&= \frac{\varepsilon_0}{4} \chi^{(2)} \left[{E_1}^2 e^{-i2\omega_1 t} + {E_2}^2 e^{-i2\omega_2 t} + 2E_1 E_2 e^{-i(\omega_1 + \omega_2)t} + 2E_1 {E_2}^* e^{-i(\omega_1 - \omega_2)t} + \left(\left|E_1\right|^2 + \left|E_2\right|^2\right)e^{0} + \text{c.c.}\right],
