An apparatus according to one embodiment includes a near field transducer positioned towards a media-facing surface side, a primary waveguide for delivering light to the near field transducer, a secondary waveguide positioned near the primary waveguide and configured for receiving light from a light source and transferring at least some of the light received thereby to the primary waveguide, and a gap layer positioned between the primary waveguide and the secondary waveguide, wherein the secondary waveguide includes an oxide of at least one of Ta, Ti, Zr, and Nb alloyed with an oxide of at least one of Si and Al. Additional embodiments are also disclosed.