  We report on the experimental observation of a cyclotron antiresonance in a
canonical 3D topological insulator Bi$_2$Te$_3$. Magneto-reflectance response
of single crystal Bi$_2$Te$_3$ was studied in 18 Tesla magnetic field, and
compared to other topological insulators studied before, the main spectral
feature is inverted. We refer to it as an antiresonance. In order to describe
this unconventional behavior we propose the idea of an imaginary cyclotron
resonance frequency, which on the other hand indicates that the form of the
Lorentz force that magnetic field exerts on charge carriers takes an
unconventional form.
