WO2015158882A2

WO2015158882A2 - Resonant-cavity wavelength-selective transmission filter

Info

Publication number: WO2015158882A2
Application number: PCT/EP2015/058362
Authority: WO
Inventors: Olivier Gauthier-Lafaye; Antoine MONMAYRANT
Original assignee: Centre National De La Recherche Scientifique
Priority date: 2014-04-17
Filing date: 2015-04-17
Publication date: 2015-10-22
Also published as: FR3020143A1; FR3020143B1; WO2015158882A3

Abstract

The invention relates to a wavelength-selective resonant-cavity transmission filter (10) comprising: a planar central portion (PC) extending in a horizontal plane XY, comprising: a coupling grating (CG) having a first periodic variation in dielectric permittivity; first and second lateral phase zones (ZL1, ZL2) devoid of periodic permittivity, and placed on either side of the coupling grating (CG) and adjacent to the latter; first and second vertical phase zones (ZV1, ZV2) devoid of periodic permittivity and placed on either side of the coupling grating (CG); first and second lateral high-reflectivity structures (HRL1, HRL2) placed on either side of the central portion (PC), adjacent to the first and second lateral phase zones, respectively, in order to laterally confine the localized mode (O_l); and first and second vertical high-reflectivity structures (HRV1, HRV2) placed on either side of the central portion (PC), for vertically confining the localized mode (O_l) and for reflecting an incident wave (Oinc) having wavelengths belonging to a spectral reflection band (Δλ) comprising the resonant wavelength, said filter forming a resonant cavity for the localized mode (O_l).

Description

Long wave selective transmission filter

with resonant cavity

FIELD OF THE INVENTION

The present invention relates to the field of selective wavelength gratings, and more particularly to transmission networks using a resonance effect at the wavelength selectively transmitted.

The present invention is applicable to wavelengths of the visible optical range, infrared and microwave frequency, typically between 400 nm and 500 μηι.

STATE OF THE ART The wavelength selective optical filters conventionally used in industry are classified according to three main families:

Optical filters in colored glass. These filters are characterized by their very good optical rejection (absorption of unwanted wavelengths of the order of 10 ^"4 to 10 ^{" 8} ). These filters, on the other hand, are not narrow spectrally.

Interferential optical filters. These filters consist of a stack of many thin layers. They can be narrow spectrally, and their filter performance can be optimized as needed. For a complex filtering function, a very large number of layers are needed, typically from 100 to 1500 thin layers.

"Dispersive" optical filters: network or prism filters. This type of filtering must be integrated into a complex optical system to function. Apart from these three large industrial families, a great deal of research work has been concentrated in recent years on resonant network filters called GMRF for "Guided Mode Resonant Filter" in English terminology, as described in US 5598300. These components can operate in reflection or transmission, and consist of a central network acting as a resonant guide, included in a multilayer dielectric structure, each layer having a thickness of λ / 2 or λ / 4. In transmission, an optimal function is obtained when the network is located in a plane of symmetry of the structure. These filters perform an ultra thin spectral filtering typically of λ / Δλ equal to a few hundred to a few thousand for a filter in transmission. A disadvantage of these filters is a low angular tolerance, which means a dependence of the filtered wavelength as a function of the angle of incidence.

More recently, a resonant cavity filter, called CRIGF for "Cavity-Resonator-Integrated Guided-mode-resonance Filter" according to the Anglo-Saxon terminology operating in reflection has been described in patent application JP2012098513, the publication of K. Kintaka , T. Majima, J. Inoue, K. Hatanaka, J. Nishii, and S. Ura, "Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization," Opt. Express 20, 1444-1449 (2012), and is shown schematically in FIG.

This filter comprises a waveguide 2, deposited on a substrate 8, a central coupling network 1 disposed on the guide and two DBR side networks, performing a distributed Bragg-type reflection function (DBR in English for Distributed Terminology). Bragg Reflector). Two phase 3 zones are arranged between the central network and the lateral networks. An incident wave 4 on the filter divides into a reflected part 5 and a transmitted part 6. The coupling network makes it possible to couple the incident wave 4 to a guided mode 7 which propagates in the guide 2 and is reflected on the mirrors DBR. The phase regions are sized to phase the propagative and contrapropagative guided modes to obtain a single peak of spectral resonance in reflection. For such phase zones two situations occur according to the wavelength of the incident wave. Off resonance, the wave does not couple to the guided modes propagative and contrapropagative and is essentially transmitted. At resonance, the wave couples to the guided modes which are then re-transmitted in reflection.

