Super resolution imaging of genetically labeled synapses in drosophila brain tissue

Understanding synaptic connectivity and plasticity within brain circuits and their relationship to learning and behavior is a fundamental quest in neuroscience. Visualizing the fine details of synapses using optical microscopy remains however a major technical challenge. Super resolution microscopy opens the possibility to reveal molecular features of synapses beyond the diffraction limit. With direct stochastic optical reconstruction microscopy, dSTORM, we image synaptic proteins in the brain tissue of the fruit fly, Drosophila melanogaster. Super resolution imaging of brain tissue harbors difficulties due to light scattering and the density of signals. In order to reduce out of focus signal, we take advantage of the genetic tools available in the Drosophila and have fluorescently tagged synaptic proteins expressed in only a small number of neurons. These neurons form synapses within the calyx of the mushroom body, a distinct brain region involved in associative memory formation. Our results show that super resolution microscopy, in combination with genetically labeled synaptic proteins, is a powerful tool to investigate synapses in a quantitative fashion providing an entry point for studies on synaptic plasticity during learning and memory formation

Kinetic studies and modeling of nylon-6 solid-state polycondensation

A kinetic model for solid-state polycondensation of nylon-6 was developed by extrapolation of the melt chemistry to the amorphous phase of the solid polymer. In this way, a phenomenological description of the process was obtained that required examining al1 possible influences of the semicrystalline structure on the kinetics, equilibrium, and transport phenomena in the polymer granules. Particularly, dramatic apparent kinetics and equilibria changes were postulated due to the confinement of reactive species in a smaller reaction volume upon crystallization. In addition, the dimensional constraint applied by the micro-morphology on the intrinsic kinetics was also assessed. The effect of crystallinity on molecular transport was examined for both the migration of by-products at millimeter scale and the diffusion of reactive functionalities at nanometer scale. In the latter case, the occurrence of a gel effect in step growth polymers was questioned based on the implication of interchange reactions. A detailed reaction scheme was formulated that incorporated the presence of terminated polymer chains, which can result from the use of chain regulators or from degradation during the melt prepolyrnerization stage. Predictions were made for the characteristic quantities of the output product, such as molar mass, polydispersity index, concentration of end-groups, water content, extent of remonomerization, and formation of the cyclic dimer. A single particle model was validated by means of batch experiments carried out in a battery of fixed-bed reactors at laboratory scale. Both the kinetics of polymerization and the evolution of the morphology were tracked, based on techniques including viscometry, titration of functionalities, chromatography, X-ray diffraction, calorimetry, and polarized light microscopy. This led to a refined picture of the complex interplay between reaction and structure. Particularly, implications of the rigid amorphous phase, meso-scale spherulitic structures, polymer skinning, or of the presence of a crystallinity gradient within the polymer were underscored. In addition, the performance of the solid-state model was extensively tested with regard to literature data. A scale-up of the single particle model was performed to deal with the dynamic operation of an industrial moving-packed bed reactor. Simulations allowed the prediction of reactor start-up and shutdown, possible fluctuation in the feed composition, and on-line grade transition. Potential implications of the developed model as a tool for on-line reactor optimization and process control were discussed, based on a parametric sensitivity analysis

Passively mode-locked 40-GHz Er:Yb:glass laser

A diode-pumped Er:Yb:glass miniature laser has been passively mode-locked to generate transform-limited 4.3-ps pulses with a 40-GHz repetition rate and 18-mW average powe

New regime of inverse saturable absorption for self-stabilizing passively mode-locked lasers

The reflectivity of a semiconductor saturable absorber mirror (SESAM) is generally expected to increase with increasing pulse energy. However, for higher pulse energies the reflectivity can decrease again; we call this a ‘roll-over' of the nonlinear reflectivity curve caused by inverse saturable absorption. We show for several SESAMs that the measured roll-over is consistent with two-photon absorption only for short (femtosecond) pulses, while a stronger (yet unidentified) kind of nonlinear absorption is dominant for longer (picosecond) pulses. These inverse saturable absorption effects have important technological consequences, e.g. for the Q-switching dynamics of passively mode-locked lasers. A simple equation using only measurable SESAM parameters and including inverse saturable absorption is derived for the Q-switched mode-locking threshold. We present various data and discuss the sometimes detrimental effects of this roll-over for femtosecond high repetition rate lasers, as well as the potentially very useful consequences for passively mode-locked multi-GHz lasers. We also discuss strategies to enhance or reduce this induced absorption by using different SESAM designs or semiconductor material

Semiconductor saturable absorber mirror structures with low saturation fluence

We present two novel semiconductor saturable absorber mirror (SESAM) designs which can exhibit more than ten times lower saturation fluence than classical SESAM devices. Design considerations and characterization data are presented. These devices are particularly suited for passively mode-locked lasers with ultra-high repetition rate

Relative timing jitter measurements with an indirect phase comparison method

We propose and demonstrate experimentally a method for the sensitive measurement of the relative timing jitter of two mode-locked lasers, which can be either free-running or timing-synchronized to a common reference oscillator. The method is based on the indirect comparison of the phases of two photodetector outputs, using a microwave oscillator, the noise of which does not affect the results, electronic mixers, and a sampling oscilloscope. We carefully analyze and experimentally demonstrate the potential of this method. Compared to phase detector methods, it has a broader scope of applications and a lower sensitivity to intensity noise. We also obtained data on the coupling of intensity to timing noise in photodetector

Passively mode-locked diode-pumped Nd:YVO4 oscillator operating at ultra-low repetition rate

We demonstrate the operation of an ultra low repetition rate, high peak power, picosecond diode pumped Nd:YVO4 passively mode locked laser oscillator. Repetition rates even below 1 MHz were achieved with the use of a new design multiple-pass cavity and a semiconductor saturable absorber. Long term stable operation at 1.2 MHz, pulse duration of 16.3 ps and average output power of 470 mW corresponding to 24 KW peak power pulses is reported. This is, to our knowledge, the lowest repetition rate high peak power pulses ever generated directly from a picosecond laser resonator without cavity dumping

Influence of the 6^1S_0-6^3P_1 Resonance on Continuous Lyman-alpha Generation in Mercury

Continuous coherent radiation in the vacuum-ultraviolet at 122 nm (Lyman-alpha) can be generated using sum-frequency mixing of three fundamental laser beams in mercury vapour. One of the fundamental beams is at 254 nm wavelength, which is close to the 6^1S_0-6^3P_1 resonance in mercury. Experiments have been performed to investigate the effect of this one-photon resonance on phasematching, absorption and the nonlinear yield. The efficiency of continuous Lyman-alpha generation has been improved by a factor of 4.5.Comment: 8 pages, 7 figure

Passively Q-switched diode-pumped Cr4+:YAG/Nd3+:GdVO4 monolithic microchip laser

the realization of high repetition rate passively Q-switched monolithic microlaser is a challenge since a decade. To achieve this goal, we report here on the first passively Q-switched diode-pumped microchip laser based on the association of a Nd:GdVO4 crystal and a Cr4+:YAG saturable absorber. The monolithic design consists of 1 mm long 1% doped Nd:GdVO4 optically contacted on a 0.4 mm long Cr4+:YAG leading to a plano-plano cavity. A repetition rate as high as 85 kHz is achieved. The average output power is approximately 400 mW for 2.2 W of absorbed pump power and the pulse length is 1.1 ns