Microcavity Effects in Thiophene-Based Oligomers
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Microcavity Effects in Thiophene-Based Oligomers G.Gigli1, M.Anni1, S. Patanè2, G.Barbarella3, L. Favaretto3 and R.Cingolani1 1 Istituto Nazionale Fisica della Materia (INFM), Dip. Ingegneria dell’Innovazione, Università di Lecce, Via per Arnesano, 73100 Lecce, ITALY 2 Istituto Nazionale di Fisica della Materia (INFM), Dip. di Fisica della Materia e Tecnologie Avanzate, Universitá di Messina, ITALY 3 Consiglio Nazionale delle Ricerche (CNR), ICOCEA, Area della Ricerca di Bologna, Via Gobetti 101, 40129 Bologna ITALY
ABSTRACT
We report on the realization and optical properties of a fully evaporated organic microcavity based on LiF-TeOx Distributed Bragg Reflectors and a substituted quinquethiophene as active material. The PL spectrum shows a strong line-width reduction, down to 10 nm, with respect to the bulk spectrum, which is about 110 nm broad. The emission at the mode wavelength shows an enhancement due to the light matter coupling in the optical resonator. The cavity mode shows a splitting for emission angles higher than about 30 degrees which continuously increases with angle up to 99 meV. This feature is observed both in angle resolved PL spectra and in angle resolved reflectance with unpolarized light. A study of angle resolved reflectance with polarized light allows us to attribute this splitting to a polarization effect, rather than to Rabi splitting due to strong coupling in the cavity.
INTRODUCTION
Organic semiconductors have recently attracted much attention for application in electroluminescent devices such as Light Emitting Diodes (LEDs) and displays. In this frame several works on organic microcavity structures demonstrated their utility to obtain a full control of the emission energy, linewidth, intensity and directionality for LED [1-9] as well as to obtain the optical feedback in laser resonators [10-11]. The commonly studied organic microcavities are realized by cladding the active material between a Distributed Bragg Reflector (DBR) and an evaporated metallic mirror [11-15], or by high temperature mechanical pressing of two half DBR cavities [16]. In this work we report on the realization and the optical characterization of a fully evaporated organic microcavity based on LiF-TeOx Distributed Bragg Reflectors and a substituted quinquethiophene as active material. This approach allows us to realize highly reproducible and ultra low cost cavities. Due to their low cost and the full compatibility with the deposition technique of the organic active material, these kind of cavities are interesting for application as organic devices, like Vertical Cavity LED (VCLED) or optically pumped laser. The cavity optical properties are studied through continuous wave photoluminescence (PL). The PL spectrum shows a strong line-width reduction, down to 10 nm, with respect to the bulk spectrum, which is about 110 nm broad.The angular properties are studied through angle resolved PL and angle resolved reflectance. The cavity mode shows a splitting for emission angles higher than about 30 degrees which continuousl
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