Abstract
Near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy retrieves structure and composition of solid state materials and soft matter. We demonstrate the first table top solution for NEXAFS based on CEP stable and high flux ponderomotively scaled high harmonic generation into the water window with potential for attosecond temporal resolution [2]. Our solution is driven by a two-stage cryogenic titanium:sapphire (Ti:Sa) laser system outputting 7 mJ, 40 fs pulses with a central wavelength of 800 nm at a repetition rate of 1 kHz. A TOPAS-HE optical parametric amplifier (OPA) wavelength converts the 800nm pulses to idler pulses at 1.85 µm with a pulse duration of 45 fs. Intrinsic carrier-to-envelope (CEP) stable pulses are generated due to the OPA’s Ti:Sa pumped white-light seed. The 1.85 µm pulses are spectrally broadened in an argon filled hollow-core fibre nd are subsequently compressed using a 3 mm thick RG 1000 filter. An f-2f interferometer takes the reflection from the RG 1000 filter and is used to control the slow CEP drift via feedback to the OPA. Single-shot CEP stability of 88.8 mrad over 1 hour is achieved. An SHG-FROG is used to characterise the pulses resulting in a pulse duration of 11.2 fs at FWHM - see Fig. 1(left). The compressed pulses have an energy of 0.46 mJ and a single shot peak-to-peak stability of 1% rms over 40 min. The sub-2-cycle pulses are focused into our HHG target in neon, resulting in a high harmonic cut-off of 400 eV. In contrast to the capillary design, a free-space geometry target is used, facilitating simple and easy alignment. The target consists of a hollow 1.5 mm aluminium tube sealed at one end, with the laser focused through a 300 µm drilled hole. Low ambient pressures in the generation chamber are maintained via differential vacuum pumping. Backing pressures in the target of up to 7 bar are thus achieved. The importance of CEP control are illustrated in the CEP dependent spectra [2] and highlight why CEP control is needed to avoid averaging over varying pre- and post-edge structures when performing spectroscopy.
© 2015 IEEE
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