Nanogap optical antennas enable enhancement of light-matter interactions owing to their great field enhancement. Interesting properties have been studied over recent years, such as high-order harmonics generation, electrically driven absorption, optical rectification and nonlinear tunnelling effect. However, as the gap size is shrunk down to the nanometer scale, losses dramatically increase and coupling efficiency of antenna with free-space decreases. This work reports conditions of perfect coupling, also called critical coupling, between a periodic array of nanogap metal-insulator-metal (MIM) antennas and free-space. We demonstrate that critical coupling is still achievable, even for the thinnest gaps.
We investigate the optical properties of nanogap MIM nanoantennas with a 2.1 nm thick gap. These plasmonic nanoantennas consist of a gold film, an insulating layer deposited by Atomic Layer Deposition, and a periodically structured gold layer. MIM nanoantennas are characterized by a tunable spectral response and a high field confinement within the nanogap. Nanometric gaps enable high coupling between the plasmons of the top and bottom metal surfaces. We demonstrated an excellent agreement between optical measurements and classical electromagnetic simulations. In particular, we observed a fluctuation in the gap thickness of 0.2 nm.
We investigate the impact of focused beams on metal-dielectric guided-mode-resonance filters. Under a planewave illumination, these filters show a resonant transmission at an easily tunable wavelength and a good angular acceptance. Although guided-mode resonance is involved, calculations show that under a focused beam the lateral extension of the electromagnetic field inside the waveguide is limited to the width of the beam. We investigate evolution of this lateral extension and the resonant transmission with the size of the beam, as well as the impact of a tilted beam. Guided-mode-resonance filtering under a focused beam is illustrated through the example of a Cassegrain microscope lens. A process for the fabrication of simple metal-dielectric filters is also presented.
We investigate a full-dielectric guided mode resonant photodiode. It has been designed to enhance the absorption by excitation of several resonances in the SWIR domain. The device consists of an InP/InGaAs/InP P-i-N heterojunction containing an active layer as thin as 90 nm on top of a subwavelength lamellar grating and a gold mirror. We successfully compared the electro-optical characterizations of individual pixels with electro-magnetic simulations. In particular, we observe near perfect collection of the photo-carriers and external quantum efficiency (EQE) of up to 71% around 1.55 μm. Moreover, compared with InGaAs resonator state-of-the-art detector, we show a broader spectral response in the 1.2-1.7 μm range, thus paving the way for SWIR low dark current imaging.
Nonlinear processes are investigated in plasmonic nanostructures which display Fabry-Perot resonances. This study focuses on the impact of the resonator geometry on the efficiency of second order effects implying 2 or 3 different wavelengths. These resonances heavily concentrate the electric field, allowing significant enhancement of the local nonlinear polarization and subsequently of the nonlinearly generated signals.
Nanophotonic devices show interesting features for nonlinear response enhancement but numerical tools are mandatory to fully determine their behaviour. To address this need, we present a numerical modal method dedicated to nonlinear optics calculations under the undepleted pump approximation. It is brie y explained in the frame of Second Harmonic Generation for both plane waves and focused beams. The nonlinear behaviour of selected nanostructures is then investigated to show comparison with existing analytical results and study the convergence of the code.
We demonstrate experimentally that plasmonic nanoantennas made of metal-insulator-metal ribbons can be used to tailor the spectral emissivity of a gold surface in the infrared. Two areas of a gold mirror sample were covered with various combinations of nanoantennas. Their emissivity was characterized thanks to a dedicated bench, based on the combination of a Fourier transform spectrometer and a microbolometer infrared camera. A near unity polarized emission on two distinct infrared bands is obtained on the respective two areas, which is coherent with theoretical predictions.
The ability to control the polarization state of an electromagnetic wave thanks to plasmonic metasurfaces is at the core of many various applications. We demonstrate both theoretically and experimentally that plasmonic planar L-shaped antennas can induce a 90°-rotation of the linear polarization of light with a nearly total efficiency in the mid-wavelength infrared. Then, we generalize these results with V-shaped antennas that can induce any rotation of the linear polarization by engineering the in-plane geometry of the antenna.
Focusing the light onto nanostructures thanks to spherical lenses is a first step to enhance the field, and is widely used in applications, in particular for enhancing non-linear effects like the second harmonic generation. Nonetheless, the electromagnetic response of such nanostructures, which have subwavelength patterns, to a focused beam can not be described by the simple ray tracing formalism. Here, we present a method to compute the response to a focused beam, based on the B-spline modal method. The simulation of a gaussian focused beam is obtained thanks to a truncated decomposition on plane waves computed on a single period, which limits the computation burden.
