I am currently working on a study involving the third-order nonlinear optical properties of doped thiourea crystals (TCP) using the Z-scan technique. For the calculation of the nonlinear refractive index (n₂), it is essential to determine the peak-to-valley difference (ΔTₚᵥ) from the normalized transmittance vs. z-position graph (open/closed aperture).Could anyone suggest a simplified, reliable method—possibly using data processing software or a mathematical fitting technique—to determine the peak-to-valley maxima accurately from normalized transmittance graphs?