Study on Micro Optical Diffusion Sensor using laser-induced dielectrophoresis (preliminary experiment of optical detection system for integrated micro optical device)

We have developed a Micro Optical Diffusion Sensor (MODS) using laser-induced dielectrophoresis, enabling small sample volume and high-speed measurement without any additives or pretreatments. All optical components are integrated in a small chip by using MEMS fabrication technique. A micro channel of MODS consists of a pair of transparent electrodes and a photoconductive layer. AC voltage is applied between transparent electrodes sealing the liquid sample, and two excitation lasers which generate the light intensity distribution are intersected on the photoconductive layer. The electric conductivity distribution of the photoconductive layer is generated by the photoconductive effect. A non-uniform electric field is formed followed by generating the dielectrophoresis, and then the sinusoidal concentration distribution is induced in a micro channel. After cutting the AC voltage, the mass diffusion occurs, and the diffusion coefficient can be obtained by observing the diffusion process of one-dimensional concentration distribution. In this paper, we have utilized the transient grating method as a detecting method of the mass diffusion because of its simplicity of integration. When probing laser enters the concentration distribution, the diffracted light is generated. The decay of the concentration distribution can be detected by the intensity change of the diffracted light, and the diffusion coefficient can be obtained from the time constant of decay. In order to verify the proposed method, preliminary experiments were conducted, and the applicability of MODS was successfully confirmed.