Yi-Ci Li

Solute migration caused by excited state absorptions

Using the Z-scan technique, we find that migration of chloroaluminum phthalocyanine in liquid ethanol can be induced by the absorption of a 19 ps laser pulse with energy exceeding a threshold but not by that of a 2.8 ns pulse depositing more energy at the solute molecules. Considering each solute molecule as an oscillator confined within a potential well, we explain, in accordance with the five-energy-band model, that solute molecules excited by a 19 ps pulse retain more translational excess energy to overcome the potential well barrier compared with those excited by a 2.8 ns pulse of equal energy. Therefore, they are more likely to migrate out of the laser beam center, weakening the solutions absorption that we detect in the Z-scan measurements. Furthermore, we theoretically infer that the 19 ps pulse-induced solute migration tends to be nonquasistatic and experimentally verify that it cannot be attributed to the Soret effect, a quasistatic process.

Control of Thermal Lensing Effect in Transparent Liquids by Femtosecond Laser Pulses

The thermal lensing effect in transparent (linear and nonlinear) molecular liquids can be modulated by disrupting continuously output 82 MHz 28 fs laser pulses at 800 nm to form trains of various widths (τt) with respect to the thermal diffusivity time τth (~2 ms). We present nonlinear refraction results for CHBr3 (bromoform) obtained by the Z-scan technique. The results show that the thermal lensing effect increases with τt when τt is less than τth, but becomes steady when τt exceeds τth and reaches 30 ms. The proposed technique of modulating the thermal lensing effect by varying τt has great application potential in information photonics and optoelectronic devices, such as variable optical attenuators, holographic recording media, optical limiters and optical switches.

Kramers-Kronig relation between nonlinear absorption and refraction of C(60) and C(70).

Using the Z-scan technique with 532 nm 16 picosecond laser pulses, we observe reverse saturable absorption and positive nonlinear refraction of toluene solutions of both C(60) and C(70). By deducting the positive Kerr nonlinear refraction of the solvent, we notice that the solute molecules contribute to nonlinear refraction of opposite signs: positive for C(60) and negative for C(70). Attributing nonlinear absorption and refraction of both solutes to cascading one-photon excitations, we illustrate that they satisfy the Kramers-Kronig relation. Accordingly, we attest the signs and magnitudes of nonlinear refraction for both solutes at 532 nm by Kramers-Kronig transform of the corresponding nonlinear absorption spectra.

Short-pulse-induced solute migration in the C49H43ClO6 + 1,2 dichloroethane solution

Using the Z-scan technique with 532 nm 19 ps laser pulses separated by two time intervals τp-ps (0.1 s and 1.0 s) sandwiching the mass diffusion time constant of the C49H43ClO6 + 1,2 dichloroethane solution, we investigate short-pulse-induced solute migration in the sample by measuring its transmittance change with τp-p variation. Preparing the sample at two concentrations, we find that τp-p reduction, from 1.0 s to 0.1 s, increases its transmittance when input pulse energy ε1 exceeds a threshold εT, which is lower for the dilute solution than the concentrated one. At two ε1s above εT for the dilute solution, τp-p-reduction-induced transmittance increase in the dilute solution, as compared to that in the concentrated solution, is more at the lower ε1 and less at the higher ε1. This differs from continuous-wave-driven thermal diffusion which always causes a larger transmittance increase in the concentrated solution by inducing a larger temperature gradient. From this study, we predict that solute migration induced by short pulses at 1064 nm is one of the undesired heating effects occurring when this solution is used to simultaneously Q-switch and mode-lock Nd:YAG lasers.

Ultrashort-laser-pulse-induced thermal lensing effect in pure H2O and a NaCl–H2O solution

Using the Z-scan technique with 82 MHz 18 femtosecond (fs) laser pulses at 820 nm, we explore the thermal lensing effect induced in pure H2O and a NaCl–H2O solution. We verify that linear absorption dominates over both two-photon absorption and stimulated light scattering (SLS) in heating of pure H2O. This differs from the situation wherein SLS dominates heating of CS2. In addition, when dissolution of NaCl into H2O at a concentration of 1 M incorporates thermal and mass diffusions into the mechanisms of the thermal lensing effect, we find that this effect is enhanced and thus determine that the sign of the Soret coefficient of NaCl is positive. Notably, use of 820 nm 18 fs laser pulses in this study, in contrast to visible continuous light commonly used in the thermal diffusion forced Rayleigh scattering technique embedded with the optical heterodyne detection scheme, eliminates the need to add inert dyes into NaCl–H2O to enhance the absorption. This avoids the artefact caused by the dyes.

Characterizing the denatured state ensemble of ubiquitin under native conditions using replica exchange molecular dynamics

The folding of peptides into three-dimensional structures has been correlated to the occurrence of biological phenomena. In order to fully understand the pathway of protein folding, the structure of the denatured state, the reference state, under native conditions should be well-defined. However, very few of the denatured proteins have been studied under native conditions because the denatured state cannot be determined under exact native conditions using known experimental approaches. Herein, we characterize the denatured state ensembles (DSE) of ubiquitin under native conditions starting from a fully extended sequence resembling a peptide generated from a ribosome using replica exchange molecular dynamics. The representative structures of DSE with specific secondary structures distinct from the native ones are discussed. This result allows us to define the protein folding pathway more unambiguously and opens the door to biomedical studies of diseases correlated to protein misfolding. Potentially, the insights into the DSE can lead to targeted design of structure-based drug.

Mechanism of short-pulse-induced solute migration in comparison to continuous-light-driven thermal diffusion

The aim of this work is to clarify the mechanism of photo-absorption-caused solute migration at the microscopic level. Experimentally, we respectively measured the short-pulse-induced and continuous-light-driven migrations of chloroaluminum phthalocyanine molecules dissolved in ethanol at two concentrations, 4.2 × 1017 cm−3 (7.0 × 10−4 M) and 1.2 × 1017 cm−3 (2.0 × 10−4 M). Theoretically, by verifying that individual solute molecules in the concentrated solution, compared to those in the dilute solution, absorb more photo energy collectively but less photo energy individually, we consider solute migration as net movement of individual solute molecules and then sequentially analyse how individual solute molecules absorb photo energy, convert the absorbed photo energy into translational excess energy intra-molecularly and carry out movement. Subsequently, by summing up movement of individual solute molecules in a unit of volume, we deduce the solute migration behaviours which coincide with the experimental results: for short pulse excitation, solute migration is more/less in the concentrated solution depending on the pulse energy; for continuous light irradiation, solute migration is always more in the concentrated solution regardless of the light power. Note that, in our theoretical deduction, the short-pulse-induced and continuous-light-driven solute migrations differ in that the former proceeds before inter-molecular relaxation becomes apparent and the latter carries on with inter-molecular relaxation practiced sufficiently. Accordingly, the former is non-quasistatic and the latter is quasistatic and thus referred to as thermal diffusion.

Study of femtosecond laser pulse induced thermal lensing effect in CS2

We experimentally verified fs laser pulse induced thermal lensing effect in CS2 with the Z-scan technique and explain this effect as a result of relaxation of stimulated Raman scattering excited libration.

Study of picosecond nonlinear refraction in C2H4Cl2 and C2H4Br2 with Z-scan technique

We investigated picosecond nonlinear refraction of simple liquids C₂H₄Cl₂ and C₂H₄Br₂ with the Z-scan technique and explained the results in virtue of various molecular motions verified by femtosecond RIKE technique.

Shrinkage of picosecond laser beam by plasma reflection in super-resolution near-field structure of SiN/Sb/SiN thin film

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.