Thermally-Tunable Liquid Lens as a Novel Alternative to the Classical Z-scan Method

This study presents a temperature-tunable liquid lens utilizing silicon oil (SO) to achieve precise focal length modulation without requiring mechanical displacement of the sample, typically required in conventional z-scan methodologies. The lens was fabricated by injecting SO into a cavity formed between two convex glass substrates, with temperature varied from 20 °C to 130 °C to enable a focal length tuning range from 69.30 mm to 77.78 mm, corresponding to an overall change of 8.48 mm. Key performance metrics include a focal length resolution of 7.2×10⁻² mm, temperature sensitivity of 1 °C, and demonstrated long-term stability under repeated thermal cycling. The lens’s functionality was validated within a z-scan setup, where it replaced fixed-focal-length optics to eliminate sample translation along the z-axis. Nonlinear refractive index measurements of silver nanoparticles dispersed in distilled water at a concentration of 13 mM yielded a value of (-1.03 ± 0.08) × 10⁻⁷ cm²/W using the proposed tunable lens, closely matching (-0.97 ± 0.06) × 10⁻⁷ cm²/W obtained via the conventional z-scan method. The proposed approach provides a practical modification to the classical Z-scan technique by allowing real-time focal control, which can simplify experimental procedures and potentially improve measurement accuracy.