Optical Kerr Effect Signal Research Articles

Anharmonic organic cation vibrations in the hybrid lead halide perovskite CH3NH3PbI3

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.

The ultrafast Kerr effect in anisotropic and dispersive media.

The ultrafast optical Kerr effect (OKE) is widely used to investigate the structural dynamics and interactions of liquids, solutions, and solids by observing their intrinsic nonlinear temporal responses through nearly collinear four-wave mixing. Non-degenerate mixing schemes allow for background free detection and can provide information on the interplay between a material's internal degrees of freedom. Here, we show a source of temporal dynamics in the OKE signal that is not reflective of the internal degrees of freedom but arises from a group index and momentum mismatch. It is observed in two-color experiments on condensed media with sizable spectral dispersion, a common property near an optical resonance. In particular, birefringence in crystalline solids is able to entirely change the character of the OKE signal via the off-diagonal tensor elements of the nonlinear susceptibility. We develop a detailed description of the phase-mismatched ultrafast OKE and show how to extract quantitative information on the spectrally resolved birefringence and group index from time-resolved experiments in one and two dimensions.

Dynamics in the BMIM PF6/acetonitrile mixtures observed by femtosecond optical Kerr effect and molecular dynamics simulations.

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for a mixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF6) ionic liquid and acetonitrile in the whole mole fractions range. The long delay line in our experimental setup allowed us to capture the complete reorientational dynamics of the ionic liquid. We have analysed the optical Kerr effect signal in the time and frequency domains with help of molecular dynamics simulations. In our approximation of the slow picosecond dynamics with a multi-exponential decay, we distinguish three relaxation times. The highest two are assigned to the reorientation of the cation and acetonitrile molecules that are in the vicinity of the imidazolium ring. The third one is recognized as originating from cation rotations and reorientation of acetonitrile molecules in the bulk or in the vicinity of the aliphatic chains of the cation. With help of the simulation we interpret the intermolecular band in the reduced spectral density, obtained from Kerr signal, as follows: its low-frequency side results from oscillations of one of the components in the cage formed by its neighbors, while the high-frequency side is attributed to the librations of the cation and acetonitrile molecule as well as the intermolecular oscillations of system components involved in specific interactions. We use this assignment and concentration dependence of the spectra obtained from velocity and angular velocity correlations to explain the mole fraction dependence of Kerr reduced spectral density.

Effects of heterodyne signals on femtosecond optical Kerr measurements

ABSTRACTThe polarization dependence of femtosecond optical Kerr effect (OKE) measurements in fused silica was investigated, which was effected greatly by the pump power. Calculations and experimental results showed that the polarization dependence of OKE signals was influenced by the optical heterodyne signals caused by the leakage through the crossed polarizers of the OKE setup. By adjusting the power ratio of the pump beam and probe beam, the polarization dependence of the OKE signals was able to be well controlled.

Giant nonlinear optical effects induced by nitrogen-vacancy centers in diamond crystals.

We investigate the effect of nitrogen-vacancy (NV) centers in single crystal diamond on nonlinear optical effects using 40 fs femtosecond laser pulses. The near-infrared femtosecond pulses allow us to study purely nonlinear optical effects, such as optical Kerr effect (OKE) and two-photon absorption (TPA), related to unique optical transitions by electronic structures with NV centers. It is found that both nonlinear optical effects are enhanced by the introduction of NV centers in the N + dose levels of 2.0×10 11 and 1.0×10 12 N +/cm 2. In particular, our data demonstrate that the OKE signal is strongly enhanced for the heavily implanted type-IIa diamond. We suggest that the strong enhancement of the OKE is possibly originated from cascading OKE, where the high-density NV centers effectively break the inversion symmetry near the surface region of diamond.

Nonlinear polarization optical response to entangled state of biexciton

We have investigated the nonlinear optical responses of biexcitons which show the polarization entanglement by using pump-probe spectroscopy. Measuring all polarization combinations of pump-probe signals and precise analysis using the idea of quantum tomography enable us to extract the specific property of the entanglement. The obtained results indicate the two kinds of the third-order nonlinear response, namely the photo-induced absorption and the optical Kerr effect. From the analysis with the optical Kerr effect signals, the polarization correlations of the optical responses are clearly derived.

