Superconducting nanowire single-photon detectors (SNSPDs) are nanodevices usually made from thin niobium nitride (NbN) films. Operated at liquid helium temperature, they can exhibit high detection efficiency with low dark-counts associated with a fast response time and a low timing jitter. Covering a broad detection range from ultraviolet to mid-infrared, SNSPDs are a very attractive alternative to silicon or gallium arsenide based semiconductor detectors for fiber based telecommunication when single-photon sensitivity and high counting rates are necessary. Efficient packaging and fiber coupling of a SNSPD is in itself a real challenge and is often a limiting factor in reaching high system quantum efficiency. Our approach makes use of a controlled expansion alloy which has been adequately heat treated to enhance its characteristics for cryogenic operation. This insures the integrity of the optical coupling at cryogenic temperatures while done at room temperature. It also provides a good attenuation for electromagnetic interference due to the high relative permeability of the nickel-iron alloy. The small form factor of this pigtailed optical fiber package makes it versatile and could be easily integrated with a commercial cryogen-free system or simply dipped into a standard helium transport Dewar. We report on our theoretical and experimental methodology to evaluate the optical coupling quality and present the optoelectronic characterization of two devices packaged in this way. Electrical simulation is studied to understand the speed limitation factor inherent to these devices and preliminary speed and jitter measurements are reported.

Superconducting nanowire single-photon detectors (SNSPDs) are nanodevices usually made from thin niobium nitride (NbN) films. Operated at liquid helium temperature, they can exhibit high detection efficiency with low dark-counts associated with a fast response time and a low timing jitter. Covering a broad detection range from ultraviolet to mid-infrared, SNSPDs are a very attractive alternative to silicon or gallium arsenide based semiconductor detectors for fiber based telecommunication when single-photon sensitivity and high counting rates are necessary. Efficient packaging and fiber coupling of a SNSPD is in itself a real challenge and is often a limiting factor in reaching high system quantum efficiency. Our approach makes use of a controlled expansion alloy which has been adequately heat treated to enhance its characteristics for cryogenic operation. This insures the integrity of the optical coupling at cryogenic temperatures while done at room temperature. It also provides a good attenuation for electromagnetic interference due to the high relative permeability of the nickel-iron alloy. The small form factor of this pigtailed optical fiber package makes it versatile and could be easily integrated with a commercial cryogen-free system or simply dipped into a standard helium transport Dewar. We report on our theoretical and experimental methodology to evaluate the optical coupling quality and present the optoelectronic characterization of two devices packaged in this way. Electrical simulation is studied to understand the speed limitation factor inherent to these devices and preliminary speed and jitter measurements are reported.

<p>The visible light photon counter (VLPC) is a high quantum efficiency (QE), Si-based, single-photon detector with high gain, low-noise multiplication, low timing jitter, and photon number resolution. While the VLPC has high QE in the visible wavelengths, the QE in the ultraviolet and infrared is low due to minimal absorption within the active layers of the device. In the ultraviolet, the absorption coefficient of Si is high and most of the incident photons are absorbed within the top contact of the device, whereas, in the infrared, Si is practically transparent. A number of applications in quantum information science would benefit from use of the VLPC if the QE was improved in the ultraviolet (e.g., state detection of trapped ions) and the infrared (e.g., long-distance quantum cryptography). This thesis describes the development of the ultraviolet photon counter (UVPC) and the infrared photon counter (IRPC), which are modified versions of the VLPC with increased QE in the ultraviolet and infrared wavelengths, respectively. The UVPC has a transparent metal Schottky contact to reduce absorption within the top contact of the VLPC, resulting in an increase in the QE in the ultraviolet by several orders of magnitude. The IRPC is a proposed device that has an InGaAs absorption layer that is wafer-fusion bonded to the VLPC. The band alignment of the resulting InGaAs/Si heterojunction is measured and shows a large discontinuity in the valence band that impedes carrier transport at the interface. A ultra-high vacuum wafer-bonding system was developed to understand the impact of the surface chemistry of the bonded wafers on the band alignment of the InGaAs/Si heterojunction of the IRPC.</p> / Dissertation

In this thesis, the development and implementation of fluorescence lifetime imaging microscopy that integrates time correlated single photon counting (TCSPC) and a confocal microscope will be described. The TCSPC method has high detection efficiency, with a time resolution limited only by the transit time spread of the detector, and directly delivers the decay functions in the time domain. TCSPC can also be used to obtain images that indicate the fluorescence resonance energy transfer (FRET) effect between critical fluorophores, an important method distinguish the difference between binding and co-localization. Estimation of distances between RET fluorophore pairs can also be established. Additionally, the effects of ion concentration, oxygen concentration, pH value, ..etc. can also be revealed.

