Effects of interplay between metal subwavelength slits on extraordinary optical transmission

In this paper we study the extraordinary optical transmission of one-dimensional multi-slits in an ideal metal film.The transmissivity is calculated as a function of various structural parameters.The transmissivity oscillates,with the period being just the light wavelength,as a function of the spacing between slits.As the number of slits increases,the transmissivity varies in one of three ways.It can increase,attenuate,or remain basically unchanged,depending on the spacing between slits.Each way is in an oscillatory manner.The slit interaction responsible for the oscillating transmission strength that depends on slit spacing is the subject of more detailed investigation.The interaction most intuitively manifests as a current distribution in the metal surface between slits.We find that this current is attenuated in an oscillating fashion from the slit corners to the center of the region between two adjacent slits,and we present a mathematical expression for its waveform.     

Single-layer metal nanolenses with tight foci in far-field

In simulations we analyze performance of plasmonic nanolenses made of a single metal layer. We consider the nanolenses in two configurations. In the first, the nanolens is a free-standing silver layer with no hole on the optical axis and double-sided concentric corrugations. In the second, the nanolens has a set of slits instead of grooves. This necessitates integrating the annular metal elements with a dielectric matrix. We examine the following parameters of the nanolenses: film thickness, diameter of an on-axis stop, and lattice constant of slits or double-sided concentric grooves, as well as depth and width of grooves. Due to radially polarized illumination lenses have foci of full widths at half maxima (FWHMs) better than half a wavelength, though foci formed by propagating waves do not decrease beyond the diffraction limit. Due to proper geometry of slits or double-sided grooves lenses have focal lengths of the order of a few wavelengths. Transmission of light through lenses with double-sided narrow grooves reaches 30% while through ones with slits exceeds 80%.     

Influence of slit width on the electromagnetic transmission of a periodic metallic grating

The light transmittance of a periodic metallic grating with varied slit widths has been investigated. The transmission peaks move to the shorter wavelength direction with an increase in the width of slits while keeping the other parameters unchanged. It was demonstrated that the slit width affects the spectral transmittance of the metal grating significantly. It was also found that the effective refractive index and cavity modes in slits are responsible for this phenomenon. Cavity modes play an important role in extraordinary transmission of the sub-wavelength aperture grating. When a complete resonant mode forms in the slits, a high transmission will appear. A wider slit results in a smaller efficient refractive index and thus affects the cavity mode in the slits. These two elements cause the transmission peaks to move to the shorter wavelength direction with widening of slits. The results obtained here may provide a useful guide to design metallic slit grating devices.     

Plasmon-assisted two-slit transmission: Young's experiment revisited

We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other.     

Can the point source method be used for design of sub-wavelength surface plasmon devices?

Integrating optical components into electronic chips needs compressing the dimensions of optical components, which are now nearly thousand times larger than those of electronic components. Surface plasmon-based devices may offer a solution to this size-compatibility problem. Shi et al. introduced a new method, called point source method, to design sub-wavelength surface plasmon devices. To test the validity of this method, we calculated the light transmission through sub-wavelength structures by the finite-difference time-domain (FDTD) method and point source method. Comparing the two results, we found that the correct light distribution through slits can be obtained by the point source method only in the comparatively far zone and the slit widths have to be selected carefully. Phase distributions obtained by the two methods are not the same and the phase difference is not constant for slits with different widths.     

原子力与光子扫描隧道组合显微镜

介绍了超高分辨光于扫描隧遭显微镜（PSTM）的计冗历程,为解决第一代（单光束照明）光千扫捕隧逼显傲镜中存在人为假象和样品光学图像与形貌图像难于分离两个难题,用“对称双光束照明方法消假象,用原子力与光子扫描隧道组合显微镜（AF／PSTM）图像分解方法分离样品光学透过率、折射率与形貌图像。研制成功新一代原子力与光子扫描隧道组合显微镜（AF／PSTM）样机。该样机在一次扫描中已获得两幅原子力显微镜图像（形貌与相位）和两幅光学图像（透过率和折射率）,有效地减少了假象,分解了样品光学折射率、透过率与形貌图像。     

Suppressed transmission of light through single slit in gold film

A new phenomenon of an obvious dip of the transmission from subwavelength slits can be obtained if we change the wavelength of the incident light in the visible regime is proposed. Theory shows that there exist maximum reflectivities if we change wavelengths for different slit widths. The theoretical results are consistent with both finite-difference time-domain (FDTD) simulations and experiments.     

