高重复频率、高功率高次谐波极紫外光源进展及应用【增强内容出版】
As a desktop-level extreme ultraviolet (EUV) coherent light source, high harmonic generation (HHG) becomes an indispensable tool in fundamental science fields such as atomic and molecular physics, biomedicine, materials chemistry, and precision spectroscopy. The maximum photon energy of high harmonics in gas extends to the soft X-ray spectral range. Based on the appropriate gating technique of high harmonics, it is possible to generate isolated attosecond pulses with tens of attoseconds pulse widths, providing feasibility for the study of electron motion in atomic and molecular systems on the attosecond time scale. In addition to being critical in basic science, HHG also serves as a coherent light source with wide industrial applications, especially in integrated circuit manufacturing and imaging detection in biomedicine. High harmonic extreme ultraviolet light sources for industrial applications require both high photon energy (100?500 eV) and higher average power (above mW). To obtain a shorter wavelength high harmonic, the mid-infrared femtosecond laser, combined with nonlinear pulse compression technology, realizes the output of keV photon energy harmonics. The shorter wavelength aids in improving imaging resolution and covering the absorption edge of high atomic number materials, which can be used for extreme ultraviolet spectrum analysis. To improve the average power of higher harmonics, it is better on one hand to use the higher repetition rate and higher power driving laser. On the other hand, improving the conversion efficiency of high harmonics is necessary, which can be realized by controlling the macroscopic propagation process of high harmonics to achieve phase matching.
In this study, we focus on the process of producing high harmonics directly in a single pass driven by high repetition rate lasers, and introduce the progress in repetition rate, single pulse energy, and average power improvement of HHG extreme ultraviolet light sources. The paper organizes in the following way: after a brief introduction in the first section, which includes the HHG three-step model, the second section reviews the work on HHG sources driven by high repetition rate lasers in recent years, with the femtosecond fiber laser being the main pump source for producing high repetition rate HHG. The main parameters from these experiments are listed in Table 1. With the development of femtosecond fiber laser techniques, such as nonlinear compression, coherent combination, and optical parametric chirped pulse amplification (OPCPA), high harmonic sources are evolving towards higher photon flux, higher cutoff photon energy, and higher repetition rates. Figures 1 and 2 present the experimental device diagrams and spectra of two significant high repetition HHG works. Figure 3 shows the distribution of the main optical parameters of HHG extreme ultraviolet sources driven by the most advanced fiber laser described in this section.
The third section discusses the key to improving high harmonic conversion efficiency, namely, phase matching in the macroscopic propagation process of HHG. By discussing the wave vector mismatch between the fundamental field and the high harmonic field, we determine how the phase-matched HHG photon energy threshold is influenced by different gas medium types, wavelengths, and pulse lengths of the driving laser, as shown in Fig.4. Considering the effect of nonlinear gas medium absorption, the effective phase matching conditions are presented in Fig.5. We introduce the scaling law that keeps HHG conversion efficiency constant by adjusting the global physical quantity under different focusing conditions, which is well utilized in the HHG experimental parameters design under tight focusing conditions for femtosecond fiber or disk lasers with high average power and relatively small pulse energy, as listed in Table 2. Then, combined with effective phase matching conditions and the scaling law, the macroscopic propagation process of two different bands of HHG in high-repetition-rate experiments is briefly discussed, as illustrated in Figs.6 and 7.
In section four, we introduce the main imaging technologies based on the extreme ultraviolet HHG source currently in use. Three different coherent diffraction imaging (CDI) techniques, conventional CDI for isolated samples, Fourier-transform holography (FTH), and ptychography are discussed in this section, as shown in Fig.8. The phase retrieval algorithm in the standard data processing procedure for CDI is also briefly introduced, as shown in Fig.9. Finally, we discuss EUV coherence tomography (ECT) technology used for object depth information detection. Figures 10 and 11 are sample reconstructions of ptychography and ECT, respectively.
With the advancement of high repetition rate and high power femtosecond laser technology, the repetition rate and photon flux of high harmonic sources continuously improve. The limitations of high power femtosecond fiber and solid-state lasers, such as long pulse widths, low single pulse energy, and narrow tuning ranges, are being overcome compared to the traditional Ti∶sapphire solid-state femtosecond laser. Various nonlinear compression techniques enable the compression of femtosecond fiber laser pulse widths to just a few cycles. With coherent combination technology, the pulse energy of high repetition rate femtosecond lasers can reach the tens of mJ level. OPCPA technology allows for tuning the driving laser wavelength over a wide range. By controlling the laser intensity and wavelength at the single-atom response level, and adjusting the self-absorption and phase matching of high harmonics during the macroscopic propagation process, new laser technologies now enable the production of extreme ultraviolet coherent light sources with the highest average power of 10 mW, the maximum photon energy of 100 eV, and the highest repetition rate of tens of MHz. Through high-repetition and high-flux extreme ultraviolet coherent sources, HHG is branching into various application scenarios beyond the scientific research laboratory, especially in the field of imaging detection. Coherent diffraction imaging and coherent tomography can achieve high spatial and material resolution of nanoscale three-dimensional structures, both transversely and longitudinally. Consequently, imaging technology and instruments based on the high photon flux HHG source are anticipated to find applications in the fields of integrated circuit manufacturing, nanomaterials, biomedicine, and more.
