Asymmetric Gratings Research Articles

基于三个非对称光纤布拉格光栅的双腔窄带宽双波长透射

基于三个非对称光纤布拉格光栅的双腔窄带宽双波长透射

An ultra-high resolution mid-infrared dual-band notch filter based on an asymmetric grating structure

An asymmetric grating structure was investigated for realizing mid-infrared (mid-IR) ultra-high resolution dual-band notch filters. The structure is constructed by an asymmetric gold grating on the stack of an Al2O3 layer and a CaF2 substrate. The grating consists of three gold strips in each unit cell, and the center-to-center distances between two adjacent gold strips are different. The filter provides two high-extinction transmission dips over a broad spectral region. One is at a wavelength of ∼3.8381μm with a full-width at half-maximum (FWHM) of ∼0.7 nm, and the other is at a wavelength of ∼3.8435μm with a FWHM of ∼1.7 nm. The quality-factor (Q-factor) of the filter is >2200. It was found that the two transmission dips result from the excited (+1) and (-1) resonance modes, respectively. Thus, there is an interesting feature that either of the two transmission dips does not split at oblique incidence.

Phase-controlled electromagnetically induced symmetric and asymmetric grating in an asymmetric three-coupled quantum well.

Fraunhofer light diffraction of a weak probe field passing through an asymmetric three-coupled quantum well, which is driven by a standing wave and two coupling laser fields, is investigated. Depending on which transitions are coupled by the probe and standing field, two schemes are considered. It is demonstrated that owing to the closed-loop transition, optical properties and the diffraction pattern of the probe field in both schemes are highly affected by the relative phase of the applied fields and can be controlled by this parameter. Moreover, it is shown that the proposed schemes have multifunction capabilities. In the first scheme, as a result of varying relative phase, the electromagnetically induced absorption phase grating turns to the electromagnetically induced gain phase grating with remarkable efficiency, while in the latter scheme, a significant result is revealed: Tuning the relative phase can lead to inducing optical parity-time symmetry, which gives rise to an asymmetric diffraction grating. Such an all-optical phase-sensitive operation could be useful in optical switching and optical communications.

Tunable infrared asymmetric light transmission and absorption via graphene-hBN metamaterials

We theoretically prove in this paper that using planar multilayer graphene-hexagonal boron nitride (hBN) metamaterials (GhMMs) can yield ultrabroadband and high-contrast asymmetric transmission (AT) and asymmetric absorption (AA) of light. The AA and AT features are obtained in the far-infrared (FIR) and mid-infrared (MIR) regions for normally incident light with transverse magnetic polarization. Here, the GhMMs are integrated with two asymmetric gratings of Ge and are composed of alternating multilayers of graphene (11 multilayers) and hBN layers (10 layers). Moreover, the total subwavelength thickness of the hybrid structures is about 3 μm, being less than half of the free-space wavelength up to nearly 50 THz. This approach—which is similar to the one introduced by Xu and Lezec [Nat. Commun. 5, 4141 (2014)] for a passive hyperbolic metamaterial operating in the visible range—is based on the excitation of high-β modes of the GhMM with different transmission characteristics. In addition to being ultrabroadband and high-contrast, AT and AA features of the proposed GhMMs can be actively tuned by varying the chemical potential of graphene. Furthermore, it is shown that an on-off switching of AT factor at FIR and selective tunability at MIR frequencies can be obtained via varying μ. Due to its subwavelength and planar configuration and active operation, these multilayer graphene-hBN metamaterials with AT and AA characteristics hold promise for integration with compact optical systems operating in the MIR and FIR ranges and are suitable for applications such as optical diodes, sensors, and thermal emitters.

On the Asymmetric Generation of a Superradiant Laser with a Symmetric Low-Q Cavity

On the basis of numerical solution to the Maxwell–Bloch equations within the one-dimensional two-level model of a superradiant laser with a symmetric cavity in which the photon lifetime is less than the incoherent relaxation time of the optical-dipole oscillations of active centers, the phenomenon of the spontaneous asymmetric generation of counter-propagating waves under continuous uniform pumping of the active medium is revealed. It is shown that the observed symmetry breaking of the spatial profiles of counter-propagating waves of the electromagnetic field and, the polarization and inversion of the population of active-medium levels in the case of a small inhomogeneous broadening of the spectral line of its working transition occurs due to the asymmetric half-wave nonlinear grating of inversion of the energy-level population produced by these waves.

