Outstanding two-photon absorption at covalent organic frameworks via Dirac points transitions

Discovering new means to increase two-photon absorption (TPA) and simultaneously achieve large TPA coefficients and substantial modulation depth is highly desirable, yet remains exceptionally challenging. We demonstrate in this study the first use of the hotspot effect of Dirac points to significantly enhance TPA in three covalent organic frameworks (COFs): TpPa, TpBD, and TpDT. These COFs exhibit TPA with 35 fs laser excitation in the wavelength range 600–1030 nm. TpBD shows the largest TPA coefficient ((5.5 ± 0.18) × 10³ cm GW⁻¹), a substantial modulation depth (>50 %), and a low optical limiting threshold (1.66 mJ cm⁻²). The TPA coefficients of the three COFs are comparable to those of single- or few-layer two-dimensional inorganic nanosheets, but the COFs exhibit much larger modulation depths. Spectral analysis and theoretical calculations reveal the crucial contribution to the TPA of band coupling at the Dirac points, and TPA occurring through a channel involving more hotspots (Dirac points) has a higher probability. Our discoveries establish Dirac point enhancement as an effective mechanism for propelling TPA to new frontiers, introducing a category of high-performance nonlinear optical materials for future optical technologies.