  Heat transfer across interfaces of graphene and polar dielectrics (e.g. SiO2)
could be mediated by direct phonon coupling, as well as electronic coupling
with remote interfacial phonons (RIPs). To understand the relative contribution
of each component, we develop a new pump-probe technique, called
voltage-modulated thermoreflectance (VMTR), to accurately measure the change of
interfacial thermal conductance under an electrostatic field. We employed VMTR
on top gates of graphene field-effect transistors and find that the thermal
conductance of SiO2/graphene/SiO2 interfaces increases by up to {\Delta}G=0.8
MW m-2 K-1 under electrostatic fields of <0.2 V nm-1 . We propose two possible
explanations for the observed {\Delta}G. First, since the applied electrostatic
field induces charge carriers in graphene, our VMTR measurements could
originate from heat transfer between the charge carriers in graphene and RIPs
in SiO2. Second, the increase in heat conduction could be caused by better
conformity of graphene interfaces un-der electrostatic pressure exerted by the
induced charge carriers. Regardless of the origins of the observed {\Delta}G,
our VMTR measurements establish an upper limit for heat transfer from unbiased
graphene to SiO2 substrates via RIP scattering; i.e., only <2 % of the
interfacial heat transport is facilitated by RIP scattering even at a carrier
concentration of 4x10^12 cm-2.
