Unidirectional Optical Transmission Research Articles

Photonic topological Weyl degeneracies and ideal type-I Weyl points in the gyromagnetic metamaterials

As a new topological phase in three-dimensional momentum space, Weyl semimetals extend the repertoire of exotic topological states. In this paper, we find the existence of photonic Weyl points in electromagnetic continuous gyromagnetic metamaterials. The dispersion of the longitudinal mode of one of the two bands forming the Weyl point is flat; thus, the Weyl degeneracy point of the system is exactly at the critical transition between the type-I and type-II Weyl points. The Fermi arc connects the projections of the Weyl points with opposite chirality at the interface between the gyromagnetic metamaterial and vacuum. Full-wave simulation results show that surface waves can bypass sharp defects and achieve robust, unidirectional optical transmission without reflection. Furthermore, taking the spatial nonlocal effects into account, the ideal type-I Weyl points can exist in this class of metamaterials. This paper sheds light on fundamental understanding of topological physics, and the study of Weyl points in metamaterials will greatly enrich the field of topological photonics.

High efficiency unidirectional optical transmission diode at 1550 nm based on H-PDLC Bragg grating-PC structure

In this paper, a method has been proposed for high efficiency unidirectional optical transmission diode at 1550 nm. We use a new composite structure, holographic polymer dispersed liquid crystal (H-PDLC) grating-photonic crystal (PC), to realize it. There exists the directional band gap for the PC structure in 1550 nm. Therefore, it is possible to use the H-PDLC Bragg grating to shift the incident angle to a particular value to satisfy the condition so that the PC energy band can be moved from the pseudo-band gap to the passband to achieve the purpose of unidirectional transmission optical diode with high-efficiency. So that when the 1550 nm beam diffracted from the H-PDLC grating to incident the PC structure, it will transmit the composite structure almost totally, but it will be completely none reflected from PC surface. The simulation results show that the forward transmittance is 94.1% and the backward transmittance is near 0. At the same time, H-PDLC grating provide a method to adjust the transmittance of the composite structure diode with its electrically-controlled characteristics. This composite structure design provides another efficient method for the realization of all-optical diodes in the field of optical communication.

Unidirectional Optical Transmission of a Dual Metallic Grating with Grooves

In this paper, a dual grating structure for unidirectional transmission is presented. The forward and backward transmission performances have been investigated by finite element method. To enhance the forward transmission and to suppress the backward transmission simultaneously, we suggested to cut grooves on the surfaces of one of the gratings, and the effects of the grooves on the optical transmission have been studied. The numerical simulation results reveal that the transmission contrast ratio and the optical unidirectional transmission of the structure can be improved markedly by properly arranging the size and the position of the grooves. The forward transmission can be more than 90%, while the backward transmission transmittance is less than 5%.

Tunable unidirectional light transmission in a graphene–metal hybrid metamaterial

ABSTRACTGraphene’s unique properties, such as strong plasmonic response and electrostatic doping, enable control over the properties of light in an active way. In this paper, we propose a graphene–metal hybrid metamaterial, which exhibits tunable wideband unidirectional light transmission. The hybrid metamaterial consists of a complementary split-ring graphene and a metallic grating. Unidirectional optical transmission with a wide bandwidth of 14.8% of the central frequency at 29.3 THz and a large tuning range of 6.6 THz is found to be achievable in simulations. The light reflection and graphene absorption are shown to be the major factors limiting the efficiency of unidirectional transmission. A large electron scattering time of graphene is beneficial for improving the transmission efficiency. The graphene–metal hybrid metamaterial enables active control over the propagation of light, which could be of interest for infrared isolation, polarization transformation, etc.

Unidirectional Optical Transmission in a Single-Layer Metallic Grating Consisting of Cambered Resonators

In order to realize unidirectional optical transmission (UOT), a single-layer metallic grating (SMG) consisting of cambered resonators (CRs) is proposed. Numerical simulation results show that the transmittance contrast ratio of the SMG can reach about 5000 with a high unidirectional transmittance of 39%. UOT results from diffraction enhancement or suppression in transmission direction. Diffraction is manipulated by the interference of electromagnetic radiation from local surface plasmons on CRs. Our single-layer structure has sub-100 nm thickness and can be used in ultracompact optical devices to manipulate light transmission.

Broadband asymmetric light transmission through tapered metallic gratings at visible frequencies.

Asymmetric transmission phenomenon has attracted tremendous research interest due to its potential applications in integrated photonic systems. Broadband asymmetric transmission (BAT) is a highly desirable but challenging functionality to achieve in the visible regime due to the limitation of material dispersion. In this paper, we propose and numerically demonstrate that a tapered-metal-grating structure (TMGS) can achieve high-contrast BAT spectra covering the entire visible region. The transmission efficiency reaches ~95% for the forward illumination and ~35% for the backward illumination at the same wavelengths, respectively, and the corresponding transmission ratio is larger than 2.5 over a broadband wavelength regime. Such a design with high performance suggests applications for unidirectional optical transmission, optical diode, and so on.

Nanoimprint-defined, large-area meta-surfaces for unidirectional optical transmission with superior extinction in the visible-to-infrared range.

Optical devices with asymmetric transmission have important applications in optical systems, but optical isolators with the modal asymmetry can only be built using magneto-optical or nonlinear materials, as dictated by the Lorentz reciprocity theorem. However, optical devices with the power asymmetry can be achieved by linear materials such as metals and dielectrics. In this paper, we report a large-area, nanoimprint-defined meta-surface (stacked subwavelength gratings) with high-contrast asymmetric transmittance in the visible-to-infrared wavelength range for TM-polarized light. The physical origin of asymmetric transmission through the meta-surface is studied by analyzing the scattering matrix.

