| Ultra-short and ultra-intense pulsed laser system is a pulsed laser with peak power Ppeak>1 TW and pulse width τpluse<100 fs,which is known as the "brightest light source" because of its ultra-strong electric field,ultra-high magnetic field,ultra-high energy density and huge optical pressure.The invention of chirped pulse amplification(CPA)technology in 1985 has greatly promoted the development of ultra-short and ultra-intense lasers and opened up many new research interests.The ultra-short and ultra-intense laser system has made some progress in the fields of high-energy particle gas pedals,extreme ultrafast light sources,and physics.And when the wavelength of the ultra-short and ultra-intense laser is expanded to the midinfrared(mid-IR)wavelength,its advantage of long wavelength brings breakthroughs and applications in strong field physics,chemistry,biology and materials science,such as high harmonic generation,attosecond pulses,infrared spectroscopy,high resolution imaging and so on.Due to the lack of gain medium for mid-IR wavelength,the high peak power laser output cannot be achieved directly using CPA technology.The optical parametric amplification(OPA)process is limited by the crystal laser damage threshold,and its conversion efficiency and output energy are low.Optical parametric chirped pulse amplification(OPCPA),which combines the advantages of CPA and OPA technologies and has become the mainstream technology for obtaining mid-IR ultra-short and ultra-intense pulsed lasers.It is well known that the nonlinear optical(NLO)crystal is the core component of the OPCPA system,and the performance of the crystal such as transmission range,laser damage threshold(LDT),effective NLO coefficient(deff)and growth habit directly affects the output performance of the mid-IR ultra-short and ultra-intense laser system.Therefore,the research on the selection,design,growth and performance optimization of mid-IR NLO crystals can favorably promote the development of mid-IR ultra-short and ultra-intense laser devices.In recent years,it has been found that langasite crystals have the characteristics of a wide transmission spectrum,high LDT,suitable birefringence,moderate deff,stable physical and chemical properties and large-size crystal growth potential,among which La3Nb0.5Ga5.5O14(LGN)crystal has more excellent comprehensive performance and meets the application of NLO crystals for mid-IR OPCPA systems.It is expected to become a beneifical material for high peak power mid-IR ultra-short and ultra-intense pulsed laser systems.The main work of this thesis is to address the demand for NLO crystals with large aperture and high optical quality,high LDT,and large deff for midIR OPCPA systems as follows:1.Growth of large-size high optical quality LGN crystalAiming at the demand of large-diameter NLO crystal in the mid-IR OPCPA system,the sub-solid phase diagram of the La2O3-Nb2O5-Ga2O3 system at 1340℃ was established,the consistent melting range of LGN phase was obtained,and the optimial initial melt components is La2O3-32.80 mol%,Ga2O3-61.80 mol%,Nb2O5-5.40 mol%.The temperature gradient distribution in the temperature field and the stress distribution in the crystal built by different types and thicknesses of insulation materials were simulated,and a suitable temperature field for the growth of large-size LGN crystal.The ideal growth shape of LGN crystals was calculated,and the suitable growth direction-c direction was selected.Finally,a 60 mm diameter LGN crystal was obtained by optimizing the growth parameters and PID parameters.The rocking curves and conoscopic patterns of the crystals indicated that the high structural perfection and optical homogeneity of the grown crystals,and a set of NLO devices for mid-IR OPCPA was processed successfully.2.Optimization of laser damage threshold of LGN crystalTo meet the requirement of high LDT for NLO crystals in the mid-IR OPCPA system,the main defect types within LGN crystals and their laser damage mechanisms were investigated.Two LGN crystals were obtained in different growth atmospheres using the Czochralski method,and their single crystal masses and crystal components were characterized.The energy band structures and absorption spectra of LGN crystals containing different point vacancy defects were calculated using the first principle,and the oxygen vacancy and color center defects were clearly identified as the main defects in LGN crystals.The distribution pattern of free electrons in the conduction band and valence band of the crystal obeys the Boltzmann distribution,which is closely related to the optical bandgap of the crystal.The linear absorption and nonlinear absorption values of LGN crystals were tested,and it is clear that the main mechanism of laser damage in LGN crystals is the linear absorption and nonlinear absorption within the crystal.The results of the LDT of LGN wafer samples after annealing in air is significantly enhanced,with a maximum of 13.1 J/cm2 for wafer samples annealed in air at 700℃,which is 24%enhanced compared with that of as-grown LGN crystals.3.Optimization of effective nonlinear optical coefficients in langasite familyTo address the problem of low conversion efficiency of LGN crystals in the mid-IR nonlinear frequency conversion process,a design strategy is proposed to increase the deff in langasite family,in which the BO6 octahedral group has a high response to an external electric field,and the design of B site can effectively regulate the deff of the crystal.We designed and grew a series of(La3(Nb1-xTax)0.5Ga5.5O14,LGNTx)solid-solution crystals for improving the defff.The phases,components,linear optical properties and NLO properties of LGNTX solidsolution crystals were comprehensively characterized.Then,the phase-matching characteristics in the process of OPA were calculated.The effectiveness of the refractive index dispersion control scheme was verified.The structure-performance relationship of component,refractive index,phase matching angles and effective nonlinear optical coefficient was established.The LGNT0.40 crystal with the largest deff was optimized and grown,whose deff is 1.7 times that of LGN crystal,which can improve its conversion efficiency in nonlinear frequency conversion processes. |
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