Effect of surface structuring onto the efficiency of the in- and out-coupling of light from a chip in Lab-on-a-chip approaches with optical detection

Optical detection methods have been implemented on micro-fluidic chips containing channels or cavities of different geometries e.g. for colorimetry or fluorescence measurements with excitation within the chip plane [1-2]. The most prominent problem of the read-out from a micro-fluidic chip is the limitation of the optical yield. Without e.g. an immersion liquid for compensation of the total reflection on the boundary, only about 11-13% of rays cross over the boundary from a polymer chip to air. One efficient method to increase the optical yield from a chip is a ray reorientation inside of the chip using an additional surface structure creating new incident refraction conditions on the boundary before rays are leaving the chip. The use of 45°-tilted mirror arrangements for in- and out-coupling of the fluorescence signal from a micro-fluidic chip and the realization of this principle for low-cost fluorescence detection systems have been published [3]. This paper includes the investigation of the effect of different tilt angles of total reflection and metallized-surface mirrors for an analyte volume emitter, using the ray-tracing simulation tool OptiCAD10. Furthermore, an estimation of the influence of a surface-emitted signal for different geometries of metallized detection cells with or without a combination with external lenses on the out-coupling efficiency will be presented. The best result of an out-coupling efficiency increase of 10 times was achieved for a combination of a structured and metallized detection cell with an external cylindrical lens.