WO2019197754A1

WO2019197754A1 - Spectral filter for electromagnetic radiation

Info

Publication number: WO2019197754A1
Application number: PCT/FR2019/050780
Authority: WO
Inventors: Mathilde MAKHSIYAN; Patrick BOUCHON; Sébastien HERON; Riad Haïdar
Original assignee: Office National D'etudes Et De Recherches Aérospatiales
Priority date: 2018-04-13
Filing date: 2019-04-03
Publication date: 2019-10-17
Also published as: FR3080191A1; FR3080191B1

Abstract

A spectral filter (10) of the Fabry-Pérot type for electromagnetic radiation comprises two parallel metal layers (1, 2) which are arranged at each side of a dielectric medium (3). One of the two metal layers is micro-structured, with a dimension (w) of zones (4) with a reduced or zero thickness which is smaller than wavelengths of a radiation which is to be filtered. The filter has values with a quality factor and/or rejection rate which are improved in relation to a filter with metal layers which are continuous and uniform.

Description

SPECTRAL FILTRATION OF ELECTROMAGNETIC RADIATION

The present invention relates to a spectral filtering method of electromagnetic radiation.

Many applications require filtering electromagnetic radiation as a function of its wavelength, in particular imaging and spectroscopy applications. For this, spectral filters of very varied types have been developed, including absorbing filters, radiation dispersion filters, interference filters, diffraction filters, and so on.

Among the interference filters, those known as Fabry-Perot type are widely used. Such a filter is based on two flat and parallel structures, one of which is partially reflective and partially transmissive, and forms the input face of the filter. The other planar structure is also partially reflective and partially transmissive when the filter is to be used in transmission, or is completely reflective when the filter is intended to be used in reflection. The two planar structures are continuous and uniform, that is to say invariant by any translations parallel to these structures, and are separated by a dielectric medium which is transparent for the radiation to be filtered. Each planar structure may be a thin metal layer, or a suitable stack of several thin layers of dielectric materials, alternatively high index and low refractive index. In known manner, such a spectral filter has resonances for particular values of the wavelength of the radiation to be filtered, which mainly depend on the thickness of the dielectric medium, and its refractive index. The quality factor of such a Fabry-Perot filter, as well as its rejection rate outside each resonance, are determined by the level of reflection of the plane structures, and beyond that by their level of absorption which reduces their reflection efficiency: the more the planar structures which limit a Fabry-Perot type filter are absorbent, the lower the quality factor and the rejection rate of this filter. For this reason, the Fabry-Perot type filters whose reflective structures are constituted stacks of thin layers of dielectric materials are more efficient than those based on reflective layers that are metallic. But their manufacture is longer, more complex and more expensive because of the number of layers to be deposited and deposition rates which are generally low for dielectric materials.

The simplest diffraction filters are each based on the implementation of a diffractive grating. Such a network, which can be designed to form a filter effective in transmission or reflection according to the constitution of the network, comprises a pattern which is repeated periodically a large number of times. In general, and especially when the network is intended to be used at normal incidence for the radiation to be filtered, the spatial period resulting from the repetition of the pattern is greater than an upper limit of a spectral interval of the radiation to be filtered, expressed in wavelength values. For the simplest diffracting gratings, the optical path of the filtered radiation is not rectilinear at the network level, which makes their implementation more complex or incompatible with certain applications, particularly certain applications within imaging objectives.

From this situation, there is a need for spectral filters of electromagnetic radiation that exhibit quality factor values and / or rejection rates which are further improved over existing filters.

An object of the invention is to provide such improved filters that are easy to manufacture, easy to implement and whose manufacturing cost is reduced.

Finally, another object of the invention is to provide such improved filters for which the resonance value which is effective for the wavelength of the radiation to be filtered can be varied simply during the manufacture of the filter, without causing significant additional cost.

To achieve at least one of these or other objects, a first aspect of the invention provides a spectral filter for electromagnetic radiation that comprises: a first metal layer, which is partially reflective and partially transmissive for a radiation to be filtered according to a wavelength of this radiation;

a second metal layer, which is parallel to the first metal layer and at least partially reflective for the radiation to be filtered; and

a dielectric medium, which is transparent for the radiation to be filtered and intermediate between the first and second metal layers.

Such a filter is intended to be used in transmission or in reflection when the radiation to be filtered is incident on the first metal layer.

