Generation of coherent phonons by coherent extreme ultraviolet radiation in a transient grating experiment

We investigate the excitation of coherent acoustic and optical phonons by ultrashort extreme ultraviolet (EUV) pulses produced by a free electron laser. Two crossed femtosecond EUV (wavelength 12.7 nm) pulses are used to excite coherent phonons at a wavelength of 280 nm, which are detected via diffraction of an optical probe beam. Longitudinal and surface acoustic waves are measured in BK-7 glass, diamond, and Bi4Ge3O12; in the latter material, the excitation of a coherent optical phonon mode is also observed. We discuss probing different acoustic modes in reflection and transmission geometries and frequency mixing of surface and bulk acoustic waves in the signal. The use of extreme ultraviolet radiation will allow the creation of tunable GHz to THz acoustic sources in any material without the need to fabricate transducer structures. The contribution by the MIT group was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-FG02-00ER15087. The JILA group graciously acknowledges support from the Department of Energy BES Award No. DE-FG02-99ER14982 and the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant No. GBMF: 4538, and J.K. acknowledges support from an SRC Fellowship. The Elettra group acknowledges the European Research Council Grant No. 202804-TIMER. A.C. and T.F. acknowledge the Swiss NSF through the NCCR-MUST