Optical resonance in a narrow slit in a thick metallic screen

Three-dimensional subwavelength confinement of terahertz electromagnetic surface modes in a coupled slit structure

We report on the three-dimensional subwavelength confinement of the electromagnetic waves at a coupled metallic slit structure beyond diffraction limit in terahertz region. Lateral confinement behavior, leading to the three-dimensional confinement, is caused by a strong funneling effect of the light which occurs at the intersection of slits with a sharp metal geometry. Tunability of the resonant frequency and the position of the light confinement is achieved by controlling the slit length and the position of the intersection of slits, respectively.

Investigation of the wave behaviors inside a step-modulated subwavelength metal slit

In this paper, we applied the modal expansion method (MEM) to investigate the wave behaviors inside a step-modulated subwavelength metal slit. The physical mechanism of the surface plasmon polariton (SPP) transmission is investigated in detail for slit structures with either dielectric or geometric modulation. The applicability of the effective index method is discussed. Moreover, as a special case of the geometric modulation, the evanescent-wave assisted transmission is demonstrated in a thin-modulated slit. We emphasize that a complete set is necessary in order to expand the wave functions in these kinds of structures. All the calculated results by the MEM are well retrieved by the finite-difference time-domain calculation.

Sound transmission through plates perforated with two periodic subwavelength hole arrays

Theoretical results on sound transmission through plates perforated with compound periodic hole arrays are presented. Calculations are obtained by means of a model under the rigid-solid assumption, which is thoroughly derived. A complex interplay between resonant transmission peaks, resonance interference, and Wood anomaly minima is observed. At high frequencies the resonant peaks overcome the Wood anomaly dips, leading to a different behaviour when compared with simple hole arrays.

Source-model technique analysis of electromagnetic scattering by surface grooves and slits

A computational tool, based on the source-model technique (SMT), for analysis of electromagnetic wave scattering by surface grooves and slits is presented. The idea is to use a superposition of the solution of the unperturbed problem and local corrections in the groove/slit region (the grooves and slits are treated as perturbations). In this manner, the solution is obtained in a much faster way than solving the original problem. The proposed solution is applied to problems of grooves and slits in otherwise planar or periodic surfaces. Grooves and slits of various shapes, both smooth ones as well as ones with edges, empty or filled with dielectric material, are considered. The obtained results are verified against previously published data.

Tuning plasmonic resonances of an annular aperture in metal plate

We present theory to describe the plasmonic resonances of a subwavelength annular aperture in a real metal plate. The theory provides the reflection, including the amplitude and phase, of radially polarized surface plasmon waves from the end faces of the aperture with a significant departure from the perfect electric conductor case due to plasmonic effects. Oscillations in the reflection amplitude and phase are observed. These oscillations arise from transverse resonances and depend on the geometry of the annulus. The theory is applied to the design of various aperture structures operating at the same resonance wavelength, and it is confirmed by comprehensive electromagnetic simulations. The results are contrasted to the perfect electric conductor case and they will be of significant interest to emerging applications in metamaterials, plasmonic sensors, and near-field optics.

Spatial quantizing a subwavelength slit to upgrade its optical properties

We propose a spatial quantization method to discretize a single subwavelength metallic slit into identical unit chains and investigate its transmission properties. Finite-difference time-domain simulations suggest that the formation of multiple fundamental plasmon resonances and their mutual coupling processes play dominant roles in the quantized structure, which eventually alters the surface plasmon energy propagation into a hopping style. Through adjusting the groove geometrical parameters, the optical transmission can be further increased by as high as about 1000% or suppressed to zero when compared with a corresponding untreated slit. We expect these findings to have potential applications in subwavelength optics.

Description of the modes governing the optical transmission through metal gratings

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 visible - near-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.

Tunable phase resonances in a compound metallic grating with perpendicular bumps and cuts

We propose a compound metallic grating with perpendicular bumps in each slit and investigate its transmission property theoretically. As the bumps are set symmetrically in the slits, the waveguide resonant peaks for the even (odd) modes exhibit a red-shift (blue-shift) compared with the resonant peaks of grating composed of bare slit. As the bumps are set asymmetrically, we show that the dips in transmission spectrum can be tuned by shifting the position and changing the size of bumps in the slit. The corresponding physical mechanisms for above phenomenon are discussed, followed by some qualitative explanations in terms of field distribution. We also investigate the optical transmission through a compound metallic grating with perpendicular bumps in one slit and cuts in another slit, and find that the dips in transmission spectrum are more sensitive to the simultaneous change of the bumps and cuts.

