Higher Harmonics open the Water Window

Spectra of the third and the fifth harmonic

Spectra of the third (left) and the fifth harmonic (right) at a fundamental wavelength of 13.7 nm measured at FLASH.

The FEL radiation has full transverse coherence

The FEL radiation has full transverse coherence, which has been demonstrated by double-slit diffraction experiments. The pictures show a double-slit diffraction pattern at a fundamental wavelength of 100 nm measured at FLASH. The slit separation is 0.5 mm in the left picture and 1 mm in the right picture.

An important feature of FEL radiation is the existence of odd higher harmonics. Not only the fundamental wavelength λ1 ≡ λ3 but also the wavelengths λ3 = λ1/3 and λ5 = λ1/5 are found in the spectrum, whereas the even harmonics λ2 = λ1/2 and λ4 = λ1/4 are forbidden by symmetry and occur only for misaligned beams.

The third harmonic of the 13.7 nm radiation has a wavelength of 4.6 nm, while the fifth harmonic has a wavelength of 2.7 nm, enabling FLASH to reach deep into the water window. This is a wavelength range that is crucially important for the investigation of biological samples because the radiation is able to “see” through water.