• In a new paper,  we report the first observation of the generation of coherent acoustic and optical phonons by extreme ultraviolet (EUV) radiation at a wavelength of 13 nm. 
• Experiments were conducted at the FERMI free electron laser (FERMI-Elettra, Trieste, Italy).
• Phonons are generated by two femtosecond EUV pulses crossed at the sample at an angle of  2.6 deg; the period of the EUV interference pattern equal to 280 nm defines the phonon wavelength. 
• The excited phonons were detected via diffraction of a time-delayed optical (400 nm) pulse. 
• We observed longitudinal and surface acoustic phonons in the tens of GHz range in diamond, BK-7 glass and Bi₄Ge₃O₁₂ ; in tghe latter material, optical phonon oscillations at 2.8 THz were also observed. 
• Phonons at a much shorter wavelengths (down to ~10 nm) can be generated by increasing the angle between the excitation pulses; however, they cannot be detected by the optical probe. New experiments using an EUV probe will be reported soon!

Fig.1 Experimental setup.

Fig. 2. Data from single crystal diamond. (a) The initial peak due to electronic response; (b) acoustic oscillations in the tail of the signal; (c) Fourier spectrum of the oscillations contains four peaks identified as harmonics and sum/difference frequencies of the longitudinal (L) and surface Rayleigh (R) modes.