This filter has a resonance when a wavelength of the radiation to be filtered is equal to a resonance value K which is determined by the geometric characteristics of the filter and the characteristics of its materials. This resonance is due to the formation, when the radiation to be filtered is incident on the first metal layer, of at least one wave in the dielectric medium which is stationary in a direction perpendicular to the metal layers. In other words, a filter as proposed by the invention has a Fabry-Perot operating principle. The propagation direction of the filtered radiation is thus collinear with that of the incident radiation, which facilitates the integration of the filter in many optical instruments, especially in imaging lenses.

In addition, at least one of the metal layers of the filter, called the micro-structured metal layer, has at least one zone in which a thickness value of this micro-structured metal layer is reduced in comparison with the same micro-metallic layer. -structured outside each zone. For this, the layer thickness is measured perpendicular to the metal layers. Possibly, the thickness of the micro-structured layer may be zero in at least some areas of this layer where its thickness is reduced according to the invention. In this case, each zone in which the thickness of the micro-structured layer is zero constitutes an opening through this layer. Possibly again, each micro-structured layer may have some of its areas of reduced thickness where this layer thickness is zero, and others of its reduced thickness areas where this reduced thickness is not zero.

According to the invention, at least one dimension of each zone of the micro-structured metal layer, when this dimension is measured parallel to the metal layers, is smaller than the resonance value which is effective for the wavelength of the radiation. Then, when the filter is used to filter electromagnetic radiation, this radiation has at least part of its spectrum corresponding to wavelengths greater than the size (s) of the zones of the micro-metallic layer. structured. In the jargon of the skilled person, such micro-structuring is called "sub-wavelength".

The inventors have indeed observed that, for the same material of the metal layers, the spectral filter of the invention is more efficient than a Fabry-Perot filter with metal layers which are invariant by any translations parallel to these layers, when the two filters - the filter of the invention and a filter as known before the invention - have the same resonance value A _r and the same value of transmission or reflection at resonance, depending on whether the filter is intended to be used in transmission or reflection. More precisely, the filter of the invention has a quality factor value and / or a rejection ratio value which is (are) improved with respect to the Fabry-Perot filter with invariant metal layers by translations. This improvement results from the presence of the zones of reduced or zero thickness in the micro-structured metal layer. Reductions in the amount of the metallic material that result from these reduced or zero thickness zones result in a reduction in the absorption by the metal of the energy of the radiation to be filtered. This absorption reduction is at the origin of the improvement of the quality factor and / or of the rejection ratio for the filter of the invention.

These improvements provided by the invention are correlated with the area of the zone or set of zones of reduced or zero thickness in each layer of the spectral filter which is microstructured. The number of these zones can then be arbitrary, including a single zone for example in the form of a succession of meanders whose trace width is smaller than the resonance value A _r , or multiple zones which are disjoint, each of which also has at least one dimension less than the resonance value A _r , and the total area may be equivalent to that of a meandering area.

In preferred embodiments of the invention, the area (s) of reduced or zero thickness of the micro-structured metal layer may (may) form a pattern that is periodically repeated along at least one direction of repetition parallel to the metal layers. In this case, a spatial period which is formed by repeating the pattern is preferably also less than the effective resonance value A _r for the wavelength of the radiation. Thus, no radiation beam emerging at a non-zero diffraction order value can be produced by diffractive grating effect from a radiation to be filtered which is sent onto the filter with a normal incidence. A quantity of stray light that could be annoying for some filter applications is avoided in this way.

In preferred first embodiments of the invention, in a pattern which is repeated periodically, the micro-structured metal layer may comprise straight metal strips which have identical widths, and which are separated by gaps devoid of metal material. These intervals, which form the zones of the micro-structured metal layer, are identical for any pairs of neighboring bands inside the layer. Such embodiments may be simple to design and manufacture, including using lithographic methods and etching methods that are well known in the art of manufacturing integrated electronic circuits.

In other embodiments, the pattern may be two-dimensional and periodically repeated along two repeating directions that are perpendicular to each other and parallel to the metal layers.

In general, for the invention, the zone (s) of reduced or zero thickness of each microstructured layer may be such that in normal incidence of the radiation to be filtered on the first layer metallic, a transmission coefficient or spectral reflection of the filter, which is effective for the transmitted or reflected radiation, respectively, is identical for two polarizations of the radiation to be filtered which are orthogonal to each other. The filter thus has a spectral response that is independent of a state of polarization of the radiation to be filtered, in normal incidence.

