WO2002086565A1

WO2002086565A1 - Photonic band gap device

Info

Publication number: WO2002086565A1
Application number: PCT/GB2002/001852
Authority: WO
Inventors: Peter John Duthie; Elizabeth May Ward
Original assignee: Bookham Technology Plc
Priority date: 2001-04-20
Filing date: 2002-04-22
Publication date: 2002-10-31
Also published as: GB0109788D0; GB2374678A

Abstract

A photonic band gap device (44) having a uniform structure comprising a regular variation of dielectric properties across the device with external actuating means (42, 43) to provide local variation in at least one selected region of the index of refraction of the structure so as to change said selected region from a photonic band gap region to a light conducting region or vice versa.

Description

PHOTONIC BAND GAP DEVICE

This invention relates to photonic band gap devices and has particular reference to photonic band gaps that are capable of variation.

Background to the Invention

Photonic band gap structures or photonic crystal devices are known. They are best understood as structures which do not permit the possibility of the existence of photons of certain energies, in the same way that the structure of semiconductor materials prohibit the existence of free electrons of certain energies.

The photonic band gap may relate to a particular polarization state. For ease of understanding it is assumed that the invention is equally effective for electric fields vectors perpendicular or parallel (TE and TM polarization) to the discontinuities (pores or tines) in the photonic band gap material. However, for certain applications the particular TE and TM polarization band gaps may be of significance.

Structures have been proposed in which there are regions with photonic band gaps and regions where photons can exist. Because of this feature photonic band gap structures can act as mirrors. If a photon attempts to cross from a region where it can exist into a region where it cannot exist it is reflected back. A particularly useful function of a photonic band gap would therefore be to act as a boundary for a waveguide, and so would permit the creation of waveguides which could have smaller radius changes of direction than would be possible with, for example, a fibre optic cable, where there is a severe limitation on the radius through which the cable can be bent and still have the desired degree of internal reflection, or an integrated optical waveguide.

Brief Description of the Prior Art

In US Patent 5 172 267, the contents of which are incorporated herein by way of reference, there is disclosed the general concept of a photonic band gap material and the way in which it can act as a reflector.

In USP 5 688 318, the contents of which are incorporated herein by way of reference, there is described a plurality of methods of varying the overall photonic band gap behaviour of a photonic band gap material by thermal excitation, physical compression or extension, electrical field excitation magnetic field variation or by the addition of a different photonic band gap material.

In PCT application WO 98/26316, the contents of which are incorporated herein by way of reference, there is described the incorporation of a non-linear material into specific regions of the photonic crystal during its production so that the resonant frequency can be adjusted after fabrication.

To date photonic band gap materials have proven difficult to make in practice. Although conceptually reasonably simple and capable of operation across the whole electromagnetic spectrum, the structures of the photonic band gap material have to be constructed at a sub-micron scale for them to work at the wavelengths of interest for optical telecommunications. The problems encountered are therefore the practical ones of manufacturing such small structures reliably on a commercial scale.

As a result of the practical problems the photonic band gap materials have yet to be produced and used on a significant commercial scale.

Summary of the Invention

The present invention therefore provides a novel and inventive way of overcoming these problems in a practical manner.

By the present invention there is provided a photonic band gap device of substantially uniform structure comprising a regular variation of dielectric properties across the device and having exteriorly thereof actuating means to provide local variation in at least one selected region of the index of refraction of the structure so as to change said selected region from a photonic band gap region to a light conducting region or vice versa.

The region may comprise an optical pathway.

The region may be permanently varied with respect to the surrounding structure, or may be temporarily or actively varied. The actuating means may be located directly on the surface of the structure or may be spaced from the structure to remotely vary the property.

By having external actuating means to vary the photonic band gap properties, the basic structure of the bulk photonic band gap material can be produced as a standard material in a straightforward manner.

By having the actuating means to vary a region of the structure exteriorly located from the bulk structure, the possibility exists for either permanent variation of properties or variable and even dynamically variable properties. Thus for example a bulk structure can be created and by varying locally the properties a region can be formed which provides a waveguide through the structure.

The structure can be created such that it has properties as a whole that do not permit the presence of photons of certain energies and then varied locally along a track to create a path for these photons through the structure. The variation could on the other hand be reversed so that the bulk structure is transparent to photons of particular energies and is varied locally to create non-permissible regions.

It will be appreciated that the variation can occur on the surface of a substrate, for example by the photonic band gap structure being etched therein and forming flexible tines. Alternatively, the variation can occur within the body of a structure, e.g. where the photonic band gap structure tines are etched into a material, on top of which are grown, or attached, further layers that leave the tine structure intact but encapsulated, and wherein the tines are held at the respective ends yet have sufficient flexibility to move under the influence of the variation inducing means. Other forms of photonic band gap structures are known.

