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We present a novel technology for investigations of biological specimens using a planar waveguide structure for optical excitation. The waveguide structure consists of a nanometer-thick high-index polymer core in a symmetric cladding environment where the bottom cladding is index-matched to the aqueous biological sample under investigation. This technique is different from other (asymmetric) waveguide excitation principles and has several unique features, including a wide tunability of the penetration depth of the evanescent field, efficient excitation of waveguide modes, straightforward multicolor excitation and high-speed imaging of, e.g., cell membranes or other structures adjacent to the chip surface. In particular, the wide range of possible penetration depths furthermore makes the technique more flexible than evanescent-wave excitation methods based on total internal reflection (e.g. TIRF microscopy). The method is well suited for evanescent-wave microscopy of biological samples, surface sensing and other applications that require illumination spread over macroscopic areas but confined to penetration depths ranging from 100 nm up to several micrometers. In the present study, we establish the practical range of penetration depths of the evanescent field and use the waveguide-excitation principle to investigate cells labelled with fluorescent markers. Cell morphology is also studied using non-labelled cells immersed in fluorescent solution. |