Organic materials that exhibit large real third-order optical nonlinearities, |Re(χ(3))|, and that also have low linear and nonlinear losses at telecommunication wavelengths may be useful for a range of all-optical signal-processing (AOSP) applications. Based on their solution linear and nonlinear optical characteristics, polymethine dyes in general, and chalcogenopyrylium-terminated heptamethines in particular, are a promising class of materials for AOSP, but the translation of their microscopic nonlinearity to device-relevant materials is hindered by significant aggregation of the molecules in high-chromophore density films. An approach to minimize aggregation these dyes, in which bulky and rigidly out-of-plane groups are introduced both in the center of the polymethine bridge and on the heterocyclic end groups, has been developed and can lead to thin-film materials exhibiting combinations of large |Re(χ(3))|, large two-photon figure-of-merit, and low linear loss that are suitable for AOSP.
 Hales J. M., Matichak J., Barlow S., Ohira S., Yesudas K., Brédas J.-L., Perry J. W. and S. R. Marder, Science, 2010, 327, 1485.
 Barlow S., Brédas J.-L., Getmanenko Y. A., Gieseking R. L., Hales J.M., Kim H., Marder S.R., Perry J. W., Risko C., Zhang Y., Materials Horizons, 2014, 1, 17.
All optical switching (AOS) applications require materials with a large nonlinear refractive index (n2) but relatively small linear and nonlinear absorption loss. The figure-of-merit (FOM), defined as the ratio between the real and imaginary parts of the second hyperpolarizability (γ), is widely used to evaluate the operating efficiency of AOS materials. By using an essential-state model, we describe the general dispersion behavior of γ of symmetric organic molecules and predict that the optimized wavelength range for a large FOM is near its linear absorption edge for cyanine-like dyes. Experimental studies are normally performed on organic solutes in solution which becomes problematic when the solvent nonlinearity dominates the total signal. This has been overcome using a Dual-arm Z-scan methodology to measure the solution and solvent simultaneously on two identical Z-scan arms and discriminating their small nonlinear signal difference. This technique significantly reduces the measurement uncertainty by correlating the excitation noise in both arms, leading to nearly an order-of-magnitude increase in sensitivity. Here we investigate the n2 and two-photon absorption (2PA) spectra of several classes of cyanine-like organic molecules and find that the results for most molecules agree qualitatively and quantitatively with the essential-state model. Many cyanine-like molecules show a relatively small FOM due to the presence of large 2PA bands near the linear absorption edge; however, an exception is found for a thiopyrylium polymethine molecule of which the maximum FOM can be < 400, making it an excellent candidate for AOS.
Electronic nonlinearities can lead to ultra-fast refractive index switching. This dynamic refractive index change can be
used to shift wavelengths as well as to mix pulses of different center wavelengths. Due to its high refractive index silicon
is suitable for tightly focusing light and generating high intensities required for such nonlinear effects, however high
nonlinear losses in silicon (two photon absorption and absorption by free carriers generated via two photon absorption)
limit transmission of high power pulses in silicon. Polymers and chalcogenide glasses have an improved nonlinear figure
of merit (ration of nonlinear effect to nonlinear losses) and also don't show free carrier absorption. Due to acceptable
levels of losses from generated free carriers, silicon organic hybrid or silicon glass heterogeneous structures offer to
achieve high conversion efficiencies and large net gain in micro photonic devices, which can be used for wavelength
conversion, parametric amplification and parametric oscillators, or for the generation of entangled photon pairs. We
show both theoretical estimates and experimental results for four wave mixing conversion efficiencies in silicon hybrid
and silicon heterogeneous structures.
We report on the nonlinear optical (NLO) transmittance and reflectance of a 20 nm-thick Ag film characterized by time-resolved
white-light continuum pump-probe experiments. The change in complex permittivity <i>Δε(t)</i> is extracted and is
fitted to the Drude model in the frequency domain and a two-temperature model in the time domain. A unified model is
presented that fully describes the dynamic NLO response of a thin Ag film that can be incorporated easily into the
modeling of more complex metal-dielectric multilayer structures designed to take advantage of the NLO response of Ag.
