铌酸锂强场太赫兹非线性时域光谱系统 下载: 875次封面文章
The strong-field terahertz (THz) time-domain spectroscopy is fundamental in strong-field THz science, technology, and applications. Strong-field THz time-domain spectroscopy is also indispensable in many fields including materials, physics, chemistry, and biology, which involve strong nonlinear interactions between strong-field THz and matter. However, the unavailability of a high-field free-space THz source with high repetition rate, excellent beam quality, and high stability hinders its development. In this study, we designed and independently developed a highly integrated strong-field THz nonlinear time-domain spectroscopy system based on a lithium niobate (LN) strong-field THz source. The proposed system is driven by a kilohertz femtosecond laser amplifier and exhibits the functions of a strong-field THz nonlinear spectrum, THz-pump THz probe (TPTP), strong-field THz-pump optical probe, optical-pump THz probe (OPTP), and THz emission spectrum. This highly integrated strong-field THz nonlinear time-domain spectroscopy system is a powerful tool for analyzing the nonlinear effects of strong-field THz waves.
We developed a strong-field THz nonlinear time-domain spectroscopy system. We employed a Ti∶sapphire femtosecond laser amplifier that provided laser pulses with a center frequency of 800 nm, pulse duration of 35 fs, repetition rate of 1 kHz, and maximum pump power of 5 mJ. The laser input to the system was split using an 80∶20 beam splitter. The transmitted beam (80%) was employed as a pump beam to generate strong-field THz waves from the LN crystal through optical rectification based on the tilted pulse front technique. The strong-field THz waves generated by the LN crystal was used to induce and probe the nonlinear effects. The residual 20% femtosecond laser energy was divided into three beams: the first for the optical pump, the second for generating a weak-field THz probing beam in a ZnTe emission crystal (ZnTe 1), and the third for electro-optic sampling of both the pump and probing THz temporal waveforms. Three delay lines were employed to synchronize the strong-field THz, optical pump, and electro-optic sampling. THz temporal waveforms were detected by an electro-optic sampling system consisting of a ZnTe crystal, quarter-wave plate, Wollaston prism, and two photodiodes for the coherent detection of THz pulses based on the principle of the electro-optic effect.
In this work, the study on LN strong-field THz nonlinear spectroscopy is summarized as follows. First, for the strong-field THz generation and detection system (Fig. 2), the energy conversion efficiency of near-infrared light to THz waves was approximately 0.22%. At the focus of off-axis parabolic mirror 2 (OAP2), the calculated peak electric field can reach 350 kV/cm. The focused THz beam profile can be detected using a THz camera and temperature-sensitive paper. Based on this strong-field THz static nonlinear spectroscopy system, we observed the nonlinear absorption caused by the intervalley scattering of doped silicon induced by strong-field THz using the Z-scan technique (Fig. 3). In addition, the strong-field THz-induced nonlinear transmission self-frequency modulation of the THz nonlinear metasurface further demonstrates the excellent ability of strong-field THz nonlinear spectroscopy in the frequency domain (Fig. 4).
Second, the pump-probe technique is an essential research method for performing strong-field THz nonlinear spectroscopy. The TPTP technique can be achieved by introducing a THz probing beam generated by a ZnTe emission crystal, a coaxially aligned THz pump (generated by LN), and a THz probe, followed by focusing on the sample with an off-axis parabolic mirror (Fig. 5). The THz probing beam was modulated by a chopper with a 500 Hz rotor frequency to obtain a pure probing signal. A THz polarizer was positioned behind the sample with its polarizing orientation perpendicular to the THz pump to further restrict the transmitted THz pump. Using the TPTP technology with a spectral resolution, we observed dynamic changes in the resonant frequency of the THz probe transmission spectra induced by a strong THz field on nonlinear THz metasurface samples. This phenomenon demonstrates that carrier production in this SRRs sample is caused by the impact ionization of high-resistance silicon.
