DEVICE AND METHOD FOR MULTISPECTRAL IMAGING IN THE INFRARED RANGE

Description

Domaine technique

[0001] The present invention relates to a device and method for multispectral imaging in the infrared. Etat de l'art

[0002] In the visible or near infrared, various means are known for forming color images using CCD or CMOS digital cameras. For example, it is possible to use spectral separation means to form images in different spectral bands on several detectors. It is also known to place a filter wheel in front of the camera and to sequentially acquire a series of images in different spectral bands. In each of these cases, the color image is reconstructed from the different images acquired in the different spectral bands.The most widely used technique, however, is the structuring of the focal plane to form a mosaic of pixelated filters, for example in the form of a structuring known as "Bayer structuring" (described in US patent 3971065) which allows images to be acquired simultaneously and with a single detector in different spectral bands in order to reproduce as closely as possible the vision of the human eye. To do this, red, green and blue filters are positioned at each of the elementary detectors of the camera (or "pixels") in the form of a mosaic of 4 pixelated filters (one red, 2 green, one blue), this pattern being reproduced over the entire surface of the detector. A "demosaicing" algorithm then makes it possible to reconstruct the color image. Pixelated filters are generally produced in the near infrared in the form of multilayer structures forming interference filters (see for example M. Lequime et al., “2 × 2-Array Pixelated Optical Interference Filters”, Proc. SPIE Vol. 9627, 96270V-1 - 96270V-7, 2015) and the technology is very well understood. In the visible range, dyes can also be used.

[0003] In the infrared, that is to say for wavelengths typically between 3 µm and 20 µm, the need for multispectral imaging also exists, not to reproduce an image similar to that detected by the eye, but to access various information such as for example the identification of a chemical species or an object thanks to its spectral signature, the temperature analysis of an emissive body, the determination of the spectral emissivity of a body, etc.

[0004] The use of multilayer structures for multispectral infrared imaging in the 3 - 5 µm band has been described but it has a number of limitations (see M. Oussalah et al. "Multispectral thin film coating on infrared detector", Proc. SPIE., Vol. 9627, 96271W-96271W-10, (2015)). In particular, since they involve a large number of layers, these components, if the materials are not chosen with great care, can become fragile as soon as they are subjected to temperature variations. Furthermore, in the infrared, the thicknesses of the layers are significant (typically greater than 1 µm) and vary from one filter to another. This not only causes technological difficulties but can result in a deterioration of performance in terms of spectral selectivity due to parasitic diffraction effects (edge ​​effects) resulting in particular from differences in thickness from filter to filter.

[0005] Other techniques have been developed for multispectral imaging in the infrared, based not on a stack of layers but on periodic subwavelength structuring of metallic layers, making it possible in particular to work with a limited number of layers.

[0006] Thus, Haïdar et al. ("Free-standing subwavelength metallic gratings for snapshot multispectral imaging", Appl. Phys. Lett. 96, 221104, (2010)) describes a multispectral infrared camera based on the use of suspended subwavelength metallic gratings. These structures exhibit remarkable transmissions at wavelengths that depend on the period. By juxtaposing several filters of different periods, it is thus possible to produce a camera with several imaging optical channels, each channel further comprising a spectral filter, which makes it possible to form for each channel an image on a given surface, typically a millimeter surface, in a given spectral band.

[0007] There FIG. 1 thus illustrates a multi-channel infrared camera 10 according to the prior art. The camera comprises a set of lenses, or microlenses, arranged for example in an enclosure 11. These lenses, referenced 12a, 12b, 12c, 12d on the FIG. 1 , are adapted for the formation of images on a detection matrix 13 formed of elementary detectors (or "pixels") 13 i . Upstream of each of the lenses 12 i, for example at the level of an input window of the enclosure 11, there is a filter matrix 14 adapted for transmission filtering in spectral bands centered on given detection wavelengths. By choosing for each of the filters referenced 14a, 14b, 14c, 14d on the FIG. 1 transmission spectral bands centered on different detection wavelengths, 4 images of different “colors” are formed on the matrix of elementary detectors 13, on detection surfaces typically of millimeter dimensions. A reading circuit 15 is adapted for processing the signals detected for each of the images and transmitting the signals to a calculation unit (not shown). This is called a “multi-channel” camera.

[0008] In Sakat et al. 2011 ("Guided mode resonance in subwavelength metallodielectric free-standing grating for bandpass filtering", Opt. Lett.36, 3054 (2011)) and Sakat et al. 2013 ("Metal-dielectric bi-atomic structure for angular-tolerant spectral filtering", Opt. Lett., 38, 425, (2013)) metallo-dielectric guided mode resonance (or "GMR" according to the abbreviation of the expression "Guided Mode Resonance") filters are described. These filters are based on a guided mode resonance in a thin dielectric layer, whose coupling to free space is ensured by a metallic grating, in particular for orders ±1 diffracted in the dielectric. Compared to the structures described in Haïdar et al., These filters may have better angular tolerance (see Sakat et al. 2013) which allows, when they are installed in a multi-channel camera configuration as illustrated in the FIG. 1 , to work with larger fields while maintaining the spectral performance of the filters.

[0009] However, in filters based on subwavelength periodic structuring as described previously, only the response of GMR filters in plane waves has been considered, possibly as a function of the angle of incidence of this wave, for use on large surfaces (typically millimeter surfaces), as in the multi-channel camera described in FIG. 1 .

[0010] For the first time, the feasibility of a multispectral infrared imaging device with metallo-dielectric GMR filters operating on surfaces of the dimension of the detection pixel is demonstrated in the present application, opening the way to new compact imagers for the instantaneous acquisition of "color" infrared images. SUMMARY

[0011] According to a first aspect, the present description relates to a device according to claim 1.

