Seminář ÚFKL: Adam Dubroka

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Adam Dubroka:
Photo-induced insulator-to-metal transition in LaCoO3 explored by femtosecond pump-probe ellipsometry

Femtosecond lasers are used to probe fast relaxation dynamics in materials after excitation by a pump pulse in the so-called pump-probe arrangement. We explore relaxation dynamics in LaCoO3 thin films by femtosecond pump-probe ellipsometry recently developed in ELI, Břežany. We have observed large spectral weight shifts corresponding to a pump-induced insulator-to-metal transition. The excited states initially decay within 200~fs via a fast relaxation most likely related to an electronic relaxation accompanied by emission of phonons. We observe in LaCoO3 and also in metallic La0.5Sr0.5CoO3 thin film a significant secondary transient structure on 4--100~ps timescale. We attribute it to a secondary wave of high-spin states excited by phonons. At longer delays between 10-1000~ps, the films essentially cool down to the state before pump via a two-component relaxation process that we tentatively attribute to thermal diffusion via optical and acoustic phonons.