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The spectral range spanning from ~100 eV to 1 keV is highly attractive for a large number scientific applications including the study of ultrafast chemical reaction in the liquid phase, the study of ultrafast demagnetization at the L-edges of 3d transition metals composing magnetic materials or, more simply, nano-imaging and micro-tomography of deep structures such as semiconductor components.
The brilliance of table-top coherent soft X-rays sources does not compete yet with large-scale synchrotron beam lines: the conversion efficiency of High order Harmonic Generation (HHG) [1], i.e. the physical process used to produce photons up to 1 keV from a near- or middle- infrared femtosecond laser, is low and the photon flux in the X-UV is actually clamped by the availability of powerful enough driving lasers. The advent of picosecond Ytterbium solid-state lasers delivering average powers in the kW range is about to change this statement. When combined with nonlinear conversion devices such as optical parametric chirped-pulse amplifiers (OPCPA), these industrial lasers can be turned into powerful tunable sources with favorable properties for HHG up to soft-x-rays [2] such as mid-infrared wavelength, few-cycle pulse duration, high peak intensity, high energy and high-repetition. Additionally, few-cycle pulses reduce the number of attosecond bursts up to, ideally, a single isolated attosecond pulse. In that case, Carrier Envelope Phase (CEP) stability and control is paramount but also ensures a shot-to-shot reproducibility of the driving electric field as well as the HHG yield and spectra.
In this talk we present the experimental results acquired during the commissioning at ELI-ALPS (Szeged, Hungary) of a supercontinuum-seeded optical parametric chirped-pulse amplifier (OPCPA) generating 4-cycle pulses at ~3.2 µm with a pulse energy >150 µJ at 100 kHz (15 W average power), a Strehl ratio >0.8 and a shot-to-shot energy stability of 0.7% over 8h. This system was optimized for long-term energy and CEP stability and exhibits a CEP noise of 65 mrad RMS over 8h. To date, this is the best recorded non-averaged CEP stability for an amplified system, independently of the wavelength, pulse duration or repetition rate. This OPCPA also delivers the highest reported peak power (~3.8 GW) at 100 kHz within the 2-4 µm wavelength range without post-compression
Last, we present our development strategy toward the extension of these high-flux OPCPA sources toward the mid- and far-infrared as well innovative ideas to adapt these sources to multi-dimensional spectroscopy.
References:
[1] M. Lewenstein et al, "Theory of high-harmonic generation by low-frequency laser fields." Phys. Rev. A 49, 2117-2132 (1994).
[2] T. Popmintchev et al, ”Bright coherent ultrahigh harmonics in the keV X-ray regime from mid-infrared femtosecond lasers”, Science 336, 1287-1291 (2012).
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