Still more generally for the invention, a material of the dielectric medium may have variations which are aligned with edges of the zone (s) of reduced or zero thickness of the microstructured metal layer, in a direction of alignment that is perpendicular to the metal layers. Such variations for the dielectric medium provide an additional degree of freedom for adjusting the resonance value A _r , while keeping a manufacturing method for the filter that can be simple. Indeed, the micro-structured metal layer or a base portion of this layer can be used as an etching mask to locally replace an initial material of the dielectric medium with another material. Such a mode of adjustment of the resonance value A _r , by local modification of the material of the dielectric medium, can also be combined with a variation of the resonance value A _r which results from an appropriate selection of the dimensions of the zones to be made. reduced or no thickness of the micro-structured layer.

Finally, the zone (s) with a reduced or zero thickness of the micro-structured metal layer, in particular at least one dimension of this (these) zone (s), can (can) be varied (s) between regions different from the filter, which are offset without overlapping relative to each other parallel to the metal layers, so that the resonance value A _r is constant within each region but varies between at least two of the regions. The region spectral filter that is thus obtained can be manufactured without increasing the number of steps of its manufacture, only by using a lithography mask that is adapted per region of the filter. Such a filter then provides several resonance values A _r which are different, while forming only one optical component. Its assembly in a multi-channel optical instrument, for example a multispectral imaging instrument or spectroscopy, remains simple. Finally, in general, a material of each metal layer may comprise at least one metal which is selected from among gold, aluminum, copper, silver, platinum, rhodium, titanium, zirconium, chromium, tungsten, or may comprise titanium nitride or zirconium nitride. In particular, the material of each metal layer may comprise an alloy based on at least one of the metals or nitrides which have just been mentioned, and / or be different or different from the material of the other metal layer. These metals or nitrides have high electrical conductivity values, and thereby reduce the absorption of radiation by the metal layers. They thus contribute to increase to an additional extent the quality factor and / or the rejection rate of the filter.

According to a configuration that is possible for a spectral filter according to the invention, the filter may further comprise a plane-face support which is transparent in a spectral range extending on either side of the resonance value A _r . Then, the second metal layer, the dielectric medium and the first metal layer form a stack on the flat face of the support.

According to another configuration also possible, the dielectric medium may consist of a self-supporting film with two parallel faces and opposite, in particular by a film of organic material. Then the first and second metal layers can be worn one-on-one by both sides of the self-supporting film.

A second aspect of the invention provides a spectral filtering method of electromagnetic radiation, which is implemented using at least one spectral filter according to the first aspect of the invention. For this, the radiation to be filtered has wavelength values that are greater than the size of each zone of reduced or zero thickness of the metal layer (s) of the filter which is (are) micro -structurée (s). To produce effective filtering, the spectral extension range of the radiation to be filtered contains the resonance value K of the filter, when this interval is expressed in wavelength values. Such a method may in particular be implemented for an application selected from a monochromatic or multispectral image acquisition, a spectroscopic analysis, a selective emission of radiation which is produced by heating the filter, etc.

When the spectral filter comprises a pattern which is repeated in a spatial period smaller than the resonance value A _r of the filter, the wavelength values of the radiation to be filtered are preferably also greater than this spatial period.

Other features and advantages of the present invention will become apparent in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

FIG. 1 is a perspective view of a spectral filter of electromagnetic radiation according to the present invention;

FIG. 2 is a diagram which compares reflectivity spectral characteristics of a filter according to FIG. 1 and of a filter known from the prior art, for a normal incidence of the radiation to be filtered;

FIG. 3 is another diagram which shows variations of a resonance value of a filter according to FIG. 1, when a dimension of metal layer openings is varied;

FIG. 4 is yet another diagram, which shows variations in the resonance value of a filter according to FIG. 1 when a spatial period of the metal layer openings is varied; and

FIGS. 5 to 7 correspond to FIG. 1 for three other embodiments of the invention.

For the sake of clarity, the dimensions of the elements shown in FIGS. 1 and 5-7 do not correspond to real dimensions or to actual dimension ratios. In addition, identical references which are indicated in different figures designate identical elements or which have identical functions.

Reference numeral 10 generally designates a spectral filter conforming to to the invention, and references 1 and 2 denote two metal layers of this filter, which are parallel and spaced from each other by an intermediate distance noted h. Each of these two layers has a thickness: ei for the metal layer 1 and e ₂ for the metal layer 2, which may be equal or different. The gap that is intermediate between the two metal layers 1 and 2 is filled by a dielectric medium 3, which is assumed homogeneous at first. This dielectric medium 3 is characterized by an optical refractive index value n, which can be assumed constant over a spectral range of use of the filter 10, but not necessarily. Modifications of the description which follows, which would be due to spectral variations of this value n, with respect to the case where this value n is substantially constant over the spectral range of use of the filter, are well known to those skilled in the art. so they will not be described again here. For example, the filter 10 may be provided for a spectral range of use which is between 0.75 μm (micrometer) and 4.0 μm, in wavelength values. In general, the spectral range of use is prescribed for each filter in a record of this filter.

