Hybrid organic-inorganic perovskites are semiconductors with disordered structures and remarkable properties for photovoltaic applications. Many theoretical investigations have attempted to obtain structural models of the high-temperature phases, but most of them are focused on the mobility of organic components and their implications in material properties. Herein we propose a set of geometric variables to evaluate the conformation of the inorganic framework at each phase of methylammonium lead iodide perovskite. We show that the analysis of these variables is required to ensure consistent structural models of the tetragonal phase. We explore the theoretical ingredients needed to achieve good models of this phase. Ab initio molecular dynamic simulation, under canonical ensemble at the experimental unit cell volume, leads to representative states of the phase. Under this scheme, PBE and van der Waals density functional approaches provide similar models of the tetragonal phase. We find that this perovskite has a highly mobile inorganic framework due to the thermal effect regardless of movement of the organic cations. Consequently, the electronic structure shows significant movements of the bands with large bandgap variations.FONDECYT of CONICYT-Chile 3150174 1150538 Ministerio de Economia y Competitividad of Spain - Comunidad de Madrid, Spain ENE2013-46624 P 2013/MAE-278

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We describe a non-interferometric ultrashort-pulse measurement technique based on frequency-resolved optical gating (FROG) with which pulses can be reconstructed directly, i.e. non-iteratively. Two different FROG spectrograms are measured, which represent the only information required to reconstruct the amplitudes and phases of two independent input pulses. The direct reconstruction method is demonstrated with a single-shot FROG setup used to obtain the spectrograms generated from two synchronized input pulses. To demonstrate and determine the reconstruction quality for complex pulses, a programmable pulse shaper is used to modify the pulses sourced from a Kerr-lens mode-locked Ti:sapphire oscillator.

Metal‐organic frameworks (MOFs) have emerged as candidate materials for nonlinear optics due to their enhanced optical and chemical stability in comparison with conventional organic crystals. However, producing large single crystals that support perfect phase matching conditions for frequency conversion is a long‐standing challenge due to the highly metastable conditions in which MOF crystals tend to self‐assemble in solution. By modulating the synthesis and growth conditions, this limitation is overcome to produce millimeter–sized Zn(3‐ptz)2 uniaxial MOF single crystals. Optimized MOF crystals with large birefringence in the visible Δn ≈ −0.3 and high transparency allow for the observation of strong second‐harmonic (SHG) and third‐harmonic generation (THG) signals for the first time, using femtosecond near‐infrared pump pulses. For conditions of type‐I SHG phase‐matching, the measured effective nonlinear coefficient of Zn(3‐ptz)2 is deff ≈ 0.10 pm V−1, the largest measured nonlinearity for MOF materials to date. The experiments quantitatively agree with first‐principles simulations based on the crystal lattice structure. The damage threshold is estimated on the order of 0.2 TW cm−2 for raw single crystals, which can be further increased with additional crystal engineering steps. The demonstration of efficient frequency up‐conversion of light with long‐range phase coherence establishes MOF single crystals as promising materials for nonlinear optical devices.