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[摘要]：Many intriguing phenomena in nature-from phase transitions to black holes-occur at singularities. A unique type of singularity common in wave phenomena, known as caustics(1,2), links processes observed in many different branches of physics(3,4). Here, we investigate the role of caustics in attosecond science and in particular the physical process behind high harmonic generation(5). We experimentally demonstrate spectral focusing in high harmonic generation, showing a robust intensity enhancement of an order of magnitude over a spectral width of several harmonics. This new level of control holds promises in both scientific and technological aspects of attosecond science(6,7). Moreover, our study provides a deeper insight into the basic mechanism underlying the high harmonic generation process, revealing its quantum nature(8) and universal properties. |
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