The present invention relates to the field of spectral filters with membranes structured on a subwavelength scale, and more particularly the field of spectral filters used for the radiation of wavelengths in the infrared spectral band.
Spectral filters are known that are made up of stacks of thin layers (interferential filters). However, since they involve a large number of thin layers, these components exhibit a fragility when they are subjected to temperature variation cycles, for example when they are arranged in a cryostat, notably for applications in the infrared. In practice, these cycles lead to an embrittlement of the structure because of the heat expansion coefficients which differ from one material to the other and therefore from one layer to the other, resulting in stresses between the layers and a risk of delamination by shearing. Furthermore, a filter operating in the infrared will require thicker layers than a filter operating in the visible, and very rapidly there will be technological difficulties linked to the thickness. In particular, the characteristics of the filter (spectral width and position) being directly linked to the thickness, it is extremely complicated to juxtapose different filters on one and the same component, which can prove useful for multispectral applications for example.
For ten or so years now, various theoretical works have predicted singular optical properties for membrane structures formed from subwavelength patterns. In R. Magnusson and S. S. Wang, “New principle for optical filters’, Appl. Phys. Lett., 61(9):1022-1024, 1992, for the first time, a theoretical study demonstrated the possibility of a selective reflection of virtually 100% in a subwavelength dielectric grating deposited on a support. A geometrical resonance mechanism has been revealed by R. Gomez-Medina (‘Extraordinary optical reflection from sub-wavelength cylinder arrays’, Optics Express, Vol. 14, No. 9, May 2006) to explain the reflection peaks in an array of cylindrical rods in the absence of plasmonic modes. Ye et al. (‘Rigorous reflectance performance analysis of Si3N4 self-suspended subwavelengths gratings’, Optics Communications 270 (2007) 133-237) have studied in more detail the influence on the reflection of a polarized incident wave TM of the optogeometrical parameters of a membrane made of dielectric material structured on a subwavelength scale, in a configuration of a self-suspended membrane. These different theoretical studies have shown that, to obtain a reflection that is selective and adjustable in wavelength, it is necessary to have a structure that has a symmetry in relation to a plane parallel to the plane of the membrane, and preferentially in a dielectric environment of low index, typically air. This assumes very strong constraints in terms of manufacture, which has considerably limited the experimental studies of these structures.
Recently, the paper by Gregory Vincent et al., ‘Large area dielectric and metallic freestanding gratings for mid-infrared optical filtering applications’, J. Vac. Sci. Technol. B26(6), 3 Nov. 2008, presented a method for manufacturing metallic or dielectric self-suspended nanostructured membranes, showing the feasibility of bandpass or band-cut filters, notably in the infrared, and possible applications in multispectral infrared cameras.
Nevertheless, the manufacture of suspended membranes produced in this way presents difficulties, notably linked to a fragility of the structure, limiting in particular the size of the membranes. Moreover, it has been proved that, in use, these filters showed a limited stability in their optical efficiency, notably due to the vibrations of the membrane when subject even to the slightest atmospheric disturbances.
One object of the present invention is to present a spectral filter with subwavelength dielectric membrane for the filtering of visible or infrared radiation which notably exhibits a better robustness and a greater stability in optical efficiency in use.
According to a first aspect, the invention relates to a spectral filter suitable for filtering an incident lightwave by reflection of said wave in a spectral band centered on at least one given first central wavelength λ0, the filter comprising a substrate with a through orifice and a membrane formed from dielectric material. The membrane is suspended above the orifice and is structured to form a set of rods organized in the form of a two-dimensional pattern repeated in two directions, the repetition of the pattern in at least one direction being periodic or quasi-periodic, with a first period less than the central wavelength λ0. The organization of the rods of a filter produced in this way has shown, notably compared to the filters of the prior art, significantly enhanced properties of robustness and of optical stability.
For example, and in a nonlimiting manner, the dielectric material is chosen from silicon dioxide, manganese oxide, silicon carbide, silicon nitride, zinc sulfide, yttrium trifluoride, alumina.
According to a variant, the width of a rod is substantially less than λ0/2n where n is the refraction index of the material of which the membrane is formed.
The rods can have a section of substantially square, rectangular or circular form, this last variant making it possible to obtain a filter of greater selectivity.
According to a first embodiment of the first aspect of the invention, the pattern has a form of parallelogram type. The membrane is then structured to form a two-dimensional grating with first rods parallel to a first direction and second rods parallel to a second direction, the first rods being formed by the repetition according to said first period of a first sub-pattern comprising at least one rod.
The first sub-pattern may comprise one or a plurality of parallel rods, making it possible to adapt the spectral response of the filter.
