Previously, we presented the experimental evidence for a degenerate frequency two beam coupling (TBC) in two photon absorbing (2PA) organic solutions. It has been well established that the two critical requirements for TBC are a nonlinear refractive index with a finite lifetime and that the interacting fields must have non-degenerate frequencies. However, degenerate frequency coupling has been shown for fields containing a time-dependent phase, i.e. a frequency chirp. This chirp can either be intrinsic to the fields or induced by self- and cross- phase modulation (S/XPM). For nanosecond pulses, the relatively small intrinsic chirp of the fields is negligible compared to the strong cumulative effects of population redistribution which generates large S/XPM. A S/XPM-mediated theoretical treatment for degenerate frequency TBC is presented along with numerical simulations using known nonlinear optical parameters to model the experimental results.
To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon)
ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings(nFAR)
and ligand topology(polyacene(L), polyphenanthrene(Z) or compact(C)). We measured ground state absorption,
fluorescence and phosphorescence spectra. We also performed nanosecond and picosecond flash photolysis
experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an
expanded series of chromophores to calculate the S1 and T1 state energies. In both the DFT results and experiment, the
ground state and phosphorescence spectra are a function of both nFAR and ligand topology. In the L chromophores, the
S1 and T1 state energies decrease linearly with nFAR. In contrast the S1 and T1 state energies of the Z chromophores
oscillate with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores.
The picosecond transient spectra show complex behavior, having spectra reflecting intersystem crossing, vibrational
cooling and solvent relaxation processes. The nanosecond transient spectra result from the T1 - Tn transition. The timeresolved
spectra show no systematic variation with structure, showing more complex behavior than previously studied
platinum acetylides having phenylene ethynylene ligands.
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. We have recently explored a new type of two photon absorbing dye
containing a platinum center with ligands that vary in length that contain electron withdrawing benzothiazole. With
increased π-π* conjugation we expect to observe a red shift in the absorption properties of the material. We have
investigated the photophysical properties of the platinum chromophores using a variety of experimental techniques.
Previously we determined that the singlet and triplet excited states are responsible for nearly all of the nonlinearity in the
nanosecond regime accept the two photon mechanism that is primarily used for excitation. Therefore we would like to
tune the photophysical properties of both the singlet and triplet excited state in these materials. To our surprise we found
there is quite a bit of red shifting due to a metal-to-ligand charge transfer from the platinum to the ligand rather than the
expected shifting due to increased π-π* conjugation. However, with increased ligand length the chromophore does take
on more π-π* character.
To learn about excited state geometry in biphenyl-containing platinum acetylides, we synthesized a series of
compounds that have biphenyl ligands. The ligands consisted of biphenyl(I), the hindered 2'-methyl biphenyl(III) and
planar fluorenyl(IV) groups. We also synthesized a "half" complex(II) consisting of one ligand attached to the platinum
atom. The optical properties of these compounds were measured by ground state absorption, phosphorescence, ultrafast
transient absorption and nanosecond transient absorption spectroscopy. DFT calculations were performed to determine
the ground state and triplet state geometries and the lowest triplet energy. TDDFT calculations were performed to
determine singlet excited state energies. Compared to the reference compound I, ground state spectra show a blue shift
in II and III and red shift in IV, showing the singlet energy is sensitive to conjugation and biphenyl twist angle.
Comparison of the phosphorescence spectra of I and II shows the triplet exciton is confined to one ligand. The time
behavior of the ultrafast excited state absorption spectrum of I shows a red shift within 1 ps from the initial spectrum.
This behavior is not seen in IV. The different behavior suggests formation of the triplet state of I is accompanied by
conversion from a non-planar to a planar conformation while IV retains a planar conformation.
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.
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.
