Ultrafast all-optical modulation in Fe-doped GaN at 1.31 and 1.55 μ m with high contrast and ultralow power

We demonstrate the possibility of all-optical modulation at 1.31 and 1.55 μm optical communication wavelengths by interband pumping of Fe-doped GaN crystals using femtosecond pump-probe techniques. Considering the lower photon energy of near-infrared probe pulses, switching time was well controlled by Fe doping from the nanosecond range to a value as fast as 10 ps for Fe concentration of 1 × 10¹⁹ cm^-3 arising from the carrier trapping effect of deep Fe acceptors, which suggests a modulation speed performance of ∼50 GHz. Simultaneously, about 50% of modulation contrast was achieved by means of optical excitation at an ultralow pump fluence of 0.5 mJ/cm². Moreover, almost no degradation of the modulation contrast and speed was observed due to Fe doping.