Spectral filter comprising coupled resonators

Abstract

A spectral filter (10) comprising coupled resonators and intended to be used in transmission mode comprises an electrically conductive film (11) with at least one slit (1) that passes through said conductive film, and at least one groove (2) that is parallel to the slit. Coupling between two Fabry-Pérot resonators formed by the slit and the groove defines a limit of a spectral transmission window of the filter. Such a filter has a high rejection rate and spectral transmission characteristics that vary little as a function of an angle of incidence of radiation to be filtered, and is easy to produce. Figure for the abstract: Figure 2a

Description

Description Title: COUPLED RESONATOR SPECTRAL FILTER Technical field

[0001] The present description relates to a spectral filter which is intended to be used for filtering electromagnetic radiation, as well as to a method for manufacturing such a filter. Prior art

[0002] Many applications require filtering electromagnetic radiation based on wavelength values ​​of this radiation, with a transmission filtering configuration. In other words, a filter must be placed on the optical path of the radiation, so as to be selectively crossed by a part of this radiation which is contained in a spectral transmission window of the filter. Very often, such filtering is sought to be effective in the infrared domain, in particular in the wavelength value interval which extends between 3 pm (micrometer) and 5 pm, commonly called band II, and with a spectral transmission window which is wider than 1 pm, sometimes up to 2 pm.

[0003] Most filters used today consist of stacks of dielectric layers, designed to form interference filters or Bragg mirror filters. However, these existing filters have the following disadvantages: - they are expensive, particularly because of the high number of layers that make up each filter, and the resulting manufacturing time; - the filtering that is obtained is strongly affected by variations in the angle of incidence of the radiation on the filter. In other words, their angular tolerance is low. Because of this, these interference filters or Bragg mirrors are not suitable for association with optics or photodetectors with large numerical aperture values; - it is very difficult to obtain such interference or Bragg mirror filters which are effective for wavelength values ​​greater than 3 pm and which have spectral transmission window widths greater than 1 pm; and - it is difficult to make mosaics with such filters which are made up by stacks of dielectric layers, while some applications require juxtaposing filters in a cross-section of a beam of radiation, in particular to simultaneously perform multiple detections of radiation which are restricted to different spectral intervals.

[0004] Furthermore, it is known that a slit formed through an electrically conductive film and narrower than the wavelength of electromagnetic radiation incident on the slit has a maximum spectral transmission when the optical frequency of the radiation corresponds to a plasmonic resonance of the slit. Such a resonance results from the combination of plasmons appearing on the surface of the conductive material at the slit, with the Fabry-Pérot resonator behavior of the slit for propagation directions that are perpendicular to the conductive film.

[0005] Furthermore, the article entitled “Enhanced transmission from a single subwavelength slit aperture surrounded by grooves on a standard detector”, by LA Dunbar et al., Applied Physics Letters, Vol. 95, 01 11 13, 2009, describes a filter that is formed by a juxtaposition of identical patterns, each pattern consisting of a slit and several grooves that are formed in a gold film. The grooves are parallel to the slit, and located at constant intervals on both sides of it, five grooves on each side. The efficiency of such a filter operating by radiation transmission results from the combination of the following two effects: - the grooves form a diffracting grating which couples the incident radiation with surface plasmons of the gold film, enhancing the intensity of the radiation at the slit; and - the slit converts surface plasmons into radiation which is transmitted between the two sides of the gold film. But such a filter still has a low angular tolerance with respect to variations in the angle of incidence of the radiation to be filtered, because of the diffracting grating effect. In addition, since the separation pitch between neighboring grooves must be matched with the value of the central wavelength of the spectral transmission window, it is not possible to produce such a filter with very small lateral dimensions. For the filters described by LA Dunbar et al. in the aforementioned article, the central wavelength of the transmission window spectral transmission is between 0.75 pm and 0.95 pm, the full width at half maximum of this spectral transmission window is about 0.15 pm, and the filter pattern size is about 14 pm x 14 pm. Technical problem

[0006] From this situation, an aim of the present invention is to propose transmission filters of a new type, which do not have the drawbacks mentioned above.

[0007] In particular, one aim of the invention is to provide transmission filters which are inexpensive to manufacture.

[0008] Another aim of the invention is to provide filters whose spectral transmission windows can have widths greater than or equal to 1 pm, or even 2 pm, in particular for filters which are effective in band II of electromagnetic radiation, i.e. between 3 pm and 5 pm, or in band III, i.e. between 8 pm and 12 pm, or straddling these two bands.

[0009] Yet another object of the invention is to provide filters whose filtering characteristics, including the rejection rate of each filter, depend little, or almost not at all, on the angle of incidence of the radiation to be filtered.

[0010] Yet another object of the invention is to provide filters which have high or very high rejection ratio values, i.e. which have spectral transmission values ​​which are zero or almost zero just beyond the limits of their spectral transmission windows.

[0011] Finally, yet another object of the invention is to provide transmission-efficient filters that can be easily combined to form filter mosaics. Summary of the invention

[0012] To achieve at least one of these aims or another, a first aspect of the invention provides a spectral filter with coupled resonators, which is intended to be used by transmission and is contained between two faces of the filter which are parallel. The use of this filter therefore comprises sending electromagnetic radiation to be filtered onto one of the faces of the filter and to use part of the radiation which is transmitted through the filter and emerges from its other face.

[0013] The filter comprises an electrically conductive film which is parallel to its faces, with at least one slit which passes through the conductive film from one face to the other, this slit containing a first medium which is transparent to the radiation to be filtered so as to form a first Fabry-Pérot resonator with first standing wave components which propagate inside the slit perpendicular to the faces of the filter. Furthermore, a width of the slit, measured parallel to the faces of the filter, is smaller than a lower limit of a spectral transmission window of the filter, expressed in wavelength values ​​of the radiation to be filtered.

[0014] The filter further comprises, for each slot, at least one groove which is formed in the conductive film parallel to the slot, is separated from the latter and open on one of the faces of the filter, has a depth less than a thickness of the conductive film, and contains a second medium which is also transparent to the radiation to be filtered so as to form a second Fabry-Pérot resonator with second standing wave components which propagate inside the groove perpendicular to the faces of the filter.

[0015] For the invention, a separation distance between the groove and the slit is also smaller than the lower limit of the spectral transmission window of the filter. In addition, the thickness of the conductive film and the depth of the groove are such that, when using the filter, a first part of the radiation to be filtered which has passed through the conductive film via the slit without propagating in the groove, and a second part of the radiation to be filtered which has propagated in the groove perpendicular to the faces of the filter in addition to passing through the conductive film via the slit, form a destructive interference for the transmission through the filter of a rejected part of the radiation, and for a wavelength which belongs to an overlap of respective spectral intervals of individual resonance of the first and second Fabry-Pérot resonators.