The central network is disposed outside the guide to ensure a non-zero but low and disturbing overlap between the propagative and contrapropagative guided modes and the central network, which ensures a resonance peak in narrow reflectivity.

These wavelength-selective reflective filters can be used as mirrors in a VCSEL laser device for Vertical Cavity Surface Emitting Laser in English terminology or in an Extended Cavity Laser Diode (ECDL) in English terminology).

These filters operate in reflection at normal incidence and therefore can not be integrated directly into the optical system detection system. Their use requires the incident beam to be separated from the return beam by an optical device, typically a beam splitter cube. The object of the invention is to overcome the aforementioned drawbacks, and more particularly to provide a selective wavelength resonant cavity filter operating in transmission. DESCRIPTION OF THE INVENTION

The present invention relates to a wavelength selective resonant cavity transmission filter comprising:

a planar central portion according to a horizontal XY plane comprising:

a coupling network having a first periodic variation of dielectric permittivity,

a first and a second phase lateral zone devoid of periodic permittivity, and disposed on both sides of the coupling network and adjacent thereto,

a first and a second vertical phase zone devoid of periodic permittivity, and disposed on both sides of the coupling network and adjacent thereto, along an axis Z perpendicular to the XY plane, the coupling network and the phase zones being configured to provide coupling of an incident wave of resonant wavelength propagating in the filter with a localized mode of the same wavelength,

first and second structures with high lateral reflectivity disposed on either side of the central portion, respectively adjacent to the first and second lateral phase zones, for laterally confining the localized mode,

first and second vertical high reflectivity structures disposed on either side of the central portion, respectively adjacent to the first and second vertical phase zones, for vertically confining the localized mode and for reflecting an incident wave having lengths of wave belonging to a reflection spectral band comprising the resonance wavelength,

said resonant cavity-shaped filter for the localized mode, capable of radiating a resonance wavelength wave that interferes with the incident waves and constructively reflected in a propagative direction and destructively in a contrapropagative direction, so that the wave transmitted at the resonant wavelength is maximized.

Advantageously, the localized mode corresponds to a stationary wave presenting extrema and zeros of electric field, two successive extrema presenting an opposite sign, and in which the coupling network is arranged so that the dielectric permittivity maxima coincide with extrema of same sign of electric field.

Advantageously, an optical thickness of the central portion in the vertical direction corresponds to an odd integer multiple of a quarter of the resonant wavelength.

According to one embodiment, the filter according to the invention comprises a third and a fourth lateral phase zones and a third and a fourth lateral high reflectivity structures, and a coupling network having a periodic variation of two-dimensional dielectric permittivity, the filter forming a three-dimensional resonant cavity. Advantageously, the filter according to the invention has a plane of symmetry. Advantageously, at least one vertical high reflectivity structure comprises a stack of dielectric layers of optical thickness substantially equal to a quarter of the resonant wavelength. According to one embodiment at least one vertical high reflectivity structure comprises a periodic structure having a period less than the resonance wavelength.

Alternatively, at least one high lateral reflectivity structure comprises a lateral network having a second periodic variation of dielectric permittivity substantially equal to half of the first periodic variation of dielectric permittivity.

According to one embodiment, at least one structure with high lateral reflectivity comprises a polished face comprising a high reflectivity treatment. According to one embodiment, the coupling network comprises an alternation of a high index material and a medium in gaseous or liquid phase, the medium being able to circulate in the filter.

According to one embodiment, the filter according to the invention comprises a plurality of central portions and associated high lateral reflectivity structures, arranged on the same substrate, each central portion having a selected resonance wavelength.

In a variant, the coupling network has a continuous variation of the periodicity of the dielectric permittivity according to an X or Y dimension. According to another aspect, the invention relates to a photo detector comprising a plurality of elementary detectors and comprising a plurality of filters. according to the invention positioned upstream of the elementary detectors, a filter being associated with an elementary detector and having a resonance wavelength chosen in a detection band of the associated elementary detector.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which:

FIG. 1 schematizes a resonant cavity filter in reflection according to the prior art.

FIG. 2 schematizes a filter according to the invention. FIG. 3 illustrates the position of the electric field extrema of the localized mode in the resonant cavity for an optimized filter.

