Resonant transmission through a metallic film due to coupled modes

Broadband Metallic Fiber-to-Chip Couplers and a Low-Complexity Integrated Plasmonic Platform

We present a plasmonic platform featuring efficient, broadband metallic fiber-to-chip couplers that directly interface plasmonic slot waveguides, such as compact and high-speed electro-optic modulators. The metallic gratings exhibit an experimental fiber-to-slot coupling efficiency of -2.7 dB with -1.4 dB in simulations with the same coupling principle. Further, they offer a huge spectral window with a 3 dB passband of 350 nm. The technology relies on a vertically arranged layer stack, metal-insulator-metal waveguides, and fiber-to-slot couplers and is formed in only one lithography step with a minimum feature size of 250 nm. As an application example, we fabricate new modulator devices with an electro-optic organic material in the slot waveguide and reach 50 and 100 Gbit/s data modulation in the O- and C-bands within the same device. The devices' broad spectral bandwidth and their relaxed fabrication may render them suitable for experiments and applications in the scope of sensing, nonlinear optics, or telecommunications.

Infrared spectral filter based on all-semiconductor guided-mode resonance

We present a theoretical and experimental study of guided-mode resonant (GMR) spectral filters made of III-V semiconductors and operating in the long-wave infrared (LWIR) wavelength range. In the scope of the colorization of infrared photodetectors, we used materials fully compatible with the epitaxial growth of Type 2 super lattice LWIR photodetectors: heavily n-doped InAsSb for the grating and GaSb for the waveguide of the GMR resonator.

Exploring surface sensitivity of Rayleigh anomaly in metal/dielectric multilayer gratings

Biosensors based on Rayleigh anomaly (RA) in metal gratings exhibit impressive bulk refractive index (RI) sensitivity and narrow linewidth. However, the electric field enhancement extends far away from surface of the gratings, which limits the application on biosensor where the RI changes are restricted at the sensor interface. To overcome this shortcoming, a novel grating composed of a 8-layer Au/Al₂O₃ stack was optimized by numerical simulation. The electric field is limited in several hundreds of nanometers from surface. The surface sensitivity increases 10 times than that of Au gratings at the detection depth of less than 400 nm. The surface index sensitivity can be improved 5 times under oblique incidence than that under normal incidence when the thickness of cover media is 20 nm.

Extraordinary optical reflection resonances and bound states in the continuum from a periodic array of thin metal plates

The creation of artificial structures with very narrow spectral features in the terahertz range has been a long-standing goal, as they can enable many important applications. Unlike in the visible and infrared, where compact dielectric resonators can readily achieve a quality factor (Q) of 10⁶, terahertz resonators with a Q of 10³ are considered heroic. Here, we describe a new approach to this challenging problem, inspired by the phenomenon of extraordinary optical transmission (EOT) in 1D structures. In the well-studied EOT problem, a complex spectrum of resonances can be observed in transmission through a mostly solid metal structure. However, these EOT resonances can hardly exhibit extremely high Q, even in a perfect structure with lossless components. In contrast, we show that the inverse structure, a periodic array of very thin metal plates separated by air gaps, can exhibit non-trivial bound states in the continuum (BICs) reflection resonances, with arbitrarily high Q, and with peak reflectivity approaching 100% even for a vanishingly small metal filling fraction. Our analytical predictions are supported by numerical simulations, and also agree well with our experimental measurements. This configuration offers a new approach to achieving ultra-narrow optical resonances in the terahertz range, as well as a new experimentally accessible configuration for studying BICs.

Impact of the slit geometry on the performance of wire-grid polarisers

Wire-grid polarisers are versatile and scalable components which can be engineered to achieve small sizes and extremely high extinction ratios. Yet the measured performances are always significantly below the predicted values obtained from numerical simulations. Here we report on a detailed comparison between theoretical and experimental performances. We show that the discrepancy can be explained by the true shape of the plasmonic structures. Taking into account the fabrication details, a new optimisation model enables us to achieve excellent agreement with the observed response and to re-optimise the grating parameters to ensure experimental extinction ratios well above 1,000 at 850 nm.

