Measurement of transient optical properties (reflectivity and transmissivity) is frequently performed in X-ray/XUV pump – optical probe experiments. The optical properties reflect the transient state of irradiated materials. Here we propose to extend the material diagnostics with an additional measurement of the transient phase change of the optical probe pulse. It can be recorded in parallel to other transient optical properties, enabling an access to the information on the complex refractive index and the thickness of the radiation-modified material layer. The latter is necessary for investigations of phase transitions progressing in XUV (and X-ray) irradiated materials. In the talk, I present the results of a computational study of XUV irradiated silicon and diamond samples performed in [1]. It demonstrates that the measurement of the optical phase from a probe pulse at correctly tuned pulse parameters can provide a signal strong enough to extract information on transient material properties. Such phase measurement, feasible with modern experimental setups, can also be a basis for an improved diagnostics tool for the temporal characteristics of an ultrashort XUV pulse.
XUV pulses at 26.2 nm wavelength were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort XUV pulses and cross correlated by ultrashort optical laser pulses. This scheme enabled for the first time the measurement of a phase transition on a timescale of ~150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that ultrashort XUV pulses can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.
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