  Phonon polaritons are promising for infrared applications due to a strong
light-matter coupling and subwavelength energy confinement they offer. Yet, the
spectral narrowness of the phonon bands and difficulty to tune the phonon
polariton properties hinder further progress in this field. SrTiO3 - a
prototype perovskite oxide - has recently attracted attention due to two
prominent far-infrared phonon polaritons bands, albeit without any tuning
reported so far. Here we show, using cryogenic infrared near-field microscopy,
that long-propagating surface phonon polaritons are present both in bare SrTiO3
and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas.
The presence of the two-dimensional electron gas increases dramatically the
thermal variation of the upper limit of the surface phonon polariton band due
to temperature dependent polaronic screening of the surface charge carriers.
Furthermore, we demonstrate a tunability of the upper surface phonon polariton
frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that
oxide interfaces are a new platform bridging unconventional electronics and
long-wavelength nanophotonics.
