Flow velocity detector in a microchip based on a photothermally induced grating.

A new photothermal technique was developed for measuring the flow velocity and making solute concentration measurements in a microchip by using the same optical and instrumental setup. Collinear pump and probe light were irradiated onto a microchip surface on which a grating pattern was fabricated. The pump light induced a temperature change with the grating pattern in a microchannel, and a refractive index change due to a subsequent temperature rise was monitored by a heterodyned diffraction signal of the probe light. The flow velocity and concentration were obtained by monitoring the motion and intensity change of the thermally induced grating, respectively. The dynamic range of the flow velocity measurement was 0.17 - 670 mm/s, which is sufficient for covering most chemical applications of a microchip. The detection limit of the concentration measurement was 2 x 10(-6) M for a rhodamine B solution.