The nonlinear concentration-dependent study of hemoglobin using the Z-scan approach

In the present work, we investigated the nonlinearity of hemoglobin using close and open-aperture Z-scan techniques at different concentrations and a constant activation laser (CW) power. Here the nonlinearity arises due to the thermal lensing phenomena. This concentration-dependent analysis revealed that the nonlinear refractive index and absorption coefficient increase with hemoglobin concentrations. The hemoglobin possesses a negative n₂ and a positive β due to the effects of self-defocusing and reverse saturation absorption. The reverse saturation absorption effect also contributes to excited-state absorption. Hemoglobin suspensions in Phosphate buffer saline (PBS) with concentrations ranging from 0.5 μmol/L to 150 μmol/L were made for the current study. Because the hemoglobin concentration in human serum is <0.6 μmol/L, we started with a lower value of 0.5 μmol/L . It is observed that haemoglobin shows nonlinearity, with a detection limit of ∼ 0.5 μmol/L. Due to the reduced haemoglobin concentration and considerable nonlinearity in the sample, the close aperture Z-Scan experiment did not show any asymmetry. We firmly believe that our straightforward and robust nonlinearity-based approach will soon open new doors for disease diagnostics based on this protein and hemoglobin detection with open and closed-aperture Z-scan setups.