Understanding Near Infrared Laser Driven Continuum White Light Emission by Graphene and Its Mixture with an Oxide Phosphor

Recent observations suggest that continuum white light generation driven by a near infrared (NIR) laser is not limited to rare earth (RE) doped phosphors, but is rather a general photophysical process of light-matter interaction as long as the NIR laser energy is efficiently absorbed. Since the proposed explanations seem to be material-dependent, a comparative investigation on the white light generation in graphene, and its mixture with a typical RE-doped oxide phosphor is reported here for a general understanding of this process. In contrast to the steady increase of white light intensity in graphene, the white light generated from the oxide phosphor and their mixtures with graphene exhibits a stepwise growth of intensity with laser power. By using the temperature-dependent upconversion luminescence as ratiometric temperature sensing, it is observed that this emission is closely correlated with the jumps in temperature with the rise of excitation power. Since the black body temperatures are much higher than the temperatures derived by the ratiometric method, it is suggested that this process can be understood by a general thermally assisted direct photon-to-photon conversion process, rather than by pure thermal radiation. The results of this work may have important implication for the understanding and design of efficient continuum white light sources.