Determining multiphoton and free carrier absorption coefficients using Fourier transform infrared spectroscopy

Abstract

We present an optical absorption model that integrates wavelength dependence of two- and three-photon absorption (2PA and 3PA) and free carrier absorption (FCA) coefficients in the below bandgap regime. The model can be termed as the characteristic function for the below bandgap absorption. The measured absorption spectrum of the PbTe film using Fourier transform infrared spectroscopy agrees well with the modeled optical properties. 2PA, 3PA, and FCA coefficients in cm−1 for the entire wavelength range can be obtained from a single measurement of the below bandgap absorption spectrum. The results indicate that the remarkable 2PA and 3PA processes occur in the wavelength range from 4.18 to 7.58 μm. In addition, we implement the model in the below bandgap absorption spectra of n-Si with the room temperature carrier concentration 1.0×10−19cm−3. The predicted 2PA values by the model in cm−1 are 5.38–0.53 cm−1 in the wavelength range from 1.1 to 2.2 μm and agree well with experimental 2PA values at 1.55 μm wavelengths by the Z-scan measurement. Our paper offers an effective method in handling the below bandgap absorption measurement.