GB2381957A

GB2381957A - Multi-layer E-M radiation filter

Info

Publication number: GB2381957A
Application number: GB0226050A
Authority: GB
Inventors: David Robert Sime Cumming; Timothy David Drysdale; Richard John Blaikie
Original assignee: University of Glasgow
Current assignee: University of Glasgow
Priority date: 2001-11-07
Filing date: 2002-11-07
Publication date: 2003-05-14
Anticipated expiration: 2022-11-07
Also published as: GB2381957B; WO2003041223A1; GB0126737D0; GB0226050D0

Abstract

There is disclosed a filter device 10 for electromagnetic radiation which is tunable across a wide range of the e-m spectrum, eg 30 GHz to 100 THz. The filter device 10 comprises at least two layers of material 6a-c; the layers 6a-c being spaced apart by a medium have a lower dielectric constant and/or electrical conductivity than a dielectric constant and/or electrical conductivity of the material; each layer 6a-c comprising a plurality of teeth 2,4 disposed in a substantially common plane; the layers 6a-c being stacked such that the teeth 2,4 of adjacent wafers interleave. The filter can be tuned for frequency of stopband, width of stop band, and the frequency of a narrow transmission peak.

Description

FILTER DEVICE FIELD OF INVENTION The present invention relates to a filter device, eg to a filter device for electromagnetic radiation. One or more embodiments of the invention relate to a Variable Photonic Band Gap (VPBG) filter device (s), which are intended to operate across at least a portion of the electromagnetic frequency spectrum.

BACKGROUND TO INVENTION Tunable filter devices are currently realised at radio and microwave frequencies and in optical systems. However, tunable filter devices are not currently available for operation in the frequency band between these two regimes that is between 30GHz and lOOTHz.

Below 30GHz, a variety of tunable filter devices are known in the art, and can be realised in both radio and microwave frequency systems, both in planar and in cavity configurations. The upper operational limit on the frequency of these devices is imposed by the properties of the semiconducting materials from which the filter devices are fabricated.

Above lOOTHz, in optical systems, tunable filter devices have been realised in micro-electrical mechanical systems (MEMS) based on Fabry-Perot cavities.

It is among objectives of one or more embodiments of one or more aspects of the present invention to provide a

tunable filter device which preferably will operate across a wide range of the electromagnetic spectrum.

SUMMARY OF INVENTION According to a first aspect of the present invention there is provided an electromagnetic radiation filter device comprising: at least two layers of material; the layers being spaced apart by a medium having a lower or higher dielectric constant and/or electrical conductivity than a delectric constant and/or electrical conductivity of the material; each layer comprising a plurality of teeth disposed in a substantially common plane; the layers being stacked such that the teeth of adjacent layers interleave.

Herein by"interleave"is meant that at least a portion of a tooth of one layer is positioned between at least a portion of two teeth of an adjacent layer when viewed in a direction substantially perpendicular to the common planes of each of the one layer and adjacent layer.

Such a device has been Termed by the inventors as a Variable Photonic Band Gap (VPBG) filter device.

There may be provided means for moving at least one layer relative to at least one other layer.

The means for moving may allow movement of the at least one layer: in a direction perpendicular to the common plane of the at least one layer; and/or in at least one direction parallel to the common plane by the at least one layer.

The at least one direction parallel to the common plane may comprise: a direction parallel to the teeth of the at least one layer and/or a direction perpendicular to the teeth of the at least one layer.

The filter device may comprise means for tuning the filter device so that the filter device may be tuned to pass or reject a particular frequency (ies) from a signal (s) incident upon the filter device.

The means for tuning may comprise the means for moving the at least one layer.

In one or more embodiments distal ends of the teeth of one layer may be further from the common plane of said one layer than distal ends of teeth of an adjacent layer are from said common plane of said one layer. In other words, the teeth of adjacent layers may overlap heightwise.

The means for moving the at least one layer may cause teeth of said one layer to align with teeth of an adjacent layer such that said teeth do not interleave.

The filter device may be adapted to operate within at least a part of the electromagnetic spectrum between 70 GHz to 100 THz According to a second aspect of the present invention

there is provided an electromagnetic radiation filter device comprising: at least two wafers of a material having a relatively high or low dielectric constant and/or electrical conductivity; the wafers being spaced apart by a medium having a relatively low or high dielectric constant and/or electrical conductivity; each wafer defining a plurality of teeth disposed in a substantially common plane; and the wafers being stacked so that the teeth of adjacent wafers may selectively intermesh.

The teeth define grooves there between.

Preferably, the filter device provides means for tuning the filter device so that the filter device may be tuned to pass or reject a particular frequency from a signal incident upon the filter device.

In preferred embodiments, the tuning means may allow the filter device to be tuned continuously across a frequency range.

Preferably, there are provided means for moving at least one of the wafers relative to the other wafer (s). In this way one may vary the degree of intermeshing and/or the lateral alignment in the plane of the wafer.

Preferably, the filter device comprises two or more wafers, and will typically include two, four or six wafers.

In such case at least intermediate wafers adjacent first and second wafers on opposing sides of the

intermediate wafer comprise a first plurality of teeth on one surface of the intermediate wafer and a second plurality of teeth on a second surface of the intermediate wafer.

Preferably, the filter device is operable in the electromagnetic radiation region between frequencies of 70GHz to lOOTHz.

One preferred wafer configuration is the so-called "woodpile"or"logpile", which comprises a stack of orthogonal dielectric gratings. Accordingly, preferred embodiments of the present invention, in which the wafers are movable relative to one another, take the form of Variable Photonic Band Gap (VPBG) filter devices. This configuration may be termed a 3-D configuration.

