S.J. Goh

Fabrication and characterization of free-standing, high-line-density transmission gratings for the vacuum UV to soft X-ray range

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.

Temporal model for quasi-phase matching in high-order harmonic generation

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.

Cluster size dependence of high-order harmonic generation

High-order harmonic generation (HHG) in clusters (instead of separate gas atoms) is of high promise due to expanded options for quasi-phase matching and because clusters appeared to offer an increased optical nonlinearity [1]. To verify the latter, we investigate HHG from noble gas clusters in a supersonic gas jet. To identify a possible dependence of HHG on the average cluster size, we change the total atomic number density in the j et over a broad range (from 3 × 1016 cm−3 to 3 × 1018 cm−3) which maximize the variation in cluster size. For disentangling the contribution to HHG from clusters and gas monomers, we perform experiments at two different reservoir temperatures (303 K and 363 K), in order to vary the liquid mass fraction, g, for the same range of cluster sizes (see variation of g in Fig. 1). We note that this is actually the first time in the evaluation of the harmonic yield in such measurements that the dependence of, g, vs. pressure and temperature is taken properly into consideration, and we determine g, reliably and consistently, to lie below 20% in the parameter range in Fig. 1. Based on measurements with a thin jet where significant variations in reabsorption and the phase-matching conditions can be neglected, we conclude that atoms in the form of small clusters (average cluster size < 1000 atoms) provide the same higher-order nonlinear response as single-atoms [2]. This implies that HHG in small clusters is based on electrons that return to their parent ions and not to neighbouring ions in the cluster. This conclusion is consistent with the measured harmonic spectra showing no obvious changes of the cut-off wavelength. Our results are in clear contrast to previous work [1] concluding that the single-atom response in small clusters (average cluster size < 700 atoms) increases with the cluster size, thereby promising a higher output than with monomers. Cluster may still increase the yield of high-order harmonic generation, however, not via the single-atom response but possibly via quasi-phase matching, as the higher mass of clusters allows for a higher density contrast in spatially structuring the nonlinear medium.

Spectral control of high-harmonic generation via drive laser pulse shaping in a wide-diameter capillary

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.