A method for writing programmable volume phase gratings into photorefractive materials using visible wavelengths in a transmission geometry, and then subsequently probing these gratings in a reflection geometry using infrared (IR) wavelengths to achieve specific angles of reflection of the probe beam is analyzed. The programmable features of these gratings include grating spacing and tilt, or K-vector magnitude and orientation. Relationships have been derived between the incidence angles of the writing beams and the corresponding reflection angles of the IR probe beam. More specifically, for a fixed angle of incidence of the probe beam, two unique writing beam angles can be found which generate a grating with the correct spacing and tilt to reflect and steer the probe beam through a wide field of desired angles.
It is well known in the literature that for a two photon nonlinear absorbing dye to be the most effective, high concentrations are needed. The problem is that most photophysical studies in solution are done at low concentration and in a solution. These low concentration studies are important for understanding inherent materials properties but it is also important to understand what happens in a material at high concentration. In addition to this, efforts have been made to study the effects of incorporating a dye into a solid matrix environment to better understand the constraints this environment has to a given material. Preliminary results for an epoxy system reveal the formation of excimers (excited state dimers) with an increase in concentration. Excimers are forming from the triplet excited state of the E1-BTF. A rate constant for this formation is 2.6 × 105 M-1 s-1. While rather slow, at greater than 50 mM concentration the excimer is readily formed with <90% efficiency. This must be considered when making nonlinear absorption measurements since the excimer will certainly contribute to the overall nonlinearity.
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.
In order to understand electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a model series of chromophores trans-Pt(PBu3)2(CCPhenyl-X)2, where X = NH2, OCH3, diphenylamino, t-Bu, methyl, H, F, benzothiazole, trifluoromethyl, CN and nitro. We collected linear spectra, including ground state absorption, phosphorescence and phosphorescence excitation spectra. We also performed DFT and TDDFT calculations on the ground and excited state properties of these compounds. The calculations and experimental data show the excited state properties are a function of the electronic properties of the substituents and the molecular conformation.
The third-order nonlinear susceptibility of crystalline Cadmium Magnesium Telluride (CdMgTe) was studies using a spatially resolved Irradiance Scan method including picosecond and nanosecond laser pulse widths at 1064nm. The samples were placed in a loosely focused beam, and a series of individual laser pulses at different energies were collected. The transmitted beam was reimaged to a CCD with a microscope objective providing a detailed objective function for numerical simulations. The nonlinear transmission results were modeled by way of a split-step nonlinear beam propagation method including diffraction, nonlinear absorption, and refraction arising from bound electrons and light-generated free carriers. The angular dependence of the third order susceptibility with respect to the electric field is also represented along with laser-induced damage thresholds.
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.
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.
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.
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.