High-throughput diffraction-assisted surface-plasmon-polariton coupling by a super-wavelength slit

We propose a novel SPP coupling scheme capable of high SPP throughput and high SPP coupling efficiency based on a slit of width greater than the wavelength, immersed in a uniform dielectric. The dispersive properties of the slit are engineered such that the slit sustains a low-loss higher-order waveguide mode just above cutoff, which is shown to be amenable to wavevector matching to the SPP mode at the slit exit. The SPP throughput and SPP coupling efficiency are quantified by numerical simulations of visible light propagation through the slit for varying width and dielectric refractive index. An optimal SPP coupling configuration satisfying wavevector matching is shown to yield an order-of-magnitude greater SPP throughput than a comparable slit of sub-wavelength width and a peak SPP coupling efficiency ≃ 68%. To our knowledge, this is the first investigation of coupling between higher-order waveguide modes in slits of super-wavelength width and SPP modes.

Absorption profile modulation by means of 1D digital plasmonic gratings

Optical simulations of 1D digital plasmonic gratings on a Silicon substrate are performed by means of the Finite Elements Method and a modal analysis. The different mechanisms of transmission of the light are elucidated. The absorption profile in Silicon can be modulated and controlled changing the geometry. Configuration maps allow to determine the different optical regimes. Surface Plasmon Polaritons and cavity-mode resonances are shown to be effectively exploitable to enhance NIR-light absorption in different shallower regions of the underlying Silicon.

Acoustic transmission enhancement through a periodically structured stiff plate without any opening

We report both experimentally and theoretically that enhanced acoustic transmission can occur in the subwavelength region through a thin but stiff structured plate without any opening. This exotic acoustic phenomenon is essentially distinct from the previous related studies originated from, either collectively or individually, the interaction of the incident wave with openings in previous structures. It is attributed to the structure-induced resonant excitation of the nonleaky Lamb modes that exist intrinsically in the uniform elastic plate. Our finding should have an impact on ultrasonic applications.

Observation of enhanced transmission for s-polarized light through a subwavelength slit

Enhanced optical transmission (EOT) through a single aperture is usually achieved by exciting surface plasmon polaritons with periodic grooves. Surface plasmon polaritons are only excited by p-polarized incident light, i.e. with the electric field perpendicular to the direction of the grooves. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. A 25 fold increase is measured as compared to the case without the dielectric film. Transmission measurements are compared with a coupled mode method and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.

Acoustic surface evanescent wave and its dominant contribution to extraordinary acoustic transmission and collimation of sound

We demonstrate both theoretically and experimentally the physical mechanism that underlies extraordinary acoustic transmission and collimation of sound through a one-dimensional decorated plate. A microscopic theory considers the total field as the sum of the scattered waves by every periodically aligned groove on the plate, which divides the total field into far-field radiative cylindrical waves and acoustic surface evanescent waves (ASEWs). Different from the well-known acoustic surface waves like Rayleigh waves and Lamb waves, ASEW is closely analogous to a surface plasmon polariton in the optical case. By mapping the total field, the experiments well confirm the theoretical calculations with ASEWs excited. The establishment of the concept of ASEW provides a new route for the integration of subwavelength acoustic devices with a structured solid surface.

Composite nanoparticle nanoslit arrays: a novel platform for LSPR mediated subwavelength optical transmission

Near infrared transmission of light through subwavelength slit arrays is shown to be significantly influenced by resonant metallic nanoparticles placed within the structure. Experimental and calculated transmission spectra show how the size, orientation of the nanoparticles, and the period of the nanoslit array influence the maximum transmission wavelength, the magnitude of the transmission, and width of the resonance. These findings suggest that the localized surface plasmon resonance (LSPR) of metallic nanoparticles and their subsequent near and far-field interactions can modulate the subwavelength transmission and bandwidth of nanoaperture array devices in optically thick metal films.