1 引言
飞秒强激光与气体介质相互作用可以辐射出高次谐波(HHG),能够提供一种重要的极紫外(EUV)相干光源,是原子分子物理[1-3]、生物医学[4]、材料化学[5-6]、精密光谱计量学[7]等基础科学领域中不可或缺的工具。气体高次谐波的产生可以用半经典的三步模型[8]来解释:气体原子或分子在强激光场的作用下首先发生隧穿电离产生自由电子;然后自由电子在激光场中加速并获得能量;最后自由电子同母离子结合并释放出阿秒量级脉冲宽度的高次谐波极紫外辐射。一方面,高次谐波最大光子能量与驱动激光光强和波长及气体介质电离势呈正相关,在适当实验条件下,其光谱范围可以扩展到软X射线波段[9-10]。另一方面,基于适当的高次谐波门控技术,可以产生脉冲宽度为数十阿秒的孤立阿秒脉冲[11-12],为在阿秒时间尺度上研究原子和分子系统中的电子运动提供了探测手段[13-15]。
高次谐波不仅在基础科学领域中具有重要作用[16-20],还是一种广泛应用在工业领域中的相干光源,尤其是在集成电路制造和生物医药等领域中的成像检测方面具有应用前景[21-22]。面向工业应用的高次谐波极紫外光源既需要提供100~500 eV的高光子能量的极紫外和软X射线辐射,还需要具有更高(mW以上量级)的平均功率。为了获得更短波长的辐射,可以采用中红外飞秒驱动激光来增大高次谐波的截止频率。这种方法已经实现了keV量级光子能量谐波的输出[9],更短波长的相干光有利于提高成像分辨率,且覆盖高原子序数材料的吸收边可用于极紫外光谱分析。为了提升高次谐波的平均功率,一方面可以采用高重复频率、高平均功率的飞秒光纤、固体激光器和光参量啁啾放大器作为高次谐波驱动源[23-25],另一方面可以控制高次谐波宏观传播过程以实现相位匹配,进而提高高次谐波转换效率。目前,光子能量在20 eV以上的单次谐波最高的平均功率接近13 mW,重复频率为1 MHz[26]。
本文重点关注高重复频率激光器直接驱动产生气体高次谐波的过程,介绍了在极紫外波段高次谐波重复频率、单脉冲能量和平均功率提升方面的研究进展。全文结构如下:第二节对近年来高重复频率激光驱动产生高次谐波相干极紫外光源的重要工作进行了回顾。飞秒光纤激光器是产生高重复频率高次谐波的主要泵源,在所有驱动光源系统中,超快飞秒光纤激光器在实验中获得了目前光子通量最高的高次谐波。然而,随着泵源重复频率的增大,激光向高次谐波单脉冲的转换效率受到传统聚焦结构中激光峰值功率下降等因素的限制。因此,第三节讨论了提升高次谐波单脉冲转换效率的关键问题,即高次谐波宏观传播过程中的相位匹配。针对新型高重复频率光纤激光器平均功率高但单脉冲能量低的特点,对基于高效率高次谐波产生实验的全局参数标度律展开了分析。气体高次谐波因其紧凑的桌面型装置结构、高时空相干性和短波长的特点,适合用作相干衍射成像的光源。随着高重复频率激光技术的进步和高次谐波光子通量的增大,高次谐波在成像检测方面的应用潜力将进一步被挖掘。第四节重点介绍了目前高次谐波极紫外光源在生物医学、集成电路成像检测技术等方面的应用,并在最后一节对高重复频率、高功率高次谐波光源在其他领域中的应用进行了展望。
2 高重复频率、高功率高次谐波光源的研究进展
钛蓝宝石固体飞秒激光器一直是产生高次谐波的主要驱动源,但是它在重复频率和平均功率等方面受到了限制。在过去的二十年里,以光纤激光器为代表的新一代飞秒激光器的发展在极大程度上改善了这些状况。实验表明,飞秒光纤激光器驱动产生的高次谐波在光子通量上有显著提高,如在26.5 eV光子能量处的平均功率达到了10 mW以上[26],在70 eV光子能量附近的平均功率达到1
飞秒光纤激光驱动产生高次谐波的实验研究开始于2009年,Boullet等[23]首次将重复频率为100 kHz、中心波长为1030 nm、脉冲宽度为270 fs、单脉冲能量为100
表 1. 光纤激光驱动产生高重复频率HHG实验的主要参数
Table 1. Main parameters of generating high-repetition-rate HHG experiment generated by fiber laser driving
|
在早期的飞秒光纤激光驱动高次谐波实验中,激光脉冲宽度受到激光增益带宽的限制,仅能达到数百飞秒。激光脉冲宽度过长不利于提高激光的峰值光强,并且会导致气体介质的电离率较高,不利于实现高次谐波产生过程中的相位匹配。为了解决这一问题,采用非线性脉冲压缩技术使得飞秒光纤激光的脉冲宽度缩短近一个数量级,从而提升了高次谐波的转换效率。2010年,Hädrich等[30]利用非线性压缩技术将脉宽为800 fs、单脉冲能量为400
除了压缩脉冲宽度外,提高激光脉冲能量也是增大激光峰值强度和提高高次谐波转换效率的有效手段。尽管飞秒光纤激光器难以达到传统钛蓝宝石激光器的单脉冲能量输出水平,但是相干合束技术在不显著改变脉冲宽度的条件下能够有效提升驱动激光系统的输出脉冲能量。2014年,Hädrich等[34]对四通道光纤激光放大器进行相干合束处理并结合非线性压缩,最终在600 kHz的重复频率下获得中心波长为1030 nm、脉冲宽度为29 fs、脉冲能量为130
高次谐波单原子响应效率随泵浦光波长的增加而迅速降低,因此使用更短波长的高重复频率光纤激光器进行驱动可以显著提高高次谐波的产生效率。2016年,Klas等[35]将非线性压缩系统处理后的激光脉冲通过偏硼酸钡(BBO)晶体进行倍频处理,得到中心波长为515 nm、平均功率为11 W、脉冲宽度为85 fs的激光脉冲,然后将515 nm倍频光束聚焦到氩气或氪气喷嘴中产生高次谐波,其单阶次光子通量达到1×1013 photon/s量级以上,在21.7 eV光子能量附近最高平均功率达到(832±204)μW,转换效率达到7.6×10-5。2017年,Zhao等[36]使用中心波长为347 nm的三倍频飞秒激光脉冲驱动氙-氩混合气体高次谐波,在10.7 eV光子能量附近产生了平均功率为1.25 mW的单阶谐波,其转换效率为2.5×10-4。2019年,Comby等[37]对掺镱光纤飞秒激光器的二、三、四次谐波飞秒激光脉冲驱动的高次谐波进行了比较研究,结果表明,在三次谐波驱动条件下,高次谐波转换效率最高,达到了2.6×10-4,在18 eV光子能量附近光子通量达到6.6×1014 photon/s,平均功率达到1.9 mW。2021年,Klas等[26]将激光二倍频与非线性压缩技术相结合,实验装置如
图 1. 使用重复频率为1 MHz的光纤激光器产生的最高平均功率HHG[26]。(a)高功率HHG实验装置;(b)利用氪气喷嘴产生的高次谐波谱以及各阶谐波对应的平均功率
Fig. 1. HHG with highest average power generated by fiber laser with 1 MHz repetition rate[26]. (a) Experimental device of high power HHG; (b) high harmonic spectrum generated using krypton gas nozzle and corresponding average power of each order harmonic
除了对光子通量进行优化外,某些应用需要更高的极紫外光子能量,例如极紫外光刻掩膜版缺陷检测需要波长为13.5 nm(光子能量为91.5 eV)的单波长光源,而生物成像应用需要水窗波段(2.3~4.4 nm,对应的光子能量为284~543 eV)光源。高次谐波截止光子能量Ecutoff=IP+3.17UP,其中
在脉冲能量一定的情况下,对驱动激光进行非线性压缩处理,既可以提高驱动激光的峰值光强,又可以降低其电离率,这个方法是获取高光子能量高次谐波的有效手段。2020年,Klas等[39]使用两级非线性脉冲压缩系统,将重复频率为75 kHz、中心波长为1030 nm、脉宽为30 fs、脉冲能量为1 mJ的激光脉冲宽度压缩至6.7 fs,单脉冲能量减小至400
值得注意的是,光参量啁啾脉冲放大(OPCPA)技术是另一种获取短周期激光脉冲宽度的有效手段,并且可以更有效地调控中心波长。2013年,Demmler等[25]使用重复频率为180 kHz的光纤激光器泵浦的光参量啁啾脉冲放大系统,获得了中心波长为918 nm、单脉冲能量为25
