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ROOM-TEMPERATURE FABRICATION TECHNIQUES FOR VERTICAL AND PLANAR ORGANIC BASED MICROCAVITIES

Conjugated polymers have been utilized in a variety of electronic and optoelectronic applications, and present the attractive prospective of the realization of an organic injection laser diode [1]. It is well known that the photoluminescence from conjugated polymers shows strong spectral line narrowing under high optical excitation intensities, which has been ascribed to amplified spontaneous emission (ASE) occurring in the polymer film, due to the optical gain via stimulated emission. The feedback necessary for laser action can be provided by distributed feedback structures (DFB), by micro-ring and micro-disk geometries or by an external microcavity [1]. Most of the previous feedback cavities are based on photonic bandgap (PBG) materials, i.e. materials with a periodical modulation of the index of refraction, exhibiting a frequency band where propagation of light is highly suppressed, in close analogy with the propagation of electrons inside a crystal. The realization of high-quality PBG cavities usually requires high temperature processing methods, that (especially if combined with oxygen atmosphere) are not compatible with organic materials, since they induce loss of adhesion to the substrate, and quenching of the optical properties [2]. We have developed new techniques for room temperature fabrication of planar (DFB) and vertical microcavities with conjugated polymers, thus preventing the deleterious effect of heating cycles on the optical properties of the light-emitting polymer. DFB planar structures are fabricated by nanoimprinting lithography [3]. The lithography onto light-emitting films was performed by room temperature nanoimprinting (Figure 1(a)). The master, a one dimensional grating with 600 nm period, was directly used as a mould for nanoimprinting, employing a precision manual press. AFM characterization of the imprinted films shows a good quality transfer of the master features to the polymer film. Vertical microcavities were instead realized by sandwiching a conjugated polymer film between two dielectrics mirrors (Figure 1(b)). High reflective TiO 2 /SiO 2 distributed Bragg reflector (DBR) mirrors were fabricated by room temperature electron beam evaporation. The DBRs were centred at 500 nm exhibiting a maximum of reflectivity as large as 99.8 % and a band of prohibit energies about 160 nm wide. The polymer film was deposited by spin-coating onto the DBR mirror and adhesion of the two cavity parts was realized by pressing the two coated mirrors in a precision manual press at 120 °C. In this case the structures itself of the cavity prevents oxygen diffusion inside the active layer, thus preserving the emission properties of the …