This thesis is concerned with a variety of topics grouped together under the general theme of periodically driven atomic systems. Periodic driving is ubiquitous in most techniques used in atomic physics, be it laser cooling, ion trapping or AC magnetic fields. An in-depth understanding of the behaviour of such systems can be provided through Floquet theory which will develop as a central theme in the following chapters. The thesis is divided in two parts: neutral atoms, and ions and biomolecules. In the first part I discuss a new ⁴¹K-⁸⁷Rb mixture experiment, built during the first year of my DPhil. This species combination has some very broad and low-loss interspecies Feshbach resonances that are instrumental for carrying out the experiments discussed in the first chapter. Unfortunately, the mixture experiment had to be put aside and our attention was shifted to Time-Averaged Adiabatic Potentials (TAAPs) and how these can be extended using multiple Radio-Frequency (RF) fields. This technique opens up the way for precise interferometric measurements. Lastly, the peculiar behaviour of Modulation Transfer Spectroscopy (MTS) of ³⁹K is investigated and a linearising transformation for four-wave mixing processes is presented. In the second part we turn our attention to charged ions and biomolecules and the techniques of ion trapping. We propose a novel technique for co-trapping charged particles with vastly different mass-to-charge ratios and thoroughly explore its consequences. The behaviour of the trap and the stability of equations with periodic coefficients in general is studied using Floquet theory. The normal modes and symmetries of the system also need to be considered in relation to the effectiveness of the sympathetic cooling of the ions. Small systems were simulated using a Molecular Dynamics (MD) approach in order to capture the effect of micromotion and other heating processes.

The development of the field of the science of ultra-cold matter has opened some exciting possibilities in exploring the quantum-mechanical processes which dominate matter interactions at the sub-microscopic scale. Although methods of cooling atoms are well established, molecular cooling is made difficult by molecules’ additional vibrational and rotational degrees of freedom. It was the goal of the research in this work to approach molecular cooling indirectly, by using broadband shaped-pulse photoassociation for the generation of tightly bound ultracold Rb₂ molecules. The experiments towards this goal conducted by our group included a pumpdecay experiment to observe the generation of ground state singlet or triplet molecules. However, attempts to observe an increase in ground state population have been unsuccessful. A pump-probe study of wavepacket dynamics in the 5s+5p electronic state was conducted in order to determine the appropriate timing for the application of an additional pulse to dump population into the ground state. Although the attempt to observe wavepacket oscillations has been unsuccessful, pump-probe studies have yielded the observation of loosely bound excited state molecules as a result of the photoassociation pulse. These results are promising as a first stage in a fully coherent pump-dump approach to stabilisation into the lowest vibrational ground state. This thesis will provide an introduction and overview to the concerns involved in addressing the problem of molecular cooling and generation. Experimental techniques will be discussed including pulsed laser systems, optical parametric amplifi- cation, and the presentation of an original design for pulse shaping with an acoustooptic modulator. The emphasis of these discussions will be on the principles and operating procedures required for the use of these devices as home-built systems. The thesis will conclude with the results of pump-probe experiments utilising the pulse shaper as a spectral cutting device.

Building complex quantum systems has the potential to reveal phenomena that cannot be studied using classical simulation. Photonics has proven to be an effective test-bed for the investigation of such quantum-enhanced technologies, however, the proliferation of bulk optical components is unlikely to be a scalable route towards building more complex devices. Instead, the miniaturisation, inherent phase stability and trivial alignment afforded by integrated photonic systems has been shown to be a promising alternative. In the first half of this thesis, we describe experiments exploiting the quantum interference of three single photons on a reconfigurable integrated photonic chip. We develop a low-loss source of single photons and introduce a low-loss silica-on-silicon waveguide architecture which enables us to show the first genuine quantum interference of three single photons on an integrated platform. A loss-tolerant, element-wise characterisation scheme is developed along with a statistical test to verify that this multi-photon circuit behaves as expected. We then make use of this three-photon interference to detail the first proof-of-principle demonstration of a new intermediate model of quantum computation called boson sampling. Finally, we perform an on-chip demonstration of the quantum teleportation protocol where all key parts --- entanglement preparation, Bell-state analysis and quantum state tomography --- are performed on a reconfigurable photonic chip. The element-wise characterisation scheme developed earlier is shown to be critical to mitigate fabricated component errors. We develop a theoretical model to account for all sources of possible error in the circuit and find good agreement with the measured teleported state fidelities, which exceed the average teleportation fidelity possible with a classical device. We identify the elements of this error budget relevant to scaling and propose improvements to chip characterisation and fabrication in order to achieve high fidelity operation. In the second half, we discuss the use of high efficiency superconducting transition edge sensors in enabling quantum experiments using more photons. We detail the installation and characterisation of these detectors in a new lab in Oxford. We achieve good photon number-resolution and high-efficiency operation. Work to integrate these detectors on the silica-on-silicon waveguide architecture is discussed and we detail the optical and thermal device modelling performed to optimise the on-chip detection efficiency. New, on-chip detectors, fabricated according to this design are shown to operate as expected and achieve high-efficiency and good energy resolution.

