IL319188A - Spectral filter comprising coupled resonators - Google Patents

Description

DESCRIPTION TITLE: SPECTRAL FILTER COMPRISING COUPLED RESONATORS 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 electromagnetic radiation to be filtered according to wavelength values of that radiation, with a transmission filtering configuration. In other words, a filter must be arranged in the optical path of the radiation, so as to be selectively passed through by a portion of this radiation that is contained within a spectral transmission window of the filter. Very often, such filtering is sought to be effective in the infrared range, particularly in the interval of wavelength values that extends between 3 µm (micrometer) and 5 µm, commonly referred to as Band II, and with a spectral transmission window that is wider than 1 µm, sometimes up to 2 µm. [0003] Most of the filters in use today consist of stacks of dielectric layers, designed to form interference or Bragg mirror filters. However, these existing filters have the following disadvantages: - they are expensive, particularly because of the large number of layers that make up each filter and the resulting manufacturing time; - the resulting filtering 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 Bragg mirror or interference filters are not suitable for use with optics or photodetectors with high numerical aperture values; - it is very difficult to obtain such interference or Bragg mirror filters that are effective for wavelength values greater than 3 µm and that have spectral transmission window widths greater than 1 µm; and - it is difficult to create mosaics with such filters, which are made up of stacked dielectric layers, whereas some applications require filters to be juxtaposed in a cross-section of a radiation beam, in particular to perform multiple simultaneous detections of radiation that are restricted to different spectral intervals. [0004] Furthermore, it is known that a slit which is formed through an electrically conductive film and which is narrower than the wavelength of electromagnetic radiation incident on this slit, has a spectral transmission maximum when the optical frequency of the radiation corresponds to a plasmon resonance of the slit. Such a resonance results from the combination of plasmons that appear on the surface of the conductive material at the slit, with the Fabry-Perot resonator behavior of the slit for propagation directions perpendicular to the conductive film. [0005] Furthermore, the paper titled "Enhanced transmission from a single subwavelength slit aperture surrounded by grooves on a standard detector", by L.A. Dunbar et al., Applied Physics Letters, Vol. 95, 011113, 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 run parallel to the slit, at constant intervals on both sides thereof, with 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 diffractive grating that couples incident radiation with surface plasmons of the gold film, exalting the intensity of the radiation at the slit; and - the slit converts the surface plasmons into radiation that is transmitted between the two sides of the gold film.

However, such a filter still has a low angular tolerance with respect to variations in the angle of incidence of the radiation to be filtered, due to the diffractive grating effect. Furthermore, since the separation pitch between adjacent grooves must be matched to 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 L.A. Dunbar et al. in the aforementioned article, the central wavelength of the spectral transmission window is between 0.75 µm and 0.95 µm, the full width at half maximum of this spectral transmission window is around 0.15 µm, and the size of the filter pattern is around 14 µm x 14 µm. TECHNICAL PROBLEM [0006] Based on this situation, one aim of the present invention is to offer transmission filters of a new type, which do not have the above-mentioned disadvantages. [0007] In particular, one aim of the invention is to offer transmission filters that 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 µm, or even µm, in particular for filters that are effective in electromagnetic radiation band II, that is, between 3 µm and 5 µm, or in band III, that is, between 8 µm and µm, or straddling these two bands. [0009] Yet another aim of the invention is to provide filters whose filtering characteristics, including the rejection rate of each filter, depend little, or hardly at all, on the angle of incidence of the radiation to be filtered. 30 id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10"

[0010] Yet another aim of the invention is to provide filters with high or very high rejection rate values, that is, with spectral transmission values that are zero or near-zero just beyond the limits of their spectral transmission windows. [0011] Finally, yet another aim 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 or another of these aims, a first aspect of the invention proposes a spectral filter with coupled resonators, which is intended to be used in transmission mode and is contained between two parallel filter faces. The use of this filter therefore comprises sending electromagnetic radiation to be filtered onto one face of the filter and using a portion of the radiation that 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-Perot resonator with first standing wave components which propagate inside the slit perpendicular to the filter faces. Furthermore, a slit width, measured parallel to the filter faces, 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 slit, at least one groove which is formed in the conductive film parallel to the slit, is separated therefrom and open on one of the filter faces, 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-Perot resonator with second standing wave components which propagate within the groove perpendicular to the filter faces. id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15"

[0015] For the invention, a separation distance between the groove and the slit is also smaller than the lower limit of the filter's spectral transmission window. In addition, the thickness of the conductive film and the depth of the groove are such that, when the filter is in use, a first portion of the radiation to be filtered that has passed through the conductive film via the slit without propagating in the groove, and a second portion of the radiation to be filtered that has propagated in the groove perpendicular to the filter faces 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 that belongs to an overlap of respective individual resonance spectral intervals of the first and second Fabry-Pérot resonators. [0016] In the present description, the individual resonance spectral interval of the first Fabry-Perot resonator extends from λf1-3·Q1 to λf1+3·Q1, where λf1 and Q1 are the resonance wavelength and the quality factor of the individual resonance of this first Fabry-Perot resonator, respectively. Similarly, the individual resonance spectral interval of the second Fabry-Perot resonator extends from λs2-3·Q2 to λs2+3·Q2, where λs2 and Q2 are the resonance wavelength and quality factor of the individual resonance of this second Fabry-Perot resonator, respectively. [0017] A filter that conforms 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, known as the individual slit resonance, results from the combination of the Fabry-Perot multi-wave interference, which occurs in the cavity formed by the slit perpendicular to the filter faces, 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. id="p-18" id="p-18" id="p-18" id="p-18" id="p-18" id="p-18" id="p-18" id="p-18"