FIG. 4 illustrates the position of the electric field extrema of the localized mode in the resonant cavity for a non-functioning filter.

FIG. 5 illustrates a first variant of a filter according to the invention with a one-dimensional variation of the dielectric permittivity of the coupling network.

FIG. 6 illustrates a second variant of a filter according to the invention with a two-dimensional variation of the dielectric permittivity of the coupling network.

FIG. 7 illustrates a variant in which the pads of the coupling network have a rectangular shape.

FIG. 8 illustrates an embodiment in which the vertical HR structures comprise a stack of dielectric layers.

FIG. 9 illustrates another variant according to which the vertical HR structures comprise a subwavelength periodic structure.

FIG. 10 schematizes a variant of the filter according to the invention having a plane of symmetry.

FIG. 11 illustrates an exemplary filter according to the invention having lateral structures comprising lateral networks.

FIG. 12 illustrates the distribution of the envelope of the electric field of the localized mode.

FIG. 13 illustrates the spectral performances of a filter according to the invention. DETAILED DESCRIPTION OF THE INVENTION

A filter 10 according to the invention is shown schematically in FIG. 2. This filter has a maximum transmission at a resonant wavelength λθ and a reflectivated high for a spectral band Δλ around λθ. The filter according to the invention operates for wavelengths included in the visible, infrared and microwave optical band, typically for wavelengths between 400 nm and 500 μηι.

To describe the operation of the filter 10, consider an incident wave on the Oinc filter, which propagates in the filter 1 0. The filter generates from the Oinc wave, by interferential superposition, a reflected wave Gold and a transmitted wave Ot which propagate in the filter then leave.

The filter 10 comprises a central portion of planar geometry PC along an XY plane. Figure 2 is a sectional plane according to for example a YZ plane. The central portion PC comprises a coupling network CG having a first periodic variation of dielectric permittivity Δε (χ). The central part PC also comprises a first phase lateral zone ZL1 and a second phase lateral zone ZL2. These zones are devoid of periodic permittivity, arranged on both sides of the coupling network CG and adjacent to it. The central portion further comprises a first vertical phase zone ZV1 and a second vertical phase zone ZV2, also devoid of periodic permittivity, disposed on both sides of the coupling network CG and adjacent to it, along an axis Z perpendicular to the XY plane.

The coupling network CG and the phase zones ZL1, ZL2, ZV1, ZV2 are configured to provide a coupling of an incident wave Oinc of resonance wavelength λθ propagating in the filter with a localized mode Oi of the same length wavelength λθ. The way to obtain optimum coupling is detailed below.

The filter 1 0 also comprises a first high-side reflectivity structure HRL1 of interferential nature adjacent to the first lateral phase zone ZL1, and a second high-side reflectivity structure HRL2 of interferential nature adjacent to the second lateral phase zone ZL2. The structures HRL1 and HRL2 are arranged on either side of the central portion PC, and their function is to laterally confine the localized mode Oi (role of lateral mirror).

The filter 1 0 also comprises a first vertical high reflectivity structure HRV1 adjacent to the first vertical phase zone ZV1, and a second vertical high reflectivity structure HRV2 adjacent to the second vertical phase zone ZV2. The structures HRV1 and HRV2 are arranged on either side of the central portion PC in a vertical plane, and have the function of vertically confining the localized mode 0 |.

Thus, the central network CG and the phase zones ZL1, ZL2, ZV1, ZV2 make it possible to couple, that is to say, to excite a localized mode Oi in the central part PC from the incident wave Oinc. This localized mode is confined laterally and vertically by high reflectivity structures HRL1, HRL2, HRV1, HRV2. It is in the presence of a "double cavity", a horizontal cavity defined by the structures HRL1 and HRL2, and a vertical cavity defined by structures HRV1 and HRV2.

Three conditions are necessary to obtain a resonant coupling.

A first condition is that the high vertical reflectivity structures HRV1 and HRV2 are configured to reflect an incident wave Oinc on a spectral reflection band Δλ comprising the resonance wavelength λθ.

Thus off resonance, that is to say for an incident wave Oinc (A) of wavelength λ included in the band Δλ but different from λθ, the filter is equivalent to a conventional high reflectivity interferential structure. Preferably the resonance wavelength λθ is located in the center of the band Δλ, the mode located vertically by the vertical cavity then being perfectly antiresonant with respect to the horizontal cavity.