By way of example, the metal layers 1 and 2 can be made of gold (Au), and the dielectric medium 3 can be made of silica (SiO ₂ ), zinc sulphide (ZnS) or polymethyl methacrylate (PMMA). ), or based on polyimide such as Kapton ^® Dupont de Nemours material, or by an epoxy type resin, for example as designated by SU8.

The thickness ei of the metal layer 1 is such that this layer is partially reflective and partially transmissive for any electromagnetic radiation in the spectral range of use of the filter 10. For example, the thickness ei may be between 5 nm ( nanometer) and 60 nm when layer 1 is gold.

In first embodiments of the invention, the thickness e ₂ of the metal layer 2 may be such that this layer 2 is also partially reflective and partially transmissive in the spectral range of use of the filter 10. In this case, the thickness e ₂ may also be between 5 nm and 60 nm when the layer 2 is gold. In known manner, the two metal layers 1 and 2 then form, with the dielectric medium 3, a Fabry-Perot filter which is effective in transmission: the layer 1 forms the input face of the radiation to be filtered, and the layer 2 forms the face of filtered radiation output. Such a spectral filter to be used in transmission has resonances when the radiation to be filtered, when it is directed on the layer 1 with a normal incidence, has a wavelength which is equal to A _r = 4-knh, where k is a nonzero natural integer which identifies successive resonances appearing for increasing wavelength values of the radiation to be filtered.

In second embodiments of the invention, the thickness e ₂ of the metal layer 2 may be such that this layer 2 is reflective with a transmission of radiation that is zero through it. In this other case, the thickness e ₂ is greater than 60 nm if the layer 2 is gold. Then, in a still known manner, the two metal layers 1 and 2 form, with the dielectric medium 3, a Fabry-Perot filter which is efficient in reflection: the layer 1 forms both the input face of the radiation to be filtered. , and the exit face of the filtered radiation. Such another filter to be used in reflection has resonances when the radiation to be filtered, when it is directed on the layer 1 with a normal incidence, has a wavelength which is equal to A _r = 2-k'-nh, where k 'is also a non-zero natural integer to identify the resonances which appear successively when the wavelength of the radiation to be filtered increases.

In a still known manner, the resonances which have just been recalled for the two cases of Fabry-Perot filtering in transmission and in reflection, are due to stationary wave structures which are produced by the radiation to be filtered in the medium. dielectric 3, according to the direction z. As indicated in FIGS. 1 and 5-7, z denotes the direction that is perpendicular to layers 1 and 2. The expressions of the resonance values K that have just been recalled are approximations that do not take into account the thicknesses ei and e ₂ of the two metal layers 1 and 2, but the person skilled in the art knows more precise expressions for these resonance values A _r which take into account these thicknesses. In the embodiments of Fabry-Perot filters known before the present invention, the metal layers 1 and 2 are continuous and homogeneous, each being invariant by any translations that are parallel to these layers, that is to say parallel to the x and y directions as shown in the figures. In contrast to these known embodiments, the present invention proposes Fabry-Perot type filters for which at least one of the two metal layers 1 and 2 is micro-structured, with at least one dimension of micro-structuring, measured parallel to the xy plane, which is smaller than a lower bound of the spectral range of use of the filter, or smaller than a lower bound of a portion of that spectral range that is useful for the intended filtering application. In particular, the micro-structuring dimension is smaller than at least some of the K resonance values of this filter. In the following, we will consider only the resonance value that corresponds to k = 1 or k '= 1, depending on whether the filter 10 is intended to be used in transmission or in reflection.

