| Organic Distributed Feedback Lasers |
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Organic conjugated polymers attract an increasing interest, since these materials, coupled to or embedded in photonic crystals (PhCs) can provide new effective media for photonics. In particular, light-emitting polymers exhibiting good performances of amplified spontaneous emission and gain cross sections over a broad spectral range (from UV to near infrared) are suitable for the realization of optically pumped laser devices. To provide the feedback required for lasing, non invasive lithographic tools have to be specifically implemented and developed, enabling patterning active conjugated materials without deteriorating the optical properties. We developed processing approaches suitable for light-emitting organics, operating at room temperature, for the realization of PhCs polymer lasers relying on distributed feedback (DFB) cavities. The impact of the lithographic process on the absolute quantum yield of the organic active layers was carefully investigated, allowing us to rule out the degradation of the emission upon processing. We fabricated DFB cavities [1,2] by nanoimprint lithography at room-temperature and high resolution masters, fabricated by e-beam lithography (EBL). The RT-NIL technique was initially ideated for non-thermoplastic oligomers, but it also revealed suitable for conjugated polymers, allowing us to obtain a highly faithful pattern transfer, without reducing the polymer emission yield. Printed polymeric DFB lasers exhibit single-mode emission, with a full width at half maximum (FWHM) of a few nm and a pump threshold of a few tens of μJ/cm2 (Fig. 1a) [1]. Printed DFB lasers based on organic dyes emitting in the NIR [2], hosted by a poly(methylmethacrylate) (PMMA) matrix, exhibit FWHM values as low as 8 Å (Fig. 1b), and strongly polarized emission, with a polarization contrast as high as 0.99. The lasing wavelength is tunable in the range 890-930 nm by adjusting the grating period, and the operational lifetime is up to 6×103 excitation pulses in vacuum environment. Recently, we investigated the possibility of directly patterning the polymer active layer by EBL, without any masking or developing/etching processes [3]. The optical and morphological properties of the exposed polymer film were investigated in depth, resulting in a smooth damage of the emission properties (in terms of photoluminescence quantum yield) of the active layer. By direct EBL, DFB structures were patterned on conjugated polymer thin films (Fig. 2a). The lasers were found to emit in the range 607-620 nm, with a linewidth of the laser emission around 1 nm (Fig. 2b) [3]. The measured threshold excitation fluence was 34 μJ/cm2 and the laser emission exhibits the typical linear growth above threshold. Our work evidences the possibility of realizing non-morphological DFB structures in a polymer layer, important for the integration of metallic electrodes and for the exploitation of electric fields for tuning the emission or for charge injection.
For more information, please contact: Dr. Andrea Camposeo (
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) Publications: [1] E. Mele, A. Camposeo, R. Stabile, P. Del Carro, F. Di Benedetto, L. Persano, R. Cingolani, and D. Pisignano, Appl. Phys. Lett. 89 Art. N. 131109 (2006). [2] P. Del Carro, A. Camposeo, R. Stabile, E. Mele, L. Persano, R. Cingolani, and D. Pisignano, App. Phys. Lett. 89 Art. N. 201105 (2006). [3] R. Stabile, A. Camposeo, L. Persano, S. Tavazzi, R. Cingolani, and D. Pisignano, Appl. Phys. Lett. 91, Art. N. 101110 (2007). |
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