Here, based on an analogy between acoustics and electromagnetism wave equations, we present an electromagnetic
resonator analogous to the Helmholtz resonator in acoustics. This structure is made of a tiny slit above a box
and exhibits appealing properties for applications such as thermal emission, bio-sensing or spectroscopy.
The design of metasurfaces able to efficiently control the polarization state of an electromagnetic wave is of importance for various applications. We demonstrate both theoretically and experimentally that plasmonic planar L-shaped antennas can induce a 90° -rotation of the linear polarization of light with a nearly total efficiency in the infrared (3-5 µm). The influence of the in-plane geometry of the nanoantenna is investigated, and it is shown that it can be engineered so that the polarization conversion occurs over a 1 µm-wide spectral band ([3.25-4.25] µm) with a mean polarization conversion efficiency of 95%. These results are experimentally confirmed on two samples with distinctive geometries.
Degenerate two-photon absorption (TPA) is investigated in a 186 nm thick gallium arsenide (GaAs) p-i-n diode
embedded in a resonant metallic nanostructure. The full device consists in the GaAs layer, a gold subwavelength grating
on the illuminated side, and a gold mirror on the opposite side. For TM-polarized light, the structure exhibits a resonance
close to 1.47 μm, with a confined electric field in the intrinsic region, far from the metallic interfaces. A 109 times
increase in photocurrent compared to a non-resonant device is obtained experimentally, while numerical simulations
suggest that both gain in TPA-photocurrent and angular dependence can be further improved. For optimized grating
parameters, a maximum gain of 241 is demonstrated numerically and over incidence angle range of (−30°; +30°). This
structure paves the way towards low-noise infrared detection, using non-degenerate TPA, involving two photons of
vastly different energies in the same process of absorption in a large bandgap semiconductor material.
Here we present a 2D slit-box electromagnetic nanoantenna inspired by the acoustic Helmholtz resonator. It is able to concentrate the energy into tiny volumes, and a giant field intensity enhancement is observed throughout the slit. Noteworthily, we have shown that this field intensity enhancement can also be obtained in three dimensional structures that are polarization independent. In the Helmholtz nanoantenna, the field is enhanced in a hot volume and not a hot point, which is of great interest for applications requiring extreme light concentration, such as SEIRA, non-linear optics and biophotonics.
Nanoresonators are used in photovoltaic applications to reduce the absorber volume by confining the electromagnetic fields. We employed a two-dimensional Maxwell equation solver to investigate the optical performance of a fully structured InP-based nanoresonator, allowing high optical absorption and being electrically compatible. We demonstrated a 60% broadband optical efficiency with a good angle tolerance: more than 40% optical efficiency for an incidence angle between 60 deg and 60 deg. We also addressed the fabrication process in identifying the main barriers and in proposing technological solutions.
Plasmonic metal-insulator-metal resonators can be designed to totally absorb an incident light. A combination of such antennas within a sub-wavelength period allows a sorting of the absorbed photons as a function of their wavelength. These structures also exhibit a high electric field enhancement in tiny volumes. We show that this enhancement can be even stronger when the resonator are illuminated with a focused light.
The design of metasurfaces able to efficiently control the polarization state of an electromagnetic wave is of importance for various applications. We demonstrate both theoretically and experimentally that plasmonic planar L-shaped antennas can induce a 90◦-rotation of the linear polarization of light with a nearly total efficiency in the infrared (3-5 µm). The nanoantenna geometry is engineered so that the polarization conversion occurs over a 1 μm-wide spectral band ([3.25-4.25] µm ) with a mean polarization conversion efficiency of 95 %. In order to validate a theoretical model describing the antenna behaviour, we investigate the polarization conversion effect as function of the incident and azimuthal angles.
Focusing the light onto nanostructures thanks to spherical lenses is a first step to enhance the field, and is widely used in applications needing strong light matter interactions. Nonetheless, the electromagnetic response of such nanostructures, which have subwavelength patterns, to a focused beam can not be described by classical optics. Here, we describe a formalism to efficiently compute the behavior of nanostructures under a focused beam based on an apodized decomposition on plane waves. Besides, each computation of a plane wave is done on only one period thanks to a conical B-spline modal method. Various examples illustrating the focusing are detailed, and in particular the possibility to move the focal spot and to tilt the focus beam.
Plasmonic lenses are based on complex combinations of nanoscale high aspect ratio slits. We show that their design can be greatly simplified, keeping similar performance while releasing technological constraints. The simplified system, called Huygens lens, consists in a central aperture surrounded by several identical single mode slits in a thin gold layer that does not rely anymore on surface plasmons. The focusing behaviour with respect to the position and number of slits is investigated, and we demonstrate the interest of this design to get compact array of lenses which are able to compensate the angle of incidence of the incoming wave.
