Wavelength division multiplexing filters including a subwavelength grating

A method of forming a structure for a wavelength division multiplexing filter, the method comprising:

• forming a first waveguide core including a first section and a second section, wherein the first section and the second section each have a first notched sidewall and a second notched sidewall opposite to the first notched sidewall;
• and
• forming a second waveguide core positioned with a first offset in a first direction relative to the first section and the second section of the first waveguide core and with a second offset in a second direction relative to the first section and the second section of the first waveguide core, wherein the second direction is transverse to the first direction,
• wherein the first waveguide core has a longitudinal axis, the first section of the first waveguide core includes a first taper and a first plurality of projections that project outwardly from the first taper transverse to the longitudinal axis, the second section of the second waveguide core includes a second taper and a second plurality of projections that project outwardly from the second taper transverse to the longitudinal axis, the first taper is adjoined to the second taper, and the first taper and the second taper narrow in opposite directions along the longitudinal axis.