D-π-A-structured two-dimensional mercury(II)-acetylide frameworks for near-infrared switchable nonlinear optics and ultrafast photonics

Two-dimensional (2D) metal-acetylide frameworks (M-AFs), a novel class of 2D materials, demonstrate significant potential in optics and photonics due to their tunable optical and electrical properties, which is achieved through the incorporation of polarizability and spin-orbit coupling of single-metal centers into the graphdiyne (novel allotrope of carbon) frameworks via metal-bis(acetylide) linkages (−C≡C−M−C≡C−). Here, 2D mercury(II)-acetylide framework nanosheets (Hg–H₂TPP) were prepared using liquid-phase exfoliation from their bulk counterparts. The incorporation of heavy Hg^II ions led to modifications in the electronic band structure, as evidenced by room-temperature photoluminescence and absorption spectra, indicating potential applications in the near-infrared (NIR) range. The nonlinear optical (NLO) properties of the 2D nanosheets were evaluated by measuring the nonlinear absorption coefficients (β). These coefficients ranged from −10.5 cm GW⁻¹ (saturable absorption SA) to 10.9 cm GW⁻¹ (reverse saturable absorption, RSA), demonstrating the nanosheets' potential as both saturable absorbers (SABs) and optical limiters. The observation that NIR-NLO properties were achieved only after the incorporation of Hg^II ions underscores the importance of material engineering in M-AF systems. To further assess the potential applications of this engineered material, Hg–H₂TPP-based SABs were developed for NIR photonic devices. By utilizing these SABs, stable Q-switched and mode-locked lasers at 1560 nm were generated, yielding pulse widths (repetition rates) of 3.56 μs (38.33 kHz) and 779 fs (7.69 MHz), respectively. The identification of these novel photonic properties and applications indicates that 2D M-AFs possess significant potential for future ultrafast nonlinear optoelectronic devices.