Tuning a semiconductor nanomaterial with large two-photon absorption cross section in the near-infrared wavelength and investigating the correlation and origins between its size and third-order nonlinear optical properties are very important in possible applications. In this work, CdSe quantum dots (QDs) with various sizes were successfully prepared, and their size-confined third-order nonlinear optical properties were investigated by Z-scan technique with 100 fs laser pulses at 800 nm wavelength. Both the two-photon absorption and nonlinear refraction were enhanced about 8.1 times with size decrease and then weakened to 2 times with further size decrease. QDs with the diameter of 4.9 nm had the largest nonlinear optical susceptibility of 7.8 × 10⁻¹² esu. The effects of photoinduced dipole moment and local electric field were proposed to explain this trend. And the intrinsic dipole moment and defects in CdSe QDs also had an effect on this nonlinear process.

CdS nanocrystals are attached on graphene nanosheets and their nonlinear optical properties are investigated by picosecond Z-scan technique at 532 nm. We found that synergistic effect between the graphene and CdS makes a major enhancement on the nonlinear optical absorption of graphene/CdS nanohybrid in comparison with cooperative effect, and the synergistic improvement is restricted by nonradiative defects in hybrid. The synergistic mechanism involving the local field theory and charge transfer evolution is proposed.

Surface oxygen-containing defects of graphene nanosheets (G NSs) are important for the nonlinear optical (NLO) properties in optoelectric devices. G NSs with different surface oxygen-containing defects are successfully tuned by a chemical reduction process, and are characterized by several experimental measurements of morphology and structure and by spectroscopy. Their NLO properties are measured with the Z-scan technique and the NLO transmission method using a 30 picosecond single-pulse laser source at 532 nm wavelength. Their NLO susceptibility is enhanced 11.8 times (from 0.38 × 10⁻¹² esu to 4.5 × 10⁻¹² esu) with the decrease in the density of surface oxygen-containing groups, while it is reduced 7.2 times (2.7 × 10⁻¹² esu) with a further decrease in oxygen. Accordingly, the nonlinear absorption coefficient of graphene is increased from 0.51 cm GW⁻¹ to 5.90 cm GW⁻¹, and then decreased to 3.41 cm GW⁻¹, indicating that the NLO response of graphene NSs should be weakened whether the component of surface oxygen-containing groups is too large or too small. The local field effect arising from surface oxygen-containing groups and isolated sp² clusters is the dominant mechanism for the NLO improvement, while the connection of small sp² clusters depresses the optical nonlinearity. An evolution from two-photon absorption to saturable absorption with the decrease of surface oxygen concentration is obtained. The intensity-related NLO absorption and refraction in NSs are also discussed. Tuning the surface oxygen-containing defects of graphene is a useful way to enhance the optical nonlinearity for potential applications in devices.

Oxygen-containing defects are very important for altering the nonlinear optical (NLO) properties of graphene. To investigate the correlation between oxygen-containing defects and the synergistic NLO response in graphene-based nanocomposites, we attached CdS nanocrystals on the surface of graphene (G) and prepared G/CdS nanohybrids (NHs) consisting of various oxygen-containing functional groups via a chemical method. The NLO absorption and refraction of G/CdS NHs under single pulse laser irradiation are enhanced by 10.8 times with the concentration decrease of surface oxygen-containing groups, which might be attributed to the local field effects and synergetic effects stemming from charge transfer between the two components. However, the optical nonlinearity is decreased with further concentration decrease, which might arise from ${\mathrm{sp}}^2$ fragment interconnection and surface defects in the NHs. The NLO absorption transformation from two-photon absorption to saturable absorption with oxygen decrease is observed, and intensity-related NLO absorption and refraction in NHs are also discussed. Meanwhile, the G/CdS NHs exhibit superior NLO properties, implying potential applications of NH material in NLO devices.

Halide perovskite quantum dots (PQDs) have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor materials thanks to their peculiar physical and electronic structures. By further improving the nonlinear optical properties of PQDs, it is expected to adapt to ultrafast photonics applications. This work reported the nonlinear optical properties of methylammonium lead bromide‐graphene (CH ₃ NH ₃ PbBr ₃ ‐G) composites synthesized by growing CH ₃ NH ₃ PbBr ₃ quantum dots directly from a graphene oxide lattice. Our experiments indicate that the combined advantages of the ultrafast charge transport properties from graphene and the strong charge generation efficiency of perovskite can be integrated together. The CH ₃ NH ₃ PbBr ₃ ‐G composite exhibited enhanced saturable absorption properties with large modulation depth and very low saturation intensity. The transient absorption spectra and carrier dynamics analysis revealed that the enhancement of the saturated absorption properties of the composites mainly arose from the ultrafast charge transfer between G and CH ₃ NH ₃ PbBr ₃ which promoted the coupling between different states. The results pave the way for the design of optical switches or mode lockers based on saturable absorbers with good performance.

CdSe-reduced graphene oxide (CdSe/RGO) composites were synthesized by a hydrothermal method. CdSe/RGO composites with different mass ratios were prepared. The structure and morphology of CdSe/RGO composites were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The synthesis of CdSe/RGO complexes was successfully demonstrated by Fourier infrared (FT-IR) and Raman spectra. CdSe nanoparticles in the CdSe/RGO composite were uniformly dispersed on the graphene surface. The study found that oxygen-containing functional groups such as hydroxyl (-OH) and carboxyl (-COOH) groups in graphene played a decisive role in the dispersion of CdSe. The third-order nonlinear optical properties of CdSe/RGO composites were measured by a single beam Z-scan technique. The experimental results showed that composites exhibited two-photon absorption and self-focusing nonlinear refraction properties. Additionally, the third-order nonlinear susceptibility of the composite material was obviously enhanced, which was mainly due to the good dispersion of CdSe nanoparticles on graphene.