Excitonic enhancement of optical nonlinearities in perovskite CH₃NH₃PbCl₃ single crystals

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding band structure and excellent optical properties. Here, we determined the excitation wavelength dependences of the two-photon absorption coefficient and the Kerr-effect-induced nonlinear refractive index of CH₃NH₃PbCl₃ perovskite single crystals by means of the Z-scan method. From theoretical analysis, we found that the electron-hole interaction, so-called exciton effect, significantly enhances the nonlinear optical responses even for the interband transitions. This interaction explains the universal relation between the exciton reduced mass and the bandgap for lead halide perovskites

Large thermal expansion leads to negative thermo-optic coefficient of halide perovskite CH₃NH₃PbCl₃

Lead halide perovskites have emerged as new optoelectronic materials owing to their outstanding optical properties. There has been increased interest in their temperature-sensitive optical properties and new optical applications have been proposed thereby. Here, we report the origin of the unusual negative thermo-optic coefficient of the halide perovskite CH₃NH₃PbCl₃, i.e., a decrease in the refractive index by an increase in temperature. From the temperature dependences of the absorption spectrum and the lattice constant and using the Lorentz oscillator model, we conclude that the negative thermo-optic coefficient below the absorption edge is predominantly determined by the large thermal expansion coefficient inherent to this soft material system. This work demonstrates that the negative thermo-optic coefficient is a distinctive phenomenon reflecting the unique electronic and lattice properties of halide perovskites

High-order harmonic generation from hybrid organic–inorganic perovskite thin films

The generation of high-order harmonics from hybrid organic–inorganic perovskites (HOIPs) is demonstrated by the excitation with a strong mid-infrared laser pulse. We prepare three types of HOIP polycrystalline thin film samples by solution processes (MAPbX3; MA = CH3NH3+;X = I, Br, Cl). The high-order harmonics from the sample (MAPbBr3) are more than tenfold stronger than those from the well-studied GaSe crystal despite their comparable bandgap energies, implying that the stronger band-to-band transition of the HOIPs causes the higher yields

Magnetic properties of the geometrically frustrated S=1/2 antiferromagnets, La2LiMoO6 and Ba2YMoO6, with the B-site ordered double perovskite structure: Evidence for a Collective Spin Singlet Ground State

Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S = 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected. La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below 150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below 20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a Weiss temperature, \theta_c, which depends strongly on the temperature range of the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence for long range magnetic order to 2K. On the other hand oscillations develop below 20K in muSR indicating at least short range magnetic correlations. Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below 150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat capacity, neutron powder diffraction and muSR data show no evidence for long range order to 2K but a very broad maximum appears in the heat capacity. The 89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/T1. These results are discussed and compared with those from other isostructural S = 1/2 materials and those based on S = 3/2 and S = 1.Comment: Accepted for publication in Phys. Rev.

Near-Band-Edge Optical Responses of CH3NH3PbCl3 Single Crystals: Photon Recycling of Excitonic Luminescence

新しい青色発光材料の出現 --有機無機ハイブリッドペロブスカイト半導体の光学特性を解明--. 京都大学プレスリリース. 2018-02-05.The determination of the band gap and exciton energies of lead halide perovskites is very important from the viewpoint of fundamental physics and photonic device applications. By using photoluminescence excitation (PLE) spectra, we reveal the optical properties of CH3NH3PbCl3 single crystals in the near-band-edge energy regime. The one-photon PLE spectrum exhibits the 1s exciton peak at 3.11 eV. On the contrary, the two-photon PLE spectrum exhibits no peak structure. This indicates photon recycling of excitonic luminescence. By analyzing the spatial distribution of the excitons and photon recycling, we obtain 3.15 eV for the band gap energy and 41 meV for the exciton binding energy

Large negative thermo-optic coefficients of a lead halide perovskite

負の屈折率温度係数を示す新しい半導体を発見 --ハロゲン化金属ペロブスカイトを用いた光学温度補償に成功--. 京都大学プレスリリース. 2019-07-22.Lead halide perovskites are promising semiconductors for high-performance photonic devices. Because the refractive index determines the optimal design and performance limit of the semiconductor devices, the refractive index and its change upon external modulations are the most critical properties for advanced photonic applications. Here, we report that the refractive index of halide perovskite CH3NH3PbCl3 shows a distinct decrease with increasing temperature, i.e., a large negative thermo-optic coefficient, which is opposite to those of conventional inorganic semiconductors. By using this negative coefficient, we demonstrate the compensation of thermally induced optical phase shifts occurring in conventional semiconductors. Furthermore, we observe a large and slow refractive index change in CH3NH3PbCl3 during photoirradiation and clarify its origin to be a very low thermal conductivity supported by theoretical analysis. The giant thermo-optic response of CH3NH3PbCl3 facilitates efficient phase modulation of visible light

Impact of spin-orbit splitting on two-photon absorption spectra in a halide perovskite single crystal

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding optical properties. Here, we report the two-photon absorption (TPA) spectra for CH₃NH₃PbBr₃ perovskite single crystals under linearly and circularly polarized laser pulses. We experimentally determined the spin-orbit splitting energy from the TPA linear-circular dichroism spectrum and found the higher-energy band around 0.8 eV above the band edge. From a theoretical analysis of the experimental data, we evaluated the exciton binding energy and the exciton reduced mass. Our findings provide essential information on the electronic structures and carrier dynamics of halide perovskites

High-Temperature Spin Crossover Behavior in a Nitrogen-Rich FeIII Based System

A nitrogen-rich ligand bis(1H-tetrazol-5-yl)amine (H3bta) was employed to isolate a new FeIII complex, Na2NH4[FeIII(Hbta)3]*3DMF*2H2O (1). Single crystal X-ray diffraction revealed that complex 1 consists of FeIII ions in an octahedral environment where each metal ion is coordinated by three Hbta2− ligands forming the [FeIII(Hbta)3]3− core. Each unit is linked to two one-dimensional (1-D) Na+/solvent chains creating a two-dimensional (2-D) network. In addition, the presence of multiple hydrogen bonds in all directions between ammonium cation and ligands of different [FeIII(Hbta)3]3− units generates a three-dimensional (3-D) network. Magnetic measurements confirmed that the FeIII center undergoes a Spin Crossover (SCO) at high temperature (T1/2 = 460(10) K)