Generalized analytical model based on harmonic coupling for hybrid plasmonic modes: comparison with numerical and experimental results

Metal nanoparticle arrays have proved useful for different applications due to their ability to enhance electromagnetic fields within a few tens of nanometers. This field enhancement results from the excitation of various plasmonic modes at certain resonance frequencies. In this article, we have studied an array of metallic nanocylinders placed on a thin metallic film. A simple analytical model is proposed to explain the existence of the different types of modes that can be excited in such a structure. Owing to the cylinder array, the structure can support localized surface plasmon (LSP) modes. The LSP mode couples to the propagating surface plasmon (PSP) mode of the thin film to give rise to the hybrid lattice plasmon (HLP) mode and anti-crossing phenomenon. Due to the periodicity of the array, the Bragg modes (BM) are also excited in the structure. We have calculated analytically the resonance frequencies of the BM, LSP and the corresponding HLP, and have verified the calculations by rigorous numerical methods. Experimental results obtained in the Kretschmann configuration also validate the proposed analytical model. The dependency of the resonance frequencies of these modes on the structural parameters such as cylinder diameter, height and the periodicity of the array is shown. Such a detailed study can offer insights on the physical phenomenon that governs the excitation of various plasmonic modes in the system. It is also useful to optimize the structure as per required for the different applications, where such types of structures are used.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Unified theory of surface-plasmonic enhancement and extinction of light transmission through metallic nanoslit arrays

Metallic nanostructures are of immense scientific interest owing to unexpectedly strong interaction with light in deep subwavelength scales. Resonant excitations of surface and cavity plasmonic modes mediate strong light localization in nanoscale objects. Nevertheless, the role of surface plasmon-polaritons (SPP) in light transmission through a simple one-dimensional system with metallic nanoslits has been the subject of longstanding debates. Here, we propose a unified theory that consistently explains the controversial effects of SPPs in metallic nanoslit arrays. We show that the SPPs excited on the entrance and exit interfaces induce near-total internal reflection and abrupt phase change of the slit-guided mode. These fundamental effects quantitatively describe positive and negative effects of SPP excitation in a self-consistent manner. Importantly, the theory shows excellent agreement with rigorous numerical calculations while providing profound physical insight into the properties of nanoplasmonic systems.

Transition from a spectrum filter to a polarizer in a metallic nano-slit array

The transition from a spectrum filter (resonant transmission) to a polarizer (broadband transmission) for TM polarized light is observed in a metallic nano-slit array as period is decreased. A theoretical model is developed and shows that the spectrum filter behavior is caused by the coupled slit/grating resonance. With decreasing period, the slit resonance is decoupled from the grating resonance, which then dominates the transmission spectrum and broadens the transmission peak. With further reducing period, the slit resonance diminishes and the peak spectrum transforms to a broadband transmission. This effect is the basis for the operation of wire grid polarizers. The transition is explained by the change of the impedance to the incoming wave.

Coupled mode enhanced giant magnetoplasmonics transverse Kerr effect

We show that the enhancement of the transverse magneto-optical Kerr effect of a smooth magnetic dielectric film covered by a noble metal grating, is strongly dependent on the precise geometry of this grating. Up till now this magnetoplasmonic enhancement was solely attributed to a nonreciprocal shift of the dispersion of the surface plasmon polariton resonances at the interface with the magnetized substrate. It is demonstrated that by hybridization of surface and cavity resonances in this 1D plasmonic grating, the transverse Kerr effect can be further enhanced, extinguished or even switched in sign and that without inverting or modifying the film's magnetization. This strong geometrical dispersion and the accompanying anomalous sign change of the magneto-plasmonic effects in such systems has never been considered before, and might find interesting applications in sensing and nanophotonics.

