Organic semiconductor microcavities, a class of devices in which a semiconductor conjugated layer is embedded between two mirrors, have attracted a lot of attention both from the fundamental and the practical viewpoint, for their possible technological applications in advanced optoelectronics. From a fundamental point of view, in such devices the optical properties of the active semiconductor layer can be dramatically altered due to quantum electrodynamics (QED) effects, whereas from an applicative point of view microcavities offer the possibility of realizing, by a single-step process, multidimensional laser arrays, with low-divergence beam emitted perpendicularly to the substrate, thus being particularly suitable for coupling with optical fibers and for telecommunication applications. The objectives of this research activity are the realization of microcavity laser devices embedding an organic active layer and the understanding of the interactions among cavity photons and primary excitations in the light-emitting material.  

Fig. 1. (a) Different mirrors realised by reactive e-beam evaporation, whose photonic band-gap is tuned in the visible and near infrared range. (b) Transmission spectra of the realized DBRs, having stop-band centred from 425 to 850 nm.

 We developed a novel approach for fabricating vertical cavity surface-emitting lasers (VCSELs) constituted by an active polymer layer embedded between two high-reflectance mirrors. Our cavities are made by Distributed Bragg Reflectors (DBRs, Fig. 1) with two alternating dielectric layers (SiO2 and TiO2), ensuring very high reflectivity (> 99%) in a wide spectral region. A specific room-temperature deposition process of oxides by reactive e-beam evaporation [1] allow us the direct evaporation of DBR mirrors on top of the active layer, thus preserving the emission properties of organic films [2]. 

Fig. 2. Scheme of a conjugated polymer-based VCSEL (Left) and emission spectra of VCSELs emitting in the visible spectral range (right).

The VCSELs rely on a conjugated polymer active layer deposited on top of the bottom mirror by spin-coating (cavity mode centered at λ/2, λ=VCSEL emission wavelength) and by a top DBR mirror directly evaporated onto the active organic (Fig. 2) [3,4]. These VCSELs can operate in the whole visible range (450-620 nm) by varying the active materials and the DBR stop-bands [5,6], with linewidth of 1-2 nm and threshold pumping fluence as low as 1.2 μJ/cm2 (measured for VCSELs embedding a blue emitting carbazole/fluorenyl derivative copolymer active film [6]). The VCSEL emission mode is essentially Gaussian, and the typical lifetime of our devices is longer than 1.5×104 pulses at an excitation density 500 times larger than threshold.
    We also performed some basic studies on emission of the vertical microcavites. The light-matter interaction resulting in the absorption and emission of photons does not only depend on the electronic properties of the active molecules, but also on the intensity of the electric field  of the incident light, which is determined by the photonic boundary conditions. In particular, 1D PhCs, such as vertical microcavities, are the simplest systems where QED effects are observable. In the strong-coupling regime, a cavity photon couples strongly to an exciton having the same energy and in-plane momentum. Polaritons can be considered as a mixture of the exciton and cavity-photon modes. We fabricated and characterized monolithic organic semiconductor microcavity, operating in the strong coupling regime [7] with a J-aggregate of cyanine dyes as active layer (in collaboration with Dr. T. Virgili at the Polytechnic of Milan). The optical properties (Fig. 3a-b) of the microcavities clearly show the presence of cavity polaritons with vacuum Rabi splitting as large as 100 meV (Fig. 3c) and polarization splitting up to 35 meV.
 

Fig. 3. (a) Cavity reflectivity at 45° vs. incident light energy. The arrows mark the energy of the LP and UP polariton. (b) Photoluminescence spectrum of the cavity (sample excited at normal incidence, PL at 42°). (c) Dispersion curves as a function of the incident angle (by reflectivity measurements). The horizontal line indicates the emission energy of bare film of J-aggregate cyanine dye (1.78 eV).

For more information, please contact: Dr. Andrea Camposeo ( This e-mail address is being protected from spam bots, you need JavaScript enabled to view it )

Publications:
[1] L. Persano, R. Cingolani, D. Pisignano, J.  Vac. Sci. Tech. B  23, 1654 (2005).
 
[2] L. Persano, E. Mele, A. Camposeo, P. Del Carro, R. Cingolani, and D. Pisignano, Chem. Phys. Lett. 411, 316 (2005).

[3] L. Persano, E. Mele, R. Cingolani, D. Pisignano, Appl. Phys. Lett. 86, Art. N. 031103 (2005).

[4] L. Persano, A. Camposeo, P. Del Carro, E. Mele, R. Cingolani, and D. Pisignano, Opt. Exp. 14 1951 (2006).

[5] L. Persano, E. Mele, P. Del Carro, R. Cingolani, D. Pisignano, M. Zavelani-Rossi, S. Longhi, and G. Lanzani, Appl. Phys. Lett. 88, Art. N. 121110 (2006).

[6] L. Persano, A. Camposeo, P. Del Carro, E. Mele, R. Cingolani and D. Pisignano, Appl. Phys. Lett. 89, Art. N. 121111 (2006).

[7] A. Camposeo, L. Persano, P. Del Carro, T. Virgili, R. Cingolani and D. Pisignano, Org. Electron. 8, 114-119 (2007).