It will be yet further appreciated that the structure could be permanently modified locally for a product of fixed properties or varied dynamically for a product that enables differing properties such as different paths to be created in the structure.

The variation in photonic band gap properties may be by applying sound waves, including sound waves beyond the limit of human hearing, to the structure to cause the structure to distort to vary the properties. The sound waves may be directed such as to form standing harmonic waves in the structure to produce the desired result.

There may be more than one source of sound to create the desired pattern.

In a further alternative, the structure may be distorted by heat to create a pathway through the structure. There may be laid onto the base structure a heating track that can be heated to bring about the local variation.

Alternatively, if the base structure has properties that can be varied by an electrical signal or a magnetic field directly, then such a mechanism can be used to effect the required variation. The structure may be a self-organising structure, and may be based on a colloid, titania, silicon or an air-gel.

The structure may be created using photo-lithography and /or holography in conjunction with etching, or by a moulding technique.

There may be pores in the structure containing a liquid crystal material having a refractive index variable in response to heat a magnetic field or an electrical field.

It will also be appreciated that the invention provides a structure in which some of the regions may be permanently and fixedly different from others.

Brief Description of the Drawings

By way of example embodiments of the present invention will now be described with reference to the accompanying drawings of which: -

Fig 1 is a schematic of transmission vs. wavelength,

Fig 2 is a schematic of a photonic band gap structure with a waveguide therethrough,

Figs 3 to 6 and Fig 11 illustrate the formation of a photonic band gap structure,

Fig 7 illustrates a two-beam illumination process,

Fig 8 shows the pattern of a two beam illumination, Fig 9 shows the pattern of two two-beam illuminations,

Fig 10 illustrates a pattern of a three-beam illumination,

Fig 12 shows a switchable waveguide photonic band gap structure according to an embodiment of the present invention,

Fig 13 shows an acoustically varied photonic band gap arrangement according to another embodiment of the invention, and

Fig 14 shows a surface variable structure that may be used in conjunction with the present invention.

Description of Embodiments of the Invention

The principle of a photonic band gap structure having a forbidden region can be clearly understood from Fig 1. This shows a graph of wavelength on the horizontal axis 1 and transmission of photons in the y-axis 2.

The structure is capable of transmitting all wavelengths below wavelength λi as shown in region 3 and above λ₂ as shown in region 4, but is incapable of transmitting light of any wavelength between λi and λ₂ as shown in region 5. The practical use of such a material is illustrated in Fig 2.

Fig 2 shows a photonic band gap material 6, which has a pathway 7 through it having a different photonic crystal structure including the specific case of uniform structure. The effect of this is that light can pass along the pathway 7 but cannot escape from the pathway into the bulk of the photonic band gap material. As a result the light is reflected along the walls of the pathway 7 and the path acts as a perfect waveguide.

Furthermore because the light cannot escape into the bulk of the photonic band gap material the path need not be straight but can be bent as shown at 8 in Fig 2. The light must follow the path and the photonic band gap material therefore has the possibility of forming a waveguide with a very small bend radius as opposed to the very much larger bend radii that are required for optical fibres and other types of integrated optical waveguides.

There are however very large practical problems in creating the structure shown in Fig 2. This is because the creation of photonic band gap structures requires the formation of structures, which have regular variation of their dielectric properties on a very small scale. Additionally the structures have to be very uniform, i.e. regular, to operate in an efficient and reliable manner.

One method of creating the structures is shown in Figs 3 to 6 and 11. These show the creation of a bulk photonic band gap structure with in-built differentiation between regions.

As shown in Fig 3, onto the base 9 of semiconductor material is deposited a layer of SiON (Silicon Oxynitride) 10 to act as an etch protection layer. Other materials could also be used to act as an etch protection layer. The base 9 itself may be formed as a three layer structure (not shown) with a first layer of InP, a waveguide layer of InGaAsP and a thin capping layer of InP to give a good surface finish. Other semi-conductor materials may also be used for the base 9.

Onto the SiON layer is deposited a layer of photo resist 11 and a mask is used to lay down the pattern of specific regions of the semi-conductor material 9 which will become photonic band gap material, e.g. corresponding to the regions 6 in Figure 2.. The masking process creates the regions 12, 13 of photo resist after exposure to hardening radiation and subsequent development, these regions delineating the areas (e.g. corresponding to the area 7 in Figure 2) which will not become photonic band gap material. For some applications, the photonic band gap regions should be aligned to the crystal plane of the wafer.