Third, we realized the OPTP technique by introducing an 800 nm pumping beam. The strong-field THz waves generated by the LN crystal can be used as the probing beam. By adjusting the incident THz field strength, the transmission signal self-modulation induced by the strong-field THz under an 800 nm pump is measured (Fig. 6). For more common OPTP applications, the weak-field THz generated by the ZnTe emission crystal is used as the probing beam, which is primarily used to investigate the ultrafast dynamics of carriers in semiconductors (Fig. 7). By adding a spectral resolution, the photoexcitation dynamics with different frequencies can be analyzed more comprehensively.
Finally, the proposed strong-field THz nonlinear time-domain spectroscopy system exhibits THz emission spectral capability. When stimulated by a femtosecond laser, the induced THz pulse carries a significant amount of physical information in its waveform. Taking the spin THz emission as an example, we demonstrate the flexibility of the system in examining the emission characteristics of the W/CoFeB/Pt thin-film structure (Fig. 8).
Strong-field THz nonlinear spectroscopy has become a critical method for studying the nonequilibrium behaviors resulting from strong THz-matter interactions. Based on the self-developed LN strong-field THz nonlinear-time-domain spectroscopy system, various experimental methods of strong-field THz nonlinear spectroscopy were studied and explained, demonstrating the unique ability and essential role of strong-field THz nonlinear spectroscopy in basic research. In addition, with a slight adjustment, the system can also be used for two-dimensional THz spectroscopy, THz electron acceleration, and the THz Kerr effect. This highly integrated and miniaturized THz time-domain spectrometer provides comprehensive research capabilities and potential for nonlinear THz spectroscopy in physics, materials, biology, and engineering applications.
1 引言
太赫兹(THz)波是指频率为0.1~10 THz的电磁波。THz波的光子能量与物质振动、旋转能级的能量范围相吻合,这使得其与物质相互作用的光谱包含着丰富的信息,从而在材料、物理、化学、生物、信息以及工程应用[1-12]等领域具有广阔的发展前景。THz时域光谱技术使用的是相干探测方法,可直接测量物质介电常数的实部和虚部信息,并反映材料在THz频段的响应。THz时域光谱技术可应用于工业环境、药物和生物医学、农业和食品、艺术品保护[13-16]等领域,这些应用主要通过测量弱场情况下材料的线性响应来获得THz频段的介电性质。