[0016] In the context of the present description, the individual resonance spectral interval of the first Fabry-Pérot resonator extends from Àn-3-Qi to Àfi+3-Qi, where An and Qi are respectively the resonance wavelength and the quality factor of the individual resonance of this first Fabry-Pérot resonator. Similarly, the interval The individual resonance spectral range of the second Fabry-Pérot resonator extends from λs2-3-Q2 to λS2+3- Q2, where λS2 and Q2 are respectively the resonance wavelength and the quality factor of the individual resonance of this second Fabry-Pérot resonator.

[0017] A filter according to the invention therefore reproduces the conditions of high transmission of radiation through the slit, when the wavelength of the radiation belongs to the resonance spectral interval of the slit. This resonance, called individual resonance of the slit, results from the combination of the Fabry-Pérot multiple wave interference which occurs in the cavity which constitutes the slit perpendicular to the faces of the filter, with plasmons which are generated by the radiation incident on the surface of the conductive material at the slit. For this reason, the filter of the invention has a high transmission value in its spectral transmission window.

[0018] The groove constitutes another cavity, also perpendicular to the filter faces, with an individual resonance of this groove which is distinct from that of the slot. This individual resonance of the groove results similarly from the combination of the Fabry-Pérot multiple wave interference which occurs in the cavity which constitutes the groove, with plasmons which are generated by the radiation incident on the surface of the conductive material at the level of the groove.

[0019] Furthermore, due to the spatial proximity between the groove and the slit, and also the spectral proximity between their respective individual resonances, the groove provides the radiation with an additional optical path to pass through the conductive film. This additional optical path combines propagation of the radiation in the groove with passage through the conductive film via the slit. In this sense, the two resonators that separately constitute the slit and the groove are coupled in the filter of the invention. An additional interference then occurs between the first part of the radiation that has passed through the conductive film only via the slit, and the second part of the radiation that has followed the additional optical path. This interference is destructive for a wavelength value that is intermediate between the respective individual resonance wavelengths of the slit and the groove.It causes the filter's spectral transmission window to decrease abruptly to a spectral transmission value that is zero. or almost zero, between these two respective individual resonance wavelengths of the slit and the groove. Thus, the filter has a significant rejection rate on the side of its spectral transmission window which is located towards the individual resonance wavelength of the groove.

[0020] The spectral transmission window of a filter which is in accordance with the invention does not depend or depends only slightly on the angle of incidence of the radiation to be filtered, because the central wavelength value of this window is fixed by the thickness of the conductive film which is effective at the location of the slit, and because the limit of this spectral transmission window results from the coupling between the slit and the groove. Thanks to this low or very low angular dependence of the spectral transmission window of the filter, the latter can be combined with optics with a large numerical aperture, in particular near or against an imaging plane of such optics.

[0021] Furthermore, a filter which is in accordance with the invention can be manufactured by etching the slot and the groove in the conductive film. Such a manufacturing method is simple and inexpensive. It also makes it possible to easily vary between different locations of the conductive film, in particular by adaptations of masks, dimensional characteristics of the filter such as the depth of the groove or the thickness of the conductive film which is effective locally at the slot. It is thus possible and simple to produce in the same conductive film a mosaic of juxtaposed filters which have different spectral characteristics.

[0022] Finally, the limit of the spectral transmission window of the filter, on the side of the individual resonance wavelength of the groove on an axis of the wavelength values ​​of the radiation to be filtered, being fixed by the depth of the groove, this depth can be selected so that the spectral transmission window has a width greater than 1 pm, or even close to 2 pm, including when the spectral transmission window of the filter is located in bands II and III.

[0023] In possible embodiments of a filter according to the invention, the conductive film may comprise a base conductive film and a stack of an electrically conductive layer and a dielectric layer, this stack being carried by the base conductive film with the dielectric layer which is intermediate between the layer conductive film and the base conductive film. The groove can then be formed in the stack above the base conductive film. Using such a stack allows the groove depth to be precisely controlled during filter manufacture. Thus, the spectral transmission window limit can be exactly the same between filters that are manufactured successively.

[0024] In preferred embodiments of the invention, the thickness of the conductive film and the depth of the groove may be such that the individual resonance wavelength λs2 of the second Fabry-Pérot resonator formed by the groove is greater than the individual resonance wavelength λn of the first Fabry-Pérot resonator formed by the slit. In this case, the groove determines the upper limit of the spectral transmission window of the filter, in terms of wavelength values. Its lower limit, to obtain a bandpass filter, may then be produced by an additional groove, in addition to the previous groove, or produced by creating a periodic repetition in the filter which produces this lower limit of the spectral transmission window by diffracting grating effect.

[0025] In the latter case, i.e. when the lower limit of the spectral transmission window of the filter is produced by a diffracting grating effect, a pattern which comprises the slit and the groove coupled with each other to produce the destructive interference when using the filter, can be repeated periodically in the filter parallel to its faces, so as to thus form the diffracting grating. Then, a repetition pitch of the pattern is adapted so that the diffracting grating produces a first-order diffraction of the radiation to be filtered for a wavelength value which is smaller than the individual resonance wavelength An of the first Fabry-Pérot resonator formed by the slit. For this, the repetition pitch of the pattern can be chosen larger than half the lower limit of the spectral transmission window of the filter, expressed in wavelength values.

[0026] In the first case, that is to say when the lower limit of the spectral transmission window of the filter is produced by an additional groove, the filter further comprises such an additional groove for each slit, this additional groove also being formed in the conductive film parallel to the slit and open on one of the faces of the filter, having a depth less than the thickness of the conductive film, and containing a third medium which is also transparent to the radiation to be filtered so as to form a third Fabry-Pérot resonator with third standing wave components which propagate inside the so-called additional groove perpendicular to the faces of the filter. The separation distance between this additional groove and the slit is also smaller than the lower limit of the spectral transmission window of the filter.Furthermore, the depth of the additional groove is such that, when using the filter, the first part of the radiation to be filtered which has passed through the conductive film through the slit without propagating in any of the grooves, and a third part of the radiation to be filtered which has propagated in the additional groove perpendicular to the faces of the filter in addition to passing through the conductive film through the slit, form another destructive interference for the transmission through the filter of another rejected part of the radiation, and for another wavelength which belongs to an overlap of the respective spectral intervals of individual resonance of the first and third Fabry-Pérot resonators.In the same way as before, for the invention, the individual resonance spectral interval of the third Fabry-Pérot resonator extends from λs3-3-Qs to λss+S-Qs, where λs3 and Qs are respectively the resonance wavelength and the quality factor of the individual resonance of the third Fabry-Pérot resonator. Furthermore, to produce the lower limit of the spectral transmission window of the filter, the depth of the additional groove is such that the individual resonance wavelength λs3 of the third Fabry-Pérot resonator which is formed by this additional groove is smaller than the individual resonance wavelength λn of the first Fabry-Pérot resonator which is formed by the slit.

