Florencio E. Hernandez

High-dynamic-range cascaded-focus optical limiter

We report experimental results of using an f/5 , cascaded-focus optical geometry for a high-dynamic-range optical limiter. The device consists of a 2-cm-thick CS(2) cell at the first focus and a reverse saturable-absorber dye in a thin cell (0.1 mm) at the second focus. The strong self-focusing in the CS(2) that is due to the ac Kerr effect and electrostriction keeps the energy at the second cell below its damage threshold. Using lead phthalocyanine in chloroform as the reverse saturable-absorption material, we clamped the maximum output energy below 1muJ for input energies up to 14.5 mJ without damage. We used a frequency-doubled, Q -switched 5-ns (FWHM) Nd:YAG laser operating at a 10-Hz repetition rate. The measured dynamic range of the device is at least 7500.

Fluorescence emission of disperse Red 1 in solution at room temperature.

In this article, we report the fluorescence emission of Disperse Red 1 in solution at room temperature and pumping at 532 nm with a 25 mW diode laser. We have measured its fluorescence quantum yield in methanol, ethylene glycol, glycerol, and phenol obtaining values as high as 10(-3) in the aliphatic alcohols. The excitation spectra of Disperse Red 1 in all four solvents as well as its excitation anisotropy in glycerol are presented. Applying a Gaussian decomposition method to the absorption spectra along with the support from the excitation spectra, the positions of the different transitions in this pseudo-stilbene azobenzene dye were determined. Solvatochromic and isomerization constraint effects are discussed. Calculations using density functional theory at TD-B3LYP/6-31G*//HF/6-31G* level were performed to interpret the experimental observations.

Untangling the Excited States of DR1 in Solution: An Experimental and Theoretical Study

We report the experimental observations and the theoretical predictions of the fully separated n-pi* and pi-pi* bands of Disperse Red One in acidified methanol solution. The analysis of the linear and two-photon absorption spectra is presented in four specific solvents. We demonstrate the possibility to establish the position of the first two excited states combining linear and nonlinear spectroscopy. Calculations using density functional theory at TD-B3LYP/6-31G*//HF/6-31G* level accurately predict the spectral region of the n-pi* and pi-pi* transitions of DR1 in all solvents.

Three- and four-photon absorption of a multiphoton absorbing fluorescent probe

The utility of multiphoton excitation processes has been the subject of increased attention due to their potential applications in biophotonics, biology, and medicine through three-dimensional fluorescence imaging and photodynamic therapy. Evidence of this are the multiple applications of two-photon absorption (2PA) in fluorescence spectroscopy and 3D imaging over the last several years because of its large effective Stoke¿s Shift and high spatial resolution.[l,2] However, because the irradiation penetration depth of 2PA is limited in medical and biological applications due to the unwanted absorption and scattering when two red photons are used, the scientific communib recently started to explore higher order absorption processes at longer wavelengths such as three- (3PA) and four-photon absorption (IPA) that minimize the scattered light losses, and reduce the unwanted linear absorption in the living organism transparency window.

Synthesis and Characterization of New Fluorene-Based Singlet Oxygen Sensitizers

The synthesis, photophysical characterization, and determination of singlet oxygen quantum yields (ΦΔ) for a class of fluorene derivatives with potential application in two-photon photodynamic therapy (PDT) is reported. It has been demonstrated that these compounds possess the ability to generate singlet oxygen (1O2) upon excitation. A photochemical method, using 1,3-diphenylisobenzofuran (DPBF) as 1O2 chemical quencher, was employed to determine the singlet oxygen quantum yields (ΦΔ) of the fluorene-based photosensitizers in ethanol. ΦΔ values ranged from 0.35 to 0.75. These derivatives may have potential application as two-photon photosensitizers when pumped via two-photon excitation in the near-IR spectral region.