Multiple extraordinary optical transmission peaks from evanescent coupling in perforated metal plates surrounded by dielectrics

We study numerically and theoretically the optical transmission of nanostructured gold films embedded in dielectric claddings. We show how multiple transmission peaks appear as the claddings thickness increases. These transmission peaks come not only from surface plasmon polariton excitations but also from the excitation of Fabry-Perot modes sustained at the claddings, coupled through the metal, as long as a periodic pattern is milled in the metal film. We propose that this structure could be used as an ultracompact all-optical switch by surrounding the metal film with Kerr nonlinear dielectric layers.

Polarized GaN-based LED with an integrated multi-layer subwavelength structure

A novel type of GaN-based LED with a highly polarized output using an integrated multi-layer subwavelength grating structure is proposed. Characteristics of both optical transmission and polarization extinction ratio of the polarized GaN-based LED with three different multi-layer subwavelength structures are investigated. It is found that both TM transmission (T(TM)) and the extinction ratio(ER) of the LED output can be effectively enhanced by incorporating a dielectric transition layer between the metal grating and GaN substrate with a lower refractive index than that of the GaN substrate. Flat sensitivity of the T(TM) on the period, duty cycle of the metallic grating, and the wide range of operating wavelength have been achieved in contrast to the conventional sensitive behavior in single-layer metallic grating. Up to 0.75 high duty cycle of the metallic grating can be employed to achieve >60dB ER while T(TM) maintains higher than ~90%, which breaks the conventional limit of T(TM) and ER being always a pair of trade-off parameters. Typical optimized multilayer structures in terms of material, thickness, grating periods and duty cycle using MgF(2) and ZnS, respectively, as the transition layers are obtained. The results provide guidance in designing, optimizing and fabricating the novel integrated GaN-based and polarized photonic devices.

Interaction of light with a metal wedge: the role of diffraction in shaping energy flow

When a light wave hits a metal wedge structure, the metal surfaces respond to the incident light by generating both free-space and surface-bound waves. Here we present a physical model that elucidates electromagnetic interactions of an incoming planar wave with a simple semi-infinite 90 degrees metal wedge. We show that a metal wedge structure possesses an intrinsic capability of directing the incident power around the corner into the forward direction. Interplay of the boundary diffraction wave and the incident and reflection waves in the near field region of a metal corner is found to form a basis of the funneling phenomena that are commonly observed in metal nanoslit structures. Theory and experiment reveal that the incident wave propagating parallel to the sidewall destructively interferes with the boundary diffraction wave forming a depleted-energy-flow region along the glancing angle direction. A physical understanding of various electromagnetic phenomena associated with a metal wedge structure confirms rich potential of the simple structure as an elemental building block of complex metal nanostructures.

Generation of radially and azimuthally polarized light by optical transmission through concentric circular nanoslits in Ag films

Optical transmission through concentric circular nanoslits is studied in experiments and numerical simulations. Polarized optical microscopic imaging shows that the optical transmission through these apertures is spatially inhomogeneous, exhibiting colored fan texture patterns. Numerical simulations show that these colored fan texture patterns originate from the cylindrical vector polarization of the transmitted beam. Specifically, the transmitted light is in-phase radially polarized at long wavelengths due to the predominant transmission of the transverse magnetic (TM) waveguide modes; and in-phase azimuthally polarized at short wavelengths due to the increased optical transmission of the transverse electric (TE) waveguide modes. Additionally, the transmission shows a peak at the wavelength of Wood anomaly and a dip at the resonant wavelength of surface plasmon excitation; and the transmitted light at these wavelengths is a mixture of azimuthally and radially polarized fields. These interesting optical transmission behaviors of circular nanoslits provide a miniaturized way to generating radially and azimuthally polarized light.

Extraordinary magnetic field enhancement with metallic nanowire: role of surface impedance in Babinet's principle for sub-skin-depth regime

We propose and analyze the "complementary" structure of a metallic nanogap, namely, the metallic nanowire for magnetic field enhancement. A huge enhancement of the field up to a factor of 300 was achieved. Introducing the surface impedance concept, we also develop and numerically confirm a new analytic theory which successfully predicts the field enhancement factors for metal nanostructures. Compared to the predictions of the classical Babinet principle applied to a nanogap, an order of magnitude difference in the field enhancement factor was observed for the sub-skin-depth regime nanowire.

