Research On Laser Generation Process And Wavelength Extension Of Electron-Phonon Weak Coupling System

Laser is an acronym for light amplification by stimulated emission of radiation.It is characterized by an extremely high degree of monochromaticity,coherence,directionality,and brightness,which is known as "the sharpest blade","the brightest light" and "the most accurate ruler".Since the advent of the first ruby laser in 1960,laser has triggered a series of breakthroughs in various fields of human society,such as laser processing,telecommunication,medicine,and autonomous driving.A vast diversity of laser wavelengths ranging from terahertz to deep-ultraviolet regions and laser media to date,e.g.,solid-state,gas,excimer,and semiconductor lasers,has played significant,even irreplaceable,roles in particular application scenarios.Among them,the all-solid-state laser,as the first invented laser,has the advantages of high efficiency,low cost,stable performance,compact structure and long lifetime,which has been attracting considerable attention for a long time.The applications of laser are generally determined by the laser wavelength.Therefore,the development of new laser crystals to meet the needs of specific laser wavelengths in practical applications is the frontier of international scientific and technological competition.However,the inherent electron energy level of rare earth ions in laser crystals is relatively fixed,and each rare earth ion can only obtain the lasers with specific wavelengths,resulting in a limited tuning range.Although hundreds of laser materials have been investigated,there are still a large number of spectral gaps that cannot be obtained by direct lasing.In order to extend the wavelength range of available sources,various methods of nonlinear frequency conversion,such as frequency doubling,difference frequency,optical parametric oscillation,and stimulated Raman scattering,have been developed.However,these methods not only need to introduce additional optical components,but also have the strict requirements on crystal and phase matching directions,which undoubtedly limits the laser efficiency and the simplicity of configurations,and cannot meet the needs of high integration and miniaturization for this information age.Besides,the processes of nonlinear frequency conversion such as frequency doubling and sum frequency have a large amplitude of wavelength regulation,which is unable to meet the needs of small amplitude regulation of laser wavelength in some situation.Hence,the development of new wavelength expansion technique to achieve direct wide-band,tunable laser output has always been an enduring pursuit in this field.The tiny vibration of atoms around their equilibrium positions in crystals is one of the most elementary motion of lattice atoms.When the inherent lattice vibration（phonon）in the crystal interacts with the electronic energy level of the rare earth ions,the energy transferred between electrons and phonons can occur during the electronic transition.This physical process is called the electron-phonon coupling effect.In the laser crystal,when the electronic transition of the active ion is coupled with the different phonon modes,the laser wavelength can be modulated by the energy exchanging between electron and phonon,thus overcoming the limitation of the inherent electronic energy level and achieving the purpose of changing the laser wavelength.According to the research,the studies of electron-phonon coupled lasers mainly focus on experiments,and there are few theoretical studies on the phonon modes involved in the coupling process.In particular,the relationship between electron-phonon coupling and crystal symmetry needs to be analyzed theoretically.In addition,most of the experimental works focus on strong coupling systems such as transition metal ions doped gain materials,and relatively few theoretical and experimental studies on rare earth ions.This is because in the weak coupling system（S<1）,the pure electronic transition dominates the fluorescence spectrum due to the lack of method of regulation,and it could not form the effective electron-phonon coupling fluorescence emission,so the related research has been neglected for a long time.This thesis focuses on two representative rare earth ions,Yb³⁺ions with quasi-three-level and Nd³⁺ions with four-level.Based on the electron-phonon coupling model,the selection rules are derived,and the rate equation of phonon-triggered population inversion is established.Represented by Yb:LuScO₃ and Nd:YVO₄ crystal,the physical mechanism of multi-phonon coupling is analyzed,and the internal relationship among electronic energy level-phonon modecoupling strength-laser output is revealed.It breaks the thinking pattern that the fluorescence spectrum limits the laser wavelength,and it also realizes the electron-phonon coupled laser output and precise wavelength control,and studies the influence of temperature on laser performance.The main works of this thesis are as follows:1.Selection rule for electron-phonon coupling effectBased on the comprehensive consideration of various calculation methods of electronphonon coupling strength,the corresponding relationship between macroscopic physical properties and crystal microstructure was found.On the basis of the symmetry of crystal structure,two phonon selection rules for electron-phonon coupling effect were established:1.The phonon mode involved in the coupling must be Raman active;2.The symmetry of participating phonon must be contained in the symmetry determined by both electron and electric dipole moments.This result provided the theoretical guidance for the realization of electron-phonon coupling laser.2.The regulation of electron-phonon coupling effect in weak coupling systemFor the weakly coupled rare earth ion system,a physical model of photon-phonon collaboratively pumped laser was proposed.By solving the rate equation of the system in steady state,the relationship between population inversion and temperature was verified.It was proved that temperature played an important part in regulating electron-phonon coupling laser.The laser process based on this collaboratively pumping mechanism had a temperature threshold in addition to the traditional optical pumping threshold,so that the laser phase diagram with two parameters of pump power and temperature can be established to analyze and describe the electron-phonon coupling laser process.3.Electron-phonon coupling in Yb:LuScO₃ crystalThe quasi-three-level Yb:LuScO₃ crystal was studied.Firstly,the zero-phonon laser performance of this crystal was characterized,including continuous-wave laser and Q-switched laser,which proved that the crystal has excellent laser performance.Due to the electron-phonon coupling effect,the fluorescence spectrum had a wide phonon sideband located around 1130 nm,and its intensity was strengthened gradually with the increase of temperature,which paved the way for tunable laser within this band.By elaborately designing the laser resonant cavity to suppress the conventional lasing inside the fluorescence spectrum,a tunable laser at 1121-1136 nm was obtained corresponding to the three-phonon case.This result represented the longest lasing wavelength achieved in Yb³⁺doped sesquioxide crystals.At the same time,for the 1 131 nm laser with the highest output power in the tuning range,the maximum output power could reach 410 mW by optimizing the resonant cavity,which is the highest result of Yb³⁺doped sesquioxide crystal reported in this band.4.Electron-phonon coupling in Nd:YVO₄ crystalThe electron-phonon coupling effect in four-level Nd:YVO₄ crystal was studied.The selection rules were applied and the phonon modes involved in the phonon-triggered process were analyzed.By controlling the temperature of crystal,it was verified that increasing temperature was an effective method to enhance the strength of electron-phonon coupling in weak coupling systems.The emission peaks in phonon-triggered region were assigned to prove the selection rule,and the experimental results were consistent with the theoretical prediction.By elaborately designing the laser resonant cavity,one-phonon process at 1176 nm and 1168 nm,two-phonon process at 1231 nm were realized in Nd:YVO₄ crystal.The influence of temperature on the electron-phonon coupling laser was also studied,including wavelength shift,the change of output power and laser threshold.In addition,keeping the optical pump power unchanged,the temperature threshold was observed for the first time by changing the cooling temperature,and the lasing phase diagram of 1176 nm laser in a two-dimensional parameter space spanned by optical pump power and temperature was established.Besides,by combining the nonlinear frequency conversion and electron-phonon coupling,the 588 nm yellow laser was realized,which can be applied to laser medicine.