Most approaches to high-capacity 3D optical data storage (ODS) require confinement of the writing action to a specified depth in the writing medium. This is achieved by a nonlinear photoresponse, usually two-photon absorption, which requires a pulsed long-wavelength source. Fluorescence photobleaching of a dye/polymer composite can be used at a short wavelength to store data at the diffraction limit in a layered storage medium. In this work, the writing response of a bleachable dye/polymer system illuminated with single pulses of various duration obtained from a modulated 405 nm wavelength CW laser was studied. A transition from a linear to nonlinear writing mechanism was observed near the microsecond time scale. Concentration-dependent measurements indicate that a photothermal mechanism accounts for the nonlinear response in the short pulse, higher power regime. This nonlinear response may be useful for realizing terabyte scale ODS in multilayered polymer media. © 2014 Optical Society of America.