Low-Frequency Molecular Responses in a Liquid upon Recording the Ultrafast Optical Kerr Effect

Low-frequency molecular rotations recorded in a region of 0–150 cm–1 with a high “signal/noise” ratio in the ultrafast optical Kerr effect (OKE), which is a third-order nonlinear optical response upon nonresonant excitation of a liquid by femtosecond laser pulses, are analyzed in detail. It is shown that the reduced Raman spectral density (RSD) derived from experimental data using the well-known deconvolution procedure is ambiguous due to the problem of separating the orientational and librational contributions to the total OKE signal. This fact significantly limits the reliability of information derived about molecular motions in a liquid. For the example of an ultrafast OKE in benzonitrile, it is shown that, in the range of 0–300 fs, rotational responses cannot be considered independent, and the application of a number of additional criteria resulting from the assumption of their correlation allows this ambiguity in deriving the RSD function from experimental data to be removed.

Influence of Europium(III) β-Diketonate Complex on Orientational Responses in Ultrafast Optical Kerr Effect

Experiments on recording the ultrafast optical Kerr effect (OKE) in solution of europium(III) β‑diketonate complexes in toluene at a concentration of 10–2 mol/L showed that the total OKE signal is not a superposition of two contributions, one of which is related to the vibrational–rotational dynamics of toluene molecules, while the other is due to the vibrational–rotational dynamics of the europium(III) complexes. This immediately indicates that the europium(III) complexes essentially change the structure of liquid toluene, which is reflected in the character of an orientational response in picosecond range.

Влияние beta-дикетонатного комплекса европия(III) на ориентационные отклики в сигнале сверхбыстрого оптического эффекта Керра-=SUP=-*-=/SUP=-

AbstractExperiments on recording the ultrafast optical Kerr effect (OKE) in solution of europium(III) β‑diketonate complexes in toluene at a concentration of 10^–2 mol/L showed that the total OKE signal is not a superposition of two contributions, one of which is related to the vibrational–rotational dynamics of toluene molecules, while the other is due to the vibrational–rotational dynamics of the europium(III) complexes. This immediately indicates that the europium(III) complexes essentially change the structure of liquid toluene, which is reflected in the character of an orientational response in picosecond range.

Низкочастотные молекулярные отклики в жидкости при регистрации сверхбыстрого оптического эффекта Керра

AbstractLow-frequency molecular rotations recorded in a region of 0–150 cm^–1 with a high “signal/noise” ratio in the ultrafast optical Kerr effect (OKE), which is a third-order nonlinear optical response upon nonresonant excitation of a liquid by femtosecond laser pulses, are analyzed in detail. It is shown that the reduced Raman spectral density (RSD) derived from experimental data using the well-known deconvolution procedure is ambiguous due to the problem of separating the orientational and librational contributions to the total OKE signal. This fact significantly limits the reliability of information derived about molecular motions in a liquid. For the example of an ultrafast OKE in benzonitrile, it is shown that, in the range of 0–300 fs, rotational responses cannot be considered independent, and the application of a number of additional criteria resulting from the assumption of their correlation allows this ambiguity in deriving the RSD function from experimental data to be removed.

Acoustically-controlled optical Kerr effect signals in bimetallic Au-Pt nanoparticles embedded in a TiO2 thin film

Nonlinear optical measurements for detecting acoustical waves were carried out in bimetallic Au-Pt nanoparticles embedded in a TiO2 thin solid film. The samples were prepared by a sol-gel method. A two-wave mixing technique was employed to explore the third order nonlinear optical properties exhibited by the nanocomposites. Interferometric measurements sensitive to acoustical perturbations were studied in the samples. The modification in the index of refraction of the nanocomposites induced by vectorial nanosecond optical irradiations was comparatively evaluated. Additionally, a single-beam transmittance experiment confirmed the absence of any important multi-photonic absorption process during our measurements. An optical Kerr effect was identified to be the main responsible effect for the third order optical nonlinearities induced by nanosecond pulses at 532nm wavelength. It is considered that advantages in acousto-optical transduction by nonlinear optical interactions can give rise to potential development of quantum information acquisition systems and high-speed sensors. Moreover, low-dimensional acousto-optical functions in nanostructured materials can be contemplated.

