Bi 12 TiO 20 (BTO)crystals have same structure as that of the Bi 12 SiO 20 (BSO)which is one of the most popular photorefractive materials.And both of them belong to cubic non centrosymmetric nonferroelectric materials.The cubic nonferroelectric oxides exhibit a number of advantages such as higher sensitivity and faster respond for applications in photorefractive.Compared to BSO,BTO have the largest electro optical coefficient and lower optical activity,especially it exhibits much high photosensitivity in the red light region that permits to use a cheap He Ne laser.It is,therefore,of high potential for applications in optical devices.
The KGd(WO 4) 2[KGW] crystal has a structure belonging to the monoclinic sys tem with space group C 2/ c and with unit cell dimensions a =1.068mm, b =1.043nm, c =0.76nm, β =130°.This crystal is an excellent host material for solid state lasers.The threshold of the laser oscillations in a Nd 3+ d oped KGW laser crystal is considerable low and has a higher emissive section.T he fluorescent concentration quench effect of the Nd 3+ ion in the KGW crystal may be weakened due to the W O covalent bond,so this crystal has a higher dopi ng concentration of active ion.Furthermore,the absorption band at 808nm of Nd 3+ in the KGW crystal,which has 12nm FWHM,is well matched with the emission w avelength of a laser diode ,a solid state laser pump that is very convenient and popular in laser science and technology today.Therefore the KGW crystal has a l aser emission with higher output and higher efficiency.Scientists of laser techn ology circles are very interested in Yb 3+ doped laser crystals because they have four advanta ges when they are compared with Nd 3+ doped laser crystals.Firstly,their flu orescent lifetimes are three or four times as many as that of Nd 3+ doped la ser crystals,which is beneficial to reserving the energy.Secondly,the heat energ y,which is formed when laser is operating,will be decreased because the pumping band is close to the upper energy level.Not only will the use ratio of energy be increased but also the damage effect to laser properties will be decreased.Thir dly,there is no absorption problems of excite state because the energy level of Yb 3+ is simple.Finally,the higher optical quality crystals are easier to gr ow because the radius of Yb 3+ is closer to that of Gd 3+ .Furthermore,Y b 3+ doped laser crystals can substitute the Ti sapphire for the psec laser .
