Molecule aggregation in groove in Fused silica

[Sciencestd]

Image (a) is a view from above of a fused silica sample, the groove depth about one micrometer and the width is about 5 micrometer, the image (b) is a side view of the facet, when I cast a solution with molecules the aggregation is more on the grooves area which is logic, but when I shine a light on the groove from the side (in image (b)) the aggregation of the molecules on the groove enhanced and almost takes the shape of the groove...
What do you think is the reason?
The light source is a laser.