Flexible spectrally tunable radiation sources for multi modal spectroscopic analysis

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The long-term vision of this project is to realise a tunable light source for simultaneous multi modal sensor applications in the UV, in the near Infrared, and in the THz-range. Our goal for the application period is to develop a sensor for simultaneous THz- and Raman spectroscopy on the basis of monolithically tunable diode lasers. THz- and Raman spectroscopy address different excitations and the corresponding additional information is interesting for several applications. As an application example, we aim to detect relevant exhaust gases via THz-spectroscopy and soot particles via Raman spectroscopy. One out of many other possible applications of such a sensor is to control packaged food: even through the packaging the quality of the food could be analysed by Raman spectroscopy, and THz spectroscopy would enable to analyse the atmosphere within the package (e.g. water content). If it was possible to realise a two-colour radiation source in which the spectral distance of the two wavelengths could be tuned electrically from 0 to 6.4 nm, one could cover the spectroscopically relevant range von 0 to 3 THz (e.g. for the gases N2O, NO, CO, NO2, SO2 and H2O which are relevant in exhaust processes) by difference frequency generation. With the corresponding spectral distances up to 7nm one could also separate broad Raman features (e.g. of amorphous carbon) from disturbing background like fluorescence or ambient light. Therefore, our project goal is to realise a diode-laser based monolithically integrated radiation source that enables a tunable two-colour operation with up to 7 nm wavelength separation. On the basis of this laser source, we aim to realize a multi-modal sensor which analyses relevant exhaust gases by THz spectroscopy and soot particles by Raman spectroscopy. A basis for this development is to improve the understanding of the laser devices with regard to their lateral design (curvature of waveguides, coupling of arms) and the understanding of the requirements for this light source given by the THz generation and Raman spectroscopy. We chose 830 nm as the center wavelength for the multi-colour diode laser. On the one hand, this is a well established wavelength for Raman spectroscopy which reduces potentially appearing fluorescence, and for which required filter elements are available. A detection of the Raman-spectra for substance identification through spectral fingerprints is possible with Silicon based detectors. On the other hand, this wavelength is also well suited for THz generation with LT-GaAs based photoconductive antennas since it fits well to the absorption spectrum of LT-GaAs.