There has been much interest in the development of two-photon absorbing materials and many efforts to understand the nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties. Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what drives both the linear and non-linear photophysical properties.

We present the first Monte Carlo simulations of the mechanism of two-photon induced molecular orientation by photo-isomerization of chromophores in solid polymer films. Two-photon absorption (TPA) occurring at intense laser pulses creates molecular orientation of photo-isomers proportionally to the square of the excitation intensity and the fourth power of the cosine of the angle θ between the TPA-transition dipole moment and the direction of the polarization of the excitation light. Photo-selection in cos⁴θ will result in the orientational redistribution of the chromophores differently from the well-known one-photon absorption process. Kinetic Monte Carlo model is proposed and illustrative "microscopic" results related to diffraction gratings inscription are presented.

Large two-photon and excited state absorption have been reported in donor-acceptor-substituted π-conjugated molecules. We have performed detailed nonlinear absorption and photophysical measurements on a system of AFX chromophores and calculate the nonlinear transmission based on an effective three-level model. A numerical model that includes far wing linear absorption has been developed and compared with an analytical three-photon absorption model. The models are in accordance and yield excellent agreement with experimental nonlinear transmission data for 0.02-M AFX solutions up to laser intensities ~ 1-5 GW/cm². We have extended our modeling efforts to include some new effects that may be anticipated in this regime, such as stimulated scattering, molecular interactions, and saturation. Effects of chirped pulses and linewidth of the pump laser on stimulated scattering are included. Self-focusing and de-focusing are also considered. We report on our experimental observations of various materials and discuss results with respect to our extended theoretical models.

In this proceeding we report the principles for a novel 3-D RET-based two-photon optical storage device based on a photochromic diarylethene (1,2-bis(2-methylbenzo[b]thiophen-3-yl) hexafluorocyclopentene (1) and a fluorene derivative (2,7-bis[4-(9,9-didecylfluoren-2-yl)vinyl] phenylbenzothiazole (2)), suitable for recording data in thick storage media. Data was recorded by linear (single-photon) and non-linear (two-photon) excitation of the closed form of diarylethene 1. The read-out mechanism is based on resonance energy transfer (RET) from fluorene derivative 2 to the closed form of diarylethene 1. Bright fluorescent data patterns were recorded onto the photosensitive film, providing excellent contrast and resolution. Two-photon characterization with a tunable mode-locked femtosecond Ti:sapphire laser gave evidence of the cycloreversion reaction from the closed to the open photoisomer by excitation at 800 nm. We demonstrated non-destructive readout of 3D data stored in multiple layers by using two-photon induced fluorescence modulation. Readout of the stored information can be done for >10,000 cycles without significantly compromising the stored data.

We present nonlinear optical spectroscopy of the model conjugated polymer MEH-PPV, a derivative of poly(paraphenylenevinylene) (PPV) and compare it with PPV. We perform multiphoton excited fluorescence spectroscopy and z-scan studies of solutions compared with third-harmonic generation, linear and nonlinear waveguide spectroscopy with intensity dependent prism coupling of thin films in the NIR region. Spectra of the nonlinear absorption coefficient and nonlinear refractive index are used to identify the figures of merit (FOM). We observe a spectral window at 1100 - 1200 nm were the application demands for all-optical waveguide switching are fulfilled. We demonstrate all-optical refractive index changes in the order of 0.001. Control of the molecular weight of MEH-PPVs enables improved film forming properties, reduced birefringence and ultimately low waveguide propagation losses < 1 dB/cm.

The concentration dependence of third order nonlinear optical susceptibility ( χ^<3> ) of copper phthalocyanine (CuPc) dissolved in tetrahydrofuran (THF) using the degenerate four wave mixing (DFWM) method at 532 nm was investigated. We present the linear dependence of the third order nonlinear optical susceptibility ( χ^<3> ) as a function of different concentration for CuPc dissolved in THF. We also calculated the second order hyperpolarizability ( γ) of CuPc solutions. Third harmonic generation (THG) measurements at the 1064 nm performed on CuPc thin films are also presented. We found that the χ^<3>_DFWM values are larger than the χ^<3>_THG ones. This variation observed in χ^<3> values, occurs probably due to the different resonance contributions in solution and solid state of CuPc. We show that in the case of DFWM measurements the one and two-photon resonant contributions at the Q and C-band increase χ^<3>_DFWM value of CuPc. In THG measurements, the three-photon resonance contribution at the Soret band gives rise to higher χ^<3>_THG value.

In this work we present the basis of the solar concentrator design, which has is located at Temixco, Morelos, Mexico. For this purpose, this place is ideal due to its geographic and climatic conditions, and in addition, because it accounts with the greatest constant illumination in Mexico. For the construction of the concentrator we use a recycled parabolic plate of a telecommunications satellite dish (NEC). This plate was totally covered with Aluminum. The opening diameter is of 332 cm, the focal length is of 83 cm and the opening angle is of 90°. The geometry of the plate guaranties that the incident beams, will be collected at the focus. The mechanical treatment of the plate produces an average reflectance of 75% in the visible region of the solar spectrum, and of 92% for wavelengths up to 3μm in the infrared region. We obtain up to 2000°C of temperature concentration with this setup. The reflectance can be greatly improved, but did not consider it as typical practical use. The energy obtained can be applied to conditions that require of those high calorific energies. In order to optimize the operation of the concentrator we use a control circuit designed to track the apparent sun position.

Steady ultrashort bright optical pulses, which are originating in single-mode erbium-doped fibers operating within a nonlinear transmission, are analytically investigated. This type of steady-state bright optical pulses can be grown in real time scale due to incoherent resculpturing of external optical pulses by the doped fiber in the traveling-wave regime of propagation. The performed analysis is related to the regimes without and with the gain saturation in a doped fiber and demonstrates that single-mode optical fibers can form and support steady-state bright pulses with various shapes of their envelopes in these regimes. Basically, these bright optical pulses have a smooth hyperbolic-secant shape, however in particular cases, both a two-side exponential and a two-side hyperbolic shapes can be also supported by the erbium-doped fibers within a nonlinear transmission.

We study one photon absorption (1PA) and two photon absorption (2PA) properties of a series of asymmetrical phthalocyanines for 2PA-based 3D optical information storage. These compounds show very large 2PA cross section, σ₂ = 10³ - 10⁴ GM, in near IR range of excitation, from 800 to 900 nm. This strong resonance enhancement effect is possible due to one-photon-allowed and narrow intermediate Q-transition, occurring in very vicinity of excitation wavelength. We report 2PA spectra, 2PA cross-sections, temperature stability, tautomerization and fluorescence quantum yields, measured in organic solvents and in polymer films in temperature range 77 - 300K. We show that the unique combination of photophysical properties make these compounds attractive candidates for fast re-writable volumetric storage.

We study the optical response of 1D photonic crystal microcavities with Kerr nonlinear and with Kerr nonlinear and dispersive defects. By means of a transfer matrix method, Kerr nonlinear and dispersive are studied on the base of δ-function and a Lorentz oscillator model, respectively. For the systems with Kerr nonlinear defects only, the localized mode frequency depends on the localized light intensity. When incident light frequency is given, with the varying incident light intensity, the system exhibits bistability. For the systems with Kerr nonlinear and dispersive defects, it is found that the threshold intensity of nonlinear bistability is much lower than the system with Kerr nonlinear only. The low threshold intensity is useful for the applications of optical switches.