Mid-infrared waveguiding in three-dimensional microstructured optical waveguides fabricated by femtosecond-laser writing and phosphoric acid etching

We fabricated a three-dimensional microstructured optical waveguide(MOW) in a single-crystal using femtosecond-laser writing and phosphoric acid etching techniques, and observed excellent midinfrared waveguiding performance with low loss of ~0.5 d B∕cm. Tracks with a periodic arrangement were written inside the yttrium aluminum garnet(YAG) crystal via femtosecond laser inscription, and then etched by the phosphoric acid(HPO) to form hollow structures. The evolution of the microstructure of tracks was investigated in detail.The function of the MOW was analyzed by different numerical methods, indicating the proposed MOW can effectively operate in quasi-single-mode pattern in the midinfrared wavelength range, which agrees well with our experiment results.