The exposed waveguide pattern is then dry etched using conventional dry- etching procedures using a plasma etch in an atmosphere of low pressure CHF₃ and O₂ to leave only the SiON at 14, 15, as shown in Fig 4.

A second layer of SiON 16, Fig 5, is then applied to the wafer covering the whole of the area and a further layer of photo resist 17 is applied, Fig 6. It will be appreciated that the wafer is now profiled with double layers for example 14, 16 of SiON in some areas. The whole structure is then exposed to two or three beam holographic exposure to create the photonic band gap pattern.

As shown in Fig 7, a beam of coherent light 18 from laser 19 is split by beam splitter 20. One half of the light beam is expanded by expansion lenses indicated generally by 21 and directed by mirror 22 onto the surface of the sample 23 being exposed. The other half of the beam is expanded by lenses 24 and directed onto the surface of the sample 23 by mirror 25 where the two beams interfere. This produces the pattern as shown in Fig 8. The eventual pattern of exposure is as shown in Fig 8 with a series of lines 26 which although they appear straight are in fact parts of very large diameter circles.

If a two beam holographic exposure is used with the laser beam split two ways and recombined onto the photoresist as shown in Figure 7 then a single set of lines is produced and the sample is then rotated through 90 degrees to provide an orthogonal array of lines 26, 27 in the form of a mesh as shown in Fig 9. This gives a substantially square array of exposed lines. Those familiar with holographic grating techniques will appreciate that gratings of geometry other than square can also be created. If three exposures are used and the lines are at 60 degrees to one another then an arrangement will be produced with effectively a network of equilateral triangles. Alternatively three-beam holographic interference may be used to generate a pattern of triangles 28, 29, 30 as shown in Fig. 10.

The photonic band gap pattern is then developed from the intersections of these lines which provide a matrix of regular variations.

The photonic band gap structure is then dry etched in two subsequent operations. In a first of the two operations, the same dry etchant as was used originally is used. In this process the wafer carrying the structure is mounted in a sealed chamber which is filled with a suitable gas such as a mixture of CH₃F and O₂ and plasma is used to etch through the silicon oxynitride into the semiconductor material at those regions where there is a single layer 16 of silicon oxynitride. Where there is a double layer 14, 16 the etching process does not reach through the double layer and unetched silicon oxynitride is still present at the bottom of the etch holes. The silicon oxynitride etching process is then replaced with a semiconductor etching process.

The differential etch rates of the mask and substrate materials are such that etching of the SiON mask is negligible during the semiconductor etching.

The result of such a semiconductor etching process is shown in Fig 11 , where the holes such as 31 , 32 are shown etched through into the semiconductor base 9, but a hole, such as hole 33, which has silicon oxynitride 14 at its base will not be etched any deeper. Thus the photonic band gap structure is etched into some areas of the semiconductor base 9 but not into other areas thereof, e.g. where a waveguide 7 such as that shown in Figure 2 is to be formed. This produces the structure with the surface having a plurality of holes or pores 31 , 32 and 33. Whereas holes such as holes like 31 and 32 pass deeply into the semiconductor base material 9, the hole 33 and those like it based on a double layer 14, 16 of silicon oxynitride pass only into the silicon oxynitride layers 14, 16.

The holes can be left empty or can be filled with a dielectric material which has a different dielectric constant from that of the base structure. Different areas could be filled with materials of different dielectric constants. In the present invention, a substantially uniform structure is formed across a device e.g. by a method such as that described above (but without any distinction between areas which are to be light conducting and those which are not).

The dimensions and spacing of the holes in this structure are so chosen that the structure, whilst acting as a photonic band gap material at the desired wavelength and hence being unavailable for photon passage at those wavelengths, is extremely close to ceasing to so act as a photonic band gap material.

To activate the photonic band gap structure in the desired areas an external control is now applied, for example to the underside 34, of the base structure so as to change selected areas from being a photonic band gap region to a transparent or optically conductive region. This external control is provided by external actuating means as described above which are independent of the light to be transmitted through the device.

In Fig 12 there is shown a pair of electrical conductors 35, 36 which are interconnected at one end and which are insulated from the base 34 of the substrate photonic band gap bulk material, by an insulating layer 37.

Electrical connectors 38, 39 permit current to be selectively passed through conductor 35 and similarly connectors 39 and 40 permit current to be passed selectively through conductor 36. By passing a current through either one of the conductors the properties of the photonic band gap material below the conductors is varied either as a result of a change in temperature or as a result of the material being responsive to the change in magnetic field around the conductors to modify the dielectric property materials of the bulk photonic band gap material to form a waveguide which corresponds to the shape of the conductor. If at each end of the conductors there is a photonic band gap waveguide, which is permanent, the system can operate as a switch.

