Carrier Dynamics and Optical Kerr Effect of Titanium Dioxide Single Crystals by Femtosecond Transient Grating Spectroscopy
Carrier Dynamics and Optical Kerr Effect of Titanium Dioxide Single Crystals by Femtosecond Transient Grating Spectroscopy
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发表时间：
2008
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通讯作者：
X. Yang;Li Wang;Y. Nakato;N. Tamai
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作者：
X. Yang;Li Wang;Y. Nakato;N. Tamai
用飞秒瞬时光栅法研究了(10 0)、(110)和(0 0 1)晶面的金红石型二氧化钛单晶的光生载流子动力学和光克尔效应。裸露的TiO2单晶在670 nm处的强度栅衍射信号被认为是由块体中Ti4+位附近的自由电子或弱俘获电子引起的。瞬时光栅的衰减曲线强烈依赖于激发强度，而与晶面类型无关。随后的强度相关驰豫用二级电子-空穴复合动力学解释，(110)和(100)晶面的速率常数均为∼2×10-11cm3 S-1。用共振偏振光栅法和非共振光强光栅法测量了裸露的二氧化钛的光克尔动力学，发现其超快响应与系统响应函数相似，表明克尔信号主要是由电子极化引起的。在超额能量为1.7 eV的二氧化钛导带电子的激发下，测得导带内超热电子的驰豫时间常数为310fs。在长时间区，观察到了1.28 GHz的声波传播，这是由于超热电子的过剩能量通过电子-声子散射和热弛豫转化为热能所致。
The photogenerated carrier dynamics and optical Kerr effect in rutile titanium dioxide (TiO 2 ) single crystals with (100), (110), and (001) faces were examined by femtosecond transient grating spectroscopy. The diffraction signal of the intensity grating in bare TiO 2 single crystal at 670 nm was considered to be due to free electrons or weakly trapped electrons nearby Ti 4+ sites within the bulk. The decay curves of transient grating were strongly dependent on the excitation intensity but not on the types of crystal faces. The subsequent intensity-dependent relaxation was interpreted in terms of second-order electron-hole recombination kinetics with the rate constant of ∼ 2x 10 -11 cm 3 s -1 for both (110) and (100) faces. The optical Kerr dynamics of bare TiO 2 examined by resonant polarization grating and non-resonant intensity grating shows an ultrafast response similar to the system response function, indicating that the Kerr signal is mainly due to electronic polarization. By the excitation of conduction band electrons of reduced TiO 2 with the excess energy of 1.7 eV, the relaxation time constant of hot electrons within the conduction band was estimated to be 310 fs. In the long time region, the propagation of acoustic wave of 1.28 GHz was observed, which was due to the excess energy of hot electron converted into the thermal energy by electron-phonon scattering followed by the thermal relaxation.