[0027] For such a two-groove-per-slot bandpass filter, a pattern that includes the slit, the groove introduced higher first, and the additional groove may be periodically repeated in the filter parallel to its faces, at a pattern repetition pitch that is preferably larger than a sum that includes the respective widths of the slit and the two grooves, and six times a skin thickness of the electrically conductive film for the lower limit of the spectral transmission window of the filter. It is thus possible to space each of the slit and the two grooves apart from the other two to prevent direct electrical coupling from occurring between two resonators of Fabry-Pérot which would be too close to each other parallel to the electrically conductive film.

[0028] Generally, a filter that is in accordance with the invention may be self-supporting, or carried by a rigid support that is transparent in the spectral transmission window of this filter. In particular, a filter that is in accordance with the invention may form a porthole, or may be carried by an optical face of a porthole.

[0029] Generally, and particularly in embodiments of the invention which are intended for applications where the radiation to be filtered has a known linear polarization, the filter may comprise several slots which are each coupled to at least one respective groove so that each slot and the groove coupled with it form the destructive interference for the transmission through the filter of the rejected part of the radiation, with the same spectral transmission window, the slots all being parallel to a common direction. Such a filter is polarizing, and it is preferably used by being oriented so that the common longitudinal direction of the slots is perpendicular to the electric field of the radiation to be filtered, which is linearly polarized.

[0030] In other embodiments of the invention which are intended for applications where the radiation to be filtered can have any polarization or a natural polarization, but also generally without limitation, the filter can likewise comprise several slits which are each coupled to at least one respective groove, again with the same spectral transmission window, the slits then being distributed into several groups distinguished by a common longitudinal direction of slits which is dedicated to each group and different from that of each other group. The filter can thus be devoid of polarizing effect. Configurations with square or hexagonal slit arrays are in particular possible.

[0031] The invention may also be used to provide a filter with two spectral transmission windows which are disjoint. For this, the filter may comprise two parallel slits which constitute respective first Fabry-Pérot resonators, with individual resonance wavelengths which are different between these two slits, each slit being coupled to at least one groove so as to form a respective destructive interference for the transmission through the filter of a rejected part of the radiation. Possibly, the two slits can be coupled simultaneously with one (two) same groove(s) which is (are) then common to them.

[0032] In particular, and usefully for many applications, a filter that is in accordance with the invention may have a spectral transmission window that is between the lower limit wavelength and the upper limit wavelength, each of the lower and upper limit wavelengths being between 1 pm and 15 pm, and the spectral transmission window having a width that is between 1 pm and 5 pm.

[0033] Still generally for the invention, the filter may have a spectral transmission value which is greater than 60%, preferably greater than 70%, even more preferably greater than 80%, in at least one wavelength value of the radiation which is included in the spectral transmission window of the filter. Simultaneously, values ​​of this spectral transmission of the filter may be less than 1%, preferably less than 0.2%, in at least one other value, or two other values, of the wavelength of the radiation which is (are) located on one side or on either side of the spectral transmission window of the filter. In other words, the filter has a rejection rate which is high or very high.

[0034] Still generally for the invention, the width of the slot may be between half and one twentieth of the lower limit of the spectral transmission window of the filter, again expressed in wavelength of the radiation to be filtered. The width of the groove, in particular when this groove determines the upper limit of the spectral transmission window of the filter, may also be between half and one twentieth of the lower limit of the spectral transmission window of the filter. When it is present to determine the lower limit of the spectral transmission window of the filter, the so-called additional groove may have a width which is between one quarter and one fortieth of this lower limit of the spectral transmission window of the filter. All these widths are measured parallel to the faces of the filter.

[0035] Finally, and again in general for the invention, and when the filter comprises at least two identical repetitions of a pattern which comprises at least the slot and the groove, any two of these repetitions which are neighbors may have between them a pitch of repetition which is smaller than the lower limit of the spectral transmission window of the filter, expressed in wavelength values ​​of the radiation to be filtered. An average value of the spectral transmission of the filter in its spectral transmission window can thus be increased, thanks to a rate of occupation of the surface of the conductive film by the coupled slots and grooves which is higher. Preferably, the repetition pitch of the pattern can further be smaller than the lower limit of the spectral transmission window of the filter multiplied by a factor equal to 0.8 / [1 + sin(Omax)], where sin( ) denotes the trigonometric function of sine, and 0max is a maximum value prescribed for the filter, for an angle of incidence of the radiation to be filtered relative to a direction which is perpendicular to the faces of the filter.This additional condition prevents a first order of diffraction by grating effect from disturbing the spectral transmission window of the filter between its lower and upper limits. The prescribed maximum value 0max may be indicated in a user manual which is attached to the filter, or which is accessible from a reference of the filter. When the lower limit of the spectral transmission window of the filter is produced by the diffracting grating effect, the prescribed maximum value 0max is preferably such that sin(0max) is smaller than 0.6.

[0036] A second aspect of the invention provides a filter mosaic, which comprises several filters which are each in accordance with the first aspect of the invention, and which are juxtaposed to constitute a mosaic arrangement, any two of the filters which are neighbors in the mosaic having respective spectral transmission windows which are different. For this, at least one of the thickness of the conductive film, the depth of the groove, that of the complementary groove if any, the medium which is contained in the slot and / or in each groove, and the pattern repetition pitch if any, varies between any two of the filters which are neighbors in the mosaic. Advantageously, the conductive film may be common to the filters of the mosaic. Furthermore, each filter of the mosaic, or at least some of them, may have lateral dimensions which are less than or equal to 10 μm.Such a mosaic of filters can be used against the imaging plane of an imaging optic, possibly with a high numerical aperture, with each filter being dedicated to a different pixel or a few neighboring pixels of a matrix image sensor that is located in this imaging plane.

[0037] A third aspect of the invention provides a method of manufacturing a filter which is in accordance with the first aspect of the invention, this method comprising the following steps: / 1 / prescribe a lower limit wavelength and an upper limit wavelength for the spectral transmission window of the filter, and calculate an average transmission wavelength of the filter from the lower and upper limit wavelengths of the spectral transmission window; 121 determine a thickness value for the conductive film such that the individual resonance wavelength λn of the first Fabry-Pérot resonator which is formed by the slit is equal to the average transmission wavelength of the filter; / 3 / determine a depth value for the groove such that the individual resonance wavelength λs2 of the second Fabry-Pérot resonator which is formed by the groove having this depth value is equal to the upper limit wavelength of the spectral transmission window of the filter; and / 4 / obtaining the conductive film with the thickness of this conductive film which is equal to the thickness value determined in step 121, and forming the slot in this conductive film, as well as the groove in accordance with the depth value of the groove determined in step / 3 / .

