Research progress on the nonlinear optical properties of graphdiyne-based materials (invited)

Significance  Graphdiyne (GDY) is a novel two-dimensional (2D) carbon allotrope featuring coexisting sp- and sp²-hybridized carbon atoms. Since its successful large-scale synthesis in 2010, GDY has demonstrated remarkable potential in diverse fields, such as catalysis, energy conversion and storage, biomedicine, sensing, and adsorption/separation technologies. Similarly, GDY exhibits exceptional nonlinear optical (NLO) properties, attributed to its distinct electronic architecture, extensive π-conjugation system, and tunable characteristics. NLO materials play a pivotal role in advanced photonic applications, serving as critical components in laser modulation systems, high-speed optical communication networks, and UV radiation shielding technologies. GDY’s exceptional NLO performance, including the strong Kerr effect, saturable absorption and ultrafast photoelectric characteristics, positions it as a superior alternative to graphene and other 2D materials. Its ability to dynamically regulate light transmission—such as selectively blocking high-intensity UV radiation while allowing low-intensity UV passage—highlights its potential in bio-protective coatings and smart optical devices.

Moreover, the breakthrough demonstration of non-reciprocal light propagation in the GDY-based photonic diodes highlights the immense potential of GDY-based materials for advancing all-optical communication systems. The nonlinear absorption coefficient (>−1 cm·GW⁻¹), low saturation intensity (<13 GW·cm⁻²), and ultrafast relaxation time (< 30 ps) of GDY collectively demonstrate its significance as a pivotal class of NLO materials.

Progress  Pioneering theoretical work by Haley et al. in 1997 first proposed the significant third-order nonlinear optical susceptibility (χ³) of graphdiyne. This prediction was experimentally validated through systematic synthesis of designed molecular analogs (annulenes), with characterization revealing exceptional two-photon absorption cross-sections in these model systems. Theoretical investigations have further established the exceptionally high first hyperpolarizability (β) inherent to graphdiyne systems, thereby positioning this carbon allotrope as a prime candidate for advanced nonlinear optical applications. The controlled synthesis of GDY was first achieved in 2010 through scalable preparation methods, marking a seminal breakthrough in carbon material engineering. This milestone has enabled broad-spectrum applications of GDY-based architectures across diverse technological domains, with particular emphasis on their implementation in nonlinear photonic systems.

GDY and its derivatives have been validated as high-performance saturable absorber (SA), demonstrating operational efficacy across dual-regime laser platforms encompassing Q-switched and mode-locked configurations. For instance, a GDY-based SA was applied to an erbium-doped fiber laser at the 1.5 μm region, successfully generating mode-locked pulses centered at 1564.70 nm with a repetition rate of 12.05 MHz and pulse duration of 734 fs. Notably, two kinds of metallated graphynes have been employed as passive Q-switchers, even exhibiting better NLO properties compared to some traditional 2D nano-materials (such as graphene, black phosphorus and MoS₂). Nevertheless, few studies have explored other NLO properties of GDY, such as second harmonic generation (SHG), two-photon absorption (2PA), optical Kerr effect. It is worth noting that the photonic diodes (GDY/SnS₂) based on optical Kerr effect has been successfully fabricated, enabling non-reciprocal light propagation at 457, 532, 671 nm. The successful construction of GDY-based photonic diodes marks a crucial step for nanophotonic devices towards high performance, miniaturization, and multifunctionality, and simultaneously provides a brand-new material platform for the development of all-optical technology.

Conclusions and Prospects  GDY, as a new carbon allotrope, has exhibited extensive π-conjugation and pronounced electron delocalization, significantly enhancing the nonlinear polarizability of the material. Specifically, the abundant delocalized π-electrons in graphdiyne are more prone to distortion and polarization, thereby endowing graphdiyne with outstanding nonlinear optical response. To date, GDY-based materials exhibit exceptional performance in several NLO applications, such as SHG, 2PA, SA and OKE. These various NLO applications of GDY-based materials further highlight the important value of GDYs and their family among the NLO materials. Nevertheless, significant challenges persist in translating GDY-based materials into practical nonlinear optical applications. Current research predominantly concentrates on saturable absorption characteristics of GDY, while other nonlinear optical phenomena intrinsic to GDY remain underexplored. Furthermore, the field requires expanded focus beyond mode-locked ultrafast fiber lasers to develop functional nonlinear photonic devices, including but not limited to optical Kerr effect-based photonic diodes.

With ongoing innovations in material design and theoretical modeling, GDY is poised to revolutionize next-generation photonic technologies. Its unique properties offer promising solutions to persistent challenges in optical engineering and device miniaturization.