Technical Field

The present invention relates to devices for guiding electromagnetic radiation. More particularly, the present invention relates to devices for producing concentrated jets of electromagnetic radiation on the micrometre or nanometre length-scale.

Background of the Invention

In optical devices it is important to be able to accurately control the path taken by electromagnetic radiation within the device. Conventional means for guiding electromagnetic radiation include optical fibres, waveguides, and microlens arrays, which can be controlled to guide and focus light onto particular structures. However, it remains difficult to control the direction of light with high precision at the micrometre length scale and below, which becomes more important as the functional structures within optical devices are miniaturised even further.

The invention is made in this context.

Summary of the Invention

According to a first aspect of the present invention, there is provided a device for guiding electromagnetic radiation, the device comprising: a body configured to transmit the electromagnetic radiation; and one or more cavities formed on a surface of the body, a refractive index within each cavity being less than a refractive index within the body, wherein each cavity is configured to guide the electromagnetic radiation passing through the body to be emitted from a region adjacent to the cavity.

The device can comprise a plurality of cavities configured to guide the electromagnetic radiation passing through the body to be emitted from a region between the cavities, and the plurality of cavities can be arranged such that the region between the cavities has a width less than or equal to 5 times a wavelength of the electromagnetic radiation.

The one or more cavities can be configured to have curved profiles in a cross-section perpendicular to the surface of the body. In the cross-section perpendicular to the surface of the body, the one or more cavities can be configured so that an angle between the surface within one of the cavities and the surface at said adjacent region increases from the centre of said cavity towards said adjacent region.

The one or more cavities can include a plurality of elongate cavities configured to be elongate in a direction parallel to the surface, so as to define one or more elongate regions between the plurality of elongate cavities.

The one or more cavities can include an array of cavities configured in a two- dimensional array on the surface, so as to define one or more localised regions between the cavities within the array.

The cavities within the array can be arranged in a close-packed fashion.

The one or more cavities can contain a material having a lower refractive index than a material from which the body is formed.

The one or more cavities can include a plurality of asymmetric cavities configured so as to cause the electromagnetic radiation to be emitted from a region between the asymmetric cavities at an angle inclined to the normal of the surface at said region.

The device can further comprise an electrically conducting layer disposed over said region adjacent to the cavity.

The device can be disposed in front of a photovoltaic element for converting the electromagnetic radiation to an electrical current, such that during use the

electromagnetic radiation passes through the device before entering the photovoltaic element, and the electrically conducting layer can form an electrode of the photovoltaic element. The device can be disposed in front of a charge coupled device (CCD) such that during use the electromagnetic radiation passes through the device before entering the CCD, and the electrically conducting layer can form an electrode of the CCD.

The device can be disposed in front of a waveguide, and the one or more cavities can be configured to guide the electromagnetic radiation to be emitted from a region aligned to an end of the waveguide, such that during use the electromagnetic radiation enters the body of the device and is guided into the waveguide. For example, the device can be disposed between an optical fibre and the waveguide to guide the electromagnetic radiation from the optical fibre into the waveguide. According to a second aspect of the present invention, there is provided an apparatus for trapping particles suspended in a fluid, the apparatus comprising: a chamber in the fluid flow path of at least one channel, for receiving the particles suspended in the fluid, the chamber comprising at least one device according to the first aspect configured to act as a trapping device, wherein the at least one device according to the first aspect is disposed in a first sidewall of the chamber such that its one or more cavities define a first elongate region that is perpendicular to the direction of flow of the fluid, such that particles suspended in the fluid are trapped.

The apparatus may further comprise a second device according to the first aspect, wherein the second device according to the first aspect may be disposed in a second sidewall of the chamber such that its one or more cavities define a second elongate region that is perpendicular to the first elongate region.

