The Research On Nonlinear Optical Properties Of Two-Dimensional Materials Such As Black Phosphorus

Nonlinear optics is the basis of photonics,and is widely used in optical information technology,laser technology,material analysis and nano-photon technology.Nonlinear optical effect is a hot topic in nonlinear optical research,and many researchers make effords to look for nonlinear optical materials with large nonlinear polarizability,high damage threshold and appropriate response tim e.In this case,two-dimensional materials have attracted wide attention because of their excellent nonlinear optical properties,and are widely used in photoelectric devices such as lasers,optical modulators and photoswitches.Graphene is the first two-dimensional material with the advantages of broadband nonlinear optical properties and fast response time.However,graphene's drawback which such as the electronic structure of zero band gap,low damage threshold and small adjustable range of modulation d epth limits its application in high energy laser.Therefore,the researchers further search for two-dimensional materials with the advantages of broadband saturation absorption,ultra-fast response time,low loss threshold,large modulation depth,high dam age threshold and low cost.In addition to graphene,other two-dimensional materials such as topological insulators,transition metal sulfides,black phosphorus appear constantly,which show different band structure,physical,chemical and electrical prope rties.In this paper,we have systematically studied the nonlinear optical properties of two-dimentional materials such as topological insulators and black phosphorus,and some results have been obtained as below:?1?Based on an efficient bottom-up synthesis technique,bismuth selenide?Bi₂Se₃?nanoplatelets with uniform morphology and average thickness down to 3-7nm have been fabricated.Its nonlinear absorption property under high power excitation have been well characterized by our Z-scan measurement system at different illumination wavelengths,and we found that the as-fabricated bilayer Bi₂Se₃nanoplatelets show unique nonlinear optical responses,that is,with a saturable optical intensity of 32 GW cm^-2(resp.3.7 MW cm^-2)and a modulation depth of 88%?resp.36%?at 800 nm?resp.1565 nm?.By implementing its saturable absorption property,we design an optical saturable absorber device based on bilayer Bi ₂Se₃ nanoplatelets through depositing them onto the end-facet of optical fiber.Stable soliton pulses with579 fs pulse width at 1571 nm are obtained.Our experimental results demonstrated that bilayer Bi₂Se₃ nanoplatelets could be a possible large-scale application in saturable absorber for mode-locked fiber laser.?2?Ultrasmall BPQDs with an average size of 2.1±0.9 nm are synthesized by using a solvothermal method in N-methyl-2-pyrrolidone?NMP?solution.Verified by femto-second laser Z-scan measurement,BPQDs exhibit excellent nonlinear optical response with a modulation depth of about 36%and a saturable intensity of about 3.3GW cm^-2.By using BPQDs as optical saturable absorber,the ultrashort pulse with a pulse duration of about 1.08 ps centered at a wavelength of 1567.5 nm is generated in mode-locked fiber laser.This is because BPQDs have the quantum confinement and edge effects.All the obtained results make us believe that BPQDs may be developed as another kind of promising nanomaterial for ultrafast photonics,and solvothermal method could be put into large scale production of BPQDs and promote the innovation of the procedures of BPQDs based opt oelectronic device fabrication.?3?We systematically studied the nature of size-dependent carrier dynamics and third order nonlinear optical?NLO?properties of black phosphorus nanosheets?BPs?.Multi-layer BPs with different sizes are synthesized by using a basic solvent exfoliation method in association with controlled gradient centrifugation.Size-dependent nonlinear saturable absorption and Kerr nonlinearity as well as ultrafast carrier dynamics of BPs is systematically studied by Z-scan and pump-probe technique.Furthermore,the ultrashort pulse with a pulse duration of about 635 fs centered at a wavelength of 1562 nm is generated by using smaller sized BPs as a saturable absorber.These results directly reveal the physical processes of size-dependent NLO properties of BPs and provide researchers with a viable approach in tailoring the NLO response of B Ps through controlling the size,paving the way towards BPs based electronics and optoelectronics applications such as ultrafast optical switches,modulators,fiber lasers,etc.?4?The stabilization of BPQDs for optical application under ambient conditions is highly challenging.Here,we present a facile approach to substantially stabilize BPQDs by making a uniform BPQDs/PMMA composite nanofiber film via an electrospinning technique.As verified by femto-second laser Z-scan measurement,the BPQDs/PMMA composite nanofiber film that ha s been stored for three months exhibits almost the same nonlinear optical properties as the fresh BPQDs.Additionally,the BPQDs/PMMA composite nanofiber film exhibits broadband nonlinear optical response ranging from the visible bandwidth?400 nm?to the mid-IR bandwidth?at least 1930 nm?.By employing the BPQDs/PMMA composite nanofiber film as an optical saturable absorber,an ultrashort pulse with the pulse duration of approximately 1.07 ps centred at the wavelength of 1567.6 nm was generated in a mode-locked fiber laser.These results suggest that the BPQDs/PMMA composite nanofiber film can combine the advantage of convenient integration and mitigation of the drawback of the easy oxidation of black phosphorous and pave the way for BP-based practical optoelectronic devices.