We investigate modal phase-matching conditions for second-harmonic generation (SHG) in tapered silica optical nanofibers, considering both bare nanofiber (BN) and coated nanofiber (CN) configurations. Our analysis is based on solving Maxwell’s equations in a perturbed nonlinear regime, incorporating second-order surface nonlinearities. In the BN case, with the fiber surrounded by air, phase matching is achieved when the effective refractive indices of the interacting modes are equal. For 1064 nm pumping, SHG is phase-matched at fiber diameters of 525 nm (HE11–HE21) and 468 nm (HE11–TM01), while for 1550 nm, it occurs at 774 nm and 692 nm, respectively. We then extend the model to include a CN configuration with engineered coatings. The introduction of a low-index Teflon® AF2400 (Polytetrafluoroethylene, PTFE) coating shifts the phase-matching diameters to 1091 nm and 1043 nm, enabling easier fiber manipulation and improved mechanical robustness, albeit at reduced optical intensity. The addition of a nonlinear PMMA/DR1 (Polymethylmethacrylate doped with Disperse Red 1) coating significantly enhances SHG efficiency, particularly when the coating thickness approaches 140 nm. SHG power conversion is analytically derived using mode orthonormality and the reciprocity theorem. Numerical simulations confirm that the Teflon® coating increases tolerance to diameter nonuniformity by nearly a factor of three compared to air. Moreover, the nonlinear PMMA/DR1 layer boosts SHG efficiency by a factor of <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11082_2025_8376_Article_IEq1.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="65" /> </InlineMediaObject> <EquationSource Format="TEX">$3.6 \times 10^5$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>3.6</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>5</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. These findings demonstrate the potential of CNs to enable efficient and robust SHG, opening new possibilities for integrated devices in nonlinear and quantum optics.

Enhancing second-harmonic generation in silica nanofibers via second-order nonlinear coating deposition

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发表日期：

2025年7月29日

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Abderrahim Azzoune,
Hocine Medjadba,
Oussama Laouedj,
Hamza Gouasmia,
Sylvie Lebrun

摘要

We investigate modal phase-matching conditions for second-harmonic generation (SHG) in tapered silica optical nanofibers, considering both bare nanofiber (BN) and coated nanofiber (CN) configurations. Our analysis is based on solving Maxwell’s equations in a perturbed nonlinear regime, incorporating second-order surface nonlinearities. In the BN case, with the fiber surrounded by air, phase matching is achieved when the effective refractive indices of the interacting modes are equal. For 1064 nm pumping, SHG is phase-matched at fiber diameters of 525 nm (HE₁₁–HE₂₁) and 468 nm (HE₁₁–TM₀₁), while for 1550 nm, it occurs at 774 nm and 692 nm, respectively. We then extend the model to include a CN configuration with engineered coatings. The introduction of a low-index Teflon^® AF2400 (Polytetrafluoroethylene, PTFE) coating shifts the phase-matching diameters to 1091 nm and 1043 nm, enabling easier fiber manipulation and improved mechanical robustness, albeit at reduced optical intensity. The addition of a nonlinear PMMA/DR1 (Polymethylmethacrylate doped with Disperse Red 1) coating significantly enhances SHG efficiency, particularly when the coating thickness approaches 140 nm. SHG power conversion is analytically derived using mode orthonormality and the reciprocity theorem. Numerical simulations confirm that the Teflon^® coating increases tolerance to diameter nonuniformity by nearly a factor of three compared to air. Moreover, the nonlinear PMMA/DR1 layer boosts SHG efficiency by a factor of $3.6 \times 10^5$ $3.6 \times 10^{5}$ . These findings demonstrate the potential of CNs to enable efficient and robust SHG, opening new possibilities for integrated devices in nonlinear and quantum optics.