Asymmetric Gratings Research Articles

Thin-Core-Fiber-Based Long-Period Fiber Grating for High-Sensitivity Refractive Index Measurement

We experimentally demonstrated the fabrication of asymmetric long-period fiber gratings (LPFGs) in thin core fiber by use of focused CO 2 laser beam. The proposed device exhibits a high extinction ratio of over 25 dB at the resonant wavelength and a narrowed 3-dB bandwidth of only 8.7 nm, which is nearly one order of magnitude smaller than that of LPFGs in conventional single-mode fibers. It also exhibits a high polarization-dependent loss of over 20 dB at resonant wavelength. The temperature and external refractive index (RI) sensitivity of the proposed structure are measured to be 46 pm/°C, within a temperature range from 25 °C to 100 °C, and 1047.3 nm/RIU, within the RI range from 1.400 to 1.440, respectively. The temperature induced error is ∼8% for RI measurement. Such long LPFGs may find potential applications of highly sensitive RI sensors in the fields of chemical and biomedical sensing.

Alternative radiative and dark mode-induced multi-broadband transmission in asymmetrical metallic grating

The phenomenon of extraordinary and multi-broadband optical transmission through sub-wavelength metallic grating with symmetry breaking has been theoretically investigated. Under normal incident light, the radiative and dark modes appear in adjacent slits of the grating with asymmetric heights. Through the destructive interference of alternative radiative and dark modes, multiple broadband transmission and enhanced light propagation is realized. The counter-propagating light circulation results in sharp dips in the transmission spectrum. These characteristics of the asymmetric grating could provide highly controllable ways to design novel devices.

A switchable dual-wavelength fiber laser based on asymmetric fiber Bragg grating Fabry–Perot cavity with a SESAM

A switchable dual-wavelength fiber laser with an asymmetric fiber Bragg grating (FBG)–Fabry–Perot (FP) cavity based a semiconductor saturable absorber mirror (SESAM) is proposed and experimentally demonstrated. The proof of concept device consists of a FGB laser with an asymmetric FBG-FP cavity, a SESAM as mode loss modulator, and a intracavity FBG as wavelength selector by changing its operation temperature. The results demonstrate the new concept of dual-wavelength fiber laser based SESAM with asymmetric FBG-FP cavity and the technical feasibility.

Direct laser writing of pyramidal plasmonic structures with apertures and asymmetric gratings towards efficient subwavelength light focusing.

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.

Low-DMD few-mode fiber with distributed long-period grating.

We propose a distributed grating-assisted few-mode fiber (DG-FMF) to reduce the differential mode delay (DMD). By introducing asymmetric long-period fiber gratings with random exposure directions in a step-index few-mode fiber during fiber manufacture process, the strong random mode coupling is generated along the entire length of the fiber. Simulation results show that mean DMD in the DG-FMF can be reduced to less than 12 ns at a distance of 100 km with a period of 514 μm and grating strength of 1×10(-6). The DMD reduction in our DG-FMF is guaranteed within the entire C-band, and in a wide temperature range from -20°C to +60°C.

Tunable terahertz radiation from arbitrary profile dielectric grating coated with graphene excited by an electron beam**Project supported by the National Basic Research Program of China (Grant No. 2014CB339801), the National Natural Science Foundation of China (Grant Nos. 61231005, 11305030, and 612111076), the Fundamental Research Funds for the Central Universities of China (Grant No. ZYGX2013J058), and the National High-tech Research and

In this paper, the enhanced terahertz radiation transformed from surface plasmon polaritons, excited by a uniformly moving electron bunch, in a structure consisting of a monolayer graphene supported on a dielectric grating with arbitrary profile is investigated. The results show that the grating profile has significant influence on the dispersion curves and radiation characteristics including radiation frequency and intensity. The dependence of dispersion and radiation characteristics on the grating shape for both the symmetric and asymmetric gratings is studied in detail. Moreover, we find that, for an asymmetric grating with certain profile, there exist two different diffraction types, and one of the two types can provide higher radiation intensity comparing to the other one. These results will definitely facilitate the practical application in developing a room-temperature, tunable, coherent and miniature terahertz radiation source.

Non-universal behavior of leaky surface waves in a one dimensional asymmetric plasmonic grating

We report on a non-universal behavior of leaky surface plasmon waves on asymmetric (Si/Au/analyte of different height) 1D grating through numerical modelling. The occurrence of the leaky surface wave was maximized (suppressing the Fabry–Perot cavity mode), which can be identified in a reflection spectrum through characteristic minimum. Beyond a specific analyte height (h), new sets of surface waves emerge, each bearing a unique reflection minimum. Furthermore, all of these minima depicted a red-shift before saturating at higher h values. This saturation is found to be non-universal despite the close association with their origin (being leaky surface waves). This behavior is attributed to the fundamental nature and the origin of the each set. Additionally, all of the surface wave modes co-exit at relatively higher h values.

Narrow-band wavelength tunable filter based on asymmetric double layer metallic grating.