自2002年倾斜脉冲波前技术[17]提出以来,铌酸锂晶体便成为稳定、可靠、高效的强THz辐射源。其他产生强场THz辐射的方法,如基于其他非线性晶体(如ZnTe)[18]或有机晶体的光学整流[19]、自旋THz发射[20]以及基于大口径光电导天线[21]、激光等离子体THz源[22]等的方案都取得了显著进步。同时,随着高功率飞秒激光技术的发展,飞秒激光泵浦的THz脉冲的输出能量已发展到mJ量级,峰值电场强度已从kV/cm增大到MV/cm[23],对物质的调控也从弱场过渡到强场,人们对THz脉冲与物质相互作用的研究也从线性调控转变为非线性调控。通过调控强场THz辐射与物质的相互作用不仅可以实现对材料非平衡物态的调控[2]、电子自旋的相干调控[24],还可以诱导材料相变[1]以及实现电子加速与操控[8]等。
作为强场THz非线性光谱技术的重要组成部分,泵浦-探测技术正逐渐成为观测强场THz诱导非线性效应的重要手段。在光学频段,对于两束具有一定时间延迟的激光来说,其中的一束能量较高且更早到达样品,用于激发样品中的电子,将其称为泵浦光;另一束能量较弱,用于探测样品被泵浦光激发后载流子的动力学过程,称为探测光。在THz频段,应用最广泛的是光泵浦-THz探测(OPTP)技术,主要用于研究半导体内载流子的超快动力学[25-26]。OPTP用弱场THz脉冲代替泵浦-探测技术中的探测光,THz光子能量在半导体载流子和声子的共振能量范围内,是研究半导体载流子和声子动力学的重要手段。类似地,将OPTP技术中的泵浦光替换为强THz脉冲,可以实现THz泵浦-THz探测(TPTP)技术,该技术可用于研究强场THz激发的载流子碰撞电离动力学[27]、等离子体非线性调制[28]、材料相变[1],以及许多新材料中远离平衡态的非平衡量子物相等。另外,随着强场THz和二维光谱技术的发展,二维光谱技术得以拓展到THz频段。二维光谱可用于揭示隐藏在传统一维线性光谱中的特征[29]。二维THz光谱的发展相较于可见光和红外频段,尚处于早期阶段。该技术目前已被用于研究凝聚态系统中大多数非共振电子和一些具有强非线性的晶格振动响应[30-31]、气相分子的THz二维旋转光谱[32]、半导体中声子的旋转光谱等[33]。此外,二维THz光谱与非共振拉曼(Raman)过程的光学激发、检测相结合,使得混合二维THz-拉曼光谱技术成为可能[34]。
除了测量材料在弱场情况下的线性响应和强场情况下的非线性响应外,THz光谱技术的另一项重要功能是用于THz发射光谱的测试。THz发射光谱技术是一种无损伤、非接触式的光谱测量手段[35],是一种分析、测量非线性光学过程的有力工具[36]。在飞秒激光激发下,瞬态载流子、偶极子、声子、激子和其他准粒子会发生变化,进一步诱导皮秒级THz脉冲辐射。辐射的THz脉冲的波形中携带着丰富的物理信息,如对掺杂类型、载流子迁移率等敏感的幅度、相位、极性和偏振等信息[37],这些信息可用于对物质的特性进行分析。在THz发射光谱技术的基础上,研究人员发展了一种激光扫描THz成像系统——激光THz发射显微镜[38],其工作原理为:将近红外飞秒激光脉冲聚焦到样品上的待探测位置并检测其THz发射光谱,移动脉冲在样品上的聚焦位置即可实现扫描成像。其成像的空间分辨率依赖于泵浦激光而非THz波的衍射极限,因此可以实现微米量级的空间分辨率。该技术可被应用于集成电路的非接触式诊断[39]、半导体材料THz发射光谱的测量[40]以及自旋THz发射[41]等。
尽管THz光谱技术的应用十分广泛,但系统往往需要根据不同的应用需求重新设计,所能实现的功能比较单一。特别地,下一代通信技术对通信频段频率和通信速率提出了更高要求,当前基于硅基材料体系的二极管、场效应晶体管体系由于损耗和迁移率等问题,很难拓展到高频微波特别是THz波段,需要在新材料、新机理和新方法等方面实现突破。
针对新材料开展THz频段非平衡物态调控研究的需求,笔者设计并搭建了一套高度集成的强场THz非线性时域光谱系统。该系统兼具非线性THz光谱测试、光泵浦-THz探测、THz泵浦-THz探测、THz发射光谱测量等能力。其中强场THz脉冲由超短飞秒激光泵浦铌酸锂晶体通过倾斜波前技术产生,在室温情况下,单脉冲能量可达6.5 μJ,转换效率约为0.22%,峰值频率位于0.4 THz处,到达样品上的最大峰值场强约为350 kV/cm。笔者设计的强场THz时域光谱系统仅使用一台飞秒激光放大器,整体光路设计紧凑,搭建在两块60 cm×90 cm的面包板上,方便移动。该系统可为THz强场非线性、泵浦-探测和THz发射光谱等研究提供全面的解决方案,并且经简单改装后可实现更多功能,具有较高的应用价值。