为了将高次谐波光子能量扩展到水窗波段,除了压缩激光脉冲的脉宽外,还需要增大激光波长。基于掺镱飞秒光纤激光脉冲的非线性压缩,2014年,Rothhardt等[44]利用脉宽为7.8 fs、脉冲能量为350
图 2. 高重复频率掺铥光纤激光器产生水窗波段HHG[47]。(a)HHG实验装置;(b)实验采集的HHG谱
Fig. 2. Water window HHG generated by high-repetition-rate thulium-doped fiber laser[47]. (a) HHG experimental setup; (b) experimentally collected HHG spectrum
目前毫焦量级飞秒激光的重复频率大多在数百kHz量级,然而光电子能谱和极紫外频率梳等应用往往需要MHz量级重复频率的高次谐波。尽管基于增强腔的高次谐波可以实现数十甚至上百MHz量级的重复频率[49],但光子晶体光纤非线性压缩的微焦飞秒激光脉冲也可以单通驱动高次谐波,重复频率达到数MHz量级。2011年,Vernaleken等[50]在光子晶体光纤中对高重复频率、单脉冲能量约为2 μJ的激光脉冲进行非线性压缩,得到波长为1030 nm、脉宽为35 fs、平均功率为20 W的驱动激光脉冲,并在氙气喷嘴靶中产生了重复频率为20.8 MHz、光子能量为20.5 eV的高次谐波,这是目前单通高次谐波达到的最高重复频率。为了提高驱动激光的脉冲能量,采用新型Kagome光子晶体光纤(具有更低的损耗、更大的传输带宽),适合对数微焦激光脉冲进行非线性压缩处理[51-53]。2015年,Hädrich等[54]利用充氪Kagome光纤进行非线性压缩,得到了能量为7
综上所述,高重复频率、高功率飞秒激光技术的进步是高光子通量高次谐波极紫外光源技术水平不断提升的原始动力。
图 3. 由最先进的光纤激光驱动产生的HHG的重复频率、单脉冲能量、光子能量和平均功率分布[23-24,26-27,29-30,32-37,39-40,42,44,47,50,54]
Fig. 3. Distribution of repetition rate, monopulse energy, photon energy, and average power of HHG generated by most advanced fiber laser driving [23-24, 26-27,29-30,32-37,39-40,42,44,47,50,54]
3 气体高次谐波的宏观传播效应与相位匹配
制备高重复频率、高平均功率高次谐波的极紫外光源不仅需要新一代飞秒激光器作为泵浦源,还需要提高高次谐波产生过程中激光到极紫外辐射的能量转换效率。提高高次谐波的转换效率可以通过多种方式实现,一方面可以通过使用双色激光场、短波长激光驱动等方式提高单原子响应效率[55-57],另一方面则需要考虑驱动激光场和高次谐波的宏观传播效应,以实现相位匹配[58]。相比于钛蓝宝石激光器在较低重复频率下输出的较大单脉冲能量,高重复频率飞秒光纤和固体激光器的单脉冲能量较低但平均功率很高,在传统松散聚焦气盒靶和毛细管靶中产生的高次谐波面临峰值光强下降和热负载增大等问题[45]。因此,需要针对新型高重复频率、高功率飞秒激光驱动下的高次谐波的宏观传输效应和相位匹配进行讨论分析。
当第q阶高次谐波光场波矢大小(
中性气体介质的材料色散是由给定压强气体中的基频激光和第q阶高次谐波的折射率差异引入的,其波矢失配量可以表示为
表 2. 松散聚焦和紧聚焦结构中的重要物理量标度律[80-82]
Table 2. Scaling laws of important parameters between loose and tight focusing regimes[80-82]
|
式中:
类似地,电离后自由电子引入的等离子体色散来自基频激光和高次谐波光场在等离子体中的折射率差异。对于基频激光,等离子体中的折射率变化量与自由电子密度成正比;而对于极紫外波段的高次谐波,可以近似认为其折射率为1。因此等离子体色散引入的波矢失配量为
式中:
当自由聚焦的激光经过光束焦点时,高斯光束在不同位置(
激光光强沿轴向的分布不均匀,导致高次谐波单原子响应量子相位差异[62],进而引起偶极矩本征相位失配。当激光作用于单个原子时,光电离产生的自由电子在光场中加速并积累一定的相位[63],近似表示为
式中:I是驱动光光强;
故斯光束在自由聚焦条件下的总波矢失配量可表示为
通过控制气体介质的电离程度和使用松散聚焦来增大瑞利长度、减小Gouy相移,是实现高次谐波相位匹配的有效方法。在松散聚焦的情况下,决定相位匹配条件的关键因素是气体介质的电离度,本课题组估算了任意光子能量的高次谐波辐射能够实现相位匹配的气体电离度范围。首先,激光光强的最小值由以该光子能量作为截止能量的有质动力势决定,在给定激光波长和脉宽条件下,根据各种光电离理论模型计算气体介质电离度的下限(
图 4. 相位匹配区间。(a)在不同波长(λ)和脉宽(τ)泵浦光的驱动下,Kr、Ar和Ne气体中满足相位匹配的最小电离度(虚线)和最大电离度(实线)随光子能量的变化;(b)Ar、Ne和He气体中的 随泵浦光波长变化的理论预测结果;(c)在波长为1030 nm的泵浦光的驱动下,Ar、Ne和He气体中的 随泵浦光脉宽的变化
Fig. 4. Phase matching regions. (a) Minimum ionization degree (dash line) and maximum ionization degree (solid line) satisfying phase matching in Kr, Ar, and Ne gases versus photon energy driven by pump light with different wavelengths (λ) and pulse widths (τ); (b) theoretically predicted in Ar, Ne, and He gases versus pump laser wavelength; (c) in Ar, Ne, and He gases versus pump laser pulse width driven by 1030 nm pump light
为了获得更高光子能量的高次谐波,增大基频激光波长是一种有效手段[66-67]。虽然最大气体电离度
减小激光脉冲宽度是获得更高光子能量高次谐波的另一种有效手段。在相同截止光强的情况下,减小激光脉宽可以显著降低最小气体电离度
前文都是基于激光自由聚焦于气体盒或气体喷嘴靶的设计展开的讨论,为了实现长距离传输激光相位匹配,基于充气毛细管波导的相位匹配技术可以实现高光子能量高次谐波的高效率产生[60-61,70]。在毛细管波导中,激光光强近似不变且不存在Gouy相移,
实验时可以通过控制波导内部的气压和电离度来补偿波导色散引起的负失配量,实现长距离相位匹配,提高低重复频率激光驱动的高次谐波转换效率[45]。在气体高电离区间(在该区间调整全局参数无法对介质色散进行有效补偿),前人采用准相位匹配技术在波导中得到了高效率高次谐波。准相位匹配技术的目的并不是实现高次谐波信号与基频光在整个介质中的相位匹配,而是周期性地调节每个相干长度中的波矢失配,防止高次谐波在介质传播距离内发生大幅度的相干相消。采用内径周期性变化的中空波导或者喷嘴阵列改变光束传播方向上的气体密度等方式,可将水窗波段的高次谐波转换效率提高2~5倍[71-73];采用反向传播光束在毛细管波导内形成相位调制光栅,可提高气体高次谐波的最大光子能量,并将同一阶次的谐波强度提升2个数量级[74]。然而随着驱动激光重复频率和平均功率的不断提高,工程上需要解决毛细管波导的散热和损伤抑制问题。
式(
式中:σabs为第q阶高次谐波的光子吸收截面,
图 5. 自吸收效应的影响。(a)HHG吸收长度随气压的变化(泵浦光波长为1030 nm);(b)在不同 / 下, 随 的变化
