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Abstract
Optical isolators provide one-way propagation and are necessary to protect laser diodes from damage and unstable operation caused by reflected light. Although magneto-optical (MO) devices can operate as isolators, achieving high-density integration using conventional direct bonding methods is difficult because a large and thick growth substrate remains on the circuits. We experimentally demonstrated a compact Mach–Zehnder interferometer-based MO isolator with Si waveguides by the µ-transfer printing of a Ce:YIG/SGGG coupon. The isolator has a footprint of 0.25 mm2 with a Ce:YIG/SGGG coupon of 50 × 800 µm2 and ∼ 1-µm thickness and achieved a maximum isolation ratio of 14 dB in telecom bands.
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1. Introduction
Communication traffic and power consumption have increased rapidly with the growth of smartphones, social networks, and cloud services. Various optical devices have been studied worldwide for constructing next-generation photonic networks. These devices must have low power consumption, high capacity, small size, high-density integration, high speed, and mass production. Silicon (Si) photonics have attracted considerable attention because of their dense and low-cost photonic integrated circuits (PICs).
Optical isolators provide one-way propagation and are necessary to protect laser diodes from damage and unstable operation caused by reflected light [1]. Bulk-type isolators are commercially available but are too large and difficult to integrate into PICs. Therefore, various waveguide-type optical isolators that can be integrated into PICs have been developed [2–7]. Magneto-optical (MO) devices enable nonreciprocal, broadband, passive operation [8]. Various structures of MO devices have been studied, such as Mach–Zehnder interferometers (MZIs), micro-ring resonators (MRR), multimode interferometers (MMIs) and 1-dimensional (1D) isolators based on Faraday rotation (FR). MRR enables compact footprint and low insertion loss while it requires strict design for critical coupling and have a narrow operating bandwidth [9–11]. MMI structures should be optimized considering the different propagation losses of transverse electric (TE) and transverse magnetic (TM) modes [12]. C. Zhang et al. demonstrated a 1D FR isolator that enables TE mode operation, but with a footprint of 4 mm × ∼1 µm, which is larger than other structures [13]. MZI isolators shows high isolation in a wide bandwidth with a small phase shifter [14–18]. Among the MO materials, cerium-substituted yttrium iron garnet Ce1Y2Fe5O12 (Ce:YIG) provides a large Faraday rotation coefficient of −4500 deg/cm at a wavelength of 1500 nm and low optical loss in the near-infrared region [2,19,20]. However, it is difficult to crystallize on Si and other semiconductor substrates. Although a direct deposition method on a Si substrate has been studied to realize poly-crystalline Ce:YIG on a YIG seed layer [10,21], it is difficult to install it in the front-end process because of its high growth temperature. Therefore, we developed a direct bonding technology in which single-crystalline Ce:YIG grown on (111)-oriented substituted gadolinium gallium garnet (GdCa)3(GaMgZr)5O12 (SGGG) is bonded to Si waveguides. After Y. Shoji et al. first reported an MZI-based MO isolator [14], we have reported MZI-based MO isolators with higher isolation, wider bandwidth, temperature-insensitive operation, and mode TE-TM converters [15–18]. Furthermore, we recently demonstrated a polarization-independent MO isolator [22]. Owing to its process temperature of ∼200°C, it can be fabricated in the back-end-of-line process. However, it is difficult to achieve high-density integration because a large and thick growth substrate of SGGG (∼mm2 × ∼300 µm) remains on the circuits.
