon pairs. </p><p>More specifically, I develop an optimized biphoton source using the nonlinear optical process of spontaneous parametric down-conversion in a second-order nonlinear crystal. I develop a formalism for predicting the two important metrics of a biphoton source: the heralding efficiency and joint count rate. I show how, from a large parameter space, one can tailor the phase matching of the nonlinear interaction to create a high quality biphoton source that produces both high heralding efficiency and high joint count rate. I achieve heralding efficiencies of 86$\pm$5$\%$ and joint count rates of 2.58$\pm$0.6 kHz per mW pump power. I show that using a collinear nondegenerate geometry allows for heralding efficiencies of up to 99.7$\%$ assuming no loss in the system. I verify the theoretical model with experimental results and find good agreement.</p><p>In the second part of the thesis I turn to manipulating the single photons born from the biphoton source for applications in creating single-photon spectrometers and time-frequency QKD systems. The security of QKD is only guaranteed if the two parties have access to a set of states called mutually unbiased states. I create a set of these states in time and their conjugate states in the frequency basis and show that I can manipulate single photon correlations in time and frequency so that an eavesdropper can be detected if she localizes a photon to a 1 ns time interval. </p><p>Additionally, in these experiments, I stretch a single photon wavepacket of 5-ps-width to a wavepacket of 8.3-ns-width and subsequently recompress it to at least the resolution of the detectors ($\sim$ 300 ps). This demonstrates a stretch factor of >1600 for a single-photon pulse using a group velocity dispersive material. To my knowledge, this is the largest reported stretch factor for a single-photon wavepacket produced by a biphoton source. The ability to stretch and recompress a single photon by this amount has applications in creating high-resolution, high-efficiency, single-photon spectrometers as well as advancing time-frequency QKD systems and other temporal pulse shaping applications.</p> / Dissertation

<p> With the advent of microsphere assisted microscopy in 2011, this technique emerged as a simple and easy way to obtain optical super-resolution. Although the possible mechanisms of imaging by microspheres are debated in the literature, most of the experimental studies established the resolution values well beyond the diffraction limit. It should be noted, however, that there is no standard resolution measurement in this field that researchers can use. The reported resolution has been based on the smallest discernible feature; although it seems logical but it is not based on the standard textbook definition, and so far it has ended to a wide range of resolution reports based on qualitative criteria which can lead to exaggerated resolution values. In addition, this method has another limitation related to its limited field-of-view. In this work, first we fabricated a novel optical component for super-resolution imaging based on an attachable polydimethylsiloxane (PDMS) thin film with embedded high index (<i>n</i>~2) barium titanate glass (BTG) microspheres. It is shown that such films can be translated along the surface of investigated structures to enhance field-of-view. Second, we introduced a method of image treatment which allows determining the super-resolution values consistent with the resolution definition in the conventional diffraction-limited optics. We demonstrated this method for a typical microsphere-assisted image where we measured the super-resolution of ~&lambda;/5.5. We also developed this technique to measure the resolution of a micro-cylindrical-assisted system. </p>

<p> In this dissertation I present theory and experimental results obtained in the Jones research group at the University of Arizona investigating the feasability of neutral Hg as a candidate for an atomic clock. This investigation includes laser-cooling and trapping of several neutral Hg isotopes as well as spectroscopy of the 6¹<i>S</i>₀ - 6³<i>P</i>₀ doubly forbidden clock transition in neutral Hg¹⁹⁹. </p><p> We demonstrate precision spectroscopy of the ground state cooling/trapping transition of neutral mercury at 254 nm using an optically pumped semiconductor laser (OPSL). This demonstration exhibits the utility of optically pumped semiconductor lasers (OPSLs) in the field of precision atomic spectroscopy. The OPSL lases at 1015 nm and is frequency quadrupled to provide the trapping light for the ground state cooling transition. We get up to 1.5 W single-frequency output power having a linewidth of &lt;10 kHz in the IR with active feedback. We frequency quadruple the OPSL in two external cavity stages to produce up to 120 mW of deep-UV light at 253.7 nm. </p><p> I give a detailed characterization of the construction and implementation of the neutral Hg vapor cell magneto-optical trap (MOT). The trap can be loaded in as quickly as 75 ms at background vapor pressures below 10^-8 torr. At reduced background pressure (&lt;10^-10 torr) the loading time approaches &sim;2 sec. </p><p> We describe construction and stabilization of a laser resonant with the Hg¹⁹⁹ clock transition and the methods employed to find and perform the experimentally delicate spectroscopy of the clock transition. We present experimental results and analysis for our initial spectroscopy of the 6¹<i>S</i>₀ - 6³<i> P</i>₀ clock transition in the Hg¹⁹⁹ isotope of neutral mercury.</p>

The scalar Huygens-Fresnel Principle describing the propagation of light is reformulated to take into account the vector nature of light and the associated directed electric and magnetic fields. A vector Huygens secondary source is developed in terms of the fundamental radiating units of electromagnetism: the electric and magnetic dipoles. The vector Huygens wavelets are incorporated into a computer model that calculates the resulting vector fields after light passes through a diffracting system by a wavefront reconstruction process similar to that originally proposed by Huygens himself in 1687. Fresnel and Fraunhofer diffraction patterns are computed for common apertures such as rectangles and circles where theoretical results are available for comparison and validation of the model. However, irregular apertures not easily described in closed mathematical form are studied as well. Both completely absorbing and infinitely conducting screens are considered as well as plane wave and spherical illumination.