[0012] An elementary metallo-dielectric guided mode resonance filter, optimized for bandpass transmission in a spectral band centered on a given detection wavelength λ d, comprises, within the meaning of the present description, a layer of dielectric material forming a single-mode waveguide at said detection wavelength λ d , and at least one metallic diffractive grating, structured according to a given pattern repeated with a given period, less than said detection wavelength. At least one diffraction grating is adapted for coupling an incident wave at said detection wavelength λ d to the mode of the waveguide.

[0013] According to one or more exemplary embodiments, a structuring pattern of the diffractive grating comprises one or more openings, of predetermined dimensions, the openings being filled with a dielectric material which may be ambient air or another dielectric material such as, for example, the dielectric material forming the waveguide or that forming a substrate. The openings may have the shapes of one-dimensional slots of given widths, slots of given widths arranged in two perpendicular directions, cross shapes, or may be circular openings, etc. Depending on the geometry of the opening, the elementary filter may be polarization selective or not.

[0014] The applicants have demonstrated unexpected remarkable properties of the behavior of metallo-dielectric guided mode resonance elementary filters when illuminated in a convergent beam over dimensions of a few periods, making possible the implementation of "pixelated" elementary filters, i.e. whose dimensions are of the order of magnitude of those of each elementary detector, or "pixel".

[0015] The shape and dimensions of the focusing spot at a given point in the field as well as the depth of focus (or image depth of field) result in a known manner from the opto-geometric characteristics of the imaging optics and the wavelength. To estimate the dimensions of the focusing spot and the depth of focus, a predetermined wavelength value can be taken, for example the wavelength λ min corresponding to the minimum wavelength that one seeks to detect with the multispectral imaging device in the infrared.

[0016] For example, for a rotationally symmetric imaging optic, the diameter ϕ given by the diffraction limit at a given wavelength λ, for example the minimum detection wavelength, can be taken as the dimension of the diameter of the focusing spot.

[0017] According to the invention, at least one of said elementary filters has an angular acceptance greater than or equal to a predetermined value, depending on the opto-geometric parameters of the device, for example the opto-geometric parameters of the image formation optics and / or of the detection matrix.

[0018] According to the invention, the angular acceptance Δθ of an elementary filter is defined by the angle of incidence of a plane wave incident on the filter with a given inclination, measured relative to a direction normal to the filter, and for which the maximum transmission is equal to half the maximum transmission of an identical plane wave incident on the filter with a zero angle of incidence (normal incidence).

[0019] According to the invention, said predetermined value is the field edge angle of the device, defined as the angle of the most inclined ray intended to reach the matrix of elementary detectors with respect to the direction normal to said matrix of elementary detectors. It depends on the dimension of the matrix of elementary detectors, the number of apertures and the focal length of the imaging optics.

[0020] According to one or more exemplary embodiments, each of said elementary filters of the matrix of elementary filters has an angular acceptance greater than or equal to the edge angle of the field of the device. Indeed, even if the angular acceptance required for an elementary filter is lower at the center of the image field, it will be possible to choose to optimize all the elementary filters of the matrix of elementary filters to obtain the greatest angular acceptance, that is to say that required for an elementary filter positioned at the edge of the image field.

[0021] According to one or more exemplary embodiments, each of said elementary filters has dimensions substantially identical to those of an elementary detector. In practice, as recalled above, an elementary filter with metallo-dielectric guided mode resonance comprises a waveguide made of dielectric material and at least one metal grating structured according to a given pattern, repeated with a given period. The dimension of an elementary filter is therefore a multiple of the dimension of a pattern and may be a little larger or a little smaller than an elementary detector. Thus, by substantially identical dimensions, it is understood that the difference between a dimension of an elementary filter and that of an elementary detector ("pixel") is less than the central wavelength of the transmission spectral band of the filter. It is however possible for an elementary filter to have a dimension equal to several times that of a pixel, for example between 2 and 4 times.

[0022] According to one or more exemplary embodiments, the elementary filters of the matrix of elementary filters are arranged in the form of zones, each zone comprising at least two elementary filters optimized for bandpass transmission in spectral bands centered on two different central wavelengths, and each zone having dimensions greater than those of the focusing spot. According to one or more exemplary embodiments, the arrangement of the elementary filters is identical in each zone.

[0023] According to a first alternative of the invention, the matrix of elementary filters comprises at least two elementary filters being optimized for bandpass transmission in spectral bands centered on two different central wavelengths, equal to two of said detection wavelengths, said at least two elementary filters each comprising a DMG (abbreviation of the English expression "Dual metallic Grating") type guided mode resonance elementary filter, optimized for bandpass transmission in a spectral band centered on a given detection wavelength λ d, comprising a layer of dielectric material forming a single-mode waveguide at said detection wavelength λ d , and two metallic diffractive gratings arranged on either side of the layer of dielectric material.Each metal grating is structured according to a given repeated pattern with a given period, less than the detection wavelength and is adapted for coupling an incident wave at said detection wavelength λ d to the mode of the waveguide. Advantageously, the periods are identical for both gratings.

[0024] According to one or more exemplary embodiments, the elementary filter with guided mode resonance of the DMG type is suspended and the two metal networks of the elementary filter are identical (same metal, same pattern, same period).

[0025] According to one or more exemplary embodiments, the elementary filter with guided mode resonance of the DMG type is deposited on a substrate made of dielectric material and the patterns of the two metal gratings of the elementary filter are different, to take into account the differences in refractive index of the dielectric materials on either side of the waveguide (air and substrate for example). Advantageously, the periods remain identical for the two gratings.

[0026] According to one or more exemplary embodiments, the matrix of elementary filters comprises at least one elementary filter with guided mode resonance of the “bi-atom” type, in which at least one metal network has a pattern with at least two openings with different dimensions, for example two slots of different widths.

