Conjugated Polymers for Ultrafast All-Optical Switching
Prof. Dr. C. Bubeck
Keywords: Third-Order Nonlinear Optics, Nonlinear Optical Susceptibility, Hyperpolarizability, Conjugated Polymers, Intensity Dependent Refractive Index, Two-Photon Absorption, Waveguide, Prism Coupling
The switching of light by light is based on the intensity dependencies of the refractive index or the absorption coefficient, which are third-order nonlinear optical processes. Conjugated polymers turned out as most suitable materials as they have the largest nonlinearities due to their highly polarizable one-dimensional π-electron system. Actual interest in their properties is growing as recent publications show that they can be combined favourably with silicon-based photonic integrated optical circuits offering unprecedented terahertz modulation frequencies, which are needed at telecommunication wavelengths in the range of 1500 nm.
This project involves measurements and model calculations of the intensity dependencies of refractive index and absorption coefficient of conjugated systems by means of strong picosecond laser pulses tunable in the near-infrared range. There, the nonlinear optical spectra of conjugated systems are usually dominated by two-photon excitations of electronic states. This causes resonance enhancements of the cubic nonlinearities. Our aim is an improved understanding of relations between chemical structure of conjugated systems, morphology and optical properties of thin film waveguides, which will help to design appropriate materials to satisfy the application criteria for all-optical waveguide switching.
The preparation of thin film waveguides with low mode propagation losses is another challenge because deleterious light scattering usually happens if sufficiently large aggregates appear in the films. This can be a significant problem with conjugated polymers or dye systems as their structures are rigid and favour nematic-like ordering phenomena, in particular at interfaces. Control of such ordering phenomena is a challenge in other research projects of organic electronics as well.
We use a Nd:YAG laser, which pumps an optical parametric generator-amplifier yielding intense pico-second laser pulses at 680 – 2000 nm for nonlinear optical spectroscopy. In the z-scan technique, the sample is moved through the focal region of a lens, which causes intensity variations to derive sign and magnitude of the nonlinear refractive index. Two-photon fluorescence excitation spectroscopy provides the two-photon absorption spectra of chromophores. Thin films are studied with third-harmonic generation spectroscopy, and prism coupling of waveguide modes, see Figures.
Recently, we showed that waveguides of the conjugated polymer MEH-PPV already satisfy the application criteria for all-optical waveguide switching, however at 1100 - 1200 nm. We continue to search for conjugated systems which show similar effects at longer wavelengths.
Fig. 1: Schematical view of prism coupling. The laser beam is totally reflected at the prism base. Its evanescent field couples through the air gap and excites a waveguide mode in the film at an appropriate entrance angle π. This causes a reduction of the reflected intensity IR depending on refractive index and absorption coefficient of the waveguide.
Fig. 2: Experimental demonstration of intensity-dependent prism coupling using a slab waveguide of the conjugated polymer MEH-PPV. The angular dependencies of the reflected intensities IR were measured at various pulse energies. This results in an ultrafast refractive index and absorbance change of the waveguide and causes a fully reversible change of IR.
Crosslinks to other projects: Optical Spectroscopy and Thin Films, Functional Dyes and Their Applications, Organic Electronics: Structure-Property Relations, Morphologies and Electronic States of Extended pi-Electron Systems, Two-Photon Nanolithography on Photo-Sensitised Organic Monolayers.
- K. Koynov, A. Bahtiar, C. Bubeck, B. Mühling, H. Meier: Effect of Donor-Acceptor Substitution on the Nonlinear Optical Properties of Oligo(1,4-phenyleneethynylene)s J. Phys. Chem. B 109, 10184-10188 (2005).
- A. Bahtiar, K. Koynov, A. Kibrom, T. Ahn, C. Bubeck: Multiphoton spectroscopy of polymers for all-optical switching Proc. SPIE 6330, 63300C-1-14 (2006).
- A. Bahtiar, K. Koynov, T. Ahn, C. Bubeck: Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV J. Phys. Chem. B 112, 3605-3610 (2008).