We present state-of-the-art high resolution transmission gratings, applicable for spectroscopy in the vacuum ultraviolet (VUV) and the soft X-ray (SRX) wavelength range, fabricated with a novel process using ultraviolet based nano imprint lithography (UV-NIL). Free-standing, high-line-density gratings with up to 10,000 lines per mm and various space-to-period ratios were fabricated. An optical characterization of the gratings was carried out in the range from 17 to 34 nm wavelength using high-harmonic generation in a capillary waveguide filled with Ne, and around 13.5 nm wavelength (from 10 to 17 nm) using a Xenon discharge plasma.

Abstract:We present a model for quasi-phase matching (QPM) in high-order harmonic generation (HHG). Using a one-dimensional description, we analyze the time-dependent, ultrafast wave-vector balance to calculate the on-axis harmonic output versus time, from which we obtain the output pulse energy. Considering, as an example, periodically patterned argon gas, as may be provided with a grid in a cluster jet, we calculate the harmonic output during different time intervals within the drive laser pulse duration. We find that identifying a suitable single spatial period is not straightforward due to the complex and ultrafast plasma dynamics that underlies HHG at increased intensities. The maximum on-axis harmonic pulse energy is obtained when choosing the QPM period to phase match HHG at the leading edge of the drive laser pulse.

For exploring the application potential of coherent soft x-ray (SXR) and extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is important to characterize the central output parameters. Of specific importance are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output energy, fluctuations of the direction of the emission (pointing instabilities), and fluctuations of the beam divergence and shape that reduce the spatial coherence. We present the first single-shot measurements of waveguided high-harmonic generation in a waveguided (capillary-based) geometry. Using a capillary waveguide filled with Argon gas as the nonlinear medium, we provide the first characterization of shotto-shot fluctuations of the pulse energy, of the divergence and of the beam pointing. We record the strength of these fluctuations vs. two basic input parameters, which are the drive laser pulse energy and the gas pressure in the capillary waveguide. In correlation measurements between single-shot drive laser beam profiles and single-shot high-harmonic beam profiles we prove the absence of drive laser beam-pointing-induced fluctuations in the high-harmonic output. We attribute the main source of high-harmonic fluctuations to ionization-induced nonlinear mode mixing during propagation of the drive laser pulse inside the capillary waveguide. References and links1. C. Wahlström, J. Larsson, A. Persson, T. Starczewski, S. Svanberg, P. Salières, P. Balcou, and A. L'Huillier, "High-order harmonic generation in rare gases with an intense short-pulse laser," Phys. Rev. A 48(6), 4709-4720 (1993). 2. P. Balcou, P. Salieres, K. S. Budil, T. Ditmire, M. D. Perry, and A. Lhuillier, "High-order harmonic-generation in rare-gases -a new source in photoionization spectroscopy," Z. Phys. D At. Mol. Clust. 34(2), 107-110 (1995 2187-2190 (1999). 8. T. Pfeifer, R. Kemmer, R. Spitzenpfeil, D. Walter, C. Winterfeldt, G. Gerber, and C. Spielmann, "Spatial control of high-harmonic generation in hollow fibers," Opt. Lett. 30(12), 1497-1499 (2005). 9. F. R. Powell, P. W. Vedder, J. F. Lindblom, and S. F. Powell, "Thin-film filter performance for extreme ultraviolet and x-ray applications," Opt. Eng. 29(6), 614-624 (1990). 10. G. Sommerer, H. Rottke, and W. Sandner, "Enhanced efficiency in high-order harmonic generation using sub-50-fs laser pulses," Laser Phys. 9(1), 430-432 (1999 and C. Jacobsen, "Generation of spatially coherent light at extreme ultraviolet wavelengths," Science 297(5580), 376-378 (2002). 14. T. J. Butcher, P. N. Anderson, R. T. Chapman, P. Horak, J. G. Frey, and W. S. Brocklesby, "Bright extremeultraviolet high-order-harmonic radiation from optimized pulse compression in short hollow waveguides," Phys. Eng. 37(9), 2454-2458 (1998). 35. Y. Tamaki, Y. Nagata, M. Obara, and K. Midorikawa, "Phase-matched high-order-harmonic generation in a gasfilled hollow fiber," Phys. Rev. A 59(5), 4041-4044 (1999). 36. S. J. Goh, H. J. M. Bastiaens, B. Vratzov, Q. Huang, F. Bijkerk, and K. J. Boller, "Fabrication a...

We investigate high-order harmonic generation (HHG) from noble gas clusters in a supersonic gas jet. To identify the contribution of harmonic generation from clusters versus that from gas monomers, we measure the high-order harmonic output over a broad range of the total atomic number density in the jet (from 3×10 16 to 3×10 18cm 3 ) at two different reservoir temperatures (303 and 363 K). For the first time in the evaluation of the harmonic yield in such measurements, the variation of the liquid mass fraction, g, versus pressure and temperature is taken into consideration, which we determine, reliably and consistently, to be below 20% within our range of experimental parameters. By comparing the measured harmonic yield from a thin jet with the calculated corresponding yield from monomers alone, we find an increased emission of the harmonics when the average cluster size is less than 3000. Using g, under the assumption that the emission from monomers and clusters add up coherently, we calculate the ratio of the average single-atom response of an atom within a cluster to that of a monomer and find an enhancement of around 100 for very small average cluster size (∼200). We do not find any dependence of the cut-off frequency on the composition of the cluster jet. This implies that HHG in clusters is based on electrons that return to their parent ions and not to neighboring ions in the cluster. To fully employ the enhanced average single-atom response found for small average cluster sizes (∼200), the nozzle producing the cluster jet must provide a large liquid mass fraction at these small cluster sizes for increasing the harmonic yield. Moreover, cluster jets may allow for quasi-phase matching, as the higher mass of clusters allows for a higher density contrast in spatially structuring the nonlinear medium.

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. 752-755 (1996). 32. G. Tempea and T. Brabec, "Nonlinear source for the generation of high-energy few-cycle optical pulses," Opt.Lett. 23(16), 1286-1288 (1998).