[0027] According to a first alternative of the invention, the matrix of elementary filters comprises at least two elementary filters being optimized for bandpass transmission in spectral bands centered on two different central wavelengths, equal to two of said detection wavelengths, said at least two elementary filters each comprising an elementary filter with guided mode resonance with single metallization on the front face, comprising a waveguide made of dielectric material with a substrate on one side and a double metal grating on the other side, the gratings having a different pattern and, according to one or more exemplary embodiments, an identical period.

[0028] In each of the examples cited above, all the elementary metallo-dielectric guided mode resonance filters of the elementary filter matrix can be identical or on the contrary, it is possible to have different elementary filters, for example between the edge and the center of the field.

[0029] According to one or more exemplary embodiments, the elementary filters of the elementary filter matrix are arranged on the same substrate, which facilitates their manufacture. They can also be suspended.

[0030] According to a second aspect, the present description relates to a method according to claim 7. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures: FIG. 1 , already described, a multi-channel infrared camera according to the prior art; FIG. 2A , a diagram of an infrared multispectral imaging device according to an example of the present description; FIG. 2B , an example of an arrangement of elementary metallo-dielectric guided mode resonance filters in an elementary filter matrix suitable for an infrared multispectral imaging device according to the present description; FIG. 3A , an exemplary embodiment of a suspended bi-atom DMG type metallo-dielectric GMR filter, suitable for an infrared multispectral imaging device according to the present description; and FIGS 3B, 3C two curves showing transmission simulations as a function of wavelength and as a function of the angle of incidence (in plane waves), in the case of an example of a suspended, bi-atom DMG filter ( FIG. 3A ); FIG. 4A , an exemplary embodiment of a metallo-dielectric GMR filter of the DMG bi-atom type on a substrate, suitable for a multispectral imaging device in the infrared according to the present description; and FIGS 4B, 4C two curves showing transmission simulations as a function of wavelength and as a function of the angle of incidence (in plane waves), in the case of an example of a bi-atom DMG filter on substrate ( FIG. 4A ); FIG. 5A , an exemplary embodiment of a metallo-dielectric GMR filter of the filter-on-substrate type with single metallization on the front face, suitable for a multispectral imaging device in the infrared according to the present description; and FIGS 5B, 5C two curves showing transmission simulations as a function of wavelength and as a function of the angle of incidence (in plane waves), in the case of an example of a filter on a substrate with front face metallization ( FIG. 5A ); FIGS. 6A et 6B , the diagram of an exemplary embodiment of a filter matrix suitable for a multispectral infrared imaging device according to the present description, comprising suspended bi-atom DMG type metallo-dielectric GMR filters, and a simulation showing the confinement of the field in the filter matrix illuminated by a convergent beam. DETAILED DESCRIPTION

[0032] There FIG. 2A illustrates an example of an infrared multispectral imaging device according to the present description. Multispectral imaging means the formation of images at at least two different detection wavelengths, or more precisely in at least two detection spectral bands centered on two different detection wavelengths. The infrared spectral band is defined in the present description as the set of wavelengths between 1 µm and 15 µm.

[0033] The multispectral infrared imaging device, referenced 20 on the FIG. 2A , comprises, for example in an enclosure 21, a detection matrix 23 comprising a set of elementary detectors 23i, or “pixels”, of predetermined dimensions, a reading circuit 25 for processing the signals delivered by the elementary detectors 23i, a processing unit 26 connected to the reading circuit 25 as well as a matrix 24 of elementary filters 24i with metallo-dielectric guided mode resonance, an example of which is illustrated in the FIG. 2B The multispectral infrared imaging device further comprises an imaging optic 22 arranged in the enclosure or outside the enclosure and adapted for the formation of images in the infrared.

[0034] The detection matrix suitable for infrared can comprise any type of known matrix detector (1D array or 2D detector), such as for example MCT (for Mercury Cadmium Tellurium), InAs, QWIP (AlGAAs / As / GaAs), superlattices (InAs / GaSb) type detectors, these detectors operating in a cooled enclosure 21. Other types of detectors suitable for operation in an uncooled environment can also be used, such as for example microbolometers.

[0035] Typically, for operation of the multispectral imaging device between 1 µm and 15 µm, it will be possible to work with elementary detectors of dimensions between 15 µm and 30 µm, arranged according to a detection strip (for example in a 288 x 4 pixel format) or according to a two-dimensional matrix (for example in a 640 x 480 pixel format). The dimensions of the detection matrix define those of the image field of the image formation device.

[0036] The imaging optics 22 are characterized by an aperture number N and a focal length F, with N = D / F, where D is the pupil diameter of the imaging optics. The imaging optics may comprise one or a plurality of lenses, formed from materials transparent to the wavelengths of interest, for example germanium.

[0037] The imaging optics 22 are suitable for forming images of a scene on the detection matrix 23. In practice, as for any optical system, it is possible to define for the imaging optics at a given wavelength an elementary focusing spot at a point in the image field and a depth of focus.

[0038] The shape and dimensions of the focusing spot at a given point in the field as well as the depth of focus (or image depth of field) are determined in a known manner, at a given wavelength, by the opto-geometric characteristics of the image forming optics.

[0039] For example, for a rotationally symmetric imaging optic, the diameter of the focusing spot ϕ can be defined by the diffraction limit, namely: ϕ = 2 , 44 λ N where N is the aperture number of the imaging optics (N = D / F, with D being the pupil diameter of the imaging optics and F being the focal length) and λ is the wavelength. For example, in the case of a spectral imaging device with an aperture number N = 3 and a wavelength λ = 4.1 µm, the diameter of the focusing spot is of the order of 30 µm.