According to a second condition, at the resonance, ie for an incident wave Oinc (AO) of wavelength λθ, the localized mode Oi is excited in the central partite PC and confined laterally and vertically by the high reflectivity structures . The localized mode takes the form of a standing wave in all directions of the space located in the central part.

The resonance wavelength λθ is a function of the pitch Λ of the coupling network CG. In fact conventionally, the vectors associated with the coupling network, the incident wave Oinc and the localized mode Oi verify the law of the networks which results in an approximate formula according to which: 2π / Λ "2π n _eff / A0

where n _eff is the effective index associated with the localized mode which is calculated from the layer indices, their thicknesses and the shape of the network and corresponds to an equivalent index.

Preferentially, the central network CG constitutes a second-order network for the localized mode Oi of wavelength λθ. It thus makes it possible to ensure a resonant coupling between the incident wave Oinc and the localized mode O | According to a third condition, the horizontal cavity is resonant along the axis of the waveguide, which results in the condition:

Leff x 2TT / n _eff = n. π n natural integer

The effective length Leff corresponds to the geometric length of the horizontal cavity including the lateral phase zones to which is added the penetration depth of the localized wave in the reflective structures HRL1 and HRL2.

All of these three conditions makes it possible to obtain the resonant coupling necessary for the invention, comprising a verticlal cavity antiresonant with respect to a resonant horizontal cavity.

The phase regions ZL1, ZL2, ZV1 and ZV2 are adjusted so that the coupling coefficient K between the incident wave and the localized mode is non-zero and maximized.

K is defined as:

K

E _moc i _e dV (1)

Where Eincident is the incident field of the incident wave Oinc, E _m0 d _e is the field of localized mode and Oi e Δ varying dielectric permittivity. It should be noted that this coefficient K is intrinsically of low value given the exponential attenuation of the incident wave produced by the vertical structure HRV1 HRV2.

We will now describe how the high reflectivity structures and the phase regions forming the cavity on the one hand, and the coupling network on the other hand, are arranged to ensure optimal coupling of the incident wave Oinc of length wavelength λθ with the localized mode Oi in the central part PC. First of all it is appropriate that the vertical optical path, ie the optical thickness of the central part PC (comprising the central part and the vertical phase zones ZV1, ZV2) corresponds to an odd integer multiple of the quarter of a wavelength. resonance (A0 / 4) to ensure that this vertical stack behaves as a reflector with high reflectivity for all off resonance wavelengths of the reflectivity band Δλ. Thus, if we disregard the coupling network CG and the localized mode in the central part, the global stack behaves as a mirror HR over the entire band Δλ.

This condition also has the consequence that the overlap between the incident wave Oinc and the dielectric permittivity variation Ae is small, the vertical structure HR ensuring an exponential attenuation of the incident wave during its penetration into the structure, which ensures the filters a spectral fineness of the resonance peak.

If the vertical optical path is chosen so that it corresponds to an even number of times λΟ / 4, the filter no longer works, it loses its monomode character and fineness. Indeed a whole range of wavelengths in the vicinity of λθ is transmitted through the complete stack by Fabry Pérot effect. The existence of these modes disrupts the operation of the filter, in particular it can make it highly multimode.

With regard to lateral optimization, FIG. 3 illustrates two cases of optimal arrangement, a first case illustrated in FIG. 3a and a second case illustrated in FIG. 3b.

The localized mode corresponds to a stationary wave in all the directions presenting extrema Mv (or bellies) and zeros (or nodes) of electric field E fixed, ie not mobile in the space.

At a given arbitrary instant t, two successive extrema (or bellies) have an opposite sign, and we denote by Mv ^" the bellies corresponding to the negative periods and by Mv ⁺ the bellies corresponding to the positive periods.

If the GC network is ignored, the lateral position of the extrema and zero of the standing wave in the cavity is determined by the lateral optical thickness DL of the resonant cavity between the lateral HR structures. periodic dielectric permittivity e. In the central part PC, there are therefore areas 30 for which the dielectric permittivity e is maximum, corresponding to index maxima.