Here, we focus on the µ-transfer printing (µ-TP) technology [23–25], which is a technology for making coupons of thin-film materials and printing them onto substrates with high accuracy. The pick-up and printing can be controlled by changing the removal speed of the silicone elastomer polymer film. This technology has been intensively developed for the heterogeneous integration of III-V semiconductor devices on Si photonic platforms [26–29]. While wafer-to-wafer and wafer-to-die bonding cause damage and stress owing to the thermal expansion mismatch between the target and hybrid materials [30], the µ-TP technology can reduce these issues because of its limited area and extremely thin coupons. This technology has attracted attention for various types of heterogeneous integration because it can minimize the footprint and amount of hybrid materials [31,32]. Other heterogeneous integration methods are remote epitaxy and diffusion-driven exfoliation [33,34]. These technologies are very consistent with µ-TP technology. However, as far as the Ref. [34] is concerned, there is still room for improvement in the exfoliation area, shape and crystal quality. In this study, we apply the µ-TP technology to integrate MO materials on Si waveguides. We successfully defined the micro-coupon structure of Ce:YIG/SGGG and demonstrated a compact MZI-based MO isolator with Si waveguides by the µ-TP.
2. Operation principle
Figure 1 shows a schematic of an MZI-based MO-isolator with 220-nm-thick and 440-nm-width Si (n = 3.48) waveguides on a 3-µm-thick SiO2 (n = 1.444) substrate and partially covered with a Ce:YIG/SGGG coupon. The 3-dB coupler divides the TM mode light equally and eliminates the TE mode light; the length and width were designed to be 5 µm and 360 nm, respectively. Nonreciprocal phase shift (NRPS) is induced by the MO effect of the Ce:YIG/SGGG coupon when external magnetic fields are applied transversely for light propagation. To realize a nonreciprocal phase difference between the two arms, a Ce:YIG/SGGG coupon is bonded to the waveguides with opposite propagation directions. In addition, the two arms are slightly asymmetric according to the length of the reciprocal phase shifter. This optical path length difference between two arms causes the reciprocal phase shift (RPS), and it does not depend on the direction of the magnetic field or the light propagation. When light propagates along the waveguide where Ce:YIG is magnetized, the propagation constant changed by the NRPS is calculated as
Fig. 1. (a) Top view and (b) cross section of a schematic of an MZI-based MO-isolator by µ-TP of a Ce:YIG/SGGG coupon.
3. Fabrication of MO isolator
Before µ-TP process, the Si waveguides were fabricated on a silicon-on-insulator (SOI) wafer. First, a 200-nm-thick SiO2 layer was deposited on an SOI wafer with a 220-nm-thick Si top layer using plasma chemical vapor deposition. Next, the ZEP520A resist was spin-coated and patterned using electron-beam (EB) lithography. The SiO2 and Si layers were then etched using CF4 and SF6 reactive-ion etching, respectively. Finally, the SiO2 mask was removed using dilute hydrofluoric acid (DHF) solution.
Figure 2 shows a schematic of µ-TP process. First, a 500-nm-thick single-crystalline Ce:YIG was grown on a 300-µm-thick SGGG by radio frequency sputtering method at 750°C. Second, the SiO2-deposited Ce:YIG/SGGG wafer was bonded upside down on a SiO2/Si handle wafer via SiO2-SiO2 surface-activated bonding at room temperature (Fig. 2(a)). Then, the back side of the SGGG was thinned from 300 µm to ∼1 µm (Fig. 2(b)). This thinning process had two steps; (1) from 300 µm to 2∼3 µm by backside grinding and chemical mechanical polishing (CMP), (2) from 2∼3 µm to ∼1 µm by ion milling with Ar ion beam irradiation. Then, photoresist was coated with thickness of ∼2 µm and coupon patterns were formed by EB photolithography (Fig. 2(c)). Subsequently, the Ce:YIG/SGGG was etched and grooves around the coupon patterns were formed by ion milling with Ar ion beam irradiation (Fig. 2(d)).
Then, free-standing structures of the Ce:YIG/SGGG coupon were formed by removing the sacrificial layer of SiO2 under the coupon using DHF. In this step, the coupon was still connected and supported by the tethers (Fig. 2(e)). To prevent the coupon from falling and sticking to the bottom Si handle substrate due to the surface tension of the rinse water, the coupons were dried by supercritical drying with liquid CO2. Figure 3(a) shows a scanning electron microscope (SEM) image of the fabricated free-standing Ce:YIG/SGGG coupon pattern.
