Study Of Novel Broadband Saturable Absorbers Technology And Its Application In Pulsed Lasers

Compared with continuous wave(CW)lasers,pulsed lasers possess many advantages such as short pulse width and high peak power,which makes it widely used in basic research,telecommunications,medical,industrial processing and so on.The most convenient and effective technical means to achieve pulsed lasers is passive Q-switching/mode-locking based on saturable absorbers(SAs).For a long time,the improvement of optical parameters of SAs for particular laser systems has been a hot topic both in the industry and research community.Although SAs may take different physical forms,semiconductor saturable absorber mirrors(SESAMs)are at present the most prevalent approach used for pulse generation.Compared with alternative technologies,a most compelling advantage of SESAMs is the ease with which device parameters can be precisely customized with great reproducibility,thanks to the use of mature semiconductor growth techniques.Up to now,SESAMs are still the most widely used and commercially available SAs.However,limited by the choices of semiconductor materials and device structure,SESAMs nevertheless exhibit a relatively narrow nonlinear optical bandwidth(～100 nm)and limited long-wavelength access,that is,typically～3 μm.With the increasing demand for pulsed lasers,there is an urgent need for stable,wideband saturable absorption devices which possess a flexible parameter control space.In this thesis,we firstly study the ultra-wideband saturable absorption of single-walled carbon nanotubes(SWNTs),and starting from the most basic low-dimensional materials to discover the potential applications of SWNTs in compact,high-reliability and ultra-wideband tunable mode-locked lasers.Secondly,we systematically study the saturable absorption of three-dimensional Dirac semi-mental(3D TDS)cadmium arsenide(Cd3As2)in the near-infrared band and its applications in pulsed lasers.Finally,we make an intensive study of parameters regulation of SAs based on Cd3As2 films.The nonlinear parameters of the Cd3As2-SA device can be actively and passively regulated by a simple current-induced temperature change and an element doping approach,respectively.These efforts further promote the practical process of the Cd3As2-SA devices.The research can be summarized as follows:(1)First of’all,we fabricated large-scale and high-quality SCNTs-SA by liquid phase processing.Then,the saturable absorption behavior and photocarrier relaxation process of SCNTs-SA were investigated by two arms transmission measurement and pump-probe measurement,respectively.Results show that SCNTs-SA with tube diameters distribution between 1.3 and 1.6 nm exhibits obvious saturable absorption effect over broad wavelength from 1.6 to 2.1 μm.Furthermore,by combining the SCNTs-SA with a diffraction grating based mirror,we demonstrated a passively mode-locked Tm/Ho fiber laser with an ultra-broadband tuning range of 200 nm(from 1860 nm to 2060 nm).(2)We have for the first time verified that molecular beam epitaxy(MBE)grown 3D TDS Cd3As2 thin-film exhibits ultrafast nonlinear absorption at the near-IR band.Firstly,by performing broadband pump-probe measurements,the photocarrier recovery time of Cd3As2 thin-film was obtained about 5 ps.Then,the typical two arms transmission measurement was employed to characterize nonlinear saturable absorption properties of Cd3As2-film.The modulation depth and saturable intensity measured at 1.96 and 1.56 μm wavelengths are 3.5%and 12 MW/cm2,5.1%and 67 MW/cm2,respectively.Finally,we obtained stable mode-locked fiber lasers at 1.96 and 1.56 μm and Q-switched fiber laser at 1.06 μm by using this emerging SA device.Our results establish Cd3As2-film as a promising SA device for wideband and tunable mode-locked lasers across the near-and mid-IR wavelength range.(3)On the one hand,an electrically contacted SA device based on 3D TDS Cd3As2 was fabricated through MBE technique and electron beam evaporation(EBE)technique.With a current-induced temperature change in the range of 297-336 K,the modulation depth of the device is found to be significantly altered from 33.4%to 75.9%(under the irradiation of a 1560 nm femtosecond laser).The simple tuning mechanism,together with the compatibility with Ⅲ-Ⅴ compounds substrate,such as GaAs,points to the potential of fabricating broadband,electrically tunable,SESAM-like devices based on emerging bulk Dirac materials,and can provide a broad support platform for in-depth study of the dynamics process of mode-locking.On the other hand,by comparing chromium-doped(Cr-doped)with manganese-doped(Mn-doped)Cd3As2 thin-films,we try to clarify the general effect of element doping on the optical properties of 3D TDS Cd3As2.Both Cr and Mn doping can regulate carrier recovery time of Cd3As2 to varying degrees,but significantly different from Cr doping,Mn doping introduces an extra long-lived photocarrier decay component(t:～-2.8 ns@ Mn:12%).The presence of a slow relaxation component is long known to be useful for SAs,as it can facilitate pulse shaping during the initial build-up of mode-locking.