Monolithic integration of broadband optical isolators for polarization-diverse silicon photonics

Integrated optical isolators have been a longstanding challenge for photonic integrated circuits (PIC). An ideal integrated optical isolator for PIC should be made by a monolithic process, have a small footprint, exhibit broadband and polarization-diverse operation, and be compatible with multiple materials platforms. Despite significant progress, the optical isolators reported so far do not meet all these requirements. In this article we present monolithically integrated broadband magneto-optical isolators on silicon and silicon nitride (SiN) platforms operating for both TE and TM modes with record high performances, fulfilling all the essential characteristics for PIC applications. In particular, we demonstrate fully-TE broadband isolators by depositing high quality magneto-optical garnet thin films on the sidewalls of Si and SiN waveguides, a critical result for applications in TE-polarized on-chip lasers and amplifiers. This work demonstrates monolithic integration of high performance optical isolators on chip for polarization-diverse silicon photonic systems, enabling new pathways to impart nonreciprocal photonic functionality to a variety of integrated photonic devices.


Introduction
Nonreciprocal optical devices are essential for controlling the flow of light in photonic systems. These devices include optical isolators placed at the output of each laser to block back-reflected light and circulators to separate signals traveling in opposite directions. Achieving optical isolation on-chip by breaking optical reciprocity has been a major goal of the integrated photonics community. 1,2 An ideal integrated optical isolator should feature several important characteristics including: monolithic integration, high isolation ratio and low insertion loss, broadband operation, polarization diversity, and multi-material platform compatibility.
Achieving these functions in a photonic integrated circuit (PIC) is a critical challenge requiring device design combined with materials development and integration.
Several approaches have been made to achieve isolation, including the use of nonlinear effects 3,4 or active modulation of refractive index 5,6 , but passive devices based on magneto-optical (MO) effects are the most attractive solutions. MO devices may be based on mode conversion via the Faraday effect 7,8 as used in bulk isolators, but the birefringence of on-chip waveguides favors devices based instead on a nonreciprocal phase shift (NRPS) including ring resonators, multimode interferometers and Mach-Zehnder interferometers (MZIs) [9][10][11][12][13][14][15][16] . The best-performing MO materials in the near-IR communications band are yttrium iron garnets substituted with Bi or Ce to increase their Faraday rotation [17][18][19][20] . Integration of garnet into silicon PICs has been accomplished via wafer bonding 21 and via monolithic integration 18,20 .
Considerable progress has been made in both device design and materials development, primarily focused on transverse magnetic (TM) mode devices in which the garnet is placed on the top or bottom surface of the waveguide. Wafer-bonded TM ring resonator (RR) isolators exhibit isolation ratio up to 32 dB and insertion loss as low as 2.3 dB 12,13 , but with low isolation bandwidth. MZIs exhibit higher bandwidth, and TM MZI devices have been fabricated on single-crystal garnets 16 or by wafer bonding 14,15 (Table 1). However, on-chip lasers produce transverse electric (TE) light whose isolation requires symmetry breaking transverse to the waveguide 22 . TE isolation has been demonstrated by Faraday rotation 8 , by device fabrication on single crystal Ce:YIG 23 , and by combination of a TM isolator with mode converters 24-26 but these solutions are large in area, difficult to integrate, lossy due to extra polarization rotators, or require complex fabrication processes.
Here we address all the aforementioned requirements for practical on-chip optical isolation by demonstrating monolithically integrated magneto-optical isolators on silicon and silicon nitride (SiN) waveguides operating for both TE and TM modes with high isolation ratio, low insertion loss, small footprint and broadband optical isolation. We demonstrate the first fully-TE broadband isolator by depositing high quality magneto-optical garnet thin films on the sidewalls of silicon interferometer waveguides, and the multi-material platform compatibility of this technology by demonstrating the first monolithic optical isolator on SiN. Both TM and TE isolators show the best performance to date among broadband optical isolators on silicon, with optical isolation up to 30 dB and insertion loss as low as 5 dB. the windows is covered with the MO film (Fig. 1c), whereas for the TE devices the waveguide top surface is masked by SiO2 such that the film only deposits on one side of the waveguide (Fig. 1b). (The NRPS cancels out if the film is deposited on both sides of the waveguide.) When the film is magnetized under a unidirectional magnetic field, nonreciprocal phase shifts of opposite sign are induced in the two interferometer arms, leading to constructive (destructive) interference of forward (backward)

