Dye-Nanotube Interaction Mechanisms Governing Nonlinear Optical Absorption in SWCNT Hybrids for Optical Switching

Abstract Controlling nonlinear optical absorption is essential both for established applications such as optical limiting and mode locking and for emerging technologies including photonic integrated circuits. Absorption modulation by hybrids on the platform of carbon nanotubes requires a rational choice of the dye. However, the relationship between the dye-nanotube interaction mechanism and the resulting nonlinear performance across different time regimes remains unclear. We synthesized noncovalent dyad hybrids of single-walled carbon nanotubes with four water-soluble dyes, Rhodamine 6G (cationic), Methylene Blue (weak π-stacking), Chicago Sky Blue (diazo, anionic), and Fotoditazin (chlorin-e6-based preparation), and characterized their femtosecond saturable absorption (SA) and nanosecond reverse saturable absorption (RSA) in the visible region using Z-scan and ultrafast pump-probe spectroscopy. We find that the enhanced nonlinear optical performance is not a simple function of linear dye absorption but is governed by the nature of the interaction between the dye and the nanotube. Rhodamine 6G forms a ground-state dark complex via electrostatic and cation−π interactions, shortening the relaxation time to 26 ps yet still enhancing nanosecond RSA through thermally induced scattering. Fotoditazin exhibits mixed static-dynamic quenching with photoinduced electron/charge transfer, creating a long-lived charge-separated state (τ3 = 495 ps) that selectively boosts nanosecond RSA even at low intensities. Chicago Sky Blue, containing photoactive diazo groups, shows reverse saturable absorption only above 8 GW/cm2, a threshold behavior that requires further study, while Methylene Blue provides no significant enhancement. These results establish a predictive framework linking the chemical nature of the dye-nanotube interaction to the nonlinear optical response, thereby enabling the design of hybrids tailored for specific temporal regimes in photonic devices.