According to a third aspect of the present invention, there is provided an apparatus for controlling the flow of particles in a fluid, the apparatus comprising: at least one channel, wherein the at least one channel comprises a first device according to the first aspect configured to act as an optical tweezing device, wherein the one or more cavities are formed in the base of a respective one of the at least one channel and define a respective elongate region through which the guided electromagnetic radiation is emitted, and said elongate region is arranged to extend along the respective one of the at least one channel to control particles suspended in the fluid flow to selectively flow along one side of the respective one of the at least one channel or the other side of the at respective one of the least one channel, using an optical tweezing effect. The apparatus may comprise a plurality of channels, wherein the elongate region may be arranged to control the particles suspended in the fluid flow to selectively flow along the respective channel of the plurality of channels or another channel of the plurality of channels. The apparatus may further comprise either: the apparatus for trapping particles according to the second aspect; or a second device according to the first aspect configured to act as a trapping device, wherein the second device according to the first aspect maybe disposed in a sidewall of the at least one channel such that its one or more cavities define an elongate region that is perpendicular to the direction of flow of the fluid, such that particles suspended in the fluid are trapped.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates an optical device comprising a plurality of concave low-refractive index cavities, according to an embodiment of the present invention;

Figure 2 schematically illustrates an optical device comprising a plurality of cavities with planar faces, according to an embodiment of the present invention;

Figure 3 schematically illustrates an optical device comprising an electrically conducting layer disposed over a plurality of light-emitting regions, according to an embodiment of the present invention;

Figure 4 schematically illustrates an optical device comprising a plurality of concave cavities containing a low-refractive index material, according to an embodiment of the present invention;

Figure 5 schematically illustrates an optical device comprising elongate cavities defining elongate light-emitting regions, according to an embodiment of the present invention;

Figure 6 schematically illustrates an optical device comprising an array of cavities defining localised light-emitting regions, according to an embodiment of the present invention;

Figure 7 schematically illustrates an optical device disposed in front of a photovoltaic cell, according to an embodiment of the present invention;

Figure 8 schematically illustrates an optical device disposed in front of a charge coupled device, according to an embodiment of the present invention;

Figure 9 schematically illustrates an optical device comprising asymmetric cavities, according to an embodiment of the present invention;

Figure 10 schematically illustrates an optical device disposed in front of a waveguide, according to an embodiment of the present invention; and

Figure 11 schematically illustrates the apparatus for selecting and imaging small-scale particles, according to the present embodiment. Detailed Description

Figure 1 schematically illustrates an optical device according to an embodiment of the present invention. The device comprises a body 100 configured to transmit light, and a plurality of cavities 102 formed on a surface of the body 100. In the orientation shown in Fig. 1, light entering the top of the device is guided through the device and emitted as focussed micro-jets 106 of light from regions 104 adjacent to the cavities 102. The regions 104 from which the concentrated jets are emitted will hereinafter be referred to as 'light-emitting regions'. Although concentrated jets are emitted from the light- emitting regions, in practice a certain amount of light will also be emitted from other parts of the surface in addition to the light-emitting regions themselves. For example, a certain amount of light will leak from the surfaces within the cavities. Therefore the term light-emitting regions' does not necessarily mean that light is only emitted from these regions of the device. Although in the present embodiment the device is configured to guide electromagnetic radiation at optical wavelengths, in other embodiments of the invention a device can be configured to guide electromagnetic radiation at any wavelength, for example microwave or radio-frequency (RF) wavelengths. A refractive index within each cavity 102 is lower than the refractive index of the material from which the body 100 is formed, and the cavities may therefore be referred to as low-refractive index cavities. In some embodiments the cavities 102 may contain a vacuum or a gas. For example, the cavities may be open to the atmosphere, or may be sealed within a device and filled with an inert gas. In other embodiments, the cavities may contain a solid material of low refractive index.

The cavities 102 can have various forms in the direction perpendicular to the plane of the cross-section in Fig. 1. For example, the cavities 102 can be formed as elongate troughs in the surface, or can be formed as spherical hollows in the surface. The cavities can also be referred to as 'depressions' or 'recesses'. Although a plurality of cavities and a plurality of light-emitting regions are illustrated in Fig. 1, in general, any number of one or more cavities and one or more light-emitting regions may be provided. For example, an optical device may include only a single concave spherical cavity, defining an annular light-emitting region around the perimeter of the cavity. In some embodiments, the light-emitting region could be reduced in size by coating part of the region adjacent to the cavity with an opaque material to block transmission of light.

