Ultrafast Optical Nonlinearities And Applications Of Graphene Analogues

Ultrafast optical nonlinear materials and their application become an advanced research hotspot in nonlinear optics.They are put forward higher requirements:ultrafast carrier response,broadband nonlinear absorption and large nonlinear coefficient.Due to its specific atomic layer structure and electronic band structure,as a new type of nonlinear two-dimensional material,graphene has the excellent electrical,thermal,optical and mechanical properties.Ultrafast carrier dynamics and broadband saturable absorption are attractive for ultrafast photonics applications,such as photodetectors,optical switchers,ultrafast fiber laser,and so on.Enlightened by the substantial advantages of graphene,researchers have started to explore the graphene analogues of layered inorganic materials.Their common characteristic is a layered structure with strong intra-layer covalent bonding and weak interlayer Van der Waals forces between the graphene analogues sheets.Due to the different composition and structure,they exhibit different physical and chemical properties,and provide a broad space for exploring new physical phenomena and application.To study the ultrafast optical nonlinearities,it will provide the basis for the nonlinear photonics application and the feasible method for regulating the nonlinear properties of graphene,and meanwhile will promote the optoelectronic applications of graphene analogues.In the paper,we investigate the ultrafast optical nonlinearities of graphene and graphene analogues,such as graphene oxide,black phosphorus and graphene/Mo S₂ nanocomposites,and obtain the following results:?1?We experimentally measure the ultrafast carrier dynamics and the broadband saturable absorption property of graphene.We find that the slow relaxation time was independence on the states or the layers of graphene.But the time would be longer with the larger pump power intensity.Because that the carrier relaxation of graphene is typically dependent on the pump-induced carrier density.Higher pump power leads to the higher optical phonon concentration.The phonon energy is coupled into the hot carrier,slowing the cooling velocity of carrier and making the slower observed relaxation time.Using open-aperture Z-scan measurement system at different wavelengths,we measure the nonlinear absorption of graphene at 400 nm,800 nm,1562 nm and 1950 nm respectively.The experimental data show that graphene has a broadband saturable absorption property,suggesting that graphene can be used as a broadband saturable absorption material in ultrafast fiber laser.?2?We experimentally investigate the ultrafast carrier dynamics and nonlinear absorption property of graphene oxide at 800 nm.Using pump-probe measurement system built by ourselves,we find that graphene oxide showed the slower relaxation than graphene under higher pump intensity.In the open-aperture Z-scan measurements,we study that graphene oxide could not only show saturable absorption,but also show the properties of reverse saturable absorption under the large input intensity.The optical limiting property would also make graphene oxide a potential broadband optical limiting material.The sp²-hybridized domains in graphene oxide kept the original properties of graphene such as saturable absorption.However,a large number of sp³-hybridized domains kept through the oxidation process would show the different properties such as the slower carrier relaxation and the reverse saturable absorption under high pump intersity.?3?We measure experimentally the ultrafast optical nonlinearities of few-layer black phosphorus at 800 nm.It is found that black phosphorus show the saturable absorption property.The saturable intensity and modulation depth are about 376GW/cm² and 3.2%,respectively.The ultrafast carrier relaxation in black phosphorus is similar to that in graphene,with two relaxation times of 0.13 ps and 1.15 ps,respectively.Black phosphorus has a direct band-gap dependent on the number of layers,making up for the deficiencies of zero-gap graphene in a certain extent.The saturable absorption property and ultrafast carrier relaxation in black phosphorus have shown a great application prospect in ultrafast and high frequency optoelectronics.?4?For the first time,we study the ultrafast carrier dynamics the broadband and enhanced nonlinear optical response of Mo S₂/graphene nanocomposites for ultrafast photonics applications.We synthesize the graphene/Mo S₂ nanocomposites,and measure the slower carrier response and the broadband enhanced nonlinear optical properties of nanocomposites.In the pump-probe measurements,the Mo S₂/graphene composites exhibit a fast delay time constant in the range of 1.3¹.4 ps and a slower constant in the 36³7 ps range.According to the open aperture Z-scan measurements at various wavelengths from the visible to the middle-infrared range,the Mo S₂/graphene nanocomposites are experimentally found to possess the enhanced and broadband saturable absorption.We consider that a covalent donor–acceptor structure was formed between graphene and Mo S₂.Thus,the immediate energy transfer from Mo S₂ to graphene could slower the carrier relaxation,and could influence the excited state absorption cross section of graphene and further enhance the light-induced bleaching effect.Taking advantage of their excellent saturable absorption property,we have further fabricated the nanocomposites based optical saturable absorbers and integrated them into a 1.5?m Erbium-doped fiber laser to demonstrate Q-switched and mode-locked pulse generation for the first time.