2 铌酸锂强场THz非线性时域光谱系统设计
笔者搭建的强场THz非线性时域光谱系统如
图 1. 基于飞秒激光放大器泵浦的铌酸锂强场THz非线性时域光谱系统
Fig. 1. Strong field THz nonlinear time domain spectrometer based on lithium niobate pumped by a femtosecond laser amplifier
上述所述剩余的20%能量由分束镜2(BS2)进一步分束。第一分束经延迟线2(DL2)后作为THz发射光谱和OPTP的泵浦光,之后经透镜聚焦后穿过离轴抛物面镜2(OAP2)的小孔入射到样品表面。延迟线2用于控制800 nm泵浦光和THz探测光之间的时间延迟。第二分束经分束镜3(BS3)后,一部分被引导入射到ZnTe发射晶体(ZnTe 1),产生弱场THz波(
剩下的800 nm飞秒光经延迟线1(DL1)被引导透过OAP4的小孔,与强场THz波、弱场THz波共同聚焦在ZnTe探测晶体(ZnTe 2)上,由ZnTe探测晶体、四分之一波片、沃拉斯顿棱镜和一对平衡探测器组成的电光采样系统通过线性电光效应进行THz脉冲的相干探测[42]。
3 强场THz静态非线性光谱
强场THz静态光谱系统如
图 2. 强场THz非线性光谱光路和强THz脉冲特性。(a)光路图,需要用反射镜替换图1中50∶50的分束片,以集中铌酸锂泵浦光的能量;(b)测量的THz脉冲能量和对应的转换效率与泵浦光功率的依赖关系;(c)在OAP2焦点处测量的直径约为1.6 mm(1/e)的聚焦THz光斑的轮廓;(d)(e)当泵浦功率为1.5 W时产生的强THz脉冲时域波形及其对应的峰值频率为0.4 THz的频谱;(f)使用液晶感温显色纸探测聚焦THz光斑
Fig. 2. Nonlinear strong field THz path and characterization of the realized strong THz pulse. (a) Optical path setup for the strong THz spectroscopy (it is necessary to replace the 50∶50 beam splitter in Fig. 1 with a mirror to concentrate the power of the pump light input to lithium niobate); (b) measured THz energy and corresponding conversion efficiency as a function of the pump power; (c) focused THz beam profile with a diameter of about 1.6 mm (1/e) measured at the focus of OAP2; (d)(e) a typical strong THz temporal waveform when the pump power is 1.5 W and its corresponding frequency spectrum with a peak frequency of 0.4 THz; (f) detection of focused THz beam profile using liquid crystal thermochromic paper
3.1 Z‑扫描强场THz非线性光谱
Z-扫描是研究THz场致非线性效应的重要方法之一,笔者使用两种硅片依赖于电场强度的非线性吸收来显示强场THz的非线性物态调控能力。一种硅片是厚度为0.3 mm的N型掺杂硅片,掺杂密度约为1015 cm-3;另一种是厚度为0.5 mm的高阻硅片(电阻率大于10 kΩ·cm)。采用Z-扫描方法改变THz电场强度,强THz脉冲正入射至样品表面,当样品放置在OAP2的焦点处时,记Z=0 mm,对应于聚焦的峰值THz强场。
图 3. 掺杂硅片和高阻硅片的相对THz透射率随Z位置的变化
Fig. 3. Relative THz transmittance of the doped and high-resistivity Si as a function of Z-position
3.2 强场THz自探测光谱
通过Z-扫描观测非线性效应不需要测量完整的THz光谱,只需要测量THz透射信号的峰值即可。利用延迟线1进行时域光谱扫描,测量THz透射波形,对其进行傅里叶变换后可以得到频域信息,从而可以观测强THz诱导的透射率谱自调制。先前的研究表明[46],在不同场强的THz入射场下,THz非线性超表面的透射谱上会出现非线性共振频率移动。该超表面由金劈裂谐振器(SRRs)矩形阵列组成,其单个单元如
图 4. 强场THz诱导的非线性频率调制。(a)TM偏振的THz入射脉冲示意图;(b)(c)在入射场强为2.5、180 kV/cm下测量的TM偏振脉冲的透射率谱及其对应的仿真结果
