  We present a magneto-optical study of the three-dimensional topological
insulator, strained HgTe using a technique which capitalizes on advantages of
time-domain spectroscopy to amplify the signal from the surface states. This
measurement delivers valuable and precise information regarding the surface
state dispersion within <1 meV of the Fermi level. The technique is highly
suitable for the pursuit of the topological magnetoelectric effect and axion
electrodynamics.
