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20世纪80年代中期, 啁啾脉冲放大技术(2018年诺贝尔物理学奖)突破了激光强度提升的瓶颈, 激光强度跨越了原子单位的门槛(1原子单位激光强度对应功率密度3.5×1016 W/cm2). 这样强的激光场可以在原子、分子中诱导出高阶非线性响应, 导致一系列新的物理现象, 其中尤其重要的是高次谐波辐射和阿秒光脉冲产生(2023年诺贝尔物理学奖). 随着强激光技术的进步, 当前激光强度已达到1023 W/cm2量级, 并在进一步提升中. 这样强的激光场能否在原子核中诱导出类似的高阶非线性响应、将“强场原子物理”推进至“强场原子核物理”? 最近的研究发现, 当前的强激光至少可以在一个特殊的原子核, 即钍-229原子核诱导出高阶非线性响应. 这得益于该原子核存在一个能量极低的激发态和超精细混合效应对于光核耦合的增强. 高阶非线性响应的触发可以极大地提升原子核的激发概率和调控效率. 类似原子, 被强激光驱动的原子核也会向外辐射高次谐波. “强场原子核物理”开始成为光与物质相互作用以及核物理研究的新前沿, 提供基于强激光的原子核激发和调控新方案, 以及基于原子核跃迁的相干光辐射新途径.
In the mid-1980s, chirped pulse amplification (Nobel Prize in Physics 2018) broke through previous limits to laser intensity, allowing intensities to exceed the atomic unit threshold (1 atomic unit of laser intensity corresponds to a power density of 3.5×1016 W/cm2). These strong laser fields can cause high-order nonlinear responses in atoms and molecules, resulting in a series of novel phenomena, among which high-order harmonic generation and attosecond pulse generation (Nobel Prize in Physics 2023) are particularly important. With the development of high-power laser technology, laser intensity has now reached the order of 1023 W/cm2 and is constantly increasing. Now, a fundamental question has been raised: can such a powerful laser field induce similar high-order nonlinear responses in atomic nuclei, potentially transitioning “strong-field atomic physics” into “strong-field nuclear physics”? To explore this, we investigate a dimensionless parameter that estimates the strength of light-matter interaction: $ \eta = D{E_0}/{{\Delta }}E $, where D is the transition moment (between two representative levels of the system), E0 is the laser field amplitude, DE0 quantifies the laser-matter interaction energy, and ΔE is the transition energy. If $ \eta \ll 1 $, the interaction is within the linear, perturbative regime. However, when $ \eta \sim 1 $, highly nonlinear responses are anticipated. For laser-atom interactions, D ~ 1 a.u. and ΔE = 1 a.u., so if E0~1 a.u., then $ \eta \sim 1 $ and highly nonlinear responses are initiated, leading to the above-mentioned strong-field phenomena. In the case of light-nucleus interaction, it is typical that $ \eta \ll 1 $. When considering nuclei instead of atoms, D becomes several (~5 to 7) orders of magnitude smaller, while ΔE becomes several (~5) orders of magnitude larger. Consequently, the laser field amplitude E0 will need to be 10 orders of magnitude higher, or the laser intensity needs to be 20 orders of magnitude higher (~ 1036 W/cm2), which is beyond existing technological limit and even exceeds the Schwinger limit, where vacuum breakdown occurs. However, there exist special nuclei with exceptional properties. For instance, the 229Th nucleus has a uniquely low-lying excited state with an energy value of only 8.4 eV, or 0.3 a.u. This unusually low transition energy significantly increases η. This transition has also been proposed for building nuclear clocks, which have potential advantages over existing atomic clocks. Another key factor is nuclear hyperfine mixing (NHM). An electron, particularly the one in an inner orbital, can generate a strong electromagnetic field at the position of the nucleus, leading to the mixing of nuclear eigenstates. For 229Th, this NHM effect is especially pronounced: the lifetime of the 8.4-eV nuclear isomeric state in a bare 229Th nucleus (229Th90+) is on the order of 103 s, while in the hydrogenlike ionic state (229Th89+) it decreases by five orders of magnitude to 10–2 s. This 1s electron greatly affects the properties of the 229Th nucleus, effectively changing the nuclear transition moment from D for the bare nucleus to $ D' = D + b{\mu _{\text{e}}} $ for the hydrogenlike ion, where D ~ 10–7 a.u., $ b \approx 0.03 $ is the mixing coefficient, $ {\mu _{\text{e}}} $ is the magnetic moment of the electron, and $ D' \approx b{\mu _{\text{e}}} \sim {10^{ - 4}} {\text{a}}{\text{.u}}{.