  With Terahertz and Infrared spectroscopy we studied the in-plane response of
an underdoped YBa2Cu3O6.6 single crystal with Tc=58(1) K in high magnetic
fields up to B=30 Tesla applied along the c-axis. Our goal was to investigate
the field-induced suppression of superconductivity and to observe the
signatures of the three dimensional (3d) incommensurate copper charge density
wave (Cu-CDW) which was previously shown to develop at such high magnetic
fields. Our study confirms that a B-field in excess of 20 Tesla gives rise to a
full suppression of the macroscopic response of the superconducting condensate.
However, it reveals surprisingly weak signatures of the 3d Cu-CDW at high
magnetic fields. At 30 Tesla there is only a weak reduction of the spectral
weight of the Drude-response (by about 3%) that is accompanied by an
enhancement of two narrow electronic modes around 90 and 240 cm-1, that are
interpreted in terms of pinned phase modes of the CDW along the a- and
b-direction, respectively, and of the so-called mid-infrared (MIR) band. The
pinned phased modes and the MIR band are strong features already without
magnetic field which suggests that prominent but short-ranged and slowly
fluctuating (compared to the picosecond IR-time scale) CDW correlations exist
all along, i.e., even at zero magnetic field.