Improvement of infrared single-photon detectors absorptance by integrated plasmonic structures

Plasmonic structures open novel avenues in photodetector development. Optimized illumination configurations are reported to improve p-polarized light absorptance in superconducting-nanowire single-photon detectors (SNSPDs) comprising short- and long-periodic niobium-nitride (NbN) stripe-patterns. In OC-SNSPDs consisting of ~quarter-wavelength dielectric layer closed by a gold reflector the highest absorptance is attainable at perpendicular incidence onto NbN patterns in P-orientation due to E-field concentration at the bottom of nano-cavities. In NCAI-SNSPDs integrated with nano-cavity-arrays consisting of vertical and horizontal gold segments off-axis illumination in S-orientation results in polar-angle-independent perfect absorptance via collective resonances in short-periodic design, while in long-periodic NCAI-SNSPDs grating-coupled surface waves promote EM-field transportation to the NbN stripes and result in local absorptance maxima. In NCDAI-SNSPDs integrated with nano-cavity-deflector-array consisting of longer vertical gold segments large absorptance maxima appear in 3p-periodic designs due to E-field enhancement via grating-coupled surface waves synchronized with the NbN stripes in S-orientation, which enable to compensate fill-factor-related retrogression.

Extraordinary optical transmission: coupling of the Wood-Rayleigh anomaly and the Fabry-Perot resonance

In this Letter, we demonstrate for the first time that by combining the effects of the Wood-Rayleigh anomaly (WRA) and the Fabry-Perot (FP) resonance, transmission efficiencies of one-dimensional metallo-dielectric gratings on substrates can be significantly improved compared to when these two phenomena work separately. Results of combining the WRA and the FP resonance can be utilized to eliminate the necessity of using the index matching technique and the core-shell structure for enhancing the performance of extraordinary optical transmission devices. Further, the outcomes of combining the WRA and the FP resonance can elucidate some of the unexplained results in the literature.

Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas

In this Letter, we demonstrate experimentally that a patchwork of four metal-insulator-metal patches leads to an unpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a 2.5 μm bandwidth at 8.5 μm. It paves the way to the design of wideband efficient plasmonic absorbers in the infrared spectrum.

Development of a polarization-insensitive thermophotovoltaic emitter with a binary grating

A wavelength-selective but polarization-insensitive thermophotovoltaic emitter was numerically developed with a binary tungsten grating and its appealing emittance spectra were demonstrated with analysis. Ranges of emitter dimensions were preliminarily confined with the excitation of the surface plasmon polariton, cavity resonance, and Wood's anomaly at specified wavelengths. Then, a hybrid scheme (the rigorous coupled wave analysis together with a genetic algorithm) was able to finely tailor the grating profile such that emittance could be significantly enhanced in the near infrared region. The peak emittance at the transverse electric and transverse magnetic polarizations was 0.997 and 0.935, respectively. The emittance was actually almost twice that from a plain tungsten plate at short wavelengths but significantly reduced at long wavelengths. Moreover, such spectral emittance is insensitive to the polarization and 5% dimension modification, making the emitter ideal for thermophotovoltaic applications. Patterns of electromagnetic fields and Poynting vectors were able to confirm the excitation of physical mechanisms.

Extraordinary optical transmission in one-dimensional gold gratings: near- and far-field analysis

One-dimensional arrays of nanoslits fabricated on silicon nitride membranes show extraordinary optical transmission. Optical characterization techniques have been used to characterize the transmission spectra and the near-field optical configuration. Experimental results have been compared with numerical simulations in order to elucidate the different modes of light propagation. Near- and far-field optical distribution is studied as a function of the polarization of light.

Polarization independence of extraordinary transmission trough 1D metallic gratings

Extraordinary optical transmission of 1D metallic gratings is studied. Experimental samples are fabricated by means of Electron Beam Lithography. The optical characterization is focused on far field transmission properties and in particular on polarization dependence of the incident light. A peculiar symmetry in transmission spectra at different polarization angles is shown; this symmetry is studied both experimentally, and numerically with FEM method. A comparison between numerical and experimental data is provided.

Nearly perfect absorption in intrinsically low-loss grating structures

The feature of enhanced absorption in two-layered grating structures is theoretically investigated. The underlying structures make the most use of resonance mechanism to achieve a nearly perfect absorption in an intrinsically low-loss medium. For standalone gratings, the maximum absorption efficiency is shown to be 50%, which is attributed to the coupling of short range (bonding) or long range (antibonding) surface plasmons with cavity resonances. By attaching a dielectric slab on top or bottom to the metallic grating, the maximum absorption efficiency can be raised to nearly 100%. The presence of guided waves in the dielectric slab causes the strong concentration of fields and reinforces the absorption to its extreme value. The efficient absorption mechanism is illustrated with the pattern of resonance fields and the distribution of power loss density. A phenomenological theory is also used to characterize the absorption anomaly in terms of complex pole and zero.

