Temperature Dependence Of Second Harmonic Generation Research Articles

Homochiral Multiferroic Cyanido-Bridged Dimetallic Complexes Assembled by C-F⋅⋅⋅K Interactions.

Cyanido-bridged dimetallic complexes are attracting attention due to their varied structures and properties. However, homochiral cyanido-bridged dimetallic complexes are rare, and making them ferroelectric is a great challenge. Introducing C-F⋅⋅⋅K interactions between the guest chiral cations and the host [KFe(CN)6 ]2- framework, gives three-dimensional cyanido-bridged dimetallic multiferroics, [R- and S-3-fluoropyrrolidinium]2 [KFe(CN)6 ] (R- and S-3-FPC). The mirror-symmetric vibrational circular dichroism (VCD) signal shows their enantiomeric nature. R- and S-3-FPC crystallize in the same chiral-polar space group P21 at 298 K. Piezoresponse force microscopy (PFM), polarizing optical microscopy, and temperature-dependent second-harmonic generation (SHG) measurements show their multiferroic properties (the coexistence of ferroelectricity and ferroelasticity), in line with the Aizu notation of 222F2. R-3-FPC shows excellent ferroelectricity with saturated polarization up to 9.4 μC cm-2 .

Temperature-dependent second-harmonic generation from color centers in diamond.

Under infrared ultrashort pulse laser stimulation, we investigate temperature-dependent second-harmonic generation (SHG) from nitrogen-vacancy (NV)-introduced bulk diamond. The SHG intensity decreases in the temperature range of 20-300°C, due to phase mismatching caused by refractive index modification. We discover that optical phonon scattering outperforms acoustic phonon scattering in NV diamond by fitting the temperature dependence of the SHG intensity using a model based on the bandgap change via the deformation potential interaction. This study presents an efficient and viable way for creating diamond-based nonlinear optical temperature sensing.

Chiral Switchable Low-Dimensional Perovskite Ferroelectrics.

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA]2CdCl4 (R-MPA+ = (R)-(-)-1-methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl2 in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P-E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-Tc, large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.

Tunable Ferroelectricity in Ruddlesden-Popper Halide Perovskites.

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)2(MA) n̅-1Pb n̅I3 n̅+1. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics.

An Order–Disorder Ferroelectric Host–Guest Inclusion Compound

The host-guest complex [(DIPA)([18]crown-6)](ClO4) (1; DIPA=2,6-diisopropylanilinium) was constructed and found to undergo a sequence of phase transitions (Ibam-Pbcn-Pna21) at T1=278 K and T2=132 K, respectively. Systematic characterizations, such as differential scanning calorimetry, heat capacity, temperature-dependent dielectric constant, and P-E hysteresis loop, reveal that the centrosymmetric-to-polar phase transition at T2 is a paraelectric-to-ferroelectric transition. The symmetry breaking was also confirmed by temperature-dependent second-harmonic generation effect and X-ray powder diffraction. The ferroelectric mechanism is attributable to the linear motion of the perchlorate counterions accompanied by the order-disorder transition of the [18]crown-6 molecules and the anions.

Temperature dependence of the spectral profile and total intensity of the second-harmonic signal of the fullerene crystal

The temperature dependence of second-harmonic generation in crystalline fullerene is simulated based on a microscopic model of orientational disorder, with special emphasis on the vicinity of the phase transition. The temperature-induced changes of the total second-harmonic intensity and of the corresponding spectral profile are treated on equal footing within a consistent theoretical framework. In addition to the previously applied approach where superexchange-type exciton transfer via the charge transfer (CT) manifold was described phenomenologically, and alternative model invoking the CT states explicitly is proposed, giving deeper insight into the underlying physics. Both models yield very good agreement with experimental results.

Surface-state relaxation dynamics on Ag(110) probed by temperature-dependent resonantly enhanced second-harmonic generation

The temperature dependence of second-harmonic generation resonantly enhanced by a surface-state transition, at the fundamental light wavelength of 712 nm, on Ag(110), was measured for six wavelengths within the surface-state transition band. A model based on a quantum-mechanical treatment of the second-order susceptibility and incorporating the occupied-surface-state energy shift with temperature was used to describe quantitatively the temperature and wavelength dependences in the resonantly enhanced second-harmonic intensity. The interpretation of the experimental data by the model requires only one variable, the homogeneous linewidth of the surface-state transition, which is determined through the fittings of the data to be 38 meV at 100 K. This transition linewidth is dominated by the homogeneous width of the unoccupied surface state. The temperature-dependent contribution toward the total transition width can be identified as electron-phonon coupling, ${\ensuremath{\Gamma}}_{e\ensuremath{-}\mathrm{ph}}=[(0.072\ifmmode\pm\else\textpm\fi{}0.014)\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{K}]\ifmmode\times\else\texttimes\fi{}T(\mathrm{K}),$ while the temperature-independent part is from scattering by defects and other carriers: ${\ensuremath{\Gamma}}_{\mathrm{imp}}+{\ensuremath{\Gamma}}_{e\ensuremath{-}e}=31\ifmmode\pm\else\textpm\fi{}2.0\mathrm{meV}.$ In addition, the electron-phonon mass-enhancement parameter, averaged over the Fermi surface, is deduced from the temperature-dependent linewidth to be $\ensuremath{\lambda}=0.13\ifmmode\pm\else\textpm\fi{}0.02.$

Second Harmonic Generation of Light in Doped Ferroelectric Polymer Films

Second-order nonlinear optical properties in polymeric system of the "host-guest" type are investigated. The ferroelectric copolymers of vinylidene fluoride with trifluoroethylene P(VDF/TrFE) are used as the polymeric matrix. P(VDF/TrFE) were doped by donor-acceptor molecules of Dispersion Orange Dye 3 (DO3). The second harmonic generation (SHG) is caused by the nonlinear polarizability of conjugated molecules containing donor and acceptor para-substituents. The temperature dependence of SHG in the region of phase transition from ferroelectric to paraelectric phase of P(VDF/TrFE) has been investigated. A manifestation of ferroelectric behaviour in the SHG in doped copolymer has been shown.

Two-dimensional ferroelectricity and phase transitions in PVDF Langmuir-Blodgett films probed by second harmonic generation

Ultrathin electrode-free ferroelectric Langmuir-Blodgett (LB) films of polyvinylidene fluoride (PVDF) with trifluoroethylene are experimentally studied by means of optical second harmonic generation (SHG) in a temperature range from 140 K to 380 K. The temperature behavior of spontaneous polarization induced SHG reveals distinct features in the vicinity of 355 K and 295 K which can be attributed to ferroelectric-paraelectric phase transitions in the bulk and at the surface of this two-dimensional structures. Lattice-stiffening transition recently observed in PVDF LB films at 160 K does not show remarkable peculiarities of temperature dependence of SHG that indicates nonpolar changes in LB film structure under this transition.

Temperature dependence of second-harmonic generation in tellurium with a CO2 laser at 10.6 μ

Temperature dependence of second-harmonic intensity in tellurium with a Q-switched CO2 laser at 10.6 μ was measured in the range 290–370 K. With increasing temperature the SHG intensity decreased rapidly, mainly due to the absorption loss of the fundamental wave. At room temperature the SHG efficiency of the order of 10−3 was achieved and d11 was estimated to be 1.7 × 10−6 esu.