| The laser is one of the greatest inventions of the 20th century.It is widely used in various fields such as scientific research,industry,medical treatment,and national defense.According to the operation modes,lasers can generally be divided into continuous-wave lasers and pulsed lasers.Thanks to the development of mode-locking technique,the duration of a single laser pulse can be compressed to the femtosecond level.Compared with the continuous wave laser,the femtosecond laser has the characteristics of extremely short pulse width,enormally high peak power,and exceptionally wide spectrum coverage.The ultrafast interaction between femtosecond laser pulses and condensed matter provides a powerful tool for solving major challenges in contemporary physics,chemistry,and biologyThe interaction between femtosecond lasers and nanomaterials provides a new tool for exploring the physical and chemical properties of nature.With the limitation of electrons on the nanoscale,low-dimensional nanomaterials,including zero-dimensional nanoparticles,one-dimensional nanotubes,and two-dimensional layered materials,exhibit extraordinary physical properties that are absent in three-dimensional counterparts.The femtosecond pump-probe technique allows the real-time observation of novel non-equilibrium dynamics of low-dimensional nanomaterials at the femtosecond level,including carrier relaxation dynamics,superconducting properties,anomalous Hall effect,charge density waves,chirality phonons,and spin valley dynamics.Femtosecond laser Z-scan spectroscopy allows us to obtain the third-order nonlinear optical parameters of low-dimensional materials such as saturation intensity and modulation depth,which plays an essential guiding role in realizing pulsed lasers.The interaction of femtosecond lasers and dielectric crystal materials has opened the door to miniature and multi-functional integrated optical devices.An optical waveguide is a high-refractive-index microstructure surrounded by a relatively low-refractive-index area.Optical waveguides can ensure long-distance and non-diffraction transmission of optical signals and are basic elements in integrated photonics systems.As the host material of waveguide devices,dielectric crystals play an indispensable role in a variety of fields due to their rich species and functionalities,and are widely used in solid-state lasers,frequency conversion,quantum optics,and many other areas.In recent years,femtosecond laser writing has become a powerful tool to micro-process transparent optical materials and prepare optical waveguides.During the interactions between the laser and transparent optical material,the energy of the femtosecond pulse is deposited into the material through a nonlinear absorption process,causing local refractive index changes to prepare different types of three-dimensional optical waveguide structuresThe combination of low-dimensional materials and dielectric crystal optical waveguides can form new integrated optical devices,such as waveguide pulse lasers.There are two main ways to combine low-dimensional materials with optical waveguides,one is the transmission mode,and the other is realized by evanescent field absorption.Since light is restricted to the micron-level space range,the pulsed waveguide laser has the advantages of high efficiency,small size,integration,and high stability.Combining different laser crystal optical waveguides and nanomaterials of different dimensions can realize pulsed lasers with different wavebands(visible and near-infrared)and different working modes(Q-switched and mode-locked)based on the waveguide platform,which greatly expands the application range of waveguide lasers.The main content of this thesis includes:1)Using femtosecond lasers to study the ultrafast non-equilibrium dynamics of low-dimensional nanomaterials;2)Using femtosecond lasers to study the nonlinear absorption properties of low-dimensional nanomaterials;3)Using femtosecond laser to modify functional crystals to fabricate optical waveguides;4)Using ion implantation technology to embed metal nanoparticles within dielectric crystals;5)Combining low-dimensional nanomaterials with optical waveguides to achieve miniatured pulsed lasing.According to the different experimental methods and the types of nanomaterials selected,the main research contents and results of this paper can be summarized as followsQuasiparticle excitation and many-body interactions in semiconductors are the basis for understanding condensed matter physics and materials science,and they also have huge application potential in photonic technology.Using chemical vapor deposition(CVD)technology,we have prepared large-scale,high-quality PdSe2 films on quartz substrates.Through ultra-low wavenumber Raman,it is revealed that PdSe2 has different anomalous interlayer interactions from other two-dimensional materials The non-equilibrium state of PdSe2 was triggered by the femtosecond laser pulse,and the band renormalization effect of the A exciton was observed,and the amount of change reached 180 meV.Using many-body perturbation theory,the physical mechanism of femtosecond-laser-induced bandgap reduction is explained.In addition,we use femtosecond laser pulses to coherently drive terahertz(THz)-level atomic oscillations between and within single atomic layers of PdSe2.The intralayer atomic oscillation frequency is 4.3 THz and the interlayer coherent phonon oscillation frequency is 0.35 THz.Combined with the bandgap renormalization effect of free carriers,we have achieved a 4.3 THz ultra-high repetition frequency modulation of the bandgap.Through broadband pump detection technology