Another preferred configuration comprises layers, wherein adjacent teeth of adjacent layers are substantially parallel. This configuration may be termed a"2-D" configuration.

The device may comprise layers that are: (a) dielectrics; or (b) conductors.

Since"perfect"dielectrics and"perfect"conductors do not exist, one may instead use, in category (a) semiconductors, and in category (b) highly conductive metals and metal alloys, for example, gold, silver, brass, aluminium.

A"perfect"conductor version of the device can be approximated by coating another material with metal, to a

depth of at least 4.6 electrical skin depths, at which thickness, electromagnetic fields (at the frequency of interest) penetrating the metal will have fallen to 1% of their original magnitude.

The electrical skin depth is equal to (nfp a) -0 5m, where f= frequency in Hertz, pm is the permeability of the material in Henrys/metre, a is the conductivity of the material in Siemens/metre.

In one form the layers may comprise silicon wafers coated in gold.

Preferably, at least one wafer may be formed of a material selected from Silicon (Si), a III-V semiconductor material, eg Gallium Arsenide (GaAs) or Indium Phosphide (InP), quartz, silica, sapphire or an appropriate dielectric medium such as a ceramic. Alternatively the at least one wafer may be formed from a metal or metallic material.

The low dielectric constant or electrical conductivity medium may be a gas, such as air, or oil, or any other displaceable fluid with low dielectric constant and/or electrical conductivity.

Alternatively, the low dielectric constant material may be a solid such as polystyrene foam and the high dielectric constant medium may be Silicone oil.

According to a third aspect of the present invention there is provided an electromagnetic filter apparatus including at least one filter device according to the first or second aspects of the present invention.

According to a fourth aspect of the present invention there is provided a method of filtering electromagnetic radiation comprising the steps of: providing at least a filter device according to the first or second aspects of the present invention; and causing electromagnetic radiation to impinge upon the at least one filter device thereby filtering at least one selected frequency from the electromagnetic radiation.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings which are: Figure 1 a perspective view of a single wafer forming part of an electromagnetic radiation filter device according to a first embodiment of the present invention; Figure 2 (a) a perspective view of a filter device, comprising four of the wafers of Figure 1, according to the first embodiment of the present invention, with the wafers in a first positional setting; Figure 2 (b) a perspective view of the filter device of Figure 2 (a) in a second

positional setting; Figure 3 (a) a schematic view of a wafer groove duty cycle of the filter device of Figure 2.

Figure 3 (b) a band gap map of a six wafer silicon filter device of Figures 2 (a) and (b); Figure 3 (c) a further band gap map of the device of Figure 3 (a); Figure 4 a perspective view of the filter device of Figure 2 (a), including an indication of the defined polarisation directions; Figure 5 (a) a perspective view of a single wafer with integrated mounting lugs forming part of an electromagnetic radiation filter device according to a second embodiment of the present invention; Figure 5 (b) photographic representations of the wafer of Figure 5 (a); Figure 6 (a) a perspective view of a filter device, comprising six of the wafers of Figure 5 (a), according to a second embodiment of the present invention with wafers in a first positional setting; Figure 6 (b) a perspective view of the filter

device of Figure 6 (a) in a second positional setting; Figures 7 (a-e) schematic views of alternate wafer separation modes within a filter device in accordance with a third embodiment of the present invention, during different operational modes; Figure 8 a graphical representation of typical operation of the filter device when operated in the "unison"mode of Figure 7 (b) and having an input polarisation rotation of 450 ; Figure 9 a graphical representation of typical operation of a filter device when operated in the'every alternate wafer"mode of Figure 7 (c) and having an input polarisation rotation of 90 ; Figure 10 a graphical representation of typical operation of the filter device when operated in the every alternate wafer mode of Figure 7 (c) and having a TM polarisation (input polarisation rotation of 0 ) ; Figure 11 a graphical representation of typical operation of the filter

device operated in a preferred"two groups"mode of Figure 7 (d) and having a TE polarisation (input polarisation rotation of 90 ' ; Figure 12 (a) a graphical representation of typical operation of the filter device operated in the preferred two groups mode of Figure 7 (d) and having a TM polarisation (input polarisation rotation of 0 ) ; Figure 12 (b) a transmission coefficient for the filter device of Figure 7 (d); Figures 13 (a-b) schematic side views of a filter device in accordance with a fourth embodiment of the present invention, and showing one actuating arrangement; Figure 14 a schematic side view of a filter device in accordance with a fifth embodiment of the present invention, and showing an alternative actuating arrangement; Figure 15 a schematic illustration of a step in the fabrication of a filter device in accordance with a sixth embodiment of the present invention; Figures 16 (a-c) alternative wafer configurations of

filter devices in accordance with a modified embodiments of the present invention.

Figure 17 a schematic perspective view of a fibre-optic communications component incorporating a filter device in accordance with any of the disclosed embodiments of the present invention; Figure 18 a graphical representation of typical operation of a modified device and having a TM polarisation; Figure 19 a graphical representation of typical operation of the modified device and having a TE polarisation; Figure 20 a graphical representation of typical operation of a further modified device and having a TM polarisation; Figure 21 a graphical representation of typical operation of a further modified device and having a TE polarisation; Figure 22 a schematic diagram of a seventh embodiment of a filter device according to the present invention;

and Figure 23 a band gap map for the filter device of Figure 22.

