On-chip optical isolation in monolithically integrated non-reciprocal optical resonators

Non-reciprocal photonic devices, including optical isolators and circulators, are indispensible components in optical communication systems. However, the integration of such devices on semiconductor platforms has been challenging because of material incompatibilities between semiconductors and magneto-optical materials that necessitate wafer bonding, and because of the large footprint of isolator designs. Here, we report the first monolithically integrated magneto-optical isolator on silicon. Using a non-reciprocal optical resonator on an silicon-on-insulator substrate, we demonstrate unidirectional optical transmission with an isolation ratio up to 19.5 dB near the 1,550 nm telecommunication wavelength in a homogeneous external magnetic field. Our device has a small footprint that is 290 mm in length, significantly smaller than a conventional integrated optical isolator on a single crystal garnet substrate. This monolithically integrated non-reciprocal optical resonator may serve as a fundamental building block in a variety of ultracompact silicon photonic devices including optical isolators and circulators, enabling future low-cost, large-scale integration. Non-reciprocal photonic devices that break the time-reversal symmetry of light propagation provide critical functionalities such as optical isolation and circulation in photonic systems. Although widely used in optical communications, such devices are still lacking in semiconductor integrated photonic systems1,2 because of challenges in both materials integration and device design. On the materials side, magneto-optical garnets used in discrete nonreciprocal photonic devices show large lattice and thermal mismatch with semiconductor substrates, making it difficult to achieve monolithic integration of garnets with phase purity, high Faraday rotation and low transmission loss3,4, and requiring wafer bonding to incorporate them on a semiconductor platform. On the device side, non-reciprocal mode conversion (NRMC) and non-reciprocal phase shift (NRPS) integrated optical isolators have large footprints with length scales from millimetres to centimetres5,6, which severely limits the feasibility of large-scale and low-cost integration. Efforts have been pursued both in the monolithic integration of iron garnet and the exploration of other magneto-optical materials with better semiconductor compatibility. Polycrystalline Y3Fe5O12 (YIG) films 3, epitaxial Sr(Ti1–xFex)O3–d (ref. 7), Sr(Ti1–xCox)O3–d (ref. 8) and Fe-doped InP films 9 have been demonstrated to have promising magneto-optical performance at a wavelength of 1,550 nm. In relation to device design, several monolithic non-reciprocal photonic devices capitalizing on optical resonance effects (for example, magneto-optical photonic crystals10, garnet thin-film based optical resonators11, silicon ring resonators with magneto-optical polymer cladding12 and modulated ring resonators using non-reciprocal photonic transitions1) have been theoretically analysed with a view to reducing the device footprint. However, the experimental realization of monolithic integrated devices on silicon has not been demonstrated so far due to material and fabrication difficulties. Currently, the only experimentally demonstrated optical isolators on silicon still rely on wafer bonding of Ce-doped yttrium iron garnet films grown on garnet single crystals to a silicon-on-insulator (SOI) Mach–Zehnder structure13 or to an SOI ring resonator14. The hybrid integrated Mach–Zehnder device has a transverse-magnetic (TM) mode isolation ratio of 21 dB and insertion loss of 8 dB (ref. 13) (with a device length of 2 mm), and a very recently reported ring resonator has an isolation ratio of 9 dB and insertion loss of 18 dB with a resonator diameter of 1.8 mm (ref. 14). Both devices featured non-uniform magnetic fields provided by bulk magnets. Compared to the hybrid solution, on-chip monolithic integration of non-reciprocal photonic devices offers high throughput, high yield, low cost and large scale, and thus has been sought for integrated photonic platforms for many years. In this Letter, we report the first monolithically integrated optical isolator on an SOI platform. The device operates under a homogeneously applied magnetic field, and uses a design based on a patterned non-reciprocal optical resonator to significantly reduce the footprint15. This device combines three essential characteristics of an on-chip isolator: a monolithically integrated design, small footprint and good isolation performance. The device structure is shown in Fig. 1a. The isolator consists of a single-mode silicon racetrack resonator fabricated on an SOI wafer with a top cladding of 1-mm-thick SiO2. Part of the SiO2 top cladding is etched to form a ‘window’, which directly exposes the underlying silicon resonator waveguide surface. A magneto-optical film is subsequently deposited on the entire sample area without the need for etching. In this work, the film consisted of a polycrystalline garnet bilayer, (Ce1Y2)Fe5O12(80 nm)/Y3Fe5O12(20 nm). By using a two-step deposition method (see Methods) with a thin YIG buffer layer, we successfully obtained phase-pure polycrystalline (Ce1Y2)Fe5O12 (Ce:YIG) films on silicon16 (Supplementary Fig. S3) in which no crystalline material other than the garnet phase was present. The values of Faraday rotation for polycrystalline YIG and Ce:YIG films were þ1008 cm and 21,2638 cm, the saturation magnetizations were 130 e.m.u. cm and 120 e.m.u. cm, and the saturation fields were 100 Oe and 200 Oe, respectively. Because of the SiO2 cladding, the optical mode interacts with the magneto-optical film only at the window region, where the silicon channel waveguide with garnet top-cladding layer provides strong NRPS of the TM-polarized mode due to the large index contrast between silicon (3.48) and magnetic garnets ( 2.2) at 1,550 nm (ref. 17). Figure 1b shows a cross-sectional scanning electron microscopy (SEM) image of the fabricated device at the window section. When an in-plane homogeneous magnetic field is applied perpendicular to