Alina Ionescu

High-peak power, passively Q-switched, composite, all-poly-crystalline ceramics Nd:YAG/Cr4+:YAG laser and generation of 532-nm green light

Output performances of passively Q-switched, composite Nd:YAG/Cr4+:YAG lasers that consisted of bonded, all-poly-crystalline ceramics Nd:YAG and Cr4+:YAG are reported. Laser pulses at 1.06 μm with 2.5-mJ energy and 1.9-MW peak power are obtained from a 1.1-at % Nd:YAG/Cr4+:YAG ceramics that was quasi-continuous-wave (quasi-CW) pumped with a diode laser. Single-pass frequency doubling with LiB3O5 (LBO) nonlinear crystal at room temperature yielded green laser pulses at 532 nm of 0.36-mJ energy and 0.3-MW peak power, with a conversion efficiency of 0.27.

Vortex activation energy in the AC magnetic response of superconducting YBa2Cu3O7 thin films with complex pinning structures

The vortex activation energy U AC in the AC magnetic response of superconductors exhibits a logarithmic variation with the screening current density J (regardless of the pinning structure details), and takes surprisingly high values in the vicinity of the DC irreversibility line, especially at low external DC magnetic fields, as often reported. This is essentially different from the behaviour of the vortex-creep activation energy at long relaxation time scales in DC magnetic measurements, and is not completely understood. We investigated the DC relaxation and the AC response for YBa2Cu3O7 films containing nanorods and nanoparticles, with the DC and AC fields oriented perpendicular to the film surface. It is shown that the large U AC values in the vicinity of the DC irreversibility line, where the critical-state-related AC signal occurs, are generated by a non-diffusive vortex motion during the AC cycle, with the mean vortex hopping length longer than the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is weak, and U AC mainly results from the strong influence of the pinning-enhanced viscous drag on the vortex hopping process. The logarithmic U AC(J) dependence is consistent with a high U AC.

AC magnetic response of superconducting YBa2Cu3O7/PrBa2Cu3O7 superlattices

Vortex activation energy UAC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [Y Ba2Cu3O7]n/[PrBa2Cu3O7]4 superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high UAC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced ...

THz-time domain spectroscopy of (001) ZnSe

Terahertz absorbance in (001) ZnSe has been experimentally studied using a femtosecond (fs) pulse train emitted from a self mode lock fiber laser. Preliminary results suggest three different regions for the phase and two different behaviors. The Drude model calculations of the complex conductivity suggest a collision frequency Δf around 0.06 THz.

A simple model of mobility transverse field dependence in MOS transistors: application to current unified analytical expressions

A new and simple model for the transverse field dependence of the channel carrier mobility is proposed and validated in bulk silicon n-MOSFETs. It is shown that the extraction of the associated parameters is fast and accurate, and the unified global current model presents good aptitudes for optimal parameter extraction.

Behavior of the Second Magnetization Peak in Self-nanostructured La 2– x Sr x CuO 4 Single Crystals

The occurrence of a second magnetization peak (SMP) on the dc magnetic hysteresis curves of superconducting single crystals with randomly distributed vortex pinning centers is quite common. However, the origin of this effect is still under debate. The investigation of the SMP in “self-nanostructured” (striped) La2x Sr x CuO4 single crystals can offer useful information about its nature. Optimally doped and overdoped specimens (x ≥ 0.15) exhibit an SMP in a large temperature interval. By decreasing x, with the external magnetic field oriented along the crystallographic c-axis, the SMP completely disappears in the doping domain of well-developed static charge and spin stripes (x ~ 1/8) and reappears for x ≤ 0.10. This behavior follows the instability of the quasi-ordered vortex solid (the Bragg vortex glass) in the presence of static stripe order (as revealed using small-angle neutron scattering experiments), which is confirmed by the determined temperature variation of the normalized vortex-creep activation energy . If the applied field is parallel to the (a, b) planes, the SMP occurs even for specimens with static stripes, accompanied by an elastic vortex creep –plastic creep crossover. The results support the scenario in which the SMP is generated by the pinning-induced disordering of the Bragg vortex glass in the dynamic conditions of dc magnetic measurements. According to this model, for a specimen without macroscopic inhomogeneities in the pinning distribution and exhibiting an SMP, no features related to an anomalous peak effect (close to the irreversibility line) or to a first-order vortex-lattice melting should appear on the dc magnetization curves or in the ac magnetic response . This leads to a simple vortex phase diagram for the investigated system.

Reactive spark plasma sintering of MgB2 in nitrogen atmosphere for the enhancement of the high-field critical current density

High density bulks (97%–99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N2) atmosphere for different heating rates (10, 20 and 100 °C min−1) and compared with reference samples processed in vacuum and Ar. N2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N2 with a heating rate of 100 °C min−1. At 2–35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

Porous and RF sputtering InP for portable THz-TDS in pharmaceutical and medical applications

We developed a technology for deposition of metal contacts/wires upon nanoporous InP thin film structures and RF sputtered InP films. Indium phosphide (InP) films were deposited onto glass substrate using RF magnetron sputtering by varying the substrate temperature (50-100°C), under constant argon pressure (6.3·10-3 Bar) and RF power (40-100 W).

Generation of high-peak power 532-nm green pulses from composite, all-ceramics, passively Q-switched Nd:YAG/Cr4+:YAG laser

Laser pulses at 1.06 μm with 2.5-mJ energy and 3.1-MW peak power have been obtained from a composite, all polycrystalline ceramics, passively Q-switched 1.1-at.% Nd:YAG/Cr4+:YAG laser that was quasi-continuous-wave pumped with diode lasers. Single-pass frequency doubling with LiB3O5 nonlinear crystal at room temperature yielded green laser pulses at 532 nm with energy of 0.36-mJ and 0.45-MW peak power; the infrared-to-green conversion efficiency was 0.27.

Study of supported phospholipid bilayers by THz-TDS

Terahertz Time-Domain Spectroscopy (THz-TDS) is a new technique in studying the conformational state of molecules. Cell membranes are important structures in the interaction with extra cellular entities. Their principal building blocks are lipids, amphiphilic molecules that spontaneously self-assemble when in contact with water. In this work we report the use of THz-TDS in transmission mode to examine the behavior of supported phospholipid bilayers (SPBs) within the frequency range of 0.2 THz to 3 THz. SPBs were obtained by vesicle adsorption method involving the spread of a suspension (50-100 μl) of small unilamellar vesicles (SUVs) or multilamellar vesicles (MLVs) dissolved in PBS (phosphate buffer solution) on a support of silicon wafers. Both SUVs and MLVs were obtained from dipalmitoyl phosphatidylcholine (DPPC) and lecithin by using the thin-film hydration method. Broadband THz pulses are generated and detected using photoconductive antennas optically excited by a femtosecond laser pulse emitted from a self-mode locked fiber laser at a wavelength of 780 nm with a pulse widths of 150 fs. THz-TDS was proven to be a useful method in studying SPBs and their hydration states. The absorption coefficient and refractive index of the samples were calculated from THz measurements data. The THz absorption spectra for different lipids in SPBs indicate specific absorption frequency lines. A difference in the magnitude of the refractive index was also observed due to the different structure of supported lipid bilayers. The THz spectrum of DPPC was obtained by using theoretical simulations and then the experimental and theoretical THz spectra were compared.