[0018] The groove forms another cavity, also perpendicular to the filter faces, with an individual resonance of this groove that is distinct from that of the slit. This individual groove resonance also results from the combination of the Fabry-Perot multi-wave interference in the cavity formed by the groove with plasmons generated by the radiation incident on the surface of the conductive material at the groove. [0019] What is more, because of 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 through the conductive film. This additional optical path combines radiation propagation in the groove with a slit for passage through the conductive film. In this sense, the two resonators constituted separately by the slit and the groove are coupled in the filter of the invention. Additional interference then occurs between the first part of the radiation that passed through the conductive film only via the slit, and the second part of the radiation that 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 provides the filter's spectral transmission window with an abrupt decay to a spectral transmission value that is zero or close to zero, between these two respective individual resonance wavelengths of the slit and groove. As a result, the filter has a high rejection rate on the side of its spectral transmission window that lies toward the individual resonance wavelength of the groove. [0020] The spectral transmission window of a filter according to the invention depends little or not at all on the angle of incidence of the radiation to be filtered, since the central wavelength value of this window is fixed by the thickness of the conductive film effective at the slit, and the limit of this spectral transmission window results from the coupling between the slit and the groove. Owing to this low or very low angular dependence of the filter's spectral transmission window, it can be combined with large numerical aperture optics, particularly proximate to or against an image formation plane of such an optical system. [0021] Alternatively, a filter according to the invention can be made by etching the slit and groove into the conductive film. This manufacturing method is simple and inexpensive. It further makes it easy to vary the filter's dimensional characteristics, such as groove depth or the conductive film thickness effective locally at the slit, between different locations on the conductive film, in particular by adapting masks. This makes it easy to produce a mosaic of juxtaposed filters with different spectral characteristics in the same conductive film. [0022] Finally, since the limit of the filter's spectral transmission window, on the individual resonance wavelength side of the groove on an axis of the wavelength values of the radiation to be filtered, is set by the depth of the groove, this depth can be selected so that the spectral transmission window has a width greater than 1 µm, or even close to 2 µm, even when the filter's spectral transmission window 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 intermediate between the conductive layer and the base conductive film. The groove can then be formed in the stack above the base conductive film. The use of such a stack allows precise control of groove depth during filter manufacture. In this way, the limit of the spectral transmission window can be exactly the same between successively manufactured filters. [0024] In preferred embodiments of the invention, the thickness of the conductive film and the depth of the groove can be such that the individual resonance wavelength λs2 of the second Fabry-Perot resonator formed by the groove is greater than the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit. In this case, the groove determines the upper limit of the filter's spectral transmission window, in terms of wavelength values. Its lower limit, to obtain a bandpass filter, can then be produced by an additional groove, in addition to the previous groove, or produced by creating a periodic repetition in the filter that produces this lower limit of the spectral transmission window by diffractive grating effect. [0025] In the latter case, that is, when the lower limit of the filter's spectral transmission window is produced by a diffractive grating effect, a pattern which comprises the slit and groove coupled together to produce the destructive interference when the filter is in use, can be repeated periodically in the filter parallel to its faces, so as to form the diffractive grating. Then, a repetition pitch of the pattern is adapted so that the diffractive grating produces a first-order diffraction of the radiation to be filtered for a wavelength value that is smaller than the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit. To achieve this, the pattern repetition pitch can be chosen to be larger than half the lower limit of the filter's spectral transmission window, expressed in wavelength values. [0026] In the first case, that is, 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 sides 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-Perot resonator with third standing-wave components which propagate within the so-called additional groove perpendicular to the filter faces. The separation distance between this additional groove and the slit is also smaller than the lower limit of the filter's spectral transmission window. In addition, the depth of the additional groove is such that, when the filter is in use, the first portion of the radiation to be filtered that has passed through the conductive film via the slit without propagating in any of the grooves, and a third portion of the radiation to be filtered that has propagated in the additional groove perpendicular to the filter faces in addition to passing through the conductive film via the slit, form another destructive interference for the transmission through the filter of another rejected part of the radiation, and for another wavelength that belongs to an overlap of respective individual resonance spectral intervals of the first and third Fabry-Pérot resonators. As before, for the invention, the individual resonance spectral interval of the third Fabry-Perot resonator extends from λs3-3·Q3 to λs3+3·Q3, where λs3 and Q3 are the resonance wavelength and quality factor of the individual resonance of the third Fabry-Perot resonator, respectively. Furthermore, to produce the lower limit of the filter's spectral transmission window, the depth of the additional groove is such that the individual resonance wavelength λs3 of the third Fabry-Perot resonator that is formed by this additional groove is smaller than the individual resonance wavelength λf1 of the first Fabry-Perot resonator that is formed by the slit. [0027] For such a bandpass filter with two grooves per slit, a pattern that comprises the slit, the groove introduced above first and the additional groove, can be periodically repeated in the filter parallel to its faces, at a pattern repetition pitch that is preferably greater than a sum that comprises the respective widths of the slit and the two grooves, and six times an electrically conductive film skin thickness for the lower limit of the filter's spectral transmission window. In this way, each of the slit and the two grooves can be spaced apart from the other two to prevent direct electrical coupling between two Fabry-Perot resonators that would be too close to one another parallel to the electrically conductive film. [0028] In general, a filter according to the invention can be self-supporting, or carried by a rigid support that is transparent in this filter's spectral transmission window. In particular, a filter according to the invention can form a port, or can be carried by an optical face of a port. [0029] In general, and in particular 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 slits which are each coupled to at least one respective groove so that each slit and the groove coupled therewith form the destructive interference for transmission through the filter of the rejected portion of the radiation, with the same spectral transmission window, the slits all being parallel to a common direction. Such a filter is polarizing, and is preferably used by being oriented so that the common longitudinal direction of the slits 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 may have any polarization or a natural polarization, but also generally without limitation, the filter may likewise comprise a plurality of slits which are each coupled to at least one respective groove, again with the same spectral transmission window, the slits then being divided into a plurality of groups distinguished by a common longitudinal slit direction which is dedicated to each group and different from that of each other group. This means that the filter has no polarizing effect. In particular, square or hexagonal slit array configurations are possible. [0031] The invention can also be used to provide a filter with two disjoint spectral transmission windows. To this end, the filter may comprise two parallel slits that constitute respective first Fabry-Perot resonators, with individual resonance wavelengths that 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 portion of the radiation. Possibly, both slits can be coupled simultaneously, with the same groove(s) then common to both. id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32"

