1. Introduction

In recent years, there have been significant advancements in the large-scale integration of photonic integrated circuits. However, achieving scalability with respect to their power consumption remains a challenge. The fundamental solution to this challenge is the implementation of nonvolatile photonic devices. In the field of photonics, a phase-change material (PCM) that uses the transition between crystal and amorphous states is a pioneering nonvolatile material [1,2]. Programmable photonic circuits and their applications to convolutional neural network have been reported [3,4]. Although their large optical absorption is being improved [5], they suffer from a limited number of repeatable changes owing to material degradation during the high-temperature process.

We have previously proposed and demonstrated nonvolatile photonic devices using two magnetic materials: magneto-optical (MO) garnet crystals and ferromagnetic metals [6,7]. MO garnet has low optical absorption and affects light owing to the MO effects. A ferromagnetic metal induces a static magnetic field owing to its remanence, which induces a magnetic field and maintains the magnetization of the MO garnet. Therefore, photonic devices are driven by the MO effect and exhibit nonvolatility. As ferromagnetic metals are widely used in magnetic random-access memories (MRAMs), many rapid repeatable magnetic switching operations are expected. Considering its application to all-optical memory [8], spatially separated reading and recording layers enable low-loss readout and efficient recording by light absorption. Cerium-substituted yttrium iron garnet (Ce:YIG) is an MO crystal with a large Faraday effect and a relatively low absorption loss at telecom wavelengths. However, it must be grown on a lattice-matched gadolinium gallium garnet substrate (SGGG) [9,10], which makes photonic integration with silicon difficult. Therefore, to date, we fabricated prototype MO switches using amorphous Si (a-Si) waveguides deposited on Ce:YIG grown on an SGGG substrate, as shown in Fig. 1(a). The first-order MO effect induces a reflectivity change at the interface with other materials and propagation constant change in planar waveguides. This effect depends on the direction of light propagation and magnetization of the MO material. An MO phase shifter is utilized for optical isolators and optical switches using a Mach–Zehnder interferometer (MZI) or microring resonator (MRR) [11–13]. A thin-film ferromagnetic metal magnet (CoFeB) was formed on the waveguide of the MO phase shifter. Nonvolatile operation with a high extinction ratio of ∼20 dB and 1-µs pulsed-current driving was experimentally demonstrated [7]. Direct deposition techniques for MO crystals on silicon have been developed, and have improved the performance, although high-temperature annealing is necessary [14,15]. These techniques are useful for flexible layouts of MO, ferromagnetic materials, and electrical wiring. High-speed switching and cryogenic applications are expected [16,17]. Magnetic nonvolatility becomes more stable at low temperatures.

 

Fig. 1. Cross section of MO phase shifter integrated ferromagnetic metal with (a) amorphous silicon waveguide formed on Ce:YIG/SGGG substrate, (b) Si waveguide with directly bonded Ce:YIG on SOI substrate, and (c) Si waveguide with directly bonded Ce:YIG on SOI substrate without electrode fabricated in this study.

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In this study, considering integration on silicon photonic platforms, a nonvolatile MO switch was fabricated and demonstrated on a silicon-on-insulator (SOI) substrate. Because Ce:YIG was directly bonded to the Si waveguide using a wafer-bonding technique [11], a thin-film magnet of CoFeB was placed beside the waveguide and beneath the Ce:YIG layer, as shown in Fig. 1(b). In addition, the device configuration for TE-TM half-mode conversion was employed to operate under a unidirectional magnetic field. In this study, an electrode (Ag) was not formed under CoFeB; however, to saturate the magnetization of CoFeB, a sufficient external magnetic field was applied using a permanent magnet as shown in Fig. 1(c).

