Nonlinear Absorption Performance Of Several New Two-dimensional Materials

In recent years, the nonlinear absorption properties of two-dimensional (2D) materials are the hot topic. The saturated absorption performance of 2D material can be used to produce the saturable absorber for the application in solid state mode-locked laser and fiber laser. Using the two photon absorption property,2D material can be used as the optical limiting material for the protection of eyes and sensitive optical device.In past decade, a series of 2D materials were be found in succession, such as graphene, graphene oxide, functionalized graphene, transitional metal dichalcogenides, boron nitride, black phosphorus, and so on. Graphene is the first founded 2D material, which was researched by scientists. Many rare properties performed by graphene, such as high optical transparency, flexible mobility, elongation and thermal conductivity, refreshed the understanding of nano-materials. The most outstanding property of graphene is the zero band gap structure, which can realize the transition of free electron in a very wide waveband. The band gap of transitional metal dichalcogenides is in a range of 1 eV-2 eV. Due to that the band gap width changes with the layer number of nanosheets, the transitional metal dichalcogenides own a quasi-continuous band gap structure. Boron nitride nanosheets can be regarded as a nonconductor, because of the wide band gap (over 5 eV). In 2014, scientist found the black phosphorus nanosheets with the band gap of 0.3 eV-2 eV, which filled up the blank leaved by other 2D nano-material.The nonlinear absorption is a kind of nonlinear effect, which only can be performed by strong coherent light, so it was found after the presence of the laser. According to the difference of absorption mechanism, nonlinear absorption can be defined as saturated absorption, two photon absorption, free charge carrier absorption, reverse saturable absorption, and so on. There are kinds of methods of measuring nonlinear absorption, but Z-scan method is the most popular method, due to the simple experimental facility, high accuracy and relatively developed theory. We chose Z-scan method to research the nonlinear absorption properties of several kinds of 2D materials, which can be summarized as follows.(1) Using the OPO laser, we researched the saturated absorption performance of graphene with different number of layers in the waveband of 410 nm～2300 nn. The modulation depth of graphene has no relationship with excitation wavelength and only depends on the number of layer. Graphene performed lower saturated intensity and high carrier concentration with the excitation of long wavelength compared to short wavelength excitation, which outstand the huge application value of graphene in intermediate infrared, far infrared, even the terahertz waveband. At the same time, we demonstrated the hourglass band structure of graphene and the existence of Dirac point. Also, we discussed the change of saturated intensity Is in three different conditions. For steady state and large facula excitation, Is is in 1 MW/cm2 order of magnitude. For steady state and small facula excitation, the order of magnitude of Is is 10 MW/cm2. In the condition of transient state excitation, Is is in 100 MW/cm2 order of magnitude.(2) We studied the nonlinear absorption properties of N-doped graphene, carboxyl of graphene oxide and F-doped graphene oxide. N-doped graphene performed obvious saturated absorption property. Due to the influence of band gap structure caused by N atoms, N-doped graphene displayed lower saturated intensity and larger two photon absorption coefficient compared to pure graphene. Through the comparison of nonlinear absorption properties with two different wavelengths excitation, N-doped graphene is kind of outstanding nonlinear material applied in infrared band.Compared to N-doped graphene, the saturated absorption property of carboxyl of graphene oxide and F-doped graphene oxide is not much obvious, but the two photon absorption performance is very strong. Compared to pure graphene oxide, carboxyl of graphene oxide can promote the nonlinear absorption performance in short wavelength waveband due to a large amount ofcarboxyl. The two photon absorption coefficient with 532 nm excitation was 8.5 times larger than that with 1064 nm excitation. Carboxyl of graphene oxide is a kind of outstanding optical limiting material applied in visible and ultraviolet band.For F-doped graphene oxide, it performs stronger nonlinear absorption property compared to pure graphene oxide due to the doping of F atoms. The fitted two photon coefficient is in the same level compared to pure graphene. Through the comparison of nonlinear absorption properties with two different wavelengths excitation, F-doped graphene oxide is kind of outstanding nonlinear material applied in infrared band.(3) The two photon absorption properties of OH-functionalized BN nanosheets were researched. The existence of hydroxyl promotes the solubility of nanosheets. In microcosmic field, hydroxyl enhances the electric polarization of B-N atoms and changes the molecule dipole moment, which motivates the two photon absorption performance. The hydroxylated boron nitride nanosheets are a potential material, which can be widely used in nonlinear optics sector as optical limiting material.(4) The nonlinear absorption properties of MoS2, WS2 were researched. Compared to MoS2 nanosheets, WS2 nanosheets displays stronger nonlinear absorption property including the saturated absorption and two photon absorption. WS2 nanosheets are more suitable to be applied in passive mode locked lasers, Q-switched lasers and eye and sensitive optical devices.(5) The black phosphorus nanosheets were successfully exfoliated adopting liquid phase exfoliation method. The nanosheets had a thickness of 7 nm-10 nm with a size of few micrometers. The nonlinear absorption performance was studied using three different excitation wavelengths. We demonstrated that black phosphorus nanosheets own better nonlinear absorption compared to graphene. Based on the analysis of different nonlinear results with three excitation wavelength (355 nm,532 nm,1064 nm), we demonstrated the huge potential of black phosphorus nanosheets in the application in infrared band as nonlinear absorption material. The black phosphorus nanosheet is a kind of broadband nonlinear absorption material.