Multiline two-dimensional guided-mode resonant filters

A novel multiline filter using a two-dimensional guided-mode resonant (GMR) filter is proposed. The filter concept utilizes the multiple planes of diffraction produced by the two-dimensional grating. Multiple resonances are obtained by matching the guided modes in the different planes of diffraction to different wavelengths. It is shown that the location and the separation between resonances can be specifically controlled by modifying the periodicity of the grating and the other physical dimensions of the structure. This is in contrast to the one-dimensional GMR filters where the location of the resonances is material dependent. Two-line reflection filter designs with spectral linewidths less than 1 nm and a controllable spectral separation of up to 23% of the short resonance wavelength are presented using rectangular-grid grating GMR structures. Three-line filters are designed in hexagonal-grid grating GMR structures with two independently controllable resonance locations.     

Dispersion relation of guided-mode resonances in multimode grating waveguide structures

It is well known that the location of guided-mode resonance (GMR) in grating waveguide structures closely tracks the leaky mode dispersion curves. In this paper, taking Bragg reflection due to periodicity and interaction between different modes into account, we first present a schematic diagram of the dispersion relations of leaky modes in multimode grating waveguide structures, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Due to the perturbation of the grating layer, the interaction between different resonance modes (transverse standing waves) is inevitable. This transverse interference will result in the non-Bragg nature resonance band gaps in the dispersion curves. Exploiting the characteristics of leaky mode dispersions over the full range of the first Brillouin zone, we hoped we could gain some insight into the relationship among the mode interactions, band gaps, and their benefits to optical elements utilizing the GMR effect in grating waveguide structures. Finally, a specific structure is analyzed.     

Guided-mode resonance filter with an antireflective surface consisting of a buffer layer with refractive index equal to that of the grating

A type of guided-mode resonance filter (GMRF) with an antireflective surface consisting of a buffer layer with refractive index equal to that of the grating is proposed, and the approximate design approach is presented. The relation between the filter linewidth and the coupling loss is used to analyze the filter properties by using different derivation methods. It is shown that the dispersion equation of the slab waveguide may provide a reliable approximation in estimating the resonance locations of the GMRF with an antireflective surface. The buffer layer functions as an intermediate layer between the grating and waveguide layers. This reduces the coupling and out-coupling of a mode of the waveguide, which results in significant reduction of the coupling loss and the filter linewidth with the antireflection condition nearly preserved. By changing the thickness of the buffer layer, different linewidths can be obtained with spectral symmetry and sideband suppression almost kept the same. The slight shift of resonance wavelengths due to the variety of the buffer layer thickness and the etching effects can be adjusted to the design value by changing the grating period. Accurate etch depth control to avoid underetching is necessary. The electric field distributions under resonance conditions shows that the buffer layer increases the mode confinement, thus narrowing the filter linewidth.     

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.     

Controlling the spectral response in guided-mode resonance filter design

Techniques for controlling spectral width are used in conjunction with thin-film techniques in the design of guided-mode resonance (GMR) filters to provide simultaneous control over line-shape symmetry, sideband levels, and spectral width. Several factors that could limit the minimum spectral width are discussed. We used interference effects for passband shaping by stacking multiple GMR filters on top of one another. A design is presented for a 200-GHz telecommunications filter along with a tolerance analysis. Compared with a conventional thin-film filter, the GMR filter has fewer layers and looser thickness tolerances. Grating fabrication tolerances are also discussed.     

Guided-mode-resonance-coupled plasmonic-active SiO(2) nanotubes for surface enhanced Raman spectroscopy

We demonstrate a surface enhanced Raman scattering (SERS) substrate by integrating plasmonic-active SiO(2) nanotubes into Si(3)N(4) gratings. First, the dielectric grating that is working under guided mode resonance (GMR) provides enhanced electric field for localized surface plasmon polaritons on the surface of metallic nanoparticles. Second, we use SiO(2) nanotubes with densely assembled silver nanoparticles to provide a large amount of "hot spots" without significantly damping the GMR mode of the grating. Experimental measurement on Rhodamine-6G shows a constant enhancement factor of 8?～?10 in addition to the existing SERS effect across the entire surface of the SiO(2) nanotubes.     

Effect of interface structure correlation on magnetoresistance of Fe/Cr multilayers

Effect of interfacial roughness on giant magnetoresistance (GMR) in Fe/Cr multilayers has been studied. A set of samples is prepared by simultaneously depositing on a set of float-glass (FG) substrates with varying rms surface roughness. This causes the correlated part of the rms roughness to vary from sample to sample. Another set of specimen is irradiated with 200 MeV Ag ions in order to induce uncorrelated roughness at the interfaces. In both the cases morphological and other microstructural features of different multilayers remained similar, thus allowing one to separate the effect of interface roughness from that of morphological changes. GMR measurements on these multilayers show that increasing interfacial roughness causes GMR to decrease nonlinearly. It is found that the effect of uncorrelated part of the roughness is much stronger than that of the correlated part.     

