Optical Spectroscopy and Thin Films

Prof. Dr. C. Bubeck

Keywords: UV-Vis-NIR Spectroscopy, Reflectometry, Fluorescence, Raman, FTIR, Prism Coupling, Waveguide, Spin-Coating, Langmuir-Blodgett Technique, Profilometry, Contact Angle

Preparation of thin films and their optical characterization are basic methods applied in many projects of polymer research. Layer thicknesses between a monolayer and several micrometers are required depending on the research topic. The optical spectroscopy of polymers yields information on the chemical structure and orientation of molecular units, electronic and vibronic excitations and relaxations, and optical materials constants such as absorption coefficient and refractive index of thin films.
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.

Experienced staff engineers are involved in the technical developments of these methods and keep the “Know How” of the instrumentation. The access to the methods is provided either as a service or by training of those scientists who need the techniques frequently to enable their own use of the instruments. The group has the following major activities:
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.

The thin film laboratory comprises all technological steps from substrate cleaning, fabrication of inorganic and organic thin films, to thickness measurements with surface profilometry and contact angle measurements. Major film preparation techniques are: Thermal evaporation and sputtering of inorganic materials (metals, semiconductors or dielectrics), spin-coating of polymers at laminar flow and inert gas conditions in a so-called glove-box, Langmuir-Blodgett technique for multilayer films of amphiphilic materials, film drawing and dipping techniques.

The development of measurement techniques is frequently required for special projects in the Institute, for example:

• IR transmission and reflection spectroscopy at grazing incidence is used to evaluate the orientation of molecular units of thin polymer films and even monolayers.
• Reflections at interfaces cause frequent problems in the transmission and reflection spectra of ultrathin films. Quantitative reflection and absorption spectroscopy in the visible and near-infrared range is used in combination with model calculations to solve this problem. This allows to determine the optical constants of thin films i.e. intrinsic absorption coefficient (see Fig. 1) and refractive index.
• A prism coupler setup is used to study planar waveguides. The refractive index, and attenuation loss of thin film waveguides and their anisotropies can be determined at many laser wavelengths

Fig. 1: Spectra of the intrinsic absorption coefficient of thin films of MEH-PPV (inset) with different molecular weights between 1: M_w = 9.3 kg/mol and 9: M_w = 1600 kg/mol.

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Crosslinks to other projects: Conjugated Polymers for Ultrafast All-Optical Switching, Synthesis of graphene molecules/Giant π-conjugated macrocycles, Functional dyes and their applications, Organic electronics: Structure-property relations, Model Membranes.

References:

1. K. Koynov, A. Bahtiar, T. Ahn, R. M. Cordeiro, H.-H. Hörhold, C. Bubeck: Molecular Weight Dependence of Chain Orientation and Optical Constants of Thin Films of the Conjugated Polymer MEH-PPV, Macromolecules 39, 8692-8698 (2006).
2. M.M. Demir, K. Koynov, Ü. Akbey, C. Bubeck, I. Park, I. Lieberwirth, G. Wegner: Optical Properties of Composites of PMMA and Surface-Modified Zincite Nanoparticles, Macromolecules 40, 1089-1100 (2007).
3. M. Campoy-Quiles, J. Nelson, P. G. Etchegoin, D. D. C. Bradley, V. Zhokhavets, G. Gobsch, H. Vaughan, A. Monkman, O. Inganas, N. K. Persson, H. Arwin, M. Garriga, M. I. Alonso, G. Herrmann, M. Becker, W. Scholdei, M. Jahja, C. Bubeck: On the determination of anisotropy in polymer thin films: A comparative study of optical techniques, Physica Status Solidi C 5, 1270–1273 (2008).