Fluorescence radiation characteristics based on evanescent wave pumping in a microfluidic chip

Abstract

A bare quartz optical fiber is implanted in a microfluidic channel of polydimethylsiloxane (PDMS) substrate. Pumping the microfluid by a continuous wave laser with a wavelength of 532 nm along the fiber axis, the fluorescent spectra from the channel filled with lower refractive index (RI) dye solution are obtained. Due to the fact that the evanescent field of the pump beam is homogeneous around fiber, the fluorescent emission from the rim of fiber is uniform. It is found experimentally that the fluorescent emission intensity decreases with the axial distance of fiber, and the intensity is very sensitive to the RI of the dye solution and the dye concentration. For the dye solution with a large RI, the emitted fluorescent intensity attenuates along the fiber axis more obviously than that of the dye solution with a small RI. For the high dye concentration solution, the emitted fluorescent intensity attenuates along the fiber axis also more significantly than that of the low dye concentration solution. Therefore, it is possible to obtain a uniform fluorescence radiation along the fiber axis by selecting a suitably smaller RI and a lower dye concentration solution. The observed experimental phenomena are well explained based on the mechanism of evanescent wave pumping fluorescent radiation. Based on the features of fluorescent emission in the microfluidic chip, a PDMS chip with three micro-channels is designed and fabricated. After injecting ethanol solutions of rhodamine 640, rhodamine B and rhodamine 6 G separately into the three channels and pumpingthese solutions by evanescent wave along the optical fiber axis, three fluorescence emissions with different wavelength ranges are successfully observed in a single PDMS chip.