[0032] In particular, and in a manner useful for many applications, a filter according to the invention can have a spectral transmission window which lies between the lower limit wavelength and the upper limit wavelength, each of the lower and upper limit wavelengths lying between 1 µm and 15 µm, and the spectral transmission window having a width which lies between 1 µm and µm. [0033] Still generally speaking, the filter can have a spectral transmission value that is greater than 60%, preferably greater than 70%, even more preferably greater than 80%, in at least one wavelength value of the radiation that lies within the spectral transmission window of the filter. At the same time, values of this spectral transmission of the filter can 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 or both sides of the spectral transmission window of the filter. In other words, the filter has a high or very high rejection rate. [0034] Again as a general rule for the invention, the width of the slit can be between half and one twentieth of the lower limit of the filter's spectral transmission window, again expressed in wavelength of the radiation to be filtered. The width of the groove, particularly when this groove determines the upper limit of the filter's spectral transmission window, can also be between half and one-twentieth of the lower limit of the filter's spectral transmission window. When it is present to determine the lower limit of the filter's spectral transmission window, the so-called additional groove can have a width that is between one quarter and one fortieth of this lower limit of the filter's spectral transmission window. All these widths are measured parallel to the filter faces. [0035] Finally, and again generally for the invention, and when the filter comprises at least two identical repetitions of a pattern that comprises at least the slit and the groove, any two of these repetitions that are adjacent may have therebetween a repetition pitch that is smaller than the lower limit of the filter's spectral transmission window, expressed in wavelength values of the radiation to be filtered. An average value of the filter's spectral transmission in its spectral transmission window can thus be increased, owing to a higher rate of occupation of the conductive film surface by the coupled slits and grooves. Preferably, the pattern repetition pitch can further be smaller than the lower limit of the filter's spectral transmission window multiplied by a factor equal to 0.8/[1 + sin(θmax)], where sin() denotes the trigonometric function of sine, and θmax is a prescribed maximum value for the filter, for an angle of incidence of the radiation to be filtered with respect to a direction that is perpendicular to the filter faces. This additional condition prevents first-order grating diffraction from disturbing the filter's spectral transmission window between its upper and lower limits. The prescribed maximum value θmax can be specified in an operating manual enclosed with the filter, or can be accessed from a filter reference. When the lower limit of the filter's spectral transmission window is produced by the diffractive grating effect, the prescribed maximum value θmax is preferably such that sin(θmax) is smaller than 0.6. [0036] A second aspect of the invention provides a filter mosaic, which comprises a plurality of filters which are each according to the first aspect of the invention, and which are juxtaposed to form a mosaic arrangement, any two of the filters which are adjacent in the mosaic having respective spectral transmission windows which are different. To this end, at least one of the thickness of the conductive film, the depth of the groove, the depth of the complementary groove if applicable, the medium contained in the slit and/or in each groove, and the pattern repetition pitch if applicable, varies between any two of the filters that are adjacent in the mosaic. Advantageously, the conductive film can be common to the mosaic filters. Furthermore, each filter in the mosaic, or at least some of them, may have lateral dimensions that are less than or equal to 10 µm. Such a mosaic of filters can be used against the imaging plane of an imaging optical system, possibly with a high numerical aperture, with each filter dedicated to a different pixel or to some neighboring pixels of a matrix image sensor located in this image formation plane. [0037] A third aspect of the invention proposes a method for manufacturing a filter which is according to the first aspect of the invention, this method comprising the following steps: /1/ prescribing a lower and an upper limit wavelength for the filter's spectral transmission window, and calculating an average filter transmission wavelength from the lower and upper limit wavelengths of the spectral transmission window; /2/ determining a thickness value for the conductive film such that the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit is equal to the mean transmission wavelength of the filter; /3/ determining a depth value for the groove such that the individual resonance wavelength λs2 of the second Fabry-Perot 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 equal to the thickness value determined in step /2/, and forming the slit in this conductive film, as well as the groove in accordance with the groove depth value determined in step /3/. [0038] In various embodiments of this method, at least one of the following additional features can be reproduced, alone or with several of them in combination: - the conductive film can be metallic, and the thickness value of this metallic film can be determined in step /2/ by dividing the individual resonance wavelength λf1 of the first Fabry-Perot resonator by 2.5 times the refractive index value of the first medium contained in the slit; - again when the conductive film is metallic, the groove depth value can be determined in step /3/ by dividing the individual resonance wavelength λs2 of the second Fabry-Perot resonator by five times the refractive index value of the second medium contained in this groove; - when the slit is associated with two grooves for determining the two lower and upper limits of the filter's spectral transmission window, step /3/ may further comprise determining a depth value for the so-called additional groove such that the individual resonance wavelength λs3 of the third Fabry-Perot resonator which is formed by this additional groove is equal to the lower limit wavelength of the filter's spectral transmission window minus half 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 further being metallic, the depth value of the additional groove can be determined in step /3/ by dividing the individual resonance wavelength λs3 of the third Fabry-Perot 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 gap and in each groove during the individual resonance of the corresponding Fabry-Perot resonator. These coefficients can be different when the film is made of other conductive materials. BRIEF DESCRIPTION OF THE FIGURES [0039] The features and advantages of the present invention will become clearer in the following detailed description of non-limiting embodiments, with reference to the appended figures, among which: [0040] [Fig. 1] is a perspective view showing a use of a filter according to the invention; id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41"

