Fabrication of a CVD-grown MoS₂/DWCNT heterostructure as a saturable absorber for a mode-locked fiber laser

Nanomaterial heterostructures play an important role as saturable absorbers (SAs) for achieving mode-locking in ultrafast fiber lasers. However, current heterostructure fabrication methods for sandwich-structured SAs in all-fiber laser applications still face several technical challenges: mechanical exfoliation exhibits limitations in thickness controllability and preparation scale, liquid-phase exfoliation suffers from polymer residues and poor material homogeneity, while conventional chemical vapor deposition (CVD) methods, although free from these limitations during material preparation, typically require wet-transfer processes from growth substrates (e.g., silicon or sapphire) to fiber end-faces that often induce material wrinkling and contamination. To enable direct dry-transfer onto fiber end-faces, this study proposes a novel approach that combines floating-catalyst CVD (FC-CVD) and gas-phase CVD (GCVD) to synthesize substrate-free, mixed-dimensional MoS₂/DWCNT heterostructures. With a fixed MoS₂ thickness of 52 nm, two heterostructures were fabricated by changing DWCNT collection time: an 88-nm-thick (36-nm-thick DWCNT) SA with modulation depth of 1.0 %, saturation intensity of 43.9 MW/cm², and nonsaturable loss of 74.0 %; and a 70-nm-thick (18-nm-thick DWCNT) SA exhibiting modulation depth of 1.4 %, saturation intensity of 50.0 MW/cm², and nonsaturable loss of 69.4 %. Both heterostructures functioned effectively as SAs in erbium-doped fiber lasers (EDFLs), generating stable conventional soliton mode-locking with pulse widths of 1.26 ps (88-nm-thick SA) and 0.78 ps (70-nm-thick SA). The experimental results demonstrate that lower nonsaturable loss not only compresses the pulse width but also reduces the soliton generation time, consistent with theoretical simulation results from the complex nonlinear Ginzburg-Landau equation. We demonstrate that the two-step CVD method for synthesizing thickness-controlled heterostructure SAs provides a simple method for tuning mode-locked pulse widths.