Femtosecond multipulse polarization spectroscopy of molecular vibrations and rotations in liquids

Simultaneous control over the vibrational and rotational molecular responses in ultrafast optical Kerr-effect signals in acetonitrile, toluene, and chloroform at room temperature is achieved by means of non-resonance excitation with two 30-fs orthogonal linearly polarized laser pulses. It is shown that the orientational response of the molecules can be suppressed and their vibrational response isolated by enhanced the delay between the exciting pulses and their relative intensity.

Cation effect in the ionic solution optical Kerr effect measurements: A simulation study

We carried out the modeling of the Optical Kerr effect (OKE) signals of several metallic chloride ionic solutions to study the microscopic origin of the cation effect on the OKE spectral features. The modeling was based on molecular dynamic simulation and the dipole-induced-dipole method for induced polarization calculation. Decent agreement was achieved between the simulation and the experiment. An extended projection method was adapted to decompose the OKE signals into the contributions from the reorientational and the collision-induced motions of the bulk and the shell water. Further analysis suggested that the different cation effects on the OKE measured relaxation time constant originate from their different water affinities. The weak water affinity of Na(+) causes the water in its first solvation shell to be only insignificantly perturbed in dynamics and frequently exchanges with water in bulk, which results in an negligible concentration dependence of the OKE time constant. The OKE time constants of Mg(2+) and Al(3+) have much stronger dependences on concentration due to their stronger water affinities, which create the more stable first solvation shells and slower water motion in the shell. Compared with Mg(2+), Al(3+) can more significantly retard the water motion outside of the shell, which causes an even stronger concentration dependence of the OKE time constant. Our study provided a microscopic picture on how the cation effect on the water dynamics is reflected in the OKE measurements.

Dynamics of RTILs: A comparative dielectric and OKE study

The dynamics of room-temperature ionic liquids (RTILs) were studied by investigating their dielectric relaxation (DR) and time-resolved optical Kerr-effect (OKE) spectra in the frequency range of ~10MHz to ~20THz. For the studied RTILs the OKE and DR spectra are dominated by a relaxation in the GHz region and extend to a relatively sharp band at around 10THz. Whilst the first feature is mainly associated with the structural relaxation of the fluid through ion rotation (α relaxation), the second indicates the short-time limit of intermolecular dynamics. The rather featureless intermediate region is mainly associated with intermolecular vibrations that are strongly coupled to hindered rotations. In contrast to other RTILs, imidazolium salts show an additional sub-α relaxation which dominates the OKE signal and is indicative of the breathing motion of rather long-lived cages.Mixed with polar solvents RTILs were found to retain their ionic liquid-like character up to relatively high levels of dilution, but with the overall dynamics considerably speeded up. Below RTIL mole fractions of ~0.2–0.4 these systems behave like conventional electrolyte solutions with more or less pronounced ion pairing.

Effect of Ion Pairing on the Solution Dynamics Investigated by the Simulations of the Optical Kerr Effect and the Dielectric Relaxation Spectra

The optical Kerr effect (OKE) and dielectric relaxation spectroscopy (DRS) signals of MgCl2 aqueous solutions were modeled based on the same group of molecular dynamics simulation trajectories. Plausible agreement between the simulations and the experiments allows us to analyze the microscopic origin of the different concentration dependences of relaxation times probed by these two techniques. Our simulations suggest that a significant amount of cations and anions associate in the solutions due to the ion pairing effect. These ion assemblies as well as the free cations form suspending clusters together with the water molecules in their cation hydration shells. The dynamics of water molecules in these clusters is significantly hindered, while that of other water molecules remains largely unaffected. The relaxation times measured by OKE and DRS have different concentration dependences because DRS probes only the rotational dynamics of water molecules outside of the cluster while OKE contains the information of dynamics of all of the water molecules in the solutions. Such findings provide us a microscopic picture on how the ion hydration affects the water dynamics in certain ionic solutions in which ion pairing plays an important role.

