Monolithically Integrable Semiconductor Waveguide Optical Isolators using III-V Semiconductor / Ferromagnet Hybrid Structures

Synthesis of III-V Semiconductor / Ferromagnetic metal or semiconductor hybrid structures is one of the hot topics in "semiconductor spintronics". Semiconductor waveguide optical isolators are ones of the most promising applications of Ill-V semiconductor / ferromagnet hybrid systems, which combine optical nonreciprocal property by ferromagnetic metals and light emission / amplification characteristics by Ill-V optoelectronics. Although free space optical isolators using ferrimagnetic garnets are commercially available with high performance and low price, they cannot be monolithically integrated with semiconductor laser diodes due to their incompatibility in material and structure with Ill-V optoelectronic devices. To realize monolithically integrable optical isolators, we have proposed semiconductor waveguide optical isolators based on the nonreciprocal loss shift. The semiconductor waveguide optical isolators based on the nonreciprocal loss shift are composed of semiconductor optical amplifier (SOA) waveguides and ferromagnetic metals. The ferromagnetic metal provides the nonreciprocal loss and the SOA compensates the forward propagation loss from the ferromagnetic metal as schematically shown in Fig. 1 [1]. Because the principle of this novel waveguide optical isolator is completely different from that of conventional free space optical isolators based on Faraday rotation, polarizers are not necessary. This is a great advantage over conventional free space optical isolators, and allows monolithic integration with edge emitting semiconductor lasers. We experimentally demonstrated TE mode semiconductor active waveguide optical isolators with ferromagnetic metal Fe at A= 1550nm. To achieve TE mode nonreciprocal loss shift, the magnetization vector of the ferromagnetic metal Fe is aligned parallel to the magnetic field vectorH of the TE mode light, perpendicular to both the waveguide and the substrate [2]. Therefore, we deposited Fe thin films on one of the InGaAsP SOA waveguide sidewalls by an electron-beam evaporator with substrates tilted, as shown in a cross-sectional image of Fig. 2. Fig. 3 shows the nonreciprocal propagation characteristics of the fabricated device of 0.7mm long with cleaved facets under a fixed permanent magnetic field 0.1 T. Here, the bias current of the SOA is 1OOmA. The devices were kept at 10°C. The single mode tunable laser diode light was of wavelength 15301560nm, intensity 5dBm, and coupled in and out of the device through lensed optical fibers. In the TE mode, the propagation light intensity was altered by a difference of 14.7dB/mm between the forward and the backward traveling light. However, the intensity change was very small (1dB) for TM mode. Because this device operates TE mode, this polarization dependence is a clear evidence of the nonreciprocal loss shift shown in Fig. 1. Fig. 4 shows the wavelength dependence of the TE mode propagation intensity and isolation ratio at a 1OOmA bias. Greater than 1 0dB/mm nonreciprocal attenuation was demonstrated over the entire wavelength range of 1530-60nm. To realize polarization insensitive waveguide optical isolators, it is necessary to realize and combine TE and TM mode nonreciprocal propagations. We also demonstrated TM mode semiconductor active waveguide optical isolators. In TM mode semiconductor waveguide optical isolators, ferromagnetic metals work as not only magneto-optical materials but also top electrodes. We demonstrated 8.3dB/mm isolation at A=1540nm in InGaAlAs SOA waveguides with epitaxially grown MnAs ferromagnetic electrodes as shown in Fig. 5 and 6. By using molecular-beam epitaxy (MBE) grown MnAs electrode, we successfully realized thermodynamically stable ferromagnetic metal electrode / p+InGaAsP interfaces with low contact resistances, and the isolation ratio and the insertion loss were improved compared with the devices with Ni/Fe polycrystalline electrodes [3]. This work was partially supported by Industrial Technology Research Grant Program in 2005 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. [1] M. Takenaka et al., 1 1th Intl. Conf. Indium Phosphide and related materials, 289, (1999). [2] H. Shimizu et al., Jpn. J. Appl. Phys., 43, L1561, (2004). [3] T. Amemiya et al., 17th Intl. Conf. Indium Phosphide and related materials, TP-41, (2005).