Selective filtering

Light Sci. Appl. 5, e16170 (2016)

A simple scheme for selecting a particular high harmonic in the vacuum-ultraviolet and extreme-ultraviolet regions could prove useful for creating a spectrally pure source of short-wavelength light for applications in spectroscopy, holography and microscopy. When intense ultrashort pulses interact with a jet of a noble gas, a number of short-wavelength high harmonics are created and filtering them can be difficult. Researchers from ETH Zurich in Switzerland have now shown that it is possible to use the generating medium itself as a filter for one particular harmonic if the set-up is arranged in a non-collinear geometry that introduces a phase mismatch for adjacent harmonics. A filter contrast of up to 10⁴ for the selected harmonic is possible with the scheme. Working with a 400 nm driving field focused to intensities of 10¹²–10¹³ W cm⁻² and various gases (Xe, Kr, Ar and Ne), the team observed selective emission of 133 nm, 80 nm or 56 nm light using the approach.