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Semiconducting polymers : A new class of solid-state laser materials

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.

Laser emission from solutions and films containing semiconducting polymer and titanium dioxide nanocrystals

Abstract 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.

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.

WHITE LIGHT FROM INGAN/CONJUGATED POLYMER HYBRID LIGHT-EMITTING DIODES

We report white light emission from InGaN/conjugated polymer hybrid light-emitting diodes (LEDs). White light sources (or sources with various colors) are achieved by combining the photoluminescence (PL) from semiconducting (conjugated) polymers with the emission from high efficiency InGaN based LEDs; the InGaN based LED provides the blue component and, simultaneously, serves as the short wavelength pump source for exciting the PL of the polymer film(s).

Plastic lasers: Semiconducting polymers as a new class of solid-state laser materials

Abstract 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.

Microstructure of thin films of photoluminescent semiconducting polymers

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.

“Plastic” lasers: Comparison of gain narrowing with a soluble semiconducting polymer in waveguides and microcavities

Gain narrowing and lasing from a soluble, highly photoluminescent conjugated polymer, poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylene vinylene) (BuEH-PPV), are compared using two resonant structures: planar waveguides and microcavities. The gain narrowing and lasing thresholds are comparable, 0.05–0.1 μJ (10 ns pulse focused to ∼1.5 mm). Gain narrowing is not observed in films on indium tin oxide (ITO) unless a cladding layer is placed between the BuEH-PPV and ITO. Single-mode microcavity lasers are obtained when a cavity resonance occurs at the wavelength where the gain of the polymer is maximum.

Polymer diodes using poly(3,4-dicyanothiophene)

Abstract 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 studies of stimulated emission and gain in solid films of conjugated polymers

Abstract 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.

Conjugated polymers as solid-state laser materials

Abstract 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.