[0038] In various implementations of this method, at least one of the following additional features may be reproduced, alone or in combination of several of them: - the conductive film may be metallic, and the thickness value of this metallic film may be determined in step 121 by dividing the individual resonance wavelength An of the first Fabry-Pérot resonator by 2.5 times the refractive index value of the first medium which is contained in the slit; - again when the conductive film is metallic, the depth value of the groove can be determined in step / 3 / by dividing the individual resonance wavelength Às2 of the second Fabry-Pérot resonator by five times the refractive index value of the second medium which is contained in this groove; - when the slot is associated with two grooves to determine the two lower and upper limits of the spectral transmission window of the filter, step / 3 / may further comprise determining a depth value for the so-called groove additional such that the individual resonance wavelength λs3 of the third Fabry-Pérot resonator which is formed by this additional groove is equal to the lower limit wavelength of the spectral transmission window of the filter reduced by half of a difference between the wavelengths of the lower and upper limits of this spectral transmission window. Step / 4 / then further comprises forming the additional groove in the conductive film in accordance with the depth value determined in step / 3 / for this additional groove; and - again when the slit is associated with two grooves to determine the two limits of the spectral transmission window of the filter, the conductive film being furthermore metallic, the depth value of the additional groove can be determined in step / 3 / by dividing the individual resonance wavelength Às3 of the third Fabry-Pérot resonator by five times the refractive index value of the third medium which is contained in this additional groove. The coefficients 2.5 and five, which are used to determine the thickness of the conductive film and the depth of each groove, when the conductive film is metallic, express the role of the surface plasmons that appear at the level of the slit and each groove during the individual resonance of the corresponding Fabry-Pérot resonator. These coefficients may be different when the film is made of other conductive materials. Brief description of the figures

[0039] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures among which:

[0040] [Fig. 1] is a perspective view illustrating a use of a filter according to the invention;

[0041] [Fig. 2a] is a sectional view of a portion of a filter according to a first embodiment of the invention;

[0042] [Fig. 2b] corresponds to [Fig. 2a] for a second embodiment of the invention;

[0043] [Fig. 3a] is a spectral transmission and reflection diagram relative to the filter of [Fig. 2a], for a normal incidence of the radiation to be filtered;

[0044] [Fig. 3b] corresponds to [Fig. 3a] for an angle of incidence of the radiation to be filtered which is equal to 50°;

[0045] [Fig. 4a] corresponds to [Fig. 2a] for a third embodiment of the invention;

[0046] [Fig. 4b] corresponds to [Fig. 2a] for a fourth embodiment of the invention;

[0047] [Fig. 4c] corresponds to [Fig. 2a] for a fifth embodiment of the invention;

[0048] [Fig. 5] is a perspective view of a filter according to a sixth embodiment of the invention; and

[0049] [Fig. 6] is a plan view of a filter mosaic according to the invention. Detailed description of the invention

[0050] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or to actual dimensional ratios. Furthermore, identical references indicated in different figures designate identical elements or which have identical functions.

[0051] As shown in [Fig. 1 ], a filter which is in accordance with the invention and designated by the reference 10 is intended to be used by transmission. The filter 10 has a generally planar shape, and is contained between two parallel faces Fi and F2. An electromagnetic radiation to be filtered, designated by RI, is sent onto one of the faces of the filter 10, for example its face F1, and the filtered part of the radiation RI, designated by T, is transmitted through the filter 10 and emerges from its face F2. For example, the radiation to be filtered RI has at least a part of its spectral distribution which is in band II or in band III. For the embodiment which is illustrated by [Fig. 1 ], the filter 10 is constituted by a repetition along an x ​​axis, of a pattern of structure M which is invariant parallel to a y axis, the x and y axes being parallel to the faces F1 and F2 of the filter 10, and perpendicular between them. Generally, it is not necessary for the invention that the structure pattern M of the filter 10 be repeated according to a repetition pitch which is constant along the x axis and, in the following, such a constancy of the repetition pitch will be mentioned when it is implemented to produce a characteristic of the filter 10. The angle of incidence of the radiation to be filtered RI, denoted 0, is then measured with respect to an axis z which is perpendicular to the face Fi, and in a plane which is parallel to both the x and z axes. [Fig. 1] shows two alternative conditions of use of the filter 10, for which the angle of incidence 0 is zero and non-zero. To obtain optimal filtering efficiency, the radiation to be filtered RI can be linearly polarized, with an electric field direction which is in the plane of the x and z axes.

[0052] According to [Fig. 2a] which is a sectional view of a part of the filter 10 in a plane parallel to the x and z axes, the structural pattern M comprises a slot 1 and at least one groove 2, which are formed in a conductive film 11. In the example of [Fig. 2a], the pattern M also comprises an additional groove 3. The groove 3 is less deep than the groove 2, and the respective depths of the two grooves are denoted p2 for the groove 2 and ps for the groove 3. e denotes the thickness of the conductive film 11 in which the pattern M is formed, preferably with several repetitions of this same pattern along the x axis. [Fig. 2a] is limited to three successive repetitions of the pattern M, but any number of repetitions can be used to obtain a filter whose size is adapted to the needs of its use. For example, the film 1 1 can be made of gold (Au), but it can alternatively be made of any other electrically conductive material, preferably metallic.Thus, the film 11 can also be made of heavily doped polysilicon, or any other degenerate semiconductor material. The film 11 can be self-supporting, or carried by a substrate (not shown) which is transparent to the radiation to be filtered RI. In the example of use illustrated by [Fig. 1], the radiation to be filtered RI is incident on the face Fi of the filter 10 and each of the grooves 2 and 3 is formed in the film 11 by being open on this same face Fi, but this is not essential and any one of the two grooves, or both, can be open alternately on the face F2 of the filter 10 from which the filtered part T of the radiation RI emerges.

[0053] [Fig. 3a] is a spectral transmission and reflection diagram of filter 10 of [Fig. 2a]. The horizontal axis marks the wavelength values ​​of the radiation to be filtered RI, denoted À and expressed in micrometers, and the vertical axis marks the spectral transmission values ​​of the filter 10, denoted T(À) and expressed as the spectral illumination of the part T of the radiation RI which is transmitted through the filter 10, normalized with respect to the spectral illumination of the radiation RI. The radiation to be filtered RI has a parallel beam structure. The spectral transmission window of the filter 10, also called bandwidth, is denoted by BP and lies between a limit value Àd at the start of the bandwidth and a limit value Àf at the end of the bandwidth. The central value of the bandwidth BP, or central transmission wavelength, is denoted Àm and can be defined / 1 1 X — 1 according to A m = — + - , and the bandwidth width BP is LBP=Àf - Àd. In the part \HAS d HAS f / general of the present description, the limit values ​​λd and λf have been called lower limits and upper limits, respectively, of the spectral transmission window of the filter. The diagram of [Fig. 3a] also shows the spectral reflection values ​​of the filter 10, denoted R(λ) and expressed as the spectral illumination of a part of the radiation RI which is reflected by the filter 10, normalized with respect to the spectral illumination of the radiation RI.