[0040] Furthermore, the depth of focus, or image depth of field, depends essentially on the aperture number of the optics used and the wavelength. It can be defined as the interval measured in the image space, in which the detector array must be placed in order to obtain a sharp image.

[0041] For example, an estimate of the depth of focus P f can be given by: P f = 2 N ϕ

[0042] Thus, for an aperture number N=3 and a focusing spot ϕ = 30µm, we obtain a depth of focus P f =180 µm.

[0043] In practice, according to the present description, the focusing spot may cover a set of at least two juxtaposed elementary detectors when the matrix 23 is formed of a line of elementary detectors and at least four juxtaposed elementary detectors when the matrix 23 is formed of several lines of elementary detectors. Because the imaging device according to the present description is intended to detect several wavelengths, the minimum detection wavelength λ min may be used for estimating the diameter ϕ of the diffraction spot and the depth of focus.

[0044] As illustrated in the FIG. 2B , the matrix 24 of elementary filters with metallo-dielectric guided mode resonance is arranged in front of the detection matrix 23 at a given distance d, less than the depth of focus, which makes it possible to avoid too great a divergence of the beams at the level of each elementary filter.

[0045] The dimensions of the elementary filters are adapted so that each elementary focusing spot formed at each point of the image field covers at least two elementary filters, these two elementary filters being optimized for bandpass transmission in spectral bands centered on two different central wavelengths, equal to two detection wavelengths. Thus, at each elementary focusing spot, elementary detectors receive light fluxes filtered in spectral bands centered around different detection wavelengths.

[0046] As an example, we have shown on the FIG. 2B a diagram of a two-dimensional 24 matrix of elementary filters 24i. In this example, it is possible to define zones Zi formed by 4 elementary filters each optimized for detection in a spectral band centered on a different detection wavelength. In practice, a focusing spot of the image-forming optics formed at the level of the filter matrix 24 and calculated for example for the smallest of the detection wavelengths, could cover a circular zone inscribed in the frame Zi.

[0047] In practice, the dimensions of an elementary filter can be substantially the same as those of an elementary detector, as illustrated in the FIG. 2B . But it is also quite possible that an elementary filter is a little larger or a little smaller than an elementary detector.

[0048] An elementary filter has dimensions such that it covers a group of two elementary detectors (case of a 1D detection array) or an elementary filter can have dimensions such that it covers a group of 4 elementary detectors (case of a 2D detection matrix) as long as the matrix of elementary filters is located in the depth of focus of the imaging optics and at an elementary focusing spot, there are at least two elementary filters adapted to transmission in spectral bands centered on two distinct detection wavelengths.

[0049] Generally speaking, we can define zones Zi formed by a larger number of elementary filters, the filters being identical or different, but each zone Zi comprising at least two elementary filters optimized for bandpass transmission in spectral bands centered on two different central wavelengths, equal to two detection wavelengths. The elementary filters can be arranged according to a given arrangement in each zone Zi. The zones Zi can all be identical, as in the example of the FIG. 2B . Advantageously, the focusing spot is small enough to be contained within a zone Zi so that there is no "overflow" from one zone to another. For example, the focusing spot is circular in shape, inscribed within a zone Zi.

[0050] As previously stated, an elementary metal-dielectric guided mode resonance (GMR) filter comprises a waveguide made of dielectric material and at least one metal grating structured according to a given pattern, repeated with a given period, for coupling an incident wave to the guided modes of the waveguide. Thus, even when the dimension of an elementary filter is substantially equal to that of an elementary detector, as illustrated in the FIG. 2B , as the dimension of an elementary filter is in practice a multiple of the dimension of a pattern of a metallic network, it could be a little larger or a little smaller than an elementary detector, with a deviation less than the detection wavelength.

[0051] As it appears on the FIG. 2A , the elementary filters 24i receive converging light beams F 0 , F 1 , the convergence of the beams at the edge of the field (beam F 1 ) being greater than that in the center of the field (beam F 0 ). In particular, a field edge angle α can be defined, defined as the angle of the most inclined ray intended to reach the matrix of elementary detectors with respect to the direction normal to said matrix of elementary detectors. It depends on the size of the matrix of elementary detectors, the number of apertures and the focal length of the image-forming optics. More precisely, the field edge angle α can be defined by: α = arctan n pix t pix + D 2 F

[0052] Or n pix is the maximum number of detection pixels (along one dimension), t pix is the pixel pitch.

[0053] For example, for a number of detection pixels on a line n pix = 640, a pixel pitch t pix = 15µm, a diameter of the forming optics D = 25 mm, a focal length F = 50 mm, we obtain a field edge angle α = 19°

[0054] Thus, all or part of the elementary filters, and in particular the elementary filters positioned at the edge of the field, may have an angular acceptance greater than or equal to the edge angle of the field of the device. According to the invention, at least one of said elementary filters has an angular acceptance measured in plane waves greater than or equal to the edge angle of the field of the device.

[0055] Various elementary metallo-dielectric guided mode resonance (GMR) filters known from the prior art can be used for implementing a multispectral imaging device in the infrared according to the present description.

[0056] Sizing GMR filters for infrared spectral filtering in a multispectral imaging device according to the present disclosure may include the following steps.

[0057] Depending on the applications, the detection wavelengths λ di and the widths Δλ i of the detection spectral bands at the considered detection wavelengths are defined. For example, for the detection of a particular chemical species, it may be interesting to seek a low detection spectral bandwidth (less than 0.5 µm) while for other applications, for example the evaluation of the emissivity of a body, it may be interesting to seek a larger detection spectral bandwidth (greater than 1 µm).

[0058] The characteristics of the detection matrix are also fixed according to the application: detection bar or 2-dimensional matrix, number of pixels (n pix ) in each direction and size of a pixel (t pix ).

