High-order laser harmonics detection in the EUV and soft X-ray spectral regions

The design and performance of a spectroscopic instrument able to simultaneously measure spectral features, angular divergence, and conversion efficiency of high-order harmonics generated by an ultrashort pulsed laser focused onto a gas jet are presented. It combines an advanced optical setup with a EUV-sensitive bidimensional detector. The grazing-incidence spectrometer consists of two optical elements: a toroidal mirror and a spherical variable-line-spaced grating with flat-field properties. The mirror focuses the radiation tangentially on the grating entrance plane, in order to increase the flux collected by the grating. The grating has a variable groove spacing with parameters optimized to produce an almost flat focal surface in the 5-40 nm region. A 40 mm diameter microchannel plate intensifier is used to acquire the spectrum. The mirror does not compensate for the grating astigmatism, so the length of the spectral lines on the spectrum is proportional to the sagittal angular divergence of the high-order harmonic emission. The absolute response of the system was measured in order to obtain the conversion efficiency of the harmonic generation process. Some experimental examples reported here show very high sensitivity, which makes the system able to work in the single-shot operation, thus allowing us to follow the dynamic evolution of the harmonic generation process.