An integrated optical waveguide is described that can measure surface concentrations of ions while immersed in an ionic solution, especially the commercially interesting case of ionic copper. Several such sensors were manufactured and tested. The theory behind this measurement technology is described. Techniques for the manufacture and experimental measurement are described. The devices made were able to measure surface concentrations on the order of 10 12 atoms per square centimeter. Several potential methods for improving the performance are given.

Upconversion coefficients in a new high-concentration erbium-doped tellurite glass were obtained a schematic study of experiments and modeling. The upconversion coefficient for ⁴I₁₃/₂ + ⁴I₁₃/₂ → ⁴I₉/₂ + ⁴I₁₅/₂ is found to be 2.74 x 10⁻¹⁸ cm³/s and for ⁴I₁₁/₂ + ⁴I₁₁/₂ → ⁴I₁₅/₂ + ²F₇/₂ is 1.09 x 10⁻¹⁸ cm³/s. The performance of high concentration Er³⁺+-Yb³⁺ -codoped phosphate fiber amplifiers and the performance of a high-power Er³⁺-Yb³⁺-codoped phosphate fiber laser were presented. From a 3.6cm-long fiber, 18 dB internal gain i.e. 5 dB/cm, for small signal input at 1535 nm, was achieved. With a cleaved facet as the output mirror, a fiber laser has been demonstrated from the same fiber with an output power of 33.8 mW at 1549.92 nm. A high slope efficiency of 40.2% was observed. Modeling results of gain and noise figure of four phosphate EDFAs with different lengths were presented with previous measured results. 34% erbium ions were found to be paired in our 3.5wt% erbium-doped phosphate fiber amplifiers. The onset of erbium concentration for pair induced quenching is suggested to be around 3wt% in phosphate fiber amplifiers.

<p> An optomechanical system consists of an optical cavity coupled to a mechanical oscillator. The system used for this work was a silica microsphere. In a silica microsphere, the optical cavity is formed by light that is confined by total internal reflection while circulating around the equator of the sphere. The mechanical oscillator is the mechanical breathing motion of the sphere itself. The optical cavity and mechanical oscillator are coupled by radiation pressure and by the mechanical oscillator physically changing the length of the optical cavity. </p><p> The optomechanical analog to electromagnetically induced transparency (EIT), known as optomechanically induced transparency (OMIT), has previously been studied in its steady state. One topic of this dissertation is an experimental study of OMIT in the time domain. The results of these experimental demonstrations continue comparisons between EIT and OMIT, while also building a foundation for optomechanical light storage. </p><p> In OMIT, an off-resonance control laser controls the interaction between on-resonance light and the mechanical oscillator. Optomechanical light storage makes use of this arrangement to store an optical signal as a mechanical excitation, which is then retrieved at a later time as an optical signal. This is done by using two temporally separated off-resonance control laser pulses. This technique is extremely flexible in frequency and displays a storage lifetime on the order of microseconds. </p><p> Use of optomechanical systems for quantum mechanical applications is hindered by the thermal background noise of the mechanical oscillator. Addressing this issue by first cooling the mechanical oscillator is costly and fraught with difficulties. The final topic presented in this dissertation deals with this issue through the use of an optomechanical dark mode. Two optical modes can interact with the same mechanical mode. The dark mode is a state that couples the two optical modes but is decoupled from the mechanical oscillator. </p><p> While our specific optomechanical system is limited by its somewhat modest optomechanical cooperativity, this conversion process can, in principle, preserve the quantum state of the signal, even at room temperature, opening the possibility for this technique to be applied in quantum information processing.</p>

This dissertation presents investigations of fundamental optical nonlinearities in semi-conductor microstructures. Two distinct effects are studied. In the first part the excitonic optical Stark effect in InGaAs and GaAs multiple quantum-well structures is investigated by means of pump-probe spectroscopy. For nonresonant excitation below the excitonic transition the direction of the shift of the resonance depends on the polarization of the pump and probe pulses. In particular, for anti-circular polarization a surprising red-shift is observed. For resonant excitation, induced absorption energetically above and below the exciton transition and bleaching of the resonance is found. Experiments using both resonant and nonresonant excitation reveal the importance of bound and unbound two-exciton states in absorption changes of the 1s heavy-hole exciton resonance. It is found that higher-order Coulomb contributions determine the intensity as well as the time dependence of the differential excitonic absorption. In addition, the influence of light-hole excitons is analyzed. It is shown that the direction of the optical Stark shift for nonresonant excitation depends also sensitively on the heavy-hole to light-hole splitting and the detuning of the pump pulse. For very high pump intensities and nonresonant excitation the absorption is split when a circularly polarized pump and a linearly polarized probe beam are used. For co-circular excitation traces of hyper-Raman gain are observed. In the second part of this dissertation, the nonlinear optical response of semiconductor microcavities in the nonperturbative regime is studied in resonant single-beam transmission and pump-probe experiments. In both types of experiment, a pronounced third transmission peak lying spectrally between the two normal modes is observed. Its dependence on the probe intensity, pump intensity, pump-probe delay, exciton-cavity detuning and pump detuning is investigated. For single-beam transmission, the energy of the third peak parallels the position of the cavity resonance. It is more pronounced for circularly polarized excitation and lasts longer than the two normal modes. For pump-probe experiments, the third peak increases with decreasing probe intensity and increasing pump intensity. Its energy is close to the low-energy side of the pump spectrum and virtually unaffected by the cavity-exciton detuning. The appearance of the third peak requires temporal overlap of pump and probe pulses. The origin of this complex nonlinearity is the quantum nature of light, which induces intraband polarizations in the presence of a coherent driving field and a finite carrier density. It is found that the coupling of the intraband polarizations via guided modes to the polarization of the fundamental longitudinal mode is responsible for the third transmission peak. A fully quantized theory reproduces the experimental observations.