强太赫兹脉冲产生及硒化镓晶体非线性效应研究 下载: 570次【增强内容出版】
The study of high-order nonlinear behavior,prompted by single-cycle or subcycle strong terahertz pulses,has been scarce,primarily due to the limitations of highly stable and high-intensity terahertz radiation sources. The strong terahertz field–matter interaction leads to new avenues for exploring and understanding various novel nonlinear phenomena. The employment of a femtosecond laser to pump a lithium niobate (LiNbO3) crystal has been considered as a major strategy for generating strong terahertz radiation. The strong terahertz pulse acquired by this technique can coherently detect and manipulate the optical properties of materials on an ultrafast timescale and can play a crucial role in discovering new phenomena and revealing new mechanisms. In this study,to explore the nonlinear effects of materials under strong terahertz pulse irradiation,a strong terahertz pump-optical probe spectroscopy system is developed by utilizing the tilted wavefront technique based on a lithium niobate crystal. Specifically,the highest terahertz output energy of 2.6 μJ and a focused terahertz peak electric field strength of 632 kV/cm are realized. Meanwhile,the nonlinear crystal gallium selenide (GaSe) is examined with this system,and we demonstrate that the strong change in the terahertz pulse-induced refractive index of GaSe is due to Pockels and Kerr effects.
A strong terahertz pump-optical probe spectroscopy system is established (Fig.1). This system operates on a Ti∶sapphire laser amplifier that can deliver laser pulses with a single-pulse energy of 2 mJ,a pulse duration of 120 fs,and a repetition rate of 1 kHz. A small portion of the femtosecond laser is divided by a beam splitter to sample the birefringence signal of the material induced by the terahertz pulse,whereas the residual 90% of the laser energy is used to pump the lithium niobate crystal to generate a strong terahertz pulse. A diffraction grating of 1800 line/mm is used to tilt the wavefront of the pump laser. A half-wave plate behind the grating changes the polarization of the pump laser from horizontal to vertical,making it parallel to the optical axis of the lithium niobate crystal. A 4f-lens geometry is used to image the pump spot with a tilted pulse front onto lithium niobate. The polarization distribution of terahertz pulses delivered from the lithium niobate is vertical. Subsequently,strong terahertz pulses are focused on the surface of the sample using a series of off-axis parabolic mirrors. A weak 780-nm probe beam overlaps with the terahertz pumping beam on the sample,and it is transmitted through the sample. An assembly of a quarter-wave plate,a Wollaston prism,and a balanced detector is used to sample changes in the refractive index of the material. A pair of wire-grid polarizers is used to tune the electric field intensity of terahertz pulses.
The transient optical responses of the GaSe crystal are measured under different terahertz peak electric fields (Fig.3). The transient signal intensity increases as the terahertz peak electric field strength increases. In essence,when strong terahertz pulses are incident on the GaSe crystals,the refractive index is modulated. Subsequently,the delayed probe light experiences birefringence induced by intense terahertz pulses as it passes through the GaSe crystal,and the two polarization components parallel and perpendicular to the terahertz polarization direction produce a phase retardation in the propagation direction. The maximum transient signals under different terahertz electric fields are extracted to analyze the potential nonlinear behavior of the experimental phenomena. A function that considers the coexistence of χ(2) and χ(3) nonlinear processes is used to fit the extracted maximum. The fitted curve can reproduce the experimental data in a better manner,indicating that the strong terahertz pulses simultaneously cause Pockels and Kerr effects in the GaSe crystal. Finally,we present the relationship between the change in the refractive index,induced by strong terahertz pulses,and electro-optic coefficient,along with the nonlinear refractive index.
A strong terahertz pulse with a single pulse energy of 2.6 μJ and peak electric field strength of 632 kV/cm is generated by employing the tilted wavefront technique based on lithium niobate crystals. The nonlinear optical response of a gallium selenide crystal is examined using a terahertz pump-optical probe system. The optical birefringence induced by sub-cycle terahertz pulses is observed,and the analysis suggests that the changes in polarization are related to Pockels and Kerr effects. This study provides a pathway for the analysis of the nonlinear effect in a medium under strong terahertz pumping and offers a reference for the determination of the electro-optic coefficient and nonlinear refractive index of materials.