2. Device structure

2.1. Operation principle

Instead of the MZI or MRR configuration, we employed a waveguide interferometer based on TE-TM half-mode conversion, as shown in Fig. 2, where a waveguide optical isolator was demonstrated on a Si platform [13]. The tapered multimode section excites the TE₀ and TE₁ modes equally, and the TE₁ mode is then converted into the TM mode. Thus, it is called a “half-mode converter.” In the central waveguide of the MO phase shifter, only the TM mode experiences an MO phase shift under the lateral magnetization of the Ce:YIG. The phase difference between the TE and TM modes determines the multimode interference between the TE₀ and TE₁ modes of the output half-mode converter. When the phase shifter length is designed so that there is a TM mode phase shift of ±π/2 for different magnetization directions, the output port of the light signal is selected by changing the magnetization. Compared with MZI and MRR, this configuration has two advantages in that it operates under a unidirectional magnetic field and a lower scattering loss for TE mode propagation at the boundary of the bonded Ce:YIG compared with that for the TM mode.

 

Fig. 2. Schematic of nonvolatile MO switch based on TE-TM half-mode conversion driven by magnetic field.

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2.2. Design of the thin-film magnet array

The MO-based nonvolatile optical switch requires high remanence to be robust to disturbance and relatively weak coercive force to be easily flip magnetization with low power current. CoFeB is a popular ferromagnetic material for MRAM and other spintronic devices which meets the requirements and a nanosecond-order fast switching speed is expected. It was deposited using a radio-frequency (RF) sputtering system. The magnetic characteristics of shaped magnets with various dimensions were characterized using a vibrating sample magnetometer, as shown in Fig. 3. We found that a thin-film magnet with a size of 4 µm × 20 µm × 75 nm shows higher remanence. The one with a width of 3 µm also shows high remanence, whereas the coercive force becomes high, which is unfavorable for low-power switching. Therefore, this dimension is the minimum to have anisotropy in lateral direction and generate a certain level of magnetic field externally.

 

Fig. 3. Measured magnetic characteristics of CoFeB thin-film magnet for various dimensions. Vertical axis illustrates magnetization normalized by the saturated magnetization (M_S) of ∼20 kG.

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The thin-film magnet was designed to provide a magnetic field of more than 100 Oe just above the waveguide, which is sufficient to saturate the Ce:YIG magnetization [18]. The required direction of magnetization is orthogonal to the direction of light propagation. A magnetic material easily aligns its magnetization along the longitudinal direction of its shape via structural anisotropy. Therefore, the thin-film magnet was separated into a rectangular shape along the MO phase shifter and arrayed on both sides of the Si waveguide. The magnet array was placed 2-µm away from the waveguide to prevent optical absorption. The arrayed gap between adjacent magnets was set to 0.7 µm, which was determined by our photolithographic fabrication accuracy and lift-off process. Because the gap causes a non-uniform magnetic field distribution along the waveguide direction, the magnet arrays were shifted on each side and a uniform distribution was expected from electromagnetic analysis with COMSOL Multiphysics, as shown in Fig. 4(a). As a result, dimensions and geometries of CoFeB array and waveguide were determined as shown in Fig. 4(b).

 

Fig. 4. (a) Magnetic field distribution along the waveguide for the placement of arrayed magnets on both sides. x-z is the plane of the photonic circuit. Red and orange rectangles indicate the waveguide and CoFeB, respectively. (b) Dimensions and geometries of CoFeB array and waveguide optimized in this study.

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2.3. Design of the MO switch

The length of the MO phase shifter was designed to be 510 µm from an MO phase shift of 6.2 mm⁻¹ for the TM mode assuming a saturated magnetization of Ce:YIG. The half-mode converters have two step-tapered waveguides, as shown in Fig. 5, starting from a 1.55-µm width, which excites the TE₀ and TE₁ modes, and then down to widths of 0.975-µm and 0.775-µm along 15-µm and 150-µm tapers, respectively, to efficiently convert the TE₁ mode into the TM₀ mode. The design methodology is described in Ref. [13]. The total device size was ∼950 µm including the input/output bending sections. It should be noted that the half-mode converter was covered by Ce:YIG because it required an asymmetric structure in the vertical direction for TE₁ and TM₀ mode conversion. The thickness of Ce:YIG grown on SGGG is ∼500 nm, for which the performance of the MO phase shifter and half-mode converter is enough insensitive.

 

Fig. 5. Schematic of TE-TM half-mode converter.