Experimental Demonstration of an Acoustic Asymmetric Diffraction Grating Based on Passive Parity-Time-Symmetric Medium

Passive parity-time-symmetric medium provides a feasible scheme to investigate non-Hermitian systems experimentally. Here, we design a passive PT-symmetric acoustic grating with a period equal to exact PT-symmetric medium. This treatment enhances the diffraction ability of a passive PT-symmetric grating with more compact modulation. Above all, it eliminates the first-order disturbance of previous design in diffraction grating. Additional cavities and small leaked holes on top plate in a 2D waveguide are used to construct a parity-time-symmetric potential. The combining between additional cavities and leaked holes makes it possible to modulate the real and imaginary parts of refractive index simultaneously. When the real and imaginary parts of refractive index are balanced in modulation, asymmetric diffraction can be observed between a pair of oblique incident waves. This demonstration provides a feasible way to construct passive parity-time-symmetric acoustic medium. It opens new possibilities for further investigation of acoustic wave control in non-Hermitian systems.

Narrow-band and high-contrast asymmetric transmission based on metal-metal-metal asymmetric gratings

A narrow-band and high-contrast asymmetric transmission (AT) device based on metal-metal-metal (M-M-M) asymmetric grating structure is proposed and investigated. Significantly distinct from previous reports, the upper and lower metallic silver (Ag) gratings are connected by a very thin metallic Ag film, without any dielectric spacer layer or subwavelength slit. Under forward incidence, the M-M-M structure supports efficient surface plasmon polaritons (SPPs) excitation and tunneling, more importantly, it promotes direct and thus high-efficiency SPPs decoupling, enabling high forward transmittance. While under backward incidence, the M-M-M structure offers not only high reflection by the Ag film but also a strong near-field coupling effect between the upper and lower gratings, which further suppresses backward transmittance, leading to near-zero backward transmittance. In addition, the M-M-M structure is optimized for narrow-band operation by employing grating groove depth effect and multiple interference effect. Numerical simulation results demonstrate that high-performance AT with high-quality factor (Q≈91), narrow-bandwidth (6.7 nm) and high contrast ratio is achieved, with forward transmittance of 0.72 and backward transmittance of 0.0015 at visible light (610 nm). Our work provides an alternative and simple way to high-performance AT devices.

Asymmetric directional coupler type contra-directional polarization rotator Bragg grating: design

We report contra-directional coupler type polarization rotation Bragg gratings using silicon waveguides. Waveguides incorporating a Bragg grating for polarization rotation and having different widths are placed near to each other. A single grating can separate the backward diffraction light. The structure is optimized considering the field strength ratio between waveguides. A distinct polarization rotation wavelength peak is obtained by 3D-FDTD simulation.

Robust Smartphone Assisted Biosensing Based on Asymmetric Nanofluidic Grating Interferometry.

Point-of-care systems enable fast therapy decisions on site without the need of any healthcare infrastructure. In addition to the sensitive detection, stable measurement by inexperienced persons outside of laboratory facilities is indispensable. A particular challenge in field applications is to reduce interference from environmental factors, such as temperature, to acceptable levels without sacrificing simplicity. Here, we present a smartphone-based point-of-care sensor. The method uses an optofluidic grating composed of alternating detection and reference channels arranged as a reflective phase grating. Biomolecules adsorbing to the detection channel alter the optical path length, while the parallel reference channels enable a direct common mode rejection within a single measurement. The optical setup is integrated in a compact design of a mobile readout device and the usability is ensured by a smartphone application. Our results show that different ambient temperatures do not have any influence on the signal. In a proof-of concept experiment we measured the accumulation of specific molecules in functionalized detection channels in real-time and without the need of any labeling. Therefore, the channel walls have been modified with biotin as capture molecules and the specific binding of streptavidin was detected. A mobile, reliable and robust point-of-care device has been realized by combining an inherently differential measurement concept with a smartphone-based, mobile readout device.

Influence of asymmetric grating structures on measurement accuracy in integrated phase grating interference-based metrology.

We present an analysis of the position error caused by asymmetry in phase grating measurement systems, demonstrating that such error is dependent on the structure of the grating and the measurement order. The asymmetry-induced error varies as a function of diffraction order, with the nature of this variation depending on the grating structure. We verify our analysis by comparing the average error obtained with a multiorder interferometer to the error simulated using an atomic force microscopy scanning profile. The deviations of different orders of asymmetry-introduced error are less than ±3 nm. This result provides an explanation of diffraction fields and measurement errors from asymmetric gratings.

