Three-photon absorption (3PA) has been observed as the dominant mechanism for nonlinear absorption in wide-bandgap hydrogenated amorphous silicon (a-Si:H-W) at 1.55 μm. The nonlinear index n2 and 3PA coefficient were measured to be 22 × 10(-17)m(2)/W and 5.0 × 10(-26) m(3)/W(2) respectively at 1.55 μm by using the z-scan method. This indicates that the figure of merit (FOM) of this material is intensity dependent. A value FOM>60 is predicted at intensities below 0.5 GW/cm(2) which is the maximum practical intensity for high-bit-rate (>160 GB/s) alloptical signal processing. The nonlinear phase change in a-Si:H-W has been compared with other common nonlinear materials (c-Si, As(2)S(3), Ge(11.5)As(24)Se(64.5)) for a 2 cm long waveguide with a-Si:H-W showing the greatest potential for integrated devices for all-optical processing with a high nonlinear index and negligible nonlinear absorption at intensities < 0.5 GW/cm(2).