Jonathas P. Siqueira

Intrachain energy migration to weak charge-transfer state in polyfluorene end-capped with naphthalimide derivative.

Polyfluorene end-capped with N-(2-benzothiazole)-1,8-naphthalimide (PF-BNI) is a highly fluorescent material with fluorescence emission modulated by solvent polarity. Its low energy excited state is assigned as a mixed configuration state between the singlet S(1) of the fluorene backbone (F) with the charge transfer (CT) of the end group BNI. The triexponential fluorescence decays of PF-BNI were associated with fast energy migration to form an intrachain charge-transfer (ICCT) state, polyfluorene backbone decay, and ICCT deactivation. Time-resolved fluorescence anisotropy exhibited biexponential relaxation with a fast component of 12-16 ps in addition to a slow one in the range 0.8-1.4 ns depending on the solvent, showing that depolarization occurs from two different processes: energy migration to form the ICCT state and slow rotational diffusion motion of end segments at a longer time. Results from femtosecond transient absorption measurements agreed with anisotropy decay and showed a decay component of about 16 ps at 605 nm in PF-BNI ascribed to the conversion of S(1) to the ICCT excited state. From the ratio of asymptotic and initial amplitudes of the transient absorption measurement, the efficiency of intrachain ICCT formation is estimated in 0.5, which means that, on average, half of the excited state formed in a BNI-(F)(n)-BNI chain with n = 32 is converted to its low energy intrachain charge-transfer (ICCT) state.

Mechanism of the Zn(II)Phthalocyanines’ Photochemical Reactions Depending on the Number of Substituents and Geometry

In this work, the synthesis and the nonlinear absorption and population dynamics investigation of a series of zinc phthalocyanines (ZnPcs) dissolved in chloroform are reported. In order to determine the relevant spectroscopic parameters, such as absorption cross-sections of singlet and triplet excited states, fluorescence relaxation times, intersystem crossing, radiative decay and internal conversion, different optical and spectroscopic techniques were used. By single pulse and pulse train Z-scan techniques, respectively, singlet and triplet excited states‘ absorption cross-section were determined at 532 nm. Furthermore, the intersystem crossing time was obtained by using both techniques combined with the fluorescence lifetime determined by time-resolved fluorescence. The radiative and internal conversion rates were determined from the fluorescence quantum yield of the samples. Such spectroscopy parameters are fundamental for selecting photosensitizers used in photodynamic therapy, as well as for many other applications.

Broadband third-harmonic generation on interfaces using femtosecond pulses

We report on third-harmonic generation (THG) in optical materials using femtosecond pulses and Z-scan method. Here we have played with beam focusing parameters and, in this way, we could track the THG signal at function of Rayleigh ranges. We observed that the femtosecond pulse has broadband spectrum and such property also affects the thirdharmonic (TH) spectrum. In this experiment we were able to distinguish the contribution of bulk and interface on the THG by measuring the intensity and spectral profile of the TH signal.

Laser induced periodic surface structuring on Si by temporal shaped femtosecond pulses.

We investigated the effect of temporal shaped femtosecond pulses on silicon laser micromachining. By using sinusoidal spectral phases, pulse trains composed of sub-pulses with distinct temporal separations were generated and applied to the silicon surface to produce Laser Induced Periodic Surface Structures (LIPSS). The LIPSS obtained with different sub-pulse separation were analyzed by comparing the intensity of the two-dimensional fast Fourier Transform (2D-FFT) of the AFM images of the ripples (LIPSS). It was observed that LIPSS amplitude is more emphasized for the pulse train with sub-pulses separation of 128 fs, even when compared with the Fourier transform limited pulse. By estimating the carrier density achieved at the end of each pulse train, we have been able to interpret our results with the Sipe-Drude model, that predicts that LIPSS efficacy is higher for a specific induced carrier density. Hence, our results indicate that temporal shaping of the excitation pulse, performed by spectral phase modulation, can be explored in fs-laser microstructuring.

Femtosecond Two-Photon Absorption Spectroscopy of Poly(fluorene) Derivatives Containing Benzoselenadiazole and Benzothiadiazole

We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.