Wide-angle transmissive filter based on a guided-mode resonant grating

A wide-angle color filter for TE-polarization is proposed based on a guided-mode resonant grating selectively coated with a metal film. The effects of the structure parameters to the resonant transmission characteristics are discussed by the rigorous coupled-wave analysis. Its optimal structure was given for a green color filter with a peak transmission of 77.6%, a full width at half-maximum (FWHM) of ～120 nm, and a good angular tolerance up to ±40°. Compared to the similar structure with a nonresonant dielectric grating, it decreases the FWHM effectively for the resonance induced by the guided-mode resonant grating. The numerical results indicate that the designed structure has a good wide-angle transmission performance.     

Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a pi phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to 10(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.     

Grating-cavity continuous-wave optical parametric oscillators for high-resolution mid-infrared spectroscopy

The use of grating as a spectral filter provides a simple way of improving wavelength tuning and stability of continuous-wave optical parametric oscillators (cw OPOs). In this paper, we discuss how to design and use such grating-cavity cw OPOs for high-resolution spectroscopy in the molecular fingerprint region at ～3μm. The first design presented in the paper is based on a metal-coated diffraction grating, which produces fast and broad wavelength tuning and high wavelength stability. The second design uses a bulk Bragg grating for high optical power and good spectral purity. We report a new Bragg-grating OPO and demonstrate its use in a Doppler-free absorption spectroscopy of CH4 at ～3.22μm. In addition, we describe a new balanced detection scheme, which can be used to improve the signal-to-noise ratio of absorption measurements if the measurement noise is limited by the intensity noise of the mid-infrared OPO.     

Insertion of a specular reflective and transmissive nano-oxide layer into giant magnetoresistance spin-valve structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.     

Comparative analysis of low-pass filters for the demodulation ofprojected gratings in 3-D adaptive profilometry

In 3-D adaptive profilometry based on structured light projection, the choice of the low-pass filter to he used in the deformed pattern demodulation is crucial. In this paper, we have studied the performance of a typical finite impulse response (FIR) and of an infinite impulse response (IIR) Butterworth low-pass filter. Adaptiveness of the filters to both coarse and small variations of the grating frequency has been investigated. The ability of the filters to adapt to coarse changes of the grating frequency has been quantified in terms of their speed of synthesis, while the ability of the filters to tolerate small variations of the grating frequency has been quantified by measuring the residual phase errors. The analysis shows that the IIR Butterworth filter performs better than the FIR filter both in the coarse and in the fine grating frequency variation cases     

Design and characteristics of polarization-insensitive resonant gratings for color filtering

A method of designing polarization-insensitive color filters with guided-mode resonance grating is presented. The influence of incident conditions on exciting waveguide mode is investigated, and we find that polarization insensitivity may occur at the full conical incidence. Using rigorous coupled-wave analysis, we mainly analyze the effects of waveguide thickness and fill factor on the diffracted efficiency of the grating filter. The final structural parameters of the devices for three primary colors are collected after optimization, and the calculated results show that the spectral reflectance of each color filter is basically identical for different polarization states of incident light. Moreover, field analysis by the finite-difference time-domain technique indicates that two symmetric modes are excited between the waveguide layer and substrate under full conical incidence, which is coincident with the previous theoretical study. The reported work will eliminate the restriction of polarization-dependence of color filters and greatly expand their application range.     

Effects of symmetrical and asymmetrical bumps on plasmonic behavior in a metallic grating

The influence was investigated of symmetrical and asymmetrical bumps on plasmonic behavior in a metallic grating. Compared with the resonant peaks of a smooth-slit grating, the peaks of odd (even) modes exhibit a blue-shift (red-shift) and the transmission properties are tunable by the size of bumps when the bumps are laid symmetrically. However, the peaks of all modes only exhibit a red-shift when the grating is engraved with a bump and a cut in the same slit (namely bend slit), and the bend slit can achieve the properties of a straight slit. Additionally, it is found that the dips in transmission spectrum can be adjusted by altering the size of the bumps if the bumps are set asymmetrically. Fabry–Pérot-like resonance, field interference and phase resonance mechanisms have been suggested for the physical origins of these observations.     