DETAILED DESCRIPTION OF DRAWINGS Referring initially to Figure 1, there is shown a perspective view of a single wafer 6 comprising a layer from which a tunable Variable Band Gap (VBG) filter device 10 (see Figure 2a) according to a first embodiment of the invention can be constructed. The wafer 6 is in this embodiment, formed of an appropriate dielectric, in this example Silicon, due to the sufficiently low loss and high dielectric constant of the material. Alternatively, the wafer 6 can be a formed of a metal. The wafer 6 is fabricated to a thickness appropriate to the particular application wavelength. In this case to fabricate the wafer 6, conventional photolithography Techniques are used to coat (for example, by spin coating or aerosol methods) and pattern a suitable polymer etch mask, for example, a positive photoresist such as Shipley S1818 photoresist or a negative photoresist such as SU-8 (not shown) onto the side of the Silicon wafer to be etched first. The mask outlines the gratings or grooves 3 to be etched, which are then Textured, or etched, by the method of Deep Reactive Ion Etching (DRIE), using multiple short etches and depositions with alternating CHF3 and SF-based chemistries until the grooves are etched to a depth appropriate to wafer thickness, ideally half the thickness of the wafer 6,

which is generally around 200 to 300 urn. The etching process is then repeated on the other side of the wafer, to form the grating or grooves 5, and the resulting arrangement is a linear, or"logpile"effect grating. Each side of the wafer 6 is thus etched to provide grooves 3,5 respectively formed at right angles to each other.

In this embodiment the grooves 3,5 on each side of the wafer 6 have the same period, or spacing. It is also the case that for grooves 3,5 the ridge/protrusion spacing width: trough/groove spacing width ratio, or mark : gap ratio, is less than 50% to allow a plurality of wafers to be slotted together. Furthermore, in this case the grooves 3,5 have been formed to a depth of half the thickness of the original wafer 6 on both sides. As the grooves 3,5 are at right angles on either side, and thus the ridges 2, 4 remain coupled where the ridges overlap, the wafer 6 does not fall apart.

It should be noted that although the etching is relatively deep, the aspect ratio is not challenging and therefore the side walls of the grooves 3,5 are typically vertical within fui 20. It should also be noted that the DRIE method is anisotropic and requires no specific crystallographic orientation. Once the etching process is completed the etch masks are removed by known methods of stripping the resist polymer with a suitable solvent, such as acetone.

It should be noted that by etching to different depths, that is by different etch depths on either side of

the wafer, a diversity of filter device characteristics may be obtained. A slight under or over-etch due to process tolerances are acceptable. In the case of a lopm underetch a thin membrane will result at the interface between the two gratings on each wafer 6. In the case of filter devices constructed with poorly conducting materials, this membrane has little effect on the operation of the device 10 because its thickness is small with regard to all the other dimensions, including A,, the wavelength of interest.

Where the wafers are made from an electrical conductor, each wafer may either be constructed from metal plate using conventional machining Techniques, such as milling, or by sputtering and optionally electroplating dielectric substrates such that the metal coating is at least several electrical skin-depths thick. Conventional machining is suited to the larger dimensions required at the lower ranges of the frequency band.

To coat one of the Silicon wafers just described, in gold, the following method could be used: Oxygen ash, de-oxidation in HC1 : H202, electron beam evaporation of 50nm of titanium and 5nm of gold, on each side of the wafer. Sputtering of 40nm of gold on each side of the wafer. Followed by electroplating of gold up to the desired thickness, which is equal to, or greater than, 4.6 skin depths or 0. 362f-O 5, which is approximately 1. 2pm at 100GHz.

Reference is now made to Figures 2 (a) and 2 (b) of the drawings which illustrate six wafers, each similar to the

wafer 6 of Figure 1, layered together to form a Variable Photonic Band Gap (VPBG) filter device 10 according to a first embodiment of the present invention. With reference firstly to Figure 2 (a), a plurality of single wafers 6 are layered together with every wafer 6 being alternately orientated by 900 before being arranged on top of one another resulting in the ridges 2 of a first wafer 6a slotting into the grooves 5 of a second wafer 6b, the ridges 4 of the second wafer 6b slotting into the grooves 5 of a third wafer 6c, and so on.

The resulting stacked wafer arrangement behaves as a tunable Variable Photonic Band Gap (VPBG) filter device with a pass band and a stop band determined by the physical dimensions and configuration of the filter device 10, as will be described below. In Figure 2 (a) the filter device 10 is shown with the wafers 6 fully interlocked, in that there are no vertical gaps between adjacent wafers. Tuning of the filter device 10 is achieved by separating the wafers 6a-f by means of mechanical adjustment, examples of adjustment mechanisms being described hereafter.

Figure 2 (b) illustrates the filter device 10 after the wafers 6 have been separated, to provide gaps between the respective wafers 6. The different configuration of the filter device 10 of Figure 2 (b) results in the position of the pass band and stop band changing, in particular the band-edge of the filter device 10 when configured as shown in Figure 2 (b) being of a lower frequency than that of the filter device 10 when configured as shown in Figure 2 (a).

Thus, simply by adjustment of the wafer separation the filter device 10 can be arranged to operate at a different frequency, as desired.

Reference is now made to Figure 3 (a), which illustrates the cross section and dimensions of two interlocking wafers 6a, 6b in more detail. The period of the rods d, and the duty cycle, or width, of each protrusion is shown as d/4, that is 25% of the rod period.

