Folding Dynamics of DNA G-Quadruplexes Probed by Millisecond Temperature Jump Circular Dichroism.

G-quadruplexes play important roles in cellular regulatory functions, but despite significant experimental and theoretical efforts, their folding mechanisms remain poorly understood. In this context, we developed a T-jump experiment to access the thermal denaturation and renaturation dynamics of short intramolecular G-quadruplexes in vitro, on the time scale of a few hundred milliseconds. With this new setup, we compared the thermal denaturation and renaturation kinetics of three antiparallel topologies made of the human telomeric sequences d[(5'-GGG(TTAGGG)3-3']/Na+ and d[5'-AGGG(TTAGGG)3-3']/Na+ and the thrombin-binding aptamer sequence d[5'-GGTTGGTGTGGTTGG-3']/K+, with those of the parallel topology made of the human CEB25 minisatellite d[5'-AAGGGTGGGTGTAAGTGTGGGTGGGT-3']/Na+. In all cases, exponential kinetics of the order of several hundred milliseconds were observed. Measurements performed for different initial temperatures revealed distinct denaturation and renaturation dynamics, ruling out a simple two-state mechanism. The parallel topology, in which all guanines adopt an anti conformation, displays much slower dynamics than antiparallel topologies associated with very low activation barriers. This behavior can be explained by the constrained conformational space due to the presence of the single-base propeller loops that likely hinders the movement of the coiled DNA strand and reduces the contribution of the entropy during the renaturation process at high temperatures.

Quantum calculation of the second-order hyperpolarizability of chiral molecules in the "one-electron" model.

Quantum calculation of the hyperpolarizabilty tensor is carried out for chiral molecules displaying a "one-electron" chirality. Calculation is made possible by introducing a chiral perturbation term in the potential energy surface. We show that a one-electron chiral molecule is intrinsically nonlinear and diplays a nonzero electric chiral hyperpolarizability. Existence of magnetic contributions is discussed, and it is shown that higher-order perturbation terms are necessary to introduce such magnetic effects in the second-order hyperpolarizability.

Pulse duration and wavelength effects on the optical limiting behavior of carbon nanotube suspensions.

We investigate pulse duration and wavelength effects on the optical limiting behavior of single-wall carbon nanotubes suspended either in chloroform or in water. The principal optical limiting effect in carbon nanotube suspensions is nonlinear scattering that is due to heat transfer from particles to solvent, leading to solvent-bubble formation and to sublimation of carbon nanotubes. We report on nonlinear transmittance measurements for pulse durations ranging from 3 to 100 ns and for wavelengths from 430 to 1064 nm. The dependence of optical limiting behavior on pulse duration and wavelength is analyzed and discussed in terms of nonlinear mechanisms.

Experimental evidence of third-order nonlinear dichroism in a liquid of chiral molecules

We present experimental evidence of the existence of a nonlinear circular dichroism in a liquid of chiral molecules. This effect is demonstrated by a light-induced circular dichroism on two enantiomeric RuTB [ruthenium(II) tris(bipyridyl)] samples after a careful analysis of the impact of the third-order nonlinear response on the optical activity of the system. These experimental results are in good agreement with a previous theoretical investigation.

Motion of hydrogen bonds in diluted HDO/D2O solutions: direct probing with 150 fs resolution

An experiment is described to study temporal variations of the hydrogen bond length in diluted HDO/D2O solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.5 microns spectral region at a 10 microJ power level is detailed next. The OH stretching band is reproduced for different excitation frequencies and different pump-probe delay times. A theory, based on statistical mechanics of nonlinear optical processes, is proposed to calculate the lowest two spectral moments. An effect is reported, the delay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the OH...O bond length directly, in real time. The corresponding time scales are of the order of 700 fs. No bond oscillations are observed.

Generation of intense highly coherent femtosecond pulses in the mid infrared.

Highly coherent mid-infrared femtosecond pulses, tunable between 2.5 and 4.4 mum, and with an average energy and duration between 2.6 and 3.6 microm of 11 microJ and 200fs, respectively, have been produced by an optical parametric amplifier setup driven at 1kHz by a 400- microJ , 800-nm pulse from a Ti:sapphire amplifier. In this system, first tunable moderate-energy femtosecond pulses in the near infrared are produced by continuum amplification in beta -barium borate, and subsequently the near-infrared pulses are amplified and frequency mixed with 800-nm radiation in potassium titanyl phosphate to produce intense tunable mid-infrared pulses. The time-bandwidth product of the mid-infrared pulses is 0.28+/-0.02 over the whole high-energy tuning range. Experimental results are compared with numerical simulations and a simple model.

Second-harmonic generation under phase-velocity and group-velocity mismatch:influence of cascading self-phase and cross-phase modulation.

Temporal and spectral characteristics of pulses resulting from second-harmonic generation of 120-fs amplified Ti:sapphire laser pulses up to 0.1 mJ at a wavelength of 815 nm in type I KDP crystal were experimentally and theoretically analyzed. Widely different behaviors were observed, according to the sign of the phase mismatch. Comparison between the theoretical simulation and experimental data demonstrates that the competition between third- and second-order nonlinear phenomena strongly modifies the structure of the pulses generated.

Measurement of ultrashort pulse durations by interferometric autocorrelation: influence of various parameters.

The interferometric autocorrelation technique for the measurement of ultrashort pulse durations is studied in detail. Effects of group velocity mismatch, group velocity dispersion, fundamental depletion, and pulse shape are carefully examined. A simple semianalytical calculation is developed that takes group velocity mismatch into account that can be used to predict the validity of this technique with real experimental parameters. A more complete calculation is also presented to analyze the effects of fundamental depletion or phase mismatch. Finally, the influence of the pulse shape is considered and a simple experimental procedure is proposed to determine whether a pulse is transform limited.

Cascaded second-order effects in the femtosecond regime in beta-barium borate: self-compression in a visible femtosecond optical parametric oscillator.

Cascaded second-order effects are studied in the femtosecond regime in beta-barium borate by use of the Z-scan technique. Large nonlinear phase shifts can be obtained near the second-harmonic-generation phasematching condition for a Ti:sapphire laser. Solution of the nonlinear propagation equations for femtosecond pulses yields good agreement with experiment and also demonstrates that the description of cascaded effects by an effective nonlinear refractive index is no longer valid in the ultrafast domain and that these effects are less efficient for ultrashort pulses. The cascade-induced negative nonlinear phase shift in beta-barium borate is shown to be responsible for the self-compression observed in a prismless femtosecond visible optical parametric oscillator.

Sub-20-fs tunable pulses in the visible from an 82-MHz optical parametric oscillator.

We have produced pulses tunable in the 590-666-nm range, with durations down to 13 fs, using an 82-MHz Ti:sapphire second-harmonic-pumped, high-bandwidth, beta-barium borate optical parametric oscillator in a fused-silica prism group-delay-dispersion-compensated, six-mirror folded ring cavity.

Subsecond grating growth in a photorefractive polymer.

We survey the dynamics of the photorefractive effect in a methyl methacrylate copolymer with the nonlinear chromophore p-nitroaniline in a pendant side group doped with a charge-transport agent, diethylaminobenzaldehyde diphenylhydrazone, a material that represents a new class of photorefractive polymer. The grating growth times are several orders of magnitude smaller than that for the previous epoxy-based photorefractive polymers and fall below 1 s at the highest intensities used. Grating competition and revelation effects suggest that charge carriers other than photogenerated holes are mobile. A sublinear dependence of growth rate on writing intensity implies that shallow traps may also be present.