Femtosecond broadband ellipsometry and magneto-optics: disentanglement of spin and charge dynamics

Condensed Matter Seminars

Speaker: Fabio Boschini, Politecnico di Milano
Date & Time: November 6, 2014, 14:00 - 15:00
Location: UBC, Hennings 318
Local Contact: Andrea Damascelli
Intended Audience: Undergraduate

Time-resolved magneto-optical Kerr effect (TR-MOKE) is a powerful technique to study ultrafast spin dynamics in magnetic materials. Since first results published on photoinduced ultrafast spin disorder in ferromagnetic systems (ultrafast demagnetization) [1], a strong debate has arisen about the reliability of the technique [2]. It is well known that magneto-optical signal originates from off-diagonal elements of the dielectric tensor but the analytical relationships of the MOKE signal depend also on diagonal terms of the dielectric tensor, i.e. the complex refractive index. I will show that the MOKE signal can be affected by non-magnetic contributions: photoinduced ultrafast charge dynamics.
By means of a broadband TR-MOKE apparatus [3] we are able to extract the temporal evolution of diagonal and off-diagonal terms of the dielectric tensor in the 1.7–2.5 eV energy range. Experimental results on CrO2 and La0.7Sr0.3MnO3 correlated materials indicate a non-magnetic contribution to the measured MOKE signal. We propose a new analysis method to extrapolate the real magnetization dynamics [4].
Finally, I will introduce our new time- and angle-resolved photoemission setup based on an Yb-laser source [5]. By means of a non-collinear optical parametric amplifier and sum-frequency generation an ultraviolet pulse with photon energy of 6.05 eV and an instrumental temporal resolution of 85 fs are established. Experimental results on BiTeI semiconductor and Bi2Se3 topological insulator materials assess the performances of our novel setup.

[1] E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot, Phys. Rev. Lett. 76, 22 (1996).
[2] B. Koopmans, M. van Kampen, J.T. Kohlhepp and W.J.M. de Jonge, Phys. Rev. Lett. 85, 844 (2000).
[3] F. Boschini, H. Hedayat, C. Piovera, C. Dallera and E. Carpene, Rev. Sci. Instr. submitted.
[4] E. Carpene, F. Boschini, H. Hedayat, C. Piovera, C. Dallera, E. Puppin, M. Mansurova, M. Münzenberg, X. Zhang and A. Gupta, Phys. Rev. B 87, 174437 (2013).
[5] F. Boschini, H. Hedayat, C. Dallera P. Farinello, C. Manzoni, A. Magrez, H. Berger, G. Cerullo and E. Carpene, Rev. Sci. Instr. submitted.