Design guidelines for ultrashort pulse generation by a Mamyshev regenerator.

We study numerically the possibility of using various gain-switched seed laser pulse parameters and fibers for a low-cost, all-fiber Mamyshev regenerator scheme. We find that for increasing pulse durations, sufficient spectral broadening will be difficult to achieve in practice and careful design of the system parameters is required for the regenerator to function. Furthermore, an optimal input peak power level can be defined for a given fiber and pulse duration that results from a balance of competing Kerr effect and stimulated Raman scattering. We also demonstrate experimental results of 3 ps pulse generation seeded by an 80 ps gain-switched diode. Our results pave the way for designing pulse-on-demand picosecond scale fiber sources for applications.

Ultrafast Dynamics of Sb-Corroles: A Combined Vis-Pump Supercontinuum Probe and Broadband Fluorescence Up-Conversion Study.

Corroles are a developing class of tetrapyrrole-based molecules with significant chemical potential and relatively unexplored photophysical properties. We combined femtosecond broadband fluorescence up-conversion and fs broadband Vis-pump Vis-probe spectroscopy to comprehensively characterize the photoreaction of 5,10,15-tris-pentafluorophenyl-corrolato-antimony(V)-trans-difluoride (Sb-tpfc-F2). Upon fs Soret band excitation at ~400 nm, the energy relaxed almost completely to Q band electronic excited states with a time constant of 500 ± 100 fs; this is evident from the decay of Soret band fluorescence at around 430 nm and the rise time of Q band fluorescence, as well as from Q band stimulated emission signals at 600 and 650 nm with the same time constant. Relaxation processes on a time scale of 10 and 20 ps were observed in the fluorescence and absorption signals. Triplet formation showed a time constant of 400 ps, with an intersystem crossing yield from the Q band to the triplet manifold of between 95% and 99%. This efficient triplet formation is due to the spin-orbit coupling of the antimony ion.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of "friction-dependent" chemical reactivity.

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity on chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent change with the molar fraction of glycerol.

Sensitized Two-NIR-Photon Z→E Isomerization of a Visible-Light-Addressable Bistable Azobenzene Derivative.

Two-NIR-photon-triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two-photon-harvesting triarylamine antenna to a thermally stable ortho-fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one-photon and two-photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy.

We monitor the time-dependent Stokes shift (TDSS) of fluorescence from the zwitterionic probe N-methyl-6-oxyquinolinium betaine in water. A spectral relaxation time τsolv = 0.57 ps (at 20.5 °C) is attributed to a solvation process involving water in the hydration layer. In this article we show that a tertiary butyl group, when attached to the chromophore, slows the dynamics to τsolv = 0.76 ps and increases the corresponding activation energy by 5 kJ/mol. In a companion paper (10.1021/acs.jpcb.7b05039), simulations suggest that the observed slow-down indicates coupling of solute vibrations to hydration water. Thus, a new angle on a thoroughly researched topic, solvation dynamics, has been opened.

Femtosecond broadband fluorescence upconversion spectroscopy: Spectral coverage versus efficiency.

Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin ß-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λF > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories.

Observing the Hydration Layer of Trehalose with a Linked Molecular Terahertz Probe.

The terahertz (THz) absorption bands of biomolecular hydration layers are generally swamped by absorption from bulk water. Using the disaccharide trehalose, we show that this limitation can be overcome by attaching a molecular probe. By time-resolving the fluorescence shift of the probe, a local THz spectrum is obtained. From the dependence on temperature and H2O/D2O exchange, it is concluded that the trehalose hydration layer is being observed. The region of dynamic water perturbation by the disaccharide encompasses the probe and is therefore larger than the first two solvation layers.