To obtain maximum coupling, the coupling network CG must be arranged so that the dielectric permittivity maxima 30 coincide with electric field extrema with the same sign as shown in figure 3a for the extrema Mv ⁺ and figure 3b for the extrema Mv ^" .This makes it possible to maximize the overlap integral between the standing wave Oi (E _m0 d _e ) and the coupling network CG (Δ e), which maximizes the coupling coefficient K defined by equation (1) (at the product level

Δ G. E ^ ocle) -

On the contrary, in JP201 209851 3, the value of the coupling coefficient K is controlled and limited to a small value by moving the coupling network away from the guided mode, or by decreasing in the integral of the relation 1 the overlap between E _m0 d _e and Δ e.

FIG. 4 illustrates the case for which the index maxima zones of the CG network are positioned on the stationary mode field nodes. In this case the overlap integral between Emode and Ae is zero and therefore the coupling is zero: the localized mode is never excited in the cavity whatever the wavelength of the incident wave. The structure behaves as if the central part did not support any stationary mode and does not have a resonant transmission peak. A wave of resonance wavelength λθ is radiated by the cavity in the form of losses. This radiated wave interferes with the incident wave Oinc, the transmitted wave Ot and the wave reflected by the vertical high reflectivity structures. The coupling made by the grating and the phase zones is configured so that the interference between the radiated wave and the incidence wave is constructive (wave in the propagating direction) and for the interference between the radiated wave and the wave. reflected are destructive (wave in a contrapropagative sense). The transmitted wave Ot resulting from these constructive interferences is thus maximized at the resonant wavelength λθ, whereas the other wavelengths of the spectral band Δλ are reflected by the vertical high reflectivity (HR) structures HRV1 and HRV2. Thus the coupling is at the origin of the excitation of the stationary mode by the incident wave and is also responsible for the "losses" which ensure the coherent retransmission of this stationary mode. Due to the combination of resonance and interference phenomena, the filter according to the invention has a high spectral fineness, and a large angular tolerance greater than interference filters and resonant cavity filters by reflection.

In addition, the value of the resonant wavelength and the fineness thereof are independent of the angle of incidence on the filter of the wave to be filtered. Transmission is maximum at normal incidence.

This large angular tolerance combined with the transmissive operation makes the use of the filter compatible with a focused beam, and therefore with the insertion of the filter according to the invention in the focal plane of an optical imaging or detection system, or directly in front of a detection system. The filter is also feasible on small surfaces and has a planar geometry, allowing the arrangement of a plurality of filters at different resonance wavelengths in 2D matrix.

In addition the filter comprises few layers (see examples below) compared to interference filters of the same spectral fineness, which allows a simpler manufacturing, and a more reliable and more robust filter.

According to a first variant illustrated in FIG. 5, the permittivity variation Δε (χ) of the coupling network CG is unidimensional, for example according to X. The resonant cavity here realizes a two-dimensional confinement (along the directions x and z).

According to a second variant illustrated in FIG. 6, the dielectric permittivity variation of the coupling network CG is two-dimensional A {x, y), the grating having, for example, a structure of pads 40, typically square or hexagonal. An advantage of two-dimensional structures is their insensitivity to the polarization of the incident wave. Alternatively, the variation of permittivity can of course be three-dimensional with A {x, y, z).

Also shown in FIG. 6 is a third variant in which the filter furthermore comprises:

a third phase lateral zone ZL3 and a third associated high lateral reflectivity structure HRL3

a fourth phase lateral zone ZL4 and a fourth structure with associated high lateral reflectivity HRL4. The filter here forms a cavity providing a three-dimensional confinement. The coupling network CG then has two directions of periodicities making it possible to excite the same guided mode independently of the polarization of the incident wave, according to these two directions of periodicity. Structures with high lateral reflectivity make it possible to confine the cavity mode horizontally in these two directions, the vertical confinement being ensured by zones HRV1 and HRV2.

This geometry is particularly suitable for an arrangement of a plurality of filters according to a matrix structure, sharing the same vertical and lateral HR structures, and the same substrate.

Thus, according to one embodiment, the filter according to the invention comprises a plurality of central portions and associated high lateral reflectivity structures, arranged on the same substrate, each central portion having a selected resonance wavelength, varying to each filters the pitch of the coupling network. The different resonance wavelengths chosen are of course included in the spectral reflectivity band of the vertical HR structure Δλ. The variation of the pitch is, for example, discrete.

According to another embodiment, the filter is made wavelength-tunable by producing a continuously variable pitch grating, that is to say that the coupling network has a continuous variation of the periodicity of the dielectric permittivity according to a dimension, X or Y, for example a fan-shaped array for a one-dimensional structure. The filter / resonance wavelength change is obtained by translation of the filter with respect to the incident wave.