Two-photon absorption and lasing properties of new fluorene derivatives

Linear photophysical properties, two-photon absorption (2PA), and lasing effects of new fluorene derivatives, 2,7-bis(4-(phenylthio)styryl)-9,9-didecyl-9H-fluorene (1) and 2,7-bis(4-(phenylsulfonyl)styryl)-9,9-didecyl-9H-fluorene (2), have been investigated. The relatively strong solvatochromic behavior of symmetrical compounds 1 and 2 can be explained by the phenomenon of symmetry breaking in the electronic distribution of the first excited state. The 2PA spectra of 1 and 2 were obtained over a broad spectral range (480–880 nm) by two-photon-induced fluorescence and open aperture Z-scan methods, in which excited state absorption and stimulated emission processes were considered in the fitting procedure. Maximum values of 2PA cross-sections ∼1300–1900 GM were obtained for 1 and 2 in the two-photon-allowed absorption bands at ∼660 nm. Efficient lasing of 1 and 2 was demonstrated in a nondispersive flat resonator with picosecond transverse pumping at 355 nm. Lasing efficiencies were ∼25–30% in this resonator, and independent of solvent.

Linear and nonlinear optical characterizations of a monomeric symmetric squaraine-based dye in solution

The photophysical properties of a symmetric squaryllium dye, namely, 2,4-bis[4-(N,N-dibutylamino)-2-hydroxyphenyl] squaraine (SQ), in its monomer form in acetone solution, have been thoroughly studied by means of one-photon absorption (1PA) and two-photon absorption (2PA), excitation anisotropy, fluorescence emission, fluorescence quantum yield, and excited state absorption. The results show that there is a strong one-photon allowed absorption band in the near IR region associated with intramolecular charge transfer. Higher one-photon allowed and forbidden singlet excited states were also revealed by absorption and excitation anisotropy. A relatively high fluorescence quantum yield (0.44) was measured for this dye. The nonlinear optical characterization of SQ in solution confirms the ability of squaraine dyes to be used as good two-photon absorbers. Additionally, it was found that this dye presents both saturable and reverse saturable absorption effects. Density functional theory calculations of the 1PA and 2PA electronic spectra of SQ were carried out to support the experimental data. A detailed analysis of the symmetry and energy of the orbitals involved in the lowest five electronic transitions is presented and discussed in relation to the behavior observed experimentally.

Viscosity dependence of optical limiting in carbon black suspensions

We measure the optical limiting behavior of carbon black suspensions in various viscosity solvents by using a 10-Hz repetition rate, 532-nm, 5-ns pulsed laser. We found that, for common solvents used in the past such as water and ethanol, the limiting behavior ceases after a few laser firings and a turnover in the limiting curve appears. This can be explained by depletion of the carbon black within the focal volume. This turnover shifts to lower energies as the viscosity of the solvent becomes greater. However, for low viscosity liquids, such as carbon disulfide or pentane, the limiting is unaffected by the repetition rate, at least for frequencies up to 10 Hz, because of diffusion of the carbon black particles. This diffusion allows fresh material to replace the irradiated volume within the time between pulses.

Thermally controlled preferential molecular aggregation state in a thiacarbocyanine dye

Herein we report the experimental and theoretical study of the temperature dependence of a thiacarbocyanine dye in its monomer, H- and J-aggregates states. We demonstrate the ability to control the ratio of monomer, H- and/or J-aggregates with heat. We link such a control to the conformation dependence of the molecule. An alternative way to gain access to the dominating species without changing the concentration as a complete switching mechanism between all the present species is proposed. The results presented in this work lead to a better understanding of thiacarbocyanine dyes behavior.

The Effect of the π-Electron Delocalization Curvature on the Two-Photon Circular Dichroism of Molecules with Axial Chirality

Herein we report on the theoretical-experimental study of the effect of curvature of the π-electron delocalization on the two-photon circular dichroism (TPCD) of a family of optically active biaryl derivatives (S-BINOL, S-VANOL, and S-VAPOL). The comparative analysis of the influence of the different transition moments to their corresponding TPCD rotatory strength reveals an enhanced contribution of the magnetic transition dipole moment on VAPOL. This effect is hereby attributed to the additional twist in the π-electron delocalization on this compound. TPCD measurements were done using the double L-scan technique in the picosecond regime. Theoretical calculations were completed using modern analytical response theory, within a time-dependent density functional theory (TD-DFT) approach, at both, B3LYP and CAM-B3LYP levels, with the aug-cc-pVDZ basis set for S-BINOL and S-VANOL, and 6-31G* for S-VAPOL. Solvent effects were included by means of the polarizable continuum model (PCM) in CH2Cl2.