Energy efficient thermo-optic phase shifters have a configuration with two sections of a waveguide adjacent for heating by a common heater. A loop section can connect the two heated waveguide sections. Further improved efficiency can be achieved in which the heated sections are curved to allow closer placement of the adjacent waveguides. The heater can be curved to follow the configuration of the curved heated waveguide sections. Energy efficiency gains can be up to approximately a factor of two over corresponding traditional thermo-optical phase shifter designs.