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Superconducting circuits can operate at higher energy efficiencies than their room-temperature counterparts and have the potential to enable large-scale control and readout of quantum computers. However, the required interface with room-temperature electronics creates difficulties in scaling up such cryogenic systems. One option is to use optical fibres as a medium in conjunction with fast optical modulators that can be efficiently driven by electrical signals at low temperatures. However, as superconducting circuits are current operated with low impedances, they interface poorly with conventional electro-optical modulators. Here we report an integrated current-driven modulator that is based on the magneto-optic effect and can operate at temperatures as low as 4 K. The device combines a magneto-optic garnet crystal with a silicon waveguide resonator and integrates an electromagnet to modulate the refractive index of the garnet. The modulator offers data rates of up to 2 Gbps with an energy consumption below 4 pJ per bit of transferred information, which could be reduced to less than 50 fJ per bit by replacing dissipative electrodes with superconductors and optimizing the geometric parameters.

In this paper, an on-chip optical circulator on the InP-membrane-on-Si (IMOS) platform is demonstrated. The circulator is composed of two multi-mode interferometers (MMIs), four polarization converters (PCs), and a Cerium-doped Yttrium Iron Garnet (Ce:YIG) die. The Ce:YIG die is adhesively bonded on the InP membrane via a 125-nm thin bonding layer. Nonreciprocal phase shift (NRPS) is employed in the presence of a transverse magnetic field. The device works as a 4-port optical circulator with a maximum optical isolation of 27.0 dB and a minimum optical isolation of 18.6 dB for the TE mode. The measured optical isolation bandwidth is 0.12 nm. Two methods are proposed in order to improve the optical isolation bandwidth.

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.

In photonic integrated circuits (PICs), optical isolators and circulators are essential to prevent lasers from backreflections and to reroute the light flow. In this paper, an integrated polarization-independent device that can be operated as an optical isolator or an optical circulator, based on an InP membrane on silicon platform, is demonstrated. A cerium-doped yttrium iron garnet die is adhesively bonded on a Mach–Zehnder interferometer, in combination with four polarization converters. The device shows maximum optical isolations of 27.0 dB for transverse-electric (TE)-mode input and 34.0 dB for transverse-magnetic TM-mode input. The device also works as a four-port optical circulator. Optical isolations of at least 18.6 dB and 16.4 dB are measured between each circulator port pair for TE- and TM-mode input, respectively. This work could remove the optical interfaces between laser and isolator for robust production. It also provides a step forward toward a multifunctional and high-density PIC.

This paper extends our previous work on the microring resonator-type serial-to-parallel conversion scheme aimed for all-optical label processing on the silicon photonic platform. In the previously proposed scheme, the architecture of the delay lines was silicon wires. This resulted in considerable propagation loss, confining multibit operation of serial-to-parallel conversion to a label packet of 2 bits. However, during a practical application, propagation loss at the delay lines has to be improved to support label packets comprising of a greater number of bits. The purpose of this article is twofold. First, we confirm that propagation loss can be lowered to sub-dB/cm values by introducing rib waveguides via simulations and experiment. Second, we demonstrate multibit serial-to-parallel conversion on a 4-bit label “1101” by utilizing a cascaded microring resonator device of which the straight sections of the delay lines are of rib waveguide architecture. In order to focus on free-carrier dispersion effect of silicon during operation, we left out a temporal delay between the pump pulses and probe bits to be extracted. As a result, serial-to-parallel conversion was viable even at peak pump power levels as low as ∼100 mW. We hereby verify that the proposed serial-to-parallel conversion scheme incorporated with rib waveguide delay lines is a realistic solution in realizing a photonics-based solution to all-optical label processing.

In this study, we propose a novel all-optical switch that is controlled by the thermo-optic effect in a silicon ring resonator with a metal cladding layer. A pump light is efficiently converted into heat through absorption by a metal layer, causing a resonant wavelength shift. The burst-switching measurement reveals a switching on/off ratio of 5.7 dB at the drop-port with a pump power of 20 mW.

We propose and numerically demonstrate a novel tunable TE-TM mode converter which consists of a Mach-Zehnder interferometer (MZI) composed of 3-dB polarization couplers. The mode conversion is controlled by the phase difference between the two MZI arms.

Optical absorption of a guided light wave in a waveguide by a metal is undesirable in many photonic integrated devices. However, the heating due to the optical absorption enables to realize novel applications such as nonvolatile optical memories and all-optical thermo-optic devices. We report an efficient heating due to the optical absorption of an evanescent field by a metal heating layer deposited on a silicon microring resonator and consequent light-to-heat conversion. We investigate the design of the microring resonator by both theoretical and experimental approaches, which enable an efficient light-to-heat conversion. An enhanced thermo-optic effect by the optical absorption of the heating layer is observed. In addition, a heating efficiency larger than 40 K/mW is expected.