According to first embodiments of a spectral filter 10 which is in accordance with the invention, as represented in FIG. 1, at least one of the metal layers 1 and 2, for example the layer 1, may consist of rectilinear and parallel metal strips that have identical widths. These strips are separated by gaps 4 which are devoid of metal material, and which have identical widths between different pairs of neighboring bands. These ranges correspond to the reduced thickness zones which have been mentioned in the general part of the present description. Since in the present case, the thickness of the micro-structured layer 1 is zero in each gap 4 between two adjacent metal strips, each gap 4 constitutes an opening through the layer 1. The references 1 a, 1 b and 1c denote three successive metal strips of the layer 1, A denotes the spatial period of the micro-structuring pattern thus formed, and w denotes the width of the separation intervals 4 between neighboring metal strips, which is also called the inter-band distance w . According to the invention, the inter-band distance w, and preferably also the spatial period A, is (are) less than the resonance value K of the filter 10. Thus, when the filter 10 is intended to be used in reflection, the period A may be less than the resonance value K = 2-n-h, where h is still the thickness of the dielectric medium 3. Such is the case, in particular, for an embodiment in which h = 1 , 2 μm, n = 1, 4 when the dielectric medium 3 consists of silica, w = 0.55 μm, A = 2.1 μm, ei = 50 nm, and e ₂ = 80 nm corresponding to a filter 10 to use in reflection. Indeed, under these conditions, A _r = 3.613 pm for the natural integer k 'equal to 1.

The diagram of FIG. 2 compares the resonance profile at normal incidence obtained for this filter 10, corresponding to the numerical values which have just been mentioned, with that of a Fabry-Perot filter as previously known. that is to say continuous and uniform metal layers. For this, the Fabry-Perot filter as known previously, called the reference filter, is parameterized to present a resonance value A _r which is identical to that of the filter 10 of the invention, as well as a residual reflectivity value when the wavelength of the radiation to be filtered is equal to A _r , which is the same as that of the filter 10. Thus, the reference filter has the following parameter values: h = 1, 191 pm, n = 1, 4 corresponding to a dielectric medium which is still made of silica, and the two metal layers being still in gold with ei = 41 nm and e ₂ = 80 nm, corresponding also to A _r = 3.613 pm.

As shown in the spectral reflectivity diagram of FIG. 2, the resonance width around the value A _r , as a function of the wavelength of the radiation to be filtered which is marked on the abscissa, is smaller for the filter 10 of FIG. the invention only for the reference filter. This corresponds to a quality factor Q which is greater, when it is defined by Q = A _r / 5A, where d1 denotes the resonance width at half height. For the two filters of the diagram of FIG. 2: Q = 90.32 for the filter 10 of the invention, and Q = 35.1 for the reference filter. In addition, the rejection ratio of the filter 10 of the invention, that is to say the reflectivity value of the filter 10 outside the resonance, is on average about 0.05 greater than the rejection rate of the filter. reference. In the context of the invention, the rejection ratio can be defined as the average value of reflectivity that is produced by a filter on the spectral band of interest when the wavelength of the radiation is less than 1 - 3 / Q times the resonance value A _r or greater than 1 + 3 / Q times this resonance value A _r , for the resonance considered (k '= 1 for FIG. 2).

The diagram of FIG. 3 shows variations of the resonance value A _r which result from changes in the inter-band distance w when all the other parameters of the filter 10 of FIG. 1 remain identical to the values mentioned above. In particular, the spatial period A is constant and equal to 2.1 μm. The horizontal axis of the diagram identifies the values of the inter-band distance w between about 0.1 pm and about 2.3 pm. For values of w which are close to 0, the resonance value A _r corresponds to the Fabry-Perot formula for continuous and uniform metal layers, taking into account an effect of the thicknesses e ₁ and e ₂ of the metal layers 1 and 2. However, the quality factor Q is low when the inter-band distance w is less than 0.4 μm, because of a too large amount of metal which causes a high level of radiation absorption. The resonance value A _r is an increasing function of the interband distance w. This variation of the resonance value A _r as a function of the interband distance w, in particular for A _r ranging from 3.58 μm to 3.68 μm under the conditions of this diagram, can be used to produce region filters. as described below with reference to FIG. 7. When the inter-band distance w is greater than about 0.8 μm, the quality factor Q becomes weak again, in particular because of the reflectivity of the metal layer. 1 which is too weak.

The diagram of FIG. 4 shows variations of the resonance value A _r which result from changes in the spatial period A, when all the other parameters of the filter 10 of FIG. 1 remain identical to the values mentioned above. In particular, the interband distance w is constant and equal to 0.55 μm. As the spatial period A increases, the resonance value A _r decreases toward the result of the Fabry-Perot formula for metal layers that are continuous and uniform. However, the quality factor Q of the filter 10 has values that are low, less than 40, when the spatial period A is less than 1.2 μm, while it is very high, greater than 90, when the period A is between 1.5 μm and 3 μm, according to the invention.

