Influence of sample depletion on Z-scan measurements of hydroxyl groups modified multi-walled carbon nanotubes dispersions

Hydroxyl groups modified multi-walled carbon nanotubes (MWNTs-OH) were dispersed in N, N-dimethylformamide (DMF), water and chloroform, respectively. Each kind of dispersions were poured in 1-mm, 2-mm and 5-mm quartz cuvettes, respectively. The nonlinear optical (NLO) and optical limiting properties of the dispersions were studied using open-aperture Z-scan technique at 532 nm in nanosecond regime. Results show that thick cuvette or solvents with excellent thermodynamic parameters allows efficient replenishment of MWNTs-OH in the intensely irradiated volume, leading to excellent optical limiting properties arising from the well known nonlinear scattering, while thin cuvette or solvents with poor thermodynamic parameters allows inefficient replenishment of MWNTs-OH, which even leads to false appearance of 'saturable absorption-like' or 'nonlinear refraction-like' behavior due to the depletion of the sample. The different replenishment efficiency of the dispersions with the same solvent in cuvettes of different thickness can be attributed to the different influence of viscous force near the cuvette sidewalls. This influence should also be noted in other nanomaterials sharing similar NLO mechanism to carbon nanotubes dispersions during Z-scan experiments.