Optical isolators and circulators are fundamental building block in photonic integrated circuits to block undesired reflections and routing light according to a prescribed direction. In silicon photonics, heterogeneous integration of magneto-optic garnet bonded on a pre-patterned silicon layer has been demonstrated to be an effective solution for manufacturing optical isolators and circulators for TM polarized light. However, most integrated semiconductor lasers emit TE polarized light, which indicates the need to find a reliable solution for this polarization. In this paper, we demonstrated broadband optical isolators and circulators for TE polarized light based on heterogeneous bonding on the silicon photonics platform. To achieve this goal, an integrated adiabatic coupler and a broadband polarization rotator are designed and optimized. The nonreciprocal behavior is induced through an energy-efficient integrated electromagnet with a minimum power consumption of 3 mW. Two isolators/circulators are fabricated with small and large free spectral range, respectively. In the former case, an optical isolation ratio as large as 30 dB is measured at 1555 nm with an insertion loss of 18 dB, while for the broadband circulator, an optical isolation larger than 15 dB is guaranteed over more than 14 nm (1.75 THz) for all port combinations with an insertion loss between 14 and 18 dB at 1560 nm. Finally, it has been theoretically shown that the insertion loss can be reduced below 6 dB with design and fabrication improvements. To the best of the authors’ knowledge, the proposed integrated TE optical circulator is the first experimental demonstration of this device in silicon photonics.

Novel magneto-optical isolator and self-holding optical switch with an a-Si:H microring resonator are demonstrated. The devices are driven by the remanence of integrated thin-film magnet and, therefore, maintain their state without any power supply.

Magneto-optical phase shift is effective to realize on-chip optical isolators. Optical isolators are fabricated on SOI platforms with isolation ratios of 30 and 16 dB for TM and TE mode input, respectively.

Magneto-optical isolators exhibit performance characteristics of a 30 dB isolation, a 20-dB isolation bandwidth of 8 nm, and a temperature-insensitive isolation. The magneto-optical switch is successfully demonstrated with a latching function. © 2020 The Author(s)

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.

Optical isolators and circulators are important elements in many photonic systems. These nonreciprocal devices are typically made of bulk optical components and are difficult to integrate with other elements of photonic integrated circuits. This article discusses the best performance for waveguide isolators and circulators achieved with heterogeneous bonding. By virtue of the bonding technology, the devices can make use of a large magneto-optical effect provided by a high-quality single-crystalline garnet grown in a separate process on a lattice-matched substrate. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer contributes to the large penetration of the optical field into a magneto-optical garnet used as an upper-cladding layer. This enhances the magneto-optical phase shift and contributes greatly to reducing the device footprint and the optical loss. Several versions of silicon waveguide optical isolators and circulators, both based on the magneto-optical phase shift, are demonstrated with an optical isolation ratio of ≥30 dB in a wavelength band of 1550 nm. Furthermore, the isolation wavelength can be effectively tuned over several tens of nanometers.

We demonstrate a novel self-holding function of a magneto-optical waveguide switch. The switching state is flipped by a pulsed current and maintained without any power supply by virtue of the nonvolatility of the thin-film magnet. Extinction ratios up to 15.4 dB were demonstrated. The switch state was controlled by a 1- $\mu \text{s}$ pulsed electrical current.

In this study, we present optical isolators and circulators fabricated by bonding cerium-substituted yttrium iron garnet (Ce:YIG) on silicon microring resonators. A novel integrated electromagnet is fabricated by depositing a metal micro-strip on the bonded chip. We experimentally prove that it can be efficiently used to control the magnetic field needed to induce the nonreciprocal phase shift effect in the Ce:YIG. The fabricated devices exhibit extremely small footprint (

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.

Optical circulators that unidirectionally route light could lead to bidirectional operations in applications in data centers and telecommunications, as well as sensors. In this work, to the best of our knowledge, we present the first realization of integrated optical circulators on silicon that are electrically driven and dynamically reconfigurable. The proposed device utilizes silicon microrings with a bonded magneto-optic cladding alongside an integrated electromagnet for nonreciprocal behavior. This novel approach does not use a permanent magnet and, for this reason, it is more attractive for packaging and further integration with lasers and other photonic devices. We use this device architecture to demonstrate 4- and 6-port optical circulators with up to 14.4 dB of isolation and propose a framework to extend the design to an arbitrary number of ports. Finally, we demonstrate that it is possible to switch the electromagnet and reconfigure the circulator on a sub-nanosecond timescale, potentially adding a new level of device functionality.

CoFe 2 O 4 films were grown on (100)-Si substrates with a MgO buffer layer for silicon photonics applications. Both CoFe 2 O 4 and MgO films were deposited by RF magnetron sputtering. The structural analysis reveals that CoFe 2 O 4 films grown with an MgO buffer layer have a single-oriented structure.

A Mach-Zehnder interferometer-type serial-to-parallel conversion device, based on free-carrier dispersion effect, is proposed, thereby reducing the pump power required for operation. Intensity modulation of probe light synchronized with pump pulses was observed via pump-probe measurements.

Optical isolators play unique roles in photonic circuits. We have developed an optical isolator with silicon waveguides, based on a Mach–Zehnder interferometer. The device characteristics were temperature dependent. We investigated the temperature dependence of the refractive index ( ${\rm d}n/{\rm d}T$ ) and the magneto-optical coefficient ( ${\rm d}\theta _f /{\rm d}T$ ) of a magneto-optical garnet Ce:YIG, and estimated these as $9.1 \times 10^{-5}$ K−1 and 44 ${\rm }^\circ \cdot {\rm cm}^{- 1} \cdot {\rm K}^{- 1} $ , respectively. We discuss the design of an athermal silicon waveguide optical isolator, in which the temperature dependences of the magneto-optical effect and refractive indices are canceled in the backward direction. The high optical isolation, greater than 20 dB, is demonstrated for temperature in the 20–60 °C range.