FIG. 5 shows another embodiment of a spectral filter 10 according to the invention, which is obtained from that of FIG. 1 by applying the periodic pattern to rectilinear metal strips in the same way along the two directions x and y. The reference 1 x denotes bands which are parallel to the direction x, and the reference 1 y denotes bands which are parallel to the direction y, similar to the bands 1 a, 1 b and 1 c of the embodiment of FIG. Thus, the inter-band distance w and the spatial period L appear in the two directions x and y. The micro-structuring of the metal layer 1 which is thus obtained consists of square openings 4 which are distributed regularly and identically in the two directions x and y. An advantage of such an embodiment, for which the micro-structuration has characteristics that are identical between the two directions x and y, is that the reflectivity of the filter 10 in normal incidence, and therefore its filtering efficiency, is independent of the polarization of the incident radiation. When the numerical values of the embodiment of FIG. 1 are taken over for the thickness h of the dielectric medium 3, the inter-band distance w, the spatial period L, and the thicknesses e ₁ and e ₂ of the metal layers 1 and 2, the resonance value K is again greater than the inter-band distance w and the spatial period L. The filter 10 of FIG. 5 therefore still has a spectral range of use, expressed in values of the wavelength of the radiation to be filtered, whose lower bound is greater than the interband distance w and the spatial period L.

FIG. 6 shows yet another embodiment of a spectral filter 10 according to the invention, which is obtained from that of FIG. 5 by using openings 4 of various shapes to constitute the micro structuring of the metal layer. 1. As shown, these openings 4 may each have dimensions that are different between the two directions x and y, especially when they are elongated. At least some of the openings 4 may also have shapes which are symmetrical between the two x and y directions, such as cross-shaped or square-shaped openings 4, whatever the orientation of these forms of openings. A Advantage of such openings which are symmetrical between the two directions x and y is to contribute to filter performance which is independent of the polarization of the radiation to be filtered. However, it is also possible to obtain a filter 10 whose filtering efficiency is independent of the polarization of the radiation to be filtered by using openings 4 which are not individually symmetrical between the two directions x and y, but whose densities local distribution in the metal layer 1 are equal. In this case, the sensitivity of the filter 10 to complementary states of polarization of the radiation to be filtered is recovered by the effect of local average in the filter surface. Still according to the invention, each opening 4 of an embodiment according to FIG. 6 has at least one dimension w in the xy plane which is smaller than the resonance value K of the filter 10. In addition, an average distance of separation which exists between respective centers of openings 4 which are adjacent, taken in pairs, is preferably also less than the resonance value A _r . Such an average separation distance between apertures has a function which is similar to the spatial period A of the embodiments of FIGS. 1 and 5.

Spectral filters which are in accordance with the invention, in particular in accordance with FIGS. 1, 5 and 6, can be manufactured at low cost by using certain known techniques of selective etching, in particular such techniques which have been developed for the manufacture of electronic circuits. integrated. For example, the metallic layer 2, continuous and uniform, can be deposited on a flat face S of a support 5 which is transparent for the radiation to be filtered. Such a support is shown in FIG. 7. A continuous and uniform layer of a material of the dielectric medium 3 can then be deposited on top of the metal layer 2, and finally the metal layer 1 on top of the layer of the dielectric medium 3. The metal layer 1 is deposited in the form of a layer which is also continuous and uniform. For the three layers, the material deposition processes that are implemented can be selected according to each material to be deposited and the desired thickness. For example, vacuum cathode sputtering can be used for metal layers 1 and 2, and plasma-assisted value phase chemical deposition, known by the acronym PECVD. for "plasma-enhanced Chemical vapor deposition" in English, can be used for silica as a material of the dielectric medium 3. Alternatively, when the dielectric medium 3 is in resin, an application method that is appropriate as a function of the nature of the resin can be used.

A photolithography technique can then be used to form, on the metal layer 1, a mask which has openings corresponding to those desired for this layer 1. The metal layer 1 is then etched through the openings of the mask to form the openings 4 (Figures 1 and 5-7). A chemical etching process, for example with aqua regia, or a dry etching process, that is to say using an etching plasma, can be used for this, depending on the metal that constitutes the layer. 1. A filter 10 which is in accordance with the invention is thus obtained, with a number of successive steps for its manufacture which is reduced.