Fig. 5. Influence of self-absorption. (a) Variation of absorption length of HHG with gas pressure (pump laser wavelength of 1030 nm); (b) versus under different /
研究高次谐波的宏观传播效应需要同时考虑相位匹配和自吸收效应,其中相位匹配由波矢失配量的倒数即相干长度[
在理想情况下
基于波矢失配和自吸收效应的高次谐波宏观传输效应的分析,以钛蓝宝石激光器为基础的高转换效率高次谐波实验取得了成功,并取得一定的经验(对大能量激光脉冲进行松散聚焦处理或者将其耦合进毛细管波导,可以提高转换效率)。然而,由于高平均功率、相对较小脉冲能量的飞秒光纤或碟片激光在高次谐波实验中的广泛应用,研究者们需要适当提高聚焦数值孔径(
Heyl等[80,82]和Rothhardt等[81]总结了在不同聚焦条件下,维持确定高次谐波转换效率的关键参数标度律,如
结合上述标度律和有效相位匹配条件,分析高重复频率、高功率飞秒光纤激光器驱动下的高次谐波的宏观传播效应。Hädrich等[34]使用重复频率为600 kHz、中心波长为1030 nm、脉宽为30 fs、单脉冲能量范围为
当以波长为13.5 nm的极紫外辐射为优化目标时,文献[39]使用重复频率为75 kHz、波长为1030 nm、脉宽为7 fs、单脉冲能量为400 μJ的激光,在氖气介质中产生了高次谐波。实验中将激光聚焦至150 μm长的氖气气体靶中,其焦斑半径为37.5 μm,当相互作用区域的气压在2.6 bar左右时,波长为13.5 nm的高次谐波的强度达到最大值。根据该实验参数计算波长为13.5 nm高次谐波的相干长度和归一化强度
图 7. 按标度律调整关键参数,在Ne介质中产生的波长为13.5 nm HHG的 (虚线)与 (实线)随气压的变化。(a)w0=37.5 μm,Lmed=150 μm;(b)w0=375.0 μm,Lmed=15 μm
Fig. 7. (dash line) and (solid line) of HHG with wavelength of 13.5 nm generated in Ne medium versus gas pressure after key parameters is adjusted according to scaling law. (a) w0=37.5 μm,Lmed=150 μm; (b) w0=375.0 μm,Lmed=15 μm
根据高次谐波转换效率不变的标度律,在保持光强不变的情况下将焦斑尺寸扩大为原来的10倍,输入的激光单脉冲能量为几十毫焦,介质长度相应扩大100倍,则在气压降低到26 mbar时,波长为13.5 nm的谐波信号强度也能达到最大值。如
4 高功率高次谐波极紫外光源的相干成像应用
极紫外高次谐波具有良好的相干性,适合作为相干衍射成像的光源。一方面,极紫外、软X射线波段的高次谐波在对样本进行成像时不会造成损伤,并且其短波长能满足高横向分辨率的需求;另一方面,高次谐波相比X射线同步辐射源有着尺寸和造价上的优势,有望实现相干衍射成像实验装置的小型化和平台化。当前,基于高次谐波极紫外光源的成像技术主要包括:相干衍射成像(CDI)技术、傅里叶变换全息成像(FTH)技术、叠层扫描成像(Ptychography)技术,以及极紫外相干断层扫描(ECT)技术等。
CDI也叫无透镜成像,是一种通过在相干光束照明的物体后方放置探测器记录散射和衍射光强,从而获取待测物体结构信息的技术,如
图 8. 相干衍射成像的不同模式。(a)传统CDI;(b)FTH;(c)叠层扫描CDI
Fig. 8. Different modes of coherent diffraction imaging. (a) Traditional CDI; (b) FTH; (c) ptychographic CDI
相干衍射成像的分辨率(
光源波长越短,数值孔径NA越大,理论上能够达到的分辨率越小,且数值孔径受限于探测器的几何尺寸和有限的光强与噪声的比值。相干衍射成像需要进行过采样以恢复样品图像,过采样的程度采用过采样率表达,将过采样率(Os)定义为衍射图谱中的散斑间隔
式中:ps表示相机单个像素的大小。过采样过程意味着需要对样品周围的无密度区域进行照明,因此相干衍射成像的分辨率不仅受限于阿贝极限,还受限于光源的相干性,入射光束的相干长度应大于样品的尺寸与无密度区域尺寸的总和。光源相干性越好,光谱带宽越小,分辨能力越强。相干衍射成像所能达到的最小空间分辨率(
式中:
2007年,美国科罗拉多大学首次将高次谐波极紫外光源用于相干衍射成像。飞秒激光在充氩空芯波导中产生了宽谱极紫外辐射,采用多层膜带通反射镜选择的29 nm波长激光,照射具有“J”形孔洞的碳膜样品,利用远场大幅面相机采集衍射光并进行相位恢复,重构得出214 nm分辨率的图像[88]。为了进一步提高分辨率,2016年,Tadesse等[89]使用飞秒光纤激光器产生了中心波长为18 nm、相对光谱带宽为1/200的高次谐波光源,结合接近0.7的成像数值孔径,使图像的最高分辨率达到了13.6 nm。
透射式光路的高次谐波相干衍射成像无法应用于厚度大于光源吸收长度的样本以及具有基底的表面结构。2012年,Gardner等[90]提出了反射式光路以进行高次谐波相干衍射成像,并在算法中加入坐标变换,对离轴的反射光路进行修正。该设计思路随后被应用于纳米表面结构的成像和缺陷检测中。2016年,Shanblatt等[91]使用波长为29.1 nm的单阶次高次谐波,实现了镶嵌在二氧化硅中的铜纳米表面结构和100 nm厚铝膜掩盖下的铜纳米线的相干衍射成像测量。高次谐波相干衍射成像也可应用于生物医学领域。2014年,Zürch等[92]使用波长为38 nm的高次谐波光源,对镀金硅胶载玻片上未标记、未染色的乳腺癌细胞进行反射式相干衍射成像,实现了癌细胞的分型。
高次谐波的转换效率和输出平均功率相对较低,这是影响相干衍射成像效率的重要因素。提高高次谐波转换效率可以实现不可重复物理过程的单发成像。2009年,Ravasio等[93]使用单脉冲能量为35 mJ的飞秒激光脉冲,利用焦距为5.5 m的长焦透镜将其松散聚焦于尺寸为10 cm、气压为2 mbar的充氩气盒中,产生了单脉冲能量为0.6 μJ、高亮度、小发散角(500
相干衍射成像的广泛应用受限于两个技术瓶颈:1)相位恢复算法的时间成本高、计算量大,而且测得的衍射图案的信噪比和过采样率也会影响收敛速度;2)测量单幅远场衍射光强分布仅能获取有限尺寸的孤立样品信息。
为了降低相位恢复算法对成像技术的影响,并提高成像鲁棒性,可以在待测物旁引入已知强度和相位、并与待测物散射光相互干涉的参考光,这样原光场的相位信息可以不借助迭代重构算法而直接从干涉图样中获取,这种方法叫作FTH技术[95],如
为了解决扩展尺寸、非孤立样品的成像问题,人们在相干衍射成像的基础上对照明光束位置进行扫描,扫描过程中记录多个位置的衍射图案,且在相邻的位置之间,光束照明区域重叠以包含冗余待测物体的信息,利用迭代算法反演物体结构。如
图 10. 基于HHG叠层扫描成像技术的成像实例。(a)首次使用HHG进行叠层扫描成像得到的样品重构图[104];(b)基于叠层扫描成像技术的小鼠海马神经元的成像结果[107]
Fig. 10. Imaging examples using HHG ptychography. (a) Sample reconstruction image obtained by ptychography using HHG for the first time[104]; (b) imaging result of mouse hippocampal neuron based on ptychography[107]
