Self-diffraction of continuous wave laser radiation in various nanomaterial suspensions

The phenomenon of spatial self-phase modulation (SSPM), i. e. the generation, temporal development, and subsequent distortion of self-diffraction ring patterns was investigated by transmitting a continuous wave 532 nm laser beam through solutions of C₆₀ and silver nanoparticle dispersions. The nonlinear optical response regarding SSPM of such systems was studied, in dependence of various solvents, wavelengths and interaction times. The observed SSPM patterns consisted of concentric diffraction rings originating from a central spot. As time progressed, the number of rings steadily increased and the overall ring structure expanded until reaching a maximum size. Thereafter, thermal convection leads to a distortion of the upper part of the ring system, while the lower rings maintained their symmetry. The experiments revealed that the temporal and spatial evolution of SSPM depends not only on the fullerene material but also strongly on thermal properties of the surrounding medium, indicating that thermal effects are the dominant mechanism. This conclusion was confirmed through additional tests: At 1070 nm (where C₆₀ does not absorb), no SSPM rings were observed. Using ultrashort 8 ps pulses at 527 nm likewise produced no SSPM, consistent with the absence of significant thermal effects on short timescales. Finally, a potential application of SSPM is proposed in the design of laser protection devices, where the effect can act as a self-limiting mechanism to protect optical components.