Fig. 4. Strong field THz induced nonlinear frequency modulation. (a) Schematic of TM polarization of the incident THz pulse; (b)(c) measured transmission spectra under TM polarization at the incident field intensities of 2.5 and 180 kV/cm and its corresponding simulation results
4 强场THz泵浦‑探测光谱
4.1 THz泵浦‑THz探测
THz泵浦-THz探测(TPTP)系统如
图 5. TPTP技术。(a)TPTP系统光路图;(b)THz泵浦和THz探测的偏振方向与SRRs取向的关系;(c)THz泵浦前后的典型THz探测透射率谱,表现出45 GHz的频率移动;(d)高阻硅衬底上THz纳米超表面的共振频率-时间延迟TPTP动力学曲线
Fig. 5. THz pump-THz probe (TPTP) technology. (a) Optical path diagram of TPTP system; (b) relation between the polarization direction of THz pump and THz probe and the orientation of SRRs; (c) typical THz probe transmission spectra before and after THz pump, showing a frequency shift of 45 GHz; (d) TPTP dynamic curve of resonant frequency-time delay for THz-nano metasurface on highly resistive silicon substrate
尽管TPTP中可用的THz泵浦场强达到了180 kV/cm,但对于一些材料而言尚不足以激发出明显的非线性效应。这里仍然使用THz纳米非线性超表面来进一步增强THz局部场强。研究表明[46],当入射的TM偏振的THz脉冲场强达到60 kV/cm时,纳米缝处增强的电场足以诱导衬底的碰撞电离,导致局部电导率提高,SRRs表现为“闭合”状态,如3.2节所述,THz透射谱中的共振频率向低频移动(是一种自共振频率调制现象)。
利用TPTP技术,观测到了THz泵浦脉冲对THz探测脉冲的共振频率调制现象。由于该THz非线性超表面对入射场的偏振敏感,要求THz泵浦和THz探测均以TM偏振作用于超表面,再考虑到THz探测光的信噪比,将SRRs含有纳米缝的臂的方向设置为与THz泵浦偏振方向成45°夹角,并通过HWP2将THz探测光的偏振方向调整成沿含有纳米缝的臂的方向,如
4.2 光泵浦‑强弱THz交替探测
光泵浦-强弱THz交替探测如
图 6. 光泵浦-强弱THz交替探测技术。(a)光泵浦-强弱THz交替探测系统光路图;(b)(c)不同入射场强下的THz透射率谱;(d)对应于(b)、(c)的仿真结果
Fig. 6. Optical pump-strong and weak THz alternate probe technology. (a) Optical path diagram of optical pump- strong and weak THz alternate probe system; (b)(c) THz transmission spectra with different incident field strengths; (d) numerical simulation results corresponding to (b) and (c)
利用该技术可以对THz非线性超表面产生的不同场强THz场诱导的高阻硅衬底的谷间散射和碰撞电离进行观测。高阻硅衬底由83 μJ/cm2泵浦通量的800 nm飞秒激光进行光掺杂,激发光生载流子。光泵浦和THz探测之间的时间延迟为46 ps,远小于高阻硅的载流子寿命。如
4.3 光泵浦‑弱场THz探测
光泵浦-弱场THz探测(OPTP)系统如
图 7. OPTP技术。(a)OPTP系统光路图;(b)(c)由ZnTe光整流产生的弱场THz探测脉冲波形及其频谱;(d)在泵浦通量为63 μJ/mm2、中心波长为800 nm的泵浦光激发下,N掺杂硅的弱场THz探测的THz时间分辨光谱;(e)在相同的泵浦光条件下,以强场THz作为探测光探测的生长在单抛蓝宝石衬底上的15 nm厚的拓扑绝缘体Bi2Te3薄膜的THz时间分辨光谱
