Abstract
We experimentally investigate spectral control of high-harmonic generation in a wide-diameter (508 μm) capillary that allows using significantly lower gas pressures coupled with elevated drive laser energies to achieve higher harmonic energies. Using phase shaping to change the linear chirp of the drive laser pulses, we observe wavelength tuning of the high-harmonic output to both larger and smaller values. Comparing tuning via the gas pressure with the amount of blue shift in the transmitted drive laser spectrum, we conclude that both adiabatic and non-adiabatic effects cause pulse-shaping induced tuning of high harmonics. We obtain a fractional wavelength tuning, Δλ/λ, in the range from -0.007 to + 0.01, which is comparable to what is achieved with standard capillaries of smaller diameter and higher pressures.
Highlights
High-harmonic generation (HHG) is a nonlinear optical process that provides coherent radiation in the form of ultra-short pulses, covering a broad spectrum including the extreme ultraviolet (XUV)
To demonstrate that the high harmonic output can be increased noticeably by using higher drive laser pulse energies in combination with a wider-diameter capillary and lower gas pressures, we measure the harmonic output pulse energy obtained with the 508 μm diameter capillary and compare it with our previous measurement [17] that had used a thin (150 μm diameter) standard capillary
This corresponds to a factor of 3.3 increase in the harmonic output from the wide capillary which is why we are interested to investigate the degree of spectral control that can be obtained at such elevated output
Summary
High-harmonic generation (HHG) is a nonlinear optical process that provides coherent radiation in the form of ultra-short pulses, covering a broad spectrum including the extreme ultraviolet (XUV). HHG has found numerous applications such as in imaging at the nanoscale [1,2], XUV interferometry [3], spectroscopy [4,5] and attosecond science [6,7,8] while future applications are under way, e.g, the spectral control of free-electron lasers via injection seeding [9]. All these applications would clearly benefit if available elevated drive laser pulse energies could be utilized to increase the high-harmonic (HH) output [10]. The reduction in control shows up as multimodal drive laser propagation and nonlinear mode coupling caused by ionization-induced defocusing, and can lead to significant spatial shot-to-shot fluctuations [17]
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