The nature of transmission resonances in plasmonic metallic gratings (original) (raw)
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Transmission resonances in plasmonic metallic gratings
Journal of the Optical Society of America B, 2011
Using the Fourier modal method (FMM) we report our analysis of the transmission resonances of a plasmonic grating with sub-wavelength period and extremely narrow slits for wavelengths of the incoming, transverse magnetic (TM)-polarized, radiation ranging from 240nm to 1500nm and incident angles from 0 0 to 90 0 . In particular, we study the case of a silver grating placed in vacuo. Consistent with previous studies on the topic, we highlight that the main mechanism for extraordinary transmission is a TM-Fabry-Perot (FP) branch supported by waveguide modes inside each slit. The TM-FP branch may also interact with surface plasmons (SPs) at the air/Ag interface through the reciprocal lattice vectors of the grating, for periods comparable with the incoming wavelength. When the TM-FP branch crosses a SP branch, a band gap is formed along the line of the SP dispersion. The gap has a Fano-Feshbach resonance at the low frequency band edge and a ridge resonance with extremely long lifetime at the high frequency band edge. We discuss the nature of these dispersion features, and in particular we describe the ridge resonance in the framework of guided-mode resonances (GMRs). In addition, we elucidate the connection of the coupling between the TM-FP branch and SPs within the Rayleigh condition. We also study the peculiar characteristics of the field localization and the energy transport in two topical examples.
Properties of TM resonances on metallic slit gratings
Applied Optics, 2012
Electromagnetic resonances on metallic slit gratings induced by TM polarized incident light have been investigated and physically interpreted. We have developed an electromagnetic model imposing surface impedance boundary conditions on the metallic grating surface from which we derive simple formulas explaining all physical properties of these resonances. It is demonstrated that Fabry-Perot (or cavity) resonances are generated by the zeroth slit mode yielding extraordinary transmission. For very narrow slits, the resonant H-field is squeezed to the slit walls and causes enhanced power losses. The excitation of surface plasmon polaritons (SPPs), however, is generated by two mode coupling. SPPs are linked to sharp absorption peaks and dips in transmittance. It is shown that these phenomena are primarily caused by the interaction of the electromagnetic fields with the finite conducting slit walls. These findings have been confirmed by measured transmittance data of gold gratings with periods of 0.5 μm, 1 μm, and 2 μm.
Transmission Resonances on Metallic Gratings with Very Narrow Slits
Physical Review Letters, 1999
In this letter we show how transmission metallic gratings with very narrow and deep enough slits can exhibit transmission resonances for wavelengths larger than the period of the grating. By using a transfer matrix formalism and a quasi-analytical model based on a modal expansion, we show that there are two possible ways of transferring light from the upper surface to the lower one: by the excitation of coupled surface plasmon polaritons on both surfaces of the metallic grating or by the coupling of incident plane waves with waveguide resonances located in the slits. Both mechanisms can lead to almost perfect transmittance for those particular resonances.
Negative Role of Surface Plasmons in the Transmission of Metallic Gratings with Very Narrow Slits
Physical Review Letters, 2002
It is generally admitted that the extraordinary transmission of metallic grating with very narrow slits is mainly due to the excitation of surface plasmons on the upper and lower interfaces of the grating. We show that the surface plasmon contribution is not the prime effect and that waveguide mode resonance and diffraction are responsible for the extraordinary transmission. Additionally and surprisingly, we reveal that the transmittance of subwavelength metallic gratings is always nearly zero for frequencies corresponding to surface plasmon excitation. This finding implies that surface plasmons play a negative role in the transmission.
Plasmonic band edge effects on the transmission properties of metal gratings
AIP Advances, 2011
We present a detailed analysis of the optical properties of one-dimensional arrays of slits in metal films. Although enhanced transmission windows are dominated by Fabry-Perot cavity modes localized inside the slits, the periodicity introduces surface modes that can either enhance or inhibit light transmission. We thus illustrate the interaction between cavity modes and surface modes in both finite and infinite arrays of slits. In particular we study a grating that clearly separates surface plasmon effects from Wood-Rayleigh anomalies. The periodicity of the grating induces a strong plasmonic band gap that inhibits coupling to the cavity modes for frequencies near the center of the band gap, thereby reducing the transmission of the grating. Strong field localization at the high energy plasmonic band edge enhances coupling to the cavity modes while field localization at the low energy band edge leads to weak cavity coupling and reduced transmission.
