Optimization of attosecond pulse generation

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains.     

Extreme ultraviolet interferometry measurements with high-order harmonics

We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2-10(20)electrons/cm>(3) are measured.     

呋喃类化合物在离子液体和硅胶负载离子液体中的Diels-Alder反应研

以1-丁基3-甲基咪唑盐作为离子液体阳离子,与三种不同阴离子BF4、PF6、Tf2N组成性质不同的三种离子液体,催化呋喃类和丁炔二羧酸酯之间发生的Diels-Alder反应,合成了一系列化合物,研究了离子液体的催化效果。研究表明,呋喃类化合物的极性越低, Diels-Alder 反应的活性越高。在含硫的呋喃类化合物的反应中,当以 [Bmim]PF6 作为离子液体时,Diels-Alder 反应的产率可达到53%。另外,当 [Bmim]PF6 离子液体被吸附在硅胶表面上时,Diels-Alder的反应产率有所提高。     

Effect of doping on structural,optical and electrical properties of nanostructure ZnO films deposited onto a-Si:H/Si heterojunction

We investigated the structural; optical and electrical properties of ZnO thin films as the n-type semiconductor for silicon a-Si:H/Si heterojunction photodiodes. The ZnO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of doping on device performance. The results show that the X-ray diffraction (XRD) spectra revealed a preferred orientation of the crystallites along c-axis. SEM images show that all films display a granular, polycrystalline morphology and the ZnO:Al exhibits a better grain uniformity. The transmittance of the doped films was found to be higher when compared to undoped ZnO. A low resistivity of the order of 2.8 × 10⁻⁴ Ω cm is obtained for ZnO:Al using 0.4 M concentration of zinc acetate. The photoluminescence (PL) spectra exhibit a blue band with two peaks centered at 442 nm (2.80 eV) and 490 nm (2.53 eV). It is noted that after doping the ZnO films a shift of the band by 22 nm (0.15 eV) is recorded and a high luminescence occurs when using Al as a dopant. Dark I–V curves of ZnO/a-Si:H/Si structure showed large difference, which means there is a kind of barrier to current flow between ZnO and a-Si:H layer. Doping films was applied and the turn-on voltages are around 0.6 V. Under reverse bias, the current of the ZnO/a-Si:H/Si heterojunction is larger than that of ZnO:Al/a-Si:H/Si. The improvement with ZnO:Al is attributed to a higher number of generated carriers in the nanostructure (due to the higher transmittance and a higher luminescence) that increases the probability of collisions.     

Enzymatic deamination of the epigenetic base N-6-methyladenine

Two enzymes of unknown function from the amidohydrolase superfamily were discovered to catalyze the deamination of N-6-methyladenine to hypoxanthine and methyl amine. The methylation of adenine in bacterial DNA is a common modification for the protection of host DNA against restriction endonucleases. The enzyme from Bacillus halodurans, Bh0637, catalyzes the deamination of N-6-methyladenine with a k(cat) of 185 s(-1) and a k(cat)/K(m) of 2.5 × 10(6) M(-1) s(-1). Bh0637 catalyzes the deamination of N-6-methyladenine 2 orders of magnitude faster than adenine. A comparative model of Bh0637 was computed using the three-dimensional structure of Atu4426 (PDB code: 3NQB) as a structural template and computational docking was used to rationalize the preferential utilization of N-6-methyladenine over adenine. This is the first identification of an N-6-methyladenine deaminase (6-MAD).     

Convenient DABCO-catalyzed one-pot synthesis of multi-substituted pyrano[2,3-c]pyrazole dicarboxylates

We developed an efficient and simple one-pot synthesis of functionalized multi-substituted 2,4-dihydro-pyrano[2,3-c]pyrazole dicarboxylates from β-ketoesters, hydrazine, dimethyl acetylenedicarboxylate and malononitrile in EtOH. This four-component one-pot reaction carried out in the presence of DABCO catalyst showed advantages over a one-pot three-component method in its simple procedure, high yield and low toxicity.     

Controllable optical delay of wideband laser pulses by means of coherent effects

We report the first experimental observation of controllable temporal delay of wideband laser pulses (input spectral bandwidth of 3.3 GHz) using electromagnetically induced transparency. We obtain this result with limited temporal distortion of our pulses and excellent values of the delay-bandwidth product. Our experimental results are in agreement with a theoretical analysis.     

A tuneable, single-mode titanium-doped-sapphire laser source with variable pulse duration in the nanosecond regime

We built and characterized a high-energy, injection-seeded, single-longitudinal-mode pulsed titanium-doped-sapphire laser with tuneable wavelength in the near-infrared and variable pulse temporal duration in the nanosecond regime. We show experimentally how the pulse duration can be easily varied either by changing the cavity length or by changing the pump energy. We successfully interpreted these results on the basis of a theoretical model which treats the operation of this type of laser as a gain-switching technique. Also, as far as the stabilization of the laser cavity, compared to the traditional techniques we employed a novel simplified solution involving the use of an avalanche photodiode.     

The effect of a strong magnetic field on the binding energy and the photoionization process in quantum-well wires

Within the effective-mass approximation, we have investigated the influence of a strong magnetic field on the ground state binding energy and the photon energy dependence of the photoionization cross-section of a shallow donor impurity in a quasi-one-dimensional rectangular quantum wire with infinite and finite potential barriers, using a variational approach. It is found that the binding energy and the photoionization cross-section as a function of photon energy were drastically dependent on the sizes of the wire, the potential well heights and the applied magnetic field.