Experimental studies of extraordinary light transmission through periodic arrays of subwavelength square and rectangular holes in metal films

We fabricate a series of periodic arrays of subwavelength square and rectangular air holes on gold films, and measure the transmission spectra of these metallic nanostructures. By changing some geometrical and physical parameters, such as array period, air hole size and shape, and the incident light polarization, we verify that both global surface plasmon resonance and localized waveguide mode resonance are influential on enhancing the transmission of light through nanostructured metal films. These two resonances induce different behaviours of transmission peak shift. The transmission through the rectangular air-hole structures exhibits an obvious polarization effect dependent on the morphology. Numerical simulations are also made by a plane-wave transfer-matrix method and in good consistency with the experimental results.     

Near-field distribution of optical transmission of periodic subwavelength holes in a metal film

Optical transmission of a two-dimensional array of subwavelength holes in a metal film has been numerically studied using a differential method. Transmission spectra have been calculated showing a significant increase of the transmission in certain spectral ranges corresponding to the excitation of the surface polariton Bloch waves on a metal surface with a periodic hole structure. Under the enhanced transmission conditions, the near-field distribution of the transmitted light reveals an intensity enhancement greater than 2 orders of magnitude in localized ( approximately 40 nm) spots resulting from the interference of the surface polaritons Bragg scattered by the holes in an array.     

Enhanced transmission through periodic arrays of subwavelength holes: the role of localized waveguide resonances

By using the rigid full-vectorial three-dimensional finite-difference time-domain method, we show that the enhanced transmission through a metallic film with a periodic array of subwavelength holes results from two different resonances: (i) localized waveguide resonances where each air hole can be considered as a section of metallic waveguide with both ends open to free space, forming a low-quality-factor resonator, and (ii) well-recognized surface plasmon resonances due to the periodicity. These two different resonances can be characterized from electromagnetic band structures in the structured metal film. In addition, we show that the shape effect in the enhanced transmission through the Au film with subwavelength holes is attributed to the localized waveguide resonance.     

Theory of extraordinary transmission of light through quasiperiodic arrays of subwavelength holes

By using a theoretical formalism able to work in both real and k spaces, the physical origin of the phenomenon of extraordinary transmission of light through quasiperiodic arrays of holes is revealed. Long-range order present in a quasiperiodic array selects the wave vector(s) of the surface electromagnetic mode(s) that allows an efficient transmission of light through subwavelength holes.     

Resonant transmission of light through finite chains of subwavelength holes in a metallic film

In this Letter we show that the extraordinary optical transmission phenomenon found before in 2D hole arrays is already present in a linear chain of subwavelength holes, which can be considered as the basic geometrical unit showing this property. In order to study this problem, we have developed a new theoretical framework, able to analyze the optical properties of finite collections of subwavelength apertures and/or dimples (of any shape and placed in arbitrary positions) drilled in a metallic film.     

Strong influence of hole shape on extraordinary transmission through periodic arrays of subwavelength holes

We show that extraordinary light transmission of periodic subwavelength hole arrays, generally attributed to surface-plasmon resonances, is strongly influenced by the hole shape. Both experiments and calculations, based on a Fourier modal method, demonstrate that a shape change from circular to rectangular increases the normalized transmission by an order of magnitude while the hole area decreases. Moreover, the spectra exhibit large redshifts (approximately 2500 cm(-1)). A comparison with the transmission of isolated holes shows that shape resonances of the rectangular holes play a dominant role.     

Surface plasmon polaritons assisted diffraction in metal films with subwavelength hole array

Metal films grown on Si wafer have been perforated with a periodic hole array and anomalous enhanced transmission in the subwavelength regime has been observed. High-order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant of Si against that of the air. Si(1,1) peak splits into two branches at oblique incidence both in TE and in TM polarization, which confirms that anomalous enhanced transmission is a surface plasmon polaritons (SPPs) assisted diffraction phenomenon.     

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.     

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.     

Directional excitation of surface plasmon polaritons in structure of subwavelength metallic holes

In this paper, we propose a structure formed by two subwavelength holes fabricated in a metal film to realize directional excitation of surface plasmon polaritons (SPPs). The holes are employed as SPP sources, and the relative phase of SPPs generated at the hole exit end can be adjusted by changing the dielectric material filled in holes. Using the difference in relative phase values of SPP for two holes filled with different dielectric media, the SPPs can interfere constructively along one direction while destructively along the opposite direction. Our theoretical analysis is verified by the three-dimensional finite-difference time-domain method. Moreover, the directional excitation of SPPs in two-hole array structure is also discussed. It is found that the effect of SPPs directional excitation is improved with the increase of the number of two-hole.     

