We present a theoretical study of a nanostructured guided-mode resonant (GMR) spectral filter operating in the long-wave infrared (LWIR) wavelength range. The component is made of III-V semiconductors: heavily n-doped InAsSb for the grating and GaSb for the waveguide of the GMR resonator. In order to study the tolerance for the fabrication process and to adjust the resonance of the filter, a geometric study is also presented.
We present a theoretical and experimental study of a nanostructured guided-mode resonant (GMR) spectral filter operating in the long-wave infrared (LWIR) wavelength range. The component is made of III-V semiconductors: heavily n-doped InAsSb for the grating and GaSb for the waveguide of the GMR resonator. Angular and temperature dependencies are also presented with the relative experimental setups.
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.
Recent studies led in the ﬁeld of infrared spectroscopy focused on the use of nanoantennas to enhance electromagnetic ﬁeld on the bonds of molecules, in order to improve detection. We propose to take beneﬁt from dipolar optical resonances in dielectric free-standing nanorod arrays as an innovative component to achieve spectroscopy in the mid-infrared wavelength range. The particularity of this component is not only to allow electromagnetic ﬁeld enhancement, but also its ability to shift spectrally the resonance according to incidence angle. Spectroscopy is thus possible on a wide wavelength range for a given geometry of the nanorods. We present here numerical studies of the impact of the size of nanorods on the reﬂection spectra. We use the permittivity of a test molecule determined by transmission spectra. In addition, we introduce the fabrication process, transmission and reﬂection measurements of nanorods.
We present a compact real-time multispectral camera operating in the mid-infrared wavelength range. Multispectral images of a scene with two differently spectrally signed objects and of a burning solid propellant will be shown. Ability of real-time acquisition will thus be demonstrated and spectra of objects will be retrieved thanks to inversion algorithm applied on multispectral images.
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.
Optical sub-wavelength structures allow to code space-varying complex transmittance functions that induce both amplitude and phase variations on a given wavefront at the micrometer scale. This paves the way to the miniaturization of optical devices based on the spatial coding of the complex transmittance. We describe here a dedicated setup in the infrared range (3-14,um) for the spatial and spectral characterization of such components. The setup combines (i) a quadri-wave lateral shearing interferometer, which enables a two dimensional measurement of phase and amplitude, and (ii) a Fourier Transform Infrared Interferometer for spectral resolution. We present both theoretical simulations and experimental results of the characterization of
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 provide the first experimental evidence of sharp resonant extinction in free-standing arrays of non-resonant
dielectric nanorods. Nearly perfect optical extinction is shown for transparent material. High-resolution optical
measurements (absolute transmission and reflection) of one dimensional gratings with very low fill factors have
been obtained. The results can be fully explained by coherent multiple scattering in arrays of non-resonant
subwavelength nanorods and are in good agreement with an analytical model.
We have recently shown that dewar-level integration of optics is a promising way to develop compact IR cameras.
Indeed, the integration of optics into the dewar leads to simple and entirely cooled optical architectures dedicated to
imaging applications with large-field of view. Here, we review the optical elements we could add in those devices to
make a hyper- or multispectral imager. Among them, we find specific focal-plane arrays with a built-in spectrometry
function, plasmonic filters combined with a multichannel optical design, and birefringent interferometers. Several optical
architectures will be detailed with first experimental results.
Sub-wavelength gratings allow to code complex transmittance functions that introduce both amplitude and
phase variations in the propagation of a given wavefront. These micro-structures are a promising technique
to miniaturize optical functions such as light polarizing, light confinement, spectral filtering... Realizations in
the visible and the infrared domain have been fulfilled: for example micro-lenses, anti-reflection coatings or
sinusoidal-transmittance can easily be coded. This technique is all the more advantageous in the mid-wavelength
infrared (MWIR) or long-wavelength infrared (LWIR) spectral range since there are only a few materials available
in this spectral range. However the characterization of these structures is problematical, since it involves phase
and amplitude measurements. It is even more complicated in the far infrared domain (8 - 14 μm), as will be
detailed. Besides, the finite size of the gratings introduces phase steps, which is well-known to be a problematic
issue. We describe here a dedicated bench to characterize sub-wavelength gratings in the LWIR spectral range.
The core of the bench is a quadri-wave lateral shearing interferometer based on a diffraction grating, which allows
a complete two-dimensional characterization of both phase and amplitude in a single measurement. We present
here theoretical and experimental results of a characterization of such a sub-wavelength grating.
Lateral shearing interferometers (LSIs) are efficient tools for optical analysis. They allow classical optical wave-front
aberrations measurements as well as the precise evaluation of abrupt steps. The basic element of an LSI
is the transmittance grating, which diffracts a number of orders (two in the case of a mono-dimensional LSI,
ideally three or four non coplanar orders in the case of bi-dimensional LSI). This brings the need for specifically
designed transmittance gratings. For instance, a mono-dimensional LSI needs a sinusoidal-shaped transmittance,
since its Fourier transform carries exactly 2 orders. Such transmittances are however either impossible or at least
extremely costly to design using classical macroscopic techniques, mainly because the usual thin film deposition
techniques require several technological steps, in order to get the desired light filtering effect.
Given these constraints, we made use of sub-wavelength structures in order to build a new class of LSI. They
are made of sub-wavelength lamellar metallic gratings specifically designed for the mid-infrared, and allow the
precise coding of the desired transmission shape all over the LSI grating.
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.