[0059] The opto-geometric characteristics of the device, in particular the aperture number N and the focal length F of the imaging optics are chosen according to the scene to be observed, the detector (spatial resolution) and the maximum size required for the device.

[0060] We define below how the elementary filters can be chosen and sized, depending on the detection wavelengths λ di sought, the width Δλ i of the detection spectral band at the detection wavelength considered and the opto-geometric parameters of the detection device. In particular, one or more types of filters (DMG, bi-atom, etc.) can be determined in the same matrix of elementary filters, as will be described later.

[0061] For each type of filter, we seek to determine the geometric parameters of the filter allowing to achieve the optical characteristics sought for the filter which are the maximum transmission T max , the resonance wavelength λ r , the resonance width Δλ and the angular acceptance Δθ. In practice, the resonance wavelength λ r sought, corresponding to the center wavelength of the transmission spectral band for which the transmission T max is maximum, will be equal to a detection wavelength λ di . The resonance width Δλ sought, corresponding to the width at half-height of the spectral response of the filter in transmission, will be equal to the width Δλ i of the detection spectral band and the angular acceptance Δθ sought can be defined as a function of the field edge angle α of the device (see equation (3) above).

[0062] As regards the angular acceptance Δθ of the elementary filters, it can advantageously be chosen to be greater than the field edge angle α for all the filters in the matrix or at least for some of the filters located at the field edge. If the angular acceptance Δθ is less than the field edge angle α, the filter continues to operate but its efficiency decreases since the transmission at resonance T max decreases and the quality factor Qi = λ di / Δλ i can be degraded.

[0063] The determination of the parameters of elementary filters, once a type of filter has been chosen, may include the following steps: (1) choice of the first parameters, (2) verification of the first parameters by numerical simulation (simulation of the transmission as a function of the wavelength and simulation of the transmission as a function of the angle of incidence) and (3) modification of the parameters according to the results of the simulation.

[0064] For step (2) of verifying the first parameters by numerical simulation, a simulation of the transmission of the filter in a convergent beam can be carried out. It comprises a decomposition of the incident convergent beam into plane waves of different angles of incidence, the simulation of the propagation of each elementary plane wave and a summation of the elementary plane waves after propagation. The applicants have shown, however, that a simulation of the transmission of the filter "in plane waves" could be carried out, because the parameters obtained with a simplified numerical simulation in plane waves were substantially similar to those obtained by means of a simulation of the transmission in convergent beams, provided that the angle of incidence of the wave remains within the range of the angular tolerance of the filter.

[0065] In any case, different known methods can be used to simulate elementary filter responses to incident electromagnetic waves. For example, a modal calculation method such as RCWA (for "Rigorous coupledwave analysis") can be used, described for example in MG Moharam et al., JOSAA 12, 1068 (1995). Finite element methods ("FEM" for "Finite Element Methods") implemented for example in the COMSOL Multiphysics ®< software or finite differences ("FDTD" for Finite Difference Time Domain) implemented in the LUMERICAL ®< software can also be used. The curves presented in the following description are calculated by a modal calculation method, and more particularly using the Reticolo calculation code for Matlab ®< (P. Hugonin and P. Lalanne, “Reticolo software for grating analysis”, Institut d'Optique, Orsay, France (2005)), in a hypothesis of plane waves and one-dimensional pattern (slits).

[0066] As previously explained, an elementary metallo-dielectric guided mode resonance filter, optimized for bandpass transmission in a spectral band centered on a given resonance wavelength λ r, comprises, within the meaning of the present description, a layer of dielectric material (refractive index nd and thickness td ) forming a single-mode waveguide at said wavelength, and at least one metallic diffractive grating (refractive index nm and thickness tm ), structured according to a given pattern repeated with a given period (p), less than said resonance wavelength. The diffraction grating is adapted for coupling an incident wave at the resonance wavelength to the guided mode. The pattern may comprise one or more openings of given dimensions, the openings being able to be two-dimensional (crosses, circular openings for example) or one-dimensional (slits).The openings are filled with a dielectric material which can be ambient air or another dielectric material such as the dielectric material forming the waveguide or that forming the substrate, depending on the different types of filters.

[0067] Three examples of designs of elementary metallo-dielectric guided mode resonance filters for the realization of a multispectral imaging device according to the present description are given below. These examples are not limiting, other geometries being possible for the realization of elementary guided mode resonance filters. In each case, a similar method for the definition of the filter parameters can be applied.

[0068] A first example is described using the FIGS. 3A - 3C .

[0069] On the FIG. 3A , only a pattern of an elementary filter 30 of dimension p is represented. In practice, the elementary filter comprises a repetition of the pattern thus represented to form a diffraction grating having a period p.

[0070] The elementary filter 30 illustrated on the FIG. 3A is of the suspended DMG ("dual metallic grating") type. It comprises a layer of dielectric material 31 forming a single-mode waveguide at the resonance wavelength λ r , and two metallic diffractive gratings 32, 33, arranged on either side of the layer of dielectric material, the whole being suspended in a fluid such as air or in a vacuum. Each metallic grating is structured according to a given pattern repeated with a given period p, less than the resonance wavelength. More precisely, in the example of the FIG. 3A , the first metal network 32 comprises a pattern with two slots 321, 322 of respective widths a 1 and a 2 and the second metal network 33 comprises a pattern with two slots 331, 332 of respective widths a 1 and a 2 identical to those of the slots of the pattern of the first network.

[0071] In the example of the FIG. 3A , the metallic networks are called "bi-atom" because they have two slits of different widths per pattern. Such a type of elementary DMG filter, bi-atom, is described for example in E. Sakat et al. 2013. Note, however, that while these filters are interesting from the point of view of angular acceptance, it is also entirely possible to design a multispectral imaging device using suspended “single-atom” DMG elementary filters, i.e. in which a pattern of a metallic diffractive grating only includes one slit, as described for example in C. Tardieu et al., Optics Letters 40, 4 (2015).