Асимметричная генерация в сверхизлучающем лазере с симметричным низкодобротным резонатором

On the basis of numerical solution to the Maxwell-Bloch equations within an one-dimensional two-level model of a superradiant laser with a symmetric cavity where a photon lifetime is less than an incoherent relaxation time of the optical dipole oscillations of active centers, we find that a spontaneous asymmetric generation of the counter-propagating waves is possible under a continuous homogeneous pumping of an active medium. We show that such a phenomenon of a symmetry breaking of the spatial profiles of the counter-propagating waves of an electromagnetic field as well as the polarization and population inversion of an active medium in the considered case of a weak inhomogeneous broadening of an operating transition is caused by an asymmetric half-wavelength nonlinear grating of the population inversion of the transition’s energy levels which is produced by these waves.

The fabrication and characterization of asymmetric gratings using the optical Talbot effect

We fabricate asymmetric gratings, which have the ratio between the opening window and period or the opening fraction, f<0.5. The gratings were fabricated by using CNC milling machine to engrave a pattern on transparent acrylic sheets. The characterization of the gratings was performed with the optical Talbot effect. The results of the Talbot pattern, as well as the intensity along the longitudinal distance, can apparently identify the opening window and grating period. The grating with the opening window of 23 μm, period of 100 μm, and thus the opening fraction of 0.23 can be obtained with our method. This type of grating can be used in many practical applications such as optical spectroscopy.

Influence of Subwavelength Grating Asymmetry on Long-Wavelength Infrared Transmittance Filters

Subwavelength dielectric gratings provide a means of narrowband spectral filtering by coupling to quasi-guided leaky modes, providing a high reflectivity background and narrowband transmittance via Fano resonance behavior. Geometrically asymmetric two-step gratings enable normal incidence filtering by accessing symmetry-protected leaky modes. The impact of asymmetry on leaky mode coupling and optical filter response are studied via simulation and described by the Fano resonance theory. The influence of asymmetry introduced by grating step height and angular cone of incidence are verified experimentally in silicon/air gratings in the long-wavelength infrared (8–14 μ m).

Ultra-broadband asymmetric transmission metallic gratings for subtropical passive daytime radiative cooling

By simultaneously reflecting solar irradiation and emitting thermal radiation to the cold universe through an atmospheric window lying within 8–13 µm of the electromagnetic spectrum, surfaces could be cooled below ambient temperature under direct sunlight. However, both humidity and cloud coverage can raise the sky emissivity, significantly intensifying thermal emission within the 8–13 µm spectrum. A radiative cooler is strongly absorptive of the additional heat load, and eventually, the cooling capacity drops in a humid environment. In this work, we suggest integrating the cooler with an asymmetric electromagnetic transmission (AEMT) window, which permits outgoing radiative transmission, but reflects incoming radiation of the same wavelengths, so as to recover the cooling performance under a humid climate. This study aims at discussing the working principle of an AEMT enhanced radiative cooler quantitatively as well as demonstrating a feasible design of an AEMT device for radiative cooling applications. First, a theoretical model on the basis of conservation of energy is developed for the prediction of cooling performance of the AEMT enhanced radiative cooling systems. Cooling power is solved for a humid semi-transparent sky condition numerically, which shows that an AEMT window with forward and backward transmittances of 0.8 and 0.4 respectively could restore the cooling power of the passive radiative cooler by 57%. Second, validated finite difference time domain (FDTD) simulations reveal that an AEMT window implemented by near-wavelength tapered metallic gratings could meet the desired transmission ratio (i.e. contrast ratio of 2 within 8–13 µm spectrum) for radiative cooling.

Underwater unidirectional acoustic transmission through a plate with bilateral asymmetric gratings

In this paper, a novel underwater unidirectional acoustic transmission (UAT) device consisting of a plate with bilateral asymmetric gratings is proposed and numerically investigated. The transmission spectra, the acoustic intensity field distributions, and the displacement field distributions are numerically calculated based on the finite element method. The transmission spectra show that the proposed device exhibits different UAT effects in three bands. The acoustic intensity field distributions demonstrate that the proposed device can realize UAT, which agree well with the transmission spectra. The mechanism is discussed by analyzing the displacement field distributions, and the UAT is attributed to the symmetric mode excited in brass plate. Furthermore, the effects of the lattice constant, the upper slit width, and the lower slit width on bands are discussed. Our design provides a good reference for designing underwater UAT devices and has potential applications in some fields, such as medical ultrasonic devices, acoustic barrier, and noise insulation.

