Assuming two degenerate photons that are linearly polarized, the TPA cross section (σ$_{TPA}$) can be calculated using the imaginary part of the third-order response property as

where α$_f$ is the fine structure constant, and the σ$_{TPA}$ unit is given as Göppert-Mayer (1 GM = 10$^{−50}$ cm$^4$ s photon$^{−1}$). The integer value N is related to the experimental setup, and in this work, N = 4 is used for all simulated TPA spectra.

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