Organic materials exhibiting strong two-photon absorption cross-sections and subsequent up-converted fluorescence have been targeted for use in a variety of applications including optical data storage, nondestructive imaging, frequency up-converted lasing, and microfabrication. In order for these materials to be useful in practical application they must either be coupled with a liquid solvent or doped into a solid host material. The purpose of this study is to examine effects of different host environments on the nonlinear photophysical properties of AF-455, a recently developed organic two-photon absorber. We present results of experiments using both emission and absorption methods to characterize the linear and nonlinear response of AF-455 dissolved in solvents of varying polarity and doped in a polymer (PMMA) matrix.
We report photophysical measurements and application of an effective three-photon absorption model that characterize the two-photon and excited state absorption in organic D-π-A chromophores. The key parameter is an effective three-photon absorption coefficient that depends on the intrinsic molecular two-photon absorption cross section and excited state photophysical properties. We measure all of these molecular parameters independently in a variety of experiments and then compare the model predictions with nanosecond nonlinear absorption measurements. We find excellent agreement with the data using only experimentally measured molecular quantities and no free parameters. We conclude that excited state absorption from both singlet and triplets states is the dominant contribution to the nonlinear transmittance loss in the nanosecond regime, and that the chief role of two-photon absorption in this regime is to populate the excited triplet state.
The nonlinear energy dynamics and charge carrier generation of a model donor-acceptor two-photon dye, AF-380, is investigated using ultrafast nonlinear spectroscopy. This material has been the focus of several investigations due to its large reported two-photon absorption cross-section, the discrepancies between long and short pulse measurements, and its use in microfabrication through holographic two-photon induced photopolymerization. It is believed that a substantial excited state absorbance can account for the difference in two-photon cross section measurements, and furthermore, that the resulting excited state exhibits a lifetime substantially long enough to affect subsequent absorption of longer pump pulses or higher repetition rates. Experimental and theoretical considerations of the electronic dynamics are discussed using a rate analysis and beam propagation code for comparison.
The diversity of biological sensing and biocatalysis is astounding. A considerable effort has been directed at not only understanding the mechanism of these biological processes, but also how this activity can be maintained or duplicated in an artificial environment. We will present work on the formation of functional optical devices that convert biological responses into optical signals through changes in diffraction efficiency and reflection angle. By incorporating biomolecules into monomer systems that can be cured using a two-photon polymerization mechanism, greater spatial resolution and increased biological viability can be achieved. The polymer can be nanopatterned using ultrafast nonlinear holography to create a functional BioMEMS device. Additionally, we will discuss the characterization of the biomolecules and the processing of the gratings that incorporate these functional proteins. This approach is a first step towards the development of a hybrid organic-inorganic composite device.
Non-cryogenic, high sensitivity infrared detection is one of the fundamental criteria for future sensor technology. To this end, we propose mimicking the natural infrared detection structures found in certain snake pit tissue. We hypothesize that the pit organ behaves like a photonic bandgap in that a regular arrangement of sub-micron micropits removes or traps visible radiation and enhances infrared radiation transmission. In order to simulate the 2- and 3-D pit surface morphology, we used holographic two- photon induced polymerization (H-TPIP), a new microfabrication technique previously reported by this group. Using the ultrafast H-TPIP procedure, we can write large area biomimetic structures into an optical resin. Due to the quadratic dependence of the absorption probability on the incident radiation intensity, molecular excitation via the simultaneous absorption of two photons has been shown to lead to improved 3-D control of photochemical or photophysical processes. Using spatial variations in the incident intensity within a photopolymerizable resin, these structures can be readily fabricated. We report our progress on duplicating the surface morphology of snake infrared pit tissue using H-TPIP.
Previously, we have reported the first demonstration of holographic two-photon induced photopolymerization (H-TPIP) in the construction of transmission holograms. This technique relies on the coupling of a two-photon absorbing chromophore and a photocurable optical resin. Several different systems have been successfully explored; all involving varied reaction pathways. Since the initial report, we have also expanded this technique to reflection holograms and some bulk structures. While the applications for this process are widely ranging, the underlying physical mechanisms still require a great deal of investigation. In this work, we report on some of the photo-physical mechanisms involved in the H-TPIP technique. Specifically, we will report on evidence for mass-transport phenomena, and the role of localized thermal loading. We also discuss a preliminary model, which examines the coupling between the chromophore's excited state population, initiation of the polymerization reaction, and localized thermal deposition.
The excited state absorption of lead phthalocyanine is investigated using ultrafast transient white light absorption spectroscopy. Singlet state absorption features are identified on the basis of time. Increases in optical density up to 1 in the near infrared region of 900 nm to 1.1 micrometers are observed by pumping at 800 nm. In addition, several artifacts of this technique are discussed, with an aim towards improvements. In particular, self-pumping of the sample via the supercontinuum probe is observed and investigated.
To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3',4,4'-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtained as a function of the number of double bonds (n equals 1 - 5). Trends in the data reflected the ordering, energy gap between and mixing of 1Bu* and 1Ag* excited state configurations. We investigated the solvatochromism of a series of (alpha) ,(omega) -di(2- dithienyl 3,3',4,4'-butyl) polyenes. Absorption (n equals 1 - 5 double bonds) were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity, giving evidence for a polar 1Bu* absorbing state and a nonpolar 1Ag* emitting state. We observed sensitivity of the absorbing and emitting states to solute-solvent dipole-dipole interactions, suggesting the dithienyl polyenes had a polar syn ground state conformation.
The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) 2, was measured by Z-scan experiments. Strong two photon absorption is indicated by high values for (sigma) 2. Solvent has strong influence in the measurement of (sigma) 2 values.
The photophysics of a series (alpha) ,(omega) -dithienyl polyenes was studied. Both one photon excited fluorescence and fluorescence originating from two photon absorption states were investigated. There is strong evidence of two photon absorption from fluorescence studies in agreement with our earlier studies involving nonlinear absorption.
We describe how defect induced one phonon absorption of far infrared radiation can be utilized for monochromatic generation of high frequency acoustic phonons in doped insulating materials. By combining this monochromatic phonon generation technique with an absorption vibronic sideband spectrometer we have been able to use time resolved high resolution phonon spectroscopy to investigate a range of fundamental dynamical processes which determine the behavior of a nonequilibrium phonon population. The combination of spectral and temporal resolution has enabled us to investigate the interaction of phonons both with electronic states and with themselves. We describe the mechanisms of energy transfer in insulating materials with particular emphasis on phonon assisted processes. While phonon assisted transfer is usually studied in a regime where a steady state phonon population stimulates energy transfer we describe results where a nonequilibrium phonon population stimulates the energy transfer between disparate Pr3+ sites in PrF3.
We discuss the generation of ultrashort far infrared (FIR) pulses using external switching of the output of a FIR superradiant cell pumped by a transversely excited atmospheric CO2 laser. The external switching mechanism consists of either one or two semiconducting wafers that have been optically excited with the second harmonic of a synchronized modelocked YAG:Nd3+ laser. The output of our FIR superradiant cell, a 50 ns FIR pulse is incident at Brewsters angle on a semiconducting wafer. On optical excitation above the band gap the (FIR) transmitting semiconductor takes on a metallic reflectance and the FIR transmission of the wafer is switched. If silicon is used as the switching medium, the long relaxation time requires that two wafers are used, one to turn on the pulse, and the second to turn it off. Using a pair of intrinsic silicon wafers the FIr pulse width can be adjusted with the ultimate limit being of the order of the width of the picosecond optical laser.
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