We have observed a very interesting feature in the mid‐infrared region of copper‐doped semi‐insulating liquid encapsulated Czochralski grown GaAs at liquid helium temperature. After gettering copper using backside mechanical damage, the transmittance of this particular feature became much weaker. The first absorption feature occurs at 1182 cm−1(0.147 eV), the second at 1460 cm−1(0.181 eV), and the third at 1750 cm−1(0.217 eV), which is a transition of an electron from the valence of three copper levels. This electronic mode absorption allows us to obtain information about the activation energies of deep‐level impurities like Cu and their relative concentration after each process. This electronic mode absorption at multilevels of Cu in GaAs is proposed for the first time using the Fourier transform infrared technique to detect deep‐level metal impurities in GaAs.