Third-order optical nonlinearities of Nb₄C₃ MXene and its application as an ultra-broadband mode-locker

MXenes have been one of the most extensively investigated nonlinear optical materials at short-wave infrared (SWIR) wavelengths. Herein, we systemically investigated the electronic and optical characteristics of Nb₄C₃ MXene. First, the electronic and linear optical properties of Nb₄C₃ MXene were theoretically estimated using density functional theory calculation, thereby revealing the metallic property of Nb₄C₃ MXene. Through Z-scan measurements, we observed large saturable absorption of Nb₄C₃ MXene at 1560 and 1910 nm wavelengths. Self-focusing and self-defocusing properties were also observed at 1560 and 1910 nm wavelengths, respectively. The effective nonlinear absorption coefficient and nonlinear refractive index of Nb₄C₃ MXene were estimated as −(2.27 ± 0.12) × 10³ cm GW⁻¹ and (1.3 ± 0.04) × 10⁻² cm² GW⁻¹ at 1560 nm, and −(2.73 ± 0.08) × 10⁴ cm GW⁻¹ and −(0.80 ± 0.01) × 10⁻¹ cm² GW⁻¹ at 1910 nm. Based on the large and broadband optical nonlinearities, a single broadband saturable absorber (SA) was fabricated to generate optical pulses at SWIR wavelengths. Stable mode-locked pulses 1.66 ps at 1559.8 nm and 1.62 ps pulse duration at 1930.3 nm were generated with a single Nb₄C₃ MXene-based SA when incorporated in an erbium-doped fiber ring laser cavity and a thulium holmium-doped fiber cavity, respectively. This work indicates that Nb₄C₃ MXene can be a high-performance material platform for broadband optical devices at SWIR or broader wavelengths.