In this thesis the effects of single photon and multi-photon excitation on protoplasts from Arabidopsis thaliana are compared. Time-lapsed micro-spectroscopy at high spatial resolution is employed to study the response of chloroplasts within the protoplasts from Arabidopsis thaliana. We have found that the fluorescence spectra of chloroplasts exhibits dramatic changes and the protoplasts are rapidly damaged under multi-photon excitation as a result of pulsed laser illumination. In contrast, single photon excitation of chloroplasts with cw laser is relatively inert to the vitality of the protoplasts. In addition to, we have built an ultrafast laser excited cryogenic micro-spectroscopy setup to study the photoluminescence of PPV thin film. We found that the spectrum of PPV¡¦s photoluminescence should shift toward longer wavelength and the non-radiative transition should be suppressed as a result of longer electron coherence length at low temperature.

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This dissertation presents the development and implementation of two novel experimental techniques for controlling atom number and phase-space density in cold atomic gases. The first experiment demonstrates the method of single-photon cooling, an optical realization of Maxwell's demon, using an ensemble of rubidium atoms. Single-photon cooling increases the phase-space density of a cloud of magnetically trapped atoms, reducing the entropy of the ensemble by irreversibly transferring atoms through a one-way wall via a single-photon scattering event. While traditional laser cooling methods are limited in their applicability to a small number of atoms, single-photon cooling is much more general and should in principle be applicable to almost all atoms in the periodic table. The experiment described in this dissertation demonstrates a one-dimensional implementation of the cooling scheme. Complete phase-space compression along this dimension is observed. The limitations on the cooling performance are shown to be given by trap dynamics in the magnetic trap. The second part of this dissertation is dedicated to the experiment built to control the atom number of a degenerate Fermi gas on a single particle level. Creating Fock states of atoms with ultra-high fidelity is a mandatory step for studying quantum entanglement on a single atom level. The experimental technique implemented to control the atom number in this experiment is called laser culling. Decreasing the trapping potential reduces the atom number in a controlled way, giving precise control over the number of atoms remaining in the trap. This dissertation details the design and construction of this experiment and reports on the progress towards the creation of neutral lithium Fock states. / text

This dissertation discusses several recently developed experimental techniques for controlling the motion of neutral atoms. While laser cooling and evaporative cooling have been extremely successful and have been in widespread use for many years, these techniques are only applicable to a few atomic species. Supersonic beams provide a general method of producing cold atoms in the co-moving frame, but their speeds are typically several hundreds of meters per second in the lab frame. Methods to slow and control atoms cooled by supersonic expansion are detailed. A method for controlling the velocity of a cold beam of ground state helium using specular reflection from single crystal surfaces is demonstrated. The velocity of the beam is shown to be continuously tunable, and beam velocities as slow as 265m/s are created from an initial beam speed of 511 m/s. Magnetism is a nearly universal atomic phenomenon, making magnetic control of atomic motion a very general technique. Magnetic stopping of supersonic beams of metastable neon and molecular oxygen is demonstrated using a series of pulsed electromagnetic coils. Neon is slowed from 446 m/s to 56 m/s, and oxygen is slowed from 389 m/s to 83 m/s, removing over 95% of the kinetic energy. The experimental technique is described in detail, and the theory and principle are discussed. An experiment for slowing and trapping of atomic hydrogen isotopes at around 100 mK using a room temperature apparatus is described. A method for further cooling of magnetically trapped hydrogen ensembles, single-photon cooling, is proposed. / text

Advances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs. / text

Recently, developments in novel and high-purity materials allow for the presence of a single, solitary crystalline defect to define the electronic, magnetic, and optical functionality of a device. The discrete nature of the active dopant, whose properties are defined by a quantum mechanical description of its structure, enables radically new quantum investigations and applications in these arenas. Finally,there has been significant development in large-scale device engineering due to mature semiconductor manufacturing techniques. The diverse set of photonic device architectures offering light confinement, guiding, and extraction is a prime example. These three paradigms – solitary dopant photonics and optoelectronics (solotronics), quantum science and technology, and device engineering – merge in the development of novel quantum photonic devices for the next generation of information processing systems. We present in this thesis a series of investigations of optical nanostructures for single optically active spins in single crystal diamond. Chapter 1 introduces the Nitrogen-Vacancy (NV) color center, summarizes its applications, and motivates the need for their integration into photonic structures. Chapter 2 describes two prototype nanobeam photonic crystal cavities for generating strong light-matter interactions with NV centers. The first device consists of a silicon nitride photonic crystal nanobeam cavity with high quality factor \(Q \sim 10^5\) and small mode volume \(V \sim 0.5*(\lambda/n)^3\). The second device consists of a monolithic diamond nanobeam cavity fabricated with the focused ion beam (FIB) directly in a single crystal diamond sample. Chapter 3 presents a high-efficiency source of single photons consisting of a single NV center in a photonic diamond nanowire. Early FIB prototypes are described, as is the first successful realization of the device achieved via reactive ion etching nanowires in a single crystal diamond containing NV centers, and finally a variation of this approach based on incorporation of NV centers in pure diamond via ion implantation. In chapter 4 we consider the optimal design of photonic devices offering both collection efficiency and cavity-enhancements and extend the model of the NV center to include photonic effects. In chapter 5 we briefly introduce a novel optically active spin discovered in a diamond nanowire. Finally, in chapter 6 we conclude with several proposals to extend this research program. / Engineering and Applied Sciences