Surface plasmon-polaritons wave fields in the vicinity of a metallic nanohole

It is shown that metal surface with a nanohole can support surface plasmon-polaritons (SPP), whose wave fields are described by Hankel functions. These plasmons can be excited by an electromagnetic wave incident at the metal surface. The optical transmission through subwavelength holes in metal films can essentially be enhanced by interaction of the incident light with surface plasmons. Dependence of excitation of the wave field of SPP on the incidence angle and on the wavelength of incident light is considered.     

Mie谐振耦合的亚波长金属孔宽带高透射传输

表面等离激元共振激发的亚波长金属孔透射较Bethe理论有大幅度的提高,然而,由于共振对频率的敏感性以及金属在光频的高损耗特性,表面等离激元共振难以实现亚波长金属孔的宽带高透射传输.本文采用放置在金属孔两边的硅纳米颗粒的Mie谐振耦合取代表面等离激元共振,实现亚波长金属孔的宽带高透射传输.全波仿真结果表明,采用Mie谐振耦合的亚波长金属孔(r/λ=0.1)光传输,透射系数超过90%的带宽达到65 nm,与表面等离激元共振诱导的透射增强相比,峰值增高了1.5倍,3 dB带宽拓宽了17倍.根据耦合模理论,建立了Mie谐振耦合亚波长金属孔透射的等效电路模型,并在临界耦合状态下反演出电路模型中的元件参数值.进一步研究发现,仅通过改变等效电路模型中的耦合系数,就可全面揭示Mie谐振耦合亚波长金属孔透射的传输规律,并得到与全波电磁仿真完全一致的结果,从而找到光与放置硅纳米颗粒的亚波长金属孔相互作用的数学表达,也给予人们在光学领域按照电路设计方法构建相应功能模块的启示.     

Surface-plasmon-assisted resonant tunneling of light through a periodically corrugated thin metal film

We present experimental results and a numerical model confirming that surface plasmons can resonantly enhance light transmission through a corrugated metal film. A new interpretation in terms of plasmon-assisted light tunneling is given to recent experiments on light penetration through periodic subwavelength holes in a thin metal film. We designed a narrow-band filter suitable for applications in optical communication by optimizing the film and the grating parameters.     

High-optical-throughput individual nanoscale aperture in a multilayered metallic film

The optical transmission of an individual subwavelength aperture in a multilayered metal film is shown to be enhanced compared with that of a homogeneous metal film. The enhancement effect is due to the light coupling to surface plasmon excitation facilitated by a film periodicity. The sensitivity of the transmission to the dielectric filling of the aperture is also shown. The latter effect can be used to switch and control the transmittance. Devices based on enhanced transmission through nanosized apertures can find applications in high-density optical and magneto-optical data storage, high-resolution microscopy, and photolithography, where nanoscale light sources with high-optical-power throughput are required, as well as in sensor applications.     

Theory of extraordinary optical transmission through subwavelength hole arrays

We present a fully three-dimensional theoretical study of the extraordinary transmission of light through subwavelength hole arrays in optically thick metal films. Good agreement is obtained with experimental data. An analytical minimal model is also developed, which conclusively shows that the enhancement of transmission is due to tunneling through surface plasmons formed on each metal-dielectric interface. Different regimes of tunneling (resonant through a "surface plasmon molecule," or sequential through two isolated surface plasmons) are found depending on the geometrical parameters defining the system.     

Critical process of extraordinary optical transmission through periodic subwavelength hole array: Hole-assisted evanescent-field coupling

We present a new explanation of the extraordinary optical transmission through subwavelength hole array in metal films. By using the classical coupled-wave theory, we analyzed the coupling process between light and the surface plasmons wave on metal films with hole array. The analysis shows clearly the physical mechanism of extraordinary optical transmission. The calculation demonstrates that, instead of energy flux directly passing through the holes, the electromagnetic modes could exchange energy by overlapping the evanescent fields under the assistance of hole array. The periodicity of the array provides the momentum-matching condition to present the transmission peaks. The theory exhibits a good agreement with the experimental results reported in every detail.     

