Broadband, semiconducting polymer optical amplifiers

Abstract

Conjugated organic semiconductors are promising materials for a wide range of optoelectronic applications. Their simple processing, strong absorption, large gain cross-sections and limited extent of concentration quenching make them very attractive for lasers [ 11 and optical amplifiers [2]. In particular, conjugated polymer optical amplifiers could have excellent compatibility with polymer optical fibre for applications in local area telecommunications. While the presence of gain in conjugated polymers is well established, only recently we reported the first direct demonstration of strong amplification, in which a weak probe beamis amplified to an intense beam [2]. In this paper we will report the operating characteristics of polymer optical amplifiers and studies of their device physics. We present data showing the dependence of the optical gain on chromophore concentration, probe wavelength, and pump and probe pulse energies. We will also present Drogress in the development of compact, solid-state polymer .optifal ainplifidrs. The experimental configuration for studying our polymer optical amplifiers is shown in figure 1. A nitrogen laser was used as the pulsed excitation source for both the polymer amplifier and a probe dye laser. Its output was split into two beams; each of which was focused to a stripe, to transversely pump the cells containing the polymer and laser dye solutions. The output beam from the dye laser (which could be tuned over the range 575 nm to 640 nm) was attenuated and then focused through the excited region of the 10 mm long amplifier cell. The optical gain was measured for amplifiers made from solutions of the conjugated polymer OCIClo-PPV. We found that a concentration of 2.0 g/l gave the highest optical gain. Gains as high as 30-43 dEl were measured across the available probe wavelength range of 65 nm, corresponding to a bandwidth of -50 THz. This gain bandwidth includes the low loss window of PMMA polymer fibres (610 - 640 nm), which implies that OCIClo-PPV could be suitable for use with such fibres. Figure 2 shows the effect of probe energy on the gain of the amplifier. Gain saturation is observed and follows the well-known relationship for a homogeneously broadened gain medium. We measured a small signal gain of 44 f 1 &/cm and a saturation energy density of 6.3 f 0.7 mJicm' at a wavelength of 600 nm. From these values we calculate a stimulated emission cross section of o= (5.3 f 0.6) x 10.'' cm2. We have also investigated the dependence of optical gain on excitation density (figure 3). We find that the gain saturates at pump energies above 100 pJ and grows only slowly with increasing pump energy. The maximum pump rate from the nitrogen laser was equivalent to an excitation density of roughly one excitation per 10 repeat units of the polymer. We discuss the importance of absorption saturation and non-radiative decay to gain saturation at high pump densities. From our results of gain in liquid solutions of conjugated polymers, we find that OCICIn-PPV shows promise as an amplifier medium for low cost, short-haul data transmission with polymer optical fibres. Finally, we will present progress in developing compact, all-solid-state configurations of optical amplifiers based on organic semiconductors.