[0054] In a known manner, the slit 1 constitutes a first Fabry-Pérot resonator, with standing wave components which propagate parallel to the z axis, and whose resonance wavelength for the part T of the radiation RI which is transmitted between the faces Fi and F2 by the slit 1 alone, is determined by the thickness e of the film 1 1 . When the film 1 1 is metallic, this resonance wavelength of the slit 1 , denoted Àn, is equal to 2.5-nre, where m is the refractive index of a first homogeneous and transparent medium which is present inside the slit 1 . Then, the thickness e of the film 1 1 can be selected to be equal to λm / (2.5-ni), so that the central value λm of the bandwidth BP is equal to the individual resonance wavelength λfi of the first Fabry-Pérot resonator which is constituted by the slit 1 .

[0055] Also known, groove 2 constitutes a second Fabry-Pérot resonator, also with standing wave components which propagate parallel to the z axis, and whose resonance wavelength for the part of the RI radiation which is reflected by groove 2 alone, is determined by the depth p2 of this groove 2. This resonance wavelength of groove 2, noted À S 2, is equal to 5-n2-p2, where 02 is the refractive index of a second homogeneous and transparent medium which is present inside the groove 2, again when the film 11 is metallic. Then, the depth p2 of the groove 2 can be selected so that the resonance wavelength A S2 is equal to the upper limit Δf of the bandwidth BP. Thus, the groove 2 determines the value of this upper limit of the bandwidth BP, with a slope of the spectral transmission T(Δ) of the filter 10 which is very high at the upper limit Δf, and with a value for the spectral transmission T(Δ) of the filter 10 which is zero or almost zero for at least one other value of the wavelength Δ which is slightly greater than Δf. The spectral transmission T(Δ) of the filter 10 then has a cut-off profile which is abrupt at the upper limit Δf of the bandwidth BP, thanks to an interference effect between a first optical path which corresponds to the passage of the radiation RI through the film 1 1 via the slit 1 , and a second optical path which corresponds to a reflection of the radiation RI in the groove 2 followed by a transmission via the slit 1 .Such a profile, which results from the two respective individual resonances of slot 1 and groove 2, and the existence of a coupling between them, is known as a Fano profile. Two conditions for the coupling to exist are that slot 1 and groove 2 are separated by a distance, measured along the x axis in this case, which is smaller than the lower limit Àd of the bandwidth BP, and that individual resonance spectral intervals of slot 1 and groove 2 have an overlap between them. These spectral intervals of individual resonance are [Àn(1 - 1 / Qi), Àfi(1 +1 / Qi)] for slit 1 when Qi is the quality factor of the individual resonance of the first Fabry-Pérot resonator constituted by slit 1, and [Às2(1 -1 / Q2), Às2(1 +1 / Q2)] for groove 2 when Q2 is the quality factor of the individual resonance of the second Fabry-Pérot resonator constituted by groove 2.The values ​​of the quality factors Q1 and Q2 can be measured experimentally on dedicated samples, provided with only slits 1 or only grooves 2. Alternatively, they can be calculated. They depend in particular on the medium contained in slit 1 or groove 2, and on their respective widths, noted h and I2.

[0056] The Fabry-Pérot resonances mentioned above for slot 1 and groove 2 are called individual slot and groove resonances, respectively, as opposed to the coupling between slot 1 and groove 2 which is used according to the invention.

[0057] In the same way as for groove 2, groove 3 constitutes a third Fabry-Pérot resonator, again with standing wave components which propagate parallel to the z axis, and whose individual resonance wavelength for the part of the RI radiation which is reflected by groove 3 alone, is determined by the depth ps of this groove 3. This individual resonance wavelength of groove 3, noted À S 3, is therefore equal to 5-ns-ps, where ns is the refractive index of a third homogeneous and transparent medium which is present inside the groove 3, again when the film 11 is metallic. Then, the depth ps of the groove 3 can be selected so that the resonance wavelength À S 3 is equal to the lower limit Àd of the bandwidth BP reduced by half the width of this bandwidth BP: À S3 = Δd - LBP / 2. Thus, groove 3 determines the value of the lower limit Δd of the bandwidth BP, with a value for the spectral transmission T(Δ) of filter 10 which is zero or almost zero for at least one value of the wavelength Δ which is slightly lower than Δd. The spectral transmission T(Δ) of filter 10 thus has a cut-off profile which is also abrupt at the lower limit Δd of the bandwidth BP, thanks to an interference between the first optical path which corresponds to the passage of radiation RI through film 11 via slit 1, and a third optical path which corresponds to a reflection of radiation RI in groove 3 followed by a transmission via slit 1.This is again a Fano profile, this time at the lower limit Àd of the bandwidth BP, with a direction of variation of the spectral transmission T(À) which is opposite to that of the profile located at the upper limit Àf of the bandwidth BP. This other Fano profile results from the coupling between slot 1 and groove 3. For this, slot 1 and groove 3 must be separated by a distance which is smaller than the lower limit Àd of the bandwidth BP, and the individual resonance intervals of slot 1 and groove 3 must have an overlap. The individual resonance interval of slot 1 is again [Àn (1 -1 / Qi), Àfi(1 +1 / Qi)], and that of groove 3 is [À. ss(1 -1 / Qs), Àss(1 +1 / Qs)] when Qs is the quality factor of the individual resonance of the third Fabry-Pérot resonator constituted by groove 3. The value of the quality factor Qs can be determined in the same way as those of Qi and Q2. It depends in particular on the third medium which is contained in groove 3, and on the width b of the latter.

[0058] According to the method of determining the dimensions of the pattern M which has just been presented, the depth p2 of the groove 2 is greater than that ps of the groove 3.

[0059] The spectral transmission characteristic T(À) of the filter 10 is almost independent of the separation distances between slot 1 and each of the grooves 2 and 3. It also does not matter whether groove 2 is closer to slot 1 than groove 3, or vice versa, as long as each groove 2, 3 is separated from slot 1 by a distance which is smaller than the lower limit Àd of the bandwidth BP.