Enhanced magneto-optical effects in magnetoplasmonic crystals

Plasmonics allows light to be localized on length scales much shorter than its wavelength, which makes it possible to integrate photonics and electronics on the nanoscale. Magneto-optical materials are appealing for applications in plasmonics because they open up the possibility of using external magnetic fields in plasmonic devices. Here, we fabricate a new magneto-optical material, a magnetoplasmonic crystal, that consists of a nanostructured noble-metal film on top of a ferromagnetic dielectric, and we demonstrate an enhanced Kerr effect with this material. Such magnetoplasmonic crystals could have applications in telecommunications, magnetic field sensing and all-optical magnetic data storage.

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.

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 resonance and suppression through double-layer subwavelength hole arrays

We present a theoretical study of acoustic waves passing through double-layer subwavelength hole arrays. The acoustic transmission resonance and suppression are observed. There are three mechanisms responsible for the transmission resonance: the excitation of geometrically induced acoustic surface waves, the Fabry-Perot resonance in a hole cavity (I-FP resonance) and the Fabry-Perot resonance between two plates (II-FP resonance). We can differentiate these mechanisms via the dispersion relation of acoustic modes supported by the double-layer structure. It is confirmed that the coupling between two single-layer perforated plates, associated with longitudinal interval and lateral displacement, plays a crucial role in modulating the transmission properties. The strong coupling between two plates can induce the splitting of the transmission peak, while the decoupling between plates leads to the appearance of transmission suppression. By analyzing the criterion derived for transmission suppression, we conclude that it is the destructive interference between the diffracted waves and the direct transmission waves assisted by the I-FP resonance of the first plate that leads to the decoupling between plates and then the transmission suppression.

Cryptosystem for plaintext messages utilizing optical properties of gratings

A cryptosystem for plaintext messages was developed with gratings and their spectral and directional optical properties. Although there were many applicable grating types and optical responses, this manuscript took an example of a binary metallic surface relief and its specular reflectance at three wavelengths to demonstrate the working principles and the capabilities of the cryptosystem. For one, a series of numbers and the grating characteristics served as the ciphertext containing plaintext messages and the information of the sender's signature. Confidential, high-density, and authentic messages could be, therefore, delivered with tiny or even virtual gratings. Second, four unique encryption/decryption keys here significantly reduced the risk of the ciphertext being easily recovered by eavesdroppers. Further manipulation of keys not only offered several strategies of enhancing the system's safety, but also allowed the coexistence of many two-party, or even multiple-entity, communications. The reflectance spectra shown here were mainly attributed to the Wood's anomaly and were numerically obtained from programs based on rigorous coupled-wave analysis.

Transmission enhancement through deep subwavelength apertures using connected split ring resonators

We report astonishingly high transmission enhancement factors through a subwavelength aperture at microwave frequencies by placing connected split ring resonators in the vicinity of the aperture. We carried out numerical simulations that are consistent with our experimental conclusions. We experimentally show higher than 70,000-fold extraordinary transmission through a deep subwavelength aperture with an electrical size of lambda/31 x lambda/12 (width x length), in terms of the operational wavelength. We discuss the physical origins of the phenomenon. Our numerical results predict that even more improvements of the enhancement factors are attainable. Theoretically, the approach opens up the possibility for achieving very large enhancement factors by overcoming the physical limitations and thereby minimizes the dependence on the aperture geometries.

Nearly perfect Fano transmission resonances through nanoslits drilled in a metallic membrane

We report nearly perfect optical transmission (87%) through freestanding metallic gratings with narrow slits, as the experimental demonstration of the theoretical prediction by Porto et al. [Phys. Rev. Lett. 83, 2845 (1999)10.1103/PhysRevLett.83.2845]. In addition, we show that the Fano line shape of transmission spectra reveals the interplay between localized and propagating surface plasmon resonances, and allows us to determine the nonradiative losses. It provides the limits for the transmission efficiency and resonance quality factor. As an illustration, a mosaic of various bandpass filters has been achieved in a single membrane.

