We report enhanced amplified spontaneous emission (ASE) and optical gain performance in a conjugated polymer (CP)-based thin film waveguide (WG) Si(100)/SiO 2 /poly[2-methoxy-5-(2 0ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) by encapsulating the active layer with a transparent dielectric film of poly(methyl methacrylate) (PMMA). With index matched SiO 2 and PMMA claddings, symmetric WGs are formed that exhibit increased mode confinement and reduced propagation loss enabling lower ASE threshold (40%) and higher optical gain (50%) compared to Si(100)/SiO 2 /MEH-PPV/air asymmetric WGs. An extremely large net gain coefficient of 500 cm À1 is achieved under picosecond pulse excitation, which is >4Â larger than values previously reported in the literature. Fabrication of symmetric WGs requires no complex processing techniques, thus offering a simple, low-cost approach for effectively controlling the ASE behavior of CP-based WGs and related optical devices. V
We report the results of a detailed investigation that addresses the influence of polymer morphology and chain aggregation, as controlled by the chemical nature of the solvent, on the optical gain properties of the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Using the variable stripe length technique in the picosecond regime, we have extensively studied the optical gain performance of asymmetric planar waveguides formed with thin MEH-PPV films spin-cast from concentrated chlorobenzene (CB) and tetrahydrofuran (THF) solutions onto thermally oxidized silicon substrates. CB and THF solvents were chosen based on their known ability to promote and effectively limit aggregate formation, respectively. Very large net gain coefficients are demonstrated, reaching values of 330 and 365 cm(-1), respectively, when optically pumping the waveguides with a maximum energy density of 85 μJ/cm(2). Our results clearly demonstrate that polymer morphology, and hence, the chain conformation dependence of the degree of aggregation in the films as controlled by the solvent, has minimal impact on the net gain. Moreover, the waveguides exhibit low loss coefficients of 10-20 cm(-1) at the ASE wavelength. These results question the importance of polymer morphology and aggregate formation in polymer-based optical devices operating at high excitation densities in the stimulated emission regime as would be characteristic of lasers and optical amplifiers.
A strong excitation pulse width dependence on optical gain is reported in thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), which suggests that previously reported gain measurements have occurred in an excitation regime that cause damage to the polymer. Symmetric waveguides Si(100)/SiO2/MEH-PPV/poly(methyl methacrylate) are fabricated and optically pumped using laser pulses having temporal widths shorter and longer than the PL decay time, resulting in transient and quasi-steady-state excitation conditions, respectively. Under quasi-steady-state conditions (8 ns pulses), a maximum gain coefficient of ∼135 cm−1 is achieved at a fluence of 2250 μJ/cm2. However, extremely large optical gain is observed under transient pumping (25 ps), reaching 700 cm−1 at a fluence of only 85 μJ/cm2; this 5× improvement in optical gain performance is achieved at the same excitation density as that for ns pulses. It is clear that our ps gain measurements more accurately represent the intrinsic net gain of MEH-PPV than prior measurements in the quasi-steady-state regime.
Aggregate formation in conjugated polymer films is one of the most important phenomena thought to influence the photophysical properties of optical devices based on these materials. In the current work, we report the results of a detailed investigation on the morphology and chain aggregation dependence of optical gain in spin-coated thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Extensive gain measurements are performed using the variable stripe length technique with picosecond pulse excitation. The polymer morphology and extent of aggregate formation in the films are controlled by thermal annealing, which is relevant to the fabrication and optimization of conjugated polymer-based optical devices. The aggregation state of the polymer chains increases with the annealing temperature, which results in a decrease in luminescence efficiency at low excitation density (≤1018 cm−3). However, the increase in aggregate formation with increasing annealing temperature does not significantly alter the optical gain; very large gain coefficients are still achieved in films containing a relatively large fraction of aggregates. Although the largest gain coefficients, 450 cm−1, are observed for as-cast (non-annealed) MEH-PPV films, very large gain coefficients of 315 and 365 cm−1 are also demonstrated for MEH-PPV films annealed at 60 and 80 °C, respectively, in spite of the enhanced packing morphology and conformational order of the polymer chains. These results are contrary to the commonly held view that aggregate formation has a detrimental effect on the amplified spontaneous emission behavior of polymer-based devices operating in the stimulated emission regime, as would be characteristic of lasers and optical amplifiers. Moreover, because aggregates promote favorable charge transport properties, our data have important implications for future development of electrically driven polymer lasers; improving carrier mobility through controlled increases in chain aggregation should provide a viable path for enhancing injection efficiency without significantly degrading optical gain.
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