Plasmonic waveguide based mid - infrared lab - on - a - chip

Dielectric-loaded plasmonic waveguides are perfectly suitable for on-chip sensing of fluids. They allow long propagation length and large mode overlaps above 96% with a the analyte. Dielectric-loading is a alternative approach to increase the confinement of mid-infrared surface plasmons without the need for sub-wavelength patterning. Direct excitation and detection is realized using chip integrated quantum cascade lasers and detectors, enabling the realization of a monolithically integrated mid-infrared lab-on-a-chip. Mid-infrared spectroscopy is an extremely powerful and versatile technique. Sensors based on plasmonic resonances have been shown to strongly enhance the interaction with the analytes. Different to plasmonic resonance spectroscopy, here, the analyte is probed through the change of the absorption through the evanescent field that penetrates into the analyte. In dielectric waveguides, this evanescent tail is only a fraction of the mode, while in surface plasmon waveguides the major part (98-99%) can interact with the analyte. Different to the visible and near-infrared, SPPs at midinfrared frequencies have very long propagation length of several ten millimeter due to the larger conductivity. However, this also goes with larger penetration into the dielectric such that the SPPs are only very weakly bound. In order to overcome this issue, we use the concept of dielectricloading. Dielectric-loading is already known from visible and near-infrared plasmonics [1], where the additional layer is mainly used to achieve lateral guiding. In the mid-infrared, there is another much more important feature: The dielectric layer produces an increase of the effective modal index, enabling the support of well confined SPPs on noble metals [2]. Propagation properties similar to spoof plasmons can be achieved without the need for sub-wavelength patterning. In contrast to the visible and near-infrared, the dielectric layer thickness (100-300nm) is only a fraction of the mode width and the mode overlap with the surrounding fluid remains more than 96%. The strong surface sensitivity due to the evanescent nature of SPPs is interesting for surface sensitive biochemical sensing applications. The used materials are bio-compatible and might be functionalized with bio-receptors. The excitation and detector of the SPPs is realized with quantum cascade technology. A special bi-functional design allows both emission and detection such that different parts on the chip can be used for laser and others for detectors [3]. Direct coupling between the active devices and the dielectric-loaded SPP waveguide is possible due to the same polarization and the optimized mode overlap with the etch depth and the thickness of the dielectric layer. The entire system serves as a flexible platform to realize plasmonic waveguide based single chip sensors [2]. The prototype sensor device is capable to detect water in isopropanol over a large range of concentrations in real-time with 50ppm resolution. Multiple units of such laser/waveguide/detector units can be used to probe different wavelength to address multiple chemicals. This work was supported by the Austrian Science Funds (FWF) through project NextLite F4909N23, as well as by the FP7 EU-project ICARUS (FP7-SEC-2011-285417). H.D. acknowledges funding through an APART Fellowship of the Austrian Academy of Sciences.