Strain-dependent anisotropic nonlinear optical response in two-dimensional functionalized MXene Sc2CT2 (T = O and OH)

Giant two-photon absorption in MXene quantum dots

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb₂C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb₂C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb₂C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb₂C changes to saturable absorption when the incident wavelength is between 400-800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb₂C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb₂C via morphology or surface termination.

All-optical Ti3C2Tx modulator based on a sandwich structure

All-optical modulators based on a MXene-Ti₃C₂T_x have recently garnered much attention due to their broadband light-matter interactions and ultrafast carrier dynamics. To investigate the modulation characteristics of pump intensity and pump light modulation frequency, we establish an all-optical modulator with a sandwich structure based on MXene-Ti₃C₂T_x/PVA (polyvinyl alcohol) film. The result shows that this modulator can achieve a high modulation depth of 12.55 dB and a modulation frequency of 50 kHz corresponding to a response time at the microsecond scale. The successful preparation of the modulator is attributed to the saturable absorption characteristics of the MXene-Ti₃C₂T_x. This modulator has great potential in all-optical communications and ultrafast optical signal processing.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS₂, GaSe, and In₂Se₃ monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

Dispersion Property and Evolution of Second Harmonic Generation Pattern in Type-I and Type-II van der Waals Heterostructures

Dispersion property and second harmonic generation (SHG) pattern of novel two-dimensional (2D) van der Waals heterostructures (vdWHs) is of great significance not only for the characterization of material symmetry but also for understanding nonlinear photophysical phenomena. Herein, we demonstrate the SHG response of 2D type-I (MoTe₂/WSe₂) and type-II (MoSe₂/WSe₂) band alignment of vdWHs. In the dispersion relation of the second-order nonlinear coefficient, the pronounced peaks of the d₁₆ element for both vdWHs are mainly contributed by resonance in the interband transition processes, whereas other elements are derived from the intraband transition processes because of the highly efficient charge transfer from WSe₂ to MoTe₂ in type-I vdWHs and the ultrafast charge separation between WSe₂ and MoSe₂ in type-II vdWHs, respectively. Besides, more nonzero nonlinear coefficient elements can participate in a nonlinear response at the oblique incidence, to which special attention needs paid. The polarization angle α-dependent SHG patterns display a rotational fourfold symmetry, whereas the azimuthal angle ϕ-dependent SHG patterns show sixfold symmetry for both type-I and type-II vdWHs at any wavelength under normal incidence. Under oblique incidence, the α-dependent (ϕ-dependent) SHG patterns will reduce to twofold (threefold) symmetry for both vdWHs. The results highlight the potential to deterministically engineer novel nonlinear optical properties for tunable anisotropic applications of nonlinear optoelectronic devices based on vdWHs.