Such relative proportions are chosen for clarity in the illustrations. The effect of the duty cycle of the teeth (rods) on the behaviour of the device, is illustrated in the band gap map of Figure 3 (b), for the case of a silicon six-wafer unison-actuated device with separation of s = d/4, where d is the wafer thickness, and with a period (A) to wafer thickness (d) ratio of A 2.29d. Since in this device setting, the wafers are intermeshed, the duty cycle (r/A) is limited to a maximum value of 0.5. There is no band gap for a duty cycle of zero, since that corresponds to no device being present. For non-zero duty cycles the band gaps move lower in frequency as the duty cycle increases. Note that the frequency F plotted on the dependant axis of the graph, represents the relationship of the actual band gap frequency f in Hz to the dimensions of the device by F = f A/c, where f is frequency in Hertz, d is the period of the grooves, and c is the velocity of light in a vacuum, 2. 998xlO'm/s.

An example of the variation of the position of the

stopband is illustrated in the band gap map of Figure 3 (c), for the case of Silicon six wafer device with a period (A) to wafer thickness (d) ratio of A = 2.29d, where d is thickness of the wafer (ie twice the groove depth). The band gap is calculated for separations in the range 0-d, where 0 < s/d < 0.5 corresponds to separations where the wafers are intermeshed, and 0.5 < s/d corresponds to separations greater than the rod depth, thus the wafers are no longer intermeshed (and are separated). Note that the frequency F plotted on the dependant axis of the graph represents the relationship of the actual band gap frequency f in Hz to the dimensions of the device by F = f d/c, where f is frequency in Hertz, d is the wafer thickness, and c is the velocity of light in a vacuum, 2.998 x 10'm/s.

The spacing of the wafers 6a, 6b is defined as s and is measured from the tip of one rod of the wafer 6a to the bottom of the opposing groove or trench of wafer 6b.

For a dielectric device, the effect of the refractive index on the position of the stopband is illustrated in the band gap map of Figure 3 (d), for the case of a silicon sixwafer unison-actuated device with separation of s = d/4, where d is the wafer thickness, and with a period (A) to wafer thickness (d) ration of A = 2.29d. As the refractive index increases, the stopband moves to lower frequencies.

Thus, if the filter device of Figures 2 (a) and 2 (b) is intended to operate with a stop band centred near 125 GHz, where Silicon has been used to form the wafers 6,

Silicon having a dielectric constant of approximately 12 (n z 3.4), then the device could be constructed with the specific dimensions of period 1.2mm, a rod; groove (mark: space) ratio of 25%, and a wafer thickness of 530 um and a grating depth of 265 pm. One of the advantages of this device is that this is not the only possible combination of dimensions that can achieve the desired band gap. For example, if the wafer thickness is constrained to a thicker value, having the effect of lowering the centre frequency of the band gap, then the period, or rod: groove duty cycle, or both, could be reduced to compensate.

Thus, where Silicon has been used to form the wafers 6, Silicon having a dielectric constant of approximately 13, and for operation at a frequency of 100GHz ( o equals 3mm) the Silicon (n equals 3.4) device has the specific dimensions of a period of 1mm, a rod: groove (mark: space) ratio of 25%, a wafer thickness of 550pm and a grating depth of 275pm.

During the operation of the filter device 10, the separation between the respective wafers is varied by a uniform amount, as required.

In Figure 4 there is illustrated the tunable Variable Photonic Band Gap filter device 10 formed of six wafers 6a to 6f which are stacked"logpile"like together with rods or protrusions 2a to 2e being disposed in grooves 5b to 5e respectively. The grating vector of the filter device is k, which is perpendicular to the uppermost or first set of rods 2f and provides the orientation of the polarisation

direction, which is described as e the angle between k and the electric field vector, where 0 < 0 < 900.

The propagation direction is shown to be normally incident on the front face 11 of the filter device 10.

Reference is now made to Figure 5 (a), which illustrates a wafer configuration for use in a tunable Variable Band Gap filter device according to a second embodiment of the present invention. As with the first described embodiment, the wafer 9 is formed of Silicon, and as before a planar wafer of Silicon has been subjected to etching to form orthogonal grooves 27,29 on either side of the wafer to a depth of 200 to 300 urn. However, in this case integral mounting lugs 30,32 have been retained at opposing edges of the wafer 9. Location holes 24 have been etched throughout the depth of the wafer 9 in a manner such that, once a number of similar wafers 9 have been stacked, the location holes of every second wafer in the stack are ; aligned.

The thickness of the mounting lugs 30,32, and depth of the mounting holes 24, may be varied as desired. The provision of the lugs 30,32 also facilitates manufacture of two or more plates from a single wafer. The etch mask is designed such that all unwanted material is etched and no processing or machining is required subsequent to the completion of the second etch and resist removal: the holes 24 are etched at the same time as the gratings.

Figure 5 (b) shows photographs of actual wafers 9 of Figure 5 (a).

Figures 6 (a) and 6 (b) illustrate a plurality of such wafers 9a to 9f in a stack arrangement forming a tunable Variable Photonic Band Gap filter device lOa according to a second embodiment of the present invention. The wafers 9a, 9f are threaded onto rods (not shown) that ensure the wafers 9a, 9f remain aligned and their"faces"parallel.

The exact configuration of the mounting rods, and the attachment of the wafers 9a, 9f to the rods, will depend on the exact mode of operation of the filter device lOa, as will be described. In Figure 6 (a) the filter device lOa is shown with, the wafers 9a, 9f fully interlocked, that is with no separation between the wafers 9a, 9f. However, in Figure 6 (b) the filter device lOa is shown only partially interlocked.

In the embodiments detailed so far, separation implemented between the individual wafers of the filter devices has been shown as being of equal distance between each wafer. However, a filter device may be arranged in a manner that permits alternative spacing, as will now be described with reference to Figures 7 (a) to 7 (e).

In Figures 7 (b) to 7 (e) there are shown four filter device embodiments, each of which is representative of a different mode of operation of a filter device according to a third embodiment of the present invention. Each of the operational modes controls a separate filter property, as will be described.

