We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65• and can be explained by a cyclotron resonance effect of the surface states.Ordered states of matter are typically categorized by their broken symmetries. With the ordering of spins in a ferromagnet or the freezing of a liquid into a solid, the loss of symmetry distinguishes the ordered state from the disordered one. In contrast, topological states are distinguished by specific topological properties that are encoded in their quantum mechanical wavefunctions [1]. Frequently, a consequence of these properties is that there are robust "topologically protected" states on the sample's boundaries. The edge states of the quantum Hall effect (QHE) are the classic example [2]. In the last few years, it was realized that another class of such topological matter may exist in 3D band insulators with large spin-orbit interaction [3][4][5][6]. These so-called topological insulators are predicted to host robust surface states, which exhibit a number of interesting properties including spin helicity, immunity to back-scattering, and weak anti -localization. There are predictions of a number of unusual phenomena associated with these surface states, including a proximity-effect-induced exotic superconducting state with Majorana fermions bound to a vortex [7,8] and an axion electromagnetic response [9,10], and proposals for applications, such as their use in terahertz (THz) devices [11].Most of the signatures of topological behavior in these materials thus far have come from surface probes such as angle resolved photoemission (ARPES) and scanning tunneling spectroscopy [12][13][14][15][16][17]. These experiments have revealed that the surface states indeed show signatures of the predicted topological properties, such as a Diraclike dispersion, chiral spin textures, and the absence of backscattering. Direct observation of the topological behavior in transport has been hampered by the lack of a true bulk insulating state. Only recently have transport experiments started to distinguish the surface contribution from the bulk [18][19][20][21].As opposed to the case of the quantum Hall effect, in topological insulators, the quantization of the offdiagonal conductivity is not a requirement for the existence of the topological state. This, along with the problem of bulk conduction, has made finding a unique signature of this state difficult. It has been proposed that topological insulators may be characterized by their electrodynamic properties [9] due to the existence of an axionic term in the action ∆L = αθ dxdtE · B, where...

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