Light trapping enhancement is a major research field in photovoltaics. Scarce and expensive resources for
semiconductor material drive the research on light management in thin absorber layer. This paper reviews some of the
known techniques, from back reflector to nanophotonic technologies such as nanowires or plasmonic-enhanced
photovoltaic devices. Light trapping enhancement can reach ~100 fold and experimental demonstrations of device
exceeding the ray optics limits have been reported.
Plasmonic lenses (PLs) are based on complex combination of various width nanoscale and high aspect ratio slits. We investigate a more simplified design keeping similar performances while releasing technological constraints. This simplification is based on an energetic analysis of the contribution of each slit relative to the entire PLs behaviour. We demonstrate that a simplified plasmonic lens (SPL) can be designed which has the same behaviours as PLs.
We show both theoretically and experimentally that metal-insulator-metal resonators can be combined within the same subwavelength period and still behave independently. This permits to conceive surface with customizable absorption, which can for instance be used in dual band absorber or in omnidirectional wideband absorber. An energetic analysis can also be applied on these more complex antennas geometries, which highlights a sorting effect: at each resonance wavelength, the photons are funneled towards the apertures of the corresponding MIM cavity.
The multispectral imaging technique consists of imaging a given scene at various wavelengths of
interest, each one containing a different spectral information. By analyzing this spectral content,
the chemical species that are present can be localized on the image and identified by reconstructing
their spectral signature. In this way, following Ebbesen's seminal work in plasmonics
, purely metallic or hybrid metallodielectric structures [2, 3] seem to be ideal candidates to
perform spectral filtering due to their extraordinary transmission efficiency  and polarization
selectivity. Moreover, their compact feature makes it possible for them to gather in wide arrays
of filters that, once integrated into a cooled infrared camera, can achieve real-time multispectral
As seen in Figure 1.d. the spectral signature reconstruction of a chemical species strongly
depends on the number of filters and their transmission spectra for the designed matrix. In
order to improve the multispectral camera, a complementary approach consists of changing the
filter design to realize a tunable filter whose spectral shape can be adjusted in real time according
to the imaged scene. We focused our attention on the superposition of subwavelength gratings
which seems to be a structure of great potential for multispectral imaging applications [6, 7].
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 lD gratings, and unpolarized or selective polarization filters with 2D gratings, which are shown to behave as crossed-lD 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.
Plasmonic metal-insulator-metal resonators can totally absorb incident light. However, it is necessary to know where the incident energy is headed and which mechanism contributes to the funneling of energy. Numerical simulations have shown the concentration of light for simple resonators but other designs may allow a sharper tuning of the absorption properties. An energetic analysis was conducted with different combinations of grooves and horizontal metal-insulator-metal resonators. It showed where the energy was preferentially funneled, which highlights the possibility of photons sorting and displays the localization of the energy dissipation in such plasmonic nanoresonator. These results are giving promising design rules for multi-spectral absorbing materials.
The B-spline modal method is adapted for the design and analysis of nanostructured devices in conical mounting.
The eigenmodes in each layer are calculated for two specific polarization states, and then combined for the
calculation of the scattering matrices. We take advantage of the sparsity of the generated matrices to decrease
the computation time, and adress the need for fast computation in complex systems. Moreover, we demonstrate
the physical interest of computing the conical response on various classical structures.
In 2010, the existence of a zero of transmission at high wavelengths different from the Rayleigh Wood anomaly
was highlighted when two identical subwavelength gratings are brought close enough. We recently revealed
the origin of this transmission extinction and the mechanism is recalled here. Furthermore, we present an
experimental setup to measure the amplitude of this extinction and study its spectral behavior when changing
geometrical parameters which represent a real technological challenge. This way a new generation of tunable
filters in the mid-infrared with perfect spectral shape control and high rejection efficiency can be designed with
practical use to gas sensing applications.
Here, we demonstrate the total extinction of the reflectivity for a transverse magnetic polarized wave on a gold
surface etched on a tiny portion of its area by both narrow and deep grooves. At the resonance, the incident
energy is funneled towards the grooves aperture and is then dissipated on the grooves sidewalls. 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 evanescent field is mainly due to the resonant
wave escaping from the groove. These high aspect ratio metallic grooves were fabricated using a mold cast
technique based on an electrolytic growth of gold. They exhibit a nearly total absorption due to a Fabry-Perot
like resonance inside the grooves. We also evidence the incidence-invariance of their spectral response, which
undoubtedly shows the localized nature of the resonances. These experimental results confirm the prediction of
total funneling of light in very narrow grooves.
