JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309-0440, USA.
Nature. 2012 Feb 1;482(7383):68-71. doi: 10.1038/nature10711.
The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs' coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation 'nuclear' clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.
光学频率梳（由一系列等间距线组成的光谱）的发展彻底改变了计量学和精密光谱学，因为它能够在微波和光学频率之间提供精确和直接的联系。频率梳技术的进一步发展是通过飞秒增强腔中的高次谐波产生，在极紫外光谱范围内产生频率梳。到目前为止，这种方法产生的梳子由于功率不足，无法应用，这一缺点也阻碍了对高次谐波产生的高重复率脉冲串的相位相干性的观察，因为这是一个非常非线性的过程。在这里，我们报告了通过将高功率近红外频率梳耦合到坚固的飞秒增强腔中，产生极紫外频率梳，达到 40 纳米的波长。这些梳子的功率足够大，我们可以观察到氩 82 纳米跃迁和氖 63 纳米跃迁的单光子光谱信号，从而证实了极紫外光中的梳子相干性。氩跃迁的绝对频率已通过直接频率梳光谱确定。由于氩原子的温度限制，跃迁的分辨率为 10 兆赫兹的线宽是该光谱区域前所未有的，对单个梳齿的线宽施加了严格的上限。由于缺乏连续波激光器，极紫外频率梳目前是将超高精度光谱扩展到 100 纳米以下光谱区域的唯一有前途的途径。在这些波长下，有广泛的应用，包括分子中电子跃迁的光谱学、单电离氦和中性氦中的束缚态和多体量子电动力学的实验检验、下一代“核”钟的发展以及利用高电荷离子的增强灵敏度搜索基本常数的变化。
