  The dielectric permittivity of a material encapsulates the essential physics
of light-matter interaction into the material's local response to optical
excitation. Dynamic, photo-induced modulation of the permittivity can enable an
unprecedented level of control over the phase, amplitude, and polarization of
light. Therefore, the detailed dynamic characterization of technology-relevant
materials with substantially tunable optical properties and fast response times
is a crucial step in the realization of tunable optical devices. This work
reports on the extraordinarily large permittivity changes in zinc oxide thin
films (up to -3.6 relative change in the real part of the dielectric
permittivity at 1600 nm wavelength) induced by optically generated free
carriers. We demonstrate broadband reflectance modulation up to 70 percent in
metal-backed oxide mirrors at the telecommunication wavelengths, with
picosecond-scale relaxation times. The epsilon near zero points of the films
can be dynamically shifted from 8.5 microns to 1.6 microns by controlling the
pump fluence. Finally, we show that the modulation can be selectively enhanced
at specific wavelengths employing metal-backed ZnO disks while maintaining
picosecond-scale switching times. This work provides insights into the
free-carrier assisted permittivity modulation in zinc oxide and could enable
the realization of novel dynamic devices for beam-steering, polarizers, and
spatial light modulators.

Optics;Materials Science