**3. Characterization techniques**

The green synthesized silver nanoparticles and Lanthanide complexes with silver nanoparticles were characterized using XRD, TEM, FT-IR, UV–visible absorption, luminescence studies, Z-scan, degenerate four-wave mixing (DFWM), and decay measurements techniques. XRD pattern of silver nanoparticles was measured on Cu-K<sup>α</sup> X-ray radiation with λ = 1.5406 Å over the range of 2θ = 5–50° at 1° min<sup>1</sup> . A transmission electron microscope (TEM) on FEI TECHNAI G<sup>2</sup> S-Twin was used to measure the diameter and morphology of nanoparticles. FT-IR spectrum was performed using Thermo-Nicolet 6700 spectrophotometer with a 500–4000 cm<sup>1</sup> range. UV–Vis spectra were carried out by JASCO-V670 UV/VIS/ NIR spectrometer with 1 nm resolution and 200–800 nm range. Luminescence emission spectra of Ln3+ complexes with silver nanoparticles were performed on HORIBA JOBIN YVON spectrophotometer with 262 nm and 350 nm excitations. Nonlinear absorption and optical limiting studies were performed using the Z-scan with laser source of Nd: YAG laser at 532 nm, 30 ps and 10 Hz repetition rate; and 1 kHz, 110 fs pulses of Ti: sapphire at wavelength 800 nm. Degenerate four-wave mixing (DFWM) measurements were studied with Ti: sapphire laser in box-car geometry. Luminescence decay studies were performed on FLS980 with an excitation source as a Xenon lamp.