[0041] [Fig. 2a] is a cross-sectional view of part 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 diagram of transmission and reflection relative to the filter in [Fig. 2a], for 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 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 planar view of a mosaic of filters 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 dimension ratios. Furthermore, identical references shown in different figures designate elements that are identical or have identical functions. [0051] As shown in [Fig. 1], a filter according to the invention and designated by reference 10 is intended to be used in transmission mode. The filter 10 has a generally flat shape, and is contained between two parallel faces F1 and F2. An electromagnetic radiation to be filtered, designated RI, is sent to one of the faces of the filter 10, for example its face F1, and the filtered part of the radiation RI, designated T, is transmitted through the filter 10 and emerges from its face F2. For example, the radiation to be filtered RI has at least part of its spectral distribution in Band II or Band III. In the embodiment shown in [Fig. 1], the filter consists of a repetition along an x axis of a structural pattern M that 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 to one another. Generally speaking, it is not necessary for the invention that the structural pattern M of the filter 10 be repeated at a repetition pitch that is constant along the x axis and, in the following, such constancy of repetition pitch will be referred to when implemented to produce a characteristic of the filter 10. The angle of incidence of the radiation to be filtered RI, noted θ, is then measured with respect to a z axis which is perpendicular to the face F1, and in a plane which is parallel to the two x and z axes. [Fig. 1] shows two alternative operating conditions for the filter 10, where the angle of incidence θ is zero and non-zero. For optimum filtering efficiency, the radiation to be filtered RI can be linearly polarized, with an electric field direction that lies in the plane of the x and z axes. [0052] According to [Fig. 2a], which is a cross-sectional view of a portion of the filter 10 in a plane parallel to the x and z axes, the structural pattern M comprises a slit 1 and at least one groove 2, which are formed in a conductive film 11. In the example shown in [Fig. 2a], the pattern M also comprises an additional groove 3. The groove 3 is shallower than the groove 2, and the respective depths of the two grooves are noted p2 for the groove 2 and p3 for the groove 3. e designates the thickness of the conductive film 11 where the pattern M is formed, preferably with several repetitions of the 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 11 can be made of gold (Au), but alternatively it can be made of any other electrically conductive material, preferably metallic. The film 11 can also be made of heavily-doped polysilicon, or any other degenerately doped semiconductor material. The film may be self-supporting, or carried by an substrate not shown that is transparent to the radiation to be filtered RI. In the usage example shown by [Fig. 1], the radiation to be filtered RI is incident on the face F1 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 F1, but this is not essential and either or both of the two grooves may be open alternately on the face F2 of the filter 10 from which the filtered portion T of the radiation RI emerges. [0053] [Fig. 3a] is a spectral transmission and reflection diagram of the filter of [Fig. 2a]. The horizontal axis marks the wavelength values of the radiation to be filtered RI, noted λ and expressed in micrometers, and the vertical axis marks the spectral transmission values of the filter 10, noted T(λ) and expressed as the spectral illuminance of the portion T of the radiation RI that is transmitted through the filter 10, normalized with respect to the spectral illuminance of the radiation RI. The radiation to be filtered RI has a parallel beam structure. The spectral transmission window of the filter 10, also known as the passband, is designated BP and lies between a limit value λd at the start of the passband and a limit value λf at the end of the passband. The central value of the passband BP, or central transmission wavelength, is denoted λm and can be defined according to λm= (λd+λf)− 1 , and the width of the passband BP is LBP = λf - λd. In the general part of this description, the limit values λd and λf have been referred to as the lower and upper limits, respectively, of the filter's spectral transmission window. The diagram in [Fig. 3a] also shows the spectral reflection values of the filter 10, noted R(λ) and expressed as the spectral illuminance of a portion of the radiation RI that is reflected by the filter 10, normalized with respect to the spectral illuminance of the radiation RI. [0054] In a known way, the slit 1 constitutes a first Fabry-Perot resonator, with standing wave components propagating parallel to the z axis, and whose resonance wavelength for the part T of the radiation RI that is transmitted between the faces F1 and F2 by the slit 1 alone, is determined by the thickness e of the film 11. When the film 11 is metallic, this resonance wavelength of the slit 1, noted λf1, is equal to 2.5·n1·e, where n1 is the refractive index of a first homogeneous and transparent medium present inside the slit 1. Then, the thickness e of the film 11 can be selected to be equal to λm/(2.5·n1), so that the central value λm of the passband BP is equal to the individual resonance wavelength λf1 of the first Fabry-Perot resonator which is constituted by the slit 1. [0055] As is also known, the groove 2 constitutes a second Fabry-Perot resonator, also with standing wave components propagating parallel to the z axis, and whose resonance wavelength for the part of the radiation RI reflected by the groove 2 alone is determined by the depth p2 of this groove 2. This resonance wavelength of the groove 2, noted λs2, is equal to 5·n2·p2, where nis the refractive index of a second homogeneous and transparent medium that 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 λsis equal to the upper limit λf of the passband BP. Thus, the groove 2 determines the value of this upper limit of the passband BP, with a slope of the spectral transmission T(λ) of the filter 10 that is very large at the upper limit λf, and with a value for the spectral transmission T(λ) of the filter 10 that is zero or almost zero for at least one other value of the wavelength λ that is little greater than λf. The spectral transmission T(λ) of the filter 10 then presents a cutoff profile that is abrupt at the upper limit λf of the passband BP, owing to an interference effect between a first optical path that corresponds to the passage of radiation RI through the film 11 via the slit 1, and a second optical path that corresponds to a reflection of radiation RI in the groove 2 followed by transmission through the slit 1. Such a profile, which results from the two respective individual resonances of the slit 1 and the groove 2, and the existence of a coupling between them, is known as a Fano profile. Two conditions for coupling to exist are that the slit 1 and the groove 2 are separated by a distance, measured along the x axis in this case, that is smaller than the lower limit λd of the passband BP, and that individual resonance spectral intervals of the slit 1 and the groove 2 have an overlap between them. These individual resonance spectral intervals are [λf1(1-1/Q1), λf1(1+1/Q1)] for the slit 1 when Q1 is the individual resonance quality factor of the first Fabry-Perot resonator constituted by the slit 1, and [λs2(1-1/Q2), λs2(1+1 /Q2)] for the groove 2 when Q2 is the quality factor of the individual resonance of the second Fabry-Perot resonator constituted by the groove 2. The values of the quality factors Q1 and Q2 can be measured experimentally on dedicated samples, provided only with slits 1 or only with grooves 2. Alternatively, they can be calculated. They depend in particular on the medium contained in the slit 1 or the groove 2, and their respective