An integrated circuit is provided with a photonic device and a spot-size converter waveguide device integrated on a common substrate. The spot-size converter waveguide device provides for transformation between a larger spot-size and a smaller spot-size corresponding to the photonic device. The spot-size converter waveguide device includes at least one of a bottom mirror and top mirror, which provide highly-reflective lower and upper cladding, respectively, for vertical confinement of light propagating through the waveguide device. The top mirror overlies opposing sidewalls of the spot-converter waveguide device, which provide highly-reflective sidewall cladding for lateral confinement of light propagating through the waveguide device. Advantageously, the highly-reflective lower cladding provided by the bottom mirror limits optical loss of the waveguide device. Similarly, the highly-reflective upper cladding and sidewall cladding provided by the top mirror limits optical loss of the waveguide device.