介质覆层下金属周期结构变化对TE波异常透射特性的影响

根据金属光栅结构变化对TE波异常透射特性影响的研究需要,建立了相应的模型。应用时域有限差分(FDTD)方法分别计算了单缝结构、多缝结构、不同宽度和不同周期等结构下的透射分布特征。研究发现添加凹槽会对金属表面能量的传递起阻碍作用。透射频域宽度随薄膜宽度增加而增加。随着狭缝宽度的增加,透射率分布曲线包络线趋于平坦,主透射峰短波长侧透射曲线分布基本不变,而主透射峰及其长波长侧的透射曲线分布变宽。这说明宽度的变化影响了表面共振模式,从而影响透射的分布。单个狭缝的透射与多周期结构相比,透射率曲线几乎重合,表明狭缝对表面模式没有调制作用,各狭缝的透射相对独立。     

Surface plasmon resonance transmission filters at 1053nm based on metallic grating with narrow slit

Metallic gratings with narrow slits can lead to special optical properties such as strongly enhancing the trans-mission and considerably strengthening the polarized effect.A narrow-band filter suitable for application in optical communication is designed by sandwiching a metallic grating between two identical dielectric films.The maximum transmission can reach 96% after optimizing the parameters of films and grating at a central wavelength of 1053 nm.It is the first time,to our knowledge,that such high transmission has been reported since the discovery of the extraor-dinarily high transmission through periodic holes or slits;moreover,the extremely polarized effect is also found in P mode of this symmetric grating.     

Modulation of Splitting Beam Angle with Metal--Nonlinear Optical Material--Metal (M-NL-M) Array Structure

We demonstrate active manipulating plasmonic signals with metal--nonlinear optical material--metal (M-NL-M) arrays consisting of slits with variant widths. The parameters of the M-NL-M array structure are derived by theoretical analysis of dispersion relationship. The splitting angle can be modulated by the incident light intensity, and verified by a nonlinear two-dimensional finite difference time domain method. The physical principle of this phenomenon is analysed from the phase of surface plasmon polaritons and Fabry--Pérot (F-P) resonance in slits     

Review of near-field optical microscopy

This review has introduced a new near-field optical microscope (NOM)—atomic force microscope combined with photon scanning tunneling microscope (AF / PSTM). During scanning, AF/PSTM could get two optical images of refractive index image and transmissivity image, and two AFM images of topography image and phase image. A reflected near-field optical microscope (AF/RSNOM) has also been developed on AF/PSTM platform. The NOM has been reviewed in this paper and the comparison between AF/PSTM & RSNOM and the commercial A-SNOM & RNOM has also been discussed. The functions of AF/PSTM & RSNOM are much better than A-SNOM & RNOM.     

Enhancement of spatial coherence by surface plasmons

We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Young's interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. The visibility of the interference pattern can be manipulated by tuning the wavelength of one of the optical sources.     

Directional and hemispherical mid-infrared transmittance through microscale slit arrays on a semi-transparent substrate at normal incidence

The directional and hemispherical transmittance through four microscale slit arrays on a semitransparent substrate were investigated at normal incidence. Materials and vertical dimensions of all arrays were the same, but lateral dimensions varied among the three simple slit arrays. The complex slit array had three identical slits and three lamellae of dissimilar widths in a profile period. The incidence covered a broad-band mid-infrared region (2.5–25 μm) in which two linear polarizations were considered. Impacts of the structure profile, incidence polarization and wavelength, and optical constants of involved materials on spectra were studied numerically with programs based on the algorithm termed rigorous coupled-wave analysis (RCWA). Near-field patterns of electromagnetic fields that correlated with unique transmittance were also plotted to identify responsible physical mechanisms. In addition, the directional transmittance spectra of fabricated samples were measured with FT-IR spectrometry. Reasonable agreement between numerical and experimental results was obtained. Unique features in spectra were consistently found, regardless of the finite substrate thickness.     

Transmission through array of subwavelength metallic slits curved with a single step or mutli-step

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with a sin- gle step or mutli-step has been explored theoretically. The transmission spectrum is simulated by using the finite-difference time-domain method. The influences of surface plasmon polaritons make the end of finite long sub-wavelength metallic slit behaves as magnetic-reflecting barrier. The electromagnetic fields in the subwavelength metallic slits are the superpo- sition of standing wave and traveling wave. The standing electromagnetic oscillation behaves like LC oscillating circuit to decide the resonance wavelength. Therefore, the parameters of adding step may change the LC circuit and influence the transmission wavelength. A new explanation model is proposed in which the resonant wavelength is decided by four factors： the changed length for electric field, the changed length for magnetic field, the effective coefficient of capacitance, and the effective coefficient of inductance. The effect of adding step is presented to analyze the interaction of two steps in slit with mutli-step. This explanation model has been proved by the transmission through arrayed subwavelength metallic slits curved with two steps and fractal steps. All calculated results are well explained by our proposed model.     