Control of optical transmission through metals perforated with subwavelength hole arrays

The transmission spectrum of a metal that is perforated with a periodic array of subwavelength holes exhibits well-defined maxima and minima resulting from, respectively, a transmission enhancement by surface plasmons and Wood's anomaly, a diffraction effect. These features occur at wavelengths determined by the geometry of the hole arrays, the refractive index of the adjacent medium, and the angle of incidence. We demonstrate control of the transmission through variation of these parameters and show that perforated metal films may form a novel basis for electro-optic devices such as flat-panel displays, spatial light modulators, and tunable optical filters.     

Terahertz radiation from interaction between an electron beam and a planar surface plasmon structure

The possibility of an electron beam exciting surface plasmons in conducting metal is discussed in this paper. A planar perfect-structure with subwavelength holes is proposed. The phenomenon that mimicking surface plasmon waves can be excited and amplified by an electron beam is proved theoretically and numerically. The mechanism of transmission through a subwavelength hole array is exploited to enhance the interaction between the electron beam and the mimicking surface plasmons.     

Nonresonant broadband funneling of light via ultrasubwavelength channels

Enhancing and funneling light efficiently through deep subwavelength apertures is essential in harnessing light-matter interaction. Thus far, this has been accomplished resonantly, by exciting the structural surface plasmons of perforated nanostructured metal films, a phenomenon known as extraordinary optical transmission. Here, we present a new paradigm structure which possesses all the capabilities of extraordinary optical transmission platforms, yet operates nonresonantly on a distinctly different mechanism. Our proposed platform demonstrates efficient ultrabroadband funneling of optical power confined in an area as small as ～(λ/500)(2), where optical fields are enhanced, thus exhibiting functional possibilities beyond resonant platforms. We analyze the nonresonant mechanism underpinning such a phenomenon with a simple quasistatic picture, which shows excellent agreement with our numerical simulations.     

Subwavelength propagation and localization of light using surface plasmons: A brief perspective

Surface plasmons at the metal–dielectric interface have emerged as an important candidate to propagate and localize light at subwavelength scales. By tailoring the geometry and arrangement of metallic nanoarchitectures, propagating and localized surface plasmons can be obtained. In this brief perspective, we discuss: (1) how surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) can be optically excited in metallic nanoarchitectures by employing a variety of optical microscopy methods; (2) how SPPs and LSPs in plasmonic nanowires can be utilized for subwavelength polarization optics and single-molecule surface-enhanced Raman scattering (SERS) on a photonic chip; and (3) how individual plasmonic nanowire can be optically manipulated using optical trapping methods.     

Resonantly enhanced transmission of light through subwavelength apertures with dielectric filling

Using both analytical and numerical methods to study transmission of light through dielectric-filled subwavelength apertures in a real metal, we have found that a propagating mode can in principle exist inside a waveguide of arbitrary small size if a particular relationship between the dielectric constants of the cladding and filling materials at the incident frequency is satisfied. Practical transmission through a subwavelength aperture of finite depth can be enhanced when the depth is such that Fabry-Pérot-like resonances are excited. For 810 nm light incident on a silicon-filled 50-nm-diameter aperture in a 200-nm-thick gold film, we found that a normalized near-field intensity ratio of 1.6 at the exit can be achieved. This resonantly enhanced transmission phenomenon may be advantageously applicable to near-field scanning optical microscopy and single-molecule spectroscopy.     

A Compact Optical Switch via Plasmonics of Subwavelength Circular‐Sharp Hole Arrays in Metal Films

We studied numerically the enhanced optical transmission (EOT) through periodic subwavelength circular‐sharp hole arrays in metallic films with different edge sharp distribution features of unit structures. Detailed studies indicate that the unit structure edge sharp distribution features strongly influence the surface plasmons (SPs). These results demonstrate that the number of edge sharp activated the localized surface plamons (LSP) resonance on the unit structure is changed by rotating the polarization of the incident light, leading to change the infrared transmittance of the array. Moreover, a compact plasmonic switch via periodic circular‐sharp hole arrays based on the dependence of SPs on unit structural edge sharp distributions is proposed. The finding provides a new idea for designing plasmonics devices, and expands the application range of metal micro‐nano structure in the field of optical communications and information processing.     