Extensive measurements and modeling of several two photon absorbing materials are described. These are used to elucidate the relative significance of various relaxation and excitation processes that come into play in nonlinear transmission (NLT) and two photon absorption cross section measurements. A reliable measurement of the one photon absorption cross sections at energies 0.5 to ~1.7 eV below the fundamental transition are presented with Voigt function fits that enable the determination of the Gaussian and Lorentzian line widths. Both a numerical model and an analytical model are developed neither of which use any adjustable parameters in comparing calculated NLT results to data. Both models fit the data relatively well over the full range of the experiment. The analytical model captures the primary causes of the nonlinear absorption in the low intensity regime and demonstrates that the nonlinear transmittance can be estimated as a simple effective three-photon process. The numerical model calculates the spatial and time dependence of three state populations and all of the transitions between these states. This model improves the quality of the nonlinear transmission fit which is due to the inclusion of the ground state absorption. Additionally an observation of a strong, long lived transient which is quenched by oxygen suggests multiphoton ionization is happening at low intensities. Thus the full range of constraints applicable to all measurements of the two photon cross section are presented.
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.
To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers. Theoretical properties of these compounds were measured by UV/Vis absorption, photoluminescence, and nanosecond flash photolysis experiments. It was found that as the number of oligomer units increased ,the transition energies decreased without saturation. The low ground state absorption and UV absorption edge gives rise to solutions that are nearly water clear. A very broad triplet state absorption extending from the absorption edge to the limits of our spectrometer is demonstrated to also be intense. These results enhance the understanding of these materials when used for nonlinear absorption applications and enable the prediction properties for materials extending this class of dyes.
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.
Two polymer systems including polymer elastomers and gels have been studied as host materials for optical limiting applications. Both systems have high laser damage thresholds (LDT), typically 20 to 35 times higher than commercial PMMA bulk materials. For the polymer elastomers, Epotek optical epoxy 301-2 and 310, the LDT increases with an increase of the molecular flexibility. We speculate that the thermo-mechanical fracture may be the mechanism for the laser induced damage. For the hydrogel system, the LDT increases with increasing water content. The mobility of the water plays a key role in determining the LDT by facilitating laser energy dissipation and self-healing. It appears that the polymer elastomer and hydrogel systems both have potential for high power laser applications.
Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.
A design method for reverse saturable absorbing (RSA) dye concentration gradient limiters, termed here the Absorption Diffraction Balance (ADB) design method, is used to produce designs for multiple chromophores and is extended to allow incident plane waves. The ADB design method is reviewed for Gaussian beams applied to a constant fluence design and a linear fluence design. These two designs are combined to allow different dyes to be used in different portions of the limiter. It is found that this hybrid design significantly enhances performance under some circumstances. It is also shown to reduce the probability of dye photodegradation. The ADB design method is extended to allow for incident plane waves or a top-hat beam profile. The field at the geometric shadow edge, expressed in terms of Lommel functions, is shown to closely match the Gaussian field when the incident irradiance, power, and second moments are the same. Since the irradiance distribution is not monotonically increasing in the focal region, the required concentration distribution has regions of negative concentration, i.e. gain regions. These designs are useful for initiating numerical nonlinear beam propagation studies.
We report the results of the laser damage threshold of an undoped optical grade epoxy elastomer and compare its performance
to that of an epoxy glassy thermoset and polymethylmethacrylate (PMMA) thermoplastic. Tests were performed in a tight
focus, f/5 geometry, with 7 ns pulses at 532 nm. The elastomer exhibits a damage threshold more than 600 times greater
than its glassy thermosetting analog and over 20 times greater than PMMA. We describe the importance that
thermomechanical properties have on improving laser damage resistance in organic polymers. Optical limiting experiments
are performed with matrices doped with silicon naphthalocyanine and zinc octabromotetraphenyl porphyrin as the
chromophores. We demonstrate the advantage of using an elastic matrix to improve the performance of solid-state organic
A new method for the design of reverse saturable absorbing (RSA) dye concentration gradient (CG) limiters is
developed and applied to produce three new designs. The radiation transport equation and the divergence behavior cI
Gaussian beams are solved together analyticaly for the dye concentration gradient and the fluence distribution in the sample.