相干衍射成像技术的侧重点是获取待测样品的横向空间信息,而对样品的深度信息不敏感。然而光学相干断层成像(OCT)技术因利用宽谱光源相干长度短的特点,可以获得样品的深度信息,得到了广泛应用[112]。光学相干断层成像技术的纵向分辨率由照明宽谱光源的相干长度或者光谱带宽决定,由于高次谐波的光谱带宽可以覆盖整个极紫外到软X射线波段,其纵向分辨率可以达到纳米量级[113]。2017年,Fuchs等[114]首次使用光子能量在30~70 eV范围内的高次谐波宽谱光源,实现了对纳米材料结构的光学相干断层成像测量,如
图 11. 用OCT测量的三维结构剖面[114]。(a)深度和横向信息;(b)深度信息
Fig. 11. Three-dimensional structural profiles measured by OCT[114]. (a) Depth and lateral information; (b) depth information
5 结束语
在高重复频率、高功率飞秒激光技术的推动下,高次谐波光源的重复频率和光子通量不断优化。相比传统钛蓝宝石固体飞秒激光器,高平均功率飞秒光纤和固体激光器的脉冲宽度长、单脉冲能量低、调谐范围窄等问题正在得到解决。利用各类非线性压缩技术,将飞秒光纤激光的脉冲宽度压缩到少周期量级;利用相干合束技术,高重复频率飞秒激光的脉冲能量可以达到数十毫焦量级;利用OPCPA技术驱动激光波长可在较大范围内调谐,同时,稳定的CEP和超短脉宽的脉冲输出有利于产生孤立阿秒脉冲。这些激光技术的进步使得满足各类场景应用需求的高重复频率、高通量高次谐波极紫外光源成为可能,通过在单原子响应层面控制激光光强和波长,在宏观传播层面控制高次谐波的自吸收与相位匹配,新一代激光技术已经能够实现毫瓦量级的平均功率、百电子伏特量级的最大光子能量、数十兆赫兹重复频率的极紫外相干光源输出。高重复频率高次谐波光源平均功率的增加将进一步丰富阿秒科学相关的基础研究内容[116-117],脉宽在几十至几百阿秒量级的高通量阿秒光源,为在电子运动的时间尺度上研究原子和分子系统提供了可行性[118],也将有助于减轻光电子辐射光谱中的空间电荷效应[119],缩短采集时间,以及提高EUV瞬态吸收光谱实验[120]和EUV泵浦-EUV探测等时间分辨测量[121]中的信噪比。利用高重复频率、高光子通量极紫外相干光源,高次谐波辐射将从前沿科学研究的实验室走向各类应用场景,尤其是成像检测领域。相干衍射成像和相干断层成像技术可以分别在横向和纵向上实现纳米尺度三维结构物体的高空间分辨、材料分辨测量,因此基于高光子通量光源的成像技术与仪器有望在集成电路制造、纳米材料、生物医学等领域中取得实际应用。
[1] Bertrand J B, Wörner H J, Salières P, et al. Linked attosecond phase interferometry for molecular frame measurements[J]. Nature Physics, 2013, 9: 174-178.
[2] Månsson E P, Guénot D, Arnold C L, et al. Double ionization probed on the attosecond timescale[J]. Nature Physics, 2014, 10: 207-211.
[3] Calegari F, Ayuso D, Trabattoni A, et al. Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses[J]. Science, 2014, 346(6207): 336-339.
[4] Dierolf M, Menzel A, Thibault P, et al. Ptychographic X-ray computed tomography at the nanoscale[J]. Nature, 2010, 467(7314): 436-439.
[5] La-O-Vorakiat C, Siemens M, Murnane M M, et al. Ultrafast demagnetization dynamics at the M edges of magnetic elements observed using a tabletop high-harmonic soft X-ray source[J]. Physical Review Letters, 2009, 103(25): 257402.
[6] Rohwer T, Hellmann S, Wiesenmayer M, et al. Collapse of long-range charge order tracked by time-resolved photoemission at high momenta[J]. Nature, 2011, 471(7339): 490-493.
[7] Cingöz A, Yost D C, Allison T K, et al. Direct frequency comb spectroscopy in the extreme ultraviolet[J]. Nature, 2012, 482(7383): 68-71.
[8] Corkum P B. Plasma perspective on strong field multiphoton ionization[J]. Physical Review Letters, 1993, 71(13): 1994-1997.
[9] Popmintchev T, Chen M C, Popmintchev D, et al. Bright coherent ultrahigh harmonics in the keV X-ray regime from mid-infrared femtosecond lasers[J]. Science, 2012, 336(6086): 1287-1291.
[10] Silva F, Teichmann S M, Cousin S L, et al. Spatiotemporal isolation of attosecond soft X-ray pulses in the water window[J]. Nature Communications, 2015, 6: 6611.
[11] Gaumnitz T, Jain A, Pertot Y, et al. Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver[J]. Optics Express, 2017, 25(22): 27506-27518.
[12] Li J, Ren X M, Yin Y C, et al. 53-attosecond X-ray pulses reach the carbon K-edge[J]. Nature Communications, 2017, 8(1): 186.
[13] Eckle P, Pfeiffer A N, Cirelli C, et al. Attosecond ionization and tunneling delay time measurements in helium[J]. Science, 2008, 322(5907): 1525-1529.
[14] Schultze M, Fiess M, Karpowicz N, et al. Delay in photoemission[J]. Science, 2010, 328(5986): 1658-1662.
[15] Kandula D Z, Gohle C, Pinkert T J, et al. Extreme ultraviolet frequency comb metrology[J]. Physical Review Letters, 2010, 105(6): 063001.
[16] Li X F, L’Huillier A, Ferray M, et al. Multiple-harmonic generation in rare gases at high laser intensity[J]. Physical Review A, 1989, 39(11): 5751-5761.
