Comparative density functional studies of pristine and doped bismuth ferrite polymorphs by GGA+U and meta-GGA SCAN+U.

We analyzed the effect of nonmagnetic dopants Al, Ga, Sc, and In at the Fe-site on the phase stability, structural, and electronic properties of different bismuth ferrite (BiFeO3) polymorphs in the framework of density functional theory with the Hubbard U correction (DFT+U). We started our consideration from the determination of the magnitude of the U parameter. First, we calculated the structural, electronic, and magnetic properties of the rhombohedral R3c-G phase of BiFeO3 within the generalized gradient approximation (GGA) and strongly constrained and appropriately normed (SCAN) meta-GGA for different values of the U. Next, we compared these results with those obtained within the parameter-free hybrid functional. After determining the optimal values of the Hubbard U parameter we analyzed the total energies between the selected BiFeO3 polymorphs without and with dopants within both GGA+U and SCAN+U. For all dopants the concentration was 12.5% which was close to their solubility limit in BiFeO3 under ambient conditions. We found that none of these dopants led to the structural phase transition. However, DFT+U calculations revealed that the doping of BiFeO3 with Al and Ga reduced the energy barrier between R3c-G and Cm-C phases whereas for Sc and In the energy difference between both phases increased. For the orthorhombic phases the considered dopants do not affect the energy barrier between them and the rhombohedral phase. In addition, the ferroelectric polarization does not change after replacing the Fe atom by the dopant for the all considered BiFeO3 polymorphs.

Mid-infrared, super-flat, supercontinuum generation covering the 2-5 µm spectral band using a fluoroindate fibre pumped with picosecond pulses.

Broadband, mid-infrared supercontinuum generation in a step-index fluoroindate fibre is reported. By using ~70-picosecond laser pulses at 2.02 µm, provided by an optical parametric generator, a wide spectrum with a cut-off wavelength at 5.25 µm and a 5-dB bandwidth covering the entire 2-5 µm spectral interval has been demonstrated for the first time. The behaviour of the supercontinuum was investigated by changing the peak power and the wavelength of the pump pulses. This allowed the optimal pumping conditions to be determined for the nonlinear medium that was used. The optical damage threshold for the fluoroindate fibre was experimentally found to be ~200 GW/cm2.

Simple all-PM-fiber laser mode-locked with a nonlinear loop mirror.

In this Letter, we present a figure-eight all-PM-fiber laser oscillator design with a nonlinear optical loop mirror as an artificial saturable absorber. Unlike previous constructions using the same mode-locking technique, our cavity is constructed entirely of polarization-maintaining (PM) fibers, making the oscillator more resistant to thermal and mechanical perturbations. Two simple and robust laser configurations that differ by the output coupling ratio (70% or 30%) are presented. The first configuration delivers high energy pulses of 3.5 nJ, and the second configuration delivers pulses of 1.6 nJ at a common repetition rate of 15 MHz. In either configuration, the pulsed operation is stable, and the laser operates in a single pulse train regime, even for pump powers approaching twice the power required for mode-locking. We have also observed that, at higher intracavity powers, stimulated Raman scattering plays a significant role.

High-power supercontinuum generation in a ZBLAN fiber with very efficient power distribution toward the mid-infrared.

We report high-power supercontinuum (SC) generation in a step-index fluorozirconate (ZBLAN) fiber with a zero-dispersion wavelength shifted to ~1.9 µm. Pumping the fluoride fiber with 2.75 W of power provided by a thulium-doped fiber amplifier, a continuous spectrum extending from ~0.85 to 4.2 µm with 2.24 W of average output power was achieved. Over 61% (1.37 W) of the total output power corresponds to wavelengths longer than 3 µm, which shows, to the best of our knowledge, the highest power conversion efficiency toward the mid-IR spectral band in relation to the output spectrum width. A linear SC power scalability up to 5.24 W, with a spectral band of ~0.9-4 µm, with repetition rate and pump power provided by a 1.55 µm fiber master-oscillator power amplifier system, is also demonstrated.