Enhancement of second harmonic generation using a novel asymmetric metal–graphene–insulator–metal plasmonic waveguide

Here, we propose a novel plasmonic structure, called asymmetric plasmonic nanocavity grating (APNCG), which is shown to dramatically enhance nonlinear optical process of second harmonic generation (SHG). The proposed structure consists of two different metals on both sides of lithium niobate and a thin layer of graphene. By using two different metals the nonlinear susceptibility of the waveguide would be increased noticeably causing to increase SHG. On the other hand, it consists of two identical gratings on one side. By two identical gratings, the pump beam is coupled to two opposing SPP waves, which interfere with each other and result in SPP standing wave in the region between the two gratings. The distance between two gratings will be optimized to reach the highest SHG. It will be shown that by optimizing the geometry of proposed structure and using different metals, field enhancement in APNCG waveguides can result in large enhancement of SHG.

Multiband guided-mode resonance filter in bilayer asymmetric metallic gratings

In this paper, a guided-mode resonances (GMRs) based multiband filter in bilayer asymmetric metallic gratings is presented. Four sharp dips are generated in the frequency range of 1.4–2.0 THz, which are induced by the split of two GMR modes (TE0 and TM0) due to the break of the structure’s symmetry. This symmetry of the structure depends on the relative position between the upper layer and lower layer gratings. Therefore, by choosing proper lateral displacement, the split of TE0 or/and TM0 modes can be eliminated. Two-, three-, and four- GMRs based polarization insensitive or sensitive filters are demonstrated numerically.

A Ring-Core Optical Fiber Sensor With Asymmetric LPG for Highly Sensitive Temperature Measurement

A compact and highly sensitive temperature sensor based on mode conversion in the ring-core fiber (RCF) incorporating an asymmetric long-period grating (aLPG) is proposed and theoretically simulated. The RCF is composed of a hollow core and a high refractive index ring. The aLPG inscribed in the upper half ring generates mode conversion between modes with different azimuthal orders within the ring. A composite resonant dip containing dual-resonant dips which are generated by mode conversion from HE $_{11x}$ to $\mathrm{T}E_{01}$ and HE $_{21x}$ can be observed in the transmission spectrum. The hollow core is infiltrated with a refractive index liquid whose material refractive index can be modulated via temperature, so the wavelength of the resonant dip can be modulated by temperature, which displays a high sensitivity of 12.92 nm/°C ( $32.3~\mu \text{m}$ /RIU). Furthermore, the relationship between 3-dB bandwidth of the resonant dip and temperature is also obtained by polynomial fitting with a high fitting degree of 99.99%.

Surface plasmon polaritons on the thin metallic film coated with symmetrical and asymmetrical dielectric gratings

Surface plasmon polartions (SPPs) on the planar metallic film coated with dielectric gratings on both sides are studied theoretically and numerically. It is found that the coupling between SPP modes can be efficiently tuned by the relative shift of the gratings on the two sides, including both the symmetrical and asymmetrical cases. The absorption due to the excitation of SPPs can be enhanced or suppressed as the coupling of SPP modes varies. A coupled mode theory is employed to analyze the coupling between these SPP modes in addition to full-wave simulations. The results obtained by the coupled mode theory and numerical simulations agree well with each other. The investigation of coupled SPP modes may have potential applications in photonic devices.

Silicon lateral-apodized add-drop filter for on-chip optical interconnection.

A lateral-apodized add-drop filter is demonstrated in a multimode asymmetric waveguide Bragg grating. This design utilizes two individual superposed gratings with the same sidewall corrugation depth. The strong side lobes of the grating filter are efficiently suppressed by mapping the target apodization profile into lateral shifts between the periods of the two gratings. Compared with other apodized technology, this device is easier to be realized. Experimental results show that the side-lobes suppression ratio can reach 18.5dB, and a bandwidth of 9.5nm is achieved by a large corrugation width of 150nm. The insertion loss at the drop port is only 0.8dB, and the extinction ratio is up to 24dB at the through port.