Ishii Nobuhisa, Kaneshima Keisuke, Kitano Kenta, Kanai Teruto, Watanabe Shuntaro, Itatani Jiro
The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan.
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamasaki, Noda, Chiba 278-8510, Japan.
Nat Commun. 2014;5:3331. doi: 10.1038/ncomms4331.
High harmonic generation (HHG) using waveform-controlled, few-cycle pulses from Ti:sapphire lasers has opened emerging researches in strong-field and attosecond physics. However, the maximum photon energy of attosecond pulses via HHG remains limited to the extreme ultraviolet region. Long-wavelength light sources with carrier-envelope phase stabilization are promising to extend the photon energy of attosecond pulses into the soft X-ray region. Here we demonstrate carrier-envelope phase-dependent HHG in the water window using sub-two-cycle optical pulses at 1,600 nm. Experimental and simulated results indicate the confinement of soft X-ray emission in a single recombination event with a bandwidth of 75 eV around the carbon K edge. Control of high harmonics by the waveform of few-cycle infrared pulses is a key milestone to generate soft X-ray attosecond pulses. We measure a dependence of half-cycle bursts on the gas pressure, which indicates subcycle deformation of the waveform of the infrared drive pulses in the HHG process.
利用钛宝石激光器产生的波形可控、周期数少的脉冲进行高次谐波产生（HHG），开启了强场和阿秒物理领域的新兴研究。然而，通过高次谐波产生获得的阿秒脉冲的最大光子能量仍局限于极紫外区域。具有载波包络相位稳定的长波长光源有望将阿秒脉冲的光子能量扩展到软X射线区域。在此，我们展示了在水窗中利用1600纳米的亚两周期光脉冲进行载波包络相位相关的高次谐波产生。实验和模拟结果表明，在碳K边附近75电子伏特带宽的单个复合事件中，软X射线发射受到限制。通过少周期红外脉冲的波形控制高次谐波是产生软X射线阿秒脉冲的一个关键里程碑。我们测量了半周期脉冲串对气体压力的依赖性，这表明在高次谐波产生过程中红外驱动脉冲波形的亚周期变形。