Extraordinary optical transmission through multi-layered systems of corrugated metallic thin films

Optical transmission through multi-layered systems of corrugated metallic thin films is investigated by rigorous electromagnetic simulations based on an exact Green tensor method. Compared to a single metal slab of equivalent thickness and volume, it was found that the multi-layered system can significantly impede the field decay, often leading to transmission greater than that expected from the Fabry-Perot resonance-like behavior exhibited by subwavelength slits in a single slab. Extraordinary optical transmission is also observable for systems of layers whose combined thicknesses are much greater than the skin depth of the metal. Structures consisting of up to five layers with a net thickness of 500 nm for the metal films were considered in our study. These findings demonstrate that an appreciable fraction of the optical power that is incident on the thin metal films can be transmitted over distances greater than their skin depth using plasmonic resonances.

Anomalous near-perfect extraordinary optical absorption on subwavelength thin metal film grating

We demonstrate analytically and numerically that the anomalous extraordinary optical absorption through thin metal grating with air slit arrays can be excited simultaneously by long and short range surface plasmonic modes supported on the surfaces of thin metal film in weak coupling while this absorption can only be excited by short range surface plasmonic mode in strong coupling. In particular, we predict localized plasmonic mode inside air slits holding double-effect to generate the anomalous extraordinary optical absorption when only this mode is present and to suppress this absorption when the pathways excited by localized and surface plasmonic modes are present simultaneously. Furthermore, we present that double-layer metal grating consisting of two identical single-layer metal gratings can be exploited to enhance absorption efficiency more than 99% and 90% for excited localized plasmonic modes and surface plasmonic modes respectively.

Enhanced transmission of transverse electric waves through subwavelength slits in a thin metallic film

By adding an array of metallic cut wires, the transmission of transverse electric (TE) waves (the electric field is parallel to the slits) through subwavelength slits in a thin metallic film is significantly enhanced. An 800-fold enhanced transmission is obtained compared to the case without the cut wires. It is demonstrated that a TE incident wave is highly confined by the cut wires, due to the excitation of the electric dipolelike resonance, and then effectively squeezed into and through the subwavelength slits.

Terahertz transmission properties of an individual slit in a thin metallic plate

We report on the terahertz transmission properties through a single slit in a thin metallic film. The properties are studied by comparing the transmissions of TE- and TM-polarized electromagnetic waves over a broad spectral range from the geometrical regime to the subwavelength limit. In the geometrical regime, the remarkable terahertz transmission due to guided modes is observed even without the contribution of surface waves. Whereas in the subwavelength limit, the surface charge oscillations associated with the TM-polarized guided mode give rise to strong transmission enhancement. The nature of the mechanisms for the terahertz transmission is elucidated using theoretical simulations of the near-field distributions and electromagnetic energy flow.

Dependence of resonant light transmission properties of a subwavelength slit on structural parameters

We perform a systematic study of the resonant transmission of visible and near-infrared (NIR) light through a single subwavelength slit in a gold film when the parameters defining the structure are varied. We further examine the optical properties of a related nanostructure, a cross with subwavelength sized features. Focused ion beam (FIB) milling was used to fabricate nanoslits and crosses with linewidths ranging from 26 nm to 85 nm. The dimensions of the structure are found to affect strongly the transmittance spectrum. For example, as the slit becomes narrower the resonance is observed to both sharpen and shift significantly. Our observations are in good agreement with our earlier numerical calculations on the optical properties of nanoslits.

Coherent electronic transport through graphene constrictions: subwavelength regime and optical analogy

Nanoelectronic devices smaller than the electron wavelength can be achieved in graphene with current lithography techniques. Here we show that the electronic quantum transport of graphene subwavelength nanodevices presents deep analogies with subwavelength optics. We introduce the concept of electronic diffraction barrier to represent the effect of constrictions and the rich transport phenomena of a variety of nanodevices. Results are presented for Bethe and Kirchhoff diffraction in graphene slits and Fabry-Perot interference oscillations in nanoribbons. The same concept applies to graphene quantum dots and gives new insight into recent experiments in these systems.

