An optical waveguide is constructed so as to comprise a non-solid core
surrounded by a solid-state material. The non-solid core has an index of
refraction which is lower than the index of refraction of the surrounding
solid state material, and light can be transmitted with a low loss
through the non-solid core. The non-solid core can extend through at
least one of multiple layers of the solid state material, wherein the
non-solid core is elevated on a substrate material above at least one
topmost layer of the multiple solid state layers lateral to the non-solid
core. In an exemplary application, the non-solid core comprises a sample
material whose light transmission, absorption, and/or interference
characteristics are to be measured.