[0060] For example, the widths h of the slot 1, L of the groove 2 and I3 of the groove 3 may be equal to Δd / 10, Δd / 10 and Δd / 20, respectively, and the first, second and third media that fill the slot 1, the groove 2 and the groove 3 may all be air, zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YF3), zinc selenide (ZnSe) or amorphous silicon (Si), when the lower limits Δd and upper limits Δf of the bandwidth BP are taken substantially equal to 8 pm and 10 pm, respectively. The successive separation distances between the slot and the grooves inside the pattern M, di2 and d23 for the embodiment of [Fig. 2a], can each be equal to Àd / 10.Each of the separation distances di2 and d23, as well as the separation distance dsi between a slit and a neighboring groove that belong to successive repetitions of the pattern M, is preferably greater than twice a skin thickness dm of the conductive material of the film 1 1 for the lower limit Àd of the bandwidth BP. In this way, the plasmons of a slit and a groove, or of two grooves, which are neighbors of each other, are not directly coupled or merged. For the embodiments where the pattern M with one slit and two grooves is repeated, a pitch of this repetition, denoted p in [Fig. 2a], is thus advantageously greater than h + I2 + I3 + 6-dm.

[0061] Finally, when the pattern M is repeated periodically along the x axis, and when the lower limit Àd of the bandwidth BP is determined by the groove 3, as just described, it is preferable that the repetition pitch p is smaller than 0.8-Àd / [1 + sin(0)]. In this way, no diffraction effect of the radiation RI by the periodic grating then constituted by the repetitions of the pattern M disturbs the spectral transmission curve T(À) in the bandwidth BP. It is possible to provide the filter 10 with a prescribed maximum value 0max for the angle of incidence 0. In this case, it is sufficient that the repetition pitch p is smaller than 0.8-Àd / [1 + sin(Omax)]. It is also possible to avoid the grating diffraction effect disturbing the spectral transmission curve T(À) as determined by groove 3, to repeat the pattern M along the x axis with repetition distances that are variable between successive repetitions.

[0062] Conversely, the lower limit Àd of the bandwidth BP can be defined by using the periodic grating diffraction effect which is obtained by repeating the pattern M along the x axis with the repetition pitch p which is constant and larger than half of the lower limit Àd desired for the bandwidth BP of the filter 10. In this case, the pattern M can be devoid of groove 3 as shown by [Fig. 2b]. The condition p < 0.8-Àd / [1 + sin(Omax)] remains applicable for a filter which conforms to [Fig. 2b]. When both conditions p > Àd / 2 and p < 0.8-Àd / [1 + sin(Omax)] are applicable simultaneously, then it is necessary that the prescribed maximum value 0max be larger than Arcsin(0.6), where Arcsin( ) denotes the reciprocal function of the sine.

[0063] In order to obtain values ​​for the spectral transmission T(À) of the filter 10 which are higher in its transmission band BP, it is advantageous for the pattern M to be repeated along the x axis each time with a repetition pitch p which is smaller than the lower limit Àd of the bandwidth BP, whether this pitch is constant or not between successive repetitions, and for the lower limit Àd of the bandwidth BP to be fixed by the groove 3 or by the grating diffraction effect without groove 3. However, and as indicated above, other reasons may lead to advantageously adopting a maximum value for the repetition pitch p which is smaller than the lower limit Àd of the bandwidth BP.

[0064] The method of manufacturing the filter 10 can then comprise at least the following successive steps: step 1: prescribing the values ​​of the lower limits Àd and upper Àf of the spectral transmission window, or bandwidth BP, of the filter 10; step 2: calculating the depth p2 of the groove 2 from the value of the upper limit Àf according to the formula: p2 = Àt / (5-n2); step 3: when the value of the lower limit Àd is to be produced by a groove 3, calculating the depth ps of this groove 3 from the value of this lower limit Àd according to the formula: ps = (Àd - LBP / 2) / (5-ns), or when the value of the lower limit Àd is to be produced by grating diffraction effect, selecting a value for the repetition pitch p of the pattern M which is larger than Àd / 2; step 4: calculating the central wavelength Àm of the bandwidth BP of the filter 10 according to the formula step 5: providing the metal film 11, with an effective thickness e which corresponds to the central wavelength λm at least at the location of the slit 1, according to the formula: e = λm / (2.5-ni); step 6: etching each slit 1 and each groove 2, as well as each groove 3 if applicable, in the metal film 11 at locations thereof as determined in steps 3 and 5; and optional step 7: filling the slits 1 and each of the grooves 2, 3 with the corresponding dielectric medium, when this medium is not air. Such a manufacturing process is economical, easily reconfigurable on demand depending on the application for which a new filter is to be manufactured, and quick to implement.

[0065] The diagram of [Fig. 3a] corresponds to the filter 10 of [Fig. 2a] when the first, second and third media that fill the slot 1 and the grooves 2 and 3 are all air. This filter 10 was designed by prescribing Δd=8.0 pm and Δf=10.1 pm, all the other dimensions of the pattern M being determined as indicated above from these two prescribed values, and the repetition pitch p of the pattern M along the x axis, constant along this axis, is equal to Δd. The maximum value of the spectral transmission T(Δ) is greater than 0.8 in a main part of the BP passband, with a profile at the top that is quite flat, and the spectral transmission T(Δ) is approximately equal to 10' 3 for the values ​​7.2 pm and 10.8 pm of the wavelength λ.

[0066] The diagram of [Fig. 3b] corresponds to that of [Fig. 3a] when the angle of incidence 0 of the radiation to be filtered RI is equal to 50° (degree), for the same filter 10 and when the illumination in transmission is evaluated in alignment with the direction of incidence, that is to say without deviation between the radiation to be filtered RI and its part T which is transmitted through the filter 10. The comparison of the two diagrams shows that the filtering characteristics are little modified by such a value of the angle of incidence 0, despite the importance of this angular value.

[0067] [Fig. 4a] shows an alternative embodiment of the invention in which the metal film 11 carries a stack called MIM, for Metal-Insulator-Metal. This stack is constituted by an intermediate layer 12 which has been deposited on the Fi face of the film 11, and by an upper layer 13 which has been deposited on the intermediate layer 12. For such an embodiment, the film 11 has been called the base conductive film in the general part of the present description. For example, the intermediate layer 12 can be made of zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YFs), zinc selenide (ZnSe) or amorphous silicon (Si) which are dielectrics for the III band of radiation, and the upper layer 13 can be made of gold. In the example shown, pattern M includes slot 1 and groove 2 only.The depth p2 of the groove 2 can be precisely controlled, using the interface between the film 11 and the intermediate layer 12 as a stop interface during a selective etching step used to form the groove 2.