widths, noted I1 and I2. [0056] The Fabry-Perot resonances that have been mentioned above for the slit 1 and the groove 2 are referred to as individual slit and groove resonances, respectively, as opposed to the coupling between the slit 1 and the groove that is used according to the invention. [0057] As with the groove 2, the groove 3 constitutes a third Fabry-Perot resonator, again with standing wave components propagating parallel to the z axis, and whose individual resonance wavelength for the part of the radiation RI reflected by the groove 3 alone is determined by the depth p3 of this groove 3. This individual resonance wavelength of the groove 3, noted λs3, is therefore equal to 5·n3·p3, where n3 is the refractive index of a third homogeneous and transparent medium that is present inside the groove 3, again when the film is metallic. Then, the depth p3 of the groove 3 can be selected so that the resonance wavelength λs3 is equal to the lower limit λd of the passband BP decreased by half the width of this passband BP: λs3 = λd - LBP/2. Thus, the groove 3 determines the value of the lower limit λd of the passband BP, with a value for the spectral transmission T(λ) of the filter 10 that is zero or almost zero for at least one value of the wavelength λ that is little less than λd. In this way, the spectral transmission T(λ) of the filter 10 has a cutoff profile that is also steep at the lower limit λd of the passband BP, owing to interference between the first optical path, which corresponds to the passage of radiation RI through the film 11 via the slit 1, and a third optical path, which corresponds to reflection of the radiation RI in the groove 3 followed by transmission through the slit 1. This is another Fano profile, this time at the lower limit λd of the passband BP, with a direction of variation in spectral transmission T(λ) that is opposite that of the profile at the upper limit λf of the passband BP. This other Fano profile results from the coupling between the slit 1 and the groove 3. To achieve this, the slit 1 and the groove 3 must be separated by a distance that is smaller than the lower limit λd of the passband BP, and the individual resonance intervals of the slit 1 and the groove 3 must have an overlap. The individual resonance interval of the slit 1 is still [λf1 (1-1 /Q1), λf1(1+1/Q1)], and that of the groove 3 is [λs3(1-1/Q3), λs3(1+1/Q3)] when Q3 is the quality factor of the individual resonance of the third Fabry-Perot resonator constituted by the groove 3. The quality factor Q3 can be determined in the same way as Q1 and Q2. In particular, it depends on the third medium contained in the groove 3, and on the width l3 of the latter. [0058] According to the aforementioned method of determining the dimensions of the pattern M, the depth p2 of the groove 2 is greater than that p3 of the groove 3. [0059] The spectral transmission characteristic T(λ) of the filter 10 hardly depends on the separation distances between the slit 1 and each of the grooves and 3. Nor does it matter whether the groove 2 is closer to the slit 1 than the groove 3, or vice versa, as long as each groove 2, 3 is separated from the slit by a distance that is smaller than the lower limit λd of the passband BP. [0060] By way of example, the widths l1 of the slit 1, l2 the groove 2 and l3 the groove 3 can be equal to λd/10, λd/10 and λd/20, respectively, and the first, second and third media filling the slit 1, the groove 2 and the groove 3 can all be air, zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YF3), zinc selenide (ZnSe) or amorphous silicon (Si), when the lower λd and upper λf limits of the passband BP are taken to be substantially equal to 8 µm and 10 µm, respectively. The successive separation distances between the slit and the grooves within the pattern M, d12 and d23 for the embodiment in [Fig. 2a], can each be equal to λd/10. Each of the separation distances d12 and d23, as well as the separation distance d31 between a slit and an adjacent groove that belong to successive repetitions of the pattern M, is preferably greater than twice a skin thickness dm of the conductive film material 11 for the lower limit λd of the passband BP. In this way, the plasmons of a slit and a groove, or of two grooves, which are adjacent to one another, are not directly coupled or combined. In embodiments where the one-slit, two-groove M pattern is repeated, a pitch of this repetition, noted p in [Fig. 2a], is thus advantageously greater than l1 + l2 + l3 + 6·dm. [0061] Finally, when the pattern M is repeated periodically along the x axis, and when the lower limit λd of the passband BP is determined by the groove 3, as just described, it is preferable for the repetition pitch p to be smaller than 0.8- λd/[1 + sin(θ)]. In this way, no diffraction effect of the radiation RI by the periodic grating formed by the repetitions of the pattern M disturbs the spectral transmission curve T(λ) in the passband BP. The filter 10 can be supplied with a prescribed maximum value θmax for the angle of incidence θ. In this case, it is sufficient for the repetition pitch p to be smaller than 0.8·λd/[1 + sin(θmax)]. To prevent grating diffraction from disrupting the spectral transmission curve T(λ) as determined by the groove 3, it is also possible to repeat the pattern M along the x axis at repetition distances that vary between successive repetitions. [0062] Conversely, the lower limit λd of the passband BP can be defined 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 greater than half the desired lower limit λd for the passband BP of the filter 10. In this case, the pattern M may be devoid of groove 3, as shown in [Fig. 2b]. The condition p < 0.8·λd/[1 + sin(θmax)] remains applicable for a filter that complies with [Fig. 2b]. When both conditions p > λd/2 and p < 0.8·λd/[1 + sin(θmax)] are applicable simultaneously, then it is necessary for the prescribed maximum value θmax to be greater than Arcsin(0.6), where Arcsin() denotes the reciprocal function of sine. [0063] In order to obtain values for the spectral transmission T(λ) of the filter 10 that 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 that is smaller than the lower limit λd of the passband BP, whether or not this pitch is constant between successive repetitions, and for the lower limit λd of the passband BP to be set by the groove 3 or by the grating diffraction effect without groove 3. However, and as indicated above, there may be other reasons for advantageously adopting a maximum value for the repetition pitch p that is smaller than the lower limit λd of the passband BP. [0064] The method for manufacturing the filter 10 may then comprise at least the following successive steps: step 1: prescribing the values of the lower λd and upper λf limits of the spectral transmission window, or passband BP, of the filter 10; step 2: calibrating the depth p2 of the groove 2 from the value of the upper limit λf according to the formula: p2 = λf/(5·n2); step 3: when the value of the lower limit λd is to be produced by a groove 3, calculating the depth p3 of this groove 3 from the value of this lower limit λd according to the formula: p3 = (λd - LBP/2) /(5·n3), or when the value of the lower limit λd is to be produced by grating diffraction effect, selecting a value for the repetition pitch p of the pattern M which is greater than λd/2; step 4: calculating the central wavelength λm of the passband BP of the filter according to the formula: λm= (λd+λf)− 1; step 5: providing the metal film 11, with an effective thickness e that corresponds to the central wavelength λm at least at the slit 1, according to the formula: e = λm/(2.5·n1); step 6: etching each slit 1 and each groove 2, as well as each groove 3 if applicable, into the metal film 11 at locations thereof as determined in steps 3 and 5; and optional step 7: filling the slits 1 and each groove 2, 3 with the corresponding dielectric medium, when this medium is not air. Such a manufacturing method is economical, easily re-parameterized on demand according to the application for which a new filter is to be manufactured, and quick to implement. id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65"