Fig. 7. Optical pump-THz probe (OPTP) technology. (a) Optical path of OPTP system; (b)(c) weak-field THz probe temporal pulse waveform generated by ZnTe crystal optical rectification and its frequency spectrum; (d) THz time-resolved spectrum of N-doped silicon probed by weak-field THz under pump excitation with central wavelength of 800 nm and pumping fluence of 63 μJ/mm2; (e) under the same pump conditions, time-resolved spectrum of 15 nm thick topological insulator Bi2Te3 film grown on a single-sided polished sapphire substrate was detected by the strong-field THz probe
在OPTP中,样品被泵浦光激发后,价带电子被激发,产生电子-空穴对。样品内载流子浓度的增加使得其对THz的吸收增加,表现为透射率迅速降低,之后随着载流子的复合,样品的THz透射率会在数十μs至数十ps的时间尺度之内逐渐弛豫至样品被激发前的状态。半导体中载流子的弛豫过程可以按指数衰减拟合,例如使用单指数衰减模型。
式中:
厚度为300 μm、晶向为〈100〉的N型掺杂硅在泵浦通量为63 μJ/mm2、中心波长为800 nm(光子能量为1.55 eV)的泵浦光激发下,瞬态变化
需要说明的是,利用
5 THz发射光谱
THz发射光谱系统如
图 8. THz发射光谱技术。(a)THz发射光谱系统光路图;(b)飞秒激光脉冲激发W/CoFeB/Pt异质结产生THz辐射;(c)(d)外加反向磁场导致辐射THz变化的原理和波形
Fig. 8. THz emission spectroscopy technology. (a) Optical path of THz emission spectroscopy system; (b) femtosecond laser pulse exciting W/CoFeB/Pt heterostructures to generate THz radiation; (c)(d) principle and waveform of the radiated THz signal variation induced by external opposite magnetic fields
THz发射光谱技术是研究THz源的重要手段之一。在常见的THz源中,光电导天线需要外加电场;非线性晶体由光整流效应产生THz波,具有信号强、频谱宽的优点,但非线性晶体中存在损伤阈值和声子吸收,导致产生的强THz辐射受限。另一种逐渐发展起来的自旋电子THz源利用飞秒泵浦光与铁磁薄膜相互作用的原理产生THz辐射,具有超宽带辐射、可实现强场辐射、廉价且高效等优点[48]。如
如
6 结论
随着对通信频段频率和通信速率要求的提高,开发THz频段功能材料的需求日益增加,而综合研究THz与物质相互作用是其中的关键。针对材料研发在强场THz非线性光谱、THz泵浦-THz探测、强场THz泵浦-光探测、光泵浦-THz探测、THz发射光谱等技术上的测试要求,笔者研发了一套铌酸锂强场THz非线性综合时域光谱系统。本文以常见的半导体材料硅为例,对多种强场THz非线性光谱技术的具体实验方法进行了研究和说明,揭示了该系统在基础研究中的综合能力和重要作用。此外,该系统还具有很强的拓展性,通过进一步改造可以实现2D-THz光谱、THz电子加速、THz克尔效应等研究功能。这种高集成、小型化的THz光谱仪为非线性THz光谱技术在物理、电子、材料、生物和工程等领域提供了应用空间。
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Article Outline
才家华, 张保龙, 耿春艳, 郝思博, 陈赛, 吴晓君. 铌酸锂强场太赫兹非线性时域光谱系统[J]. 中国激光, 2023, 50(17): 1714012. Jiahua Cai, Baolong Zhang, Chunyan Geng, Sibo Hao, Sai Chen, Xiaojun Wu. Lithium Niobate Strong‑Field Terahertz Nonlinear Time‑Domain Spectroscopy System[J]. Chinese Journal of Lasers, 2023, 50(17): 1714012.