Amplified spontaneous emission and lasing in conducting polymers and fluorescent dyes in opals as photonic crystals

Journal ArticleSpectral narrowing of photoluminescence (PL) and evolution of sharp emission lines upon optical excitation have been observed in opals made of SiO2 spheres infiltrated with conducting polymers such as OOPPV and MDDOPPV and also fluorescent dyes such as rhodamine 6G, NK-3483, and coumarin 120. Their emission properties are dependent on the sort of the opal and the solvents used for infiltration. With increasing optical excitation intensity, spectral narrowing and evolution of sharp lines have been observed in the green opal infiltrated with OOPPV, MDDOPPV, or rhodamine 6G but not in the infiltrated red and purple opals. With a solvent having a refractive index similar to that of SiO2, the evolution of the sharp emission lines is greatly suppressed and the lines exhibit a blueshift with decreasing refractive index. In NK-3483 and coumarin 120 which show red and purple PL, respectively, evolution of sharp lines is observed when they were infiltrated in red and purple opals, respectively, but not in other opals. These results are discussed in terms of amplified spontaneous emission and multimode lasing due to optical feedback in the opal matrix with a periodic structure

Evidence for excimer photoexcitations in an ordered ?-conjugated polymer film

Journal ArticleWe report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations, we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to energy states appearing below the continuum band threshold of the single strand

Two-photon absorption spectra of luminescent conducting polymers measured over a wide spectral range

Journal ArticleWe report the two-photon absorption (TPA) spectra of poly(2,5-dibutoxy-p-phenylene acetylene) (PPADBO), poly(2,5-dioctyloxy-p-phenylene vinylene) (PPV-DOO), and poly(3-hexylthiophene) (PAT6) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution, we measured a strong two-photon allowed transition in ail three materials which we attribute to the mAg essential state. In the case of PPA-DBO and PPV-DOO, TPA peaks were coincident with dispersion in the nonlinear refractive indices as detected by reduced aperture Z scan. In all three polymers this peak occurs at approximately 1.3 the bandgap energy (Eg^). The excitonic nature of the excited electronic states in PPA-DBO is indicated by the lack of a TPA band at or near the 1 Bu exciton position. Saturation was observed in the nonlinear index of refraction near spectral peaks, as well as an apparent reverse Kramers-Kronig effect

Studies of resonant and preresonant femtosecond degenerate four-wave mixing in unoriented conducting polymers

Journal ArticleDegenerate four-wave mixing with femtosecond time resolution is used to measure the magnitude and transient response of the third-order nonlinear optical susceptibility x(3) (ω;ω, -ω,ω) at 620 nm in nonoriented conducting polymers including polydiacetylene, polyacetylene, polyaniline, polydiethynylsilane, polythiophene, and polythiophene derivatives. Resonant and nonresonant excitations influence the magnitude and transient response of xc3'. The electronic response is instantaneous for preresonant excitation, but for resonant excitation it has ultrafast and slow components which illustrate photoexcitation dynamics. The magnitude Of X (3) for all of the polymers is in the range from 10 - 10 to 5 X 10 - 8 esu depending on the energy difference between the laser excitation and the polymer absorption maximum

Femtosecond dynamics of the nonlinear optical response in polydiethynylsilane

Journal ArticleWe have measured the femtosecond dynamics of the nonlinear optical response xc3' in polydiethynylsilane ( C4H2SiBu2)x, a novel class of r-conjugated polymers incorporating Si, using degenerate four-wave mixing and photoinduced absorption techniques. In resonance conditions at 620 nm we found x (3) = 3 X 10 - 9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at high laser intensities. This material is an excellent candidate for nonlinear optical devices in the sub-THz frequency range

Two-photon absorption measured in the presence of strong one-photon saturation in cumulene-containing polymer

Journal ArticleThe Z-scan technique is used to extract the real and imaginary third order nonlinear susceptibilities, Rex(3) and Imx(3), respectively. A series of Z scans were conducted at 590 nm (near one-photon resonance) on a cumulene-containing polymer, poly(p-phenylene-l,4-diphenyl-l,2,3-butatriene) or PPC3, prepared in solution. At very high peak focal point intensities two-photon absorption is seen superimposed on a one-photon saturation signature in open aperture Z scans. A negative real third-order nonlinear susceptibility is also observed in closed aperture Z scans. We describe a procedure to extract the desired third-order nonlinear susceptibilities by conducting Z scans at various peak focal point intensities and then fitting the results by adjusting Rex(3) and saturation intensity. Molecular second hyperpolarizability, is calculated to be (-1.4+2.2i)xl029 esu which is almost 40 times larger than that measured in a monomer equivalent, and scaling nonlinearly with chain length. Z scans were also conducted at 780 nm (below one-photon resonance) on PPC3 and also gives a molecular second hyperpolarizability that is greater than that seen in the monomer. The higher value of molecular second hyperpolarizability measured near one-photon resonance is attributed to the existence of a real state at the first transition in a two-photon process. The extent of the exciton wave function obtained from the saturation intensity, is 60 A , larger than a single repeat unit

Ultrafast studies of laser-action in poly-(p-pherylene vinylene) films

Journal ArticleThe recent demonstration of emission spectral narrowing in ∏-conjugated polymer has stirred widespread interest to the phenomenon of laser-action in these materials. It is now generally believed that spectral narrowing in superior polymer films is caused by amplified spontaneous emission (i.e. single pass stimulated emission (SE)) enhanced by the op-QThF4 Fig. 2. Picosecond SE dynamics of DOO-PPV at 630 nm (a) constant L (=600 pm) for various P. (b) At constant intensity (=300 pJ/cm2 per pulse) for various L

Ratiometric spectral imaging for fast tumor detection and chemotherapy monitoring in vivo

We report a novel in vivo spectral imaging approach to cancer detection and chemotherapy assessment. We describe and characterize a ratiometric spectral imaging and analysis method and evaluate its performance for tumor detection and delineation by quantitatively monitoring the specific accumulation of targeted gallium corrole (HerGa) into HER2-positive (HER2 +) breast tumors. HerGa temporal accumulation in nude mice bearing HER2 + breast tumors was monitored comparatively by a. this new ratiometric imaging and analysis method; b. established (reflectance and fluorescence) spectral imaging; c. more commonly used fluorescence intensity imaging. We also tested the feasibility of HerGa imaging in vivo using the ratiometric spectral imaging method for tumor detection and delineation. Our results show that the new method not only provides better quantitative information than typical spectral imaging, but also better specificity than standard fluorescence intensity imaging, thus allowing enhanced in vivo outlining of tumors and dynamic, quantitative monitoring of targeted chemotherapy agent accumulation into them

Investigating photoexcitation-induced mitochondrial damage by chemotherapeutic corroles using multimode optical imaging

We recently reported that a targeted, brightly fluorescent gallium corrole (HerGa) is highly effective for breast tumor detection and treatment. Unlike structurally similar porphryins, HerGa exhibits tumor-targeted toxicity without the need for photoexcitation. We have now examined whether photoexcitation further modulates HerGa toxicity, using multimode optical imaging of live cells, including two-photon excited fluorescence, differential interference contrast (DIC), spectral, and lifetime imaging. Using two-photon excited fluorescence imaging, we observed that light at specific wavelengths augments the HerGa-mediated mitochondrial membrane potential disruption of breast cancer cells in situ. In addition, DIC, spectral, and fluorescence lifetime imaging enabled us to both validate cell damage by HerGa photoexcitation and investigate HerGa internalization, thus allowing optimization of light dose and timing. Our demonstration of HerGa phototoxicity opens the way for development of new methods of cancer intervention using tumor-targeted corroles