With reference firstly to Figure 7 (a), there is shown the"initial position"of elements of the wafers 48a to 48f

from which a filter device 40 is formed; in these Figures the grooves are not drawn (for simplicity) and the wafers are shown un-interlocked. Figure 7 (a) represents schematically the basic initial operational position of the filter device 40, with only a small separation, which includes being interlocked by half the groove depth, between each wafer 48.

With reference to Figure 7 (b), there is shown an example of the"unison"mode, which controls the centre frequency of the stop band of the filter device. When operating in unison mode, one wafer 48a is fixed with the other wafers 48b to 48f being movable with respect to wafer 48a, and each of the wafers being movable apart or together, equidistantly, in the style of an accordion.

This increases the effective period of the logpile, shifting the stop band to longer wavelengths.

In contrast, the filter device arrangement shown in Figure 7 (c) is one in which every second wafer, in this case wafers 48a, 48c and 48e, are fixed in position with wafers 48b, 48d and 48f being moveable, as a group, with respect to the fixed wafers, such that a fixed distance is maintained between the moving wafers. This results in asymmetric spacing between the odd and even wafers of the filter device and creates a different periodicity for the vertical and horizontal grooves and spaces between the wafers. This mode of operation, referred to as the"every alternate wafer mode", controls the width of the stop band of the filter device.

Another mode of operation is illustrated in Figure 7 (d), referred to as the"two groups"mode of operation, which controls the presence and centre frequency of a transmission peak within the stop band. In this arrangement wafers 48a, 48b and 48c are fixed however wafers 48d, 48e and 48f are moveable, as a group, with respect to fixed wafers 48a, 48b and 48c. This arrangement provides equal spacing between the first three and last three wafers of the filter device with a variable separation being possible between the wafers 48c and 4Bd, thus creating a variable size air gap between the two groups.

In this mode, the narrow transmission peak is introduced into the stop band by treating the stack as a Fabry Perot cavity with Bragg mirrors. The addition of photonic crystal structures increases Q by reducing induced current in the lossy Silicon dielectric. Thus a low loss filter device can be constructed in Silicon in the frequency range of interest. Alternatively, photonic crystals made with conductive substrates can be used to realise the reduced loss. For this form of actuation the filter device should be made of an even number of wafers, at least four with two groups of two. The wafers are evenly spaced initially to create a stopband that suppresses all frequencies of interest. The variation of the gap between the two groups of wafers is varied. This alters the frequency of the single mode transmission peak. For frequencies within the stopband, the filter device

performs like a narrow bandpass filter device with a tunable centre frequency. The position of the peak depends

on the rotation of the input linear polarisation. For 9 = 0 , the peak is expected to be lower in frequency than for 9 = 90 . For intermediate rotations, such as 0= 45 , both peaks are expected to be present.

Another mode of operation is illustrated in Figure 7 (e) referred to as the"lateral shift"mode of operations which controls the presence of a defect mode resonance and location of the first passband, when the substrate is a perfect electrical conductor. In this arrangement, wafers 48a, 48c and 48e are fixed, however 48b, 48d, 48f are moveable as group in the lateral direction with respect to fixed wafers 49a, 48c and 48e. This arrangement provides equal spacing between all the wafers in the direction of propagation, but allows the periodicity of the rods to be disturbed in the lateral direction. In this mode, for TE polarisation, the metal substrate device blocks all transmission from OHz to the first passband, in a plasmonlike stop band. The number of peaks in the first passband is related to the number of wafers used in the device, and when two wafers are used there is a narrow passband, the centre frequency of which may be adjusted by shifting the second wafer laterally, in the plane of the wafer. In the second band gap, a defect mode resonance occurs when the wafer is shifted, and its frequency may also be controlled, but to a lesser extent. This mode allows the tunable first

passband to be nearly lossless. In this mode, for TM polarisation, there is no transmission until a narrow, tunable, passband occurs at a frequency above that of the second passband in the TE polarisation. This tunable passband is also of use.

It should be noted that more than one of these modes of operation may be used simultaneously within a filter device arrangement.

In Figure 8 there is shown a graphical representation of the transmission coefficient for a filter device constructed to operate in the"unison mode"as shown in Figure 7 (b). The transmission coefficient is plotted for four different separation positions, in each case with each wafer of the filter device separated by an equidistant amount, the wafer spacing being defined as s in Figure 3 (a). In this illustration the input polarisation rotation is 45 . As can be seen, the centre frequency of the stop band decreases as the separation between the wafers of the filter device, in steps of 80 urn, is increased. In particular, Figure 8 shows a 25GHz shift in centre frequency for a 150pm change in wafer separation. The stop band width is constant for all wafer separations. The same shifting behaviour is produced for all rotations of incident linear polarisation, 00 < e < 900.

In Figure 9 is shown a graphical representation of the transmission coefficient simulated whilst a filter device was operating in the alternate wafer mode, as illustrated in Figure 7 (c). The transmission coefficient is plotted for

four different positions, with the wafer offset being incremented by 50pm between each measurement taken. The input polarisation rotation is 900. As can be seen the width of the stop band is reduced as the wafers are moved further from the initial central position, that is as the offset increases the width of the stop band decreases.

In Figure 10 is shown a graphical representation of the transmission coefficient simulated with the filter device operating in the"alternate wafer"mode of operation, however in this case the input polarisation is 0 . The transmission coefficient is plotted for four different separation positions, that is four different offset positions. As can be seen the width of the stop band decreases as the wafers are moved further from the initial equal separation position, that is as the wafers are increasingly offset. It can also be seen that as the wafer

offset is increased the centre frequency shifts down.