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3. Characterization

3.1. Fabrication

Si waveguides were formed on an SOI wafer with 220-nm-thick Si and 3-µm-thick buried oxide layers using electron-beam (EB) lithography and reactive-ion etching. The EB resist of ZEP-520A was positive type and patterned on the side-cladding regions of the waveguide. In the MO phase shifter, a wider area of the Si layer was removed and a CoFeB thin-film magnet was formed by photolithography and RF sputtering deposition. Ce:YIG film was grown on 300-µm thick SGGG wafer. Finally, a 1.5-mm-square Ce:YIG die was directly bonded to the entire circuit region using surface-activated bonding with oxygen plasma treatment. The strong bonding was achieved at an annealing temperature of 200 °C for 30 min. Because CoFeB has a thickness that is 75 nm lower than that of the Si layer, it did not hinder die-scale bonding. Figure 6(a) shows a microphotograph of the fabricated device.

 

Fig. 6. (a) Fabricated MO switch with thin-film magnet array on both sides of Si waveguide and Ce:YIG directly bonded as an upper cladding layer. (b) Measured transmission spectra after magnetizing the thin-film magnet.

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3.2 Measurement of optical switching

Broadband light around the C-L band wavelengths from the amplified spontaneous emission source was input to the fabricated device, and the output transmission spectra were measured using a spectrum analyzer. The magnetization of the thin-film magnet was in the + H or -H directions, as depicted in Fig. 6(a), by an external magnetic field using a permanent magnet. Figure 6(b) shows the measured transmittances after magnetization and removal of the permanent magnet. Therefore, the spectral change owing to the MO phase shift is attributed to the remanence of the thin-film magnet. A maximum extinction ratio of 19.9 dB was obtained at a wavelength of 1575.3 nm.

The transmittance of a straight waveguide adjacent to the device propagating under the bonded Ce:YIG region, but was not surrounded by CoFeB thin-film magnets, was measured as a reference. The breakdown of the ∼43-dB loss would be the 1 dB propagation loss of the waveguide, a 2.3-dB absorption loss of the MO material, and a coupling loss of ∼20 dB between the fiber and device at each facet. The absorption loss was calculated from the propagation losses of TE and TM modes considering different mixture ratio and different optical confinement factors of TE and TM modes along each section with the material loss of ${\alpha _{\textrm{Ce}:\textrm{YIG}}} = 40\; \textrm{dB}/\textrm{cm}$. The difference between the reference waveguide and a device peak of ∼3.4 dB corresponded to the excess loss of the device. Consequently, the insertion of the MO switch, excluding the fiber-coupling loss, was estimated to be 6.7 dB. The junction loss at the boundary between the Ce:YIG and air cladding regions of 0.2 dB was estimated for TE mode propagation with a Lumerical FDTD simulation.

4. Conclusions

A nonvolatile MO switch was demonstrated on a silicon photonic platform. Thin-film magnets of CoFeB have the magnetization of Ce:YIG and consequently provide nonvolatility. The wafer-bonding technique enables the integration of good-quality MO crystal films during low-temperature annealing; therefore, it is compatible with the back-end-of-line process.

In this study, the CoFeB magnetization was controlled using an external magnetic field. This control can be achieved by a current-induced magnetic field when electrodes are formed beneath the CoFeB. The additional bonding area can be reduced by improving the manipulation method for the small Ce:YIG die or by developing a transfer printing technique for thin-film MO crystals [19,20]. By introducing tapered Ce:YIG pattern, not only absorption loss due to extra bonding region but also junction loss are reduced. Excess loss of the half-mode converter can be reduced if tapered structures are installed at the input waveguide connections. The schematic of future prospected structure is shown in Fig. 7. The insertion loss will be improved to be ∼2 dB. The operation bandwidth is dominated by the wavelength dependence of phase difference between TE and TM modes in the MO phase shifter. It can be wider if the dispersions of both modes are adjusted by changing the waveguide geometries. Applications to photonic neural networks, which have already been investigated with PCMs [3,21], can be explored when the magnetization is controlled by a light pulse, as is the case with optical memory [8].

 

Fig. 7. Future prospected nonvolatile MO switch driven by electric current. Silver electrode is formed beneath CoFeB magnet array. Ce:YIG/SGGG thin-film sheet with narrow taper is bonded by a µ-transfer printing technique. Tapers are installed at the input waveguide connections.

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