Felix O. Saouma

Broadband studies of the strong mid-infrared nonlinear optical responses of KPSe6

One-dimensional noncentrosymmetric KPSe6 is an excellent mid-infrared (mid-IR) nonlinear optical (NLO) material possessing reversible phase-change behavior. In this special issue we present a broadband NLO study on its crystalline and glassy phases as well as fundamental material characterizations. The compound has a bandgap of ∼ 2.1 eV and exhibits intrinsic second harmonic generation even in the glassy phase. The crystalline compound is type-I phase-matchable and shows strong second- and third-order NLO responses over a broad wavelength range (1.0 – 3.0 μm) with excellent optical transparency. Based on the measured NLO coefficients χ(2) ≃ 142.8 pm/V and χ(3) ≃ 4.7 × 105 pm2/V2, we propose that KPSe6 can be utilized for numerous mid-IR NLO applications.

Mn2FeWO6 : A new Ni3TeO6-type polar and magnetic oxide.

Mn(2+)2 Fe(2+)W(6+)O6 , a new polar magnetic phase, adopts the corundum-derived Ni3TeO6 -type structure with large spontaneous polarization (PS) of 67.8 μC cm(-2), complex antiferromagnetic order below ≈75 K, and field-induced first-order transition to a ferrimagnetic phase below ≈30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.

Impact of two-photon absorption on second-harmonic generation in CdTe as probed by wavelength- dependent Z-scan nonlinear spectroscopy

We present a nonlinear optical (NLO) probe technique, wavelength-dependent Z-scan nonlinear spectroscopy (WDZNS), which can be utilized for assessing broadband NLO properties of materials. Unlike typical Z-scans, WDZNS can spectrally monitor the frequency-doubled output as a function of wavelength λ as well as input intensity I. Based on WDZNS we have investigated the strong impact of two-photon absorption (TPA) on second-harmonic generation in CdTe over a broad TPA range. This complicated NLO effect is characterized by the λ-dependent TPA coefficient, which is consistent with a simple two-band model. The relative second-order NLO dispersion derived from WDZNS is also consistent with previous measurements.

Multiphoton Absorption Order of CsPbBr3 As Determined by Wavelength-Dependent Nonlinear Optical Spectroscopy

CsPbBr3 is a direct-gap semiconductor where optical absorption takes place across the fundamental bandgap, but this all-inorganic halide perovskite typically exhibits above-bandgap emission when excited over an energy level, lying above the conduction-band minimum. We probe this bandgap anomaly using wavelength-dependent multiphoton absorption spectroscopy and find that the fundamental gap is strictly two-photon forbidden, rendering it three-photon absorption (3PA) active. Instead, two-photon absorption (2PA) commences when the two-photon energy is resonant with the optical gap, associated with the level causing the anomaly. We determine absolute nonlinear optical dispersion over this 3PA-2PA region, which can be explained by two-band models in terms of the optical gap. The polarization dependence of 3PA and 2PA is also measured and explained by the relevant selection rules. CsPbBr3 is highly luminescent under multiphoton absorption at room temperature with marked polarization and wavelength dependence at the 3PA-2PA crossover and therefore has potential for nonlinear optical applications.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

The effect of grain improvement on carrier lifetime in perovskite solar devices

The grain size of the organometallic halide perovskite, methylammonium lead iodide (MAPbI3) was modified through a solvent annealing technique. Dimethyl sulfoxide (DMSO) solvent was found to coalesce perovskite grains leading to improved carrier lifetimes and solar device performance. Samples with three ratios (1.0, 1.1, and 1.2) of PbI2/(CH3NH3I) were used for this study. SEM images of the film surface showed improvement in the grain size with the amount of the solvent used during the annealing. In addition, the number of pinholes was also reduced by solvent annealing. The increased grains directly lead to increased carrier lifetimes and the device efficiency. The optimal device efficiency was observed for the ratio 1.1 of PbI2/(CH3NH3I) and the solvent volume of 5 μL. As the solvent volume increased further, segregated regions of lead rich grains formed among the large grains, which in turn reduced the device efficiency.

Erratum: Mn2FeWO6: A New Ni3TeO6-Type Polar and Magnetic Oxide (Advanced Materials (2015) 27 (2177) DOI 10.1002/adma.201405244)

In this work, we present the high pressure and temperature (HPT) synthesis of Mn 2 FeWO 6 —a new magnetic and polar member of the A 2 BB′O 6 -type corundum derivative family, and the results of the crystal structure, SHG activity, magnetic phase diagram, and electrical/dielectric properties. First-principles calculations are also carried out to better understand its structure, polarization, complex magnetic properties, and to point to new materials design with multifunctional properties. Polycrystalline black Mn 2 FeWO 6 was prepared at 1673 K for 1 h at 8 GPa. Original lab powder X-ray diffraction (PXD, Figure S2, Supporting Information) indicates a rhombohedral cell ( a ≈ 5.3 Å, c ≈ 13.9 Å) of the main phase and a small wolframite impurity (Figure S3a,b and Table S1, Supporting Information). Figure 1 a presents the RT synchrotron powder X-ray diffraction (SPXD) patterns of Mn 2 FeWO 6 , which could correspond to any of the R 3 or R -3 structure types discussed above. However, the high-angle annular dark fi eld scanning transmission electron microscopy (HAADF-STEM) images (Figure 1 b and S4, Supporting Information) clearly show Fe/W ordering, and thus rule out IL prototype for Mn 2 FeWO 6 . Polar magnets are one of the most promising materials for multiferroics and spintronics. Recently, the A 2 BB′O 6 -type corundum derivatives have drawn much interest due to their remarkable magnetic and electronic properties with polar structures and multiferroic and second harmonic generation (SHG) behavior. [ 1–6 ] These structures are related to perovskite, but unlike perovskites, which have corner-sharing B site octahedra, in these phases, with unusually small A-site cations, both the A and B sites are octahedrally coordinated. The corner-, face-, and edge-sharing octahedra form a 3D lattice. [ 2,7 ] The A 2 BB′O 6 -type corundum derivatives can crystallize in the centrosymmetric ilmenite (IL, space group R -3), or in non-centrosymmetric LiNbO 3 (LN, R 3 c ) , or ordered ilmenite (OIL, R 3), or Ni 3 TeO 6 (NTO, R 3) structure, as shown in Figure S1 (Supporting Information). The stabilization of any one of these structures depends on the cation arrangement and their ordering/disordering degree, which are affected by the cationic size and charge differences, electron confi guration, and synthesis conditions. [ 1,8–11 ] The general structural features of A 2 BB′O 6 -type corundum derivatives with octahedral coordination of all metal sites favor the formation of practical magnetic and multiferroic/magnetoelectric materials by incorporation of magnetic transition metal ions. Recently, near room temperature (RT) ferrimagnetic (FiM) behavior of Mn 2 FeSbO 6 ( T C ≈ 270 K) [ 1 ] and antiferromagnetic (AFM) Ni 3 TeO 6 with nonhysteretic colossal magnetoelectricity [ 4 ] were reported. More recently, we reported