Reciprocity is a fundamental physical principle that applies to a variety of technological fields such as mechanics, acoustics, electronics, and photonics. For photonic systems especially, breaking reciprocity using nonreciprocal materials is a fundamental challenge and opportunity, which is both of scientific interest and of technological importance. This not only allows for the development of key photonic components such as optical isolators and circulators on chip, but also provides novel ways to transport and process data in photonic systems. Over the past several decades, developing integrated nonreciprocal photonic materials has been one of the most challenging and actively studied topics within the photonics research community. In this issue of MRS Bulletin, several representative research directions toward realizing integrated nonreciprocal photonic materials and devices are summarized. The six articles in this issue showcase cutting-edge progress in this field and exciting opportunities for the future.

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.

This article discusses recent studies of on-chip integration of a plasmonic isolator on a Si substrate and a hybrid isolator on an InP substrate. The key characteristics of the plasmonic isolator are reviewed and future prospects are discussed. A method to enhance the magneto-optical figure of merit (FOM) and reduce the propagation loss of a surface plasmon in order to realize the proposed design of the plasmonic isolator is described. One hundred percent enhancement of the FOM and 20× reduction of propagation loss in the optimized ferromagnetic plasmonic structure are experimentally demonstrated.

Optical isolators, devices that only allow unidirectional light propagation, constitute an essential building block for photonic integrated circuits. For near-infrared communications wavelengths, most current isolator designs rely on the incorporation of magneto-optical (MO) materials to break time-reversal symmetry, such as iron garnets or magnetically substituted semiconductors. MO garnets form the backbone of traditional bulk isolators, but suffer from large lattice and thermal mismatch with common semiconductor substrates, which has significantly impeded their integration into on-chip optical isolators. Materials innovations over the past few years have overcome these barriers and enabled monolithic deposition of MO oxide thin films on silicon using techniques such as pulsed laser deposition and magnetron sputtering. On-chip optical isolator devices with polarization diversity in the telecommunication band have been demonstrated based on these materials. This article reviews the latest technological breakthroughs in MO oxide material growth as well as device design and integration strategies toward practical implementation of on-chip optical isolation.

A novel multiferroic thin-film heterostructure exhibiting both ferromagnetic (FM) and ferroelectric (FE) properties, as well as magneto-optic (MO) and electro-optic (EO) properties, was fabricated via a wet chemical route. Oxide buffer layers were used to allow the growth of ferroelectric lanthanum modified lead magnesium niobate titanate (La:PMNT) layer on top of ferromagnetic bismuth and aluminum substituted yttrium iron garnet (BiAl:YIG). X-ray diffractometer (XRD) analysis confirmed the formation of both crystalline structures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the surface and cross-section morphologies of the resulted heterostructure. Multiferroic properties of the film were investigated.

The development of recording materials has been a critical component in the advancement of optical data storage. The continual need for improved performance in both consumer and business applications has pushed forward the capabilities of optical storage. In this issue of MRS Bulletin, we review some of the important developments in the materials behind established technologies such as compact discs, digital versatile discs, and magneto-optical recording; the emerging technology of blue recording; and two technologies that seek to extend the performance roadmap for optical storage, multilayer and holographic recording.

This work presents a study of electro-optic and magneto-optic films made by a Metal-Organic Chemical Liquid Deposition (MOCLD) method. Electro-optic thin film, La-modified Pb(Mg1/3Nb2/3)O3-PbTiO3 (PLMNT) and magneto-optic thin film, rare earth doped yittrium iron garnet (YIG) have been grown at different conditions. Low temperature growth on buffered semiconductor substrates has been studied for semiconductor device integration. High quality PLMNT film with EO coefficient of 1x10-16 (m/V)2 was obtained with MOCLD. Doped and undoped YIG onto MgO and glass substrates and also onto buffered semiconductors were successfully deposited using MOCLD method. Several of these films had successful rotations that were of device quality. Based on these high quality functional films, two dimensional photonic bandgap waveguide structures were designed and simulated.