We present the experimental study of a new design of band-pass filter based on guided-mode resonances in a
free-standing metal-dielectric structure with subwavelength gratings. Component 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. Experimental angularly resolved spectra are presented: they reveal
the role of the diffracted orders and of the waveguide eigenmode in the resonance. Spectra have a typical profile of
Fano resonances: we show that this profile is due to interferences between a direct transmission channel through
the 0th order, and an indirect transmission channel which results from the excitation by the ±1 diffracted orders
of a waveguide eigenmode.
We will present a brief overview of the interest in subwavelength gratings for spectral filtering in the mid-infrared wavelength range. Guided-mode, plasmonic and dipolar resonances will be considered. We will particularly focus on components fabricated in our laboratories, achieving band-pass or cut-band filtering. Optical characterization will be shown, revealing resonances with high quality factors. Multispectral camera has been realized by integrating our components into a cooled infrared focal plane array.
We propose an experimental demonstration of a THz modulator with a visible optical command. The device is a n-doped
GaAs grating with subwavelength dimensions. The principle of this modulator is the control of the THz resonant
absorption by surface waves supported by the grating. This absorption is modulated with low power visible light, leading
to a modulation of the reflected THz beam. From experimental polarized THz reflectivity measurement of the grating,
we show that a depletion layer at the surface of the doped GaAs has to be taken into account to correctly describe the
observed resonant absorption. From experimental observation and modeling we are able to ascribe this absorption to the
coupling of incident THz light with surface plasmon-phonon polariton mode propagating along each wall of the grating.
Thus, each wall acts as a nano-antenna that resonantly absorbs light. The grating can be viewed as a metamaterial
composed of individual resonators. The theoretical model indicates that the reflectivity dip linked to the surface wave is
sensible to the electronic density in the walls of the grating. We performed an experiment to measure the THz
reflectivity while illuminating the grating with visible photons having energy higher than the bandgap of GaAs. The
created photoelectrons change the effective mode index, leading to a shift of the resonant absorption frequency. This
demonstrates the modulation of THz radiation around 8.5 THz with a visible optical command at room temperature.
Plasmonic has demonstrated the ability to enhance performances of photodetectors at a
resonant wavelength. Absorption in a photodetector can reach 100% using nanophotonic
plasmonic array. Plasmonic devices are confining light at the interface metal/dielectric, as a
consequence, detection volume is smaller (100 to 1000 times) than in usual photodetectors
leading to a decrease in dark current of infrared photodetectors and therefore a higher working
temperature. The second consequence of a short detection volume is a higher collection
efficiency of photocarriers as the transit time is smaller than the lifetime.
Here, we address the need for fast computation of subwavelength structures. It allows fast conception of optical
devices. We present a modal method based on B-splines formulation which solves Maxwell equations. The two
assets of this method are to use non uniform B-splines permitting to adapt the mesh to the structure, and to
produce sparse matrices which permit to speed up the computation. As an illustration, we make use of this
method for the design and analysis of variously shaped infrared optical devices.
Subwavelength dielectric and metallic gratings embedded in vacuum can act as highly-resonant spectral filters. We review the theoretical principles for the design of symmetric dielectric and metal gratings to conceive efficient optical filters in the mid and far infrared range, and we show how both resonance width and resonance wavelength can be tuned. We describe an original process for the fabrication of free-standing SiC gratings, and we present the first samples obtained with 10 &mgr;m period. Experimental angularly resolved transmission spectra show evidences of their filtering properties.
Subwavelength metallic structures are used to design gratings with a great variety of transmittance levels. Such gratings can answer growing needs for complex transmittance devices, particularly useful for wave-front analysis applications. Having in mind the conception of a perfectly sinusoidal transmittance for the mid-infrared, we have decided to test the ability of subwavelength lamellar gratings to code the transmittance with several levels. In order to calibrate gratings transmission, as a function of the fill factor, we have designed, realized and measured samples made of six 2mm x 2mm gratings, with transmittance ranging from 10% to 95%. Experimental results for TM- and TE-polarized light are reported and analysed.
New concepts for efficient light absorption in nanoscale metal-semiconductor-metal photodetectors are analyzed from both theoretical and experimental point of view. They are based on sub-wavelength metallic gratings which allows light confinement in tiny volumes (< 100 nm) close to electrodes (< 100 nm). Two photodetector structures are proposed: (i) a resonant-cavity-enhanced subwavelength metal-semiconductor-metal photodetector, and (ii) a nanoscale metal-semiconductor grating photodetector. External quantum efficiency as high as 9 % has been obtained in 40 x 100 nm2 cross-section GaAs wires, limited by fabrication technology. These results show promising features for highly efficient and ultrafast photodetectors.