Impact of Sub-Picosecond Timing Errors on Complex Conductivity Extraction in Terahertz Time-Domain Spectroscopy of Ultrathin FAPbI3 Films

Terahertz time-domain spectroscopy (THz-TDS) is widely used to determine the complex conductivity of thin films from transmission measurements. For ultrathin films with thicknesses much smaller than the THz wavelength, however, the retrieved complex conductivity becomes highly sensitive to small phase errors in the measured transmission spectrum. In this work, we investigate the impact of sub-picosecond temporal uncertainty on the extraction of complex conductivity spectra in THz-TDS. Transmission measurements were performed on 400-nm-thick FAPbI ₃ (Formamidinium Lead Iodide) films coated on dielectric substrates, and the spectra were analyzed using a Lorentz oscillator model. Small temporal offsets between sample and reference waveforms introduce systematic phase errors that predominantly affect the imaginary component of the conductivity. Numerical analysis shows that temporal deviations on the order of tens of femtoseconds can lead to significant distortions in the retrieved conductivity spectra. The analysis further indicates that these phase errors primarily modify the high-frequency permittivity parameter in Lorentz oscillator fitting, while the resonance frequency, oscillator strength, and damping constant remain largely unaffected. A phase-correction procedure is demonstrated to mitigate these effects and improve the reliability of conductivity extraction for nanometer-scale films. These results highlight the importance of precise phase alignment in THz-TDS of ultrathin materials.