The effect of gas-density gradient on high-harmonic generation from neon-filled cells using annular and Gaussian laser beams

Abstract

An experimental study was performed to explore the effect of gas-density gradient in laser propagation direction on high-harmonic generation from neon-filled cells using a 50 fs annular and Gaussian Ti:sapphire laser beam. It was observed that, despite 20% lower power, the photon flux of the high-harmonics generated using an annular beam under optimum parameter conditions (∼2.5 × 1010 photons/sec for the 37th order in a 5 mm long cell) is on par with the maximum photon flux generated using a full Gaussian beam (∼2 × 1010 photons/sec for the 37th order in a 15 mm long cell). To elucidate the underlying mechanism for the experimental observation, a numerical simulation of the propagation of both the annular and Gaussian laser beams inside the cell was performed. The simulation was extended to estimate the high-harmonic intensity, after incorporating the effect of laser defocusing, the electron trajectory resolved phase-matching, and gas-density gradient. The dominant role of short electron trajectories was observed in the case of the annular beam, whereas, in the case of the Gaussian beam, a contribution of both short and long trajectories was found. Our analysis shows that, in neon-filled cells, the gas-density gradient present at the laser exit end of the cell plays a dominant role in achieving a high photon flux using an annular laser beam. Further, the annular beam not only provides a higher flux but also has lower divergence and higher coherence. This study will be useful in attosecond pulse metrology as well as in imaging applications viz coherent diffractive imaging.