[0065] The diagram in [Fig. 3a] corresponds to the filter 10 in [Fig. 2a] when the first, second and third media filling the slit 1 and the grooves 2 and 3 are all air. This filter 10 has been designed specifying λd=8.0 µm and λf=10.1 µm, all 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 λd. The maximum value of the spectral transmission T(λ) is greater than 0.8 in a major part of the passband BP, with a profile at the top that is fairly flat, and the spectral transmission T(λ) is about equal to 10-3 for the 7.2 µm and 10.8 µm values of the λ wavelength. [0066] The diagram in [Fig. 3b] corresponds to that in [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 transmitted illuminance is evaluated in alignment with the direction of incidence, that is, without deviation between the radiation to be filtered RI and its part T which is transmitted through the filter 10. A comparison of the two diagrams shows that the filtering characteristics are little altered by such a value of the angle of incidence θ, despite the significance of this angular value. [0067] [Fig. 4a] shows an alternative embodiment of the invention wherein the metal film 11 carries a stack called MIM, for Metal-Insulator-Metal. This stack consists of an intermediate layer 12 deposited on face F1 of the film 11, and a top layer 13 deposited on the intermediate layer 12. For such an embodiment, the film 11 has been referred to as base conductive film in the general part of this description. For example, the intermediate layer 12 can be made of zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YF3), zinc selenide (ZnSe) or amorphous silicon (Si), which are dielectric for radiation band III, and the top layer 13 can be made of gold. In the example shown, the pattern M comprises the slit 1 and the groove 2 only. The depth pof 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, wherein 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 manufacture of the filter 10 with a dielectric material such as zinc sulfide (ZnS), germanium (Ge), magnesium fluoride (MgF2), yttrium fluoride (YF3), zinc selenide (ZnSe) or amorphous silicon (Si). Each of the two slits 1 and 1' individually determines, as described above, a central wavelength value of a passband that is separate from that determined by the other slit. The passband determined by the slit 1 is located at shorter values of the wavelength λ for the radiation to be filtered RI than the slit 1', with a minimum of the spectral transmission T(λ) between the two passbands 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 in [Fig. 4b] may have a first passband extending from 8.1 µm to 9.9 µm, produced by coupling the slit 1 with both grooves 2 and 3, and a second passband extending from 10.2 µm to 10.8 µm, produced by coupling the slit 1' also with both grooves 2 and 3. The two passbands are separated by a minimum spectral transmission T(λ) which is equal to about 0.07 for the wavelength value 10.05 µm. [0069] [Fig. 4c] shows another way of creating a filter 10 with two separate passbands. Instead of being differentiated by the refractive index value of their respective dielectric filling 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 the slit 1 and the slit 1'. For example, both slits 1 and 1' can be filled with air, the metal film 11 has thickness e at slit 1, and has a reduced thickness e' at slit 1'. Possibly, the reduced thickness e' can be achieved by etching a recess RT into the film 11 on one or both faces F1 or F2. In practice, it may be sufficient for the recess RT to be formed on only one edge of the slit 1', as shown. [0070] All the above-described embodiments of the invention feature slits and grooves that extend only parallel to the y axis. These filters therefore have a polarizing effect, that is, their spectral transmission characteristics T(λ) under normal incidence, corresponding to the zero value for angle 0, each vary between the linear polarization direction 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 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 has a first distribution along the x axis of coupled slits and grooves extending parallel to the y axis, and a second distribution along the y axis of coupled slits and grooves extending parallel to the x axis, both distributions being identical and only transposed from the x axis to the y axis. The resulting filter 10, as shown in [Fig. 5], is non-polarizing, that is, its spectral transmission characteristic T(λ) for θ=0 is identical between the two linear polarization directions, parallel to the x axis and y axis, for the radiation to be filtered RI. [0071] Finally, since a filter according to the invention can be made from the conductive film 11 only by combining masking, etching and possibly also dielectric material deposition steps, 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 λd and upper λf limits of the passband BP that are different from one zone to another, and by carrying out the masking, etching and possibly also dielectric material deposition steps with patterns that are parameterized according to these values, a multitude of different, juxtaposed filters can be manufactured simultaneously in the same conductive film 11. In particular, a mosaic of filters conforming to the invention can be obtained in this way, where the filters are distributed in zones according to a determined distribution network. [Fig. 6] shows a possible example of such a mosaic, which is designated globally by reference 100, and has a square filter distribution network with four different filter patterns assigned to zones Z1, Z2, Z3 and Z4, respectively. [0072] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments that have been described in detail above, while retaining at least some of the cited advantages. In particular, the following modifications can be implemented, depending on each application for which a filter according to the invention is intended: - the filter can be effective in any spectral interval for the radiation to be filtered, for example with a passband of between 30 µm and 32 µm. For this purpose, the thickness of the conductive film and the depth of each groove are adapted to the desired values of the bandwidth limits, as are any dielectric materials used, but the principles for determining these thickness and depth values remain identical to those presented above; - a filter according to 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 forming the filter is not necessarily metallic, and may alternatively be made of a degenerately doped semiconductor material, such as heavily-doped polysilicon; and - the pattern comprising the slit and at least one groove is not necessarily repeated periodically in the conductive film, but can be repeated with separation distances that are variable between neighboring repetitions.