CHANNEL MONITORED CHANNEL DUPLEXER FOR OPTIMIZATION OF BIDIRECTIONAL PASSIVE OPTICAL NETWORK

Method of optimizing the optical network transmission in access network has been investigated in many years. Unidirectional optical transmission system is the earliest method of delivering the information. In recent years, bidirectional optical transmission system is the most popular network and shall be the first right of refusal to deploy nowadays. It is justify enough by the massive deployment of the popular state-of-the-art network named Passive Optical Network (PON) in Fiber To The Home (FTTH) technologies. Combining 3 wavelengths includes (1) 1310nm, (2) 1490nm, and (3) 1550nm within a fiber is the method used on Gigabit Capable Passive Optical Network (GPON) or Gigabit Ethernet Passive Optical Network (GEPON/EPON). Combining 2 different wavelengths for uplink and downlink on Small Form Pluggable (SFP) lasers also has been a method used to optimized and saved the fiber infrastructure. Compared those techniques, the research optimization focusing on introducing a passive optical duplexer that combined the same wavelength from both end with the element of monitoring via different wavelength to confirm the network availability. In the design, a unidirectional converter able to operate at a nominal 1310nm or 1550nm windows shall be demonstrated up to 10Gbps Ethernet signal.

One-way light transmission in compact SOI structures

Nonreciprocal transmission of the light is fundamental in all-optical processing systems and, it is in a great demand in advanced optical communication networks. Optical devices based on one-way light transport are essential to prevent interference between various integrated components of the optical networks. The integration of these devices in a single platform is a challenging task due to material incompatibilities between the integrated devices. We report all-silicon-on-insulator waveguides where the asymmetric light propagation is obtained. These waveguides are coupled using micro rings with modified gaps. We show that unidirectional optical transmission of the light can be accomplished by non-symmetric distribution of micro rings and gaps between the waveguides. We also show the time domain snapshot of the light propagation from left to right and from right to left behaves differently. The advantageous properties of the proposed structure are the compatibility with the integrated photonics and can serve as a main building block for designing integrated silicon nanophotonic devices.

Asymmetric optical transmission through periodic arrays of cone air holes in a metal film.

The metal film perforated with a micro-nano cone air hole (CAH) can serve as an optical device having asymmetric transmission. In this paper, we numerically investigate the metal film perforated with periodic arrays of cone air holes (PACAH) and find that the metal PACAH can also implement asymmetric optical transmission. In the short wave band of visible light, Al PACAH shows the most obvious effect of asymmetric optical transmission. And in the middle-long wave band, Ag PACAH shows the effect best. When R(L) (the radius of large end of hole) = 600 nm, R(S) (the radius of small end of hole) = 150 nm, t (the film thickness) = 1500 nm, and the substrate with a dielectric constant of 2.4 is placed on the side adjacent to the large end of hole, for Al PACAH (the CAHs arranged in a triangular array), the average forward transmittance and the average transmission ratio (the ratio of forward transmittance to backward one) in the region of 400 nm-600 nm are 28.4% and 7.6, respectively. Particularly, the forward transmittance and the transmission ratio at 448 nm are 43.0% and 12.4 respectively. And for Ag PACAH (the CAHs arranged in a square array), the average forward transmittance and the average transmission ratio in the region of 550 nm-800 nm are 25.4% and 5.7, respectively. Particularly, the forward transmittance and the transmission ratio at 611 nm are 36.5% and 8.5, respectively. This metal PACAH shows potential of application in secrecy equipment, such as a unidirectional optical transmission wall.

Multichannel optical diode with unidirectional diffraction relevant total transmission

We will show that broadband unidirectional optical transmission with a total transmission maximum inside the band can be obtained for linearly polarized incident waves in the nonsymmetric photonic crystal gratings made of isotropic linear materials at a fixed nonzero or zero angle of incidence. Being based on the merging of diffraction and dispersion effects, the basic physical mechanism studied exploits the transmission channels associated with higher orders, for which asymmetry in the coupling conditions at the two grating interfaces appears when spatial inversion symmetry is broken. Total transmission in one direction and zero transmission in the opposite direction can be obtained due to hybridization of Fabry-Perot type resonances with a diffraction anomaly that yields a diode-like operation regime. Single-beam deflection and two-beam splitting can be obtained, for which transmission can be (nearly) total, if the corrugated side is illuminated. In contrast to the previous studies, it is also shown that unidirectional transmission can appear only at a fixed frequency and only due to diffractions, when total transmission occurs at the noncorrugated-side illumination, being in agreement with the Lorentz Lemma.

On-chip optical isolation in monolithically integrated non-reciprocal optical resonators

Scientists report the fabrication of a nonreciprocal optical resonator with a small length footprint of 290 µm on a silicon-on-insulator substrate. The device achieves unidirectional optical transmission with an isolation ratio of up to 19.5 dB near the telecommunications wavelength of 1,550 nm in a homogeneous external magnetic field.

Unidirectional optical transmission in dual-metal gratings in the absence of anisotropic and nonlinear materials

We predict the unidirectional optical transmission in dual-metal grating structures composed of two gratings with different structures in the absence of anisotropy and nonlinearity. The zero-order unidirectional transmission is achieved. Based on the unique property and by modulating the structural parameters, the transmittance approaches to 0% and 60% in the two opposite directions, respectively.