Effect of Photoperiod and Acute Stress on Facilitative Glucose Transporter GLUT3 in Siberian Hamsters (Phodopus sungorus)

Metabolic activity in mammalian brains is dependent upon glucose utilization.  Transport of glucose to the neurons requires mediation by a family of known as facilitative glucose transporter proteins (GLUTs).  The GLUT3 isoform is solely responsible for facilitating glucose transport in mammalian brains. Studies show that age-related brain dysfunction and disease, as well as reductions in learning and memory correlate with decreased GLUT3 expression. Winter is a particularly difficult time to reproduce and survive because of the energy bottleneck caused by the coincidence of low ambient temperatures with reduced food availability. Individuals cope with this energetic bottleneck by engaging tradeoffs among expensive physiological processes through development of specific adaptations including adjustments in metabolic rate, body mass, reproduction, and immune function. Non-tropical animals determine time-of-year by monitoring photoperiod (day length) in order to evoke the appropriate suite of adaptations. However, such metabolic trade-offs can be challenged by stressors. To test the combined effects of acute stress and photoperiod on GLUT3 expression, male adult Phodopus sungorus were housed in either long or short photoperiods for 14 weeks. After 14 weeks, half of the animals from each photoperiod were subjected to restraint stress immediately prior to brain collection. Brain tissue from each animal was excised and used in GLUT3 gene expression analysis. The findings of this study demonstrate that photoperiod and acute stress do not impact the facilitative glucose transporter GLUT3 at the RNA expression level. Further research needs to be conducted in order to determine the precise mechanism which regulates the expression of the GLUT3 transporter in neurons of the hippocampus

Modelling the optical and electronic transport properties of AlGaAs and AlGaN intersubband devices and optimisation of quantum cascade laser active regions

Terahertz quantum cascade lasers (THz QCLs) have many potential applications such as medical and security screening. While their output power has recently exceeded 1 W, their highest operating temperature is currently limited to approximately 200K due to mechanisms such as thermal backfilling and non-radiative phonon emission between lasing states. To achieve higher operating temperatures, theoretical models are key to suppressing these degradation mechanisms either through further design optimisation or new material systems. This work investigates the opto-electronic properties of state-of-the-art intersubband devices in AlGaAs/GaAs and AlGaN/GaN material systems as well as the applications of QCLs. A density matrix model is investigated and used to predict the electron distribution, gain and current density in an arbitrary QCL active region. This model is validated with a comparison to rate equation, non-equilibrium Green’s function, and experimental data for AlGaAs/GaAs QCLs. Novel designs using tall AlAs barriers to suppress leakage current are modelled, and the effect of long and short range interface roughness is investigated. An increased sensitivity to roughness is shown for tall barrier structures which have a larger conduction band offset discontinuity and thinner epitaxial layers. The model is then used to optimise both AlGaAs and AlGaN QCL structures to propose new designs for a desired emission wavelength. The use of the density matrix approach to model possible applications is demonstrated by modelling the origin of the self-mixing (optical feedback) interferometry terminal voltage variations. It is shown that the self-mixing voltage amplitude is highly dependent on the differential resistance of the QCL, and the increased sensitivity of a particular QCL is explained. The feasibility of nitride QCLs is shown by comparing the calculated and experimental absorption linewidth of near-infrared and THz AlGaN/GaN quantum wells grown by molecular beam epitaxy. Finally, a novel adaptation of the density matrix approach is used to investigate the transport properties of nitride resonant tunnelling diodes alongside sequential tunnelling devices. This allows the extent of transport due to bound defect states and interface roughness values to be estimated

Micromotion-induced Limit to Atom-Ion Sympathetic Cooling in Paul Traps

We present and derive analytic expressions for a fundamental limit to the sympathetic cooling of ions in radio-frequency traps using cold atoms. The limit arises from the work done by the trap electric field during a long-range ion-atom collision and applies even to cooling by a zero-temperature atomic gas in a perfectly compensated trap. We conclude that in current experimental implementations this collisional heating prevents access to the regimes of single-partial-wave atom-ion interaction or quantized ion motion. We determine conditions on the atom-ion mass ratio and on the trap parameters for reaching the s-wave collision regime and the trap ground state

