采用超快半导体光电导开关的宽带太赫兹(THz)时域频谱(TDS)系统,研究了同族的KTiOPO4(KTP)晶体与RbTiOPO4(RTP)晶体在0.5-2.0 THz波段的光学声子振荡特性。虽然这两种晶体具有相似的晶格结构特征,但它们的THz时域频谱却显示出很大的差别:一是RTP晶体对THz波的吸收比KTP晶体强得多;二是当晶体的z轴平行于THz波电场振动方向时,RTP晶体的光学声子共振吸收峰较KTP晶体的数量更多。利用多洛仑兹振子伪谐振介电模型对两种晶体的复介电常数进行拟合,得到了对应于晶格弱振动的各个光学声子的特征参数。结果表明,这些吸收峰都是晶体中沿z轴排列的K+(或Rb+)相对于PO4六面体和TiO6八面体晶格振动造成的光学外振动模。但是由于Rb的原子量大,对周边原子或基团的键力作用更强更复杂,因此形成的晶格振动模式也更加多样化。

By use of photoconductive switches triggered by the ultrafast femtosecond pulses in the terahertz (THz) time-domain spectroscopy (TDS) system,the optical phonon oscillation characteristics of isomorphs KTiOPO4 (KTP) and its crystallographic isomorphs RbTiOPO4 (RTP) crystals are studied in the 0.5-2.0 THz range. Although the two crystals have quite similar lattice structure,their terahertz spectrums reveal different characteristics:the absorption of the RTP crystal is stronger than that of the KTP crystal,and the peaks of related optical phonon oscillation of the RTP crystal are more than that of the KTP crystal while the polarization of terahertz field parallels to the z-axis of the crystals. With pseudo-harmonic dielectric model,the complex dielectric constant curves of the two crystals are calculated,and the parameters related to the optical phonon resonances are obtained. The results demonstrate that all these phonon oscillation peaks in terahertz range are corresponding to the optical external modes which are caused by the vibrations of the K (or Rb) ions with the screw of PO4 tetrahedral and TiO6 octahedral. But since the Rb ions have more atomic weight,their chemical band strength with surrounding atoms or groups is correspondingly stronger and complex,which leads to the diversity of optical phonon oscillation modes.