We report on the linear and nonlinear optical properties of Ag/Au multi-metal thin films and Fabry-Perot
resonator cavities. The linear optical properties of these multi-metal layers, having different mass distributions
and Ag/Au ratios with thicknesses around 15 nm, resemble those of electrically continuous metal layers. The
optical losses introduced by interband transitions in the Au layers are reduced to achieve peak transmittances
of 76 % around 550 nm. Using femtosecond-pulsed white-light continuum pump-probe experiments we show
that the nonlinear optical response of such multi-metal layers is comparable to that of neat Au thin films.
Low-finesse Fabry-Perot resonators fabricated with such multi-metal layers, combine the large NLO response
of Au with a transmittance of 60% and a spectral bandwidth that covers the visible spectral range.
Two-photon fluorescence microscopy is a powerful tool for the study of dynamic cellular processes and live-cell imaging. Many commercially available fluorescent probes have been used in multiphoton-based imaging studies despite exhibiting relatively low two-photon absorption cross-section values in the tunability range of ultrafast Ti:sapphire lasers commonly used in multiphoton microscopy imaging. Furthermore, available fluorophores may be plagued with low fluorescence quantum yield and/or photoinstability (i.e., photobleaching) on exposure to the high peak power and photon density provided by the ultrafast laser source. To address the demand for better performing dyes, we prepare fluorophores tailored for multiphoton imaging. These fluorophores are based on the fluorene ring system, known to exhibit high fluorescence quantum yield (>0.7) and high photostability. Furthermore, an amine-reactive fluorescent probe for the covalent attachment onto amine-containing biomolecules is also prepared. Epi-fluorescence and two-photon fluorescence microscopy images of H9c2 rat cardiomyoblasts stained with an efficient two-photon absorbing fluorene fluorophore is demonstrated. Additionally, single-photon spectral characteristics of the amine-reactive fluorophore, as well as the two-photon absorption cross sections of its model adduct in solution, and spectral characterization of a bovine serum albumin (BSA) as a model bioconjugate are presented.
Organic compounds that undergo strong nonlinear, multiphoton absorption have been gaining greater interest, mainly in the developing fields of multiphoton fluorescence imaging, optical data storage, 3-D microfabrication, and photodynamic therapy. Systematic studies have shown that conjugated organic molecules with large delocalized π electron systems show very large nonlinear optical effects. Two-photon absorbing chromophores have also been incorporated into dendrimers to increase two-photon absorptivity. A cooperative enhancement of two-photon absorption (2PA) has been observed, such as in the linkage of branched chromophores through a common amine group and chromophore-metal complexes. This enhancement may be related to extensive two-dimensional π-delocalization in these molecules. Herein, we describe the synthesis, structural characterization and photophysical study of a series of compounds (model, oligomer, and polymer) with symmetric molecular structure of the D-π-D motif and branched D-π-D dendrimeric structures based on substituted fluorene derivatives. Femtosecond 2PA cross sections were very large for some derivatives (over 10,000 GM) and often exhibited substantial solvent effects.
Fluorescent dyes and probes are key components in multiphoton based fluorescence microscopy imaging of biological samples. While many commercially available fluorescent dyes have sufficed, most exhibit relatively low two-photon absorption (2PA) cross-section values in the tunability range of Ti:sapphire lasers commonly used in multiphoton microscopy imaging. Furthermore, available fluorophores may be plagued with either low fluorescence quantum yields and/or the additional problem of rapid photobleaching upon exposure to the high peak powers provided by fs laser sources. In order to address the demand for better performing dyes for two-photon based imaging, we have prepared a new series of reactive fluorophores tailored for multiphoton imaging. These fluorophores are based upon the fluorene ring system, known to exhibit high fluorescence quantum yields, typically > 0.7, and possess high photostability. They have been functionalized with moieties to act, e.g., as efficient amine-reactive fluorescent probes for the covalent attachment onto, e.g., proteins and antibodies. The synthesis and the single-photon spectral characteristics, as well as measured two-photon absorption cross sections of the reactive fluorophores in solution will be presented. Spectral characterizations of bovine serum albumin (BSA) conjugated with the new reactive probe will also be presented.