Enhanced light transmission through cascaded metal films perforated with periodic hole arrays

We report experimental results on enhanced light transmission through two periodically perforated metal films separated by a layer of dielectric. A perforated metal film (single metallic structure) exhibits extraordinary optical transmission, and when two such perforated metal films are spaced by a dielectric layer (cascaded metallic structure), the transmission is further increased. The maximum transmission of the cascaded metallic structure, which depends on the distance between the two metal films, can be more than 400% greater than that of a corresponding single metallic structure. It is proposed that the coupling of surface plasmon polaritons between the two metal films is involved in the process.     

Anomalous transmission of electromagnetic wave through periodic arrays of subwavelength slits arranged on thin metal films

We study the optical properties of periodic structures, each of which consists of arrays of subwavelength slits in thick films that are placed directly against a continuous thin film. We find that the transmittance of the entire system including the thin film is higher than the transmittance of the isolated thin film alone. Our investigations are performed through numerical simulations with the Fourier modal method.     

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.     

Enhanced optical transmission through a nanoslit by coupling light between periodic strips and metal film

Surface plasmon resonance （SPR） is obtained by exciting the surface plasmon （SP） at the metal and dielectric interface, which can greatly enhance the extraordinary optical transmission （EOT） through a nanoslit milled in the metal film. We present a structure with a 50-nm-wide silver nanoslit for EOT by coupling light into the dielectric interlayer between periodic strips and a metal film. When the period of the metallic strips is equal to the wavelength of the SPR, the transmission efficiency of 187.6 through the nanoslit is enhanced. The metallic strip width over the nanoslit is optimized to improve transmission efficiency, and the maximal efficiency of 204.3 is achieved.     

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.     

Two resonances effect for light transmission assisted with surface plasmon through perforated metal film

When light is incident on a nonplanar metal surface, an oscillating dipole is excited at the entrance openings. We show that the enhanced transmission assisted with surface plasmon (SP) through a perforated metal film results from two different SP resonances effects: (i) zero-order Fabry-Perot resonance effect where each air hole can be considered as a section of metallic waveguide, forming a low-quality-factor resonator, and many dipoles are arranged inside the nanoholes region; and (ii) structure-factor-induced charges self-tunnelling effect due to the well-recognized surface structure periodicity, where the positive or negative charges can respectively tunnel into the right surface through the metal walls. Furthermore, when light transmits through the double-layer perforated metal films, the different transport behaviors are also clearly shown, which convinces the existence of dual SP resonances effect.     

Mechanism of the superenhanced light transmission through 2D subwavelength coaxial hole arrays

By using FDTD numerical simulations, we show that mechanism that is different from surface plasmon polaritons set up by the periodicity at the in-plane metal surfaces may account for the superenhanced light transmission through coaxial hole arrays. We propose that resonant cavity-enhanced light transmission is responsible for it. When an axis is introduced into a hole, slits of definite length are formed. We suppose that a coaxial hole will support the standing waves of Fabry–Pérot-like modes with frequency higher than its cutoff frequency if its gap is small enough in comparison to the wavelength of the incident light and if the metal film is thick enough.     

Role of localised surface plasmon polaritons coupling in optical transmission through double-layer metal apertures

In this paper, we investigate the optical properties of the double-layer metal films perforated with single apertures by analysing the coupling of localized surface plasmon polaritons (LSPPs). It is found that the amplitude and the wavelength of transmission peak in such a structure can be adjusted by changing the longitudinal interval D between two films and the lateral displacements d_{x} and d_{y} which are parallel and perpendicular to the polarization direction of incident light, respectively. The variation of longitudinal interval D results in the redshift of transmission peak due to the change of coupling strength of LSPPs near the single apertures. The amplitude of transmission peak decreases with the increase of d_{y} and is less than that in the case of d_{x}, which originates from the difference in coupling manner between LSPPs and the localized natures of LSPPs.     

Multiple directional enhanced light source through a periodic metal grating structure

Higher emission rates and controllable emission direction are big concerns when it comes to finding a good single photon source. Recently, surface plasmons are introduced to this application, as they can manipulate and enhance the luminescence of single emitters. Here, we experimentally achieve a wide-area multiple directional enhanced light source through periodic metal grating structures. The surface-plasmon-coupled emission can have multiple precisely emission angles by just changing the period of the grating. Our result indicates that metal plasmonic grating can be used as a productive quantum device for unidirectional quantum light sources in quantum optics.     

Propagation of surface plasmon polaritons through gradient index and periodic structures

We study propagation of surface electromagnetic waves along a metallic surface covered by various layered dielectric structures. We show that strong radiative losses, typical for scattering of a surface wave, can be considerably suppressed when a single dielectric step is substituted by gradient index or periodic layered structure.