The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength integrated photonics.

Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism (spins and/or magnetic field) and electricity (electric dipoles and/or electric field). In spite of the long research history in the whole twentieth century, the discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this exciting area. The availability of more multiferroic materials and improved magnetoelectric performance are approaching to make the applications within reach. While seminal review articles covering the major progress before 2010 are available, an updated review addressing the n...

Multiferroic materials and magnetoelectric physics: symmetry, entanglement, excitation, and topology

Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism and electricity. The discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this exciting area. The availability of more multiferroic materials and improved magnetoelectric performance are approaching to make the applications within reach. While seminal review articles covering the major progress before 2010 are available, an updated review addressing the new achievements since that time becomes imperative. In this review, following a concise outline of the basic knowledge of multiferroicity and magnetoelectricity, we summarize the important research activities on multiferroics, especially magnetoelectricity and related physics in the last six years. We consider not only single-phase multiferroics but also multiferroic heterostructures. We address the physical mechanisms regarding magnetoelectric coupling so that the backbone of this divergent discipline can be highlighted. A series of issues on lattice symmetry, magnetic ordering, ferroelectricity generation, electromagnon excitations, multiferroic domain structure and domain wall dynamics, and interfacial coupling in multiferroic heterostructures, will be revisited in an updated framework of physics. In addition, several emergent phenomena and related physics, including magnetic skyrmions and generic topological structures associated with magnetoelectricity will be discussed.

Multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe2O4 films on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates using pulsed laser deposition. Upon applying an electric field, the in-plane magnetization of the heterostructures increases and the out-of-plane magnetization decreases. Sharp and reversible changes in magnetization under electric field were also observed for the poled sample. The relative change in magnetization-electric field hysteresis loops were obtained for both the in-plane and out-of-plane magnetizations. Analysis of the results suggests that the electric field induced change in magnetic anisotropy via strain plays an important role in the magnetoelectric coupling in the heterostructures.

Electric field manipulation of magnetization at room temperature in multiferroic CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to –10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 μm.

/pdf/high-extinction-ratio-silicon-polarization-beam-splitter-2xekg0jinx.pdf

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations

Mode-division multiplexing on an integrated photonic chip is critical for future optical networks. High-order mode multiplexing is desired to increase the transmission capacity. Here, we propose and experimentally demonstrate a silicon on-chip high-order mode (de)multiplexer using subwavelength grating (SWG) structure, which supports 11-mode (de)multiplexing on a TE-polarized light (TE0–TE10). The proposed mode (de)multiplexer comprises three directional couplers and seven SWG-based directional couplers. Measurement results show that all the 11 channels have low crosstalk values (−15.4 to −26.4 dB) and low insertion losses (0.1 to 2.6 dB) at 1545 nm. To the best of our knowledge, our device achieves the highest-order-mode (de)multiplexing on a silicon photonic chip.

Silicon High-Order Mode (De)Multiplexer on Single Polarization

DOI: 10.1002/adom.201801191 plasmonic or dielectric nanoantennas.[20] Mode conversions between the trans-electric (TE) modes TE0, TE1, and TE2 were achieved with a length of ≈10 μm at 1550 nm using silicon nanoantennas on LiNbO3 or Si3N4 waveguides by simulations.[20] A nanoscale mode converter was theoretically proposed[11] and experimentally demonstrated[21] by introducing a periodic perturbation in its effective refractive index along the propagation direction and a graded effective index profile along its transverse direction. Mode conversion between TE0 and TE1 modes was successfully achieved with a length of 23 μm at 1550 nm. However, no experimental results on the crosstalk of the TE mode conversion were provided in the previous reports.[20,21] It is highly desired to miniaturize the device footprint and realize a compact waveguide mode converter. In this article, we propose and demonstrate a compact silicon waveguide mode converter based on all-dielectric metasurface structure with a tilted subwavelength periodic perturbation. The mode coupling coefficient changes along the propagation direction, which can help the mode conversion process. Our proposed waveguide mode converter can be scaled to realize arbitrary waveguide mode conversion. As examples, we experimentally demonstrate two silicon waveguide mode converters employing all-dielectric metasurface structures that can convert TE0 mode to TE1 mode and TE0 mode to TE2 mode, respectively. The mode conversion lengths are 5.75 and 6.736 μm for the TE0-to-TE1 and TE0-to-TE2 waveguide mode converters, respectively, which are shorter than previously reported results.[20,21] Mode demultiplexers are cascaded after the metasurface mode converter to measure the crosstalk values and conversion losses. The insertion losses are lower than 1 dB and the crosstalk values are below −10 dB in a wavelength range of 20 nm for the TE0-to-TE1 and TE0-to-TE2 waveguide mode conversions. Figure 1a depicts the 3D view of the proposed waveguide mode converter based on dielectric metasurface structure with a tilted subwavelength periodic perturbation. Propagation of optical field in a perturbed dielectric structure can be described by the coupled mode theory. Attributed to the perturbation on the silicon waveguide, one waveguide mode can be coupled into another mode, and the amplitude of each waveguide mode along the propagation direction is determined by a set of differential equations[11,25]

Compact Silicon Waveguide Mode Converter Employing Dielectric Metasurface Structure

We propose and experimentally demonstrate an ultra-compact and highly efficient polarization splitter and rotator based on a silicon bent directional coupler structure. The TM-to-TE cross-polarization coupling occurs between the two parallel bent waveguides, if the phase matching condition is satisfied. Efficient polarization splitting and rotating are simultaneously achieved. The device is fabricated by a single step of exposure and etching. The measured peak TM-to-TE polarization conversion efficiency reaches 96.9%. The TM-to-TE conversion loss is lower than 1 dB in the wavelength range of 1544 nm–1585 nm, and the insertion loss for the TE polarization is lower than 0.3 dB in the wavelength regime of 1530 nm–1600 nm. The cross talk values are lower than −20 and −18 dB for the TE- and TM-polarizations over a wavelength range of 70 nm, respectively. The coupling length of the polarization splitter and rotator is 8.77μm. To the best of our knowledge, our device achieves the shortest coupling length.

/pdf/ultra-compact-and-highly-efficient-silicon-polarization-48tqwhwuov.pdf

Yu He

Papers

Electric field manipulation of magnetization at room temperature in multiferroic CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations

Silicon High-Order Mode (De)Multiplexer on Single Polarization

Compact Silicon Waveguide Mode Converter Employing Dielectric Metasurface Structure

Ultra-compact and highly efficient silicon polarization splitter and rotator