Critical coupling and extreme confinement in nanogap antennas.

Nanogap antennas are compelling structures for squeezing light into ultrasmall volumes. However, when gaps are shrunk to the nanometer scale, the mode losses dramatically increase. In this Letter, we report the conditions of critical coupling between the arrays of nanogap resonant metal-insulator-metal (MIM) antennas and free space. Adapting the antenna density, critical coupling is achievable for any thickness of insulator, from 100 down to 0.1 nm. The fundamental optical mode can be described as continuous transitions through three types of modes: a perfect MIM mode, coupling between the MIM mode and surface plasmon polariton, and a gap plasmon mode. We found that the space between adjacent antennas is an essential parameter to perform critical coupling for thinner gaps. These results pave the way towards understanding extreme confinement in nanogap antenna structures such as MIM or nanoparticle arrays.

Ultrathin mono-resonant nano photovoltaic device for broadband solar conversion.

Nano-resonators can be used in photovoltaics to drastically improve the ability of the device to absorb light and generate photo-carriers, therefore enabling a reduction of the absorber volume. Conventionally, the harvest of the spectrally broad solar spectrum is achieved via the tedious engineering of multiple optical resonances. In this paper, we propose a breakthrough approach, which consists in reducing the solar spectral range with a spectral conversion layer to match only one resonance that can then be easily designed. We use a Maxwell solver and a ray-tracing code to optimize the nano-resonator and its spectral converter. We show that 66.2% optical efficiency can be theoretically achieved in less than 40 nm mean thick absorber while leading to device design enabling collection of photo-generated carriers.

Pixel-sized infrared filters for a multispectral focal plane array.

We present a theoretical study of guided-mode-resonance filters made of two sub-wavelength metallic gratings and a dielectric waveguide, with lateral geometries compatible with the size of infrared focal plane array pixels. Contrary to most of the studies described in the literature, we consider here a focused beam on finite-sized filters, and we investigate the optical properties of a mosaic of 30 µm-long filters. We demonstrate the spectral filtering ability and low crosstalk of such components. We also study the impact of an oblique beam onto a filtering mosaic. We finally discuss the opportunities offered by these results for a new generation of multispectral infrared cameras, and we give an example of a simple architecture.

Field extension inside guided-mode-resonance filters under a focused beam.

We present a theoretical study of the mid-infrared guided mode resonance spectral filters made of two subwavelength metallic gratings and a dielectric waveguide under a focused beam with a finite spot size. This Letter shows that, at the resonant wavelength, the lateral extension of the electromagnetic field in the waveguide is close to the width of the beam. We compare the performance of filters using gratings with a one-slit pattern and gratings with a two-slit pattern, and we show that the latter gratings (biatom gratings) provide a higher transmission and a better limitation of field extension, due to an improved angular acceptance. These results open new perspectives for pixel-sized infrared filters.

Integration of nanostructured planar diffractive lenses dedicated to near infrared detection for CMOS image sensors.

This paper deals with the integration of metallic and dielectric nanostructured planar lenses into a pixel from a silicon based CMOS image sensor, for a monochromatic application at 1.064 µm. The first is a Plasmonic Lens, based on the phase delay through nanoslits, which has been found to be hardly compatible with current CMOS technology and exhibits a notable metallic absorption. The second is a dielectric Phase-Fresnel Lens integrated at the top of a pixel, it exhibits an Optical Efficiency (OE) improved by a few percent and an angle of view of 50°. The third one is a metallic diffractive lens integrated inside a pixel, which shows a better OE and an angle of view of 24°. The last two lenses exhibit a compatibility with a spectral band close to 1.064 µm.

Two-mode model for metal-dielectric guided-mode resonance filters.

Symmetric metal-dielectric guided-mode resonators (GMR) can operate as infrared band-pass filters, thanks to high-transmission resonant peaks and good rejection ratio. Starting from matrix formalism, we show that the behavior of the system can be described by a two-mode model. This model reduces to a scalar formula and the GMR is described as the combination of two independent Fabry-Perot resonators. The formalism has then been applied to the case of asymmetric GMR, in order to restore the properties of the symmetric system. This result allows designing GMR-on-substrate as efficient as free-standing systems, the same high transmission maximum value and high quality factor being conserved.

Extraordinary transmission in optical Helmholtz resonators.

Optical Helmholtz resonators (OHRs) have been adapted from acoustics designs for light absorbing structures, exhibiting extreme light confinement. Here, extraordinary transmission of light is theoretically demonstrated through symmetric OHRs, comprising a cavity with two λ/500 narrow slits on either side. This device has appealing features to act as a spectral bandpass filter in the context of multispectral imaging, in particular its high angular tolerance because of the localized nature of the resonance. Besides, the cavity can be modeled as an inductor and the two slits can be modeled as capacitors, the whole design acting as a LC circuit thus preventing any harmonic features.

Electromagnetic modelization of spherical focusing on a one-dimensional grating thanks to a conical B-spline modal method.

Focusing light onto nanostructures thanks to spherical lenses is a first step in enhancing the field and is widely used in applications. Nonetheless, the electromagnetic response of such nanostructures, which have subwavelength patterns, to a focused beam cannot be described by the simple ray tracing formalism. Here, we present a method for computing the response to a focused beam, based on the B-spline modal method adapted to nanostructures in conical mounting. The eigenmodes are computed in each layer for both polarizations and are then combined for the computation of scattering matrices. The simulation of a Gaussian focused beam is obtained thanks to a truncated decomposition into plane waves computed on a single period, which limits the computation burden.