It is demonstrated that the fluence distribution for a given CG can be found which is independent of the dye kinetics. A
new figure ofmerit for gradient RSA limiters is defmed and used to compare designs. These new designs are shown to be
significantly better than a constant fluence design. These designs are useful for initiating numerical nonlinear beam
Conventional guest-host optical limiting materials utilize either a liquid solvent or solid as the matrix for nonlinear absorbing chromophore dopants. Concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, low viscosity liquid solutions can not retain a concentration gradient while polymer solid matrices damage at low laser fluences. We report on a novel approach of using an elastic polymer and viscoelastic gels for guest- host optimal limiting matrices. We achieve high bulk laser damage thresholds in the hosts and maintain a concentration gradient of the chromophore. By softening the epoxy we significantly enhance its bulk laser damage threshold. We characterize this effect by measuring the damage threshold as a function of viscoelastic properties. In addition, optical limiting was demonstrated in all the hosts doped with nonlinear phthalocyanine chromophores.
Z-scan profiles very different than the typical peak-valley signatures have been observed. In the closed over open ratio data, a feature centered around z equals 0 and with normalized transmission greater than 1 has been observed in three classes of materials. Experimental artifacts are ruled out. In one class, tolane melts, the usual peak and valley signature is not observed even at low irradiances. In the other two material classes, C60 fullerene solutions and benzophenone-thiophene polymer solutions, the usual signature is observed at low irradiances. A model based on the Gaussian decomposition method is used to interpret the data. This analysis leads to the interpretation of the observations as a diffractive effect arising from the distorted amplitude profile due to nonlinear absorption.
We study one free base and seven metallo- octabromotetraphenylporphyrins by several techniques. In a pico-second pump-probe experiment, we monitor the transient transmission of each sample up to 11 ns after it is irradiated by an intense laser pulse. Combined with the results from time-resolved fluorescence spectroscopy, we propose a simple model to interpret the transmission data. We attribute the reduction in the transmission to triplet state absorption and extract the triplet state absorption cross-sections, as well as the lifetimes from the time dependent transmission data. In a separate experiment where the transmission of a nanosecond laser pulse is measured with various input energies, our measurement in the cross- sections predicts the correct optical limiting behavior. We assess the overall optical limiting performance of all 8 samples by direct comparison with C60 at the same ground state transmission.
We demonstrate optical limiting in a unique guest-host system which uses neither the typical liquid or solid host. Instead, we dope a gelatin gel host with a water soluble Copper (II) phthalocyaninetetrasulfonic acid, tetrasodium salt (CuPcTs). We report on the gelatin's viscoelasticity, laser damage threshold, and self healing of this damage. The viscoelastic gelatin has mechanical properties quite different than a liquid or solid. Our laser measurements demonstrate that the single shot damage threshold of the undoped gelatin host increases with decreasing gelatin concentration. The gelatin also has a much higher laser damage threshold than a stiff acrylic. Unlike brittle solids, the soft gelatin self heals from laser induced damage. Optical limiting test also show the utility of a gelatin host doped with CuPcTs. The CuPcTs/gelatin matrix is not damaged at incident laser energies 5 times the single shot damage threshold of the gelatin host. However, at this high laser energy the CuPcTs is photo bleached at the beam waist. We report photo bleached sites by annealing the CuPcTs/gelatin matrix.
Complex formations in C60 solutions and the reverse saturable absorption (RSA) properties have previously been observed. In a wide survey of solvents conducted in our laboratory, several solvents exhibited complexing behavior with C60 when analyzed with UV-Vis and fluorescence spectroscopy. We have studied the formation of complexes in C60/toluene solutions when benzonitrile, N-methyl-pyrrolidinone, and triphenylamine, and diethylaniline were added. Formation of the complexes was followed with UV-Vis spectroscopy. Several plots of the complexes were done to determine the equilibrium constant and the change in molar extinction coefficient in each system. We report the behavior of the reverse saturable absorption (RSA) properties as a function of complexing agent and concentration.