Interaction of trapped ions with trapped atoms

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 179-186).In this thesis, I present results from two Paul-trap based ion traps carried out in the Vuletić laboratory: the Atom-Ion trap for collision studies between cold atoms and cold ions, and the Cavity-Array trap for studying the interaction between ionic ensembles and photons. The Atom-Ion trap overlaps a surface-planar ion trap with a magneto-optical trap (MOT) for neutral atoms. The initial results of this system were loading of a shallow surface-planar ion trap at an unprecedented high rate of 4. 105 s-1 and isotopic purity by photoionization from the MOT. We demonstrate the first collisions between trapped atoms and trapped ions in the Langevin collision regime between Yb+ and Yb. A measurement of the Langevin rate constant through charge-exchange collisions between [alpha]Yb+ and [beta]Yb over three orders of magnitude in collision energy down to 3 yueV follows. The measured rate coefficient of 6 - 10-10 cm 3 s-1 is in good agreement with the Langevin model based on theoretical predictions of the polarizability of Yb. The theory and limits of sympathetic cooling of ions by localized cold atoms at low temperature is outlined. Measurements of momentum-transfer collisions between Yb+ and Rb are presented indicating that momentum-transfer collisions affect the ion energy at the Langevin rate. Finally, the fabrication and assembly of the Cavity-Array trap are presented. The Cavity-Array trap overlaps a high-finesse optical cavity with a linear array of Paul traps in order to reach the high co-operativity limit with trapped ions. Initial results from loading of the Cavity- Array trap are shown, indicating successful overlap of the optical cavity mode with the ion trapping region and the ability to load individual sites of the array ion trap.by Andrew T. Grier.Ph.D

Anomalous tumbling of colloidal ellipsoids in Poiseuille flows

Shear flows cause aspherical colloidal particles to tumble so that their orientations trace out complex trajectories known as Jeffery orbits. The Jeffery orbit of a prolate ellipsoid is predicted to align the particle's principal axis preferentially in the plane transverse to the axis of shear. Holographic microscopy measurements reveal instead that colloidal ellipsoids' trajectories in Poiseuille flows strongly favor an orientation inclined by roughly $\pi/8$ relative to this plane. This anomalous observation is consistent with at least two previous reports of colloidal rods and dimers of colloidal spheres in Poiseuille flow and therefore appears to be a generic, yet unexplained feature of colloidal transport at low Reynolds numbers.Comment: 5 pages, 4 figure

Bright Source of Cold Ions for Surface-Electrode Traps

We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap loading rate of $4\times10^{5}$ Yb$^{+}$ ions/s exceeds that attained by photoionization (electron impact ionization) of an atomic beam by four (six) orders of magnitude. Traps as shallow as 0.13 eV are easily loaded with this technique. The ions are confined in the same spatial region as the laser-cooled atoms, which will allow the experimental investigation of interactions between cold ions and cold atoms or Bose-Einstein condensates.Comment: Paper submitted to PRL for review on 2/1/0

One-dimensional array of ion chains coupled to an optical cavity

We present a novel hybrid system where an optical cavity is integrated with a microfabricated planar-electrode ion trap. The trap electrodes produce a tunable periodic potential allowing the trapping of up to 50 separate ion chains spaced by 160 $\mu$m along the cavity axis. Each chain can contain up to 20 individually addressable Yb\textsuperscript{+} ions coupled to the cavity mode. We demonstrate deterministic distribution of ions between the sites of the electrostatic periodic potential and control of the ion-cavity coupling. The measured strength of this coupling should allow access to the strong collective coupling regime with $\lesssim$10 ions. The optical cavity could serve as a quantum information bus between ions or be used to generate a strong wavelength-scale periodic optical potential.Comment: 15 pages, 6 figures, submitted to New Journal of Physic

Observation of Cold Collisions between Trapped Ions and Trapped Atoms

We demonstrate a double-trap system well suited to study cold collisions between trapped ions and trapped atoms. Using Yb$^+$ ions confined in a Paul trap and Yb atoms in a magneto-optical trap, we investigate charge-exchange collisions of several isotopes for collision energies down to 400 neV (5 mK). The measured rate coefficient of $6 \times 10^{-10}$ cm$^{3}$s$^{-1}$, constant over four orders of magnitude in collision energy, is in good agreement with that derived from a semiclassical Langevin model for an atomic polarizability of 143 a.u.Comment: 4 pages, 4 figures; Revision 1/V2: Revised in response to PRL Referees' comment