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Optofluidic dye lasers have recently attracted much interest as potentially efficient light sources for integration
on lab-on-a-chip micro-systems. However, dye bleaching resulting in limited life-time could limit the applications
of such devices in lab-on-a-chip technology. Typically, the problem of dye bleaching is addressed by employing a
continuous convective flow of liquid-dissolved dye molecules, compensating the bleaching caused by the external
optical pump. In previously reported optofluidic light sources the required convective dye replenishing flow has
been achieved by external fluid handling apparatus (syringe pumps), on-chip microfluidic pumps, or by means
of capillary effect. We have investigated the bleaching dynamics that occur in optofluidic light sources where a
liquid laser dye in a micro-fluidic channel is locally bleached due to optical pumping. A simple one-dimensional
diffusion model is used to explore the characteristic evolution of the local un-bleached dye concentration in the
optically pumped or bleached volume of the device. In the absence of convective flow, the decay of the local
dye concentration in the optically pumped volume is governed by the diffusion rate and the resulting lifetime
of the device is mainly limited by the capacity of the fluidic reservoirs. Generic microfluidic platforms typically
allow for device layouts with a large volume ratio between the fluidic reservoir and the region being optically
pumped. These conclusions drawn from the simple model are supported by basic experiments. Our investigations
reveal the possibility that such optofluidic dye laser devices may potentially be operated for days by diffusion
without the need for a convective flow. Relying on diffusion rather than convection to generate the necessary
dye replenishment significantly simplifies optofluidic dye laser device layouts, omitting the need for cumbersome
and costly external fluidic handling or on-chip microfluidic pumping devices.
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