Extended optical waveguide theory with magneto-optical effect and magnetoelectric effect.

Optical waveguide theory is essential to the development of various optical devices. Although there are reports on the theory of optical waveguides with magneto-optical (MO) and magnetoelectric (ME) effects, a comprehensive theoretical analysis of waveguides considering these two effects has not yet been published. In this study, the conventional waveguide theory is extended by considering constitutive relations that account for both MO and ME effects. Using the extended waveguide theory, the propagation properties are also analyzed in a medium where metamaterials and magnetic materials are arranged such that MO and ME effects can be controlled independently. It has been confirmed that the interaction between MO and ME effects occurs depending on the arrangement of certain metamaterials and the direction of magnetization. This suggests a nonreciprocal polarization control that rotates the polarization in only one direction when propagating in plane wave propagation and enhances the nonreciprocal nature of the propagating waves in waveguide propagation.

Compact magneto-optical isolator by µ-transfer printing of magneto-optical single-crystal film on silicon waveguides.

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.

High-speed modulation in a waveguide magneto-optical switch with impedance-matching electrode.

A magneto-optical switch responding to signal with 200 ps rise time was demonstrated. The switch uses current-induced magnetic field to modulate the magneto-optical effect. Impedance-matching electrodes were designed to apply high-frequency current and accommodate the high-speed switching. A static magnetic field generated by a permanent magnet was applied orthogonal to the current-induced ones and acts as a torque and helps the magnetic moment reverse its direction which assist the high-speed magnetization reversal.

Light-induced thermomagnetic recording of thin-film magnet CoFeB on silicon waveguide for on-chip magneto-optical memory.

Thermomagnetic recording is a technique used as a writing process for magneto-optical (MO) drives. Despite their significant advantages, such as rewritability, nonvolatility, reliability, and large cycling endurance, MO drives are rarely used today because of the complex drive systems that must deal with magnetic field and lightwave simultaneously. This study reports on the light-induced thermomagnetic recording of a ferromagnetic thin-film CoFeB on a Si photonic platform. Lightwave guided in the Si waveguide evanescently coupled to the thin-film magnet and underwent optical absorption, resulting in heating and a decrease in coercive force. Therefore, we observed magnetization reversal with an applied magnetic field for both continuous and modulated light pulses using a magneto-optical Kerr effect microscope, and the light-induced thermomagnetic recording was experimentally demonstrated on a Si photonic platform. The proposed scheme enables the realization of on-chip MO memories on the Si photonic platform in which neither bulky free-space optics nor mechanical rotation systems are required.

TE-mode magneto-optical isolator based on an asymmetric microring resonator under a unidirectional magnetic field.

Optical isolators are necessary components in photonic integrated circuits to ensure system stability by blocking the back-reflected light waves. In this study, a transverse electric mode magneto-optical isolator based on an asymmetric microring resonator without additional polarization rotators in series is demonstrated. Cerium-substituted yttrium iron garnet was integrated with silicon-on-insulator substrates by wafer bonding to break the Lorentz reciprocity on-chip after applying a unidirectional magnetic field. The isolator shows a 22 dB isolation ratio and a 4.3 dB insertion loss at an operation wavelength of 1572.62 nm.

Mode-evolution-based TE mode magneto-optical isolator using asymmetric adiabatic tapered waveguides.

As an indispensable component in the photonic integrated circuits, the design and fabrication of optical isolators, particularly in the transverse electric (TE) polarized mode, is a long-standing challenge. Herein, we present a TE mode magneto-optical isolator using adiabatic tapered waveguides to realize conversions between designated modes. The isolator exhibits an ultranarrow structure of 1.27 µm × 1498 µm. We demonstrate that the device functions under a TE mode input with a maximum isolation ratio of 15 dB and an insertion loss of 5 dB at a wavelength of 1537.3 nm.

Nonvolatile magneto-optical switches integrated with a magnet stripe array.

Nonvolatile optical switches are promising components for low-power photonic integrated circuits with multiple functionalities. In this study, we experimentally demonstrate magneto-optical switches integrated with a magnet array. Optical switches in both microring and Mach-Zehnder configurations are fabricated on a high-quality single-crystalline magneto-optical material Ce:YIG. The switched state is alternated by a current-induced magnetic field from an integrated electromagnet and remained without any external power supply owing to the nonvolatile magnetization of thin-film magnets. Subsequently, the arbitrary level control of optical transmission is demonstrated by changing the magnetization state of integrated thin-film magnets with a current applied in the microring gate switch, and a maximum switching ratio over 25 dB is achieved in the Mach-Zehnder switch. The latching operation is presented with a 1-µs pulsed voltage.

Low-loss waveguide optical isolator with tapered mode converter and magneto-optical phase shifter for TE mode input.