Claims (14)

1. Claims 1. A spectral filter (10) comprising coupled resonators and intended to be used in transmission mode and contained between two parallel faces (F1, F2) of the filter, a use of the filter comprising sending electromagnetic radiation to be filtered onto one of the filter faces and using a portion of the radiation which is transmitted through the filter and emerges from the other filter face, 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-Perot resonator with first standing wave components which propagate inside the slit perpendicular to the filter faces, a width (l1) of the slit, measured parallel to the filter faces, being smaller than a lower limit (λd) 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 (11) 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 that is also transparent to the radiation to be filtered, so as to form a second Fabry-Perot resonator with second standing wave components that propagate inside the groove perpendicular to the filter faces, the filter (10) being characterized in that a separation distance between the groove (2) and the slit (1) is also smaller than the lower limit (λd) of the filter's spectral transmission window, and in that the thickness (e) of the conductive film (11) and the depth (p2) of the groove (2) are such that, when the filter (10) is in use, a first portion of the radiation to be filtered that has passed through the conductive film via the slit (1) without propagating in the groove, and a second portion of the radiation to be filtered that has propagated in the groove perpendicular to the filter faces (F1, F2) 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 that belongs to an overlap of respective individual resonance spectral intervals of the first and second Fabry-Pérot resonators, the individual resonance spectral interval of the first Fabry-Perot resonator extending from λf1-3·Q1 to λf1+3·Q1, where λf1 and Q1 are a resonance wavelength and a quality factor of the individual resonance of said first Fabry-Perot resonator, respectively, and the individual resonance spectral interval of the second Fabry-Perot resonator extending from λs2-3·Q2 to λs2+3·Q2, where λs2 and Q2 are a resonance wavelength and a quality factor of the individual resonance of said second Fabry-Perot resonator, respectively.