Fig. 3. (a) SEM image of a Ce:YIG/SGGG coupon. (b) Microscope image of fabricated MZI-based MO-isolator by µ-TP.
The Ce:YIG/SGGG coupon was picked up using an as-patterned polydimethylsiloxane (PDMS) stamp with soft-nanoimprinting lithography (Fig. 2(f)). Then, the Ce:YIG/SGGG coupon was transfer-printed onto the Si waveguides after an O2 plasma hydrophilization treatment for both sample surfaces (Fig. 2(g)). Alignment was performed by a zoom optical microscope and a visible camera. Alignment accuracy is already evaluated at ±410 nm ($3\mathrm{\sigma }$) the details of alignment and transfer process are described in our paper [28]. This value is enough for our device that does not require an optical interlayer coupling structure. To strengthen the bonding, the chip was heated at 140°C for 5 min. Figure 3(b) shows a microscopic image of the fabricated MO isolator with a 50 × 800 µm2 Ce:YIG/SGGG coupon.
4. Device characterization
4.1. Bonding interface
Figures 4(a)–(c) show scanning transmission electron microscopy (STEM) images of the cross sections of the Si waveguide with Ce:YIG/SGGG bonded by µ-TP. The buried resin on both sides of the waveguide was used for mechanical support of the sliced sample during the STEM observation. The interlayer thickness between the Ce:YIG and Si was approximately 10 nm. Energy dispersive X-ray spectroscopy (EDX) was performed for elemental analysis. Figures 4(d) and 4(e) show the EDX spectra of the upper cladding and interlayer, respectively. Molybdenum (Mo) was obtained from a STEM observation sample. According to the STEM results, the upper cladding was Ce1Y2Fe5O12 (Ce:YIG), as expected, and the interlayer was found to be SiO2. EDX was performed along the arrow in Fig. 4(c). Figures 4(f) and 4(g) show the elemental distributions along the vertical axis, and Fig. 4(g) shows a superimposition of the STEM image. From these results, the elements in the upper cross section of the Si waveguide were Ce:YIG/SiO2(10 nm)/Si from the top. The SiO2-based interlayer was formed by O2 plasma treatment [35].
Fig. 4. (a)–(c) STEM images of the cross sections of the Si waveguide with Ce:YIG/SGGG bonded by µ-TP. EDX spectra of (d) Ce:YIG and (e) interlayer between Ce:YIG and Si waveguide. (f) The elemental distribution from top Ce:YIG to bottom Si waveguide. (g) Superimposition with the STEM image and the element distribution.
4.2. Optical measurement
The fabricated MO isolator was characterized by measuring its transmission spectra. The TM input light from the amplified spontaneous emission light source was injected into the center port of the device using a focusing-lens module. The transmitted light output from the center port of the device was coupled to another lens module and measured using a spectrum analyzer. The Ce:YIG was magnetized transverse to light propagation by applying an external magnetic field using a permanent magnet, as shown in Fig. 5. Figure 6 shows the measured fiber-to-fiber transmittances. First, we measured a reference MZI without a Ce:YIG layer (w/o Ce:YIG). The fabricated MO isolator was measured without an external magnetic field (w/ Ce:YIG, $H = 0$). Subsequently, the Ce:YIG was magnetized from left to right and right to left, as shown in Fig. 3(b), using an external magnet and measured (w/ Ce:YIG, $H + $ and w/ Ce:YIG, $H - $, respectively). Here, we adopted longer ${L_{\textrm{RPS}}}$ intentionally so that a few resonant peaks could be observed in the wavelength range. The wavelength range between adjacent peak and bottom corresponds to the π phase shift due to the wavelength dependence of reciprocal phase shifter. The transmittance changed depending on the direction of the magnetic field, which corresponds to the forward and backward propagation of the isolator. If π phase shift for the nonreciprocal phase shift (${\theta _{\textrm{NRPS}}} ={\pm} \mathrm{\pi }/2$) were achieved for the opposite directions of the magnetic field, peak of the forward