Ultrafast NonlinearOptical Response of Pb- and Zn/Li-ModifiedGermanate Glasses for All-Optical Switching

Germanate glasses are emerging materials for nonlinear photonics owing to their broad IR transparency, dual GeO4/GeO6 coordination, and high refractive index. Two compositions, 70GeO2–10K2O–20PbO (GKP) and 70GeO2–10K2O–10Li2O–10ZnO (GKLZ), were synthesized to establish structure–property correlations. Both contain 10 mol % K2O as a common alkali modifier, while Pb2+ (GKP) or Zn2+/Li+ (GKLZ) act as the main modifiers controlling structural and nonlinear behavior. FTIR and Raman confirmed Ge–O–Ge bending, ring-breathing modes, and Q2/Q3 tetrahedra, whereas XPS O 1s spectra revealed low-binding-energy NBO/modifier-coordinated oxygen contributions and higher-binding-energy BO components, confirming Pb–O linkages in GKP and Zn–O/Li–O environments in GKLZ. Band gap energies extracted from Tauc plots were 3.67 eV (GKP) and 4.43 eV (GKLZ), while refractive indices at 500 nm reached 1.74 and 1.67, respectively. The dispersion of the nonlinear refractive index (n2), determined via nonlinear ellipse rotation (NER), ranges from (25 – 16) × 10–20 m2/W for GKP and (18 – 13) × 10–20 m2/W for GKLZ, with GKP exhibiting ∼1.3× higher values due to enhanced Pb2+ polarizability. The Sheik-Bahae framework indicates that predominant contributions to the n2 dispersion of the samples is 2PA and the Raman effect. Ultrafast optical Kerr gate measurements reveal a response limited by the laser pulse (∼165 fs), confirming that n2 of the samples originate from an ultrafast electronic nonlinearity. These combined features, high n2, low nonlinear loss, and subpicosecond response highlight germanate glasses as promising platforms for ultrafast photonics, including all-optical switching and optical limiting.