Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Photothermal therapy (PTT) has emerged as a promising cancer therapeutic modality with high therapeutic specificity, however, its therapeutic effectiveness is limited by available high‐efficiency photothermal agents (PTAs), especially in the second near‐infrared (NIR‐II) biowindow. Here, based on facile liquid‐exfoliated FePS3 nanosheets, a highly efficient NIR‐II PTA with its photothermal conversion efficiency of up to 43.3% is demonstrated, which is among the highest reported levels in typical PTAs. More importantly, such Fe‐based 2D nanosheets also show superior Fenton catalytic activity facilitated by their ultrahigh specific surface area, simultaneously enabling cancer chemodynamic therapy (CDT). Impressively, the efficiency of CDT could be further remarkably enhanced by its photothermal effect, leading to cancer synergistic PTT/CDT. Both in vitro and in vivo studies reveal a highly efficient tumor ablation under NIR‐II light irradiation. This work provides a paradigm for cancer CDT and PTT in the NIR‐II biowindow via a single 2D nanoplatform with desired therapeutic effect. Furthermore, with additional possibilities for magnetic resonance imaging, photoacoustic tomography, as well as drug loading, this Fe‐based 2D material could potentially serve as a 2D “all‐in‐one” theranostic nanoplatform.

All the optical properties of materials are derived from dielectric function. In spectral region where the dielectric permittivity approaches zero, known as epsilon-near-zero (ENZ) region, the propagating light within the material attains a very high phase velocity, and meanwhile the material exhibits strong optical nonlinearity. The interplay between the linear and nonlinear optical response in these materials thus offers unprecedented pathways for all-optical control and device design. Here the authors demonstrate ultrafast all-optical modulation based on a typical ENZ material of indium tin oxide (ITO) nanocrystals (NCs), accessed by a wet-chemistry route. In the ENZ region, the authors find that the optical response in these ITO NCs is associated with a strong nonlinear character, exhibiting sub-picosecond response time (corresponding to frequencies over 2 THz) and modulation depth up to ≈160%. This large optical nonlinearity benefits from the highly confined geometry in addition to the ENZ enhancement effect of the ITO NCs. Based on these ENZ NCs, the authors successfully demonstrate a fiber optical switch that allows switching of continuous laser wave into femtosecond laser pulses. Combined with facile processibility and tunable optical properties, these solution-processed ENZ NCs may offer a scalable and printable material solution for dynamic photonic and optoelectronic devices.

Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4−10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting for photocurrent generation based on a photoelectrochemical cell. Our joint theoretical and experimental results suggest that GeSe sheets of atomic thickness could be a new two-dimensional semiconductor that can be exploited for potential applications in optoelectronics and photonics.

We report that non-contact self-referencing temperature sensors can be realized with the use of core-shell nanostructures. These lanthanide-based nanothermometers (NaGdF4:Yb(3+)/Tm(3+)@Tb(3+)/Eu(3+)) exhibit higher sensitivity in a wide range from 125 to 300 K based on two emissions of Tb(3+) at 545 nm and Eu(3+) at 615 nm under near-infrared laser excitation.