Photonic Crystal Exhibits Enhanced Faraday Rotation

Daniel S. Burgess

Researchers at Royal Institute of Technology in Stockholm, Sweden, have reported a multilayer structure they call a magneto-optical photonic crystal that displays 140 percent greater Faraday rotation at 748 nm than a single-layer bismuth iron garnet film of the same thickness. They suggest that the new material may find applications in magnetic field sensors.

Sören Kahl, who was recently awarded a doctorate from the university and who performed the work with professor Alexander M. Grishin, explained that magneto-optical Faraday rotation is magnetically induced circular birefringence. As linearly polarized light passes through a crystal that is magnetized or that is in a magnetic field, the plane of polarization is rotated by an angle proportional to the optical path length of the light inside the crystal. The effect is used in photonics in optical isolators that permit the propagation of light in only one direction as well as in magnetic field sensors.

Current Faraday rotators, he said, are single crystals or epitaxial films. The single-crystal devices, however, are rather large, making their use in applications such as integrated optics difficult. And even the films display thicknesses on the order of 500 µm, so alternative material systems are desirable.

Kahl and Grishin therefore investigated the use of stacked films of iron garnets, specifically bismuth and yttrium iron garnets. Designed for use with 750-nm light, the stack featured four heteroepitaxial layers of 81-nm-thick yttrium iron garnet (YIG) atop 70-nm-thick bismuth iron garnet (BIG), a 279-nm-thick central layer of BIG, and four layers of BIG atop YIG. To fabricate the stack, they employed pulsed laser deposition using an LPX305i 248-nm KrF excimer laser from Lambda Physik.

The researchers measured Faraday rotation in the stack with a lab-built setup that incorporated a PC2000 fiber optic spectrometer made by Ocean Optics Inc. Compared with a BIG film of the same thickness, Faraday rotation increased from –2.6°/µm to –6.3°/µm.

Kahl explained that the magneto-optical photonic crystal achieves this enhanced Faraday rotation because the structure increases the effective optical path length of the central BIG layer. Light that enters the crystal is reflected back and forth from the YIG/BIG and BIG/YIG layers and through the BIG many times before it escapes. Each time it does so, the angle of rotation is increased.

Now that they have demonstrated that these materials are suitable for use in Faraday rotators, the question remains whether there are commercial applications that would benefit from their use. Kahl noted that the level of sophistication of the isolators in optical communications is so high that substantial optimization would be necessary before magneto-optical photonic crystals could compete on the market. Their use in magnetic-field sensors, however, may be realized in the near-term, he suggested.