  Femtosecond laser writing is a powerful technique that allows rapid and
cost-effective fabrication of photonic integrated circuits with unique
three-dimensional geometries. In particular, the possibility to reconfigure
such devices by thermo-optic phase shifters represents a paramount feature,
exploited to produce adaptive and programmable circuits. However, the
scalability is strongly limited by the flaws of current thermal phase shifters,
which require hundreds of milliwatts to operate and exhibit large thermal
crosstalk. In this work, thermally-insulating three-dimensional microstructures
are exploited to decrease the power needed to induce a 2{\pi} phase shift down
to 37 mW and to reduce the crosstalk to a few percent. Further improvement is
demonstrated when operating in vacuum, with sub-milliwatt power dissipation and
negligible crosstalk. These results pave the way towards the demonstration of
complex programmable integrated photonic circuits fabricated by femtosecond
laser writing, thus opening exciting perspectives in integrated quantum
photonics.
