Highly sensitive transient reflection measurement in extreme ultraviolet region for tracking carrier and coherent phonon dynamics.

A highly sensitive method for detecting transient reflection in the extreme ultraviolet (XUV) region was developed on the basis of high-order harmonics for tracking carrier and coherent phonon dynamics. The use of lock-in detection and boxcar integration enables us to observe optical modulation (ΔR/R) as high as 1 × 10, and the data acquisition takes only four minutes. XUV transient reflections of bismuth exhibited exponential decay originating from excited carriers and periodic oscillation originating from A optical phonons. The linear power dependence of the electronic and phonon amplitudes indicated that one-photon excitation occurred under the experimental conditions. The cosine of the initial phase of the phonon oscillation revealed that a displacive excitation mechanism contributed to phonon generation. The phonon parameters obtained by the XUV and NIR probes were consistent even though their penetration depths were different. The result indicated that the XUV and NIR pulses probe the same excited region, which should be near the surface due to the short penetration depth of the NIR pump pulses. The present highly sensitive means of detecting XUV transient reflections in solid-state materials could be utilized for detecting attosecond dynamics in the future.