Microporous diffractive filter for infrared rejection in high-harmonic generation setups

High-harmonic-generation beamlines require efficient suppression of the co-propagating infrared driver while transmitting broadband short-wavelength radiation. Existing solutions rely on absorption or reflection and are limited by bandwidth, angular constraints, and low damage thresholds. We demonstrate microporous filters as transmissive diffractive elements that suppress infrared by redistributing it into off-axis diffraction orders while preserving short-wavelength transmission. A scalar diffraction model links open-area ratio to transmission. Experiments show on-axis infrared suppression down to 1.1% while transmitting broadband short-wavelength radiation over the 8-19 nm range. Microporous filters withstand average intensities up to 160 W/cm(2) without degradation, yielding >10 & times; spectral purity improvement and enabling power scaling in high-harmonic-generation beamlines.