J. Sadeghi, G.A. Cooksey
UMD & National Institute of Standards and Technology,
Keywords: Integrated optics, Beam shaping, Lab-on-a-Chip
Summary:Microfluidic systems enable well defined environments for biological studies and precious samples. Many measurements involve optical interrogation, yet precise positioning of light inside microfluidic devices remains challenging. Often, lasers or other light sources are projected onto a sample through lenses, such as a microscope objective. Though this strategy affords spatial detail, it can be difficult to set up and replicate the configuration, and it is typically restricted to a single measurement area with limited temporal resolution. Alternatively, integrated optical elements have been developed as robust strategies to take light from a macroscale source to a microscale measurement region, which usually involves an optical fiber as an intermediate optical carrier . Uniform illumination profiles would promote more robust scattering and fluorescence measurements, making them less sensitive to particle or cell trajectory and easier to model , but such profiles have proved elusive. Another challenge is converting the cylindrical projection of most light sources into a rectangular profile that is typical for microchannels. Here, we present a straightforward approach to uniform, rectangular light profiles in microfluidic systems with a single waveguide structure. Although the transmission properties of tapered waveguides are well known [3,4], the study of near-field profiles of tapered fibers is incomplete. This paper fills in that gap by studying the beam shape from an integrated linear tapered waveguide (ILTW). Propagation of rays through various ILTW configurations was simulated, as shown in Figure 1. Microfluidic devices containing the ILTWs were fabricated as previously reported [5-7]. Light input from an optical fiber is collimated and made uniform in 2D at the exit aperture of a diverging ILTW, which is demonstrated from excitation/emission profiles of flowing fluorescent dyes as shown in Figure 1C. Overall, we provide the first demonstration of a novel lens-less method of creating sharp-edged, uniformly collimated light profiles within LoC platforms. The method requires only single layer fabrication and permits facile interface with sources and detectors through robust integration of optical fibers, which makes it promising for measurement systems such as flow cytometers. References  A. L. Washburn and R. C. Bailey, Analyst, vol. 136, no. 2, pp. 227–236, 2011.  J. Zhao and Z. You, Biomicrofluidics, vol. 10, no. 5, 2016, doi: 10.1063/1.4963010.  C. L. Chiu and Y. H. Liao, Int. J. Opt., vol. 2019, 2019, doi: 10.1155/2019/4270612.  B. K. Garside, T. K. Lim, and J. P. Marton, Appl. Opt., vol. 17, no. 22, 1978, doi: 10.1364/ao.17.003670.  P. N. Patrone, G. Cooksey, & A. Kearsley,. Phys. Rev. Appl. 11, (2019).  G. A. Cooksey, P. N. Patrone, J. R. Hands, S. E. Meek, and A. J. Kearsley, Anal. Chem., vol. 91, no. 16, pp. 10713–10722, Aug. 2019.  J. Sadeghi, P. N. Patrone, A. J. Kearsley, and G. A. Cooksey, “Optofluidic flow meter for sub–nanoliter per minute flow measurements,” Journal of Biomedical Optics, In press, 2022.