Analytical description of subwavelength plasmonic MIM resonators and of their combination.

We show that a periodic array of metal-insulator-metal resonators can be described as a high refractive index metamaterial. This approach permits to obtain analytically the optical properties of the structure and thus to establish conception rules on the quality factor or on total absorption. Furthermore, we extend this formalism to the combination of two independent resonators.

Metal-dielectric bi-atomic structure for angular-tolerant spectral filtering.

We theoretically study metal-dielectric structures made of bi-atomic metallic gratings coupled to a guided-mode dielectric resonator. The bi-atomic pattern grating allows tailoring of the Fourier spectrum of the inverse grating permittivity in order to adapt the frequency gap and obtain a flat dispersion band over a wide angular range. A significant enhancement (two-fold) of the angular tolerance as compared to a simply periodic structure is obtained.

Fast modal method for crossed grating computation, combining finite formulation of Maxwell equations with polynomial approximated constitutive relations.

We present a modal method for the fast analysis of 2D-layered gratings. It combines exact discrete formulations of Maxwell equations in 2D space with polynomial approximations of the constitutive equations, and provides a sparse formulation of the eigenvalue equations. In specific cases, the use of sparse matrices allows us to calculate the electromagnetic response while solving only a small fraction of the eigenmodes. This significantly increases computational speed up to 100×, as shown on numerical examples of both dielectric and metallic subwavelength gratings.

Mason's rule and signal flow graphs applied to subwavelength resonant structures.

Widely used for the design of resonant electronic devices, Mason's scalar rule is adapted here to the study of resonant subwavelength optical structures. It turns out to be an efficient formalism, especially when dealing with multiple wave interference mechanisms. Indeed it allows to comprehend the underlying physical mechanisms of the structure in a straightforward way and fast analytical formulae can be retrieved. As an illustration, we apply it to the study of dual metallic gratings, which appear to be promissing optical filters as their spectral shape can be tailored according to needs.

Free-standing guided-mode resonance band-pass filters: from 1D to 2D structures.

We study experimentally and theoretically band-pass filters based on guided-mode resonances in free-standing metal-dielectric structures with subwavelength gratings. A variety of filters are obtained: polarizing filters with 1D gratings, and unpolarized or selective filters with 2D gratings, which are shown to behave as two crossed-1D structures. In either case, a high transmission (up to ≈ 79 %) is demonstrated, which represents an eight-fold enhancement compared to the geometrical transmission of the grating. We also show that the angular sensitivity strongly depends on the rotation axis of the sample. This behavior is explained with a detailed description of the guided-mode transmission mechanism.

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.

Complex optical index of single wall carbon nanotube films from the near-infrared to the terahertz spectral range.

We retrieve the complex optical index of single-walled carbon nanotube (CNT) films in the 0.6-800 µm spectral range. Results are obtained from a complete set of optical measurements, reflection and transmission, of free-standing CNT films using time domain spectroscopy in the terahertz (THz) and Fourier transform infrared (IR) spectroscopy in the visible-IR. Based on a Drude-Lorentz model, our results reveal a global metallic behavior of the films in the IR, and confirm their high optical index in the THz range.

Harvesting light at the nanoscale by GaAs-gold nanowire arrays.

A nanoscale metal-semiconductor-metal photodetector with a 40 nm-thick GaAs absorbing layer has been studied numerically and experimentally. A gold nanowire array is the top mirror of a Fabry-Perot cavity and forms interdigitated Schottky contacts. Nearly perfect absorption is achieved in TE polarization. It is shown numerically that the gold nanowire array induces light absorption in GaAs nanowires with tiny sections (100 nm × 40 nm). High external quantum efficiency (η > 40 %) is demonstrated.

Perfect extinction in subwavelength dual metallic transmitting gratings.

We investigate the strong electromagnetic coupling that settles in dual metallic grating structures. This coupling is evidenced to lead to a perfect optical extinction in the transmission spectrum. The behavior of this perfect extinction that strongly depends on the longitudinal space and the lateral displacement between the two gratings can be explained by a simple model that describes the interference between a propagating mode and a couple of evanescent modes. The results show that the electromagnetic transmission of the structure can be tuned by controlling the position of this perfect transmission extinction and thus pave the way to new types of infrared tunable filters.

Guided mode resonance in subwavelength metallodielectric free-standing grating for bandpass filtering.

We present the experimental study of a free-standing metallic guided-mode resonant structure, for bandpass filtering applications in the mid-IR wavelength range. Structure consists of a subwavelength gold grating with narrow slits deposited on a silicon nitride membrane. High optical transmission is measured with up to 78% transmission at resonance. Angularly resolved spectra are presented revealing Fano-type resonance.

Light funneling mechanism explained by magnetoelectric interference.

We investigate the mechanisms involved in the funneling of optical energy into subwavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magnetoelectric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.

Reduced scattering-matrix algorithm for high-density plasmonic structures.

We describe a method to compute S-matrix interface terms using a selection of eigenmodes. When solving the modal equation, the computation of left and right eigenvectors leads to rectangular eigenmodes matrices. Expressions of S-matrix interface terms are then expressed so as to allow for a significant reduction of the computation cost. The reduction is even further decreased in the case of the B-spline modal method, which deals with sparse matrices. Its convergence is illustrated on a high-density plasmonic structure and compared to a full modal method.