Optimization of a Thermal Lens Microscope for Detection in a Microfluidic Chip

Abstract

The optical configuration of a thermal lens microscope (TLM) was optimized for detection in a microfluidic chip with respect to the flow velocity, and the pump and probe beam parameters (beam waists, offsets, and mode mismatching degree). It was found that an appropriate pump–probe beam offset for a certain flow velocity would provide not only a higher sensitivity but also a better response linearity of TLM over three orders of magnitude of sample concentration. Diffraction-limited pump beam excitation is advantageous for space-resolved measurement, while a larger pump beam with 10 times lower power density is favorable for higher sensitivity at given experimental conditions. As an application, TLM was used to study the diffusion of azobenzene in a microfluidic chip. Diffusion profiles at different distances from the mixing point were recorded by scanning the TL signal along the cross section of the microchannel. By fitting the diffusion profiles to a theoretical model of mass transfer in a microchannel, diffusion coefficients of azobenzene in octane and methanol were determined to be $5\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$ and $6\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$, respectively.