The nonlinear absorption in a series of compounds has been investigated using the open aperture Z-scan technique. The compounds investigated were diphenyl acetylene, diphenyl butadiene, diphenyl butadiyne, 1,4-bis (phenylethynyl) benzene, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl. Solutions of the compounds were made in chloroform and showed no linear absorption at the laser wavelength. The Z-scan measurements were carried out using 35 ps laser pulses at 532 nm. Nonlinear absorption was observed for all of these materials, and the nonlinear absorption coefficient (beta) has been determined from this data. The nonlinear absorption process for two of the compounds, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl, is most likely caused by a two-photon absorption to a two-photon allowed state. The nonlinear absorption mechanism for the remaining compounds appears to be two-photon absorption followed by linear absorption from this excited state. The refractive nonlinearity has been more difficult to determine, and in most cases appears to be indistinguishable from the solvent contribution.
This paper presents the results of a comparison study among three carbon-based solutions: a carbon-black suspension, C60 in toluene, and C60 in chloronaphthalene. The carbon-black suspensions were used as baseline samples for comparison with the C60 solutions and were fabricated with the same transmission at 694 nm as the C60 samples. A ruby laser operating both with and without the Q-switch was used in this study. The limiting results of the C60 materials are compared with the predicted results from a five-level model previously used to calculate the optical limiting performance of C60 at 532 nm.
The nonlinear absorptive properties of C60 are of significant interest especially in the near IR. In this paper we describe spectral emission measurements on C60 solutions. In toluene solutions the laser induced emission in the fluence regime of 3 to 1500 J/cm2 excited by nanosecond and picosecond pulses is measured. In C60/toluene solutions a broad double feature is observed with peaks at 692 nm and 732 nm matching the published fluorescence spectra of C60. No evidence for breakdown is apparent in the C60 solutions though there is in the neat toluene. A survey study to determine solvent effects on the electronic states of C60 is reported. Thirteen solvents were chosen from among those in which C60 is soluble to give a wide range of dielectric constant and polarity. The molar extinction coefficient and fluorescence is measured in these solutions. A large solvent induced increase of the near IR molar extinction coefficient is observed in most solvents relative to toluene. Evidence for complex formation is observed. The changes of these spectra relative to that observed for toluene solutions indicate that solvent induced change of the quantum efficiency of triplet production is possible.
The nonlinear absorptive properties of C60 are of significant interest. In this paper we describe spectral emission measurements on C60/toluene solutions and neat toluene in the fluence regime of 3 to 1500 J/cm2 excited by nanosecond and picosecond pulses. Ten emission lines between 547 nm and 635 nm are observed in both toluene and C60 solutions. These lines are found to correlate well with published Raman spectra of toluene. In C60 solutions an additional broad double feature is observed with peaks at 692 nm and 732 nm matching the published fluorescence spectra of C60. At the highest fluences a broad new feature appears at 655 nm, grows nonlinearly, and saturates in the C60 solution. In the blue spectral region, at wavelengths shorter than the pump wavelength, a plethora of lines are observed in neat toluene between 340 and 520 nm and found to correlate with molecular disassociation products. No evidence for breakdown, however, is apparent in the C60 solutions. Both neat toluene and C60 solutions show evidence of photochemical reaction.
Nonlinear absorption dynamics are studied in C60 solutions by measuring the nonlinear energy transmittance and transmitted temporal profiles of Q-switched doubled Nd:YAG pulses. The rate equations are solved for a five level system using a combination of analytical and numerical techniques via Mathcad with model parameters taken from the literature. Very good agreement is found with the nonlinear energy transmittance. The slight disagreement between the calculated and temporal profiles is discussed.
The optical limiting behavior of trans-diphenyl-1,3-butadiene (DPB), 2,2'-3- methyldithienylethylene (MDTE), 2,2'-dithienyl-1,3-butadiene (DTB) and trans- 2,2'-dibenzthienylethylene (DBTE) in chloroform was studied. Solutions of these compounds are nonresonant at 532 nm. The effects of laser beam spot size at 532 nm on the onset of limiting and the temporal profiles of the transmitted laser pulses were examined. The results indicate two photon absorption as the major contributor to the optical limiting. Two photon absorption cross sections were estimated from the optical limiting data.