According to a variant illustrated in FIG 7 the pads 50 have a rectangular shape. This structure allows the cavity to resonate at two resonant wavelengths M O, λ20. The two directions of periodicity allow to excite each a mode in its direction, the two modes being at different wavelengths. Each cavity direction is then adjusted independently to resonate over a specific wavelength.

According to an embodiment illustrated in FIG. 8, at least one vertical HR structure, the two (HRV1 and HRV2) in FIG. 8, comprises a stack dielectric layers of optical thickness substantially equal to one quarter of the resonance wavelength, ie A0 / 4. The stack consists of an alternation of layers of different index n _a , n _b .... This alternation constitutes a high reflectivity structure for any incident wave of wavelength in the range Δλ around λθ and incident according to an angle of incidence close to normal.

The vertical phase zones ZV1, ZV2 are, for example, layers of optimized thickness consisting of the material of index close to _nb . The central part outside the phase zones consists of a tri-layer structure Ec. The coupling network CG formed on a substrate having an index n _C c and is disposed between two layers C1 and C2 of the same index

For the localized mode to be confined in the central part, this central part must make a waveguide when it is inserted inside the stack. One solution for this is that n _C c and n _PC are greater than all the indices present in the stack. Another solution is that n _PC or n _G c is greater than the lowest index present in the stack and that the thickness of this high index layer is greater than the other layer thicknesses present in the stack.

According to an embodiment also illustrated in FIG. 8, at least one lateral HR structure (the two structures HRL1 and HRL2 in FIG. 8) comprises a lateral network having a second periodic variation of dielectric permittivity substantially equal to half of the first variation. periodic dielectric permittivity, that is to say that the pitch of the lateral network is substantially equal to Λ / 2. In fact, preferably, the lateral network is a first-order network for the localized mode at the wavelength λθ. The realization of lateral HR structures in the form of identical networks close to the central coupling network has the advantage of making the central portion and the lateral HR structures in a single planar manufacturing step.

The lateral phase zones are zones of constant index, for example equal to the index n _PC or n _C G, the thickness of these zones in the direction of the variation of permittivity of the central network is determined to optimize the coupling as explained previously. According to another embodiment, at least one structure with high lateral reflectivity comprises a polished face comprising a high reflectivity treatment.

FIG. 9 illustrates another variant of the filter according to the invention, according to which at least one vertical high reflectivity structure (the two structures HRV1, HRV2 in FIG. 9) comprises a periodic structure having a period δ less than the length of resonance wave λθ. These components called sub-wavelength networks are known, the wave incident on these structures is decomposed into several Bloch modes interfering destructively in the direction of incidence propagation and thus allowing a high reflectivity structure to be realized on a large scale. spectral range and with high angular tolerance.

FIG. 10 schematizes a preferred variant of filter according to the invention having a plane of symmetry Ps.

A filter having this symmetry can be made from two identical parts each made on a substrate which are then connected, for example by laminating substrates. The two half-structures can thus be manufactured simultaneously on the same substrate which is subsequently cut in half, the two half-structures being thereafter glued face to face. This ensures perfect symmetry, including a perfect match of the HRV1 and HRV2 multilayers, phase zones, both in thickness and composition and positioning of the coupling network in the center of the vertical stack.

Table I below illustrates an example of a stack for a filter according to the invention of resonance wavelength λθ = 81 6 nm based on a glass stack (index 1 .47) / nitride (index 2.04). as shown schematically in Figure 10.

Material Thickness (nm) Denomination Function

Glass - Substrate

Nitride 104 ¼-wave blade Stack HRV1 Silica 144 Blade ¼ wave

Nitride 104 Blade ¼ wave

Silica 144 Blade ¼ wave

Nitride 120 ZV1 Central part PC including coupling network and thickness

Silica 100 Total optical network ¾ wave layer to achieve

Nitride 120 ZV2 a HR stack.

Silica 144 ¼ Wave Blade Stack HRV2

Nitride 104 Blade ¼ wave

Silica 144 Blade ¼ wave

Nitride 104 Blade ¼ wave

Infinite glass Substrate

Table I

The vertical phase zones ZV1 and ZV2 consist of a 120 nm thick nitride layer.

