Recent results of a seismically isolated optical table prototype designed for advanced LIGO

The Horizontal Access Module Seismic Attenuation System (HAM-SAS) is a mechanical device expressly designed to isolate a multipurpose optical table and fit in the tight space of the LIGO HAM Ultra-High-Vacuum chamber. Seismic attenuation in the detectors' sensitivity frequency band is achieved with state of the art passive mechanical attenuators. These devices should provide an attenuation factor of about 70dB above 10Hz at the suspension point of the Advanced LIGO triple pendulum suspension. Automatic control techniques are used to position the optical table and damp rigid body modes. Here, we report the main results obtained from the full scale prototype installed at the MIT LIGO Advanced System Test Interferometer (LASTI) facility. Seismic attenuation performance, control strategies, improvements and limitations are also discussed

N/P GaAs concentrator solar cells with an improved grid and bushbar contact design

The major requirements for a solar cell used in space applications are high efficiency at AMO irradiance and resistance to high energy radiation. Gallium arsenide, with a band gap of 1.43 eV, is one of the most efficient sunlight to electricity converters (25%) when the the simple diode model is used to calculate efficiencies at AMO irradiance, GaAs solar cells are more radiation resistant than silicon solar cells and the N/P GaAs device has been reported to be more radiation resistant than similar P/N solar cells. This higher resistance is probably due to the fact that only 37% of the current is generated in the top N layer of the N/P cell compared to 69% in the top layer of a P/N solar cell. This top layer of the cell is most affected by radiation. It has also been theoretically calculated that the optimized N/P device will prove to have a higher efficiency than a similar P/N device. The use of a GaP window layer on a GaAs solar cell will avoid many of the inherent problems normally associated with a GaAlAs window while still proving good passivation of the GaAs surface. An optimized circular grid design for solar cell concentrators has been shown which incorporates a multi-layer metallization scheme. This multi-layer design allows for a greater current carrying capacity for a unit area of shading, which results in a better output efficiency

Thresholds For Dielectric-Breakdown In Laser-Irradiated Diamond

We report on picosecond laser-induced damage experiments that were carried out on a natural type-IIa diamond and a thick specimen of high-quality chemically vapor-deposited (CVD) diamond. In conjunction with earlier measurements performed elsewhere on an \u27\u27optically thick\u27\u27 single crystal, it is shown that for spot sizes (2omega) ranging from 3 to 60 mum, the breakdown field strength (E(BD)) at the damage threshold of diamond obeys a pattern best described as follows: E(BD) congruent-to A/square-root 2omega, where A = 30.7 and 38.7 MV mu1/2/cm at 532 and 1064 nm, respectively. The case of CVD diamond demonstrates that if problems arising from localized high absorption at the deposition surface can be avoided, this material should be of much promise for contemplated high-power free-electron laser window applications

Cell biological mechanisms of activity-dependent synapse to nucleus translocation of CRTC1 in neurons.

Previous studies have revealed a critical role for CREB-regulated transcriptional coactivator (CRTC1) in regulating neuronal gene expression during learning and memory. CRTC1 localizes to synapses but undergoes activity-dependent nuclear translocation to regulate the transcription of CREB target genes. Here we investigate the long-distance retrograde transport of CRTC1 in hippocampal neurons. We show that local elevations in calcium, triggered by activation of glutamate receptors and L-type voltage-gated calcium channels, initiate active, dynein-mediated retrograde transport of CRTC1 along microtubules. We identify a nuclear localization signal within CRTC1, and characterize three conserved serine residues whose dephosphorylation is required for nuclear import. Domain analysis reveals that the amino-terminal third of CRTC1 contains all of the signals required for regulated nucleocytoplasmic trafficking. We fuse this region to Dendra2 to generate a reporter construct and perform live-cell imaging coupled with local uncaging of glutamate and photoconversion to characterize the dynamics of stimulus-induced retrograde transport and nuclear accumulation

Simulation of underground gravity gradients from stochastic seismic fields

We present results obtained from a finite-element simulation of seismic displacement fields and of gravity gradients generated by those fields. The displacement field is constructed by a plane wave model with a 3D isotropic stochastic field and a 2D fundamental Rayleigh field. The plane wave model provides an accurate representation of stationary fields from distant sources. Underground gravity gradients are calculated as acceleration of a free test mass inside a cavity. The results are discussed in the context of gravity-gradient noise subtraction in third generation gravitational-wave detectors. Error analysis with respect to the density of the simulated grid leads to a derivation of an improved seismometer placement inside a 3D array which would be used in practice to monitor the seismic field.Comment: 24 pages, 12 figure