For intermediate polarisation rotations, such as 9 = 450, the response is the sum of the filter device characteristics for 00 < e < 900.

In Figure 11 is shown the graphical representation of the transmission coefficient for the filter device when operating in the"two wafer group"mode of operation, as illustrated in Figure 7 (d). The transmission coefficient is plotted for two different positions, those are wafer 48c

and 48d having a separation of 100m and wafer 48c and 48d having a separation of 200pm. The input polarisation rotation is 900. As can be seen the position of the narrow

transmission peak in the stop band shifts down as the spacing between the two groups is increased.

In Figure 12 (a) the transmission coefficient for the filter device when operating in the"two wafer group"mode is shown in this case with the input polarisation being 0 .

The transmission coefficient is plotted for two different positions of the wafer groups, that is, with a separation

of 100pu between wafers 48c and 48d and a separation of 200um between wafers 48c and 48b. As can be seen the position of the narrow transmission peak in the stop band shifts down in frequency as the spacing between the two groups is increased.

In Figure 12 (b) the transmission coefficient for the filter when operating in the"lateral shift"mode with two metallic wafers is shown in this case with TE polarisation.

The transmitted coefficient is plotted for two different positions of the wafer groups, that is, with all the wafers

in the normal positions of the"log-pile"like arrangement, and with wafers 48 (b), 48 (d), 38 (f) diagonally shifted one quarter of a period. As can be seen, the position of the first passband shifts down in frequency as the lateral shift is increased and a defect mode resonance appears in the second band gap.

Reference is now made to Figures 13 (a) and 13 (b) which illustrate a filter device 60, in particular a Variable Photonic Band Gap (VPBG) filter device in accordance with a fourth embodiment of the present invention, including an actuating arrangement.

The filter device 60 comprises a housing including two retainers 62a, 62b. Four wafers 64 are mounted between the retainers, the lowermost wafer 64a resting on portions of the retainers 62a, 62b, and the other wafers being located and supported by three pairs of wedge wheels 66, the wheels being mounted on keyed spindles 68. Compression springs 70 are provided between the uppermost wafer 64d and a respective portions of the retainers 62a, 62b. With the exception of the lowermost wheels 66a, the wheels 66 may can slide up and down on the spindles 68, but rotate in unison with the spindles 68. As the spindles 68 are rotated, a varying thickness of wedge-shaped wheel 66 is inserted between the wafers 64, and the separation of the wafers 64 changes, in unison. A single annular retainer may alternatively be used within which the wafers 64 are mounted.

Reference is now made to Figure 14, which is a schematic side view of a filter device 80 in accordance with a fifth embodiment of the present invention, and showing an alternative actuating arrangement. In particular, two group of photonic band gap wafers 82a, 82b are fixed to a piezostack 84 via a lever mechanism 86: a typical piezostack provides a very small displacement ( < 1%

of stack length) but a large force (typically lOON). In this example, a 10mm stack 84 provides 3pm of displacement.

Thus, to achieve 30pm of displacement s between the two wafer groups a lever ratio of 1 : 10 is required. This provides 10 N of force, more than sufficient for operation

of the micro-machined structures of the filter device.

It will be appreciated that the invention is also applicable to the near infrared wavelengths used for fibreoptic communications (BOO to 1600nm). Scaling the geometries of the filter device into the sub-micron range, and using dielectric materials that are transparent at these wavelengths (including Silicon) allows wavelength filtering and polarisation control devices to be fabricated. At these much smaller device dimensions the Techniques described above for fabricating individual plates and mechanically interlocking them may no longer be appropriate.

Fabricating devices that operate at these wavelengths may utilise batch processing and micromachining Techniques that have been developed for fabricating Micro-ElectroMechanical Systems (MEMS).

An example of such a fabrication process is shown in Figure 15, which is a schematic illustration of a step in the fabrication of a filter device in accordance with the disclosed embodiments of the present invention. A dielectric substrate material 91 is first patterned with a grating structure 92 of the required depth, period and duty cycle to form the base layer for the device. Upon this patterned substrate 91 is deposited a material to act as a spacer 93, the surface of which is patterned with grooves running parallel to those on the substrate 94, followed by a layer of dielectric material 95, the surface of which is patterned with grooves running either parallel or

perpendicular to those on the substrate 96, depending on the desired mode of operation. This process in repeated until the desired number of layers is achieved. Following this, the sacrificial layers are removed using a chemical etching process.

In order to prevent the device falling apart when the alternating sacrificial layers are removed, a support structure is required, akin the mounting rods for the longer wavelength device shown in Figure 5. Such rods may be formed in a number of ways, for example, by machining cavities through the whole structure and then filling these with a metallic material using an electroplating process.

Actuation of this"self-aligned"device is achieved by piezoelectric or electrostatic control of the separation of the now freestanding dielectric layers. In the case of piezoelectric actuation, small sections of piezoelectric ceramic materials may be added to the layers between theinterlocking dielectric layers during fabrication, and voltages applied to these piezoelectric sections give rise to plate movement. In the case of electrostatic actuation, thin conductive coatings are added to the surfaces of the dielectric layers during fabrication, and application of voltages to these layers causes the plates to move.

Alternatively, the spacing between the layers may be filled by an electro-optic material, or the spacer layer 93 may itself be electro-optic. Application of different voltages to thin conducting coatings on the surfaces of the dielectric layers (which could be added during the layer-

by-layer device fabrication), has the same effect as varying the physical separation between the layers.