The central layer Ec is a layer of silica, constituted in at least one direction X and / or Y of an alternation of silica (index 1 .54) and air (index 1) of identical thickness (filling ratio of 50%) so as to form the central network and the lateral networks, as shown in FIG. 11a (sectional view) and FIG. 11b (plan view in the XY plane of the networks). The period of the network Λ is equal to 485 nm.

We have:

2 x 120 nm x 2.04 + 100 nm x 1 .22 = 612 nm = 3 x 1/4 x 81 6 nm

With 1.22 average index of the network layer Silica / air.

The phase zones ZL1 and ZL2 separate the central network from the lateral networks and consist of the same material as one of the materials constituting the networks here silica. Their thickness is equal to 0.13.Λ (Λ period of the CG network) is 63.05 nm. The optical thickness of these zones, optimized to obtain a calibration of the field maxima with the same sign on the high permittivity areas of the CG network as explained above, is a function of the optical thickness of the network layer and the phase zones. vertical. FIG. 12 illustrates the envelope of the electric field 90 of the wave coupled in the vertical stack according to Z of the filter, corresponding to the localized mode Oi of wavelength λθ = 81 6 nm. Curve 91 illustrates the distribution of index of the vertical stack of the filter according to Z. The field is located in the central part PC.

FIG. 13 illustrates the spectral performance of the filter of Table I, the transmitted intensity Τ (λ) of the transmitted wave Ot in FIG. 13a and the reflected intensity R (A) of the reflected wave Gold in FIG. 13b when illuminated by an incident wave focused on the filter with an incidence cone of +/- 1 ° 55 °. It can be seen that the filter has a very high spectral selectivity in transmission, despite a non-planar incident wave, with a spectral resonance width of:

δλ / λθ = 0.04% (0.33 nm to 81 6 nm).

The very small value of δλ is due to the resonant effect of the cavity. The optical coupling with the latter adds to the reflectivity a Fano resonance whose spectral width varies proportionally with the coupling factor. The realization of a transmissive filter of the same spectral width is impossible according to a CRIGF type design "Cavity-Resonator-Integrated Guided Mode-resonance Filter". The realization according to a GMRF type design "Guided Mode Resonant Filter" is possible but said filter would then have an angular tolerance well below 0.5 °. Finally, obtaining the same spectral fineness according to a GMRF-type design requires the production of a stack of a larger number of layers, the fineness of the resonance of the invention being increased by the three-dimensional optical confinement of the mode. located. A broad band Δλ around λθ, greater than +/- 15 nm is reflected by the vertical HR structure, the central part PC being configured to be an integral part of the vertical HR structure out of resonance.

The resonance wavelength λ 0 of the filter component is tunable by changing the period of the networks considered, for a given vertical structure HR. For example, the wavelength tunability is obtained by a modification of the central part: mechanical modification of the size of the phase zones ZL1, ZL2, ZV1, ZV2 for example, or modification of the period of the gratings, for example by contraction / expansion, typically thermal, networks, or modification of the effective index of the mode localized (by electro-optical effect on the index of the network layer for example).

The resonance wavelength λο and the spectral resonance width δλ are insensitive to the incidence angle of the incident wave 0 |. The resonance spectral width of the δλ filter is adjustable by modifying the K coupling between the grating, the incident wave and the localized mode, that is to say by modifying, for example, the contrast of index between the two constituent materials. teeth and the trough of the coupling network or the fill factor of the network (that is to say the ratio of the lengths of the teeth and troughs), or the positioning of the network CG compared to extrema of the localized mode. When the angle of incidence is not normal to the filter, only the maximum transmission is impacted: Tmax = T (A0) decreases and therefore Rmin = R (A0) increases.

According to another variant, the coupling network CG and the lateral networks, if any, comprise an alternation of a high index material and a medium in a gaseous or liquid phase (index typically between 1 and 1 .7), the medium liquid or gaseous being able to circulate in the filter. The circulation of the liquid or gas makes it possible to make the index variable (by modification of the type or composition of the liquid / circulating gas for example). The variation of the index of the liquid / gas then allows a tunability wavelength of resonance of the filter. This type of filter is applicable as a tunable filter, or to probe the optical index of the liquid / gas and thus its composition. Finally, the absorption of the liquid / gas at the resonance wavelength is likely to substantially modify the optical response of the component. The filter component according to the invention can be used in this case as an instrument for measuring the absorption of the medium.

