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内容記述 |
Burst Intensification by Singularity Emitting Radiation (BISER) in underdense relativistic laser plasma is a bright source of temporarily and spatially coherent XUV and x-ray radiation [1-3]. BISER scales favorably with the laser power and can produce 1012 photons per pulse (with ~100 TW laser power, as in our experiment with the J-KAREN-P laser [4,5]). BISER originates from relativistic plasma singularities and has a nano-scale source size. BISER spectra have hundreds of electronvolt (eV) bandwidth and, according to the simulations [2], close to the transform-limited duration, which promises Terawatt pulses shorter than the atomic unit of time (24 attoseconds).We performed high-resolution measurements of BISER spectra and found harmonic-like spectral fringes with separation much finer (down to 0.12 eV) than the initial driving laser frequency, ~1.5 eV. Further, BISER spectra contained several fringe spacings in one shot.To explain the observations, we propose the concept of alloharmonics [6,7], i.e. spectral interference of different harmonic orders from different emission moments in a (slightly) nonperiodic generation process.The alloharmonics phenomenon generalizes classical harmonics and is critical in attosecond physics, optical frequency combs, and other fields where nonperiodic processes can generate harmonics. The discovery of alloharmonics emphasizes that x-ray spectrographs with much higher resolving power than typically used, and simultaneously with wider bandwidth, are demanded in experiments with harmonics and harmonic-like spectra. |
