| A laser is a device based on the stimulated emission of electromagnetic radiation,with the characteristics of high intensity,good directionality and high coherence.The laser is one of the most important inventions of the 20th century.Since the invention of the first laser by T.Maiman in 1960,laser devices and technologies have developed rapidly for 60 years.Now lasers have played an irreplaceable role in the fields of industry,medicine,communication,science and consumer electronics.Hence laser has been known as one of the new "Four New Inventions" that can drive human development.Depending on output wavelength,a laser can be classified as an ultraviolet laser,visible laser and infrared laser.And infrared lasers can be subdivided into near-infrared lasers,mid-infrared lasers and far-infrared lasers.Among them,the mid-infrared 3-5 μm spectral region covers the atmospheric transmission window and the absorption peaks of various gases and chemical functional groups.The characteristics determine that the lasers operating in the mid-infrared spectral region have significant applications in many fields,such as medical treatment,military,industry,etc.Nonlinear frequency conversion,mainly including difference frequency generation,optical parametric oscillation,and optical parametric amplification has the advantages of high output power and wide tuning range,and has become the most common approach to midinfrared laser.So far,both theories and materials of nonlinear frequency conversion are relatively mature.Since the development of lasers over the past sixty years,the basic nonlinear mixing theory has been established.As we all know,phase-matching technology is the necessary condition for efficient nonlinear frequency conversion.Currently the analysis and practice of various phase-matching techniques play significant roles in nonlinear devices.Therefore,further theoretical research on phase-matching will certainly promote the development and improvement of nonlinear optics theory.The nonlinear optical crystal is the core of nonlinear mixing.Depending on the difference in crystal properties,nonlinear crystals can be classified as oxides and non-oxides.Oxides such as β-barium metaborate(β-BaB2O4,BBO),lithium triborate(LiB3O5,LBO),potassium titanyl phosphate(KTiOPO4,KTP),potassium titanyl arsenate(KTiOAsO4,KTA),magnesium-doped lithium niobate(MgO:LiNbO3),etc.,have advantages of large band gaps,high laser damage threshold,easy growth,but they are limited to the poor infrared transparency.Non-oxides such as sulfur gallium silver(AgGaS2,AGS),gallium silver selenide(AgGaSe2,AGSe),zinc germanium phosphorus(ZnGeP2,ZGP),gallium arsenide(GaAs),etc.,generally have high nonlinear coefficient and wide range of infrared transparency,but most of this type of crystals have the disadvantage of low damage threshold and difficult growth.In addition,they can’t be pumped by more conventional and powerful near 1-μm laser systems.In recent years,several langasite(LGS)family crystals have attracted more and more attention owing to their excellent midinfrared nonlinear frequency conversion properties,especially La3Ga5.5Nb0.5014(LGN).The development of mid-infrared laser sources based on LGN will contribute to break through the bottleneck of mid-infrared laser development.The main work of this paper is as follows:1.Originating from the rotation of polarization plane of linearly polarized beams(including natural rotation and Faraday rotation),the process of second harmonic generation in optically rotatory nonlinear crystals is theoretically studied.We found that the introduction of geometric phase can contribute to compensate for wavevector mismatch of wave-wave interactions.We analyzed the electric polarization intensity in the optically rotatory nonlinear crystal,then obtained the second harmonic output amplitude under the approximate condition through the coupled wave equation in the nonlinear medium.Finally,we obtained the phasematching condition,and discussed the realization this condition in practical materials.This work further improves the nonlinear frequency conversion theory and provides a phasematching method for optically rotatory nonlinear materials.It will have broad application prospects in the field of optoelectronic chips involving Faraday rotation and nonlinear frequency conversion.2.Based on the refractive index dispersion equation of LGN crystal,we calculated the relationship between the phase matching angle and wavelength of nonlinear frequency downconversion at the pump wavelength of 1.064μm.We found that LGN crystal can achieve phasematching conditions in the whole mid-infrared region.In addition,we further calculated the walk-off angle,acceptance angle and acceptance bandwidth corresponding to different parameter output wavelengths,which laid the foundation for the subsequent experiments.3.In order to assess the practicality of LGN in the mid-infrared region,we studied the difference frequency generation performance of LGN crystals in the 2.8-3.5 μm and 4.45.7 μm spectral region.Firstly,we designed experiments of mid-infrared nonlinear frequency conversion employing LGN crystal.A pulsed laser with a wavelength of 1.064 μm was used as the pump laser,and LGN-based difference frequency generation for mid-infrared radiation was realized.In this work,we obtained μJ-level mid-infrared nanosecond pulses,with the tuning wavelength of 2.8-3.5 μm and 4.4-5.7 μm.In the tuning region of 2.8-3.5 μm,the output performance of difference frequency generation under type-I phase-matching was preliminarily explored,where the maximum output pulse energy was 15.53 μJ(@3.3 μm).In the tuning region of 4.4-5.7 μm,the output performance of type-Ⅰ and type-Ⅱ difference frequency generation was further studied,giving a maximum output pulse energy of 13.1μJ@4.5 μm.The comparison between the output performance of LGN and other oxide nonlinear crystals shows LGN has good performance in the long-wave mid-infrared(>4.5 μm).This work shows LGN will have important prospects in the mid-infrared spectral region. |
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