Two-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. In CO2, a robust destructive interference was seen consistent with the known separation of the oxygen atoms that are active in HHG. In O2, a harmonic enhancement was found indicating constructive interference. A good agreement was found with a simple two-centre interference model that includes the angular distribution function of the sample. The effective momentum of the electron wave was determined from the spectral position of these interferences. Ellipticity-dependent studies in CO2 clearly show how the destructive interference can be ‘switched off’ by increasing the degree of ellipticity and thus shifting the effective resonance condition.

We propose a different approach to obtain phase-matched generation of high-order harmonics based on the use of pulsed Bessel beams as pump pulses. By means of the "coherence map" technique, we show that it is possible to maximize the generation of a chosen harmonic of interest by properly adjusting the phase front tilt of the pulsed Bessel beam to compensate the mismatch arising from material and plasma dispersion and atomic phase.

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The autocorrelation function of the intensity scattered from cylindrical rough surfaces is analytically obtained with the Kirchhoff scalar diffraction theory. It is shown that, in contrast to the case in which planar rough surfaces scatter radiation, this function, related to the speckle size, depends on the statistical parameters that characterize the surface and on the scattering direction. This result suggests a new, to our knowledge, optical method that can be applied to the characterization of cylindrical rough surfaces, such as in on-line quality assessment, in manufacturing processes. The calculated theoretical expression was tested, showing good agreement with experiments.