We have performed nonlinear spectroscopic measurements to investigate the chemical structure/nonlinear optical property relations for a set of alkyl fluorene derivatives. The characterization method we have utilized is a femtosecond white-light continuum (WLC) pump-probe spectrometer that can rapidly characterize an organic sample’s nondegenerate two-photon absorption (2PA) spectrum. The nature of these experiments requires sophisticated data analysis. In particular, the relative group velocity mismatch between the pump and probe, which are at different frequencies, makes these pulses walk through each other within the thickness of the sample. For widely different frequencies, this can severely diminish the 2PA signal strength. However, given careful analysis, we have found good agreement with well-known semiconductor samples. Confidence in this method has allowed us to investigate the effects of solvism, electron-withdrawing character, conjugation length, and symmetry on the two-photon absorbing properties of these molecules. We have found an optimum solvent polarity as well as electron-withdrawing character which serves to maximize the strength of the 2PA in these materials. Different synthesis avenues have provided us with two different methods of extending the conjugation length that increases the nonlinearity as well. Finally, investigations of molecules with disparate symmetry have allowed us to identify the symmetry of the excited states. In addition, we present the first experimental study of the intermediate state resonance enhancement of nondegenerate 2PA in organic molecules. Using a simplified sum-over-states expression, we make comparisons between experiment and theory.
Two-photon fluorescence microscopy is a prominent tool in biological imaging analysis. Many commercially available fluorescent dyes currently being used have sufficed for multiphoton based imaging of biological samples. While measured two-photon cross-sections (in Goppert Meyer, GM units) of some of the dyes are available, many exhibit relatively low two-photon cross-section values in the tunability range of Ti:sapphire lasers commonly used in multiphoton microscopy imaging. For example, Bodipy FL exhibits a maximum GM unit of 18 at 925 nm, compared to a range of 2-4 GM units from 775 - 875 nm. Furthermore, available fluorophores may be plagued with either low fluorescence quantum yield and/or the additional problem of rapid photobleaching upon exposure to the high peak power provided by the fs laser source. In order to address the demand for better performing dyes for two-photon based imaging, we have prepared a new series of reactive fluorophores tailored for multiphoton imaging. These fluorophores are based upon the fluorene ring system, known to exhibit high fluorescence quantum yields, typically > 0.7, and possess high photostability. They have been functionalized with various moieties to act, e.g., as efficient amine-reactive fluorescent probes for the covalent attachment onto amine-containing biomolecules. Single-photon spectral characteristics, as well as measured two-photon cross sections of a reactive fluorophore and its model conjugate in solution, as well as spectral characterizations of a bovine serum albumin (BSA) conjugate are presented.
We report image formation via single and two-photon photoinduced fluorescence changes in a polymeric medium with two-photon fluorescence readout of multiplayer structures. Photoinduced acid generation in the presence of a two-photon fluorescent dye possessing strongly basic functional groups (7-benzothiazolyl-9,9-didecyl-2,2-(N,N- diphenylamino)fluorene underwent protonation upon exposure with UV or near-IR (740 nm fs pulses). Solution studies demonstrate formation of monoprotonated and diprotonated species upon irradiation, each resulting in distinctly different absorption and fluorescence properties. The fluorescence of the original, neutral, fluorophore is quenched upon monoprotonation with a concomitant increase in fluorescence at longer wavelengths due to the monoprotonated form. Hence, two channel two-photon fluorescence imaging provides 'positive' or 'negative' image readout capability. Results of solution and solid polymer thin films experiments are presented.