Abstract
We develop and implement an experimental strategy for the generation of high-energy high-order harmonics (HHG) in gases for studies of nonlinear processes in the soft x-ray region. We generate high-order harmonics by focusing a high energy Ti:Sapphire laser into a gas cell filled with argon or neon. The energy per pulse is optimized by an automated control of the multiple parameters that influence the generation process. This optimization procedure allows us to obtain energies per pulse and harmonic order as high as 200 nJ in argon and 20 nJ in neon, with good spatial properties, using a loose focusing geometry (f#≈400) and a 20 mm long medium. We also theoretically examine the macroscopic conditions for absorption-limited conversion efficiency and optimization of the HHG pulse energy for high-energy laser systems.
Highlights
High-order harmonics generated by the nonlinear interaction of an intense ultrashort laser pulse with atoms or molecules are used in many fields of physics
Since the discovery of the High-order harmonic generation (HHG) process over two decades ago,12,13 its conversion efficiency has been progressively improved by optimizing the macroscopic phase-matching conditions and the microscopic single atom response
We present measurements of high-order harmonics generated in argon and neon
Summary
High-order harmonics generated by the nonlinear interaction of an intense ultrashort laser pulse with atoms or molecules are used in many fields of physics. High-order harmonic generation (HHG) sources are well established in many research areas such as attosecond science or femtosecond spectroscopy and have become interesting for high-resolution imaging, free-electron-laser seeding, and nonlinear optics in the XUV range. . Most applications of HHG sources benefit from harmonic pulses with high pulse energy. This requirement is difficult to achieve due to the low conversion efficiency of the generation process. Since the discovery of the HHG process over two decades ago, its conversion efficiency has been progressively improved by optimizing the macroscopic phase-matching conditions and the microscopic single atom response. High-order harmonic generation has been carried out in different conditions, such as high-pressure jets, gas cells, semi-infinite media, and capillaries. Phase-matching optimization using loosely focused (possibly self-guided) fundamental fields has led to conversion efficiencies of ∼10−7 in neon,15 ∼10−5 in argon, and slightly below 10−4 in xenon. By modifying the generation field, e.g., by combining the fundamental field with one or more of its harmonics, the microscopic single atom response has been controlled on the subcycle level leading to enhanced HHG signals and/or generation of even-order harmonics.
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