A spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first propagation mode and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and comprising: a lower waveguiding structure having a first end and a second end opposed to the first end of the lower waveguiding structure, the first end of the lower waveguiding structure being configured to couple to the first waveguide and to receive light therefrom or transmit light thereto in the first propagation mode, the lower waveguiding structure being characterized by a first effective refractive index; an upper waveguiding structure having a first end and a second end opposed to the first end of the upper waveguiding structure, wherein the first end of the upper waveguiding structure is configured to couple to the second waveguide and to transmit light thereto or receive light therefrom in the second propagation mode, and wherein the upper waveguiding structure is characterized by a second effective refractive index; and wherein the upper waveguiding structure comprises a first plurality of high-index elements disposed within a cladding structure and extending substantially along the waveguiding axis, each of the first plurality of high-index elements having a refractive index higher than that of its surrounding environment; wherein the first plurality of high-index elements comprises an array of high-index elements arranged in at least two rows of multiple high index elements that are vertically spaced apart and above the lower waveguiding structure; wherein at least part of the upper waveguiding structure extends at least partially over the lower waveguiding structure so as to define a coupling region, wherein a crossing of the first effective refractive index and the second effective refractive index occurs, thereby enabling evanescent coupling of light between the lower waveguiding structure and the upper waveguiding structure. a lower waveguiding structure having a first end and a second end opposed to the first end of the lower waveguiding structure, the first end of the lower waveguiding structure being configured to couple to the first waveguide and to receive light therefrom or transmit light thereto in the first propagation mode, the lower waveguiding structure being characterized by a first effective refractive index; an upper waveguiding structure having a first end and a second end opposed to the first end of the upper waveguiding structure, wherein the first end of the upper waveguiding structure is configured to couple to the second waveguide and to transmit light thereto or receive light therefrom in the second propagation mode, and wherein the upper waveguiding structure is characterized by a second effective refractive index; and wherein the upper waveguiding structure comprises a first plurality of high-index elements disposed within a cladding structure and extending substantially along the waveguiding axis, each of the first plurality of high-index elements having a refractive index higher than that of its surrounding environment; wherein the first plurality of high-index elements comprises an array of high-index elements arranged in at least two rows of multiple high index elements that are vertically spaced apart and above the lower waveguiding structure; wherein at least part of the upper waveguiding structure extends at least partially over the lower waveguiding structure so as to define a coupling region, wherein a crossing of the first effective refractive index and the second effective refractive index occurs, thereby enabling evanescent coupling of light between the lower waveguiding structure and the upper waveguiding structure.