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The polymer chain distribution in sclution-cast thin films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) has been investigated by X-ray diffraction. The results show that regardless of the solvent, chain orientation in cast films is anisotropic; the PPV backbones and the planes defined by the benzene rings within the PPV backbones are predominantly parallel to the film plane. When drop-cast from tetrahydrofuran (THF), the preferred solvent for preparation of MEH-PPV films with low threshold in gain narrowing experiments, this anisotropy in chain orientation is more pronounced, and the crystallinity is higher.

A brief review of the recent emergence of semiconducting π -conjugated polymers as solid-state laser materials is given. Photopumped gain narrowing and lasing in undiluted submicron thick polymer films have been demonstrated. The dramatic collapse of the emission linewidth (to as little as 7 nm) occurs at very low pump-energy thresholds. Gain narrowing is found in over a dozen different polymers representing a variety of molecular structures and is explained in terms of amplified spontaneous emission (ASE) in planar waveguides, which allows the distance traveled by emitted photons to exceed the short intrinsic gain lengths. Prospects for producing electrically pumped solid-state polymer diode lasers using this class of materials are discussed in the context of the low-threshold gain narrowing in submicron-thick films.

Two major hurdles to producing solidstate polymer laser diodes are identified: the presence of a photoinduced absorption (PA) which inhibits the stimulated emission (SE) necessary for lasing; and t...

The recent discovery of gain narrowing in conjugated polymers has placed new emphasis on understanding the details of stimulated emission (SE) and gain in these plastic laser materials. In this Letter, the results of ultrafast SE and photoinduced absorption (PA) measurements on films of poly(2-butyl-5-(2′-ethyl-hexyl)-1,4-phenylenevinylene) (BuEH-PPV), a conjugated polymer that demonstrates gain narrowing above threshold, are presented. The ≈ 60 ps SE gain decay time of BuEH-PPV is significantly longer than that of most other conjugated polymers, allowing ample time for lasing to occur. We argue that the existence of gain is limited by a dynamic blue-shift of the PA resulting from interchain interactions.

We demonstrate optically pumped lasing in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. Lasing is evidenced by a dramatic collapse of the emission line width (to as little as 7 nm) at very low pump energy thresholds (~10 μJ/cm 2 ). Laser action is found in over a dozen different conjugated polymers representing a variety of molecular structures, including poly( p -phenylenevinylene), poly( p -phenylene) and polyfluorene derivatives; lasing wavelengths in these materials span the visible spectrum. The short gain lengths in these conjugated polymers are attributed to the high density of chromophores, the large density of states associated with the interband (π-π * ) transition in quasi-one-dimensional systems, and the Stokes shift which minimizes self-absorption and enhances stimulated emission in the absence of excited state absorption. The observation of lasing in this new class of solid-state laser materials is explained in terms of simple planar waveguiding structures which allow the distance traveled by emitted photons to readily exceed the short intrinsic gain lengths. The dependence of the threshold and the gain narrowed line width on the solvent from which the film is spin cast suggests that chain packing can be used to control lasing in some of these materials. The prospects for producing electrically pumped solid state polymer diode lasers using this class of materials are discussed in the context of the lowthreshold gain narrowing in submicron films.

Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium. The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.

We report laser emission from solutions and dilute blend films containing a semiconducting polymer, poly(2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV), and titanium dioxide (TiO2) nanocrystals under pulsed optical excitation at 532 and 435 nm. The TiO2 nanoparticles multiply scatter photons in the active polymer medium such that gain exceeds loss above a critical excitation threshold. Above threshold, the emission spectrum narrows significantly. Solid state lasing is observed from free standing films of MEH-PPV and TiO2 in polystyrene. This is the first demonstration of lasing with a semiconducting polymer in the solid state as the active medium.

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.

We have made polymer distributed feedback (DFB) lasers by spin casting films of poly(2-butyl, 5-(2'-ethyl-hexyl)-1,4-phenylene vinylene), BuEH-PPV, onto gratings in SiO/sub 2/ cladding films on silicon substrates. The gratings had periods of 170-185 nm and depths of up to 30 nm and were made by holographic lithography and reactive ion etching. The lasers were photopumped by 10 nsec pulses of 435 nm light from a doubled Nd:YAG laser equipped with a hydrogen Raman cell. Above a threshold of 60 W/cm/sup 2/, a narrow line emission from DFB lasing was detected at 563 nm, which is the wavelength where Bragg reflections were expected for this laser. Above a threshold of 2200 W/cm/sup 2/, a broader peak characteristic of amplified spontaneous emission (ASE) was detected at 547 nm. When other lasers with different grating periods were studied, similar behavior was observed with the DFB peak always occurring at the wavelength predicted by the Bragg equation. We studied the competition between distributed feedback lasing and amplified spontaneous emission. One of the limitations of these initial semiconducting polymer DFB lasers is that the feedback is so strong that light at the Bragg wavelength cannot travel much further than approximately 10 /spl mu/m before it is reflected. We will present our current work on making waveguide structures with weaker feedback by introducing an intermediate cladding layer between the grating and gain polymer and on distributed Bragg reflector (DBR) lasers, which have a smooth gain region with short grating reflectors at each end. We will also present work which is in progress for improving the quality of the waveguides.