Although in the described arrangements groove depths etched on the planar wafers to form the wafers of the device have been described as being equal on both sides, however the grooves may be etched asymmetrically, that is the grooves on one side of the wafer may be etched to a greater depth than the grooves on the other side. Grooves need not be square, nor protrusions right angled, that may instead be rounded or triangular, as illustrated in Figures 16 (a), 16 (b) and 16 (c) of the accompanying drawings.

Once fabricated, a filter device or polarisation control device may be incorporated into the fibre-optic system using conventional methods, such as is shown in Figure 17. The self-aligned VPBG device 97 is mounted with a free-space gap between two optical fibres 98 and 99. A conventional or graded-index (GRIN) lens 100 is used to collimate the light from the input fibre 98 and direct it through the PBG device 97. Another lens 101 focuses the light back into the output fibre 99. The whole arrangement is mounted on a fixed base structure and hermetically sealed within a casing.

Further scaling of device dimensions into the nanometre range provide applications for visible or ultraviolet wavelengths (200 to 800nm) devices.

Referring now to Figures 18 and 19, there is illustrated simulation results for a modified device according to the present invention. The modified device

comprises six wafers of 525 urn Silicon, with gratings (layers) having a period of 1200pm, evenly spaced by s=d/2 (ie on boundary between being intermeshed and being separated). The separation between the two groups is Oum (ie fully intermeshed, s = 0) and 130pm (half intermeshed, s = d/4). There are two peaks that may be of use-one just inside the lower band edge (around 100GHz) and one at around 140 GHz. The lower peak has a higher Q for the TE polarisation.

Referring now to Figures 20 and 21, there is illustrated simulation results for a further modified device dimensionally the same as the modified device according to Figures 18 and 19, but made from a conductor.

One will note in this case that a TM input polarisation provides no transmission, but a TE input polarisation proves a single peak.

Referring now to Figure 22, there is shown a seventh embodiment of a filter device according to the invention.

In this device adjacent teeth of adjacent layers are parallel. This embodiment of the VPBG filter device may therefore be formed in a 2-dimensional (2-D) form.

The 2-D filter comprises wafers with grids lying in a single direction only. The adjacent wafers may still interlock, and thus it may be operated in all the same ways as the preceding"3-D"filter device. For example, the "unison"mode, and"every alternate wafer"modes may be used. Here we show an additional example, where a single layer in the 2-D filter is shifted from side to side. The

actuation scheme of this"Single-layer left-right"-mode is illustrated in Figure 22. The lateral shift, b, is measured from the nominal starting position of the wafer, as shown in Figure 22. For this example, the groove depth is represented by d, the wafer separation by s, and the groove period by A.

The band gap map in Figure 23 was calculated for a loss-less Silicon device (n = 3.4), with seven sets of rods in the propagation direction. The plotted TE and TM band gaps correspond to an attenuation of the transmitted signal exceeding 20dB. The modeling was done on the basis of the crystal having an infinite lateral extent, although the diagram only shows 10 layers across the device. The separation was set at s = d/2, while the period A = 2d, and the duty cycle of the grooves is 25%. The frequency is specified in Terms of the device dimensions in order to allow scaling of the results. In this case, F = f A/c, where F is the number on the frequency axis, d is groove depth in metres, c = 3 x 10"metres/second is the speed of light, and f is the actual frequency in Hertz.

The band gap features shown in Figure 23 are as follows. The lowest TE band gap frequency may be shifted up, by moving the rod away from its starting position.

Towards the extremes of movement, the band splits. In the second TE band gap, a more prominent split is observed, with the frequency of the split increasing with b (the absolute value of b). There is an overlap between the second TE and the first TM band gaps.

Applications for a filter device in accordance with the present invention are found in, for example, communications, where its frequency agile nature reduces the number of components required to serve user channels in a wide band system. The tunable VPBG filter device Technique lends itself to use in reconfigurable switches, cross connects and add-drop multiplexors for wavelength division multiplexing (WDM) systems because a single filter device can be made to select any individual channel at will. Reconfigurable switches allow more effective use of bandwidth than passive switches with fixed filter devices.

In a similar vein, the tunable VPBG filter device Technique will provide miniaturisation benefits and hardware cost savings through part count reduction in secure wireless mobile communications units that employ channel hopping as part of the encryption regime. Only a single filter device is required instead of a fixed frequency filter device for each channel that is part of the hopping sequence.

The tunable VPBG filter device Technique also finds use in high speed wireless local area networks where the tunable filter device Technique will allow fast setup and flexible reconfiguration by enabling each network node's base station to transmit and receive on multiple channels.

This feature is particularly useful for increasing network reliability and speed by reducing data errors due to cross talk between channels in areas such as large office

buildings where the wireless LAN spectrum is highly congested.

The narrow bandpass filter device mode of the present invention will enable new and important spectroscopy systems for use in the THz frequency band, where many molecules have their fundamental resonance. It is currently difficult to generate a narrowband THz signal, thus tunable narrowband THz sources in analogy to the tunable modelocked lasers of fibre-optical systems) are not currently available. In THz spectroscopy of molecules, the tunable filter device is used on the signal generation side to allow illumination of the sample with narrow band sources, and on the output or measurement side, in conjunction with a power meter, to produce frequency specific output data.

Organic imaging, primarily medical, benefits from the tunable filter device because greater image contrast can be achieved with an imaging system that is capable of shifting bands of operation. Greater image contrast allows better and easier identification of tissue types and boundaries.

This information is currently expensive to obtain with existing non-invasive Techniques. Inorganic imaging is also improved with the tunable filter device. For example, in the THz band it is possible to image watermarks in banknotes and semiconductors inside of their packaging. It is also possible to improve security in sensitive areas such as airports by providing THz band information about baggage contents.

