Figure 3: Focussing of light in the optofluidic waveguides.

(a) Observed light propagation under different flow conditions. At a high flow rate of Q_clad=50 μl min⁻¹, the light is confined in the core due to the step-index distribution. At Q_clad=0.5 μl min⁻¹, the diffusion-induced gradient of the refractive index causes the light to repeatedly merge. With increasing r, the length of the first section becomes larger, for example, P_1/6=270 μm, P_1/3=300 μm and P_1/2=340 μm. The focussing period also increases, for example, the first period for P_1/3 is 300 μm, the second is extended to 490 μm and the third is 590 μm. (Scale bar equals 300 μm). (b) The first section lengths as a function of the flow rate of the core fluid at r=1/6 (square), r=1/3 (circle) and r=1/2 (triangle), showing an approximately linearly decreasing relationship over a Q_clad range of 0.1–1.0 μl min⁻¹.