We propose and demonstrate a novel low-loss waveguide optical isolator with tapered mode converter and magneto-optical phase shifter. The principle of operation of the isolator is based on the superposition of the TE and TM modes. The two different modes become direction-dependent due to a magneto-optical phase shift affecting the TM mode. We designed a tapered mode converter in order to generate the TE and TM modes with equal amplitude when the waveguide is excited with a TE mode input. We successfully demonstrated that the fabricated device acts as an isolator showing a different transmittance between forward and backward directions. The maximum isolation measured is 16 dB at a wavelength of 1561 nm for a TE mode input.

Integrated broadband Ce:YIG/Si Mach-Zehnder optical isolators with over 100 nm tuning range.

We demonstrate integrated optical isolators with broadband behavior for the standard silicon-on-insulator platform. We achieve over 20 dB of optical isolation across 18 nm of optical bandwidth. The isolator is completely electrically controlled and does not require a permanent magnet. Furthermore, we demonstrate the ability to tune the central operating wavelength of the isolator across 100 nm, which covers the entire S + C telecom bands. These devices show promise for integration in optical systems in which broadband isolation is needed such as wavelength multiplexed systems or optical sensors.

Development of SAAP3D force field and the application to replica-exchange Monte Carlo simulation for chignolin and C-peptide.

Single amino acid potential (SAAP) would be a prominent factor to determine peptide conformations. To prove this hypothesis, we previously developed SAAP force field for molecular simulation of polypeptides. In this study, the force field was renovated to SAAP3D force field by applying more accurate three-dimensional main-chain parameters, instead of the original two-dimensional ones, for the amino acids having a long side-chain. To demonstrate effectiveness of the SAAP3D force field, replica-exchange Monte Carlo (REMC) simulation was performed for two benchmark short peptides, chignolin (H-GYDPETGTWG-OH) and C-peptide (CHO-AETAAAKFLRAHA-NH2). For chignolin, REMC/SAAP3D simulation correctly produced native ß-turn structures, whose minimal all-atom root-mean-square deviation value measured from the native NMR structure (except for H) was 1.2 Å, at 300 K in implicit water, along with misfolded ß-hairpin structures with unpacked aromatic side chains of Tyr2 and Trp9. Similar results were obtained for chignolin analog [G1Y,G10Y], which folded more tightly to the native ß-turn structure than chignolin did. For C-peptide, on the other hand, the α-helix content was larger than the ß content on average, suggesting a significant helix-forming propensity. When the imidazole side chain of His12 was protonated (i.e., [His12Hip]), the α content became larger. These observations as well as the representative structures obtained by clustering analysis were in reasonable agreement not only with the structures of C-peptide that were determined in this study by NMR in 30% CD3CD in H2O at 298 K but also with the experimental and theoretical behaviors having been reported for protonated C-peptide. Thus, accuracy of the SAAP force field was improved by applying three-dimensional main-chain parameters, supporting prominent importance of SAAP for peptide conformations.

Amorphous-Si waveguide on a garnet magneto-optical isolator with a TE mode nonreciprocal phase shift.

We fabricated a magneto-optical (MO) isolator with a TE mode nonreciprocal phase shift. The isolator is based on a Mach-Zehnder interferometer composed of 3-dB directional couplers, a reciprocal phase shifter, and a nonreciprocal phase shifter. To realize TE mode operation in the optical isolator, we designed a novel waveguide structure composed of a hydrogenated amorphous silicon waveguide with an asymmetric MO garnet lateral clad on a garnet substrate. The isolator operation is successfully demonstrated in a fabricated device showing the different transmittances between forward and backward directions. The maximum isolation of the fabricated isolator is 17.9 dB at a wavelength of 1561 nm for the TE mode.

Three-dimensional nanostructuring in YIG ferrite with femtosecond laser.

With the goal of creating magneto-optical devices, we demonstrated forming nanostructures inside a substrate of cerium-substituted yttrium iron garnet (Ce:YIG) by means of direct laser writing. Laser irradiation changed both the optical and magnetic properties of Ce:YIG. The measurements showed that the refractive index was increased by 0.015 (about 0.7% change) and the magnetization property was changed from hard to soft to decrease the coercivity. This technology enables the formation of 3-dimensional optical and magnetic nanostructures in YIG and will contribute to the development of novel devices for optical communication and photonic integration.

Magneto-optical non-reciprocal devices in silicon photonics.

Silicon waveguide optical non-reciprocal devices based on the magneto-optical effect are reviewed. The non-reciprocal phase shift caused by the first-order magneto-optical effect is effective in realizing optical non-reciprocal devices in silicon waveguide platforms. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer enhances the magneto-optical phase shift, which reduces the device footprints. A surface activated direct bonding technique was developed to integrate a magneto-optical garnet crystal on the silicon waveguides. A silicon waveguide optical isolator based on the magneto-optical phase shift was demonstrated with an optical isolation of 30 dB and insertion loss of 13 dB at a wavelength of 1548 nm. Furthermore, a four port optical circulator was demonstrated with maximum isolations of 15.3 and 9.3 dB in cross and bar ports, respectively, at a wavelength of 1531 nm.

