  Plasmonic metasurfaces exhibiting collective responses known as surface
lattice resonances (SLRs) show potential for realizing tunable and flat
photonic components for wavelength-selective processes, including lasing and
optical nonlinearities. However, post-fabrication tuning of SLRs remains
challenging, limiting the applicability of SLR-based components. Here, we
demonstrate how the properties of high quality factor SLRs are easily modified
by breaking the symmetry of the nanoparticle surroundings. We break the
symmetry by changing the refractive index of the overlying immersion oil simply
by controlling the ambient temperature of the device. We show that already
modest temperature changes of 10{\deg}C can increase the quality factor of the
investigated SLR from 400 to 750. Our results demonstrate accurate and
reversible modification of the properties of the SLRs, paving the way towards
tunable SLR-based photonic devices. On a more general level, our results
demonstrate how symmetry breaking of the surrounding dielectric environment can
be utilized for efficient and potentially ultrafast modification of the SLR
properties.