Study on the Third-order Nonlinear Optical Properties of bis(tetraethylammonium) bis(1,3-dithiole-2-thione-4,5-dithiolato)cadium

A dmit2- salt: bis(tetraethylammonium)bis(1,3-dithiole-2-thione-4,5-dithiolato)cadium (CADMIT) was synthesized. The Optical Kerr Effect (OKE) signal of its acetonitrile solution was measured by femtosecond optical Kerr gate technique. Using CS2 OKE signal as reference signal measured under identical conditions, the third-order optical nonlinear susceptibility, c(3), of the sample solution was obtained to be about 2.98 x 10-14 esu at the concentration of 1.57 x 10-3 M. The second-order hyperpolarizability of its molecular was estimated to be as large as 1.23 x 10-32 esu. Its response time was about 195 fs, which is believed to be the contribution from the delocalized electrons.

Low frequency response of methanol/acetone mixtures: Optical Kerr effect and molecular dynamics simulations

The low frequency spectra of methanol/acetone mixtures were obtained from the Fourier transform of the time resolved optical Kerr effect (OKE) signal. In order to further investigate the effect of the mole fraction of acetone on the low frequency response, we have performed molecular dynamics simulations and calculated the OKE response from the anisotropic polarizability correlation function. We analyze the low frequency response by decomposing it into molecular, induced and cross term. We suggest that the hydrogen bonding (HB) manifests itself at higher frequencies, which is mainly visible in the induced contribution to the low frequency response of methanol.

Ultrafast third-order optical nonlinearities in DMSO

The third-order optical nonlinearities of dimethyl sulfoxide (DMSO) are investigated using the optical Kerr effect (OKE) and Z-scan techniques with femtosecond pulses. We are able to fit the OKE signal theoretically by convolving the autocorrelation of the incident pulse intensity profile with the impulse responses of the samples, illustrating the instantaneous nonlinear response of DMSO. We verify that the purely electronic cloud distortion causes the observed signal for DMSO. The Z-scan technique is used to estimate the third-order susceptibility of DMSO, which is about a quarter of that for CS2 and mainly comes from the nonlinear refraction with an intensity of less than 140 GW/cm2.

Pulsed laser induced switching of birefringence in nematic phase of photochromic molecules

Fast and dynamic switching of liquid-crystalline photochromic system birefringence induced by pulsed laser has been observed. The system consisted of photochromic molecules of 4-heptyl-4′-methoxyazobenzene showing liquid-crystalline nematic state close to the room temperature. Experiment of dynamic birefringence switching was done in optical Kerr effect (OKE) set-up, where for the sample excitation picosecond pulsed laser was used. Simultaneously, HeNe laser was served as a probe beam source. Measurements were done for different voltages applied to the sample. Rise time constant was in the range of microseconds. Full reversibility of the OKE signal was observed.

Ultrafast third-order nonlinear optical properties of ZnPc(OBu)_6(NCS)/DMSO solution

The ultrafast third-order nonlinear optical properties of ZnPc(OBu)(6)(NCS)/dimethyl sulfoxide solution are investigated by optical Kerr effect (OKE) and pump-probe techniques with femtosecond pulses. Using CS(2) as the reference, the third-order susceptibility of the sample solution is obtained as about 1.40x10(-14) esu at a concentration of 3.00x10(-5) M. The second hyperpolarizability for the sample solution is deduced to be as large as 2.35x10(-31) esu. The dynamic characteristics of the nonlinear optical response and the origins of the OKE signal can be attributed to excited singlet state populations.