On the transmission of diffuse light through thick slits

We study the transmission of diffuse light through thick slits. For perfectly conducting slits and in-plane s-polarized illumination, the transmittance curves present a staircaselike behavior as a function of the aperture width, where the steps mark the appearance of new propagation modes. In contrast, with p-polarized illumination the transmittance increases linearly with the aperture width, with only some perturbations in the positions that correspond to the appearance of new modes. Out-of-plane incidence and more realistic assumptions about the slit, such as finite conductivity and roughness, are also discussed.

Transmission through metallic array slits with perpendicular cuts

The transmission of normally incident plane wave through an array of subwavelength metallic slits modified by perpendicular cuts has been explored. The cuts in middle of slit affect the even and odd modes of slits in different manners. To the best of our knowledge, this is the first work to investigate the influence of cuts on higher modes of slit, which is quite different from that on the fundamental mode studied in previous papers. Shifting the cuts along the vertical slits, we can get two kinds of resonances, which can be excited when the cuts locate at the center of electric or magnetic antinodes. In addition, we propose a new explanation model, in which we ascribe the resonance to three reasons: the F-P cavity theory, the surface current flow, and the surface charges. Irrespective of even and odd modes, the increase in the length of current flow or decrease in ability of accumulating charges dominates when the cut lies at different locations, which corresponds to a red or blue shift of resonant wavelengths. All calculated results are well explained by our proposed model.

Plasmonic focusing with a coaxial structure illuminated by radially polarized light

We propose and analyze a plasmonic lens that is illuminated by a radially polarized light. The lens is made of a coax-like geometry consisting of an annular dielectric slit surrounded by metal. Focusing efficiency is enhanced by the use of a circular grating assisting the coupling of light into surface plasmons. Further enhancement is obtained by introducing a circular Bragg grating on top of the structure, reflecting the surface plasmon modes that are propagating in the counter-focus direction. Using the Finite-Difference Time-Domain approach we investigate the transmission and the focusing mechanisms, and study the effect of structural parameters on the performance of the plasmonic lens.

Optical resonances on sub-wavelength silver lamellar gratings

The optical response of sub-wavelength silver lamellar gratings has been theoretically investigated. Two distinct types of resonance have been predicted for incident radiation with E-field perpendicular to the long axis of the wires. The first resonance has been identified as a cavity mode resonance that is associated with transmission enhancement. The second resonance has been identified as an entirely new horizontal plasmon resonance on the incident (and transmission) surfaces of the wires of the grating. Normal surface plasmon modes are investigated on discontinuous gratings, and their relation to those found on continuous gratings is highlighted by focusing on the perturbation effect of the discontinuities. It is shown that the new horizontal plasmon mode is in no way related to the well known diffractively coupled surface plasmon, and is shown to have a particle plasmon-like nature. It is therefore termed a horizontal particle plasmon, and may be either an uncoupled horizontal particle plasmon resonance (a 1-dimensional particle plasmon) or a coupled horizontal particle plasmon resonance (a 2-dimensional particle plasmon) depending on the height of the grating. It is shown that this resonance may result in a reflection efficiency that is very high, even when the grating would be optically thin if it were a homogeneous film, therefore, it behaves as an inverse wire grid polariser as it reflects more TM than TE incident radiation.

Transmission resonances of electromagnetic wave through metallic gratings: phase and field characterizations

We investigate experimentally and numerically the transmission resonances of electromagnetic wave through a metallic grating of periodic slits as well as an array of subwavelength holes. The far-field transmission (magnitude and phase) spectra and the detection of surface electric field are used to characterize two kinds of transmission resonances (Fabry-Perot-like and structural factor resonance) occurring to the slit grating. In addition, the comparison between the slit grating and the subwavelength hole array reveals the effect of the aperture property on the phase and field of resonantly transmitted electromagnetic wave.

Extraordinary sound screening in perforated plates

We report extraordinary effects in the transmission of sound through periodically perforated plates, supported by both measurements and theory. In agreement with recent observations in slit arrays, M. H. Lu et al. [Phys. Rev. Lett. 99, 174301 (2007)10.1103/PhysRevLett.99.174301], nearly full transmission is observed at certain resonant frequencies, pointing out similarities of the acoustic phenomena and their optical counterpart. However, acoustic screening well beyond that predicted by the mass law is achieved over a wide range of wavelengths in the vicinity of the period of the array, resulting in fundamentally unique behavior of the sound as compared to light.