propagation and bottom of the backward propagations would be coincident. A maximum isolation ratio (IR) of 14 dB was obtained at a wavelength of 1567.1 nm. The wavelength shift was 6.5 nm which corresponds to 56.5% of the π phase shift between the two directions. The additional loss caused by µ-TP was 11 dB obtained from the difference between the transmittance peaks without and with µ-TP in no magnetic field conditions. The main cause of this additional loss was thought to be the mode mismatch between the air cladding and the Ce:YIG cladding. The value was estimated to be 3.7 dB/interface by simulation. Because there were two coupon interfaces, the total loss of the mode mismatch was estimated to be 7.4 dB. In addition, assuming that the extinction coefficient of Ce:YIG is 1.22 × 10−4 [36], the propagation loss of Ce:YIG was simulated to be 29 dB/cm. The waveguide length of Ce:YIG cladding on each arm was approximately 400 µm, so the total propagation loss of Ce:YIG was estimated to be approximately 1.2 dB. The remaining 2.4 dB loss was thought to be caused by the surface roughness of Ce:YIG and the interlayer.
Fig. 6. Fiber-to-fiber transmission spectra of the reference MZI without Ce:YIG layer and the MO isolator without and with applying external magnetic field.
The phase shift was smaller than ideal value for π phase shift. There are two possible reasons for this. The first is the influence of the SiO2 interlayer. Our simulation results confirmed that the remaining10-nm-thick SiO2 between the Ce:YIG and Si waveguides reduced the transmittance by 23%. Another reason is the insufficient magnetization of Ce:YIG due to the distance between the Ce:YIG and the permanent magnet. In the current measurement setup, the permanent magnet could not be placed closer than 2 mm from the device surface because of physical interference from the in/out-coupling lens modules. The applied magnetic field was expected to be approximately 50 Oe at a distance of 2 mm, whereas a magnetic field of 100 Oe was required to saturate the magnetization of Ce:YIG in the film plane [36]. These problems can be solved by making enough chip size or approaching the permanent magnet without conflict with the lens modules. Assuming that the phase shift reaches to π, the IR is estimated to be approximately 20 dB at the maximum transmittance of the forward propagation. In addition, the insertion loss at higher wavelengths for the maximum IR is improved by enough phase shift.
5. Conclusion
We demonstrated a silicon MZI-based MO isolator by µ-TP of Ce:YIG/SGGG coupon. While the previous MO isolator fabricated by a direct bonding has a footprint of 2.25 mm2 depending on Ce:YIG/SGGG (∼ 300-µm thickness) die [15–18], in this work, the isolator had a footprint of only 0.25 mm2 with a Ce:YIG/SGGG coupon of 50 × 800 µm2 and ∼ 1-µm thickness. In other words, we reduced the footprint and thickness of Ce:YIG/SGGG to ∼1/10 and ∼ 1/300 of the previous isolator, respectively. We achieved the maximum IR of 14 dB at a wavelength of 1567.1 nm and the wavelength shift was 6.5 nm which equals to 56.5% of the π phase shift between forward and backward direction. In addition, it operated under a unidirectional magnetic field, whereas the previous isolator was required to apply magnetic fields in antiparallel directions to each arm. Compared with direct deposition methods [10,21], our µ-TP process requires a lower bonding temperature and is compatible with back-end-of-line processes. In addition, µ-TP technology can be applied isolator configurations of not only MZIs but also MRRs and MMIs. In other words, µ-TP technology can be expected to expand the possibilities of MO devices.
Funding
New Energy and Industrial Technology Development Organization (JPNP16007, JPNP20004); Japan Society for the Promotion of Science (22K18805, 23H04802, 23K03982, 23KJ0908).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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