Compact 2 × 2 polarization-diversity Si-wire switch.

A polarization-independent 2 × 2 switch based on silicon-wire waveguides has been realized with a compact size of 600 × 500 µm². Polarization-independent operation was achieved with a polarization-diversity technique which implements polarization splitters, TE-TM intersections, and Mach-Zehnder switches. The extinction ratios of the 2 × 2 switch for TE, TM, and a mixed polarization at a wavelength of 1550 nm were measured to be larger than 30 dB, 25 dB, and 30 dB, respectively. The measured switching powers for the TE and TM polarizations were 25 and 55 mW, respectively. The measured polarization-dependent loss was lower than 1 dB. The differential group delay (DGD) between the TE and TM modes was also evaluated using the Mueller matrix method, which was in good agreement with the values estimated from the path lengths for each mode. A path-length-compensated switch was fabricated, whose DGDs for all paths were indeed as small as ~2 ps, mainly from the access waveguides. The switch could provide an important route to develop ultra-compact polarization-independent integrated circuits based on silicon-wire waveguides.

GaInAsP/InP MZI waveguide optical isolator integrated with spot size converter.

We fabricated a waveguide optical isolator with a GaInAsP guiding layer integrated with spot size converters (SSCs) for efficient coupling to optical fibers. The isolator is constructed with a Mach-Zehnder interferometer (MZI), which is composed of multi-mode interference (MMI) couplers, as well as nonreciprocal and reciprocal phase shifters. The nonreciprocal phase shifter is constructed with a magneto-optical cladding layer directly bonded to a semiconductor guiding layer. The performance of the GaInAsP waveguide optical isolator was demonstrated with a maximum optical isolation of 28.3 dB at a wavelength of 1558 nm for the TM mode.

MZI optical isolator with Si-wire waveguides by surface-activated direct bonding.

We fabricate a Mach-Zehnder interferometer-based optical isolator using a silicon-wire waveguide with magneto-optic garnet cladding using direct bonding techniques. Using Si-wire waveguides, the size of the device is greatly reduced from that of our previous device. We investigate surface-activated direct bonding with nitrogen plasma treatment, which shows better bonding results than oxygen plasma treatment. A large magneto-optic phase shift of 0.8π and an optical isolation of 18 dB are obtained at a wavelength of 1322 nm.

Ultra-small, self-holding, optical gate switch using Ge2Sb2Te5 with a multi-mode Si waveguide.

We report a multi-mode interference-based optical gate switch using a Ge(2)Sb(2)Te(5) thin film with a diameter of only 1 µm. The switching operation was demonstrated by laser pulse irradiation. This switch had a very wide operating wavelength range of 100 nm at around 1575 nm, with an average extinction ratio of 12.6 dB. Repetitive switching over 2,000 irradiation cycles was also successfully demonstrated. In addition, self-holding characteristics were confirmed by observing the dynamic responses, and the rise and fall times were 130 ns and 400 ns, respectively.

Direct Wafer Bonding and Its Application to Waveguide Optical Isolators.

This paper reviews the direct bonding technique focusing on the waveguide optical isolator application. A surface activated direct bonding technique is a powerful tool to realize a tight contact between dissimilar materials. This technique has the potential advantage that dissimilar materials are bonded at low temperature, which enables one to avoid the issue associated with the difference in thermal expansion. Using this technique, a magneto-optic garnet is successfully bonded on silicon, III-V compound semiconductors and LiNbO3. As an application of this technique, waveguide optical isolators are investigated including an interferometric waveguide optical isolator and a semileaky waveguide optical isolator. The interferometric waveguide optical isolator that uses nonreciprocal phase shift is applicable to a variety of waveguide platforms. The low refractive index of buried oxide layer in a silicon-on-insulator (SOI) waveguide enhances the magneto-optic phase shift, which contributes to the size reduction of the isolator. A semileaky waveguide optical isolator has the advantage of large fabrication-tolerance as well as a wide operation wavelength range.

Low-crosstalk 2 x 2 thermo-optic switch with silicon wire waveguides.

We demonstrate a low-crosstalk 2 x 2 thermo-optic switch with silicon wire waveguides. The device is based on a 2 x 2 array of Mach-Zehnder interferometer (MZI) switches. Lowest crosstalk levels of -50 dB and -30 dB are obtained for 'bar' and 'cross' switching states, respectively. An intersection in the switch is important for low-crosstalk operation. The power consumption of one MZI element switch is 40 mW and the total power consumption of the device is at most 160 mW.

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide.

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.