A composite oxide includes agglomerated particles which have an average particle diameter of 20 m or less, which are composed of a plurality of metallic element oxides being in form of fine particles having an average diameter of 50 nm or less, and which have a surface and an inner portion whose metallic element distributions differ with each other. The heatsink is in thermal communication with one or more solid state emitters, and may define a cup-like cavity containing a reflector. The light source may further comprise a plurality of light extractors configured to extract at least a portion of the first light upon establishing optical contact with the light guide.