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

Abstract

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 systems 1,2 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 loss 3,4 , 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 centimetres 5,6 , 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 Y 3 Fe 5 O 12 (YIG) films 3 , epitaxial Sr(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 crystals 10 , garnet thin-film based optical resonators 11 , silicon ring resonators with magneto-optical polymer cladding 12 and modulated ring resonators using non-reciprocal photonic transitions 1 ) have been theoretically analysed with a view to reducing the device footprint. However, the experimental realization of monolit...

Optimized Magneto-optical Isolator Designs Inspired by Seedlayer-Free Terbium Iron Garnets with Opposite Chirality

“…However, the results shown in Figure 1 (b) suggest that even a 20nm-thin seedlayer, as used in Ref. 28, reduces the net MO effect of the garnetcladded SOI waveguides by almost 30% (as seen in all of the curves by comparing 0 vs. 20nm seedlayer thickness). If a 45nm YIG seedlayer would allow complete crystallization of an ideal Ce:YIG cladding, thereby obtaining Faraday rotation as high as -4500 °/cm, 45nm would still separate the guided mode from the active cladding.…”

“…The opposite chiralities of TIG and Bi:TIG enables QPM to be extended into a novel "push-pull" garnet waveguide that provides much higher net Faraday rotation, as shown by the black line in Figure 5(b). Push/pull QPM isolators were also simulated using TIG and monolithically-integrated Ce:YIG, which has much higher reported values of Faraday rotation (-1263°/cm 28 and -3700°/cm 48 ). These devices had the same (1.6 µm x 0.8 µm) cross-sections.…”

“…Such a seedlayer therefore limits the amount of light that reaches the Ce:YIG. Unfortunately, even with the seedlayer, most researchers report lower MO values (eg: -1263°/cm) 28 for this integrated Ce:YIG than that of a Ce:YIG grown on doped GGG (gadolinium gallium garnet) substrate by sputter epitaxy (-4500°/cm) 18,31 . The usual explanation is incomplete crystallization.…”

“…The first monolithic Si-integrated isolator was achieved using was achieved using an SOI racetrack resonator with a Ce:YIG cladding 28 . However, monolithic integration of this doped garnet required a two-step deposition process in which an ultra-thin (≈ 20 nm) seedlayer of undoped YIG was grown and annealed prior to deposition of the doped layer 30 .…”

“…Some examples that highlight the race for improved device design are: a 4mm long Si interferometer with wafer-bonded Ce:YIG (IR=17dB) 18 , a 1.8mm (diameter) silicon ring with wafer-bonded Ce:YIG (IR=9dB) 19 and a 920µm long Si interferometer with adhesive-bonded Ce:YIG (IR=25dB) 22 . The shortest reported isolator device to date is a 290µm Si ring resonator with a sputtered Ce:YIG cladding (IR= 19.5dB) 28 . However, these NRPS devices only operate with one mode, typically transverse magnetic (TM) unless side-coating can be achieved 32 .…”