E.K. Miller

SEMICONDUCTING POLYMER DISTRIBUTED FEEDBACK LASERS

We have fabricated photopumped distributed feedback lasers by spin-casting thin films of the semiconducting polymer poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylenevinylene) over gratings in silicon oxide. The lasers have two modes that each have a linewidth of 0.2 nm. The lasing wavelength was tuned from 540 to 583 nm by adjusting the period of the gratings.

Index of refraction and waveguiding in thin films of a conjugated polymer which exhibits stimulated emission

We report variable angle spectroscopic ellipsometry (VASE) measurements on thin films of a soluble derivative of poly-paraphenylenevinylene (PPV) in the emission region of the spectrum. We find that the films are birefringent, with a higher and more dispersive index of refraction for light polarized with its electric field in the plane of the film. Physically, this anisotropy is explained by preferential in-plane orientation of the polymer chains; a result of this orientation is that on typical substrates such as glass and quartz, only transverse-electric waveguide modes propagate in the polymer, while transverse-magnetic modes are evanescent. The implications of these data for amplified spontaneous emission and design of in-plane laser structures are discussed.

Reflectance anisotropy spectroscopy of oriented films of semiconducting polymers

We present reflectance anisotropy spectroscopy (RAS) data for aligned films of luminescent conjugated polymers. Ultrathin films (5–10 nm) spin-cast onto friction-deposited poly-tetrafluoroethylene (PTFE) show birefringence and dichroism indicating alignment of the polymer chains with the PTFE axis. The observed dichroism agrees very well with recently published polarized ultraviolet absorption spectra. In particular, the spectrum of a derivative of poly (para-phenylene) shows the onset of a perpendicular-polarized absorption feature at 5 eV, consistent with recent theoretical predictions. Thick films, prepared by gel-processing in polyethylene (PE), also show characteristic optical anisotropy, and the RAS results are interpreted in terms of the ellipsometric functions, Ψ and Δ.

Static and dynamic photoluminescence (PL) quenching of polymer :quencher systems in solutions

Quenching of the PL from conjugated polymers in solution by charge transfer to electron acceptors was studied by PL measurements while varying the quencher concentration, the ion concentration in buffered aqueous solutions, and the temperature. For the aqueous solutions, the PL quenching was greatly enhanced by static quenching originating from the formation of a weakly bound complex between the anionic conjugated polymer and cationic electron acceptors. The estimated Coulomb binding energy was - 150 meV. In contrast, for MEH-PPV and PCBM in 1,2-dichlorobenzene, dynamic quenching was identified as the basic mechanism.

Observation of photoinduced charge transfer in conducting polymer/fullerene composites using a high-bandgap polymer

Abstract We report photoinduced electron transfer in films of poly(3-decyloxy-1,4-phenylene) (DO-PPP) blended with C 60 , observed by infrared photoexcitation spectroscopy. The photoinduced IR-active vibrational (IRAV) structure and electronic subgap absorption features of the DO-PPP are greatly enhanced upon adding just two percent C 60 - The spectral signature of the C 60 anion at 1.15 eV is clearly observed as a peak superimposed on the weak background between the midgap states of the polymer. The system is pumped at 2.41 eV, below the bandgap of the polymer, but within the visible absorption tails of both the polymer and of the C 60 .

Characterization of semiconducting polymer laser materials and the prospects for diode lasers

Abstract By measuring the gain and loss in thin film planar waveguides using a standard technique developed for inorganic laser materials, we show that the narrow-line emission from photopumped waveguides of the conjugated polymer poly(2-butyl-5-(2′-ethyl-hexyl)-1, 4-phenylenevinylene) (BuEH-PPV) results from amplification of spontaneous emission (ASE). The narrowed linewidth of the ASE spectrum is determined by gain saturation. The techniques presented offer a simple and useful way for evaluating conjugated polymer as materials for solid state lasers. The prospects for making polymer diode lasers are discussed.

Reflectance studies of soluble polypyrrole doped with dodecylbenzene sulfonic acid

Abstract Recent progress in the processing of conducting polypyrrole (PPy) from solution has made possible the spin-casting of optical quality films or varying thickness. We present the results of reflectance studies of soluble PPy films doped with dodecylbenzene sulfonic acid (DBSA) over a wide spectral range (.01 eV –6 eV) at room temperature. The reflectance exhibits a plasma resonance at 1.7 eV and increases sharply at the lowest frequencies. These are the spectral features expected for a material with a partially filled conduction band. However, the corresponding optical conductivity and the real part of the dielectric function show low-frequency features characteristic of a conductor on the insulating side of the metal-insulator (M-I) transition. We conclude that there exists a finite density of localized states at the Fermi level, typical of a Fermi glass. The processibility and stability of this material make it attractive for applications such as transparent electrodes.

Infrared photoexcitation spectra of conducting polymer/methanofullerene films

Abstract We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)-methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index, ΔN = Δn(ω)+iΔϰ(ω), are obtained. The results indicate that the PIA spectra are dominated by Δϰ(ω), while the PIR spectra are dominated mainly by Δn(ω). The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials.

Photoinduced changes in the complex index of refraction resulting from photoinduced charge transfer in conjugated polymer/fullerene blends

We report steady-state photoinduced absorption (PIA) and photoinduced reflectance (PIR) in films of MEH-PPV and BCHA- PPV blended with fullerene-based acceptors. Absorption from the metastable charge-transferred state is probed by PIA; the modulated absorption spectrum causes changes in the real part of the index of refraction, (Delta) n, which can be measured directly by PIR. The charge transfer gives rise to pronounced features in (Delta) n, including vibrational structure in the mid- and near-IR. Our measurements over a wide spectral range allow quantitative comparison of (Delta) n obtained from PIR with that obtained from Kramers- Kronig transformation of the PIA data. We find good agreement throughout the IR, indicating that our method for measuring (Delta) n is useful as an analytical tool for optical characterization and for prediction of optical spectral ranges for nonlinear optical response.

Nonlinear optical changes in the complex index of refraction in conducting polymer-methanofullerene films evaluated by photoexcitation spectroscopy

We report photoinduced absorption and photoinduced reflectance spectra of poly(3-octyl thiophene)- methanofullerene films. The data allow direct evaluation of the corresponding changes in the complex refractive index, (Delta) N equals (Delta) n((omega) )+i(Delta) (kappa) ((omega) ), of the films. As a results of the efficient photoinduced intermolecular charge transfer, the magnitudes of (Delta) n((omega) ) and (Delta) (kappa) ((omega) ) are significantly enhanced over those in either of the component materials, with (Delta) n and (Delta) (kappa) approximately equals 10-2 in the infrared at laser pump intensity of only 50mW/cm2. Moreover, the photoinduced absorption shows new features at 1.2 eV and 1.6 eV, both of which are associated with excited state absorptions of the methanofullerene anion. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials, indicating that conducting polymer/methanofullerene films are promising as high- performance nonlinear optical materials.