A planar lightwave circuit, PLC, that includes a first waveguide having a core, wherein the first waveguide includes: 1) a high-contrast-waveguide region comprising a first waveguide portion in which the core is a multi-layer core that collectively supports propagation of a light signal having a first mode-field profile, the multi-layer core having; a) a first layer comprising a first material, the first layer having a first thickness (tl); b) a second layer comprising the first material, the second layer having a second thickness that is greater than the first thickness; and c) a third layer that is between the first layer and the second layer, the third layer comprising a second material, wherein the third layer has a third thickness that is less than or equal to 500 nm; 2) a low-contrast waveguide region comprising a second waveguide portion in which the core is a single-layer core that includes the first layer and not the second layer, wherein the core in the second region supports propagation of the light signal such that it has a second mode-field profile; and 3) a spotsize converter that optically couples the first waveguide portion and the second waveguide portion, wherein the spotsize converter includes a transition region in which the core includes; a) the first layer; b) the second layer, wherein the second layer has a thickness that changes monotonically from the second thickness to extinction; and c) the third layer; wherein the transition region is between the first waveguide portion and the second waveguide portion, and wherein the transition region is operative for adiabatically transitioning the mode-field profile of the light signal between the first mode-field profile and the second mode-field profile; wherein the third layer enables optical coupling of the first layer and second layer in the first waveguide portion such that the first layer, second layer, and third layer collectively support propagation of the light signal such that it has the first mode-field profile; and the PLC further includes: 4) a phase shifter disposed on the first waveguide portion, the phase shifter including a first electrode layer, a second electrode layer, and a piezoelectric layer that is between the first electrode layer and the second electrode layer, wherein the phase shifter is operative for inducing stress in the first waveguide portion in response to a control signal. 1) a high-contrast-waveguide region comprising a first waveguide portion in which the core is a multi-layer core that collectively supports propagation of a light signal having a first mode-field profile, the multi-layer core having; a) a first layer comprising a first material, the first layer having a first thickness (tl); b) a second layer comprising the first material, the second layer having a second thickness that is greater than the first thickness; and c) a third layer that is between the first layer and the second layer, the third layer comprising a second material, wherein the third layer has a third thickness that is less than or equal to 500 nm; a) a first layer comprising a first material, the first layer having a first thickness (tl); b) a second layer comprising the first material, the second layer having a second thickness that is greater than the first thickness; and c) a third layer that is between the first layer and the second layer, the third layer comprising a second material, wherein the third layer has a third thickness that is less than or equal to 500 nm; 2) a low-contrast waveguide region comprising a second waveguide portion in which the core is a single-layer core that includes the first layer and not the second layer, wherein the core in the second region supports propagation of the light signal such that it has a second mode-field profile; and 3) a spotsize converter that optically couples the first waveguide portion and the second waveguide portion, wherein the spotsize converter includes a transition region in which the core includes; a) the first layer; b) the second layer, wherein the second layer has a thickness that changes monotonically from the second thickness to extinction; and c) the third layer; a) the first layer; b) the second layer, wherein the second layer has a thickness that changes monotonically from the second thickness to extinction; and c) the third layer;