According to another aspect, the invention relates to a photodetector comprising a plurality of elementary detectors and comprising a plurality of filters according to the invention positioned upstream of the elementary detectors, a filter being associated with an elementary detector and having a wavelength. of resonance chosen in a detection band of the elementary detector associated. The photodetector according to the invention is applicable in spectroscopy or hyperspectral imaging systems.

Claims

1. Filter (1 0) in wavelength selective resonant cavity transmission comprising:

a planar central portion (PC) according to a horizontal XY plane comprising:

a coupling network (CG) having a first periodic variation of dielectric permittivity,

a first and second phase lateral zones (ZL1, ZL2) devoid of periodic permittivity, and arranged on both sides of the coupling network (CG) and adjacent to it,

a first and a second vertical phase zone (ZV1, ZV2) devoid of periodic permittivity, and disposed on both sides of the coupling network (CG) and adjacent thereto, along an axis Z perpendicular to the XY plane; ,

the coupling network (CG) and the phase zones (ZL1, ZL2, ZV1, ZV2) being configured to provide a coupling of an incident wave (Oinc) of resonant wavelength (λθ) propagating in the filter with a localized mode (Oi) of the same wavelength (λθ),

first and second structures with high lateral reflectivity (HRL1, HRL2) disposed on either side of the central portion (PC), respectively adjacent to the first and second lateral phase zones, for laterally confining the localized mode (Oi )

a first and a second vertical high reflectivity structures (HRV1, HRV2) disposed on either side of the central portion (PC), respectively adjacent to the first and second vertical phase zones, to vertically confine the localized mode (Oi ) and for reflecting an incident wave (Oinc) having wavelengths belonging to a reflection spectral band (Δλ) comprising the resonance wavelength,

said filter forming a resonant cavity for the localized mode (Oi), able to radiate a resonant wavelength wave (λθ) which interferes with the incident waves and is reflected constructively in a propagative direction and destructively in a contrapropagative direction, so that the transmitted wave (Ot) at the resonant wavelength (λθ) is maximized.

2. The filter of claim 1 wherein the localized mode (Oi) corresponds to a standing wave having extrema (Mv) and electric field zeros, two successive extrema having an opposite sign, and wherein the coupling network (CG ) is arranged so that the maxima (30) of dielectric permittivity coincide with extrema of the same sign (Mv ^" , Mv ⁺ ) of electric field.

3. Filter according to one of the preceding claims wherein an optical thickness of the central portion (PC) in the vertical direction (Z) corresponds to an odd integer multiple of a quarter of the resonance wavelength (λθ).

4. Filter according to one of the preceding claims further comprising a third and a fourth (ZL3, ZL4) phase lateral zones and a third and a fourth (HRL3, HRL4) structures with high lateral reflectivity, and a coupling network ( CG) having a periodic variation of two-dimensional dielectric permittivity, said filter forming a three-dimensional resonant cavity.

5. Filter according to one of the preceding claims having a plane of symmetry (Ps).

6. Filter according to one of the preceding claims wherein at least one vertical high reflectivity structure (HRV1, HRV2) comprises a stack of dielectric layers of optical thickness substantially equal to a quarter of the resonant wavelength.

7. Filter according to one of the preceding claims wherein at least one vertical high reflectivity structure (HRV1, HRV2) comprises a periodic structure having a period less than the resonance wavelength (λθ).

8. Filter according to one of the preceding claims wherein at least one structure with high lateral reflectivity comprises a lateral network having a second periodic variation of dielectric permittivity. substantially equal to half of the first periodic variation of dielectric permittivity.

9. Filter according to one of the preceding claims wherein at least one structure with high lateral reflectivity comprises a polished face comprising a high reflectivity treatment.

10. Filter according to one of the preceding claims wherein the coupling network comprises an alternation of a high index material and a gas phase or liquid medium, said medium being able to flow in the filter.

1 1. Filter according to one of the preceding claims comprising a plurality of central portions and associated high side reflective structures, arranged on the same substrate, each central portion having a selected resonance wavelength.

12. Filter according to one of the preceding claims wherein the coupling network has a continuous variation of the periodicity of the dielectric permittivity according to an X or Y dimension.

13. Photo detector comprising a plurality of elementary detectors and comprising a plurality of filters according to the invention positioned upstream of the elementary detectors, a filter being associated with an elementary detector and having a resonance wavelength chosen in a detection band. of the associated elementary detector.