and first-principles calculations,we have constructed intuitive microscopic images of electron-phonon and exciton-phonon coupling,revealing the coupling mechanisms of the two different coherent phonons and different types within and between layers.Using low-energy silver ion implantation technology,spherical silver(Ag)nanoparticles are prepared inside yttrium vanadate(YVO4)crystals.Through cross-sectional transmission electron microscopy and linear absorption spectroscopy,the successful synthesis of embedded nanoparticles and the localized surface plasmon resonance effect are proved.Based on the femtosecond laser transient absorption experiment,the carrier dynamics process of the Ag:YVO4 composite structure from visible light to near-infrared is revealed.Based on the femtosecond laser Z-scan spectrum,it is proved that the composite system embedded with silver nanoparticles has excellent saturation absorption characteristics in the near-infrared band and can be applied to ultrafast optical switches.Using the interaction of femtosecond laser and YVO4 crystal,we prepared the photonic-like lattice cladding optical waveguide in Nd:YVO4 crystal for the first time,and the fluorescence characteristics of the waveguide region of the microscopic fluorescent surface have been well preserved.Based on the embedded nanoparticle and optical waveguide structure,the generation of 1 ?m band Q-switched laser was realized,and the single pulse energy reached 298 mW.High repetition rate,especially for mode-locked lasers with repetition frequencies above 1 GHz,has great research value in many applications,such as high-speed optical communication systems,optical frequency combs,and high-speed optical sampling.Using the interaction of femtosecond laser and YVO4 crystal,we fabricated a circular cladding optical waveguide structure in Nd:YVO4 crystal.Using graphene,MOS2 and Bi2Se3 three typical two-dimensional materials as saturable absorbers,a 6.5 GHz ultra-high fundamental repetition frequency Q-switched mode-locked pulsed laser was realized in an Nd:YVO4 crystal optical waveguide pumped by a 808 nm laser The output signal to noise ratio is greater than 50 dB,and the shortest pulse width reaches 26 ps.Exploring new two-dimensional materials with excellent nonlinear optical properties will help improve the performance of pulsed waveguide lasers.Based on the new graphene/WS2 heterojunction structure and femtosecond laser direct writing Nd:YVO4 crystal optical waveguide,a high-efficiency passively Q-switched waveguide laser with a working wavelength of 1064 nm is realized,with a maximum output power of 275 mW and a slope efficiency of 37%.Compared with single graphene or WS2 saturable absorber under the same conditions,the waveguide pulsed laser output based on graphene/WS2 heterojunction has higher pulse energy and higher slope efficiency;two-dimensional materials can be adjusted by metal nanoparticles Modifications can improve its optical properties and device parameters.The Graphene/Ag nanocomposite was synthesized on top of the surface of graphene by laser ablation method.Through femtosecond laser Z-scan spectroscopy,the nonlinear optical performance of graphene modified with Ag nanoparticles has been significantly improved,the saturation intensity is reduced by 56 times,and the modulation depth is increased by 19 times.Based on the higher modulation depth and lower saturation intensity,we applied it to the Nd:YVO4 cladding optical waveguide platform to obtain a shorter mode-locked pulse;based on theory and experiment,we systematically studied new two-dimensional materials The physical properties of rhenium diselenide(ReSe2),and through femtosecond laser Z scanning technology,it is found that it has lower saturation intensity and modulation depth than other two-dimensional materials,and it is easier to achieve continuous mode-locked pulses in the waveguide platform.In the experiment,we further realized the passive continuous mode-locked waveguide laser based on ReSe2,which produced an ultra-high repetition rate continuous mode-locked pulse laser with a pulse width of 29 ps and an operating frequency of 6.5 GHzUsing femtosecond laser micro-nano processing technology,micron-level circular cladding optical waveguide structures of different sizes are fabricated in barium metaborate(?-BBO)crystals.Based on the end-face coupling system,the transmission loss and near-field light intensity distribution of each waveguide at 400 nm and 800 nm are measured.The study found that the cladding optical waveguide based on ?-BBO crystal showed good transmission characteristics in the TM polarization direction of 400 nm and 800 nm.At 800 nm wavelength,the minimum transmission loss is 0.19 dB/cm.Confocal Raman microscopy technology shows that the physical properties of the waveguide region are well preserved.Using Rsoft(?)software,the transmission characteristics at 400 nm and 800 nm wavelengths were simulated.Using all femtosecond laser micro-nano processing methods,different types of ridge optical waveguides are prepared in neodymium-doped yttrium aluminum garnet(Nd:YAG)crystals,including Y-branch optical waveguide devices with different branch angles.Experiments show that the ridge optical waveguide prepared by the all-femtosecond laser supports both TE and TM polarization transmission and has low transmission loss.Based on MoSe2 as a saturable absorber,the Y-branch device is integrated with a monolithic waveguide pulsed laser device to achieve a Q-switched mode-locked pulse output in the 1 ?m band with a repetition frequency of up to 7.7 GHz. |
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