Ultrafast Optical Kerr Effect Research Articles

Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties.

We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetra-exponentials. The fastest decay (t1) is the same as pure water's at all concentrations within error, while the second component (t2) slows slightly with concentration. The slower components (t3 and t4), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperature-dependent studies (15-95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τcslow, the correlation time of the slow dynamics (weighted averages of t3 and t4). Remarkably, all four viscosity vs 1/τCslow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

Modification of local and collective dynamics of water in perchlorate solution, induced by pressure and concentration

The presence of ions induces perturbations in the water network, these structural and dynamic modifications can extent over space scales overcoming the local solvation shell: aqueous solutions of sodium perchlorate (NaClO4) are characterized by extended phenomena of structure breaking of the solvent network. The aim of the present work is the experimental investigation of the interplay between the local structural modifications induced by the perchlorate ions and the collective dynamical properties of the solvent. Ultrafast Optical Kerr Effect (OKE) and time resolved infrared absorption are the experimental techniques adopted: OKE is mostly sensitive to the collective properties of the sample, while transient IR provides access to local properties of the solvent. Classical Molecular Dynamics (MD) simulations support the analysis of the experimental results. All experiments and simulations are performed at room temperature, varying the concentrations (0–6 M) and varying the applied pressure (10−4–1.3 GPa). Experiments and computer simulations confirm that pressure and concentration have convergent effects on the water dynamics, due to the analogous modification of the short-range liquid structure that cancel some dynamical anomalies typical of pure water. Both local and collective dynamic observables point to structural properties as responsible for their peculiar pressure and concentration dependence.

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.

Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation.

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

Effect of Local Structure on Rotational Contributions in the Ultrafast Optical Kerr Effect in Pure Liquid Acetonitrile and Dimethyl Sulfoxide

The relationship between the structures of liquid acetonitrile (AN) and dimethyl sulfoxide (DMSO) and rotational contributions in the ultrafast optical Kerr effect is considered. It is shown that the structure of associates in the form of chains impedes the emergence of induced orientational anisotropy of molecules in liquid DMSO. In liquid AN, antiparallel dimers are capable of reorientation under the action of a pump laser field; both dimers and monomers contribute to the libration response.

Possibilities of Double-Pulse Femtosecond Polarization Spectroscopy Based on Registration of the Ultrafast Optical Kerr Effect

Spectroscopic data are presented for the motion of molecules in a liquid during double-pulse excitation and registration of the ultrafast optical Kerr effect (OKE). It is shown that selective spectroscopy of individual molecular motions can be achieved by choosing certain parameters of the double-pulse sequence.

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.

Analyzing the Signal of the Ultrafast Optical Kerr Effect by Considering the Correlation between the Rotational Responses of Molecules in a Liquid

The signal of the ultrafast optical Kerr effect (OKE) in liquids, including toluene C7H8, nitrobenzene C6H5NO2, and acetonitrile C2H3N is analyzed. It is shown that considering the correlation between the orientational and librational responses allows us to determine the rise time of the orientational response associated with the inertial nature of molecular rotations over a time interval of 300–500 fs after the impact of a laser pulse, and to increase the accuracy of determining the form of the Raman spectral density function.

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.

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.

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.

Selective Spectroscopy Based on the Multiple-Pulse Excitation of the Ultrafast Optical Kerr Effect

The use of multiple-pulse laser excitation to control molecular responses that produce the ultrafast optical Kerr effect in order to perform selective spectroscopy of the Raman-active motions of molecules in liquids is discussed. Theoretical analysis and experimental data indicate that multiple-pulse laser excitation is a powerful spectroscopic tool that allows unique information to be obtained about low-frequency molecular dynamics in a liquid.

Control of Molecular Dynamics in Benzonitrile and Femtosecond Spectroscopy of the Ultrafast Optical Kerr Effect

The decomposition of the recorded signal of the ultrafast optical Kerr effect (OKE) in benzonitrile into its components corresponding to different molecular contributions is considered. An optical scheme based on two-pulse nonresonant pumping with the registration of a test laser pulse is devised for this purpose.

Frustration vs Prenucleation: Understanding the Surprising Stability of Supersaturated Sodium Thiosulfate Solutions.

Gibbs classical nucleation theory predicts that a supersaturated solution will have transient nuclei that flitter in and out of existence. Only when one of these nuclei becomes larger than a critical size, will the solution crystalize. Recently, nonclassical nucleation theories have invoked the presence of prenuclei possibly associated with a liquid-liquid phase separation. However, there are few experimental observations of such prenuclei. Here, we use ultrafast optical Kerr-effect spectroscopy to measure the temperature-dependent low-frequency (sub-gigahertz to terahertz) anisotropic Raman spectra of supersaturated aqueous sodium thiosulfate solutions. Clear evidence of clusters is obtained in the spectra. However, on the basis of the inferred stability of these clusters, it appears that they frustrate rather than promote the formation of crystals. This would explain the surprising stability of supersaturated sodium thiosulfate and similar solutions.