According to a variant, the first direction and the second direction are substantially at right angles.
According to a variant, the second rods are also formed by the repetition according to a second period of a second sub-pattern comprising at least one rod per period.
According to a variant, the second period is less than the central wavelength λ0.
According to a first example, the second period is identical to the first period and the first and second sub-patterns are similar, rendering the structure symmetrical and making it possible notably to produce a filter that is insensitive to the polarization of the incident wave. According to a second example, the second period is different from the first period, allowing, for example, a spectrally selective filtering according to the polarization of the incident wave.
According to a variant, two second adjacent rods are spaced apart by a minimum distance, substantially greater than three times the central wavelength λ0. The filter then has an optical response close to that of a filter with a membrane structured with one dimension, while having an enhanced robustness and reliability.
According to another embodiment, the pattern may comprise rods arranged in at least three different directions, notably making it possible to obtain a better angular acceptance while retaining a certain degree of insensitivity to the polarization of the incident wave.
According to a second aspect, the invention relates to a multispectral matrix comprising a plurality of spectral filters according to the first aspect suitable for filtering different central wavelengths, the membranes of the filters being suspended above one and the same substrate. Such a matrix exhibits a robustness and an optical stability despite the greater dimensions and retains a constant thickness, the filtering wavelength of each filter being determined by the patterns of the structured membrane and not its thickness.
According to a third aspect, the invention relates to an infrared imaging system comprising an infrared detector and a filter according to the first aspect or a multispectral matrix according to the second aspect, said filter or said matrix being used in transmission mode or in reflection mode.
According to a variant, the imaging system comprises means for rotating the filter or the matrix, making it possible to vary the angle of incidence of the incident wave on said filter(s) in order to obtain one or more wavelength-tunable filters.
According to a fourth aspect, the invention relates to a method for manufacturing a spectral filter suitable for filtering by reflection of an incident wave in a spectral band centered on at least one first given central wavelength λ0 comprising:
According to a variant, the method also comprises an isotropic etching of the rods, for example by immersion of the duly obtained filter in a solution of a diluted acid allowing for a controlled attack of the material of which the rods are made in order to round and/or reduce the section of said rods in a controlled manner.
Other features and advantages of the invention will emerge from reading the following description, illustrated by the figures in which:
The membrane is formed from dielectric material. “Dielectric material” should be understood to mean, generally, a material or a stack of materials whose dielectric permittivity has a positive real part and an imaginary part that is zero or very low compared to the real part.
The membrane is structured to faun a set of rods organized in the form of a two-dimensional pattern, the pattern being repeated in two directions. The pattern may comprise rods arranged in two directions, it is then, for example, or parallelepipedal, rectangular or square form. It may take other forms, with rods arranged in at least three directions, for example a hexagon form, or indeed exhibit a complex structure with rods arranged according to an outline and within this outline, as will be described hereinbelow. In
The method thus described makes it possible to obtain a suspended structured membrane 30, the two-dimensional pattern of which makes it possible to confer a rigidity on the structure. Notably, the presence of rods arranged in different directions makes it possible to prevent a transversal movement of the rods in case of vibrations during use. The applicants have thus found a significantly better stability in optical performance levels, making it possible to test filters produced in this way in conditions of use, which had not been possible hitherto with the suspended membranes of the prior art.
With the method described previously, rods are obtained with a section that is substantially square or rectangular. According to a preferred variant of the method, it is possible to obtain rods whose sectional form tends toward a rounded form. For this, the sample undergoes an isotropic etching of its rods, for example by dipping it in a solution of a diluted acid, which chemically attacks the material of which the rods are made. The isotropic etching is faster on the edges of the rods. It makes it possible to round and then reduce the section of the rods in a controlled manner. Rods of very small sections can thus be manufactured easily. In the case of rods of silicon nitride, this chemical etching can be done, for example, in a dilute solution of hydrofluoric acid (HF), for a few minutes.
The applicants have shown that rods with a substantially round section allowed, notably by reduction of the size and of the roughness of the rods, for a better selectivity in the filtering function.
The comparison of the spectra 41 and 42 in
As with a structured membrane in one dimension, the cut-off wavelength depends on the period of the rods 32 spaced apart with a subwavelength period and the filter obtained is polarizing, only the polarization TE being reflected by the resonant mechanism. On the other hand, the wave transmitted at the cut-off wavelength is polarized according to the polarization TM. Such a filter can be used in transmission mode (band-cut filter) or in reflection mode (bandpass filter), for example in an imaging system.