Tetracarboxy-phthalocyanines: From excited state dynamics to photodynamic inactivation against Bovine herpesvirus type 1

Herein we present the excited state dynamic of zinc and aluminum tetracarboxy-phthalocyanines (ZnPc and AlPc) and its application in the photodynamic inactivation (PDI) of Bovine herpesvirus type 1 (BoHV-1) in vitro. The excited state dynamic provides valuable data to describe the excited state properties of potential optical limiters and/or photosensitizers (PSs), such as: the excited state cross-sections, fluorescence lifetime and triplet state quantum yield. The excited state characterization was performed using three different Z-scan techniques: Single Pulse, White Light Continuum and Pulse Train. Considering the photodynamic inactivation of BoHV-1, an initial viral suspension containing 105.75TCID50/mL was incubated with the PSs for 1h at 37°C under agitation and protected from light. The samples were placed in microtiter plates and irradiated (180mW/cm2). During irradiation, a sample was taken every 15min and the viability of the virus was evaluated. The results show that both phthalocyanines were efficient against viruses. However, a higher photodynamic efficiency was observed by ZnPc, which can be attributed to its higher triplet and singlet quantum yields. The results presented here are important for animal health (treatment of BoHV-1) and also open up a field of studies to use AlPc and ZnPc as potential agents against a wide range of microorganisms of veterinary interest.

Effect of SLM pixelation on two-photon fluorescence by applying an off-centered quadratic spectral phase mask

Spatial light modulators (SLMs) have become a valuable tool to shape ultrashort laser pulses, which have found innumerous applications in different areas of ultrafast science. Despite the fact that this family of devices can be used to produce almost any arbitrary pulse shape, as long as enough bandwidth is available, there are limitations related to their discrete nature that may lead to distortions in the expected pulse shape. Here we investigate such collateral temporal distortions by quantifying the effect imposed by the discrete nature (pixelation) of liquid-crystal-based SLMs on the two-photon excited fluorescence signal of a conjugated polymer, when the central position of a quadratic spectral phase mask is scanned across the laser bandwidth. We conclude that the observed temporal distortions are related to replica pulses generated by an interplay between the additional linear phase resultant of an off-centered quadratic phase mask and pixelation of the applied phase mask.

Enhancing multi-photon induced excitonic emission of ZnO single crystals by shaping fs laser pulses

We report on the control of multi-photon excited emission of the ZnO exciton band via spectral phase modulation of the femtosecond excitation pulses. It was observed that the optimum spectral phase that enhances the exciton emission results in a pulse-train temporal profile, whose separation corresponds to an energy of 82 meV, which is close to the energy of LO phonon sidebands in ZnO emission. Such a result suggests that exciton–LO phonon coupling can be explored to coherently enhance the exciton emission in ZnO single crystals with respect to the defect luminescence band.

Nonlinear characterization of fs-laser written Gorilla Glass waveguides

In this work, we performed the nonlinear characterization of Gorilla Glass waveguides produced by fs-laser microfabrication with different writing parameters. Their nonlinear refractive index n2 was determined by the dispersive-scan technique. We found that the waveguides n2 values are lower than the one for the pristine material and they depend on the writing parameters. Raman spectroscopy measurements showed important structural modifications related to non-bridging oxygens (NBOs) on the fs-laser irradiated glass, which may lead to the reduction of the third-order polarizability.

Investigation of the nonlinear absorption spectrum of all-trans retinoic acid by using the steady and transient two-photon absorption spectroscopy

This work investigates the two-photon absorption (2PA) spectrum of all-trans retinoic acid (ATRA) in DMSO solution, employing the wavelength-tunable Z-scan and white-light pump-probe techniques with femtosecond pulses. Our results showed that ATRA presents two 2PA allowed bands at 280 nm (2hν = 560 nm) and 365 nm (2hν = 730 nm) with 2PA cross-section values of 34 GM and 40 GM, respectively. The 2PA band at 280 nm was ascribed to the transition from ground to high energy excited states with contribution from the real intermediate excited state (11Bu+-like). The 2PA band at 365 nm was attributed to an overlap of 11Bu+-like and 21Ag−-like excited states, which are essential to describe the photochemical processes that this class of organic materials play in nature. Through solvatochromic measurements and by using the two-energy level approach, we found that the 11Bu+-like and 21Ag−-like states contribute, respectively, 48% and 52% of the lowest energy 2PA band of ATRA in DMSO solution. From femtosecond transient spectroscopy we verified that when ATRA is excited at 775 nm (2PA excitation) its excited state absorption (ESA) spectrum presents a red-shift of ∼5.5 nm in comparison to the same spectrum excited at 387.5 nm (1PA excitation), corroborating the interpretation about the 2PA spectrum for the ATRA in DMSO.