[0072] In step (1) of determining the first parameters, the thickness and the refractive index tg and ng of the waveguide are first chosen. tg and ng are chosen to be sufficiently small so that the waveguide is single-mode at the desired resonance wavelength λ r. They thus respect the condition: 0 < 2 λ r t d n d 2 − 1 1 / 2 < 1

[0073] The period p and the dielectric index ng are then adjusted so that a plane wave at normal incidence at the resonance wavelength is diffracted in only 3 orders in the waveguide and only in order 0 in free space (incident medium or transmission medium). For this, we rely on the known laws of transmission gratings.

[0074] The width of the slits can then be adjusted, knowing that in this case we are looking for two slits of different widths (a 1 ≠ a 2 ). Wide slits allow for high transmission at resonance but reduce the quality factor. To obtain a finer resonance, narrower slits are required.

[0075] In practice, the applicants have shown that at the detection wavelengths of interest (e.g. between 3µm and 5µm), the filter parameters can be chosen within the following ranges of values: Metallic networks 32,33 in gold (Au), silver (Ag), or copper (Cu) tm between λ r / 100 and λ r / 10 p < λ r ; a 1 < λ r / 4 , a 2 < λ r / 4 , a 1 ≠ a 2 ; Dielectric material, e.g. silicon carbide (SiC) or silicon nitride (SiN) td between λ r / 20 and λ r / 2; For a spectral range at the highest wavelengths, e.g. in the 8-12 µm range, typical dimensions will naturally be larger.

[0076] In step (2), the optical characteristics of each filter are verified by numerical simulations with the first chosen parameters.

[0077] To do this, we calculate the transmission spectrum of this filter in order to obtain the maximum transmission over the simulated wavelength range and its wavelength position ( FIG. 3B ), corresponding respectively to the value of the transmission at the resonance T max of the filter and to the resonance wavelength λ r . We also obtain the width at half-maximum Δλ of the resonance. We also calculate ( FIG. 3B ) the evolution of the transmission for the fixed wavelength λ r when the angle of incidence of the plane wave is modified. We deduce the angular tolerance of the filter Δθ which corresponds to the angle for which the transmission drops to half its value under normal incidence.

[0078] The curves illustrated on the FIGS. 3A et 3B thus illustrate the transmission as a function of wavelength and the transmission as a function of the angle of incidence for a guided mode resonance filter such as that described in the FIG. 3A with the following parameters: period p = 3 µm, a 1 = 0.2 µm, a 2 = 0.7 µm, tm = 0.1 µm and td = 0.65 µm, nd = 2.15 (SiNx) and nm is given by a Drude model of gold.

[0079] The simulations give for this filter: λ r = 4.01 µm, T max = 75%, Δθ = 17° and Δλ = 120 nm.

[0080] Step (3) of filter design includes a possible modification of the parameters for optimization according to the desired characteristics. For example, a shorter resonance wavelength can be obtained by decreasing the period p. A greater angular tolerance can be obtained by increasing the guide index. A finer spectral resonance can be obtained by decreasing the width of the two slits. However, each time a parameter is modified to change the value of one of the optical characteristics, the other parameters must be readjusted, otherwise there is a risk of degrading another of the optical characteristics. It is also possible to use an optimization algorithm to find the best parameters, such as particle swarm optimization (Mehrdad Shokooh-Saremiand et al., "Particle swarm optimization and its application to the design of diffraction grating filters," Opt. Lett. 32, 894-896 (2007)).

[0081] A second example is described using the FIGS. 4A - 4C .

[0082] On the FIG. 4A as on the FIG. 3A , only a pattern of an elementary filter 40 of dimension p is represented. In practice, the elementary filter comprises a repetition of the pattern thus represented to form a diffraction grating having a period p.

[0083] The elementary filter 40 illustrated on the FIG. 4A is of the DMG (“dual metallic grating”) two-atom type, with substrate. It comprises a layer of dielectric material 41 (thickness td , refractive index nd ) forming a single-mode waveguide at the resonance wavelength λ r , and two metallic diffractive gratings 42, 43 (thicknesses t m1 , t m1 , refractive indices n m1 , n m2 ), arranged on either side of the layer of dielectric material. It further comprises a substrate 44 of refractive index n sub (n sub < nd ) on which are deposited in this example the layer of dielectric material 41 and the metallic diffractive grating 42. Each metallic grating is structured according to a given pattern, the pattern being repeated for each grating with a given period p less than the resonance wavelength. More precisely, in the example of the FIG. 4A , the first metal network 32 comprises a pattern with two slots 421, 422 of respective widths b 1 and b 2 and the second metal network 43 comprises a pattern with two slots 431, 432 of respective widths a' 1 and a' 2 .

[0084] In practice, we can start from a suspended DMG filter as described in the FIG. 3A for a first estimation of the parameters in order to obtain the desired characteristics. A suitable substrate is then chosen and the widths of the grating slots in contact with the substrate are adjusted. We are looking for a substrate with the lowest possible optical index and a waveguide with a high optical index in order to maintain a single-mode waveguide. The period p is chosen so that only the order 0 diffracted by the gratings propagates in the substrate, as for the suspended guide. In this example, unlike the example of the FIG. 3A , the widths and / or thicknesses of the slits of the grating 42 are different from those of the grating 43 to compensate for the change in index of the substrate. Such a filter is described for example in C. Tuambilangana et al., Optics Express 23, 25 (2015).

[0085] As before, although bi-atomic metal lattices are presented in the example of the FIG. 4A , it would also be possible to optimize the parameters for single-atom DMG filters on substrate.