Theory of electromagnetic wave transmission through metallic gratings of subwavelength slits
Journal of Optics A: Pure and Applied Optics, 2007
We present FDTD calculations for transmission of light and other electromagnetic waves through periodic arrays of slits in a metallic slab. The results show resonant, frequency dependent, transmittance peaks for subwavelength widths of the slits which can be up to a factor of ten with respect to those out of resonance. Although our conclusions agree with previous work by Lezec and Thio as regards both the magnitude of the enhancement and the lack of contribution of surface plasmon polaritons of the metal surface to this effect, we derive an interpretation from a theory that deals with emerging beam-Rayleigh anomalies of the grating, and with Fabry-Perot resonances of the perforated slab considered as an effective medium.
Efficient excitation of cavity resonances of subwavelength metallic gratings
Journal of Applied Physics, 2005
One dimensional rectangular metallic gratings enable enhanced transmission of light for specific resonance frequencies. Two kinds of modes participating to enhanced transmission have already been demonstrated : (i) waveguide modes and (ii) surface plasmon polaritons (SPP). Since the original paper of Hessel and Oliner [1] pointing out the existence of (i), no progress was made in their understanding. We present here a carefull analysis, and show that the coupling between the light and such resonances can be tremendously improved using an evanescent wave. This leads to enhanced localisation of light in cavities, yielding, in particular, to a very selective light transmission through these gratings.
Journal of the Optical Society of America A, 1999
We have investigated, both theoretically and experimentally, the intensity of p-polarized light diffracted by a metallized diffraction grating when surface plasmons are excited and when they are not. Experiments were made on modulated silver surfaces, and light in the near and far fields was detected by a sharpened optical fiber. Good agreement between experiment and theory (based on the differential method) was found for the intensity collected by the fiber for several angles of incidence in near and far fields. These angles of incidence were chosen to produce amplitudes of the electromagnetic field that displayed many differences in the nearfield and far-field zones. The near-field study, both experimental and theoretical, gave us extra information about the sample. Indeed, the near field is composed of evanescent waves, which describe the high spatial frequencies of the surface and thus its subwavelength structures. Analysis of the electromagnetic fields diffracted in far and near fields by the grating permits refinement of the parameters of the sample (thickness of silver) and of its environment (thickness and refractive index of a mixed air-water layer deposited on the sample). Finally, we present and analyze the electromagnetic field diffused by the grating when the surface was oxided after a long period in ambient air.
Description of the modes governing the optical transmission through metal gratings
Optics Express, 2011
An analytical model based on a modal expansion method is developed to investigate the optical transmission through metal gratings. This model gives analytical expressions for the transmission as well as for the dispersion relations of the modes responsible for high transmission. These expressions are accurate even for real metals used in the visiblenear-infrared wavelength range, where surface plasmon polaritons (SPP's) are excited. The dispersion relations allow the nature of the modes to be assessed. We find that the transmission modes are hybrid between Fabry-Pérot like modes and SPP's. It is also shown that it is important to consider different refractive indices above and below the gratings in order to determine the nature of the hybrid modes. These findings are important as they clarify the nature of the modes responsible for high transmission. It can also be useful as a design tool for metal gratings for various applications.
Finite gratings of many thin silver nanostrips: Optical resonances and role of periodicity
AIP Advances, 2013
We study numerically the optical properties of the periodic in one dimension flat gratings made of multiple thin silver nanostrips suspended in free space. Unlike other publications, we consider the gratings that are finite however made of many strips that are well thinner than the wavelength. Our analysis is based on the combined use of two techniques earlier verified by us in the scattering by a single thin strip of conventional dielectric: the generalized (effective) boundary conditions (GBCs) imposed on the strip median lines and the Nystrom-type discretization of the associated singular and hyper-singular integral equations (IEs). The first point means that in the case of the metal strip thickness being only a small fraction of the free-space wavelength (typically 5 nm to 50 nm versus 300 nm to 1 μm) we can neglect the internal field and consider only the field limit values. In its turn, this enables reduction of the integration contour in the associated IEs to the strip median lines. This brings significant simplification of the scattering analysis while preserving a reasonably adequate modeling. The second point guarantees fast convergence and controlled accuracy of computations that enables us to compute the gratings consisting of hundreds of thin strips, with total size in hundreds of wavelengths. Thanks to this, in the H-polarization case we demonstrate the build-up of sharp grating resonances (a.k.a. as collective or lattice resonances) in the scattering and absorption cross-sections of sparse multi-strip gratings, in addition to better known localized surface-plasmon resonances on each strip. The grating modes, which are responsible for these resonances, have characteristic near-field patterns that are distinctively different from the plasmons as can be seen if the strip number gets larger. In the E-polarization case, no such resonances are detectable however the build-up of Rayleigh anomalies is observed, accompanied by the reduced scattering and absorption.