Ultralong-Range Rydberg Molecules in a Divalent-Atomic System

We report the creation of ultralong-range Sr$_2$ molecules comprising one ground-state $5s^2$ $^1S_0$ atom and one atom in a $5sns$ $^3S_1$ Rydberg state for $n$ ranging from 29 to 36. Molecules are created in a trapped ultracold atomic gas using two-photon excitation near resonant with the $5s5p$ $^3P_1$ intermediate state, and their formation is detected through ground-state atom loss from the trap. The observed molecular binding energies are fit with the aid of first-order perturbation theory that utilizes a Fermi pseudopotential with effective $s$-wave and $p$-wave scattering lengths to describe the interaction between an excited Rydberg electron and a ground-state Sr atom.Comment: 5 pages, 2 figure

A Cross Section of Oscillator Dynamics

The goal of this research is to explore criteria sufficient to produce oscillations, sample some dynamical systems that oscillate, and investigate synchronization. A discussion on linear oscillators attempts to demonstrate why autonomous oscillators are inherently nonlinear in nature. After describing some criteria on second-order dynamics that ensure periodic orbits, we explore the dynamics of two second-order oscillators in both autonomous and periodically driven fashion. Finally, we investigate the phenomena of synchronization with the nonlinear phase-locked loop. Methods of analysis are exemplified as they become relevant including Poincaré; maps and the Zero-One test for chaos. The Poincaré-Bendixson theorem is used to demonstrate the existence of periodic orbits in R2 under extraordinarily general conditions. Liénard\u27s equation and theorem are introduced, which provide an intuitive parameterization for a class of oscillators. Liénard\u27s equation is a second- order, ordinary differential equation that characterizes an oscillator with respect a state dependent damping function and a restoring force function. Liénard\u27s theorem establishes sufficient criteria under which the Liénard\u27s equation has a unique, stable, limit cycle. The Duffing equation conforms with the Liénard equation, yet produces limit cycles without satisfying Liénard\u27s theorem. Our Duffing dynamics are explained in the context of a nonlinear spring model. We survey the parameter space, which form both pitchfork and hyperbolic potential wells with respect to the displacement. These two wells characterize the bifurcations between the four fundamental undamped dynamical modes. One interesting result is that chaotic trajectories of the Duffing equation are able to quickly shed light on a multitude of quasi-periodic trajectories at the boundaries of the Poincaré map. Next we introduce an oscillator that is similar to many engineered oscillators. The Van der Pol (VDP) oscillator model is presented in the context of a nonlinear current source in parallel with an inductor, a capacitor, and a resistor. It provides a net negative conductance destabilizing the equilibrium, and is tamed into global stability by increasing damping by the square of the voltage. The VDP oscillator is the opposite of the Duffing equation in that its nonlinearity is in the damping function, with a linear restoring force function. Like the VDP oscillator, many engineered oscillators are self-excited, autonomous systems that produce limit cycles. Finally, we investigate the process of synchronization with the phase-locked loop (PLL). Synchronization is a nonlinear process in which systems entrain their frequencies to external signals or other systems. Naturally occurring PLLs lie at the foundation of synchronization. We describe the basic topology of the PLL. Interestingly, the phase model introduced conforms with Liénard\u27s equation and is similar to the model used for the Josephson junction and the driven pendulum. Perhaps explaining the prevalence of synchronization, we show that almost any nonlinear functional can serve as a phase detector. We briefly demonstrate a phase-lock of two oscillators with phase- noise analysis. Finally, we report on the nonlinear behavior of the PLL when subjected to a modulated input

Rydberg dressed spin-1/2 Fermi gases in one dimension

The emergent phases of strongly correlated spin-1/2 Fermi gases of Rydberg dressed atoms in a one dimensional optical lattice are theoretically investigated. At weak coupling a bosonization description is used to demonstrate the ability to drive alternating quantum phase transitions between distinct Luttinger liquids. At strong coupling the ground state develops non-trivial phase separation exhibiting Luttinger liquid ''puddles'' separated by magnetic domain walls due to the interplay of the incommensurate filling and the Rydberg core length scale. These phases can be detected in ultracold gases of Rydberg atoms made from $^6$Li.Comment: 10 pages, 8 figure