Tzer-Hsiang Huang

LIFETIME DETERMINATION FOR HIGH-LYING EXCITED STATES USING Z SCAN

We report a method of determining the lifetimes of the high‐lying excited singlet states of dye molecules in solution. The method is based on the Z‐scan technique and the observation of saturation of excited state absorption in dye solutions using laser pulses having Gaussian temporal and spatial distributions. This leads to the important finding that the use of laser pulses as long as tens of picosecond can resolve subpicosecond lifetimes. The method is demonstrated by measuring an upper excited singlet state lifetime of 900 fs using 30 ps full width at half maximum input pulses on a chloroaluminum phthalocyanine dye.

Mechanism of reverse saturable absorption in chloro-aluminum phthalocyanine solution studied with Z-scan

Abstract Using the Z -scan technique with 532 nm laser pulses of dual widths in the picosecond regime, we distinguish between two-photon absorption (TPA) and excited state absorption (ESA). To account for reverse saturable absorption in chloro-aluminum phthalocyanine dissolved in methanol both TPA and ESA, rather than either one, need be invoked.

Studies of nonlinear absorption and refraction in C60/toluene solution

Abstract Nonlinear absorptive and refractive properties of C60/toluene solution are studied using the Z-scan technique and a 532 nm laser delivering single picosecond (ps) pulses and trains of ps pulses separated by 7 nanoseconds (ns). With single ps pulses, reverse saturable absorption and positive nonlinear refraction are observed and attributed to population transitions among the singlet states. With ps pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed and is attributed to heat-induced temperature rise and population relaxing to triplet states.

Electronic energy dissipation in chloro-aluminum phthalocyanine/methanol system following nonlinear interaction with a train of picosecond pulses

Using the Z-scan technique, we study the nonlinear interaction of a dilute methanolic solution of chloro-aluminum phthalyocyanine with a ∼70 ns long train of laser pulses, each of 18 ps duration. The observations are explained by taking into account, in addition to the photoexcitation of a ground state molecule by a single pulse, thermal lensing and photoabsorption by an excited molecule (singlet or triplet) produced by previous pulses. A microscopic picture of the heating process in the solution is provided.

Mass transport following impulsive optical excitation

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.

Negative nonlinear refraction obtained with ultrashort laser pulses.

We present nonlinear refraction results for liquids methanol and acetic acid obtained with the Z-scan technique and 28 femtosecond (fs) 800 nm laser pulses. In contrast to the positive lensing effect obtained previously with picosecond and nanosecond laser pulses, a negative lensing effect is observed. The associated mechanism features the third-order polarization arising from the nonlinear response of the molecular skeletal motion that is driven into resonance through its electrostatic coupling to the valence electron cloud distorted by the fs laser field.

The ultrafast dynamics of liquid CBrCl3 studied with optical Kerr effect and Raman scattering

The ultrafast inter- and intramolecular dynamics of liquid CBrCl3 are studied with a femtosecond laser in a pump/probe optical Kerr experiment. The prompt part of the observed Kerr signal is found to comprise not just electronic response but also coherent coupling. As reported in the literature, the weak and delayed part of the signal is attributed to three kinds of intermolecular dynamics: diffusive reorientation, polarizability distortion, and molecular libration with decay times, respectively, of 2 ps, 500 fs, and 190 fs. Included in the delayed Kerr signal are long lasting oscillations which are ascribed to quantum interference involving four normal vibrations of the molecule. By a careful fitting of both the time domain (Kerr) and frequency domain (Raman) data with existing theories the four vibrations are identified as ν2 = 422 cm−1, ν3 = 246 cm−1, ν5 = 295 cm−1, and ν6 = 191 cm−1, with dephasing times, 1.1 ps, 1.7 ps, 1 ps, and 1 ps, respectively.

Hyperpolarizabilities of the m‐substituent phenyl amine based chromophores determined from the hyper‐Rayleigh scattering and two photon absorption induced fluorescence

The technique of hyper‐Rayleigh scattering is used to determine and compare the first hyperpolarizabilities (β) of five structurally similar m‐substituent phenyl‐amine based chromophores using both internal and external standards. Contribution from the two photon absorption induced fluorescence to the observed signal is carefully removed. The chromophores with the m‐substituted phenyl ring, that stabilizes the maximal charge‐transfer state through resonance effect, are found to have larger static β values and redshifted electronic absorption peaks (λmax) compared with the para‐nitroaniline (pNA) chromophore. The electron‐withdrawing m‐substituent of the chromophore is found to have the effect of reducing both the β and λmax values. Its effect on β is less pronounced when the π‐electron reservoir is large. The enhancement of β value and the redshift λmax are observed to be consistent with the extent of the π electron delocalization.

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.

Hyper-Rayleigh scattering of thiophene-incorporated polyene chromophores with π-configuration locking

Abstract The hyper-Rayleigh scattering (HRS) technique is used to determined and compare the first hyperpolarizabilities (β) of thiophene-incorporated polyene chromophores with and without a configuration locked trans-triene bridge. To ensure the purity of the HRS signal, special attention is paid to its dependence on the fundamental intensity and the spectral content of the scattered radiation including the two-photon absorption induced fluorescence. From the HRS results, it is found that the configuration-locked trans-triene bridge enhances the β values of the chromophores studied. The β-enhancement is more pronounced when the electron acceptor CHCC(CN)2 in the chromophores is replaced with C2(CN)3.