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The substrate effect is an important issue in the properties of two-dimensional transition metal dichalcogenides (2D TMDs). Quantitatively determining the dependence of the photoluminescence (PL) emission properties and the excitonic behavior of single-layer 2D materials in a specific dielectric environment would provide helpful guidance for the rational design of substrates for high performance 2D TMD PL emission devices. Here, using a WSe2 monolayer on different substrates as a model system, it is demonstrated that the PL emission intensities can drastically change depending on the substrate effect. From the analysis of the excitonic behavior, the results reveal that the spectral weight between the neutral and charged excitons in the PL spectra is significantly modified by the substrate types, and the weight factor is dependent on the laser excitation density. The charged exciton binding energy is obviously negatively related to the substrate dielectric constant. Furthermore, the change trends of the binding energy of the monolayer WSe2 on different substrates are basically the same for the increase of the excitation density. These results suggest that the choice of the substrate plays a significant role in the modulation of the PL properties and exciton states of atomically thin WSe2; hence, substrate engineering should be carefully considered in the design of future 2D devices.

The bandgap, electrical, and optical properties of PtSe2 depend dramatically on the vertical stacking and fabrication method. Here, we study the nonlinear absorption properties of the PtSe2 films composed of both semiconducting and semimetallic phases in a single film. These PtSe2 films exhibit remarkable thickness-dependent saturable absorption for femtosecond pulses at 400 nm and 800 nm. The saturation intensities decrease with the increase in the film thickness due to the accompanied increase in the semimetallic component and are much smaller than the reported values of PtSe2 synthesized by thermally assisted conversion. The saturable absorption characteristics are confirmed by time-resolved spectroscopies. The nonlinear refractive indexes of these PtSe2 films should be smaller than 1 × 10–12 cm2/W. Our results imply that the optical nonlinearities of PtSe2 could be flexibly tuned by the synthesis method and thickness.

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices.

We present a femtosecond optical heterodyne detection of the optical Kerr effect study on Chemical Vapor Deposition-grown monolayer MoS2 films at 800 nm. The third-order nonlinear optical susceptibility (i.e., χ xxyy ( 3 )+ χ xyyx ( 3 )) of monolayer MoS2 is determined to be 1.4 × 10−9 esu, and the ultrafast temporal response process indicates that the susceptibility origins from nonresonant electronic polarization. Based on Kleinman symmetry, susceptibility tensor elements are determined, and further, the nonlinear refractive indexes of any elliptically polarized light could be calculated for MoS2. These results will benefit the application of MoS2 in nonlinear photonic devices.

We report on a general method for broadband responsivity enhancement in graphene photodetectors based on the sandwiched graphene structure under total internal reflection. The optical absorption is ∼25% for transverse electric waves for pure monolayer graphene, and the responsivity of pure monolayer graphene photodetectors is 0.012 A/W, which is one or two orders of magnitude larger than the normal incidence excitation. The enhanced responsivity covers a wide wavelength range from 300 to 1550 nm. Further, this method is not limited by the device, and it is a general method used to improve the light-graphene coupling, thus increasing the responsivity of graphene photodetectors largely. And this method allows large area preparation to meet the needs of the free space light detections.

Graphene saturable absorbers (GSAs) have been widely applied in ultra-fast mode-locked fiber lasers. Thanks to the broadband advantage of graphene, we theoretically and experimentally demonstrate the variation of the modulation depth of GSA by employing the effect of cross absorption modulation. This method provides an easy and efficient way to modulate the characteristics of GSA. By varying the modulation power, we realize an all-fiber fundamental mode-locked fiber laser and a harmonic mode-locked fiber laser with tunable output pulse width. Results show that the output pulse widths of the two fiber lasers can be tuned more than 40%, and the lasers have high wide application potential on nonlinear optical bio-imaging and offer an advantageous front end for extreme-power laser technologies.

Based on the polarization-sensitive absorption of graphene under conditions of total internal reflection, we demonstrate the fabrication and reading of transparent and flexible multi-layer-film optical data storage media based on graphene recording layers. We report a realization of the process of data writing-transferring-reading by repeatedly transferring recorded graphene and its strong polarization effect. The reading results show a high signal-to-noise ratio and stability and low crosstalk interference between the layers. In addition, the graphene-based multi-layer-film optical data storage medium has a high transparency and flexibility. The high signal-to-noise ratio remains stable after the structure is bent 1000 times.

It is shown that graphene exhibits strong polarization-dependent optical absorption under total internal reflection. Compared with universal absorbance of 2.3%, larger absorption was observed in monolayer, bilayer, and few-layer graphenes for transverse electric (TE) wave under total internal reflection. Our result indicates that reflectance ratio of transverse magnetic wave to TE waves can easily provide the information of number of graphene layers. Furthermore, the enhanced light-graphene coupling in a wide spectral range will be great potential in many applications such as photodetector, photovoltaics, and optical sensor.

Ultrafast carrier dynamics and saturable absorption of few-layered graphene oxide, well-dispersed in organic solvent, are studied using femtosecond pump-probe and Z-scan techniques. The results demonstrate that few-layered graphene oxide has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of reduced graphene oxide. Fast carrier relaxation combined with well solution processing capability arises from the large fraction of sp2 carbon atom inside the few-layered graphene oxide sheet together with oxidation mainly existing at the edge areas. This superiority of few-layered graphene oxide will facilitate potential applications of graphene for ultrafast photonics.

The nonlinear optical properties of graphene oxide (GO) were investigated at 532 nm in nanosecond and picosecond regimes. Results show that two-photon absorption dominates nonlinear absorption process of GO in the case of picosecond pulses, while excited state nonlinearities play an important role in the case of nanosecond pulses. Additionally, we compared nonlinear optical properties of three different dimensional carbon-based materials (two-dimensional graphene, one-dimensional carbon nanotube, and zero-dimensional fullerene) in nanosecond and picosecond regimes, respectively. The nonlinear mechanism of GO is distinctly different from nonlinear scattering of carbon nanotube and excited state nonlinearity of fullerene.