Principles to tailor the saturable and reverse saturable absorption of epsilon-near-zero material

Indium tin oxide (ITO) films have recently emerged as a new class of functional materials for nonlinear optical (NLO) devices due to their exotic properties around epsilon-near-zero (ENZ) wavelength. Here, we experimentally investigated and tailored the NLO absorption properties of ITO films. The NLO absorption response of ITO films is investigated by using the femtosecond Z-scan measurement technique at two different wavelengths of 1030 nm (out of ENZ region) and 1440 nm (within ENZ region). Interestingly, we observed conversion behavior from saturable absorption (SA) to reverse saturable absorption (RSA) at 1030 nm with the increasing incident laser intensity, whereas only SA behavior was observed at 1440 nm. We demonstrate that SA behavior was ascribed to ground-state free electrons bleaching in the conduction band, and RSA was attributed to three-photon absorption. Moreover, results reveal that ITO film shows more excellent SA performance at 1440 nm with a nonlinear absorption coefficient of ∼−23.2  cm/GW and a figure of merit of ∼1.22×10−16  esu·cm. Furthermore, we tailored the SA and RSA behaviors of ITO films at 1030 and 1440 nm wavelengths via post-annealing treatment. The modulatable NLO absorption was ascribed to the changing of free-carrier concentration in ITO films via annealing treatment. The experimental findings offered an inroad for researchers to tailor its NLO absorption properties by changing the free-carrier concentration through chemical modification such as annealing, oxidation, or defect implantation. The superior and tunable nonlinear optical response suggests that ITO film might be employed as a new class material with potential applications in novel optical switches or optical limiters to realize the all-optical information process.