It is an immense pleasure to be praised with the 2023 ELEX Best Paper Award for my research publication in the prestigious IEICE Electronics Express journal, entitled “Small magnetless integrated optical isolator using a magnetized cobalt ferrite film”. I would like to briefly introduce the background of this research here.
Optical isolators are nonreciprocal optical devices that allow light propagation in a single direction, preventing unwanted backward light reflections in photonic circuits. These devices typically consist of magnetized bulky magneto-optical (MO) materials to break Lorentz reciprocity and achieve optical isolation. An integrated optical isolator on Silicon photonic integrated circuits (Si-PICs) is highly desired for the optimal operation of active devices such as integrated lasers.
Great efforts have been made to integrate traditional MO materials such yttrium iron garnet (YIG) on Si platforms, but crystal growth of garnets on silicon is challenging due to their material discrepancy. So far, Si optical isolators based on YIG have been achieved by heterogeneous integration methods such as direct bonding [1], adhesive bonding [2], or micro transfer printing [3]. Isolators using polycrystalline YIG deposition on silicon have also been demonstrated [4], but the lower MO material quality and the required high temperature process are a concern for practical application on Si-PICs.
In order to solve the Si integrated isolator problem, we propose the use of cobalt ferrite (CFO) as novel MO material for the realization of optical isolators. The material discrepancy between CFO and Si can be solved using an appropriate buffer layer such as magnesium oxide. In addition, The Faraday rotation coefficient (FR) at 1,550 nm wavelength of cobalt ferrite is multiple times larger than that of Ce:YIG, permitting the realization of reduced footprint nonreciprocal devices. However, the large optical absorption at telecom wavelengths of spinel materials is a performance-limiting factor for practical devices. In this research, we demonstrate a compact and monolithically integrated optical isolator on a silicon-on-insulator (SOI) substrate. This isolator uses a self-magnetized CFO film deposited by a sputtering method on a Si racetrack micro-ring waveguide structure with a reduced footprint of only 40 x 80 um. The CFO film is self-biased by its own remanent magnetic field, allowing operation without an external magnet, reducing device bulk, and improving fabrication throughput. Isolation operation near 1550 nm wavelength with an isolation ratio of 9.6 dB was achieved.
References：
[1] Shoji, Y., et al., Applied Physics Letters, 92(7), 071117 (2008).
[2] Ghosh, S., et al., Optics Express, 20(2), 1839 (2012).
[3] Minemura, D., et al., Optics Express, 31(17), 27821 (2023).
[4] Bi, L., et al., Nature Photonics, 5(12), 758–762 (2011).
[5] Serrano-Núñez, et al., Applied Physics Express, 13(6), 062002 (2020).
Biography：
Mario Serrano received his Ph.D. degree in Electronic and Electrical Engineering from Tokyo Institute of Technology in 2022. His Ph.D. degree research was focused on the development and integration on Silicon Photonics of cobalt ferrite films with high Faraday rotation. In 2022, he joined Fujitsu Optical Components Ltd, where his research interests are ultra high-speed germanium photodiodes on silicon photonics, and integrated silicon modulators based on large electro-optic effect materials.