In this study, micro optical diffusion sensor (MODS) is developed to measure the diffusion coefficient of biological sample such as protein with small sample volume in a short time. By utilizing the laser-induced dielectrophoresis (LIDEP), an additives-free and a high-speed sensing can be realized in the proposed method. The photoconductive layer is essential material for LIDEP, and the high-contrast photoconductivity change is required to form the concentration distribution of the biological sample using LIDEP. In this paper, a hydrogenated amorphous silicon (a-Si:H) is deposited as the photoconductive layer by a reactive RF magnetron sputtering method, which is a simple method compared with other technique such as a plasma-enhanced chemical vapor deposition. a-Si:H was deposited in several sputtering conditions and evaluated the applicability to the sensing device. A measurement device was fabricated, and the diffusion phenomenon of the lattice-shaped concentration distribution was detected by the device.