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<p>This paper presents a comprehensive study of third-order optical nonlinearity in nanostructured Ag/Au composite films coated with Al<sub>2</sub>O<sub>3</sub> dielectric layers of varying thickness. Using Z-scanning, performed in both nanosecond (1064 and 532&#xa0;nm) and femtosecond (800, 600, and 400&#xa0;nm) modes of laser excitation, the coefficients of nonlinear refraction n<sub>2</sub>, nonlinear absorption coefficients β, and saturation intensity I<sub>s</sub> were determined. In the nanosecond mode, a change in the sign of n<sub>2</sub> was observed depending on the thickness of the dielectric layer, which is explained by the competition between thermal nonlinearity and electronic contribution, as well as the tunneling of hot carriers through an ultra-thin Al<sub>2</sub>O<sub>3</sub> layer. In the femtosecond mode, the nonlinear response was exclusively electronic, with maximum values of n<sub>2</sub> and β observed at 400&#xa0;nm due to interband excitation in gold and local plasmonic amplification. Increasing the thickness of the Al<sub>2</sub>O<sub>3</sub> layer affected the degree of electromagnetic field localization and the efficiency of nonlinear absorption. The results obtained demonstrate that nanoscale structuring of the dielectric environment allows controlling not only the magnitude but also the sign of χ<sup>(3)</sup>, which opens up prospects for the creation of tunable nonlinear photonic elements – optical limiters, switches, and modulators.</p>

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Tunable third-order optical nonlinearity in Ag/Au nanostructures with Al2O3 coating layer under femtosecond and nanosecond excitation

  • 期刊论文

  • 发表日期：

    2025年12月17日

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  • Valentyn Rudenko,
  • Viktor Styopkin,
  • Volodymyr Liakhovetskyi,
  • Alexandr Brodin

摘要

This paper presents a comprehensive study of third-order optical nonlinearity in nanostructured Ag/Au composite films coated with Al2O3 dielectric layers of varying thickness. Using Z-scanning, performed in both nanosecond (1064 and 532 nm) and femtosecond (800, 600, and 400 nm) modes of laser excitation, the coefficients of nonlinear refraction n2, nonlinear absorption coefficients β, and saturation intensity Is were determined. In the nanosecond mode, a change in the sign of n2 was observed depending on the thickness of the dielectric layer, which is explained by the competition between thermal nonlinearity and electronic contribution, as well as the tunneling of hot carriers through an ultra-thin Al2O3 layer. In the femtosecond mode, the nonlinear response was exclusively electronic, with maximum values of n2 and β observed at 400 nm due to interband excitation in gold and local plasmonic amplification. Increasing the thickness of the Al2O3 layer affected the degree of electromagnetic field localization and the efficiency of nonlinear absorption. The results obtained demonstrate that nanoscale structuring of the dielectric environment allows controlling not only the magnitude but also the sign of χ(3), which opens up prospects for the creation of tunable nonlinear photonic elements – optical limiters, switches, and modulators.