Graphene Saturable Absorption Mirror Controllable Equipment And Optical Properties

With the birth of high-power pulsed lasers,the nonlinear optics have ushered in a new development opportunity.With the rapid development of ultrafast laser technology,the traditional semiconductor saturable absorbers have no longer fulfilled the requirement of broadband lasers using passive mode-locking technology.The key strategy is naturally to develop saturable absorbing materials with broadband strong saturation absorption,and faster relaxation time.As an ultra-thin nonlinear optical crystal,graphene has many advantages such as wide-band saturable absorption,electrically tunable optical properties,high laser damage threshold and ultrafast response time,thus providing a promising prospects in the fields of laser technology such as pulse laser modulation,high-order harmonic generation and four wave mixing.However,the existing graphene saturable absorbers(GSAs),usually prepared by the solution coating of graphene powder,still have the problems such as poor film quality,abundant graphene defects,and large optical loss.In order to address above challenges,this thesis focuses on the controllable growth,broadband response and nonlinear optical properties of GSAs for developing high-performance,long-wavelength mode-lock lasers.The main contents are listed as follows:(1)In order to overcome the drawbacks such as abundant defects and low laser damage threshold of solution-processed graphene films,graphene films with high crystallinity,high uniformity and less impurities were grown by chemical vapor deposition(CVD)method.The system of plasma-enhanced chemical vapor deposition(PECVD)was home-made,which realized the low-temperature preparation of GSA mirror without using metal catalysts.Graphene films with different thickness were directly grown on the quartz surface,avoiding the film damage and the residual of metal impurity caused by graphene transfer process,thus to maintain the purity of graphene optical glass.The morphology,crystal quality and layer thickness of samples were characterized by optical microscopy,Raman spectroscopy,transmission and scanning electron microscopies as well as selective electron diffraction.The growth mechanism of graphene on insulating substrates was explored.The nonlinear absorption characteristics of GSA mirrors were probed by Z-scan technique.The influence of film thickness on the saturation absorption was revealed.Our results showed that,the nonlinear absorption coefficient was 2.49?13.65×10-14 mW-1,the average number of layers was 3?62,the unsaturated loss was 0.41?3.2%,and the modulation depth was 1?28%.It provides a platform for developing the massively produced GSA mirrors.(2)In order to fulfill the requirement of broadband-saturable absorbers,a low-temperature controlled growth method of graphene on the CaF2(111)substrate was provided.The uniform graphene films with high crystallinity were obtained on the CaF2 substrates.The high crystalline quality and defect features of graphene films were revealed by Raman spectrum analysis.The as-prepared graphene films were multilayer and polycrystalline.The nonlinear absorption properties of graphene/CaF2 saturable absorption mirrors were probed by Z-scan measurement technology.The nonlinear absorption coefficient is proportional to the layer thickness,while inversely proportional to the film transmittance.The modulation depth increases with the growth time of graphene films,and the unsaturated loss is comparable to that of the graphene grown on copper substrate.The results show that,the nonlinear absorption coefficient is 2.61 × 10-14 mW-1,the average number of layers is 4,the unsaturated loss is 0.79%,and the modulation depth is 2.8?11.2%.It opens up a new way for the design and preparation of long-wavelength GSA mirrors.