1 引言
得益于飞秒激光技术的快速发展,可见-近红外波段的超快非线性过程已经得到了广泛的研究,相应的研究手段相对成熟。然而,受限于高稳定性、高强度的太赫兹脉冲辐射源,关于单周期或亚周期的强太赫兹脉冲诱导的高阶非线性行为(如太赫兹高次谐波[1-2]、克尔效应[3-4]、饱和吸收[5]、自相位调制[6]及交叉相位调制[7]等)的研究鲜有报道。亚周期内的瞬态介电常数调制对应于材料的光学性质在很宽的频率范围内的变化。研究亚周期强太赫兹场与物质的相互作用对于探索、理解各种新奇的非线性现象具有重要意义,时间分辨的强太赫兹光谱已成为研究不同物质系统中超快动力学及非线性光与物质相互作用的强有力的光谱学工具。同时,利用亚周期高强度脉冲可以实现多种元激发的相干调控。光学介质的折射率随光强变化是非线性光学中的一种基本现象,这种现象导致了光的自作用效应[8],包括光的自相位调制、自聚焦或自散焦。透明介质中引起这些自作用效应的非线性响应的强度依赖于介质的线性和非线性折射率系数,因此,这些参数的测定对于非线性晶体的应用具有重要的意义[9]。
目前,产生强太赫兹脉冲辐射的方法有多种。其中,LiNbO3晶体有着非常大的有效非线性系数(d33=168 pm/V)[10]、非常高的损伤阈值及较大的能隙,避免了双光子吸收的发生,因此基于LiNbO3晶体的光学整流效应成为利用光子学方法获取强太赫兹脉冲源的主要的手段之一[11-12]。为了实现晶体中的速度匹配,需要利用超快脉冲的倾斜波前技术[13]。该技术利用衍射光栅使飞秒激光脉冲的强度前沿倾斜,使得抽运光的群速度等于太赫兹脉冲的相速度,进而获得高效率的太赫兹脉冲输出[14]。利用该技术产生的亚周期强太赫兹脉冲源,可以在超快时间尺度上相干探测、操控材料的光学特性,进而研究各类基础物理,这种方法有助于发现新现象、揭示新机理[15]。
GaSe是一种六方层状结构的负单轴晶体,室温下的带隙为2.0 eV[16],具有较大的二阶非线性系数(d22=54 pm/V)[17]和较宽的光学透明窗口,特别是太赫兹波段的吸收系数很小(αTHz=0.07 cm-1)[18]。作为一种综合性能优异的非线性材料,GaSe在超快光电子器件和非线性光学晶体等领域中有着广泛的应用。在太赫兹技术领域中,GaSe可作为太赫兹发射器[19-21]和太赫兹探测晶体[22-23],相关研究已经有了大量报道,而强太赫兹电磁脉冲诱导的非线性响应还未得到揭示。当GaSe作为太赫兹探测晶体时,基于与太赫兹电场成正比的普克尔斯效应(二阶非线性效应),可对太赫兹电场进行重构。三阶的克尔效应则与太赫兹电场的平方成正比。当太赫兹电场足够强时,可以同时诱导普克尔斯效应与克尔效应。
为了探索强太赫兹脉冲抽运下材料的非线性效应,本文基于LiNbO3晶体,采用倾斜波前技术,实现了2.6 μJ的单太赫兹脉冲能量输出,聚焦后的场强可达632 kV/cm。在强太赫兹辐射源的基础上搭建了强太赫兹抽运-光探测光谱系统。利用该系统对非线性晶体GaSe进行了测试研究,观测到亚周期太赫兹脉冲诱导的GaSe在光学频段上的折射率系数扰动,其是二阶的普克尔斯效应和三阶的克尔效应引起的。
2 实验装置
用于太赫兹非线性效应研究的实验构置如
图 1. 强太赫兹抽运-光探测系统的构置图
Fig. 1. Configuration diagram for strong terahertz pump-optical detection system
采用热释电太赫兹功率计对样品位置处的太赫兹功率进行了测量,测量到最大的功率为2.6 mW。使用刀片法对太赫兹光斑大小进行测量,测量到的水平和竖直方向的结果如
图 2. 强太赫兹脉冲的表征。(a)刀片法在水平方向上的测量结果;(b)刀片法在竖直方向上的测量结果;(c)太赫兹脉冲的时域波形;(d)太赫兹脉冲的频域谱
Fig. 2. Characterization of strong terahertz pulse. (a) Measurement results of knife-edge method in horizontal direction; (b) measurement results of knife-edge method in vertical direction; (c) time-domain waveform of terahertz pulses ; (d) frequency domain spectrum of terahertz pulses