Origin of superenhanced light transmission through two-dimensional subwavelength rectangular hole arrays

Superenhanced light transmission through subwavelength rectangular hole arrays have been reported and some investigations have been made into the physical origin of this phenomenon [K.J. Klein Koerkamp et al., Phys. Rev. Lett. 92, 183901 (2004)]. In our current work, by performing FDTD (finite difference in the time domain) numerical simulations, we demonstrate that mechanism that is different from surface plasmon polaritons set up by the periodicity at the in-plane metal surfaces may account for this superenhanced light transmission. We suggest that for arrays of rectangular holes with small enough width in comparison to the wavelength of the incident light, standing electromagnetic fields can be set up inside the cavity by the surface plasmons on the hole walls with its intensity being substantially enhanced inside the cavity. So resonant cavity-enhanced light transmission is predominant and responsible for its superenhanced light transmission. Rectangular holes behave as Fabry-Pérot resonance cavities except that the frequency of their fundamental modes is restricted by their TM cutoff frequency. However we believe that both localized surface plasmon modes and surface plasmon polaritons set up by the periodicity at the in-plane metal surfaces have their shares in extraordinary optical transmission of rectangular hole arrays especially when the width of the rectangular hole is not small enough and the metal film is not thick enough.     

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66.Bz; 42.79.Gn; 42.79.Vb     

Extraordinary light transmission through a metal film perforated by a subwavelength hole array

It is shown that, depending on the incident wave frequency and the system geometry, the extraordinary transmission of light through a metal film perforated by an array of subwavelength holes can be described by one of the three mechanisms: the “transparency window” in the metal, excitation of the Fabry–Perot resonance of a collective mode produced by the hybridization of evanescence modes of the holes and surface plasmons, and excitation of a plasmon on the rear boundary of the film. The excitation of a plasmon resonance on the front boundary of the metal film does not make any substantial contribution to the transmission coefficient, although introduces a contribution to the reflection coefficient.     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

Role of shape in middle-infrared transmission enhancement through periodically perforated metal films

We report experimental results on the role of aperture shape in middle-infrared enhanced transmission through a metal film perforated with a periodic aperture array. The transmission spectrum is highly dependent on the aperture shape. When the aperture shape changes from circular to cross-dipole or the cross-dipole arm length increases, the transmission spectrum exhibits a large redshift. The enhanced transmission is proposed to be an interplay of surface plasmon polaritions at the metal surface and localized surface plasmons (LSPs) inside the apertures. The shift of the transmission bands is proposed to be due to the redshift of the LSP resonance spectrum of the individual apertures. The role of shape in enhanced transmission in the cascaded structure is also studied.     

Evanescent tunneling of an effective surface plasmon excited by convection electrons

We introduce the subwavelength transmission of an effective surface plasmon beyond the light zone via the proximity interaction of convection electrons with a metal grating. A comparative analysis of dielectric homogenization and a finite-difference-time-domain simulation shows that out-of-phase-like modes (pi modes) have strong transmission below the cutoff frequency relying on the parametric condition of structural dimension and electronic energy. The synchronous spatial field and charge distribution of the pi mode system confirms the evanescent tunneling effect of the electron-coupled plasmons.     

Broadband near-field interference spectroscopy of metal nanoparticles using a femtosecond white-light continuum

We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.     

Interference-induced enhancement of intensity and energy of a quantum optical field by a subwavelength array of coherent light sources

Recently, we have shown a mechanism that could provide great resonant and nonresonant transmission enhancements of the classical (nonquantum) light waves passed through subwavelength aperture arrays in thin metal films not by the plasmon–polariton waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. We now present a quantum reformulation of the model. The Hamiltonian describing the phenomenon of interference-induced enhancement and suppression of both the intensity and energy of a quantum optical field is derived. The basic properties of the field energy determining by the Hamiltonian are analyzed. Normally, the interference (addition) of two or more waves causes enhancement or suppression of the light intensity, but not the light energy. The model shows that the phenomenon could be observed experimentally, for instance, by using a subwavelength array of the coherent quantum light-sources (one- and two-dimensional subwavelength apertures, fibers, dipoles, and atoms).