[0068] [Fig. 4b] shows yet another variant of the invention, in which the pattern M comprises two slits 1 and 1', and the grooves 2 and 3. For example, the slit 1 is filled with air during use of the filter 10, and the slit 1' is filled during the manufacture of the filter 10 with a dielectric material such as zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YFs), zinc selenide (ZnSe) or amorphous silicon (Si). Each of the two slits 1 and 1' individually determines, in the manner described above, a central wavelength value of a passband which is separated from that determined by the other slit. The bandwidth that is determined by slit 1 is located at shorter values ​​of the wavelength λ for the radiation to be filtered RI than slit 1', with a minimum of the spectral transmission T(λ) between the two bandwidths down to possibly very low values.The two grooves 2 and 3 then simultaneously determine the respective lower and upper limits of the two passbands, by their respective couplings with one and the other of the two slits 1 and 1' which are effective simultaneously. By way of illustration, the filter 10 of [Fig. 4b] may have a first passband which extends from 8.1 pm to 9.9 pm, produced by the coupling of the slit 1 with the two grooves 2 and 3, and a second passband which. extends from 10.2 pm to 10.8 |im, produced by the coupling of the slit 1 ' also with the two grooves 2 and 3. The two passbands are separated by a minimum of the spectral transmission T(À) which is equal to about 0.07 for the wavelength value 10.05 pm.

[0069] [Fig. 4c] illustrates another way of making a filter 10 with two passbands which are separated. Instead of being differentiated by the refractive index value of their respective filling dielectric media, the two slits 1 and 1' can be differentiated by effective values ​​for the thickness e of the conductive film 11, which are different between slit 1 and slit 1'. For example, both slits 1 and 1' can be filled with air, the metal film 11 has the thickness e at the location of slit 1, and has a reduced thickness e' at the location of slit 1'. Possibly, the reduced thickness e' can be achieved by etching a recess RT in the film 11 on one of the faces F1 or F2, or even both. In practice, it may be sufficient if the RT indentation is formed only on one edge of the 1' slot, as shown.

[0070] All the embodiments of the invention that have been described above use slits and grooves that extend only parallel to the y axis. These filters therefore have a polarizing effect, that is to say that their spectral transmission characteristics T(Δ) at normal incidence, corresponding to the zero value for the angle 0, each vary between the linear polarization direction that is parallel to the x axis for the radiation to be filtered RI, and that parallel to the y axis. A non-polarizing filter 10 can be obtained by using the same slits and grooves, with patterns and distributions that are identical along both the x and y axes.In other words, the filter comprises a first distribution along the x-axis of coupled slots and grooves which extend parallel to the y-axis, and a second distribution along the y-axis of coupled slots and grooves which extend parallel to the x-axis, the two distributions being identical and only transposed from the x-axis to the y-axis. The filter 10 which is thus obtained, as shown in [Fig. 5], is not polarizing, that is to say that its spectral transmission characteristic T(À) for 0=0 is identical between the two directions of linear polarization, parallel to the x-axis and to the y-axis, for the radiation to be filtered RI.

[0071] Finally, since a filter according to the invention can be manufactured from the conductive film 11 only by combining masking, etching, and possibly also of deposition of dielectric material, it is easy to vary the spatial parameters of etching and / or deposition between adjacent areas of the film. Thus, by assigning values ​​for the lower limits Àd and upper Àf of the bandwidth BP which are different from one area to another, and by carrying out the steps of masking, etching, and possibly also of deposition of dielectric material, with patterns which are parameterized according to these values, a multitude of different and juxtaposed filters can be manufactured simultaneously in the same conductive film 1 1. In particular, a mosaic of filters according to the invention can be obtained in this way, in which the filters are distributed by areas according to a determined distribution network. [Fig.6] shows a possible example of such a mosaic, which is designated overall by the reference 100, and which has a square filter distribution network with four different filter patterns respectively assigned to zones Z1, Z2, Z3 and Z4.

[0072] It is understood that the invention may be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages cited. In particular, the following modifications may be implemented, depending on each application for which a filter according to the invention is intended: - the filter can be effective in any spectral range for the radiation to be filtered, for example with a bandwidth which is between 30 pm and 32 pm. For this, the thickness of the conductive film and the depth of each groove are adapted for the desired values ​​of the bandwidth limits, as are the dielectric materials possibly used, but the principles for determining these thickness and depth values ​​remain identical to those which have been presented; - a filter which is in accordance with the invention can be used in one direction or the opposite direction for the face of the filter on which the radiation to be filtered is incident; - the conductive material of the film which constitutes the filter is not necessarily metallic, and may alternatively be constituted by a degenerate semiconductor material, such as heavily doped polysilicon; and - the pattern which includes the slit and at least one groove is not necessarily repeated periodically in the conductive film, but it can be repeated with separation distances which are variable between neighboring repetitions.

Claims

Claims

1. A spectral filter (10) with coupled resonators, intended to be used by transmission and contained between two faces (Fi, F2) of the filter which are parallel, a use of the filter comprising sending electromagnetic radiation to be filtered onto one of the faces of the filter and using a portion of the radiation which is transmitted through the filter and emerges from the other face of the filter, the filter (10) comprising an electrically conductive film (11) which is parallel to the faces (F1, F2) of said filter, with at least one slit (1) which passes through the conductive film from one face to the other, said slit containing a first medium which is transparent to the radiation to be filtered so as to form a first Fabry-Pérot resonator with first standing wave components which propagate inside the slit perpendicular to the faces of the filter, a width (h) of the slit, measured parallel to the faces of the filter,being smaller than a lower limit (Àd) of a spectral transmission window of the filter, expressed in wavelength values ​​of the radiation to be filtered, the filter (10) further comprising, for each slit (1), at least one groove (2) which is formed in the conductive film (1 1) parallel to the slit, is separated from said slit and open on one of the faces (F1, F2) of the filter, has a depth (P2) less than a thickness (e) of said conductive film, and contains a second medium which is also transparent to the radiation to be filtered so as to form a second Fabry-Pérot resonator with second standing wave components which propagate inside the groove perpendicular to the faces of the filter, the filter (10) being characterized in that a separation distance between the groove (2) and the slit (1) is smaller than the lower limit (Àd) of the spectral transmission window of the filter,and in that the thickness (e) of the conductive film (1 1 ) and the depth (P2) of the groove (2) are such that, when using the filter (10), a first part of the radiation to be filtered which has passed through the conductive film via the slit (1 ) without propagating in the groove, and a second part of the radiation to be filtered which has propagated in the groove perpendicular to the faces (F1, F2) of the filter in addition to passing through the conductive film via the slit, form a destructive interference for the transmission through the filter of a rejected part of the, radiation, and for a wavelength which belongs to an overlap of respective spectral intervals of individual resonance of the first and second Fabry-Pérot resonators, the spectral interval of individual resonance of the first Fabry-Pérot resonator extending from Ån-3-Qi to Ån+3-Qi, where An and Qi are respectively a resonance wavelength and a quality factor of the individual resonance of said first Fabry-Pérot resonator, and the spectral interval of individual resonance of the second Fabry-Pérot resonator extending from Ås2-3-Q2 to Ås2+3-Q2, where Ås2 and Q2 are respectively a resonance wavelength and a quality factor of the individual resonance of said second Fabry-Pérot resonator.

