Wavelength dependence and figures of merit of ultrafast third-order optical nonlinearity of a conjugated 3,3'-bipyridine derivative

Experimental and theoretical investigations on the optical and electrochemical properties of π-conjugated donor-acceptor-donor (DAD) compounds toward a universal model

A series of nine (9) donor-acceptor-donor (DAD) π-conjugated small molecules were synthesized via palladium catalyzed Stille aromatic cross-coupling reactions by the combination of six (6) heterocycle building blocks (thiophene, furan, thiazole, 2,1,3-benzothiadiazole, 2,1,3-pyridinothiadiazole, thienothiadiazole) acting as electron donating (thiazole, furan, thiophene) and electron deficient (benzothiadiazole, pyridinethiadiazole, thienothiadiazole) units. These model compounds enable determining the correspondence between the theoretical and experimental optical and electrochemical properties for the first time, via Density Functional Theory (DFT), time-dependent DFT, UV-Vis spectroscopy, and cyclic voltammetry, accordingly. The obtained theoretical models can be utilized for the design and synthesis of new DAD structures with precise optical bandgaps, absorption maxima, and energy levels suitable for different optoelectronic applications.

Z-scan technique for investigation of the noninstantaneous optical Kerr nonlinearity

By treating laser-induced optical Kerr nonlinearity as a noninstantaneous decaying process, we present the pulse-duration-dependent Z-scan analytical expressions for an arbitrary aperture and an arbitrary nonlinear magnitude. This theory has the capacity to characterize the third-order nonlinear refraction induced by a laser pulse with its temporal duration being much longer than or comparable to the recovery time of the nonlinear effect. Through Z-scan measurements at different pulse durations, the nonlinear refractive coefficient and the recovery time could be determined unambiguously and simultaneously. Furthermore, the theory can be utilized to confirm whether the measured optical Kerr nonlinearity is instantaneous or noninstantaneous with respect to the given pulse duration.

Solvent effect on two-photon absorption and fluorescence of rhodamine dyes

For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.

Molecular structure-property correlations from optical nonlinearity and thermal-relaxation dynamics

We apply ultrafast single beam Z-scan technique to measure saturation absorption coefficients and nonlinear-refraction coefficients of primary alcohols at 1560 nm. The nonlinear effects result from vibronic transitions and cubic nonlinear-refraction. To measure the pure total third-order nonlinear susceptibility, we removed thermal effects with a frequency optimized optical-chopper. Our measurements of thermal-relaxation dynamics of alcohols, from 1560 nm thermal lens pump and 780 nm probe experiments revealed faster and slower thermal-relaxation timescales, respectively, from conduction and convection. The faster timescale accurately predicts thermal-diffusivity, which decreases linearly with alcohol chain-lengths since thermal-relaxation is slower in heavier molecules. The relation between thermal-diffusivity and alcohol chain-length confirms structure-property relationship.

A Sensitive Technique for Two-Photon Absorption Measurements: Towards Higher Resolution Microscopy

High repetition rate (HRR) lasers are essential in multiphoton microscopy for satisfactory signal to noise at low average powers. However, HRR lasers generate thermal distortions in samples even with the slightest single photon absorption. Using an optical chopper with HRR lasers ("blanking") we demonstrate a femtosecond z-scan setup that effectively eliminates thermal as well as small linear absorption effects and precisely measures two-photon absorption (TPA) cross-sections of chromophores. Accurate measurement of TPA cross-sections in biologically relevant chromophores is especially important since the TPA spectrum is considerably different in regions with even minute linear absorption. Such blanking measurements with chopper also show enhanced fluorescence efficiency of the chromophores.