It is possible to reach the radiation-reaction–dominated regime in today’s high-intensity laser facilities, using the collision of a wakefield-accelerated GeV electron beam with a 30 fs laser pulse of intensity 10²² Wcm^-2. This would demonstrate that the yield of high energy gamma rays is increased by the stochastic nature of photon emission: a beam of 10⁹ electrons will emit 6300 photons with energy > 700 MeV, 60 times the number predicted classically. Detecting those photons, or a prominent low energy peak in the electron beam's post-collision energy spectrum, will provide strong evidence of quantum radiation reaction; we place constraints on the accuracy of timing necessary to achieve this. This experiment would provide benchmarking for the simulations that will be used to study the plasmas produced in the next generation of laser facilities. With focused intensities > 10²³ Wcm^-2, these will be powerful enough to generate high fluxes of gamma rays and electron-positron pairs from laser–laser and laser–solid interactions. It will become possible to test the physics of exotic astrophysical phenomena, such as pair cascades in pulsar magnetospheres, and explore fundamental aspects of quantum electrodynamics (QED). To that end we will discuss: classical theories of radiation reaction; QED processes in intense fields; and a Monte Carlo algorithm by which the latter may be included in particle-in-cell codes. The feedback between QED processes and classical plasma dynamics characterises a new regime we call QED-plasma physics.

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 variation of a magnetic field in the vicinity of a zero-energy resonance allows highly vibrationally excited molecules (‘Feshbach molecules’) to be produced from an ultracold atomic gas. In this thesis, we study the dynamics of this process. We begin by studying the dissociation of Feshbach molecules, showing that in the limit of a sudden jump the shape of the spectrum of dissociated atoms can act as a probe of the zero-energy resonance. For some resonances, such jumps are within reach of current experiments. We also study the intermediate region between sudden jumps and asymptotically wide, linear ramps. It is shown from a precise derivation how the latter limit leads to a universal spectrum with a shape independent of the implementation of the two-body physics, provided that the near-resonant scattering properties are correctly modelled. We then turn to the dynamics of Feshbach molecule production from thermal and condensed gases. Our microscopic quantum dynamics approach includes the exact twobody evolution as an input to the many-body calculations. We show that in the long-time limit, and the Markov limit for the interactions, the non-Markovian Boltzmann equation (NMBE) we derive for the one-body density matrix reduces to the normal Boltzmann equation. In the limit of short times and small depletion of the atomic gas, the molecule production efficiency can be calculated by thermally averaging the two-body transition probability density. This thermal averaging technique is applied to studies of the formation of Feshbach molecules using a magnetic field modulation that is near-resonant with the molecular bound state energy. The continuum is shown to have a significant effect on both the dynamics and efficiency of this process. We examine the dependence of the molecule production efficiency on the duration, amplitude and frequency of the modulation, as well as the temperature and density of the gas. This method of producing molecules is effective for a wide range of bound state energies, but requires sufficient variation of the two-body energy levels with magnetic field. Lastly, we implement the NMBE for the case of a fast linear ramp across a Feshbach resonance. The solution of this equation is made feasible by including a large part of the required computation in the kernel, which is calculated in advance. The NMBE allows predictions of the molecule production efficiency which go beyond the thermal averaging technique by accounting for the depletion and rethermalisation of the continuum. In the limit of small depletions, the two approaches give the same results. As the depletion increases, the two approaches differ due to many-body effects limiting the maximum possible molecule production efficiency. We have observed this in our simulations by considering higher-density gases. We have therefore shown the suitability and practicability of this beyond mean-field approach for application to further problems in the production of Feshbach molecules from ultracold gases.

Understanding the electronic structure and dynamics of cyclic organic molecules is becoming increasingly the subject of investigations from different perspectives due to their unique chemical and physical properties. Since they are largely involved in the biochemistry of living organisms, studies on this class of compounds are also valuable to understand biologically relevant complex systems. Compared to other techniques, high-order harmonic generation (HHG) has been increasingly considered as a powerful spectroscopic tool with Angstrom spatial and attosecond temporal resolutions. This thesis demonstrates that high-order harmonic spectroscopy is capable of providing structural and dynamical information on the electronic systems of representative cyclic organic molecules comprising randomly oriented five-membered or six-membered rings. The first part of this thesis shows that the HHG from these molecules is sensitive to their aromatic character, which results from the de-localized pi electrons, and can potentially be a useful qualitative measure of aromaticity. We show that the advantage of utilizing HHG in this direction stems from the result that only pi molecular orbitals, associated with aromatcity, are responsible for the HHG emission in aromatic systems. The capability of HHG to distinguish cyclic isomers is demonstrated in the case of xylene molecules. Supported by numerical calculations, differences in the isomers are attributed to both tunnel ionization and photorecombination, the first and last steps of HHG. These results enable further HHG-based time-resolved studies of the dynamics associated with isomeric effects that these molecules exhibit. The present work also challenges the well-established prediction that strong field ionization from a molecular orbital is suppressed along nodal planes, where the electron density is zero. In fact, our study shows that considerable tunnel ionization in some cyclic molecules can occur near or along nodal planes. This unusual ionization is reported to have its signature on the quantitative and qualitative dependence of harmonic yield on laser ellipticity. The high symmetry displayed by the cyclic molecule, 1,4 cyclohexadiene, is shown to leave its imprints on the HHG in the form of structural interferences even if the target is randomly oriented. Two-color HHG from this molecule also indicates that hole dynamics could be involved in the generation process. A general study on high harmonic spectroscopy of the Cooper minimum in molecules is also reported. The presence of this minimum could affect the interpretation of harmonics spectra in any molecule containing S or Cl atoms. The molecular environment is shown to influence the position of this spectral modulation.

