Observation of Floquet-Bloch states and photo-induced time-reversal symmetry breaking in topological insulators
The topological
insulator (TI) is a new phase of matter that exhibits quantum-Hall-like
properties, even in the absence of an external magnetic field.
Understanding and characterizing unique properties
of these materials can lead to many novel applications such as current
induced magnetization or extremely robust quantum memory bits. In this
talk, I will discuss recent experiments in which we used
novel time and angle-resolved photoemission spectroscopy (ARPES)
to directly probe and control properties of Dirac Fermions.
The unique
electronic properties of the surface electrons in a topological
insulator are protected by time-reversal symmetry. Breaking such
symmetry without the presence of any magnetic ordering may lead
to an exotic surface quantum Hall state without Landau levels.
Circularly polarized light naturally breaks time-reversal symmetry, but
achieving coherent coupling with the surface states is challenging
because optical dipole transitions generally dominate.
Using time- and angle-resolved photoemission spectroscopy, we show that
an intense ultrashort mid-infrared pulse with energy below the bulk
band gap hybridizes with the surface Dirac fermions of a topological
insulator to form Floquet-Bloch bands. The photon-dressed
surface band structure is composed of a manifold of Dirac cones evenly
spaced by the photon energy and exhibits polarization-dependent band
gaps at the avoided crossings of the Dirac cones. Circularly polarized
photons induce an additional gap at the Dirac
point, which is a signature of broken time-reversal symmetry on the
surface. These observations establish the Floquet-Bloch bands in solids
experimentally and pave the way for optical manipulation of topological
quantum states of matter.