Optical Nonlinearity of Semiconducting Cd(II) Metallogel in the Femtosecond Regime with Two‑, Three‑, and Four-Photon Absorption

The charge transport characteristics of a citraconic acid-directed supramolecular metallogel of Cd­(II), named as Cd-CA, are extensively explored by fabricating a Schottky barrier diode using an Al/Cd-CA/ITO configuration, revealing remarkable semiconducting attributes in the present study. An estimated conductivity of 2.06 × 10^–4 S/m, rectification ratio (J_+1V/J_–1V) of 62.38, and barrier height Φ_B = 0.65 eV of the Cd-CA metallogel-based Schottky diode are obtained. Additionally, we probe the broadband nonlinear optical (NLO) properties of the Cd-CA metallogel through the femtosecond Z-scan method at excitation photon energies below its band gap (E_g = 4.81 eV) at 2.98, 2.34, 1.65, and 1.24 eV. Intriguingly, the gel exhibits positive nonlinear refraction and multiphoton absorption (MPA), including two-, three-, and four-photon processes. The material exhibits (i) two-photon absorption at a pump photon energy of 2.98 eV, (ii) simultaneous two- and three-photon absorption at 2.34 eV, (iii) enhanced three-photon absorption at 1.65 eV excitation, and (iv) four-photon absorption with a low optical limiting threshold at 1.24 eV. A nonlinear refraction (n₂) of 2.29 × 10^–6 cm²/GW and nonlinear absorption (β) of 14.13 × 10^–2 cm/GW are obtained at 65 GW/cm² with an excitation wavelength of 530 nm. This work elucidates the factors behind semiconducting and NLO properties of the Cd-CA metallogel, presenting a potential path for its utilization in optical switches and limiters.