2. The filter (10) according to 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 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. The filter (10) according to claim 1 or 2, wherein the thickness (e) of the conductive film (11) and the depth (p2) of the groove (2) can be such that the individual resonance wavelength λs2 of the second Fabry-Perot resonator formed by the groove is greater than the individual resonance wavelength λfof the first Fabry-Perot resonator formed by the slit (1).

4. The filter (10) according to claim 3, wherein a pattern (M) which comprises the slit (1) and the groove (2) coupled together to produce the destructive interference when the filter is in use, is repeated periodically in the filter parallel to the faces (F1, F2) of said filter, so as to form a diffractive grating, and a repetition pitch (p) of the pattern is adapted so that the diffractive grating produces a first-order diffraction of the radiation to be filtered for a wavelength value that is smaller than the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit.

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

6. The filter (10) according to claim 3, further comprising, for each slit (1), at least one additional groove (3) which is formed in the conductive film (11) parallel to said slit and open on one of the faces (F1, F2) of the filter, has a depth (p3) 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-Perot resonator with third standing wave components which propagate within said additional groove perpendicular to the filter faces, wherein a separation distance between the additional groove (3) and the slit (1) is also smaller than the lower limit (λd) of the filter's spectral transmission window (10), wherein a depth (p3) of the additional groove (3) is such that, when the filter (10) is in use, the first portion of the radiation to be filtered that has passed through the conductive film (11) via the slit (1) without propagating in any of the grooves, and a third portion of the radiation to be filtered that has propagated the additional groove perpendicular to the filter faces in addition to passing through the conductive film via the slit, form another destructive interference for the transmission through the filter of another rejected part of the radiation, and for another wavelength that 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-Perot resonator extending from λs3-3·Q3 to λs3+3·Q3, where λs3 and Q3 are a resonance wavelength and a quality factor of the individual resonance of said third Fabry-Perot resonator, respectively, the depth (p3) of the additional groove (3) further being such that the individual resonance wavelength λs3 of the third Fabry-Perot resonator formed by said additional groove is smaller than the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit (1).

7. The filter (10) according to claim 6, wherein a pattern (M) which comprises the slit (1), said groove (2) and said additional groove (3), is periodically repeated in the filter parallel to the faces (F1, F2) of said filter, at a pattern repetition pitch (p) which is greater than a sum comprising the width (l1) of the slit (1) and the respective widths (l2, l3) of the groove and said additional groove, and six times a skin thickness of the electrically conductive film for the lower limit (λd) of the spectral transmission window of the filter.

8. The 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 slit (1) and the groove (2), any two of said repetitions of the pattern which are adjacent having therebetween a repetition pitch (p) smaller than the lower limit (λd) of the spectral transmission window of the filter, expressed in wavelength values of the radiation to be filtered.

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

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 (λf) for the filter's spectral transmission window (10), and calculating an average filter transmission wavelength (λm) from the lower and upper limit wavelengths of the spectral transmission window; /2/ determining a thickness value (e) for the conductive film (11) such that the individual resonance wavelength λf1 of the first Fabry-Perot resonator formed by the slit (1) is equal to the mean transmission wavelength (λm) of the filter (10); /3/ determining a depth value (p2) for the groove (2) such that the individual resonance wavelength λs2 of the second Fabry-Perot resonator which is formed by the groove having this depth value is equal to the upper limit wavelength (λf) of the spectral transmission window of the filter (10); and /4/ obtaining the conductive film (11) with the thickness (e) of said conductive film equal to the thickness value determined in step /2/, and forming the slit (1) in said conductive film, as well as the groove (3) in accordance with the groove depth value (p2) determined in step /3/.

11. The method according to claim 10, wherein the conductive film (11) is metallic, and the thickness value (e) of said metallic film is determined in step /2/ by dividing the individual resonance wavelength λf1 of the first Fabry-Perot resonator by 2.5 times the refractive index value of the first medium contained in the slit (1).

12. The method according to claim 11, wherein the depth value (p2) of the groove (2) is determined in step /3/ by dividing the individual resonance wavelength λs2 of the second Fabry-Perot resonator by five times a refractive index value of the second medium contained in said groove.

13. The 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 (p3) for the so-called additional groove (3) such that the individual resonance wavelength λs3 of the third Fabry-Perot resonator which is formed by this additional groove is equal to the lower limit wavelength (λd) of the filter's spectral transmission window minus half a difference between the wavelengths of the lower (λd) and upper (λf) limits of said spectral transmission window, and step /4/ further comprises forming the additional groove (3) in the conductive film (11) in accordance with the depth value (p3) determined in step /3/ for said additional groove.

14. The method according to claims 12 and 13, wherein the depth value (p3) of the additional groove (3) is determined in step /3/ by dividing the individual resonance wavelength λs3 of the third Fabry-Perot resonator by five times a refractive index value of the third medium which is contained in said additional groove.