Nonlinear absorption conversion of epsilon-near-zero multilayer metamaterial at optical frequencies.

Modern all-optical logic switches demand selective, precise, and rapid transmission of optical information. In this study, we investigate an epsilon-near-zero (ENZ) metamaterial composed of silver (Ag) and magnesium fluoride (MgF₂), which demonstrates a low conversion threshold, strong nonlinear response, and nonlinear absorption conversion. Particularly noteworthy is its highest nonlinear absorption (β≈-2 × 10⁶cm/GW) occurring at the ENZ point (695 nm) under deposited condition. This research marks the first discussion of nonlinear absorption conversion in the ENZ multilayer metamaterial. The deposited metamaterial sample exhibits saturation absorption (SA), attributed to ground state free electron bleaching, while annealed sample shows a transition from SA to reverse saturation absorption (RSA) due to a three-photon absorption effect. Annealing significantly reduces the laser power threshold required for this conversion process, indicating reduced risk of laser-induced damage. Furthermore, the wavelength shift of the largest RSA (γ≈1.93 × 10⁴ cm³/GW²) in the annealed sample aligns with the expected redshift direction of the ENZ region (735 nm). Our metamaterial design achieves enhanced nonlinear absorption and low-power absorption conversion, which holds significant potential for applications in all-optical logic switches.