Device design and fabrication
propagating waves and optical isolation. The design therefore uniquely features a small footprint, large bandwidth, and compatibility with a simple unidirectional magnetization scheme. The simulated modal profile is shown in Fig. 1d  1b and 1c. The garnet thin films also exhibit excellent crystallinity and chemical homogeneity up to the Si/MO oxide interface for both devices, evidenced by high resolution tunneling electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analysis presented in Supplementary Fig. S1.    (cross-sectional SEM). It worth noting that unlike the TM resonator isolator design demonstrated previously 10 , the window can extend along the entire resonator without cancelling out NRPS as the magnetic field is applied along the out-of-plane direction.
In our SiN device, the window covers the resonator device except the coupling section to avoid changing the coupling condition to the bus waveguide.
Transmittance spectra of forward and backward propagation light are displayed in  The ring resonator device has a narrow bandwidth compared to MZI devices. The resonance frequency can be thermally tuned, and ring devices have been integrated in several applications such as optical communication modules, sensors and frequency combs. We also fabricated a MZI TE SiN device following a similar geometry to that of the Si TE MZI presented in Supplementary Fig. S5, showing that broadband SiN TE mode isolators are also possible. This isolator achieved a maximum isolation ratio of 18 dB, insertion loss of 10 dB, and 10 dB isolation bandwidth of 3 nm at 1581 nm wavelength.

Discussion
To benchmark the performance of our device,  and benefits from judicious control of the deposition oxygen partial pressure to drive higher Ce 3+ /Ce 4+ ratios ( Supplementary Fig. S2).  cladding. An additional 250 nm PECVD silicon oxide was further deposited onto the wafer to completely isolate the optical mode from interacting with Ce:YIG deposited in next steps. Next, a second EBL process using a positive resist (ZEP520A) was carried out to pattern the window regions. Finally, for TM devices, buffered oxide etch (BOE) was used to expose the silicon waveguide surface. For TE devices, RIE using a gas mixture of CHF3 and Ar ambient was applied to etch down silicon oxide top cladding and exposed one sidewall of the silicon waveguides. A piranha solution was used to clean the samples to remove any fluorinated polymer generated during the etching process. The as-fabricated devices were loaded into the PLD chamber for magneto-optical thin film deposition. Thin film deposition utilized a KrF excimer laser source which operates at 248 nm and at a repetition rate of 10 Hz. The fluence of the laser was determined to be 2.5 J/cm 2 . The distance between target and substrate was fixed at 5.5 cm. 50 nm thick YIG thin films were first deposited onto the substrate at 450 ºC and then rapid thermal annealed (RTA) at 900 ºC for 5 minutes for fully crystallization. Finally, 100 nm thick Ce:YIG thin films were deposited at 650 ºC onto the devices.

Device characterization:
The optical isolators were characterized on a fiber butt coupled waveguide test station. A LUNA Technology OVA 5000 was used to emit laser light from 1520 nm to 1610 nm. The transmitted light was then acquired by the OVA to analyze polarization dependent transmission spectra. In a different set-up, a free-space polarization control bench was used to obtain TE or TM polarized light before coupling to a polarization maintaining (PM) fiber. The linear polarized light was then butt coupled to the device for transmittance measurements with a lens tipped PM fiber. The testing methods were detailed in Supplementary materials. All devices were tested at least 3 times by reversing light propagation directions. The samples were maintained at room temperature with ± 0.2 ºC accuracy during the test.