Investigation On The Optical, Second Harmonic And Fs Laser Damage Propetries In Pure And Doped Gase Crystals

Nonlinear optical crystal is the important material foundation of photoelectrictechnology, especially laser technology. It can be used for laser frequencyconversion and signal storage and plays an important role in both modern high-techand military.The key physical properties that render the layered GaSe as one of the bestnonlinear crystals for parametric frequency conversion (PFC) are the broadtransparency range from0.62to20μm, the large second-order nonlinearity of54pm/V and the high damage threshold that can be similar to that of the so-called“mid-IR standard crystal” ZnGeP2. However, the layered structure of GaSe leads toextremely weak mechanical properties: almost zero hardness by Mohs scale andeasy cleaving that limit large size crystals application.It is found that the mechanical properties of GaSe can be improved significantlyby doping with S, In, Te, Al and so on, which makes it more suitable to be used inPFC systems.The optical, second harmonic and damage properties of several kinds of dopedGaSe crystals are investigated in this thesis.1. The optical and second harmonic properties of GaSe:Er crystalsCentimeter-sized Er-doped single crystals were grown from the meltsGaSe:Er(0.025,0.1,0.5,1and2at.%) by modified Bridgman technology.Incorporation of Er atoms into GaSe structure is confirmed by fixing specifictransformation features in the short-and long-wave absorption edges, Raman spectra and EDX patterns. It is found that Er content in all grown ingots issurprisingly lower to that in the melt compositions. Real Er content is ascertained as0.009,0.019,0.033,0.042and0.048at.%, respectively. The phase matching anglesunder fs OPG and CO2laser pump are measured. No visible differences can befound between phase matching angles for GaSe:Er crystals and that for pure GaSe,which also confirms the low concentration of Er. The optimal doping of Er in GaSecannot be determined by absorption spectroscopy. Optimal doping of0.033at.%ofEr is established from SHG experiment that results in about20%increased efficientnonlinearity. It is found that improved optical quality is a reason for increased SHGefficiency.2. The optical and second harmonic properties of GaSe:Te crystalsGaSe crystals with0.05~10mass.%Te-doping are studied. GaSe:Te (≤5mass.%) crystals show the hexagonal structure like ε-GaSe. It is found that thecentimetre-sized single crystal of GaSe:Te (≤0.5mass.%) possesses high opticalquality. The phase matching angles under fs OPG and CO2laser pumping aremeasured. No visible differences can be found between phase matching angles forGaSe:Te crystals and that for pure GaSe. Nevertheless, the improved opticalquality and damage threshold lead to increased frequency conversion efficiency.From the available and obtained data, it is proposed that the optimal doping levelof such heavier elements as In, Te, Er in GaSe is below0.5mass.%for nonlinearoptical applications.3. The optical properties of GaSe:InS crystalsSingle crystals are grown from the melt of GaSe and InS (1,5,20mol.%) thatare identified as ε-polytype of quaternary solid solution crystals Ga1-yInySe1-xSx.Chemical composition, lattice structure and optical properties have beeninvestigated that shown its usefulness as parametric frequency converters that forthe first time were confirmed experimentally. Lower nonlinear optical losses to that for solid solution crystals GaSe1-xSxand higher damage threshold to that forpure GaSe and solid solution crystals Ga1-xInxSe are their advantages. It isproposed that Ga1-yInySe1-xSxcrystals with independce of S and In concentrationmay be more attractive in nonlinear applications.4. The fs laser damage properties of GaSe:S and GaSe:In crystalsThe impact of fs pulses on doped crystals GaSe:S (0.5,1,2,3,7,10mass.%)and GaSe:In (0.5,1.32,2,2.32mass.%) has been studied in comparison with thaton pure GaSe crystal. It is established that visual criterion in the determination ofthe crystal damage threshold under0.8μm fs pulse pumping is not consistentbecause observation of the black matter damage spots on the crystal surfaces donot decrease noticeably both the transparency and frequency conversion efficiency.The pump intensity limit can be readily determined from the transparency decaycurve measurements and that is still reversible after a decline down to themagnitude of0.1choosing an acceptable transparency level. High advantages inthe limit pump intensity as up to40~50%for optimally In-doped crystal GaSe:In(2mass.%) and up to4.5times for optimally S-doped crystal GaSe:S (3mass.%)crystals have been demonstrated under expose to fs Ti:Sapphire laser systempulses. The nonlinear multiphoton absorption has been identified as key factorlimiting the fs Ti:Sapphire laser system pulse pump intensities.