Exploratory photonics

Overview

All-optical signal processing is becoming a key issue with the steady increase of bandwith demand in telecommunication and IT industries. Significantly higher channel rates require new concepts and device structures. In this context organic semiconductor devices offer some advantages with respect to traditional materials and devices. In particular, they feature easy processing, low cost and flexibility. If successful, lasers and ultrafast all-optical switches based on organic semiconductors could become important due to their integration potential on arbitrary substrates and the tunability of the lasing emission.

Organic lasers are improved by incorporating a photonic crystal that consists of a thin layer of TiO2. The TiO2 increases the index contrast and the confinement in the waveguide. Thus the mode coupling is increased, which results in larger feedback given to the lasing modes. The larger feedback leads to smaller devices as the interaction length is decreased to achieve the lasing threshold. To employ a photonic crystal consisting of a high-index material in this way gives rise to new design criteria. These criteria are investigated and employed to design an optimum organic photonic crystal laser. Devices have been fabricated according to optimum parameters and characterized. The measured spectral features of the laser agree very well with the predictions obtained from simulations.

Furthermore, Fabry-Perot cavities are fabricated by incorporating an organic nonlinear Kerr material between two dielectric mirrors. Using femto-second pump and probe measurements we characterize these hybrid 1-D photonic band gap structures for various organic materials.

Promising organic materials are C60 and MEH-PPV. By varying the pump beam wavelength across the cavity resonance we are able to delineate the various underlying nonlinear processes. It turns out that, in the spectral region from 780 to 880 nm, nonlinear absorption dominates the signal. However, for larger wavelengths of around 1300 to 1500 nm, refractive nonlinear effects dominate the signal. Comparing these measurements with computations, we are able to quantify both the refractive and absorptive nonlinear coefficients of various organic materials.