In this paper, the optical properties of asymmetric double layer metallic gratings are presented theoretically. The asymmetric structure is achieved by two main factors: one corresponding to moving alternatively metal nanowires of the top layer metallic grating, the other corresponding to possessing different thickness of the top and down layer metallic gratings. Our proposed structure shows one remarkable narrow-band transmission dip at normal incidence, which is distinct different from that of symmetric structure. The results are further confirmed by using different numerical computation methods, and explained by the analytical model of Fano-like resonance. We find that, only when the thickness of the down layer metallic grating has certain fixed value, transmission dip can be transformed from two to only one dip even if the existence of symmetry breaking. However, the wavelength position of the dip can be easily controlled by adjusting the thickness of the top layer metallic grating without the need to modify the structure period, and the width of metal nanowire. Moreover, the influence of other structure parameters on the dip is also investigated. Surprisingly, in order to keep the appearance of one dip in the transmission spectrum of designed structure, there is a good linear approximation between the refractive index of waveguide layer and the thickness of down layer metallic grating, and the relation of waveguide layer thickness and the thickness of down layer metallic grating satisfy secondary polynomial fitting. This work can be used to develop subwavelength metallic-grating-based and narrow-band tunable wavelength filters.

Gap solitons attached to a gapless layer

We consider linear and nonlinear modes pinned to a grating-free (gapless) layer placed between two symmetric or asymmetric semi-infinite Bragg gratings (BGs), with a possible phase shift between them, in a medium with the uniform Kerr nonlinearity. The asymmetry is defined by a difference between bandgap widths in the two BGs. In the linear system, exact defect modes (DMs) are found. Composite gap solitons pinned to the central layer are found too, in analytical and numerical forms, in the nonlinear model. In the asymmetric system, existence boundaries for the DMs and gap solitons, due to the competition between attraction to the gapless layer and repulsion from the reflectivity step, are obtained analytically. Stability boundaries for solitons in the asymmetric system are identified by means of direct simulations. Collisions of moving BG solitons with the gapless layer are studied too.

Active graphene plasmonic grating for terahertz beam scanning device

We have proposed an electrically controllable terahertz (THz) beam scanning device based on asymmetric graphene plasmonic grating. It is proved that the dispersion relation of surface plasmon waves can be efficiently manipulated on the periodic metal–dielectric–graphene plasmonic structure by an external bias voltage, and thus the derivation angle of the THz beam can be actively tuned. The numerical results show that the radiation angle of THz waves through this device can be tuned from −18° to 18° with the bias changing from 53.5mV into 368.8mV at the operating frequency of 0.5THz. This electrically active beam scanning device has important application in the THz imaging, radar and communication systems.

Mueller matrix ellipsometric detection of profile asymmetry in nanoimprinted grating structures

Mueller matrix ellipsometry (MME) is applied to detect foot-like asymmetry encountered in nanoimprint lithography (NIL) processes. We present both theoretical and experimental results which show that MME has good sensitivity to both the magnitude and direction of asymmetric profiles. The physics behind the use of MME for asymmetry detection is the breaking of electromagnetic reciprocity theorem for the zeroth-order diffraction of asymmetric gratings. We demonstrate that accurate characterization of asymmetric nanoimprinted gratings can be achieved by performing MME measurements in a conical mounting with the plane of incidence parallel to grating lines and meanwhile incorporating depolarization effects into the optical model. The comparison of MME-extracted asymmetric profile with the measurement by cross-sectional scanning electron microscopy also reveals the strong potential of this technique for in-line monitoring NIL processes, where symmetric structures are desired.

Dual-wavelength erbium-doped fiber laser with asymmetric fiber Bragg grating Fabry-Perot cavity

A novel dual-wavelength fiber laser with asymmetric fiber Bragg grating (FBG) Fabry-Perot (FP) cavity is proposed and experimentally demonstrated. A couple of uniform FBGs are used as the cavity mirrors, and the third FBG is used as intracavity wavelength selector by changing its operation temperature. Experimental results show that by adjusting the operation temperature of the intracavity wavelength selector, a tunable dual-wavelength laser emission can be achieved. The results demonstrate the new concept of dual-wavelength lasing with asymmetric FBG FP resonator and its technical feasibility.

Scatterometry specialized for a highly asymmetric triangular grating on a transparent substrate

We present a specialized scatterometry method to measure the groove profiles of highly asymmetric triangular gratings. Compared with the conventional scatterometry working in a specular way, this method utilizes diffraction spectra of the reflected ±1st orders and is good at measuring this kind of asymmetric grating with a higher sensitivity. In our work, diffraction efficiency angular spectra at a single wavelength are measured and passed on to a parameter optimization process to retrieve three profile defining parameters. Final results are compared with the ones from an atomic force microscope and discrepancies are discussed and explained.