} $ That is to say, the existence of the 1s electron increases the light-nucleus coupling matrix element by approximately three orders of magnitude, leading to the five-orders-of-magnitude reduction in the isomeric lifetime. With the minimized transition energy ΔE and the NHM-enhanced transition moment D’, it is found that $ \eta \sim 1 $ for currently achievable laser intensities. Highly nonlinear responses are expected in the 229Th nucleus. This is confirmed by our numerical results. Figure (a) shows the nuclear isomeric excitation probabilities for 229Th89+ as a function of laser intensity. Note that the isomeric state has been split into two states with total angular momentum quantum number F = 2 and F = 1 due to hyperfine interaction, and the excitation probabilities to both of these levels are shown. One can see the nonlinear “bursts” above the intensity 1017 W/cm2: a four-order-of-magnitude increase in laser intensity from 1017 to 1021 W/cm2 leads to a 14-orders-of-magnitude increase in excitation probability to the 10% level (per nucleus per laser pulse). In contrast, for the bare nucleus 229Th90+ without NHM, the dependency of the excitation probability on the laser intensity remains linear across the whole intensity range up to 1023 W/cm2 and the absolute excitation probability remains low (~ 10–15). Correspondingly, the intense laser-driven 229Th89+ system emits secondary light in the form of high harmonics, and the spectra of four different laser intensities are shown in Fig. (b). These spectra share similarities with those from laser-driven atoms but also have different features. In conclusion, it appears feasible to extend “strong-field atomic physics” to “strong-field nuclear physics”, at least in the case of 229Th. “Strong-field nuclear physics” is emerging as a new frontier in light-matter interaction and nuclear physics, providing opportunities for precisely exciting and controlling atomic nuclei with intense lasers and new avenues for coherent light emission based on nuclear transitions. -
Keywords:
- strong-field atomic physics /
- laser-nuclear physics /
- highly nonlinear responses /
- high harmonic generation
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图 1 (a)两能级原子示意图, $ {{\Delta }}E $是两能级之间能量差, $ \hbar \omega $是激光光子能量; (b)计算中所用激光脉冲示意图; (c)脉冲结束时刻原子激发概率随着激光场强的依赖关系, 黑色圆点是TDSE数值结果, 红色虚线是一阶含时微扰理论计算结果, $ \eta = D{E_0}/{{\Delta }}E $是相互作用能量与跃迁能量的比值
Fig. 1. (a) Illustration of a two-level atom, $ {{\Delta }}E $ is the energy difference between the two levels and $ \hbar \omega $ is the energy of the laser photon; (b) illustration of the laser pulse used in the calculation; (c) end-of-pulse excitation probability versus laser intensity. Black dots are numerical results from TDSE, and the red dashed line is the result from first-order time-dependent perturbation theory, $ \eta = D{E_0}/{{\Delta }}E $ is the ratio between the interaction energy and the transition energy.
图 2 (a)—(c)诱导电偶极矩Dind(t)及其(d)—(e)傅里叶变换得到的频谱; 激光强度分别为1011 W/cm2 (a), (d); 1014 W/cm2 (b), (e); 1016 W/cm2 (c), (f)
Fig. 2. (a)–(c) Induced dipole moment Dind(t) and (d)–(e) the harmonic spectra from Fourier transform; the laser intensity is 1011 W/cm2 (a), (d); 1014 W/cm2 (b), (e); 1016 W/cm2 (c), (f).
图 3 (a)钍–229裸核(229Th90+)最低两个核能级相关参数; (b)类氢离子(229Th89+)的超精细劈裂与混合效应. 裸核的激发态寿命在103 s量级, 而类氢离子由于超精细混合效应, 激发态寿命大幅缩短为10–2 s量级
Fig. 3. (a) The lowest two energy levels of the bare thorium-229 nucleus (229Th90+); (b) hyperfine splitting and state mixing in the hydrogen-like ionic state (229Th89+). Note that the lifetime of the nuclear excited state in the bare nucleus is on the order of 103 s, while the lifetime reduces dramatically to the order of 10–2 s due to hyperfine mixing effect.
图 4 (a) 光学谐振腔中的原子, 其中原子态可以与谐振腔中的光场态之间耦合形成缀饰态; (b)离子中的原子核, 其中原子核态可以与电子态之间耦合形成类似的耦合态
Fig. 4. (a) Atom in a light cavity, where atomic states can couple with the light states, forming dressed states; (b) nucleus in an ion, where nuclear states can couple with the electronic states, forming coupled states.
图 5 (a)脉冲结束时刻钍–229核激发概率(Th89+与Th90+两种情况)与激光场强的依赖关系, 该图源自文献[21](已授权); (b) 4个不同激光场强下的Th89+高次谐波频谱, 该图源自文献[21](已授权)
Fig. 5. (a) Nuclear isomeric excitation probability at the end of the laser pulse (for both Th89+ and Th90+) as a function of laser intensity, from Ref. [21] with permission; (b) harmonic spectra of Th89+ under four different laser intensities, from Ref. [21] with permission.
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