Impacts of geometric modifications on infrared optical responses of metallic slit arrays

This work numerically investigates optical responses (absorptance, reflectance, and transmittance) of deep slits with five nanoscale slit profile variations at the transverse magnetic wave incidence by employing the rigorous coupled-wave analysis. For slits with attached features, their optical responses can be much different due to the modified cavity geometry and dangled structures, even at wavelengths between 3 and 15 microm. The shifts of cavity resonance excitation result in higher transmittance through narrower slits at specific wavelengths and resonance modes are confirmed with the electromagnetic fields. Opposite roles possibly played by features in increasing or decreasing absorptance are determined by the feature position and demonstrated by Poynting vectors. Correlations among all responses of a representative slit array, the angle of incidence, and the slit density are also comprehensively studied. When multiple slit types coexist in an array (complex slits), a wide-band transmittance or absorptance enhancement is feasible by merging spectral peaks contributed from each type of slits distinctively. Discrepancy among infrared optical responses of four selected slit combinations is explained while effects of slit density are also discussed.

Plasmonic crystal for efficient energy transfer from fluorescent molecules to long-range surface plasmons

Corrugated metallic thin film structures that do not support short-range surface plasmon modes but do support long-range modes are discussed. The coupling efficiency of the energy of excited fluorescent molecules to long-range modes is theoretically calculated using the rigorous coupled wave approach. The obtained maximum coupling efficiency is found to be 55%, more that two times higher than the efficiency of uncorrugated metallic thin films.

Tailoring radiative and non-radiative losses of thin nanostructured plasmonic waveguides

Thin nanostructured metal films allow to control radiative and non-radiative losses of surface plasmon polariton modes without changing their group velocities. This effect is studied in plasmonic waveguides made of thin gold films drilled with very narrow slits and deposited on a GaAs substrate. The analysis is supported by high-resolution angle-resolved transmission measurements and rigorous electromagnetic calculations. We show that the excitation of air/gold and gold/GaAs surface waves leads to Fano-type resonances with specific light localization into the slits. As a result, gold/GaAs surface waves induce a modulation of radiative and non-radiative losses of air/gold surface waves. The minimum and maximum of the Fano-type resonance introduce two propagation regimes. In the radiative propagation regime, the losses due to the absorption are negligible, whereas an efficient inhibition of free-space coupling is demonstrated in low-loss propagation regime.

Modal method for conical diffraction on a rectangular slit metallic grating in a multilayer structure

The modal method is applied to the problem of conical diffraction on a rectangular slit metallic grating lying on an arbitrary multilayer medium. In the approximation of the surface impedance boundary condition on the grating walls, a single matrix equation is obtained, whose coefficients are expressed simply by the reflectivities on the different layers. A simple and comprehensive treatment is thus obtained for virtually any multilayer system. The method is illustrated for the case of a cavity formed by a planar metallic mirror and a grating, as well as the system formed by a doped layer with Drude susceptibility in a substrate below the grating. The method could be useful for the design of near- and far-infrared devices.

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.

Extraordinary optical transmission in the ultraviolet region through aluminum hole arrays

We investigated the extraordinary optical transmission phenomenon in the UV range by fabricating large-area, free-standing aluminum hole arrays using extreme UV interference lithography and shadow thermal evaporation. Transmission spectra show strong peaks in the UV region resulting from both surface plasmon polariton and localized surface plasmon excitations. The results indicate that the high plasmon frequency of Al is directly responsible for the presence of strong resonance peaks in the UV region, which supports the role of plasmonic phenomena in the extraordinary transmission. The simple fabrication method enables large-area production of such structures for research and industrial production purposes.

Electromagnetic coupling between a metal nanoparticle grating and a metallic surface

The electromagnetic coupling between a two-dimensional grating of resonant gold nanoparticles and a gold metallic film is investigated. We report on the observation of multipeaks in the extinction spectra attributed to resonant modes of the hybrid system, resulting from the coupling between the localized plasmon of the nanoparticles with the underlying surface plasmon mode. Simulations based on the Fourier modal method give good agreement with the experimental measurements and allow for the identification of the respective contributions.