A Liquid Optical Tip via Control of Flow Rate

Abstract

Efficient energy interconversion between light propagation and the localized field of light is important for highly-sensitive biological and chemical detectors. Solid optical tip can effectively enhance optical intensity for the measurement of nanoscale single molecule imaging. However, it lacks dynamic control mechanisms and is difficult to realize a smooth interface which may result in serious loss of scattering. Liquid can be dynamically controlled and the interfaces are optically smooth. Recently, liquid waveguides are reported to exhibit various advantages of dynamic, cheap and low optical loss. In this paper, a liquid optical tip in a microchannel controlled by flow rates is reported. In the design of the optofluidic chip, the core flow stream of the liquid waveguide is formed as Calcium Chloride (CaCl2) flow stream, and the cladding flow stream is formed as dioionzed (DI) water flow stream. The diffusion of CaCl2 between the microfluidic laminar flows establishes a gradient refractive index distribution to make an optical tip. For the optical system, laser source with central wavelength of 633 nm is used as input. The microchannel has a height of 80 µm and a width of 100 µm. The diffusion coefficient is 1 × 10-9 m2/s. The original refractive index of CaCl2 solution and deinized (DI) water are 1.442 and 1.332, respectively. The optical intensity at the optical tip is increased by 15 times sing Finite-Difference Time-Domain (FDTD) method. Thus, light be guided to form a sharp optical tip through the control of liquid. The on-chip optical tip has potential applications in biological, chemical and medical solution detectors.