Waveguides constitute the basic components of integrated laser devices and optical amplifiers. Guiding light favours the interaction with the active material and the amplification by stimulated emission. Waveguides made by active light-emitting polymers are studied for the possibility of realizing flexible and low cost laser devices and optical amplifiers, and can be easily fabricated by spin-coating on substrates with lower refractive index compared to the organics (Fig. 1a). Patterning of the active layer by soft-lithographic methods also provides lateral field confinement and better waveguiding or amplification. Upon UV excitation of gain polymer active layers, amplified spontaneous emission (ASE) is easily recognised (Fig. 1b) by the appearance of a narrow (FWHM<10 nm) and coherent peak in the photoluminescence spectrum. The proper choice of the substrate, in terms of refractive index, and of the polymer thickness provides the waveguiding of only one mode (generally TE polarized), thus allowing to obtain a spectrally narrow, coherent and polarized emission at the waveguide edge (Fig. 1c). One main issue of polymer waveguide is the self-absorption in the active material, increasing the waveguide losses.  
    A powerful way for overcoming self-absorption is red-shifting the emission wavelength in a spectral region with minimal residual self-absorption. In polymer blends, energy transfer by dipole-dipole coupling (Förster transfer) occurs, if the emission spectrum of the host overlaps with the absorption spectrum of the guest, and that the dipoles are sufficiently close to each other.
    The waveguiding properties (in terms of losses and gain coefficients) of polymer blend waveguides were carefully characterized in our laboratories [1]. In most cases the ASE output intensity follows an exponential law (Beer’s law): (α= loss coefficient), confirming the predominance of absorption losses (Fig. 2). The measured loss coefficient where of the order of 10^-1 cm-1 (corresponding to about 1 dB cm^-1), which is the lowest reported for active organic waveguide. The investigation of the gain properties of the blend waveguide evidences a maximum gain coefficient of about 8 cm^-1 [2].
 

Fig. 1. (a) Schematic sketch of a polymer waveguide, composed by a low refractive index substrate (generally quartz) and an active polymer layer. (b) ASE spectrum of a conjugated polymer waveguide. (c) TE and TM components of ASE from a polymer waveguide.

Fig. 2. (a) Scheme of the set-up used to characterised the waveguide emission and properties. The blend waveguide losses are determined by fixing the length (l = a few mm) of the optical pumped region (i.e. the optical pumping beam stripe), and by moving the optical pump stripe away from the emission edge of the sample, thus producing the increasing of the length (x) of the un-pumped region. This takes into account losses resulting from self absorption and from possible edge effects (scattering, reflections etc.). (b) ASE output vs. length (x) of the non-excited region, for a conjugated polymers blend with different host/guest concentrations. The continuous and dashed lines are the best fits to an exponential decay law.

An interesting finding concerns the polarization properties of the waveguided light. We measured the ASE polarization state and calculated the polarization contrast, C, defined as:

         (1)

where I_^║ and I_⊥  are the intensity of the edge emitted light with polarization parallel and perpendicular to the waveguide plane, respectively. The blend waveguided light shows a significant increase of the polarization contrast, with a maximum of 0.65, at the ASE emission wavelength [1]. Typical values for randomly oriented polymer films are around 0.2 and values of 0.88-0.93 were obtained only in artificially oriented films. This is likely determined by a partial orientation of the donor-acceptor polymer molecules, or to the difference in the cut-off thickness of the waveguide for TE and TM polarizations.

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] A. Camposeo, E. Mele, L. Persano, D. Pisignano and R. Cingolani, Phys. Rev. B 73 165201 (2006).
 
[2] A. Camposeo, E. Mele, L. Persano, D. Pisignano and R. Cingolani, Opt. Lett.  31 1429 (2006).