Theory of resonant acoustic transmission through subwavelength apertures

A complete landscape is presented of the acoustic transmission properties of subwavelength apertures (slits and holes). First, we study the emergence of Fabry-Perot resonances in single apertures. When these apertures are placed in a periodic fashion, a new type of transmission resonance appears in the spectrum. We demonstrate that this resonance stems from the excitation of an acoustic guided wave that runs along the plate, which hybridizes strongly with the Fabry-Perot resonances associated with waveguide modes in single apertures. A detailed discussion of the similarities and differences with the electromagnetic case is also given.

Optical control over surface-plasmon-polariton-assisted THz transmission through a slit aperture

We demonstrate optical control over the transmission of terahertz (THz) radiation through a single subwavelength slit in an otherwise opaque silicon wafer. The addition of periodic corrugation on each side of the wafer allows coupling to surface plasmon polaritons, so that light not impinging directly on the slit can contribute to the transmission. A significant enhancement of the THz transmission can be achieved through control of the surface wave propagation length by excitation at optical wavelengths. The observed transmission increase is in distinct contrast to the reduction reported for photoexcitation of arrays of holes in semiconductors.

Design of a subwavelength bent C-aperture waveguide

We present a design for a subwavelength C-shaped optical waveguide with a 90 degrees junction that efficiently transports light while maintaining tight confinement with an exit spot size of lambda/7. Finite-difference time-domain simulations of perfect electric conductors and Au C-aperture waveguides are performed for optical frequencies. A design resonant near 780 nm is presented, with a spot size of 107 nm x 107 nm and an energy density enhancement factor of 10 for a bent waveguide of total length 1.4 microm.

Some properties of the optical resonances in a single subwavelength slit

Diffraction of TM-polarized waves by a slit in a thick screen of infinite conductivity is treated. The case of an arbitrary incident beam wave is considered. We study the resonances that appear when the wavelength of the incident beam wave is larger than the slit width, i.e., the subwavelength regime where a one-mode model for the slit can be considered. High anomalous values (resonances) of the transmission coefficient, the angular diffracted energy, and the magnetic field within the slit are analyzed. A simple linear relationship to determine the resonant wavelengths is proposed. We show that the transmission coefficient, the normal diffracted energy, and the magnetic field within the cavity are linear functions of the resonant wavelength and the thickness of the screen. Additionally and surprisingly, we reveal that under certain conditions the incident beam wave via the diffraction can give a suppressed light transmission; i.e., a minimum in the transmission is obtained where a maximum is expected.

Enhanced transmission and reflection of few-cycle pulses by a single slit

We show that a physical mechanism responsible for the enhanced transmission and reflection of ultrashort (few-cycle) pulses by a single subwavelength slit in a thick metallic film is the Fabry-Perot-like resonant excitation of stationary, quasistationary, and nonstationary waves inside the slit, which leads to the field enhancement inside and around the slit. The mechanism is universal for any pulse-scatter system, which supports the stationary resonances. We point out that there is a pulse duration limit below which the slit does not support the intraslit resonance.

Light in tiny holes

The presence of tiny holes in an opaque metal film, with sizes smaller than the wavelength of incident light, leads to a wide variety of unexpected optical properties such as strongly enhanced transmission of light through the holes and wavelength filtering. These intriguing effects are now known to be due to the interaction of the light with electronic resonances in the surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes. This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics.

Low-loss subwavelength metal C-aperture waveguide

We present a design of a linear optical waveguide that utilizes a C-shaped metallic nano-aperture that efficiently transports light while maintaining a spot size of lambda/10. The performance of a C-aperture waveguide is superior to both a regular ridge waveguide and other surface plasmon based metal nano-optical waveguides. The energy transport mechanisms are explained by the coupling of an aperture surface resonance and the thickness resonances inside the guide channel. Finite-difference time-domain simulations of gold C-aperture waveguides are performed for a 1.5 microm wavelength incident plane wave. The 1/e decay length in power transmission is predicted to be approximately 2.5 microm. The total power throughput is 1.66 for the 2.55 microm long guide, with an intensity 6 times that of the incident wave at a distance 120 nm from the exit plane, having a spot size of 150 nm.

