C.A. Froud

Soft-x-ray wavelength shift induced by ionization effects in a capillary

Coherent soft x rays are produced by high-harmonic generation in a capillary filled with Ar gas. We demonstrate that the tuning of the harmonic wavelengths with intensity and chirp arises from changes in the Ar ionization level. Control over the tuning can be achieved either by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blueshift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization, rather than directly on any chirp imposed on the fundamental wavelength.

Molecular variation of capillary-produced soft x-ray high harmonics

The production of coherent soft x-rays via high harmonic generation (HHG) from Ar, N2 and N2O in a gas-filled capillary waveguide is reported. Odd harmonics of the driving laser intensity are observed as a function of gas pressure up to energies of 50 eV. By employing a simple phase matching theory that incorporates the spatial and temporal dependences of ionization within the waveguide, good agreement with the HHG spectral envelope has been obtained, indicating phase matching is the dominant factor in shaping the spectral envelope of the harmonics.

Spatiotemporal phase-matching in capillary high-harmonic generation

We present a simple phase-matching model that takes into account the full spatiotemporal nature of capillary high-harmonic generation. Spectra predicted from the model are compared to experimental results for a number of gases and are shown to reproduce the spectral envelope of experimentally generated harmonics. The model demonstrates that an ionization-induced phase mismatch is limiting the energy of the generated harmonics in current capillary high-harmonic generation experiments. The success of this model shows that phase-matching processes play a dominant role in determining the emission from capillary high-harmonic generation.

EUV off-axis focusing using a high harmonic source

High Harmonic Generation is a well established technique for generating Extreme Ultraviolet radiation. It is a promising technique for both structure and spectroscopic imaging due to both the high flux and coherence of the source, and the existence of multiple absorption edges at the generated wavelengths. To increase the flux, a focussing device can be used. Here we present focussing results for a Mo/Si spherical mirror that has been used in an off-axis arrangement, and give extensive analysis of the resulting astigmatic focus and its consequence on diffractive imaging. The astigmatic beam exists as a vertical and horizontal focus, separated by a circle of least confusion. With the help of a theoretical model we show that the most intense part of the beam is always the second line foci and that the phase at the focus is strongly saddle-shaped. However, this phase distortion cannot explain the significant interference peak splitting that is experimentally observed in our diffraction patterns. Instead we propose that the beam quality is degraded upon reflection from the multilayer mirror and it is this asymmetric phase distortion that causes the diffraction peak splitting.

Molecular control of the evolution of capillary-generated soft X-ray high harmonics

High harmonic generation from targets of Ar, N2 and N2O in a gas-filled capillary has been studied. A clear shift in the weighting of the harmonic intensity distribution with decreasing ionization energy is reported.

Microscale diffraction measurements with a high harmonic soft x-ray source

High harmonic generation in a gas filled capillary provides a high spatial coherence, collimated, soft x-ray source. We present simple diffraction measurements and theoretical simulations that demonstrate this coherence.

Optimisation of X-ray harmonic generation in capillaries by pulse shaping using an acousto-optic programmable filter

This study presents an acousto-optic programmable filter (AOPDF) that provides rapid, highly specific shaping of 0.3 laser pulses, which is used to change both the intensities and positions of generated X-ray harmonic orders. Because of the simple and accurate adjustment of spectral phase offered by AOPDF, this ability to modify and shift the X-ray spectrum can be used to produce frequency-agile X-ray tunable sources, as well as providing improved insight into the generation process in capillaries in which the key phase matching is modified by processes which depend on pulse shape.