Reconfigurable diffraction gratings based on elastomeric microfluidic devices

This paper describes the fabrication and operating principles of a reconfigurable diffraction grating based on a microfluidic device. The device consists of an array of microscopic channels (50 μm wide and 20 μm deep) defined by the conformal contact between a transparent elastomeric material that has an embossed surface relief and a flat glass substrate. The microchannels can be filled reversibly with fluids (gas, aqueous solutions, or organic solutions). The difference in index of refraction between the fluid in the array of microchannels and the structural elastomeric solid generates a difference in the phase of the light passing through the device; absorption by the fluid changes the amplitude of the light. Both of these effects give rise to diffraction. The gratings are reconfigured by pumping liquids with different indices of refraction and optical densities through the microchannels. In these experiments, the devices exhibited maximum depths of modulation of approximately 20 dB and switching times of approximately 50 ms; the volume of liquid sampled by the incident light was about 8 nl. The potential application of these devices as sensing elements in micro total analysis systems (μTAS) and as actuators is evaluated.