Control of laser-induced nonadiabatic molecular alignment in dissipative media

Laser-induced nonadiabatic alignment in nitrogen and mixtures of nitrogen with Ar gas was investigated using the ultrafast optical Kerr effect in the collinear pump-probe setup. We find that dissipative media produce desired alignment characteristics in distinct manners to decoherence and to population relaxation. The results show that the intense laser alignment can be used as a control tool of molecular dynamics in dissipative environments.

Empirical Analysis of Optical Kerr Effect Spectra: A Case for Constraint.

Ultrafast optical Kerr effect (OKE) spectroscopy is a widely used method for studying the depolarized, Raman-active intermolecular dynamics of liquids. Through appropriate manipulation of OKE data, it is possible to determine the reduced spectral density (RSD), which is the Bose-Einstein-corrected, low-frequency Raman spectrum with the contribution of diffusive reorientation removed. OKE RSDs for van der Waals liquids can often be fit well to an empirical function that is the sum of a Bucaro-Litovitz function and an antisymmetrized Gaussian (AG). Although these functions are not directly representative of specific intermolecular dynamics, the AG fit parameters can provide useful insights into the microscopic properties of liquids. Here we show that fits using the AG function are typically not well-determined, and that equally good results can be obtained with a wide range of fitting parameters. We propose the use of a physically motivated constraint on the amplitude of the AG function, and demonstrate that this constraint leads to more intuitive trends in the fit parameters for temperature-dependent RSDs in 1,3,5-trifluorobenzene and hexafluorobenzene.

Resolving and Controlling Photoinduced Ultrafast Solvation in the Solid State.

Solid-state solvation (SSS) is a solid-state analogue of solvent-solute interactions in the liquid state. Although it could enable exceptionally fine control over the energetic properties of solid-state devices, its molecular mechanisms have remained largely unexplored. We use ultrafast transient absorption and optical Kerr effect spectroscopies to independently track and correlate both the excited-state dynamics of an organic emitter and the polarization anisotropy relaxation of a small polar dopant embedded in an amorphous polystyrene matrix. The results demonstrate that the dopants are able to rotationally reorient on ultrafast time scales following light-induced changes in the electronic configuration of the emitter, minimizing the system energy. The solid-state dopant-emitter dynamics are intrinsically analogous to liquid-state solvent-solute interactions. In addition, tuning the dopant/polymer pore ratio offers control over solvation dynamics by exploiting molecular-scale confinement of the dopants by the polymer matrix. Our findings will enable refined strategies for tuning optoelectronic material properties using SSS and offer new strategies to investigate mobility and disorder in heterogeneous solid and glassy materials.

Measurement of ultrafast optical Kerr effect of Ge–Sb–Se chalcogenide slab waveguides by the beam self-trapping technique

We present a reliable and original experimental technique based on the analysis of beam self-trapping to measure ultrafast optical nonlinearities in planar waveguides. The technique is applied to the characterization of Ge–Sb–Se chalcogenide films that allow Kerr induced self-focusing and soliton formation. Linear and nonlinear optical constants of three different chalcogenide waveguides are studied at 1200 and 1550 nm in femtosecond regime. Waveguide propagation loss and two photon absorption coefficients are determined by transmission analysis. Beam broadening and narrowing results are compared with simulations of the nonlinear Schrödinger equation solved by BPM method to deduce the Kerr n2 coefficients. Kerr optical nonlinearities obtained by our original technique compare favorably with the values obtained by Z-scan technique. Nonlinear refractive index as high as (69±11)×10−18m2∕W is measured in Ge12.5Sb25Se62.5 at 1200 nm with low nonlinear absorption and low propagation losses which reveals the great characteristics of our waveguides for ultrafast all optical switching and integrated photonic devices.

Phonon-like Hydrogen-Bond Modes in Protic Ionic Liquids

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra.

Dielectric Relaxation of the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethyl Sulfate: Microwave and Far-IR Properties.

Dielectric relaxation of the ionic liquid, 1-ethyl-3-methylimidazolium ethyl sulfate (EMI+ETS-), is studied using molecular dynamics (MD) simulations. The collective dynamics of polarization arising from cations and anions are examined. Characteristics of the rovibrational and translational components of polarization dynamics are analyzed to understand their respective roles in the microwave and terahertz regions of dielectric relaxation. The MD results are compared with the experimental low-frequency spectrum of EMI+ETS-, obtained via ultrafast optical Kerr effect (OKE) measurements.