Tunable 19 × 10 GHz L-band FP-SOA based multi-wavelength mode-locked fiber laser

We present a multi-wavelength mode-locked fiber ring laser incorporating a semiconductor optical amplifier (SOA) and a Fabry-Perot semiconductor optical amplifier (FP-SOA). Because the gain of the SOA is depleted by an external injection optical signal, the SOA acts as a loss modulator. The FP-SOA serves as a tunable comb filter. The presented laser source can generate 19 synchronized wavelength channels with the extinction ratio of about 21 dB, each mode-locked at 10 GHz, and mode-locked pulse width is about 40 ps. Oscillation wavelengths band can be tuned by adjusting the bias current of the SOA, and wavelength spacing also can be changed by using a tunable optical delay line (ODL) or a temperature controller. The polarization-insensitive devices ensure that the output power is rather stable. This fiber laser has potential applications in longer waveband (L-band) within the low-attenuation window.     

Colour characterization of a Morpho butterfly wing-scale using a high accuracy nonstandard finite-difference time-domain method

In certain species of moths and butterflies iridescent colours arise from subwavelength diffractive structures. The optical properties of such a structure depend strongly on wavelength, incidence angle and state of polarization of illuminating radiation and on the viewing angle. Such structures can be analyzed only by solving Maxwell's equations, but since analytical solutions exist for only a few simple, highly symmetric structures numerical methods must be employed. We investigated the optical properties of butterfly wings in two dimensions by simulating a scale structure using a high accuracy version of nonstandard finite-difference time-domain algorithm. The simulated structure is a computer-generated model of a certain quasi-periodic arrangement of tree-like structures observed in the transmission electron micrograph (TEM) image of a transverse cross-section of a single scale from Morpho butterfly wings. We assumed that the structure is made of a slightly lossy dielectric material. We checked the accuracy and validity of our approach, by computing scattered field intensities due to an infinite cylinder and compared the results with analytical calculations using Mie theory. Next we deduced the wavelength dependence of a real refractive index and an absorption coefficient for the ground scales on the wings of Morpho sulkowskyi butterfly by computing the reflectivity and transmissivity spectrum of a scale at normal incidence, and comparing with experimental measurements. Finally, we calculated the tristimulus values and corresponding colour coordinates for various viewing directions from the scale's far-field reflectivity and transmissivity spectra to characterize its colour rendering abilities.     

Analysis of Center Wavelength Shift of VCSEL Light in AWG for WDM PON Applications

Center wavelength shift of vertical cavity surface emitting laser light in arrayed waveguide gratings is verified with mathematical and experimental analysis.It is induced by the linearly increasing trend of optical power of vertical cavity surface emitting laser by bias current increase.It is retrieved effectively to the original center wavelength by simple correction method of compensation.This was done for application of vertical cavity surface emitting laser as a light source in optical line terminal of wavelength division multiplexing-passive optical network.     

Transmission through metallic array slits curved with perpendicular waveguides

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with perpendicular waveguides has been explored. Two-dimensional electromagnetic fields inside and near the metal film are simulated by the finite-difference time-domain (FDTD) method. The perpendicular waveguides in the middle of slits affect the modes of slits in two different manners: commonly, the extended length of the waveguides leads to the increase of the resonant wavelength; shifting the waveguides location along the vertical slits, red or blue shift of resonant wavelengths can be achieved, however to some confirmed length of the perpendicular waveguides, there is the only one resonant wavelength. In addition, a new explanation model is proposed in which the resonance is ascribed to three reasons: the Fabry-Perot cavity theory, the surface current flow, and the surface charges. All the calculated results are well explained by our proposed model.     

Role of surface plasmon polaritons in anomalous transmission of an electromagnetic wave through two arrays with subwavelength slits

The anomalously large transmission of an electromagnetic wave through structures consisting of two periodic arrays of subwavelength slits in films has been investigated. The conditions ensuring zero transmittance of this system have been determined. The role of surface plasmon polaritons in transmission anomalies has been analyzed. An analysis of the systems consisting of three arrays of slits has revealed specific features of the transmittance that are independent of the number of slits. It has been demonstrated that, at a wavelength corresponding to the excitation of a surface plasmon polariton in the gap between two periodic arrays of subwavelength slits, the transmittance is zero (i.e., transmission is suppressed). The investigation has been carried out using numerical simulations with the Fourier modal method.     

Determine locking range of coherently combined fiber lasers joined by fiber couplers

When the wavelength difference of two fiber lasers, sharing a common reflection mirror with the aid of an optical coupler, is tuned to fall into a certain range, the array lasers can be locked to generate coherent radiation with a wavelength somewhere between the two constituent field wavelengths. Based on the fact that the laser should oscillate at the wavelength with the lowest losses and the band widths of the reflection curves of the laser tuning elements are finite, the locking range of the laser array has been quantified. For the tuning element (i.e., fiber gratings) selected in this work, the calculations show that the locking range is mainly dependent on the coupling coefficients of the gratings which ever is smaller.