Heavily-doped Semiconductor Nanocrystal-based Saturable Absorbers And Pulsed Lasers

The rapid development in many areas,like optical communication,biomedicine,military defense,fundamental research,precise material processing,etc.,relies largely on the progress of pulsed laser sources which serve as powerful tools in these areas.As a general rule of human social development,technological advances,in turn,raise higher requests for the tools.As is the case here for pulsed lasers,technologists from different backgrounds are demanding laser pulses with ultrashort pulse duration,high power,high pulse repetition rate and multi-wavelengths.Laser pulse generation mainly relies on the saturable absorber,of which the commercial available saturable absorber SESAM become more and more difficult to meet the urgent demands for advanced pulsed laser sources due to its own limitations,therefore it is of particular urgency to develop new saturable absorbers.Considering the developing trend of pulsed laser sources and to develop a new saturable absorber,we come up with a new concept for saturable absorber based on the localized surface plasmon resonance?LSPR?absorption of heavily-doped semiconductor nanocrystals.We exploited two kinds of material systems?chalcogenide and oxide nanocrystals?for saturable absorber,and systematically studied their nonlinear optical absorption properties and ultrafast carrier dynamics.A super-broadband saturable absorption response corresponding to the broadband LSPR absorption of the chalcogenide nanocrystals,covering a spectral region of 0.8-3.0 ?m,was found.We further realized a super-broadband?1.0 to 3.0 ?m?saturable absorber in pulsed lasers based on these chalcogenide nanocrystals.Besides,we realized a saturable absorber in the optical communication band based on the superior nonlinear optical response in the epsilon-near-zero?ENZ?region of heavily-doped oxide nanocrystals.The work in this thesis mainly includes the following parts:1.The nonlinear optical properties and ultrafast carrier dynamics of off-stoichiometric Cu_2-xS nanocrystals were studied.The nonlinear absorption coefficient ?,the imaginary part of the third-order optical susceptibility Im??3?,and the figure of merit?FOM?at the LSPR absorption peak?1300 nm?of Cu_2-xS nanocrystals,are estimated to be?-161 cm/GW,-1.39×10-10 esu,and 3.79×10^-15 esu cm,respectively,based on the Z-scan measurements combined with theoretical calculations;besides,the optical modulation depth can be as high as 63.3%.These nonlinear optical parameters are obviously better than the recently rising two-dimensional materials based saturable absorbers,like graphene,transition-metal dichalcogenides,black phosphorus,etc.Furthermore,broadband nonlinear optical test demonstrates that the nonlinear optical modulation depth of the Cu_2-xS nanocrystals is closely connected with the LSPR absorption intensity and the largest modulation depth can be achieved at the LSPR peak;in other words,Cu_2-xS nanocrystals show super-broadband saturable absorption response which is related to the LSPR absorption.On the other hand,according to the pump-probe test,the transient spectral response decay follows a biexponential behavior,with a faster decay component?corresponding to electron-phonon interaction?characterized by a short lifetime of?315 fs followed by a slower part of?34 ps?corresponding to phonon-phonon interaction?,both of which are an order of magnitude faster than that in noble metals.2.Super-broadband saturable absorber for pulsed lasers was first constructed based on the Cu_2-xS nanocrystals,which fully demonstrates its ability to operate in a super-broad spectral region.We constructed several pulsed lasers based on Cu_2-xS nanocrystals:1)mode-locked fiber laser operating at 1.5 ?m band,with central wavelength of 1562.6 nm,repetition rate of 7.28 MHz,pulse width of 295 fs and signal-to-noise ratio?SNR?of 50 dB.Besides,the output optical spectrum remained almost no change during two-hour operation according to the laser stability test.2)mode-locked bulk laser operating at 1.0?m band,with central wavelength of 1030 nm,repetition rate of 84.17 MHz,pulse width of 7.8 ps and signal-to-noise ratio?SNR?of 60 dB.3)Q-switched fiber laser operating at 3.0 ?m band,with central wavelength of 2769 nm,minimum pulse width of 0.75 ?s,maximum output power of 214 mW,and maximum repetition rate of 90.7 kHz.3.A concept of optical-parameter-tunable saturable absorber based on the widely tunable LSPR peak of copper chalcogenide nanocrystals was proposed.Tunable carrier density between 9.7×10²¹ Cm^-3 and 3.18x10²¹ Cm^-3 can be obtained through controlling the doping conditions,which results in tunable LSPR peak between 800 nm and 1500 nm.Through controlling the doping level of Sn,we can adjust the LSPR peak of Cu-Sn-S nanocrystals from 1350 nm to the optical communication window near 1550 nm,where the nonlinear optical absorption properties was studied and found to show superior saturable absorption response with optical modulation depth to be 52.6%;besides,the carrier recovery speed can be further enhanced with Sn doping,reducing the faster decay lifetime from 315 fs to 266 fs.Based on these Cu-Sn-S nanocrystals as saturable absorber,we realized mode-locked femtosecond pulse generation in Er-doped fiber laser,and the laser stability test demonstrated that almost no change could be found in the performance of the laser under 10-hour continuous operating state.4.The nonlinear optical absorption properties and ultrafast carrier dynamics were studied in the ENZ region of ITO colloidal nanocrystals,which were further used to construct a saturable absorber and integrated into a laser to achieve pulse generation.In the ENZ region,ITO nanocrystals were found to show excellent saturable absorption properties,of which the nonlinear optical absorption coefficient can be as high as?-50 cm/GW,according to the Z-scan results.Pump-probe test indicated that the optical modulation depth in the ENZ region of ITO nanocrystals can even reach 160%and the carrier decay process can fully complete in-450 fs with a decay lifetime of only?100 fs,implying an ultrafast and ultra-sensitive optical modulation property in the ENZ region of ITO nanocrystals.Finally,based on the ENZ ITO nanocrystals?ENZ region located at 1.5 ?m band?as saturable absorber,we obtained mode-locked pulse generation at 1.5 ?m with pulse width of 593 fs in an Er-doped fiber laser.