Waveguide Design at Infrared Wavelength With Asymmetric Dielectric Surface Gratings

In this paper, we propose waveguide design at infrared wavelength with asymmetric dielectric surface gratings. Surface gratings are able to generate the surface plasmon polariton effect at the outlet of radiated light through an aperture. The property and the shape of gratings are key factors behind generating collimated beams from subwavelength slits or holes surrounded by corrugated surfaces. Grating design is conducted by structural optimization based on the phase field method. The design objective is set to maximize the Poynting vector in the measuring area, and it is focused on finding a clear shape of grating structure for the manufacturing feasibility to be taken into account. The simulation and optimization process is performed by using the commercial package COMSOL associated with the MATLAB programming.

Periodic dielectric structures for light-trapping in InGaAs/GaAs quantum well solar cells

We study dielectric diffraction gratings for light-trapping in quantum well solar cells and compare their performance with plasmonic and Lambertian light-trapping structures. The optimum structural parameters are identified for symmetric uni-periodic, symmetric bi-periodic and asymmetric bi-periodic gratings. The enhancement in short-circuit current density from the quantum well region with respect to a reference cell with no diffraction grating is calculated. The ratio of this enhancement to the maximum achievable enhancement (i.e. no transmission losses) is 33%, 75% and 74%, respectively for these structures. The optimum asymmetric and symmetric bi-periodic structures perform closest to Lambertian light-trapping, while all three optimum grating structures outperform optimum plasmonic light-trapping. We show that the short-circuit current density from the quantum well region is further enhanced by incorporating a rear reflector.

Polarization-based refractive index sensor using dual asymmetric long-period gratings in ridge waveguides

We propose a sensor scheme operating in the wavelength band of 1460-1530 nm (S band) that utilizes two orthogonally polarized eigenmodes in a ridge-waveguide geometry by employing a pair of nonidentical asymmetric long-period gratings. Numerical simulations show a high sensitivity ~4900 nm/RIU (refractive index unit) over the refractive index range 1.33-1.34 exhibiting a relative resonance shift of 1 pm for an index change of ~10(-7). Such high resolution sensors enable detection of minute changes with potential applications to various biochemical industries.

Visual Stimulation Reverses the Directional Preference of Direction-Selective Retinal Ganglion Cells

Direction selectivity in the retina is mediated by direction-selective ganglion cells. These cells are part of a circuit in which they are asymmetrically wired to inhibitory neurons. Thus, they respond strongly to an image moving in the preferred direction and weakly to an image moving in the opposite (null) direction. Here, we demonstrate that adaptation with short visual stimulation of a direction-selective ganglion cell using drifting gratings can reverse this cell's directional preference by 180°. This reversal is robust, long lasting, and independent of the animal's age. Our findings indicate that, even within circuits that are hardwired, the computation of direction can be altered by dynamic circuit mechanisms that are guided by visual stimulation.

All-fiber bandpass filter based on asymmetrical modes exciting and coupling

A low cost all-fiber bandpass filter is demonstrated by fabricating an asymmetric long-period fiber grating (LPFG) in an off-set splicing fiber structure of two single mode fibers in this paper. The main principle of the filter is that the asymmetric LPFG written by single-side CO2 laser irradiation is used to couple the asymmetric cladding modes excited by the offset-coupling of the splicing point between the single mode fiber and the grating, and the left core mode of the splicing point cannot be coupled to the right fiber core, hence the interference effect is avoided. So the bandpass characteristics in the transmission spectrum are achieved. The designed filter exhibits a pass band at a central wavelength of 1565.0nm with a full-width at half-maximum bandwidth of 12.3nm.

An analysis on mode selection by V–I transmission matrix in DBR laser with asymmetric fiber gratings

The V–I Transmission Matrix Method (VITMM) which is well known in the microwave engineering field was firstly applied to analyze the output spectra of the Distributed Bragg Reflector (DBR) laser, formed by asymmetric fiber gratings as resonator mirrors. One mirror is the uniform Bragg grating and the other is chirped grating. A theoretical model of grating was established, and then a numerical simulation of the mode selection in DBR laser with asymmetric fiber gratings was presented. Simulation results show that VITMM, with calculation error less than 0.1%, could save the calculation time compared to the Rouard method. In the experiment, the setup design of the single-longitudinal-mode laser output at 1544.7nm was carried out, and the result, which lasted about 10min, observed on an optical spectrum analyzer, demonstrates the feasibility of VITMM to address the mode output issues of DBR fiber laser.

Acoustic transmission through asymmetric grating structures made of cylinders

Based on the multiple scattering theory, we have developed a highly efficient numerical method to calculate the acoustic transmission and reflection coefficients for complex grating structures made of cylinders. This method can provide the detailed information for each independent diffraction order, which is particularly useful in studying the exotic transmission behavior through spatially asymmetric gratings. Our numerical results manifest that, in general, such gratings produce asymmetric transmission if only the frequency is high enough to create the nonzero order diffraction. An intuitive and valid scheme is further proposed to suppress the transmission along one of the incident directions, producing an almost unidirectional acoustic transmission. Such kind of unidirectional transmission could be very useful in designing acoustic rectifying devices.