No description available.

This thesis outlines the high intensity tabletop attosecond extreme ultraviolet laser source at the Institute for the Frontier of Attosecond Science and Technology Laboratory. First, a unique Ti:Sapphire chirped pulse amplifier laser system that delivers 14 fs pulses with 300 mJ energy at a 10 Hz repetition rate was designed and built. The broadband spectrum extending from 700 nm to 900 nm was obtained by seeding a two stage Ti:Sapphire chirped pulse power amplifier with mJ-level white light pulses from a gas filled hollow core fiber. It is the highest energy level ever achieved by a broadband pulse in a chirped pulse amplifier up to the current date. Second, using this laser as a driving laser source, the generalized double optical gating method is employed to generate isolated attosecond pulses. Detailed gate width analysis of the ellipticity dependent pulse were performed. Calculation of electron light interaction dynamics on the atomic level was carried out to demonstrate the mechanism of isolated pulse generation. Third, a complete diagnostic apparatus was built to extract and analyze the generated attosecond pulse in spectral domain. The result confirms that an extreme ultraviolet super continuum supporting 230 as isolated attosecond pulses at 35 eV was generated using the generalized double optical gating technique. The extreme ultraviolet pulse energy was ∼100 nJ at the exit of the argon gas target.

This thesis focuses on simulating, designing, and characterizing a narrow-linewidth continuous-wave (CW) Ytterbium (Yb) master oscillator fiber amplifier (MOFA) system. The primary goal was to achieve a watt-level output at 1087 nm with a narrow 1 GHz linewidth, making the MOFA well-suited for serving as a pump source in Terahertz (THz) generation experiments. This system can essentially be divided into two main parts: a narrow-linewidth Yb fiber seed laser (master oscillator), responsible for delivering a stable and narrow-linewidth signal power, and a Yb fiber amplifier for scaling this signal power.The all-fiber Yb seed laser produced a maximum output power of 11.31 mW at 1087.39 nm with a narrow linewidth of approximately 1.86 GHz. The narrow-linewidth operation was achieved by incorporating a fiber Bragg grating (FBG) as a resonator mirror and an intracavity FBG based Fabry-Perot interferometer as a bandpass filter, allowing only specific portion of the FBG reflected spectrum to pass through. A simulation was subsequently developed to identify the optimal Yb gain fiber lengths and pump powers required to realize an efficient Yb fiber amplifier with high gains. Finally, by employing a 10-meter Yb fiber amplifier, seed laser’s output power was boosted to 0.86 W, resulting in a 23.1 dB gain, achieved with a notable power conversion efficiency of 33 %. The final amplified spectrum exhibited a Gaussian-like profile centered at 1087.12 nm, with a bandwidth of less than 12.7 GHz. By further scaling the output power to watt-level range, this Yb MOFA system has the potential to be utilized as a pump source in Terahertz (THz) generation research. / I detta examensarbete presenteras simuleringar, konstruktion och karaktärisering av en kontinuerlig smalbandig Ytterbium (Yb) fiberlaser och förstärkare. Målet med arbetet är att uppnå en effekt runt en watt med en centervåglängd på 1087 nm och en bandbredd på 1 GHz som kan användas vid generation av terahertzstrålning (THz-strålning). Systemet kan delas in i två delar: en Yb fiberlaser som levererar en stabil och smalbandig signal och den andra är en Yb fiber baserad förstärkare för amplifikation av signalen. Yb fiberlasern hade en maxeffekt på 11.31 mW med en centervåglängd på 1087.39 nm och en bandbredd på 1.86 GHz. Den smalbandiga operationen var uppnådd genom att inkorporera en fiber bragg-gitter (FBG) som formade en resonator, med två ytterligare FBG element som agerade som en intra-kavitär Fabry-Perot interferometer vilket begränsar vilka våglängder som kan propagera. Ett program för att simulera den optimala längden på Yb fiberförstärkaren samt vilken pumpeffekt som maximerade den förstärkta signalen togs fram för projektet. Det resulterade i en förstärkare med en 10 m lång Yb fiber som kunde förstärka fiberlasern till 0.86 W, vilket motsvarade till en 23.1 dB förstärkning med en effektivitet på 33%. Den slutgiltiga signalen hade ett spektrum med gaussisk profil och en bandbredd under 12.7 GHz. Med ytterligare skalning på effekten genom fler förstärkare kan system som helhet användas vid generation av THz strålning.