Electrical Tuning of the Fifth-Order Optical Nonlinearity of Antimony-Doped Tin Oxide

In this work, the electrical tuning of the fifth-order nonlinear absorption of antimony-doped tin oxide (ATO) by ionic liquid gating is demonstrated. The pristine ATO film exhibits two-photon-induced excited-state absorption (2PA-ESA) with laser excitation at 1030 nm. The fifth-order nonlinear absorption coefficient (γ_eff) of the ATO film can be monotonically modified in the range of 0.51 to 3.46 cm³ GW⁻² by varying the sample bias, with a maximum enhancement factor of 6.8. The fundamental processes occurring during electrical tuning are revealed. The electrostatic and electrochemical capacitance is responsible for the modification in the number of free carriers in the conduction band of ATO. The electrical modulation of the nonlinear absorption of the ATO is ascribed to the voltage-dependent diameter of the undepleted core of ATO. A smaller voltage results in the charging of ATO and a larger undepleted core, and consequently the size of the active component for 2PA-ESA is larger. The electrochemical capacitance results from the chemisorption of H⁺ and OH⁻ on the surface of the ATO, the larger water content in the ionic liquid affording a larger modulation range for the number of free carriers and the γ_eff.