According to a first improvement of the invention, optional, a dimension of the openings 4 which are formed in the metal layer 1, for example the interband distance w for embodiments according to FIG. 1, can be varied between regions. 10 shows a first value w ₁ for the inter-band distance that is used in the region R ₁ of the filter 10, and a second value w ₂ , smaller than w _i , which is used for the inter-band distance in the region R ₂ , close to the region Ri. A variation of the resonance value A _r between these two regions of the filter 10 results, in particular from the diagram of FIG. 3 when the spatial period A is the same in the two regions R 1 and R ₂ . Such a variation in size for the openings 4 of the metal layer 1 can be achieved by adapting the photolithography mask which is used to define these openings. No additional step in the manufacturing process of the filter 10 is therefore necessary. The resulting filter, with resonance values K which are different between different regions of the filter, can be used in a multispectral imaging instrument or spectroscopy. It then simultaneously defines the spectral sensitivity windows for several optical acquisition channels that are juxtaposed in parallel in the instrument. The spectral window of each acquisition channel can thus be defined precisely while being very fine and spectrally close to the windows of the other channels. In addition, the filter can be assembled in the device in a single mounting operation, common for all channels. The assembly of the apparatus can thus be faster than if independent filters were used for all the channels.

According to a second improvement of the invention, which is also optional, the dielectric medium 3 can be varied between zones 3b of the spectral filter 10 which are exposed through the openings 4 of the metal layer 1, and complementary zones 3a of the filter Which are covered by the metal layer 1 between the openings 4. For example, the dielectric medium 3 may be silica (SiO ₂ ) in the zones 3a, and a silicon oxynitride (SiO _x N _y ) in the zones 3b. Then, by average field effect, such a dielectric medium 3 which is composite appears in the operation of the filter 10 as a medium whose effective value of refractive index is intermediate between the respective values of refractive index of the two materials which constitute this medium 3: the refractive index value of the silica in the zones 3a, and that of the silicon oxynitride in the zones 3b, for the example considered.

The embodiment of the invention which is represented in FIG. 7 combines the two improvements which have just been mentioned. The difference in resonance value A _r which is thus obtained between the two regions R 1 and R ₂ results from two contributions which are combined. The first contribution corresponds to the diagram of FIG. 3, as already mentioned, and the second contribution is produced by the variation in proportion between the two constituent materials of the dielectric medium 3, which exists between the regions R 1 and R ₂ . The combination of the two contributions makes it possible to obtain resonance values which have a greater difference between them.

Numerous modifications can be introduced with respect to the filters according to the invention which have just been described with reference to the figures, while retaining at least some of the advantages mentioned. Among these modifications, the following are mentioned in a non-limiting way: the spectral filter may be designed to be used in transmission, instead of a filter to be used in reflection. For a filter effective in transmission, the metal layer 2 has a thickness e ₂ which is reduced, for example, to the thickness ei of the metal layer 1, in order to produce a residual transmission of radiation that is not zero;

- In the case of a transmission spectral filter, the two metal layers 1 and 2 may each have openings, these openings may be identical or not between the two layers, and aligned or not in the direction z which is perpendicular to the layers. 1 and 2 ;

- the two metal layers 1 and 2 may be formed on both opposite sides of a self-supported film of uniform thickness, for example one type of polyimide film Kapton ^®. The spectral filter 10 which is thus obtained is particularly light and fast to manufacture, since the synthesis of the dielectric medium 3 by a material deposition process is no longer necessary;

the two improvements which are combined in the embodiment of FIG. 7 can each be implemented without the other improvement;

- When one of the two metal layers 1 and 2 has openings 4 according to the invention, it is possible to deposit on this metal layer, called the base metal layer, a conformal metal layer. The conformal metal overlayer covers with an additional thickness of metal that is constant the dielectric medium 3 in the openings 4 and the base metal layer between the openings 4. Then the base metal layer and the conformal metal overlayer together constitute the metal layer 1 or 2 within the meaning of the invention. The resulting metal layer, or composite, thus has a thickness value which is reduced in the areas corresponding to the openings 4 of the base metal layer, in comparison with its thickness. outside these areas, but non-zero. A thickness profile of such a composite metal layer is shown in broken lines in FIG. 7, and designated by the reference P. Such an embodiment of at least one of the metal layers 1 and 2 makes it possible to adjust precisely its reflectivity level;

finally, all the numerical values which have been mentioned can be modified, in particular to obtain filters which have different resonance values, these values possibly being between 0.5 μm and 10 μm.

A spectral filter in accordance with the invention can be advantageously used for many applications, among which the acquisition of monochromatic or multispectral images, and spectroscopic analysis. Such applications are well known, so there is no need to describe them again here.

Another type of application may be the spectral control of a thermal radiative emission. For this, a spectral filter which is in accordance with the invention, or a portion of material to which the filter is applied, is heated, for example by means of an electrical resistance which is inserted into the portion of material. The thermal emission of radiation that is caused by the heating is then concentrated in the spectral range where the reflectivity of the filter is the lowest, that is to say around each resonance value A _r .