In the embodiment shown in Fig 13 the variation is effected by a pair of piezo electric transducers 42, 43 attached to the bulk photonic band gap material 44.

By setting up standing waves in the bulk photonic band gap material using the piezo-electric devices 42, 43 the dielectric constant of certain regions is varied to give a pathway through the material. Optimum effects can be obtained when the acoustic impedance of the piezo-electric devices and the photonic band gap devices are matched.

It will be appreciated that the actuating means used to alter the refractive index of the selected regions, generally applies the influence which effects this, e.g. the distortion, heat, electric or magnetic field, in a direction transverse to the direction of propagation of light through the device. In many cases, this means that the actuating means is located on, or applies said influence through, a surface of the device other than that through which light enters or exits the device. In the above example, the structure is formed as a photonic band gap material and modified by the external actuating means to become light conducting in selected areas. The arrangement may, however, be reversed with the structure being formed to be light conducting and selected areas modified by the external actuating areas to become non-conducting.

As shown in Fig 14, the surface 45 of the photonic band gap material 46 may be formed with a series of rods 47 or tines by an etching technique similar to that shown in Figs 3 to 10 and then a standing wave 48 can be set up in the surface to give the desired effect of variation in local regions of the dielectric constant so as to form regions where light photons can pass surrounded by regions where the are reflected at the boundary.

Examples of bulk photonic band gap materials for use at telecommunication wavelengths are synthetic materials fabricated out of solid material using photolithographic or holographic techniques in combination with etching techniques; micro-moulding techniques akin to sand moulds with molten solids; micro- moulding techniques in which material exchange takes place, akin to hot molten solid displacing wax.

The bulk photonic band gap material could also be a self organising structure such as that which can be produced utilising a colloidal suspension which is slowly dried to form a regular structure. An example of this is the use of colloidal titania to form air sphere crystals, see for example JEGJ Wijnhoven and WL Vos, Science 281 ,802 (1998).

Silicon based macro-porous photonic band gap materials have also been proposed , see R Hillebrand and W Hergbert Solid State Communications 115 (2000) 227-232. . The silicon based self-organising structures can be fabricated as a triangular lattice see Leonard et al Appl. Phys. Lett 75 3063 (1999) and these lattices may be infiltrated with a liquid crystal material which has a refractive index dependant on temperature. See Leonard et al Physical Review B 3^rd series Vol 61 , No 4 Jan 200pp2389 to 2391.

Aluminium oxide based self-organising structures could also be used.

In an alternative, the self-organising structures can be produced utilising the gel membrane techniques described in USP 6 123 845.

Claims

1 ) A photonic band gap device of substantially uniform structure comprising a regular variation of dielectric properties across the device and having exteriorly thereof actuating means to provide local variation in at least one selected region of the index of refraction of the structure so as to change said selected region from a photonic band gap region to a light conducting region or vice versa.

2) A device as claimed in claim 1 in which the region comprises an optical pathway.

3) A device as claimed in claim 1 or 2 in which the region is permanently varied with respect to the surrounding structure.

4) A device as claimed in claim 1 or 2 in which the region is actively varied.

5) A device as claimed in any one of claims 1 to 4 in which the structure is a self-organising structure.

6) A device as claimed in claim 5 in which the self-organising structure is based on a colloid, titania, silicon or a an air-gel. 7) A device as claimed in any one of claims 1 to 4 in which the structure is created using photo-lithography and/or holography in conjunction with etching.

8) A device as claimed in any one of claims 1 to 4 in which the structure is created by a moulding technique.

9) A device as claimed in any one of the preceding claims in which there are pores in the structure containing a liquid crystal material having a refractive index variable in response to heat a magnetic field or an electrical field.

10) A device as claimed in any one of claims 1 to 9 in which the actuating means is located directly on the surface of the structure.

11 ) A device as claimed in any one of claims 1 to 9 in which the actuating means is spaced from the structure to remotely vary the index of refraction of said selected region.

12) A device as claimed in any one of claims 1 to 11 having a photonic band gap surface on one side thereof and the actuating means is mounted on the opposite side of the device. 13) A device as claimed in any one of claims 1 to 12 in which the actuating means applies a magnetic field to said selected region or heats said selected region.

14) A device as claimed in any one of claims 1 to 4 in which the device has a plurality of tines on its surface capable of movement to form a photonic band gap or become light conducting.

15) A device as claimed in any claim 14 in which sound waves are used to move the tines.

16) A device as claimed in any one of claims 1 to 15 in which the structure is varied locally by mechanical deformation of parts of the structure.

17) A device substantially as described herein with reference to and/or as illustrated in the accompanying drawings.