Photo-induced ultrafast spin current in a topological insulator

We may be approaching a revolution when it comes to electronic devices and computing based on spintronics. In a recent letter in JPCM, Davide Bugini and his team at Politecnico di Milano investigate the topological insulator Bi2Se3 using ARPES, with promising results for opto-spintronic devices. Read on to find out more from the authors:

Faster and more power efficient devices are imperative, however; our technology is currently approaching the fundamental scaling limit, i.e. computational speed is essentially limited by high-energy consumption. Therefore, a breakthrough is needed for the next-generation of computational and storage devices. Physicists and material scientists are trying to add an additional degree of freedom to the electronic charge (i.e. the “information carrier” in the actual electronics devices): electronic spin, a purely quantum property, giving rise to the so-called “spintronics”. In this race, topological insulators (TIs) represent promising. Even though TIs behave as insulators, they have a spin-polarized metallic surface in which a spin current can easily flow.

However, complete control of spins is still puzzling. Magnetic fields naturally couple with spins, but the realization of confined micro- or nano- magnetic fields is technologically complicated and requires a large amount of energy. Luckily, circularly polarized light can control spins too. In fact, ultrashort light pulses have been used to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers for many years. Moreover, light pulses are renowned for being easily-switchable allowing a fast spin control.

In our recent letter, we report the first experimental evidence of a direct coupling between circularly-polarized ultrashort light pulses and the spin-polarized population of the Dirac cone located in the unoccupied surface states of Bi2Se3, the prototypical TI (see figure 1).

Figure 1: (a) Symmetrized ARPES map of the unoccupied Dirac-cone measure with p-polarized pump and probe beams and (b) ARPES map of the asymmetry signal due to the circularly polarized pump beam populating the unoccupied Dirac-cone. Red dashed line in panel (a) follow the Dirac-cone dispersion. Black dashed line in panel (b) are taken from panel (a). © Davide Bugini 2017.

Our state-of-art time- and angle-resolved photoemission setup, exploiting its 70-fs temporal resolution, allowed us to track the establishment of a flow of spin-polarized electrons in momentum-space i.e. a photon-induced spin-current. We provide evidence of a transient spin-current in TIs simply triggered by direct optical coupling to an empty spin-polarized topological state. Our fascinating results are just a preview of the next-generation ultrafast opto-spintronic devices which promise to revolutionize our electronic technology.


About the Authors

Group website

Davide Bugini is a PhD candidate at Politecnico di Milano and at Center for NanoScience and Technology (CNST) of Italian Institute of Technology (IIT@POLIMI). He got his MSc in Physics at Università degli Studi di Milano in 2015. His current research fields concern ultrafast electrons dynamics in quantum materials like topological insulators, ultrafast dynamics of magnetization in magnetic materials and quantum phase transitions (eg. charge-density waves in quasi-2D materials) investigated by time-resolved optical spectroscopies, time-resolved magneto-optical Kerr-effect and time- and angle-resolved photoemission spectroscopy.

Fabio Boschini is a Postdoc fellow at the Stuart Blusson Quantum Matter Institute at UBC since March 2015. He got his MSc in Physics Engineering (2011) and my Ph.D. in Physics (2014) at Politecnico di Milano, Italy. He currently studies ultrafast electronic dynamics in quantum materials like cuprate superconductors, topological insulators and Weyl semimetals employing time- and angle-resolved photoemission technique. Furthermore, he is also performing equilibrium and out-of-equilibrium resonant x-ray scattering measurements on cuprates to study the complex interplay between the charge-order and the superconductivity.

Hamoon Hedayat is a research fellow in the Department of Physics, Politecnico di Milano, Italy, where he received his PhD degree in 2014. His area of expertise is investigating ultrafast physical phenomena in solids using femtosecond optical spectroscopy approach. His main field of interest is the study of electronic properties of quantum materials by time- and angle-resolved photoemission spectroscopy (trARPES).

Cristian Manzoni received his PhD in Physics at the Physics Department of Politecnico di Milano. From 2006 to 2009 he was Post Doc at the same institution. In 2009 he joined as Post Doc the Max Planck Research Group for Structural Dynamics, at the Center for free Electron Laser Science (Hamburg, Germany), working on time resolved measurements on high-temperature semiconductors in the mid-infrared and THz spectral range. Since 2010 he is researcher at National Research Council of Italy, at the Institute of Photonics and Nanotechnology. His main scientific activity focuses on the synthesis of few-optical-cycle laser pulses from amplified Ti:sapphire and Yb:doped laser systems, and on their application to  time-resolved spectroscopy and nonlinear processes. He is co-author of more than 80 peer-reviewed journal and proceedings articles, 4 book chapters on ultrabroadband nonlinear optics and spectroscopy, and more than 120 contributions to international conferences. He holds 1 international patent.

Claudia Dallera is associate professor at the Department of Physics of the Politecnico di Milano. Her research activity focuses on the electronic structure and dynamics of strongly correlated systems, studied by resonant x-ray spectroscopies and time-resolved reflectivity and time- and angle- resolved photoemission.

Giulio Cerullo was born in Milano, Italy, in 1965. He is a Full Professor in the Physics Department, Politecnico di Milano, Milano, Italy, where he leads the Ultrafast Optical Spectroscopy Laboratory. His research interests include a broad area known as “Ultrafast Optical Science,” and concerns on the one hand pushing our capabilities to generate and manipulate ultrashort light pulses, and on the other hand using such pulses to capture the dynamics of ultrafast events in molecular and solid-state systems.

Ettore Carpene received his PhD in Physics at the II Physics Institute of the University of Goettingen (Germany) in 2002. Since 2009 he is researcher at Institute of Photonics and Nanotechnology of the National Council of Research (CNR) in Italy. His main scientific activity focuses on time-resolved spectroscopy and ultrafast electronic and spin dynamics in magnetic and strongly correlated materials.


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