[17] Ferray M, L’Huillier A, Li X F, et al. Multiple-harmonic conversion of 1064 nm radiation in rare gases[J]. Journal of Physics B Atomic Molecular Physics, 1988, 21(3): L31-L35.
[18] Paul P M, Toma E S, Breger P, et al. Observation of a train of attosecond pulses from high harmonic generation[J]. Science, 2001, 292(5522): 1689-1692.
[19] Hentschel M, Kienberger R, Spielmann C, et al. Attosecond metrology[J]. Nature, 2001, 414: 509-513.
[20] Corkum P B, Krausz F. Attosecond science[J]. Nature Physics, 2007, 3: 381-387.
[21] Zürch M, Rothhardt J, Hädrich S, et al. Real-time and sub-wavelength ultrafast coherent diffraction imaging in the extreme ultraviolet[J]. Scientific Reports, 2014, 4: 7356.
[22] Miao J W, Ishikawa T, Robinson I K, et al. Beyond crystallography: diffractive imaging using coherent X-ray light sources[J]. Science, 2015, 348(6234): 530-535.
[23] Boullet J, Zaouter Y, Limpert J, et al. High-order harmonic generation at a megahertz-level repetition rate directly driven by an ytterbium-doped-fiber chirped-pulse amplification system[J]. Optics Letters, 2009, 34(9): 1489-1491.
[24] Emaury F, Diebold A, Saraceno C J, et al. Compact extreme ultraviolet source at megahertz pulse repetition rate with a low-noise ultrafast thin-disk laser oscillator[J]. Optica, 2015, 2(11): 980.
[25] Demmler S, Rothhardt J, Hädrich S, et al. Generation of high photon flux coherent soft X-ray radiation with few-cycle pulses[J]. Optics Letters, 2013, 38(23): 5051-5054.
[27] Rothhardt J, Hädrich S, Shamir Y, et al. High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules[J]. Optics Express, 2016, 24(16): 18133-18147.
[28] Cabasse A, Machinet G, Dubrouil A, et al. Optimization and phase matching of fiber-laser-driven high-order harmonic generation at high repetition rate[J]. Optics Letters, 2012, 37(22): 4618-4620.
[29] Lorek E, Larsen E W, Heyl C M, et al. High-order harmonic generation using a high-repetition-rate turnkey laser[J]. The Review of Scientific Instruments, 2014, 85(12): 123106.
[30] HädrichS, RothhardtJ, KrebsM, et al. Short wavelength generation at high repetition rate by direct high harmonic generation[C]∥International Conference on Ultrafast Phenomena, July 18-23, 2010, Snowmass, Colorado. Washington, DC: OSA, 2010: MD2.
[31] Hädrich S, Rothhardt J, Krebs M, et al. High harmonic generation by novel fiber amplifier based sources[J]. Optics Express, 2010, 18(19): 20242-20250.
[32] Hädrich S, Krebs M, Rothhardt J, et al. Generation of µW level plateau harmonics at high repetition rate[J]. Optics Express, 2011, 19(20): 19374-19383.
[33] KirscheA, KlasR, GebhardtM, et al. Continuously tunable high photon flux high harmonic source at 50-70 eV[C]∥2021 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), June 21-25, 2021, Munich, Germany. New York: IEEE Press, 2021.
[34] Hädrich S, Klenke A, Rothhardt J, et al. High photon flux table-top coherent extreme-ultraviolet source[J]. Nature Photonics, 2014, 8: 779-783.
[35] Klas R, Demmler S, Tschernajew M, et al. Table-top milliwatt-class extreme ultraviolet high harmonic light source[J]. Optica, 2016, 3(11): 1167-1170.
[36] Zhao Z G, Kobayashi Y. Realization of a mW-level 10.7-eV (λ=115.6 nm) laser by cascaded third harmonic generation of a Yb: fiber CPA laser at 1-MHz[J]. Optics Express, 2017, 25(12): 13517-13526.
[37] Comby A, Descamps D, Beauvarlet S, et al. Cascaded harmonic generation from a fiber laser: a milliwatt XUV source[J]. Optics Express, 2019, 27(15): 20383-20396.
[38] Bucksbaum P H, Freeman R R, Bashkansky M, et al. Role of the ponderomotive potential in above-threshold ionization[J]. Journal of the Optical Society of America B, 1987, 4(5): 760-764.
[39] Klas R, Eschen W, Kirsche A, et al. Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser[J]. Optics Express, 2020, 28(5): 6188-6196.
[40] TschernajewM, HädrichS, KlasR, et al. High repetition rate high harmonic generation with ultra-high photon flux[C]∥2021 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), June 21-25, 2021, Munich, Germany. New York: IEEE Press, 2021.
[41] Popmintchev D, Hernández-García C, Dollar F, et al. Ultraviolet surprise: efficient soft X-ray high-harmonic generation in multiply ionized plasmas[J]. Science, 2015, 350(6265): 1225-1231.
[42] BussJ H, PetevM, GolzT, et al. High repetition rate extreme ultraviolet source driven by tunable OPCPA[C]∥OSA Nonlinear Optics 2021, August 9-13, 2021, Washington, DC. Washington, DC: Optica Publishing Group, 2021: NTu1A.6.
[43] Krebs M, Hädrich S, Demmler S, et al. Towards isolated attosecond pulses at megahertz repetition rates[J]. Nature Photonics, 2013, 7: 555-559.
[44] Rothhardt J, Hädrich S, Klenke A, et al. 53 W average power few-cycle fiber laser system generating soft X rays up to the water window[J]. Optics Letters, 2014, 39(17): 5224-5227.
[45] Popmintchev D, Galloway B R, Chen M C, et al. Near- and extended-edge X-ray-absorption fine-structure spectroscopy using ultrafast coherent high-order harmonic supercontinua[J]. Physical Review Letters, 2018, 120(9): 093002.
[46] Fu Y X, Nishimura K, Shao R Z, et al. High efficiency ultrafast water-window harmonic generation for single-shot soft X-ray spectroscopy[J]. Communications Physics, 2020, 3: 92.
[47] Gebhardt M, Heuermann T, Klas R, et al. Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre[J]. Light, Science & Applications, 2021, 10(1): 36.
[48] Pupeikis J, Chevreuil P A, Bigler N, et al. Water window soft X-ray source enabled by a 25 W few-cycle 2.2 µm OPCPA at 100 kHz[J]. Optica, 2020, 7(2): 168-171.
[49] Pupeza I, Zhang C K, Högner M, et al. Extreme-ultraviolet frequency combs for precision metrology and attosecond science[J]. Nature Photonics, 2021, 15: 175-186.
[50] Vernaleken A, Weitenberg J, Sartorius T, et al. Single-pass high-harmonic generation at 20.8 MHz repetition rate[J]. Optics Letters, 2011, 36(17): 3428-3430.
[51] Jocher C, Eidam T, Hädrich S, et al. Sub 25 fs pulses from solid-core nonlinear compression stage at 250 W of average power[J]. Optics Letters, 2012, 37(21): 4407-4409.