Claims

A method of spectrally filtering electromagnetic radiation, implemented using at least one spectral filter (10) which comprises:

a first metallic layer (1), partially reflecting and partially transmissive for the radiation to be filtered according to a wavelength of said radiation;

- a second metal layer (2), parallel to the first metal layer (1) and at least partially reflective for the radiation to be filtered; and

a dielectric medium (3), transparent for the radiation to be filtered and intermediate between the first (1) and second (2) metal layers,

the filter (10) being intended to be used in transmission or in reflection when the radiation to be filtered is incident on the first metal layer (1), at least one (1) of the metal layers, called the microstructured metal layer, having at least one zone (4) in which a thickness value of said micro-structured metal layer is zero or smaller in comparison with said micro-structured metal layer outside said at least one zone, the thickness being measured perpendicular to the metal layers (1, 2),

the radiation to be filtered having wavelength values which are greater than the size of each zone (4) of each microstructured layer (1) of the filter,

the method being characterized in that the filter (10) has a resonance when a wavelength of the radiation to be filtered is equal to a resonance value K which is determined by the geometric characteristics of the filter and the material characteristics of said filter , said resonance being relative to a formation, when the radiation to be filtered is incident on the first metal layer (1), at least one wave in the dielectric medium which is stationary in a direction perpendicular to the metal layers (1, 2), at least one dimension (w) of each zone (4), measured parallel to the layers metallic (1, 2) being smaller than the resonance value K which is effective for the wavelength of the radiation,

and in that the method is implemented for an application that is selected from monochromatic or multispectral image acquisition, spectroscopic analysis, and selective radiation emission that is produced by heating the filter (10).

The method according to claim 1, wherein said at least one area (4) of the micro-structured metal layer (1) forms a pattern which is periodically repeated along at least one direction (x) of repetition parallel to metal layers (1, 2).

The method of claim 2, wherein a spatial period (A) formed by repeating the pattern is less than the resonance value K which is effective for the wavelength of the radiation.

4. The method of claim 3, wherein the wavelength values of the radiation to be filtered are greater than the spatial period (A) which is formed by repeating the pattern.

The method according to any one of claims 2 to 4, wherein the micro-structured metal layer (1) comprises rectilinear metal strips (1a, 1b, 1c) which have identical widths, and which are separate at intervals devoid of metal material, said gaps forming the zones (4) of the micro-structured metal layer and being identical for any pairs of adjacent strips within said micro-structured metal layer.

The method according to any one of claims 2 to 4, wherein the pattern is two-dimensional and periodically repeated along two repeat directions (x, y) which are perpendicular to each other and parallel to the metal layers (1, 2). .

7. Method according to any one of the preceding claims, wherein said at least one zone (4) of each microstructured layer (1) is such that at normal incidence of the radiation to be filtered on the first metal layer (1) , a spectral transmission or reflection coefficient of the filter which is effective for the transmitted or reflected radiation, respectively, is identical for two polarizations of the radiation to be filtered which are orthogonal to each other.

The method according to any one of the preceding claims, wherein the dielectric medium (3) has variations of a material of said dielectric medium which are aligned with edges of said at least one area (4) of the micro metallic layer. -structured (1), in an alignment direction (z) which is perpendicular to the metal layers (1, 2).

9. Method according to any one of the preceding claims, wherein said at least one zone (4) of the microstructured metal layer (1), in particular at least one dimension of said at least one zone (w), is varied. between regions (Ft-i, Ft ₂ ) different from the filter, said regions being offset without overlapping with respect to each other parallel to the metal layers (1, 2), so that the resonance value A _r is constant at the within each region, but varies between at least two of said regions.

The method according to any one of the preceding claims, wherein a material of each metal layer (1, 2) comprises at least one metal selected from gold, aluminum, copper, silver, platinum, rhodium, titanium, zirconium, chromium, tungsten, or comprises titanium nitride or zirconium nitride.

A method according to any one of claims 1 to 10, wherein the spectral filter (10) further comprises a plane-face support (5) which is transparent in a spectral range extending from else of the resonance value A _r , and wherein the second metal layer (2), the dielectric medium (3) and the first metal layer (1) form a stack on the flat face of the support.

12. A method according to any one of claims 1 to 10, wherein the dielectric medium (3) is constituted by a self-supporting film with two parallel and opposite faces, and the first (1) and second (2) metal layers are carried one-on-one by both sides of the self-supporting film.