2. The filter (10) of claim 1, wherein the conductive film (11) comprises a base conductive film and a stack of an electrically conductive layer (13) and a dielectric layer (12), the stack being carried by the base conductive film with the dielectric layer being intermediate between the conductive layer and the base conductive film, and wherein the groove (2) is formed in the stack above the base conductive film.

3. Filter (10) according to claim 1 or 2, wherein the thickness (e) of the conductive film (1 1 ) and the depth (P2) of the groove (2) are such that the individual resonance wavelength Às2 of the second Fabry-Pérot resonator formed by the groove is greater than the individual resonance wavelength An of the first Fabry-Pérot resonator formed by the slot (1 ).

4. Filter (10) according to claim 3, wherein a pattern (M) which comprises the slit (1) and the groove (2) coupled with each other to produce the destructive interference when using the filter, is periodically repeated in the filter parallel to the faces (F1, F2) of said filter, so as to form a diffracting grating, and a repetition pitch (p) of the pattern is adapted so that the diffracting grating produces a first-order diffraction of the radiation to be filtered for a wavelength value which is smaller than the individual resonance wavelength An of the first Fabry-Pérot resonator formed by the slit.

5. Filter (10) according to claim 4, wherein the repetition pitch (p) of the pattern is greater than half the lower limit (Àd) of the spectral transmission window of the filter, expressed in wavelength values.

6. Filter (10) according to claim 3, further comprising, for each slot (1), at least one additional groove (3) which is formed in the conductive film (11) parallel to said slot and open on one of the faces (Fi, F2) of the filter, has a depth (ps) less than the thickness (e) of said conductive film, and contains a third medium which is transparent to the radiation to be filtered so as to form a third Fabry-Pérot resonator with third standing wave components which propagate inside said additional groove perpendicular to the faces of the filter, wherein a separation distance between the additional groove (3) and the slot (1) is also smaller than the lower limit (Àd) of the spectral transmission window of the filter (10), wherein a depth (ps) of the additional groove (3) is such that, when using the filter (10), the first part of the radiation to be filtered which has passed through the conductive film (1 1 ) through the slit (1 ) without propagating in any of the grooves, and a third part of the radiation to be filtered which has propagated the additional groove perpendicular to the faces of the filter in addition to passing through the conductive film through the slit, form another destructive interference for the transmission through the filter of another rejected part of the radiation, and for another wavelength which belongs to an overlap of the respective individual resonance spectral intervals of the first and third Fabry-Pérot resonators, the individual resonance spectral interval of the third Fabry-Pérot resonator extending from λss-3-Qs to λss+3-Qs, where λ S3 and Q3 are respectively a resonance wavelength and a quality factor of the individual resonance of said third Fabry-Pérot resonator, the depth (ps) of the additional groove (3) being further such that the individual resonance wavelength At S 3 of the third Fabry-Pérot resonator formed by said additional groove is smaller than the individual resonance wavelength Àn of the first Fabry-Pérot resonator formed by the slit (1).

7. Filter (10) according to claim 6, wherein a pattern (M) which comprises the slot (1), said groove (2) and said additional groove (3), is periodically repeated in the filter parallel to the faces (Fi, F2) of said filter, according to a pitch (p) of repetition of the pattern which is greater than a sum which comprises the width (h) of the slot (1) and the respective widths (I2, b) of the groove and said additional groove, and six times a skin thickness of the electrically conductive film for the lower limit (Àd) of the spectral transmission window of the filter.

8. Filter (10) according to any one of the preceding claims, comprising at least two identical repetitions of a pattern (M) which comprises at least the slot (1) and the groove (2), any two of said repetitions of the pattern which are neighbors having between them a repetition pitch (p) smaller than the lower limit (Àd) of the spectral transmission window of the filter, expressed in wavelength values ​​of the radiation to be filtered.

9. The filter (10) of claim 8, wherein the repetition pitch (p) of the pattern (M) is further smaller than the lower limit (Àd) of the spectral transmission window of the filter multiplied by a factor equal to 0.8 / [1 + sin(Omax)], where sin( ) denotes a trigonometric function of sine, and 0max is a maximum value prescribed for the filter, for an angle of incidence of the radiation to be filtered relative to a direction which is perpendicular to the faces of the filter.

10. A method of manufacturing a filter (10), said filter being in accordance with any one of claims 1 to 9, the method comprising the following steps: / 1 / prescribing a lower limit wavelength (λd) and an upper limit wavelength (λt) for the spectral transmission window of the filter (10), and calculating an average transmission wavelength (λm) of the filter from the lower and upper limit wavelengths of the spectral transmission window; / 2 / determine a thickness value (e) for the conductive film (1 1 ) such that the individual resonance wavelength Àfi of the first Fabry-Pérot resonator formed by the slit (1 ) is equal to the average transmission wavelength (Àm) of the filter (10); / 3 / determine a depth value (P2) for the groove (2) such that the individual resonance wavelength At S2 of the second Fabry-Pérot resonator which is formed by the groove having said depth value is equal to the upper limit wavelength (Àf) of the spectral transmission window of the filter (10); and / 4 / obtaining the conductive film (1 1 ) with the thickness (e) of said conductive film which is equal to the thickness value determined in step 121, and forming the slot (1 ) in said conductive film, as well as the groove (3) in accordance with the depth value (ps) of the groove determined in step / 3 / .

11. A method according to claim 10, wherein the conductive film (11) is metallic, and the thickness value (e) of said conductive film is determined in step 121 by dividing the individual resonance wavelength λn of the first Fabry-Pérot resonator by 2.5 times a refractive index value of the first medium which is contained in the slit (1).

12. Method according to claim 11, according to which the depth value (ps) of the groove (2) is determined in step / 3 / by dividing the individual resonance wavelength À S 2 of the second Fabry-Pérot resonator by five times a refractive index value of the second medium which is contained in said groove.

13. A method according to any one of claims 10 to 12, wherein the filter (10) is according to claim 6, and step / 3 / further comprises determining a depth value (ps) for the additional groove (3) such that the individual resonance wavelength λ S3 of the third Fabry-Pérot resonator which is formed by said additional groove, is equal to the lower limit wavelength (Àd) of the spectral transmission window of the filter reduced by half of a difference between the wavelengths of the lower (Àd) and upper (Àf) limits of said spectral transmission window, and step / 4 / further comprises forming the additional groove (3) in the conductive film (1 1 ) in accordance with the depth value (ps) determined in step / 3 / for said additional groove.

14. Method according to claims 12 and 13, according to which the depth value (ps) of the additional groove (3) is determined in step / 3 / by dividing the individual resonance wavelength À S 3 of the third Fabry-Pérot resonator by five times a refractive index value of the third medium which is contained in said additional groove.

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