[52] Emaury F, Dutin C F, Saraceno C J, et al. Beam delivery and pulse compression to sub-50 fs of a modelocked thin-disk laser in a gas-filled Kagome-type HC-PCF fiber[J]. Optics Express, 2013, 21(4): 4986-4994.
[53] Mak K F, Seidel M, Pronin O, et al. Compressing μJ-level pulses from 250 fs to sub-10 fs at 38-MHz repetition rate using two gas-filled hollow-core photonic crystal fiber stages[J]. Optics Letters, 2015, 40(7): 1238-1241.
[54] Hädrich S, Krebs M, Hoffmann A, et al. Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources[J]. Light: Science & Applications, 2015, 4(8): e320.
[55] Jin C, Wang G L, Wei H, et al. Waveforms for optimal sub-keV high-order harmonics with synthesized two- or three-colour laser fields[J]. Nature Communications, 2014, 5: 4003.
[56] Severt T, Troß J, Kolliopoulos G, et al. Enhancing high-order harmonic generation by controlling the diffusion of the electron wave packet[J]. Optica, 2021, 8(8): 1113-1121.
[57] Wang H, Xu Y M, Ulonska S, et al. Bright high-repetition-rate source of narrowband extreme-ultraviolet harmonics beyond 22 eV[J]. Nature Communications, 2015, 6: 7459.
[58] 石顺祥. 非线性光学[M]. 2版. 西安: 西安电子科技大学出版社, 2012.
ShiS X. Nonlinear optics[M]. 2nd ed. Xi’an: Xidian University Press, 2012.
[59] Balcou P, Salières P, L’Huillier A, et al. Generalized phase-matching conditions for high harmonics: the role of field-gradient forces[J]. Physical Review A, 1997, 55(4): 3204-3210.
[60] Rundquist A, Durfee C G, Chang Z, et al. Phase-matched generation of coherent soft X-rays[J]. Science, 1998, 280(5368): 1412-1415.
[61] Kazamias S, Douillet D, Weihe F, et al. Global optimization of high harmonic generation[J]. Physical Review Letters, 2003, 90(19): 193901.
[62] Salières P, L’Huillier A, Lewenstein M. Coherence control of high-order harmonics[J]. Physical Review Letters, 1995, 74(19): 3776-3779.
[63] Guo C, Harth A, Carlström S, et al. Phase control of attosecond pulses in a train[J]. Journal of Physics B Atomic Molecular Physics, 2018, 51(3): 034006.
[64] Lewenstein M, Salières P, L’Huillier A. Phase of the atomic polarization in high-order harmonic generation[J]. Physical Review A, 1995, 52(6): 4747-4754.
[65] Yudin G L, Ivanov M Y. Nonadiabatic tunnel ionization: looking inside a laser cycle[J]. Physical Review A, 2001, 64(1): 013409.
[66] Popmintchev T, Chen M C, Bahabad A, et al. Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum[J]. Proceedings of the National Academy of Sciences of the United States of America, 2009, 106(26): 10516-10521.
[67] Popmintchev T, Chen M C, Cohen O, et al. Extended phase matching of high harmonics driven by mid-infrared light[J]. Optics Letters, 2008, 33(18): 2128-2130.
[68] Shiner A D, Trallero-Herrero C, Kajumba N, et al. Wavelength scaling of high harmonic generation efficiency[J]. Physical Review Letters, 2009, 103(7): 073902.
[69] Weissenbilder R, Carlström S, Rego L, et al. How to optimize high-order harmonic generation in gases[J]. Nature Reviews Physics, 2022, 4: 713-722.
[70] Durfee C G, Rundquist A R, Backus S, et al. Phase matching of high-order harmonics in hollow waveguides[J]. Physical Review Letters, 1999, 83(11): 2187-2190.
[71] Gibson E A, Paul A, Wagner N, et al. Coherent soft X-ray generation in the water window with quasi-phase matching[J]. Science, 2003, 302(5642): 95-98.
[72] Seres J, Yakovlev V S, Seres E, et al. Coherent superposition of laser-driven soft-X-ray harmonics from successive sources[J]. Nature Physics, 2007, 3: 878-883.
[73] Willner A, Tavella F, Yeung M, et al. Coherent control of high harmonic generation via dual-gas multijet arrays[J]. Physical Review Letters, 2011, 107(17): 175002.
[74] Zhang X S, Lytle A L, Popmintchev T, et al. Quasi-phase-matching and quantum-path control of high-harmonic generation using counterpropagating light[J]. Nature Physics, 2007, 3: 270-275.
[75] Constant E, Garzella D, Breger P, et al. Optimizing high harmonic generation in absorbing gases: model and experiment[J]. Physical Review Letters, 1999, 82(8): 1668-1671.
[76] Kazamias S, Daboussi S, Guilbaud O, et al. Pressure-induced phase matching in high-order harmonic generation[J]. Physical Review A, 2011, 83(6): 063405.
[77] Cooper J W. Photoionization from outer atomic subshells. A model study[J]. Physical Review, 1962, 128(2): 681-693.
[78] Lewenstein M, Balcou P, Ivanov M Y, et al. Theory of high-harmonic generation by low-frequency laser fields[J]. Physical Review A, 1994, 49(3): 2117-2132.
[79] Lindner F, Stremme W, Schätzel M G, et al. High-order harmonic generation at a repetition rate of 100 kHz[J]. Physical Review A, 2003, 68(1): 013814.
[80] Heyl C M, Coudert-Alteirac H, Miranda M, et al. Scale-invariant nonlinear optics in gases[J]. Optica, 2016, 3(1): 75-81.
[81] Rothhardt J, Krebs M, Hädrich S, et al. Absorption-limited and phase-matched high harmonic generation in the tight focusing regime[J]. New Journal of Physics, 2014, 16(3): 033022.
[82] Heyl C M, Güdde J, L’Huillier A, et al. High-order harmonic generation with μJ laser pulses at high repetition rates[J]. Journal of Physics B: Atomic, Molecular and Optical Physics, 2012, 45(7): 074020.
[83] Harth A, Guo C, Cheng Y C, et al. Compact 200 kHz HHG source driven by a few-cycle OPCPA[J]. Journal of Optics, 2018, 20(1): 014007.
[84] Takahashi E J, Nabekawa Y, Midorikawa K. Low-divergence coherent soft X-ray source at 13 nm by high-order harmonics[J]. Applied Physics Letters, 2004, 84(1): 4-6.
[85] Fienup J R. Phase retrieval algorithms: a comparison[J]. Applied Optics, 1982, 21(15): 2758-2769.
[86] Abbe E. Beiträge zur theorie des mikroskops und der mikroskopischen wahrnehmung[J]. Archiv Für Mikroskopische Anatomie, 1873, 9(1): 413-468.
[87] Spence J C H, Weierstall U, Howells M. Coherence and sampling requirements for diffractive imaging[J]. Ultramicroscopy, 2004, 101(2/3/4): 149-152.
[88] Sandberg R L, Paul A, Raymondson D A, et al. Lensless diffractive imaging using tabletop coherent high-harmonic soft-X-ray beams[J]. Physical Review Letters, 2007, 99(9): 098103.