Various modifications may be made to the invention as

hereinbefore described without departing from the scope of the invention.

For example, the wafers may be fabricated using various Techniques such as mechanical cutting or milling. It is also the case that the device can be formed having even or odd number of wafers with a minimum of two wafers.

Further, although in the disclosed embodiments the radiation is shown as incident normal to the device, it will be appreciated that the device may operate with radiation incident at angles off the normal.

Claims

CLAIMS 1. An electromagnetic radiation filter device comprising: at least two layers of material; the layers being spaced apart by a medium having a lower or higher dielectric constant and/or electrical conductivity than a dielectric constant and/or electrical conductivity of the material; each layer comprising a plurality of teeth disposed in a substantially common plane; the layers being stacked such that the teeth of adjacent layers interleave.
2. An electromagnetic radiation filter device as claimed in claim 1, wherein there is provided means for moving at least one layer relative to at least one other layer.
3. An electromagnetic radiation filter device as claimed in claim 2, wherein the means for moving allows movement of the at least one layer: in a direction perpendicular to the common plane of the at least one layer; and/or the at least one direction parallel to the common plane of the at least one layer.
4. An electromagnetic radiation filter device as claimed in claim 3, wherein the at least one direction parallel to the common plane comprises: a direction parallel to the teeth of the at least one layer; and/or a direction

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perpendicular to the teeth of the at least one layer.
5. An electromagnetic radiation filter device as claimed in any preceding claim, wherein the filter device comprises means for tuning the filter device so that the filter device is tuned to pass or reject a particular frequency (ies) from a signal (s) incident upon the filter device.
6. An electromagnetic radiation filter device as claimed in any preceding claim where dependent upon any of claims 2,3 or 4, wherein the means for turning comprises the means for moving the at least one layer.
7. An electromagnetic radiation filter device as claimed in any preceding claim, wherein distal ends of teeth of one layer are further from the common plane of said one layer than distal ends of teeth of an adjacent layer are from said common plane of said one layer.
8. An electromagnetic radiation filter device as claimed in any preceding claim, wherein the means for moving the at least one layer can, in use, selectively cause teeth of said one layer to align with teeth of an adjacent layer such that said teeth do not interleave.
9. An electromagnetic radiation filter device as claimed in any preceding claim, wherein the filter device is

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adapted to operate wherein at least a part of the electromagnetic spectrum between 70 GHz and 100 THz.
10. An electromagnetic radiation filter device comprising: at least two wafers of a material having a relatively high or low dielectric constant and/or electrical conductivity; the wafers being spaced apart by a medium having a relatively low or high dielectric constant and/or electrical conductivity; each wafer defining a plurality of teeth disposed in a substantially common plane, and the wafers being stacked so that the teeth of adjacent wafers may selectively intermesh.
11. An electromagnetic radiation filter device as claimed in claim 10, wherein the filter device provides means for tuning the filter device so that the filter device may be tuned to pass or reject a particular frequency from a signal incident upon the filter device.
12. An electromagnetic radiation filter device as claimed in claim 11, wherein the tuning means allows the filter device to be tuned continuously across a frequency range.
13. An electromagnetic radiation filter device as claimed in any of claims 10 to 12, wherein there are provided means

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for moving at least one of the wafers relative to the other wafer (s).
14. An electromagnetic radiation filter device as claimed in any of claims 10 to 13, wherein the filter device comprises at least two wafers.
15. An electromagnetic radiation filter device as claimed in claim 14, wherein the filter device comprises two, four or six wafers.
16. An electromagnetic radiation filter device as claimed in any of claims 1 to 15, wherein at least intermediate wafers adjacent first and second wafers on opposing sides of the intermediate wafer comprise a first plurality of teeth on one surface of the intermediate wafer and a second plurality of teeth on a second surface of the intermediate wafer.
17. An electromagnetic radiation filter device as claimed in any of claims 10 to 16, wherein the filter device is operable in the electromagnetic radiation region between frequencies of 70 GHz to 100 THz.
18. An electromagnetic radiation filter device as claimed in any of claims 1 to 9 or claims 10 to 17, wherein adjacent teeth of adjacent layers are octagonal or parallel to one another.

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19. An electromagnetic radiation filter device as claimed in any of claims 1 to 9 or claims 10 to 17, or claim 18, wherein at least one layer/wafer is formed of a material selected from Silicon (Si), a III-V semiconductor material, eg Gallium Arsenide (GaAs) or Indium Phosphide (InP), quartz, silica, sapphire or a dielectric medium such as a ceramic.
20. An electromagnetic radiation filter device as claimed in any of claims 1 to 9 or claims 10 to 17, or claim 18, wherein at least one layer/wafer is formed from a metal or metallic material.
21. An electromagnetic radiation filter device as claimed in any preceding claim, wherein the lower/low dielectric/constant electrical conductivity medium is selected from a gas such as air or from oil.
22. An electromagnetic radiation filter device as claimed in claim 1, wherein the low dielectric material is polystyrene foam and the high dielectric material is Silicone oil.
23. An electromagnetic radiation filter apparatus including at least one filter device according to claims 1 to 22.
24. A method of filtering electromagnetic radiation

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comprising the steps of: providing at least a filter device according to any of claims 1 to 22 causing electromagnetic radiation to impinge upon the at least one filter device thereby filtering at least one selected frequency from the electromagnetic radiation.
25. An electromagnetic radiation filter device as hereinbefore described with reference to the accompanying drawings.
26. An electromagnetic filter apparatus as hereinbefore described with reference to the accompanying drawings.
27. A method of filtering electromagnetic radiation as hereinbefore described with reference to the accompanying drawings.