One-way diffraction grating

Diffraction gratings are elementary tools for much of optics and spectroscopy. Here, at microwave frequencies, we provide a new perspective on these fundamental structures. A transmission diffraction grating is presented that has diffracted beams emanating from one surface only. It can thus function either as a transmission grating with no reflected orders (other than zero) or, in the reverse configuration, as a partially transmitting structure with diffracted orders in reflection only.

Rigorous theory of the diffraction of Gaussian beams by finite gratings: TM polarization

Diffraction of TM-polarized Gaussian beams by N equally spaced slits (finite grating) in a planar perfectly conducting thick screen is treated. We extend to the TM polarization case the results of a previous paper where the TE polarization was considered. The far-field diffraction patterns, the transmission coefficient tau, and the normally diffracted energy E as a function of several optogeometrical parameters are analyzed within the so-called vectorial region. The existence of constant-intensity angles in the far field when the incident beam wave is scanned along the N slits is shown. In addition, the property E=Ntau/lambda, valid in the scalar region, is extended to the TM polarization case in the vectorial region, lambda being the wavelength. The coupling between slits is analyzed, giving an oscillating amplitude-decreasing function as the separation between slits increases, where the period for these oscillations is the wavelength lambda. Finally, the extraordinary optical transmission phenomena that appear when the wavelength is larger than the slit width (subwavelength regime) are analyzed.

Controlling strong electromagnetic fields at subwavelength scales

We investigate the optical response of two subwavelength grooves on a metallic screen, separated by a subwavelength distance. We show that the cavity, Fabry-Perot-like mode, already observed in one-dimensional periodic gratings and known for a single slit, splits into two resonances in our system: a symmetric mode with a small Q factor, and an antisymmetric one which leads to a much stronger field enhancement. This behavior results from the near-field coupling of the grooves. Moreover, the use of a second incident wave allows control of the localization of the photons in the groove of our choice, depending on the phase difference between the two incident waves. The system acts exactly as a subwavelength optical switch operated from far field.

Theory of surface plasmon generation at nanoslit apertures

In this Letter, we study the scattering of light by a single subwavelength slit in a metal screen. In contrast with previous theoretical works, we provide a microscopic description of the scattering process by emphasizing the generation of surface plasmons at the slit apertures. The analysis is supported by a rigorous formalism based on a normal-mode-decomposition technique and by a semianalytical model that provides accurate formulas for the plasmonic generation strengths. The generation is shown to be fairly efficient for metals with a low conductivity, such as gold in the visible regime. Verification of the theory is also shown by comparison with recent experimental data [H. F. Schouten, Phys. Rev. Lett. 94, 053901 (2005)].

Remarkable zeroth-order resonant transmission of microwaves through a single subwavelength metal slit

A slit in a thick metal plate that is extremely subwavelength will not transmit microwaves polarized parallel to it. It is shown here that cuts perpendicular to the slit allow parallel polarized radiation to resonantly transmit. Furthermore, a zero-order mode may be excited within the slit, the frequency of which, to first order, is independent of the plate depth. Remarkably, for this novel type of resonance, the field in the slit oscillates with a constant phase and little amplitude variation throughout the plate depth, while the resonant wavelength tends to infinity as the slit width approaches zero.

Tunneling mechanism of light transmission through metallic films

A mechanism of light transmission through metallic films is proposed, assisted by tunneling between resonating buried dielectric inclusions. This is illustrated by arrays of Si spheres embedded in Ag. Strong transmission peaks are observed near the Mie resonances of the spheres. The interaction among various planes of spheres and interference effects between these resonances and the surface plasmons of Ag lead to mixing and splitting of the resonances. Transmission is proved to be limited only by absorption. For small spheres, the effective dielectric constant of the resulting material can be tuned to values close to unity, and a method is proposed to turn the resulting materials invisible.

Microwave transmission through a single subwavelength annular aperture in a metal plate

The resonant transmission of a small annular aperture, with a diameter much smaller than the radiation wavelength, in a thin metal plate is studied at microwave frequencies. It transpires that such an annular aperture supports several resonant guided modes, including those that are not quantized in the azimuthal direction. Such modes have resonant frequencies that are largely independent of the diameter of the annular aperture, thus being supported by annular apertures that tend to zero radius. The transmittance of such a structure at microwave frequencies is detailed and compared with the predictions of a finite element method model.

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.