[0086] The curves illustrated on the FIGS. 4A et 4B (step 2) illustrate the transmission as a function of wavelength and the transmission as a function of angle of incidence for a guided mode resonance filter such as that described in FIG. 4A with the following parameters: period p = 2 µm, a 1 = 0.12 µm, a 2 = 0.62 µm, b 1 = 0.15 µm, b 2 = 0.65 µm, t m1 = 0.1 µm, t m2 = 0.05 µm, td = 0.6 µm, ng = 2.84 (SiC) and nm is given by a Drude model of gold.

[0087] The simulations give for this filter: λ r = 3.98 µm, T max = 92%, Δθ = 20° and Δλ = 160 nm.

[0088] A step (3) of modifying the parameters to optimize the parameters according to the desired characteristics can be carried out as previously described.

[0089] A second example is described using the FIGS. 5A - 5C .

[0090] On the FIG. 5A as on the FIG. 3A , only a pattern of an elementary filter 50 of dimension p is represented. In practice, the elementary filter comprises a repetition of the pattern thus represented to form a diffraction grating having a period p.

[0091] The elementary filter 50 illustrated on the FIG. 5A is of the single metallization type on the front face, with substrate. It comprises a layer of dielectric material 51 (thickness td , refractive index nd ) forming a single-mode waveguide at the resonance wavelength λ r , and two metallic diffractive gratings 52, 33 (thickness t m1 and t m2 , refractive index nm ), arranged this time on the same side of the layer of dielectric material. It further comprises a substrate 54 of refractive index n sub (n sub < ng ) on which the layer of dielectric material 51 is deposited (substrate on the side opposite to the side carrying the gratings). Each metallic grating is structured according to a given pattern, repeated for each grating with a given period p , less than the resonance wavelength. More precisely, in the example of the FIG. 5A , the first metal network 52 comprises a pattern with two slots 521 of identical widths b' 1 and the second metal network 53 comprises a pattern with a single slot 531 of width a" 1 .

[0092] In practice, we can also start from a suspended DMG filter as described in the FIG. 3A for a first estimation of the parameters in order to obtain the desired characteristics (but in a single-atom configuration). We then choose a suitable substrate and adjust the width and thickness of the single slit of each network to change the quality factor and the angular tolerance.

[0093] The curves illustrated on the FIGS. 5A et 5B (step 2) illustrate the transmission as a function of wavelength and the transmission as a function of angle of incidence for a guided mode resonance filter such as that described in FIG. 5A with the following parameters: period p = 1.5 µm, a" 1 = 0.2 µm, b' 1 = 0.1 µm, t m1 = 0.1 µm, t m2 = 0.13 µm, td = 0.63 µm, nd = 2.15 (SiNx) and nm is given by a Drude model of gold.

[0094] The simulations give for this filter: λ r = 3.89 µm, T max = 70%, Δθ = 15° and Δλ = 320 nm.

[0095] A step (3) of modifying the parameters to optimize the parameters according to the desired characteristics can be carried out as previously described.

[0096] Applicants have demonstrated that the metallo-dielectric GMR filters described above can operate in a convergent beam, and on surfaces the size of the detection pixel.

[0097] There FIG. 6A thus illustrates a sectional view of a matrix 24 of elementary filters with guided mode resonance comprising in this example elementary filters 24 A , 24 B , each adapted for resonant transmission in a spectral band centered on the wavelengths λ A , λ B respectively. In this example, a matrix 24 is assumed to be formed of a line of elementary filters and adapted for filtering in an infrared multispectral imaging device equipped with an array of elementary detectors. However, it could just as well be a 2D matrix of elementary filters adapted for filtering in an infrared multispectral imaging device equipped with a 2D matrix of elementary detectors. Each filter comprises a diffraction grating formed of a few periods of a pattern such that the dimensions of a filter are substantially the same as the dimensions of a pixel.

[0098] The behavior of the filters under focused beam F 0 is studied, the light beam F 0 comprising a whole range of wavelengths, including the wavelengths λ A , λ B . The applicants have shown that the spreading of the electric and magnetic fields in the waveguide at resonance is limited, the electromagnetic field being localized at the wavelength λ A in the filter 24 A and at the wavelength λ B in the filter 24 B . Thus at the output of the filter matrix, beams F a and F b are obtained respectively at the central wavelengths λ A and λ B .

[0099] There FIG. 6B thus shows the results of a numerical simulation calculating the intensity of the magnetic field within an elementary filter, the simulation being carried out with a matrix of elementary filters as represented on the FIG. 6A . For this simulation, each filter 24 A , 24 B is chosen to be of the suspended two-atom DMG type, as illustrated for example in the FIG. 3A , optimized respectively at resonance wavelengths λ A = 4 µm and λ B = 4.7 µm. Insert 30 on the FIG. 6A illustrates in detail a 24 A filter, limited to a pattern. The characteristics of the filters are as follows. For the 24 A filter, period p A = 3 µm, number of periods = 5, slot widths a 1A = 0.2 µm, a 2A = 0.5 µm. For the 24 B filter, period p A = 3.7 µm, number of periods = 4, slot widths a 1B = 0.1 µm, a 2B = 0.7 µm. For both filters 24 A , 24 B , the simulation is carried out with a dielectric material layer formed in SiN and a metal network in Au. Furthermore, t mA = t mB = 0.1 µm and t dA = t dB = 0.65 µm.

[0100] For the simulation, the filters are illuminated with a focused beam F 0 with a half-opening angle of 9° and at wavelength λ B . It can be seen that although these are guided mode resonance filters, the electromagnetic field is well localized in filter B and that it alone transmits the incident radiation.