[89] Tadesse G K, Klas R, Demmler S, et al. High speed and high resolution table-top nanoscale imaging[J]. Optics Letters, 2016, 41(22): 5170-5173.
[90] Gardner D F, Zhang B S, Seaberg M D, et al. High numerical aperture reflection mode coherent diffraction microscopy using off-axis apertured illumination[J]. Optics Express, 2012, 20(17): 19050-19059.
[91] ShanblattE R, PorterC L, GardnerD F, et al. Quantitative chemically-specific coherent diffractive imaging of reactions and diffusion at buried interfaces using a tabletop EUV nanoscope[C]∥Computational Optical Sensing and Imaging 2016, July 25-28, 2016, Heidelberg, Germany. Washington, DC: OSA, 2016: CT4C.1.
[92] Zürch M, Foertsch S, Matzas M, et al. Cancer cell classification with coherent diffraction imaging using an extreme ultraviolet radiation source[J]. Journal of Medical Imaging, 2014, 1(3): 031008.
[93] Ravasio A, Gauthier D, Maia F R N C, et al. Single-shot diffractive imaging with a table-top femtosecond soft X-ray laser-harmonics source[J]. Physical Review Letters, 2009, 103(2): 028104.
[94] Huijts J, Fernandez S, Gauthier D, et al. Broadband coherent diffractive imaging[J]. Nature Photonics, 2020, 14: 618-622.
[95] Winthrop J T, Worthington C R. X-ray microscopy by successive Fourier transformation[J]. Physics Letters, 1965, 15(2): 124-126.
[96] Sandberg R L, Raymondson D A, La-O-Vorakiat C, et al. Tabletop soft-X-ray Fourier transform holography with 50 nm resolution[J]. Optics Letters, 2009, 34(11): 1618-1620.
[97] Tadesse G K, Eschen W, Klas R, et al. High resolution XUV Fourier transform holography on a table top[J]. Scientific Reports, 2018, 8(1): 8677.
[98] Guizar-Sicairos M, Fienup J R. Holography with extended reference by autocorrelation linear differential operation[J]. Optics Express, 2007, 15(26): 17592-17612.
[99] Gauthier D, Guizar-Sicairos M, Ge X, et al. Single-shot femtosecond X-ray holography using extended references[J]. Physical Review Letters, 2010, 105(9): 093901.
[100] Abbey B, Whitehead L W, Quiney H M, et al. Lensless imaging using broadband X-ray sources[J]. Nature Photonics, 2011, 5: 420-424.
[101] Whitehead L W, Williams G J, Quiney H M, et al. Diffractive imaging using partially coherent X rays[J]. Physical Review Letters, 2009, 103(24): 243902.
[102] Thibault P, Dierolf M, Menzel A, et al. High-resolution scanning X-ray diffraction microscopy[J]. Science, 2008, 321(5887): 379-382.
[103] Maiden A M, Rodenburg J M. An improved ptychographical phase retrieval algorithm for diffractive imaging[J]. Ultramicroscopy, 2009, 109(10): 1256-1262.
[104] Seaberg M D, Zhang B S, Gardner D F, et al. Tabletop nanometer extreme ultraviolet imaging in an extended reflection mode using coherent Fresnel ptychography[J]. Optica, 2014, 1(1): 39-44.
[105] Zhang B S, Gardner D F, Seaberg M D, et al. High contrast 3D imaging of surfaces near the wavelength limit using tabletop EUV ptychography[J]. Ultramicroscopy, 2015, 158: 98-104.
[106] Baksh P D, Odstrčil M, Kim H S, et al. Wide-field broadband extreme ultraviolet transmission ptychography using a high-harmonic source[J]. Optics Letters, 2016, 41(7): 1317-1320.
[107] Baksh P D, Ostrčil M, Miszczak M, et al. Quantitative and correlative extreme ultraviolet coherent imaging of mouse hippocampal neurons at high resolution[J]. Science Advances, 2020, 6(18): eaaz3025.
[108] Mamezaki D, Harada T, Nagata Y, et al. Imaging performance improvement of coherent extreme-ultraviolet scatterometry microscope with high-harmonic-generation extreme-ultraviolet source[J]. Japanese Journal of Applied Physics, 2017, 56(6S1): 06.
[110] Tanksalvala M, Porter C L, Esashi Y, et al. Nondestructive, high-resolution, chemically specific 3D nanostructure characterization using phase-sensitive EUV imaging reflectometry[J]. Science Advances, 2021, 7(5): eabd9667.
[111] Mancini G F, Karl R M, Shanblatt E R, et al. Colloidal crystal order and structure revealed by tabletop extreme ultraviolet scattering and coherent diffractive imaging[J]. Optics Express, 2018, 26(9): 11393-11406.
[112] Huang D, Swanson E A, Lin C P, et al. Optical coherence tomography[J]. Science, 1991, 254(5035): 1178-1181.
[113] Fuchs S, Blinne A, Rödel C, et al. Optical coherence tomography using broad-bandwidth XUV and soft X-ray radiation[J]. Applied Physics B, 2012, 106(4): 789-795.
[114] Fuchs S, Wünsche M, Nathanael J, et al. Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source[J]. Optica, 2017, 4(8): 903-906.
[115] Wiesner F, Wünsche M, Reinhard J, et al. Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography[J]. Optica, 2021, 8(2): 230-238.
[116] 戴晨, 汪洋, 缪志明, 等. 基于飞秒激光与物质相互作用的高次谐波产生及应用[J]. 激光与光电子学进展, 2021, 58(3): 0300001.
[117] 董嘉豪, 梁青青, 许亮, 等. 气体高次谐波产生中的角动量守恒[J]. 激光与光电子学进展, 2023, 60(15): 1526001.
[118] 谢端, 银燕, 周泓宇. 基于强激光与等离子体波导激发高亮度、圆偏振高次谐波理论研究[J]. 光学学报, 2022, 42(21): 2114001.
[119] Keunecke M, Möller C, Schmitt D, et al. Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline[J]. The Review of Scientific Instruments, 2020, 91(6): 063905.
[120] Hütten K, Mittermair M, Stock S O, et al. Ultrafast quantum control of ionization dynamics in krypton[J]. Nature Communications, 2018, 9: 719.
[121] González-Castrillo A, Martín F, Palacios A. Quantum state holography to reconstruct the molecular wave packet using an attosecond XUV-XUV pump-probe technique[J]. Scientific Reports, 2020, 10(1): 12981.
魏子娟, 高熙泽, 孟翔宇, 李政言, 张庆斌, 兰鹏飞, 陆培祥. 高重复频率、高功率高次谐波极紫外光源进展及应用[J]. 中国激光, 2024, 51(7): 0701001. Zijuan Wei, Xize Gao, Xiangyu Meng, Zhengyan Li, Qingbin Zhang, Pengfei Lan, Peixiang Lu. High Harmonic Extreme Ultraviolet Light Source with High Repetition Rate and Power[J]. Chinese Journal of Lasers, 2024, 51(7): 0701001.