The applicants have shown that the resonant reflection mechanism revealed both by theory and experimentally could be explained by a multiple scattering mechanism. In other words, for a given wavelength which depends on the geometry of the structure, a coherence is observed between the waves scattered by each of the rods of the structure. This is reflected in the observation at said wavelength of a specular reflection, said wavelength depending on the angle between the incident light and the normal to the plane of the membrane, as is shown in
According to an exemplary application, such a filter can be used to analyze the polarization of a scene. For example, the polarization analysis system may comprise an infrared imaging system with said spectral filter optimized for filtering at a given cut-off wavelength in the infrared spectral band, a detector sensitive to the cut-off wavelength of the filter and a device for rotating the polarization of the incident wave. If the incident wave comprises a component with a linear polarization, which is, for example, the case of an infrared radiation emitted by an artificial object (of vehicle or building type for example), the signal measured in transmission mode will be variable with the position of the polarization rotation device (and minimal, for example, when the incident polarization is TE). If the incident wave is purely non-polarized (typically the case of an infrared radiation emitted by a natural object, of vegetation type), the signal in transmission mode will be constant regardless of the position of the polarization rotation device.
According to another variant of the invention, second rods 34 can be arranged periodically according to a period T2 of the order of the period T1 of the first rods 32. The periodic arrangement of the first rods 32 in a direction D1 with a period T1 makes it possible to obtain a filtering effect around a first cut-off wavelength λ1 that is a function of T1 for a component of the incident electrical field parallel to the direction D1. The periodic arrangement of the second rods 34 with a period T2 close to T1 makes it possible to obtain a filtering effect at a second cut-off wavelength λ2 close to λ1 for a component of the incident electrical field parallel to the direction D2. A spectral filter with a membrane structured in this way allows, for example, for a selective wavelength filtering, produced by selecting the polarization of the incident wave.
As for the examples described previously, such a filter can be used in reflection mode or in transmission mode, for example in an imaging system.
According to another variant of the invention, the membrane can be structured to form a two-dimensional grating with first rods 32 parallel to a first direction and second rods 34 parallel to a second direction, the first rods being formed by the repetition according to the first period (T1) of a first sub-pattern 320 comprising a plurality of rods. Such a structure makes it possible to obtain a multi-resonant filter for a component of the electrical field parallel to the direction of the first rods.
In the embodiment illustrated in
In other embodiments, the first sub-pattern and/or the second sub-pattern may comprise more than two rods. The rods of the first sub-pattern and/or of the second sub-pattern may be spaced apart regularly within the sub-pattern or be spaced apart irregularly and may be of different section. These adjustments make it possible to adapt the spectral response of the nanostructured membrane to obtain specific optical effects.
For example, the structure may be symmetrical relative to the bisector of the directions D1 and D2 with the same number of sub-patterns per period, making it possible to produce a spectral filter that is insensitive to the polarization.
According to a variant, the pattern according to which the rods are organized may be repeated quasi-periodically, that is to say with a period with slow variation. In practice, it appears that the filtering function is effective when the number of repetitions of the pattern is at least equal to the quality figure of the filter, defined as the ratio of the central filtering wavelength to the spectral width at mid-height. Thus, typically, for a filter suitable for filtering at 3 μm and a spectral width at mid-height of 0.1 μm, the aim will be to arrange at least thirty rods in the direction of periodicity (for a simple pattern consisting of one rod). The applicants have shown that if the period varies slowly, that is to say by a value substantially less than the spectral width at mid-height for a number of rods substantially equal to the quality figure, it would be possible to retain the filtering function while making the filtering wavelength slip. For example, the variation of the period may be a linear function of the distance, in the direction of periodicity of the pattern.
It is then possible to produce, for example for a spectro-imager function, a filter structured in two directions. In the first direction, the quasi-periodic repetition provides a filtered response for which the cut-off wavelength λ0 varies continuously from one end to the other of the filter, covering an entire spectral range. For example, a filter 10 mm long in this first direction makes it possible to cover the entire transmission band II of the atmosphere (3 to 5 microns) with a spectral offset of Δλ/5 over Q rods where Q is the quality figure and Δλ the width at mid-height of a periodic filter. In the second direction, a minimum periodicity substantially equal to three times the wavelength provides, for example, a non-filtered transmission.
Although described through a certain number of detailed exemplary embodiments, the spectral filter and the method for producing the spectral filter according to the invention comprise different variants, modifications and refinements which will obviously become apparent to the person skilled in the art, given that these different variants, modifications and refinements fall within the scope of the invention, as defined by the following claims.
Number | Date | Country | Kind |
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1055226 | Jun 2010 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/060694 | 6/27/2011 | WO | 00 | 3/11/2013 |