为了获取样品位置处太赫兹脉冲的时域波形和场强,用厚度为0.5 mm、取向为<110>的ZnTe晶体进行了电光取样测量。在该过程时,强太赫兹电场会导致采集到的波形失真,为了避免这种现象,通过旋转第一个线栅偏振片大幅减小太赫兹脉冲的强度。探测到的太赫兹脉冲的时域信号以及傅里叶变换后的频域信号分别如
式中:WTHz为太赫兹脉冲的能量;τ为太赫兹脉冲的脉宽;A为太赫兹脉冲的光斑面积;
用于实验的GaSe晶体尺寸为15.0 mm×15.0 mm×0.5 mm,取向为c切。实验时利用线栅偏振片对太赫兹功率进行调节以进行太赫兹场强依赖的实验。
3 分析与讨论
式中:ω为探测光的角频率;c为光速。当探测光从厚度为d的GaSe晶体中射出时,经历的相位延迟为
平衡探测器输出的差分电压信号与探测光的两个正交分量的相位差关系[3]为
式中:ΔV为平衡探测器输出的电压之差;V0为电压之和;
图 3. 归一化强太赫兹脉冲诱导的非线性光学响应。(a)抽运太赫兹脉冲在不同峰值场强下诱导的相位延迟随时间的演化,插图为632 kV/cm场强下测得的信号与太赫兹时域脉冲的对比;(b)诱导的相位延迟的峰值与太赫兹峰值场强的依赖关系及拟合曲线
Fig. 3. Nonlinear optical response induced by normalized strong terahertz pulse. (a) Evolution of phase retardation induced by pumped terahertz pulse over time under different peak field strengths with comparison between measured signal at field strength of 632 kV/cm and terahertz time-domain pulse shown in inset; (b) dependence of peak value of induced phase retardation on terahertz peak field strength and fitted curve
我们通过提取强场太赫兹诱导的信号的峰值[
式中:
当综合考虑二阶和三阶非线性效应时,GaSe晶体的折射率与太赫兹电场(E)的关系为
式中:
折射率变化由普克尔斯效应(太赫兹脉冲诱导的折射率变化Δn1=αE)和克尔效应(太赫兹脉冲诱导的折射率变化Δn2=βE2)引起。接下来我们从折射率椭球方程出发,推导强太赫兹脉冲诱导的GaSe晶体折射率变化与电光系数和克尔系数之间的定量关系。
对于普克尔斯效应,GaSe的一次电光系数张量为
式中:非零张量元γ12=-γ22=γ61。
GaSe为单轴晶体,当未施加太赫兹电场时,no=nx=ny,ne=nz,其中no为o光的折射率;ne为e光的折射率;nx、ny、nz分别为GaSe晶体中x、y和z轴方向的折射率。折射率椭球方程为
假定沿晶体的x轴施加太赫兹电场E,有Ex=E,Ey=Ez=0,其中Ex、Ey、Ez分别表示施加在晶体x、y和z轴方向上的太赫兹电场。折射率椭球方程变为
通过坐标变换可得
再利用泰勒公式,得到新坐标系下的折射率为
式中:
对于克尔效应,GaSe的二次电光系数张量为
当未施加太赫兹电场时,折射率椭球方程为
沿晶体的x轴施加太赫兹电场E,折射率椭球方程变为
可求得新坐标系下的折射率为
探测光沿z轴传播,其折射率变化为
式中:K为克尔电光系数;λ为探测光的波长。式(
另一方面,从非线性光学理论易知,在普克尔斯效应和克尔效应中,外加的太赫兹电场诱导的折射率变化分别为
式中:n2为非线性折射率系数。
当实验测得ΔV和V0后,便可以得到强太赫兹脉冲诱导的相位延迟Δϕ,从而进一步得到折射率的变化Δn。通过
4 结论
利用波前倾斜的飞秒激光抽运LiNbO3晶体,得到了单脉冲能量为2.6 μJ、峰值强度为632 kV/cm的亚周期强太赫兹脉冲辐射源。为了研究各种材料在强场太赫兹脉冲抽运下的非线性响应,在此基础上搭建了强太赫兹抽运-光探测光谱系统,并利用该系统对非线性晶体GaSe进行了测试研究。实验发现,强场太赫兹抽运同时导致了普克尔斯效应和克尔效应。最后给出了折射率变化与太赫兹电场之间的关系式,其为强太赫兹抽运下非线性系数的测定提供了参考。
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