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A four-level system is a typical model describing the process of lasing. It is widely used to model gain media. PhotonicsGAIN-0D simulates the local response (0-dimenstion) of a generic 4-level system interacting with one or two sequential incident light pulses. The code uses the local electric field, to compute the population kinetics at each energy level, and monitors the polarization evolution in time domain. Then, the effective susceptibility is calculated in frequency domain through the Fast Fourier transform (FFT).