[0101] An array of elementary metallo-dielectric guided mode resonance filters suitable for a multispectral imaging device according to the present description can be manufactured according to known methods described for example in the articles referenced in the present description. The array can be deposited on a substrate or suspended.

[0102] The arrangement of the elementary filter array near the elementary detector array can then be done in various ways. For example, the elementary filter array can be arranged without gluing using spacers. It can also be glued, using a transparent adhesive in the filtering wavelength range. To prevent reflections at the interfaces at the substrate or the adhesive, an anti-reflection layer can be added if necessary. The bonding can be done in several ways. For example, by turning the filter over and gluing the upper part with the gratings to the detector array. Or, as another example, in the case of a filter on a substrate whose typical thickness, greater than 300 µm, is generally greater than the depth of focus, the substrate can be thinned by mechanical polishing or chemical etching and the substrate bonded to the detector array.

[0103] Although described through a number of detailed exemplary embodiments, the infrared multispectral imaging device and method according to the present description include various variations, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variations, modifications and improvements are part of the scope of the invention, as defined by the following claims.

Claims

1. A device (20) for infrared multispectral imaging suitable for detecting at least one first and one second detection wavelength, comprising: - a detection matrix array (23) comprising a set of elementary detectors (23i) of preset dimensions forming an image field of given dimensions; - an image-forming optic (22) having a given aperture number (N) and a given focal length (F) , said number and length being suitable for forming, at every point of the image field, an elementary focal spot, said focal spot covering a set of at least two juxtaposed elementary detectors; - a matrix array (24) of elementary metallodielectric guided-mode-resonance filters (24i), said matrix array being arranged in front of the detection matrix array (23) at a distance smaller than a focal depth of the image-forming optic, the dimensions of the elementary filters being chosen so that each elementary focal spot formed at each point of the image field covers at least two elementary filters, said elementary filters being optimized for passband transmission in spectral bands centered on two different central wavelengths, equal to two of said detection wavelengths said at least two elementary filters each comprising an dual-metallic-grating (DMG) guided-mode-resonance elementary filter comprising a layer made of dielectric material (31, 41, 51) forming a waveguide that is single mode at said associated detection wavelength, and two metallic gratings arranged on either side or one elementary guided-mode-resonance filter with a single metallization on its front face, comprising a waveguide made of dielectric material with, on one side, a substrate and, on the other side, a double metallic grating, wherein at least one of said elementary filters has an angular acceptance measured in plane waves higher than or equal to a field-edge angle of the device, where the field-edge angle is defined as the angle of the most inclined ray intended to reach the matrix array of elementary detectors with respect to a direction normal to said matrix array of elementary detectors, and for which the maximum transmittance is equal to half the maximum transmittance of an identical plane wave incident on at least one of the elementary filters with an angle of incidence of zero.

2. The device for infrared multispectral imaging as claimed in the preceding claim, wherein each of said elementary filters has dimensions substantially identical to those of one elementary detector.

3. The device for infrared multispectral imaging as claimed in any one of the preceding claims, wherein said elementary filters (24i) of the matrix array (24) of elementary filters are arranged in the form of zones (Zi), each zone comprising at least two elementary filters optimized for passband transmission in spectral bands centered on two different central wavelengths, and having dimensions larger than those of the focal spot.

4. The device for infrared multispectral imaging as claimed in any one of the preceding claims, wherein each of said at least two elementary filters comprises the elementary DMG guided-mode-resonance filter and the elementary DMG guided-mode-resonance filter is suspended and the two metallic gratings are identical.

5. The device for infrared multispectral imaging as claimed in any one of the preceding claims, wherein each of said at least two elementary filters comprises the elementary DMG guided-mode-resonance filter and the elementary DMG guided-mode-resonance filter is deposited on a substrate made of dielectric material and the two metallic gratings of the elementary filter are different.

6. The device for infrared multispectral imaging as claimed in any one of the preceding claims, wherein each of said at least two elementary filters comprises the elementary DMG guided-mode-resonance filter, and the elementary DMG guided-mode-resonance filter comprises at least one elementary guided-mode-resonance filter of the bi-atomic type, in which said at least one metallic grating has a pattern with at least two apertures of different dimensions.

7. A method for infrared multispectral imaging suitable for detecting at least one first and one second detection wavelength, comprising: - forming an image of a scene by means of an image-forming optic of given aperture (N) and acquiring said image by means of a detection matrix array comprising a set of elementary detectors of preset dimensions forming an image field of given dimensions, the image-forming optic (22) forming at every point of the image field an elementary focal spot covering a set of at least two juxtaposed elementary detectors; - filtering light beams focused by said image-forming optic by means of a matrix array (24) of elementary metallo-dielectric guided-mode-resonance filters, said matrix array being arranged in front of the detection matrix array (23) at a distance smaller than a focal depth of the image-forming optic so that each elementary focal spot formed at each point of the image field covers at least two elementary filters, said elementary filters being optimized for passband transmission in spectral bands centered on two different central wavelengths, equal to two of said detection wavelengths said at least two elementary filters each comprising an dual-metallic-grating (DMG) guided-mode-resonance elementary filter comprising a layer made of dielectric material (31, 41, 51) forming a waveguide that is single mode at said associated detection wavelength, and two metallic gratings arranged on either side or one elementary guided-mode-resonance filter with a single metallization on its front face, comprising a waveguide made of dielectric material with, on one side, a substrate and, on the other side, a double metallic grating, wherein at least one of said elementary filters has an angular acceptance measured in plane waves higher than or equal to a field-edge angle of the device, where the field-edge angle is defined as the angle of the most inclined ray intended to reach the matrix array of elementary detectors with respect to a direction normal to said matrix array of elementary detectors, and for which the maximum transmittance is equal to half the maximum transmittance of an identical plane wave incident on at least one of the elementary filters with an angle of incidence of zero.