非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术，通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡，振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为，抽运-探测实验构置中，前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明，非线性光学克尔效应可以作为一种全新的手段，在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.

Ultrafast polarization modulation of laser pulses at terahertz frequencies via optical Kerr effect

Polarized light has already been widely used for photography and display technologies. Magneto-optical Faraday effect, i.e., the light polarization rotates in the magnetic field applied to the material in the direction of light propagation, plays a crucial role in the interaction between light and spin. Faraday effect allow us to understand the nature of magnetization in condensed materials. As an effect opposite to the Faraday effect, the magnetization can be induced in a transparent medium exposed to a circularly polarized electromagnetic wave, which is called inverse Faraday effect. Knowledge of the mechanism provides the opportunities of modulation devices in photonics, ultrafast opto-magnetism and magnonics. In this paper, we experimentally demonstrate a proof-of-concept ultrafast polarization modulation by employing circularly polarized light to demonstrate a strengthened terahertz (THz) frequency Kerr modulation signal, at room temperature. By using the transient pumpprobe spectroscopy with the reflected geometry, we are able to demonstrate the feasibility of such an ultrafast magneto-optical polarization modulation at 0.19 THz in a paramagnetic Li:NaTb (WO₄)₂ crystal with a thickness of 3 mm. The time-resolved modulation signal is explained by the interaction between two counter-propagating laser pulses (central photon energy of 1.55 eV) within the crystal via the optical Kerr effect. We find that the amplitude of the modulation increases with the pump fluence increasing, while the modulation frequency is dependent neither on the pump fluence nor on polarization of pump beam. However, it can further be found that the phase and amplitude of the transient Kerr modulation are strongly dependent on the helicity of the circularly polarized pump pulses. Indeed, these oscillating signals may be mistaken for spin excitation modes. The present findings allow us to get an insight into the transient magneto-optical dynamical process in transparent medium. In addition, the polarization modulation of ultrashort laser pulse on a picosecond time scale will facilitate all-optical data processing, as well as the polarization-dependent ultrafast dynamics in various material systems, which span from condensed matter to molecular spectroscopy. In this regard, our experimental results provide a possibility for designing novel all-optical (magneto-optical) modulators operating at THz clock frequencies. The magneto-optical polarization response modulated at THz frequencies may have new possibilities for designing all-optical devices, such as ultrafast modulators.