Generation of self-mode-locked resembling pulses in a fast gain-switched thulium-doped fiber laser.

We report on a generation of self-starting mode-locked resembling (MLR) pulses in an all-fiber, gain-switched Tm(3+)-doped fiber laser operating at 2 µm wavelength, which we believe to be the first demonstration of such an approach. The laser delivers 100% modulated MLR pulses within an envelope of ~30 ns gain-switched pulse at a repetition rate of 30 kHz. The maximum average output power is 0.4 W and the maximum peak-power of MLR pulses can be as high as 1.1 kW. The performance of the laser is described.

Mid-IR supercontinuum generation in a ZBLAN fiber pumped by a gain-switched mode-locked Tm-doped fiber laser and amplifier system.

We demonstrate a novel method of mid-infrared (mid-IR) supercontinuum (SC) generation with the use of a 2 µm gain-switched self-mode-locked thulium-doped fiber laser. SC radiation ranging from ~1.9 to 3.8 µm wavelength, generated in a single-mode ZBLAN fiber with a zero-dispersion wavelength (ZDW) shifted to ~1.9 µm, is reported. An average output power of 0.74 W with 0.27 W at wavelengths longer than 2.4 µm was measured. It is, to the best of our knowledge, the first report on such an approach to generate a mid-IR SC in optical fibers.

MicroRNA-146a negatively regulates the immunoregulatory activity of bone marrow stem cells by targeting prostaglandin E2 synthase-2.

The molecular mechanisms that regulate the immune function of bone marrow-derived mesenchymal stem cells (BMSCs) are not known. We have shown previously that freshly isolated BMSCs when induced to express neuronal stem cell markers lose immunoregulatory function when transferred into mice sensitized to develop experimental autoimmune encephalomyelitis. Recently, microRNAs (miRs) have been shown to be involved in the regulation of several immune responses in both innate and acquired immunity. We now show that among several differentially expressed miRs, miR-146a was strongly upregulated in neuronally differentiated when compared with miR-146a expression in freshly isolated BMSCs or control BMSCs cultured in parallel but in nondifferentiating medium. Inhibition of miR-146a with a selective antagomir restored the immunoregulatory activity of nBMSCs. We mapped miR-146a to its multiple predicted target mRNA transcripts and found that miR-146a was predicted to block PGE2 synthase (ptges-2). We then showed that Ptges-2 was directly targeted by miR-146a using a luciferase reporter assay. Furthermore, increased expression of miR-146a in BMSCs correlated with inhibition of PGE synthase-2 and inhibition of PGE2 release. Accordingly, inhibition of miR-146a restored synthesis of PGE2. These data support the conclusion that miR-146a plays a critical role in the control of the immunoregulatory potential of BMSCs.

Immunoregulatory function of bone marrow mesenchymal stem cells in EAE depends on their differentiation state and secretion of PGE2.

Bone marrow mesenchymal stem cells (BMSC)-induced amelioration of experimental autoimmune encephalomyelitis (EAE) was diminished with neuronal differentiantion of BMSC (nBMSC). BMSC secreted large amounts of PGE2, compared to nBMSC, which correlated with higher efficacy to EAE inhibition. EAE mice treated with PGE2 inhibitor, meloxicam showed decreased serum levels of PGE2 and in parallel decreased inhibitory effect on EAE course. In addition, high levels of PGE2 secretion correlated with high expression of indoleamine-2,3-dioxygenase (IDO). Meloxicam blocked IDO expression in BMSC transferred mice indicating functional relation between PGE2 and IDO induction. The current findings demonstrates PGE2 involvement in BMSC-induced inhibition of EAE and provides a mechanistic link between BMSC-derived PGE2 and IDO-dependent immunoregulation of this autoimmune condition.