The recent emergence of semiconducting (pi) -conjugated polymers as solid-state laser materials has been made possible by the availability of numerous high photoluminescence efficiency polymers with emission wavelengths spanning the visible spectrum. As the first important step in exploring the feasibility of electrically pumped polymer laser diodes, we have demonstrated photopumped gain narrowing in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. The dramatic collapse of the emission line width (to as little as 7 nm) occurs at very low pump energy thresholds (approximately 10 (mu) J/cm2 per 10 ns pulse). Gain narrowing is found in over a dozen different polymers representing a variety of molecular structures and is explained in terms of amplified spontaneous emission in planar waveguides which allow the distance traveled by emitted photons to exceed the short intrinsic gain lengths. The short gain lengths arise from the high density of chromophores, the large density of states associated with the (pi) -(pi) * transition in quasi-one-dimensional systems, and the Stokes shift which minimizes self- absorption. Prospects for producing electrically pumped solid-state polymer diode lasers using this class of materials are discussed in the context of the low-threshold gain narrowing in submicron thick films.

Ultrafast stimulated emission (SE) and photoinduced absorption (PA) measurements were performed on films of a new alkyl-substituted conjugated polymer, BuEH-PPV. The 60 ps SE decay time of BuEH-PPV is significantly longer than that of other conjugated polymers. The SE lifetime is limited by a dynamic blue-shift of the PA. The presence of the persistent SE opens the possibility of using pnoton confinement by scattering to produce lasing polymer devices.

A polymer blend containing MEH-PPV and a conjugated polyquinoline, poly(2,6-(4-phenylquinoline)), PQ, is obtained by first dissolving PQ in common organic solvents by complexation with a functionalized protonic acid and then mixing with a solution of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV. Transmission electron microscopy (TEM) of blend films reveals an extremely homogeneous and fine fibrillar interconnected dispersion of the PQ-complex for blends containing ~1% PQ, suggesting that there is a large interfacial area between the MEH-PPV and PQ-complex regions. In devices with the simple structure ITO/polymer blend/Al, EL external quantum efficiency is significantly enhanced by the blending, reaching a value of 0.08% photons/electron for blend films with a nominal PQ content of about 1.5%. This is greater than that of ITO/MEH-PPV/A1 devices by a factor of more than 20.

We have succeeded in the in-situ polymerization of thin, highly homogeneous films of poly(3,4-dicyanothiophene), PDCTh, on conducting substrates by heating the precursor monomer, 2,5-diiodo-3,4-dicyanothiophene. As a result of the presence of two cyano substituents on the thiophene backbone, PDCTh is an electronegative polymer, with depressed HOMO (6.7 eV) and LUMO (3.6 eV) levels. We demonstrate a polymer diode using PDCTh and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV. The device, which has the structure Au/PDCTh/MEH-PPV/Au, has rectification ratios in the current voltage characteristics in excess of 10 3 and is a photodiode with a dc sensitivity at -3 V reverse bias of 4×10 −4 A/W or a quantum yield of 0.1% electrons/photon

Ultrafast transient absorption, stimulated emission (SE), and photoluminescence (PL) measurements were performed on solid films of the conducting polymer: poly[2-butyl-5-(2′-ethyl-hexyl)-1,4-phenylenevinylene](BuEH-PPV). The ground state absorption and luminescence of BuEH-PPV are similar to those of unsubstituted PPV, indicating that alkyl substitution does not significantly alter the electronic structure of the ground state. The PL lifetime and PL quantum yield of BuEH-PPV, however, are ≥ 900 ps and over 0.6, respectively. These numbers are nearly 3 times longer and higher than those for unsubstituted PPV. Moreover, BuEH-PPV exhibits a strong, relatively long-lived SE (≥ 60 ps). The SE decays more quickly than the PL due to an interfering photoinduced absorption which undergoes a dynamic blue-shift to mask the SE at later times. These observations taken together offer a prescription for controlling the excited state electronic structure of conjugated polymers, and open the possible production of optically or electrically pumped solid polymer lasers.

The morphologies of polymer blends based on MEH-PPV have been studied by using transmission electron microscopy. It is shown that by controlling the preparation conditions, the phase separation in the blend films results in nano-scale network structure. These nanostructured self assembled polymer blends, as a novel class of materials, offer enhanced electronic and optical properties.