Bruce A. Reinhardt

Two-photon absorption and optical-limiting properties of novel organic compounds

The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.

High-density three-dimensional optical data storage in a stacked compact disk format with two-photon writing and single photon readout

Using a polymer block doped with a highly efficient two-photon dye, we have achieved a high density data storage with gray-scale control in multiple planes as stacked compact disks at a separation of 10 μm. The absorption and fluorescence of the dye at the written spot shift to a longer wavelength, permitting an easy fluorescence mode readout with a linear excitation using an inexpensive laser source. The storage capacity in this case is estimated to be 1012 bits/cm3.

Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials☆

Abstract Optical limiting and stabilization via two-photon absorption (TPA) in organic molecule-doped solid materials have been investigated. The nonlinear materials are epoxy rod and a composite glass rod doped with the same organic dopant (2,5-benzothiazole 3,4-didecyloxy thiophene). An ultrashort laser source with 0.5 ps pulsewidth and 602 nm wavelength was employed. The transmissivity of these two materials has been measured as a function of the input beam intensity. The measured results can be well fitted based on the assumption that TPA is the predominant mechanism producing the observed optical limiting behavior. Also, optical stabilization behavior is observed, and, at ∼930 MW/cm 2 input intensity level, the output intensity fluctuation is three times less than the input intensity fluctuation.

Third- and fifth-order optical nonlinearities in organic materials

Abstract We measure the nonlinear optical properties of solutions of a bisbenzethiozole-substituted thiophene compound (BBTDOT) and didecyloxy substituted polyphenyl (DDOS) using the Z -scan technique with 532 nm picosecond pulses. Both compounds exhibit two-photon absorption (2PA) and excited-state absorption (ESA) from the 2PA generated excited states. We measure the magnitude and sign of the real (refractive) and imaginary (2PA) parts of the third-order hyperpolarizability, and the excited-state absorptive and refractive cross sections. We observe third-order self-focusing in BBTDOT and self-defocusing in DDOS while both show excited-state defocusing. All these effects were previously observed and modeled in semiconductors giving insight into the nonlinearities occurring in these organic materials.

Two-photon fluorescence imaging and spectroscopy of nanostructured organic materials using a photon scanning tunneling microscope

Photon scanning tunneling microscopy and spectroscopy using femtosecond two-photon excitation are demonstrated. The measurement of both intensity dependence and spectral dependence is performed on a two-photon chromophore. A subdiffraction-limited resolution is obtained, and the domain-size dependence of spatial and spectral features is observed, which indicates the high degree of molecular order in the isolated nanoparticle. It is shown that the light confinement due to a quadratic dependence of the fluorescence intensity leads to an optical contrast enhancement with a coated probe.

Energy transfer coupling of two-photon absorption and reverse saturable absorption for enhanced optical power limiting.

We introduce a new approach to enhancing the optical power-limiting function at near-IR wavelength (~800 nm) by coupling effective two-photon absorption in one molecule with excited-state absorption in another molecule. We experimentally demonstrate this approach by using a strong two-photon absorbing dye, AF-380, and a strong reverse saturable absorber, C(60) . A nanosecond time-resolved experiment is used to show that energy transfer from AF-380 to C(60) generates triplet excitation in C(60) that further absorbs the pump beam to enhance the power-limiting function.

Influence of two‐photon absorption on third‐order nonlinear optical processes as studied by degenerate four‐wave mixing: The study of soluble didecyloxy substituted polyphenyls

We have investigated the influence of two‐photon absorption on the third‐order nonlinear optical properties of model organic molecules using the technique of degenerate four‐wave mixing (DFWM). A theoretical formulation developed here shows that the presence of two‐photon absorption, which is related to the imaginary part of the third‐order susceptibility χ(3), leads to an enhancement of the effective third‐order nonlinearity and to the appearance of effects caused by the formation of two‐photon generated excited states. The dynamic behavior of the nonlinearity is then governed by the properties of excited molecules. The nonlinear effects also involve contributions which depend on the fifth power of the electric field. We have performed a systematic study of third‐order nonlinear optical properties of alkoxy (–C10H21OCH) substituted p‐polyphenyl oligomers using the technique of time‐resolved degenerate four‐wave mixing with subpicosecond pulses at 602 nm. Experimentally determined values of the second‐ord...

Comparative reactivity of thiophene and 3,4‐(ethylenedioxy)thiophene as terminal electropolymerizable units in bis‐heterocycle arylenes

The monomers bis(2-thienyl)-9,9-didecylfluorene, BTDF, and bis(3,4-(ethylenedioxy)thien-2-yl)-9,9-didecylfluorene, BEDOT-DF, have been synthesized and electropolymerized to the corresponding conducting polymers. The potential for the electropolymerization of BTDF was found to be dependent on the solvent composition. In CH 2 Cl 2 , polymer film deposition is achieved only at potentials higher than 1.3 V vs. Ag/Ag + , while in a 30/70 mixture of CH 2 Cl 2 /CH 3 CN the polymerization is efficient at 0.9 V. BEDOT-DF polymerizes at significantly lower potentials and more rapidly than BTDF. The electron-donating alkoxy substituents of the EDOT units lead to stabilization of the cation radical intermediates allowing the electropolymerization to proceed at 0.55 V. The neutral polymers are insoluble in common organic solvents and are stable to 300°C under nitrogen. Upon oxidation, both polymers show two intragap transitions at intermediate doping levels due to the formation of bipolaronic states and the oxidized polymers exhibit conductivities up to 10 -4 S/cm. The redox-stimulated ion transport characteristics, studied by the electrochemical quartz crystal microbalance (EQCM) indicates that the electrolyte anions are the dominant mobile species.

Photoluminescence Studies and Read/Write Process of a Strong Two-Photon Absorbing Chromophore

The optical properties of a fluorene-based chromophore exhibiting a high two-photon absorption cross section have been investigated both in the pure state and as a guest–host system in poly(N-vinylcarbazole). Upon irradiation with a helium cadmium (He–Cd) laser at 325 nm, the guest–host sample exhibits a blueshift with a maximum emission at 459 nm. Information written onto the samples in the blue can also be read using an Ar+ laser. The chromophore undergoes a chemical change upon irradiation in air, and has been found to no longer exhibit upconversion after this transformation. Infrared analysis of the sample (the chromophore) before and after irradiation was carried out, and the spectra suggest the formation of a new conjugated species. Other applications we have developed using this phenomenon include the successful writing of optical device structures in polymer films where the AF-50 is a guest material.

Nonlinear optical properties of novel thiophene derivatives: Experimental and abinitio time‐dependent coupled perturbed Hartree–Fock studies

We report in this paper experimental and theoretical studies of the nonlinear optical properties of a new class of compounds which differ from traditional second‐order molecules in the sense that a commonly used electron donor chromophore is replaced by a thiophene ring. The molecular second‐order nonlinear optical coefficients, β, as determined by the electric field‐induced second‐harmonic generation technique, for 2‐nitro‐1‐(2‐thienyl)ethene and 4‐nitro‐1‐(2‐thienyl)‐1,3‐butadiene are, respectively, 1 and 3 times that of para‐nitroaniline. Ab initio time‐dependent coupled perturbed Hartree–Fock results are in agreement with the experimental findings. A Mulliken population analysis indicates that the sulfur atom in the thiophene ring acts as an electron donor. Both the experimental and the theoretical results confirm that a thiophene ring acts as an efficient donor, giving rise to highly efficient second‐order nonlinear optical properties. We also show that with this type of chromophore one can use a thi...