Efficient stimulated Raman scattering-based wavelength converters using tapered optical nanofibers immersed in nonlinear liquids

Tapered optical nanofibers (ONFs) with radii comparable to or smaller than the wavelengths of visible and near-infrared light have become versatile tools for studying nonlinear optical processes, owing to their strong evanescent fields and light-confining properties. We explore evanescent stimulated Raman scattering (SRS) in tapered silica ONFs immersed in various nonlinear liquids. The influence of nonlinear liquid composition on SRS characteristics is examined by analyzing factors such as refractive index, Raman gain coefficient, and volumetric ratios of Raman-active nonlinear liquids like benzene and toluene mixed with other nonlinear solvents such as perfluorohexane and cyclohexane. Using numerical modeling, we examine the nonlinear interactions between the guided modes of the silica ONF and the surrounding nonlinear liquid, aiming to determine the optimal ONF radius and liquid composition that maximize modal Raman gain at a 532 nm pump wavelength. Our results show that higher Raman gain can be achieved with shorter ONF lengths and larger radii compared to previous studies. For example, the fundamental <i>H</i><i>E</i>₁₁ mode reaches the highest Raman gain of 3.65<i>m</i>^-1⋅<i>W</i>^-1 at a 140 nm radius when submerged in perfluorohexane. Additionally, combining high-index Raman-active nonlinear liquids with lower-index solvents ensures the composite refractive index remains below that of silica, which helps maintain efficient light transmission and enhances the Raman effect. Optimizing the interplay between the ONF radius and nonlinear liquid composition leads to significant improvements in Raman gain, achieving enhancements of up to 3.5 times compared to ethanol-based systems. These findings reduce fabrication complexity for ONFs, expand the operational range of the Raman effect, and increase the laser-induced damage threshold, making them ideal for applications in telecommunications, wavelength conversion, optical sensing, and signal amplification.