0.5-keV Soft X-ray attosecond continua

Abstract

Attosecond light pulses in the extreme ultraviolet have drawn a great deal of attention due to their ability to interrogate electronic dynamics in real time. Nevertheless, to follow charge dynamics and excitations in materials, element selectivity is a prerequisite, which demands such pulses in the soft X-ray region, above 200 eV, to simultaneously cover several fundamental absorption edges of the constituents of the materials. Here, we experimentally demonstrate the exploitation of a transient phase matching regime to generate carrier envelope controlled soft X-ray supercontinua with pulse energies up to 2.9±0.1 pJ and a flux of (7.3±0.1) × 10(7) photons per second across the entire water window and attosecond pulses with 13 as transform limit. Our results herald attosecond science at the fundamental absorption edges of matter by bridging the gap between ultrafast temporal resolution and element specific probing.

Figure 1. Experimental setup showing pressure dependence and spectral coverage.

Top: pressure scans in Ne (a) and He (b). Clearly visible is the C K-shell absorption edge due to hydrocarbon residue in the beamline. Middle: experimental setup consisting of a high-pressure effusive target, a free-standing IR filter, and the home-built spectrograph that consists of a 2,400 lines per mm gold coated grating and a cooled X-ray CCD. Bottom right: (c) a spectrum from HHG in He (2 min integration time) with the reachable K-shell absorption edges indicated by solid vertical lines and L-shell absorption edges indicated by dashed vertical lines. The two graphs below show absorption measurements using foils of 200 nm of carbon (d) and titanium (e) where the K-edge at 284 eV and L_2,3-edges at 456 eV, are clearly evident.

Figure 2. CEP controlled soft X-ray emission and its spectral influence.

Shown in a,c are spectra generated from helium (6 bar) and neon (3.5 bar), respectively, which were acquired for varying CEP in steps of 90 mrad and with 30 s integration time each. The solid lines in b,d show the dramatic change of the spectral amplitude for two different CEP values; these values are indicated by the coloured vertical lines in a,c. The excellent CEP stability of the system results in clearly resolved spectra which repeat—as expected—with π rad periodicity. This is shown by the dashed lines which are acquired for a π rad CEP offset compared with the matching solid coloured lines—see b,d.

Figure 3. Spatio-temporal phase matching maps as function of target pressure in He.

Calculated on-axis phase mismatch as a function of propagation position and time within the pulse for 500 eV radiation generated in helium for our experimental conditions and target pressures of 2–12 bar (a–f; data for neon can be found in the Supplementary Information). Dark red indicates good phase matching and the back dotted oval area encloses the z–t space in which the field strength is sufficient to generate 500 eV radiation. At low pressure, phase matching occurs across the entire z–t range. At high pressure, good phase matching occurs only transiently within a narrow temporal window but across the entire Rayleigh length (here similar to the encircled spatial range).