Realization of a time-compensated monochromator exploiting conical diffraction for few-femtosecond XUV pulses

Complete test of publication follows. The use of the XUV/X-ray radiation emitted in the interaction of femtosecond laser beam with the matter calls for optical instrumentation with unique characteristics. The peculiarities of this class of source are very challenging: in the case of harmonics generation the emission is discrete although tunable in the EUV, XUV and soft X-ray spectral region, highly collimated, of extremely short duration, presently in the attosecond regime, and the source is of very small spatial dimensions. A time compensated monochromator for the selection and utilisation of the harmonics is here presented. Its design prevents the negative effect of time broadening induced by selecting a single harmonic order with a conventional dispersing type of instrument. Time-delay compensated monochromators requires the use of at least two gratings in subtractive and compensated dispersion, in order to impose the two requirements: 1) for all the rays transmitted by the instrument and having the same wavelength, the differences in the optical-path lengths that are caused by the first grating have to be compensated by the second grating; 2) two rays with different wavelengths within the spectrum of the pulse to be selected have to follow a path with the same optical length and have to be focused on the same point. Both these conditions are satisfied by a scheme with two equal concave gratings mounted with opposite diffraction orders. A new configuration for a time-delay compensated monochromator based on the conical diffraction has been recently theoretically analyzed. It envisage the use of two blazed gratings in the conical diffraction mounting, in which the light approaches the grating in a plane parallel to the direction of the grooves. It consists of a generation chamber, where the ultra-short Ti:Sapphire laser interacts with the gas jet generating the HHs, and of a monochromator divided in two equal sections, each of them with two toroidal mirrors and a plane blazed grating. The first mirror of each section acts as a collimator, the second one acts as a condenser. The first section creates a spectrally dispersed image of the source on an intermediate plane, where a slit carries out the spectral selection of the HHs. Only a selected portion of the spectrum, i.e. a single harmonic or a set of few harmonics, is propagating through the slit toward the second section that compensates for the temporal delay introduced by the first grating and gives an image of the source on the output plane. The wavelength scanning is performed by rotating the gratings around an axis lying at the centre of the grating surface and parallel to the grooves. The optical setup and the XUV experimental characterization of the instrument will be detailed in the paper.