Far field of gratings with rough strips

In this work, we analyze the far-field pattern produced by a grating made of strips with two different random roughness levels. The efficiency and shape of the diffraction orders is obtained, which are shown to depend on the statistical properties of roughness. We assume for the calculations that the grating can be used in a mobile mechanical system. A preliminary experimental approach which partially corroborates the theoretical results is also performed.     

Nonreciprocal microwave transmission in metastructures with transversely magnetized ferrite plate and a grating of resonant elements

Nonreciprocal transmission of a linearly polarized electromagnetic wave at a ferromagnetic resonance (FMR) frequency has been observed in a metastructure comprising a transversely magnetized ferrite plate and a grating of resonant elements. The nonreciprocity of wave transmission is observed for the metastructure arranged along the axis of a rectangular waveguide and even in the free space between transmitting and receiving waveguides, where the effect does not take place when there is no grating. The observed phenomenon is explained by the formation of a surface wave with elliptic or circular polarization on the grating. The nonreciprocity reaches maximum (>35 dB) under the conditions of mutual influence between the FMR and the resonance of grating elements for certain values of the certain frequency and magnetic field, which depend on the distance between the ferrite plate and the grating. The nonreciprocal effects have been observed for grating elements in the form of double split rings, polyhedral loops, and dipoles. The results may be of interest for the development of new nonreciprocal devices and multifunctional metastructures such as decoupling elements for quasi-optical systems and two-frequency decoupling filters for counterpropagating waves in the gigahertz and terahertz range.     

Co／Cu／Co三明治膜的巨磁电阻效应研究

采用超高真空电子束蒸发方法在硅单晶衬底上制备了Ｃｏ／Ｃｕ／Ｃｏ三明治膜,研究了衬底晶向、过渡工层材料和生长室温度对三明治膜中巨磁电阻效应的影响;结合原子力显微镜表面形貌观察,探讨了三明治膜表面（界面）组糙度与其巨磁电阻效应的内在关系;还分析了三明治膜经高温热退火后巨磁电阻效应退化的物理机制。     

Tunable dual-band nearly perfect absorption based on a compound metallic grating

Traditional metallic gratings and novel metamaterials are two basic kinds of candidates for perfect absorption. Comparatively speaking, metallic grating is the preferred choice for the same absorption effect because it is structurally simpler and more convenient to fabricate. However, to date, most of the perfect absorption effects achieved based on metamaterials are also available using an metallic grating except the tunable dual(multi)-band perfect absorption. To fill this gap, in this paper, by adding subgrooves on the rear surface as well as inside the grating slits to a free-standing metallic grating, tunable dual-band perfect absorption is also obtained for the first time. The grooves inside the slits is to tune the frequency of the Cavity Mode(CM) resonance which enhances the transmission and suppresses the reflectance simultaneously. The grooves on the rear surface give rise to the phase resonance which not only suppresses the transmission but also reinforces the reflectance depression effect. Thus, when the phase resonance and the frequency tunable CM resonance occur together, transmission and reflection can be suppressed simultaneously, dual-band nearly perfect absorption with tunable frequencies is obtained. To our knowledge, this perfect absorption phenomenon is achieved for the first time in a designed metallic grating structure.     

Tunable, oblique incidence resonant grating filter for telecommunications

We have designed a tunable, oblique-incidence resonant grating filter that covers the C band as an add-drop device for incident TE-polarized light. We tune the filter by tilting a microelectromechanical systems platform onto which the filter is attached. The fabrication tolerances as well as the role of finite incident-beam size and limited device size were addressed. The maximum achievable efficiency of a finite-area device as well as a scaling law that relates the resonance peak width and the minimum device size is derived. In good agreement with simulations, measurements indicate a negligible change in shape of the resonance peak from 1526 nm at a 45 degrees angle of incidence to 1573 nm at a 53 degrees angle with a full width at half-maximum of 0.4 nm. In this range the shift of the peak wavelength is linear with respect to changes in the angle of incidence.     

Random phase mask as a model of rough surface. Part I. Theory

There have been numerous attempts to correlate light scattering measurements with the characteristics of surface roughness. Another but considerably more difficult approach is to solve the inverse scattering problem. A number of different empirical models of surface roughness have been used to characterize surfaces including the sine grating, triangle grating (echelette), and rectangle grating. In this paper, we use, for the first time, the random phase mask model, which is a two-dimension orthogonal grating with a stochastic distribution of square “defects” with size a. We describe our calculations of the polarizing characteristics of the random phase mask and discuss the influence of each of the parameters of defects on polarizing angles. The analysis was carried out for multiple-angles-of-incidence ellipsometric measurements.