  A room-temperature infrared spectroscopy study of elemental tellurium at
pressures up to 8.44 GPa in the energy range 0.015-2 eV is reported. Optical
signatures of the high-pressure polymorphs are investigated and compared to the
results of density-functional band-structure calculations. A Drude peak is
first seen in the optical conductivity around 3.5 GPa indicating a
semiconductor-to-metal transition within trigonal Te-I. A sharp increase in the
Drude spectral weight and dc-conductivity around 4.3 GPa signals the
transformation toward the triclinic Te-II polymorph. An absorption peak around
0.15 eV appears above 5 GPa concomitant with the gradual transformation of
Te-II into the structurally similar but incommensurately modulated Te-III.
Microscopically, this peak can only be reproduced within a sufficiently large
commensurate approximant, suggesting the low-energy optical response as a
fingerprint of the structural modulation.