The shape of each cavity 102 is configured to guide light passing through the body 100 to be emitted from the light-emitting regions 104 adjacent to the cavity 102. In the present embodiment the cavities 102 are formed to have a curved profile in a cross- section perpendicular to the surface from which light is emitted, as illustrated in Fig. 1. In the present embodiment the cavities 102 are concave in profile, such that the angle between the surface within one of the cavities 102 and the surface at an adjacent one of the light-emitting regions 104 increases from the centre of said cavity 102 towards said light-emitting region 104. That is, the sides of the cavity 102 become steeper closer to edge of the cavity 102, where the light-emitting region 104 is located. Although in the present embodiment this is achieved with a curved profile, in other embodiments a similar effect may be achieved with a facetted profile.

The surface features in the optical device, that is, the cavities and light-emitting regions, can have widths on the micrometre (μηι) or sub-μιτι scale. Preferably, when a plurality of cavities are arranged to define a light-emitting region between the cavities, the light-emitting region between the cavities has a width less than or equal to 5 times a wavelength (λ) of the electromagnetic radiation. When the distance between adjacent cavities is increased beyond 5λ, the intensity of light within the micro-jet emitted from the region between the cavities decreases by more than 25%. However, each individual cavity can have a width much greater than the wavelength. By way of an illustrative example only, experiments carried out by the inventors have demonstrated the micro- jet effect in devices with cavities between 100 nanometres (nm) and 10,000 μπι in width, and with light-emitting regions between the cavities from o μπι (i.e. adjacent cavities meeting at a point) to 10 μπι in width. However, these dimensions are merely exemplary, and in other embodiments the cavities and light-emitting regions may have widths beyond these ranges. The micro-jets emitted from the light emitting regions can extend for several tens of μπι beyond the surface of the device, and can be tightly focussed with widths of a few hundred nm.

In the present embodiment, the cavities have curved profiles in cross-section. The effect of a curved interface between high- and low-refractive index materials is to provide a more intense and highly collimated light jet at the adjacent light emitting regions. However, micro-jets can still be produced by cavities with inclined planar faces, as opposed to the curved concave cavities shown in Fig. 1. An example of an optical device comprising a plurality of cavities with planar faces is schematically illustrated in Fig. 2, according to another embodiment of the present invention. Like the device of Fig. l, the device shown in Fig. 2 comprises a body 200, a plurality of cavities 202 formed in the surface of the body 200, and a plurality of light-emitting regions 204 from which micro-jets 206 of light are emitted. As in the embodiment of Fig. 1, the cavities 202 illustrated in Fig. 2 may be elongate in the direction

perpendicular to the plane of the cross-section, or may be truncated. Figure 3 schematically illustrates an optical device comprising an electrically conducting layer disposed over a plurality of light-emitting regions, according to an embodiment of the present invention. Like the devices of Figs. 1 and 2, the device of the present embodiment comprises a body 300, a plurality of cavities 302 formed in the surface of the body 300, and a plurality of light-emitting regions 304 from which micro-jets 306 of light are emitted.

In addition, the device comprises an electrically conducting layer 308 disposed over each light-emitting region 304. The electrically conducting layer 308 may be directly in contact with the surface of the body 300, or other intervening layers maybe deposited between the light-emitting region in the surface of the body 300 and the electrically conducting layer 308. Any suitable conductive material maybe used for the electrically conducting layer 308, for example metal, conductive polymer, or carbon. Also, experiments by the inventors have shown that micro-jets are still emitted even through relatively thick layers that would normally be considered opaque.

In the present embodiment a separate electrically conducting layer is provided over each light-emitting region. However, in other embodiments a continuous layer spanning the plurality of cavities and light-emitting regions could be provided, for example by depositing a low-refractive index material within the cavities to form a substantially planar surface on which a continuous conductive layer can be deposited. One such example is illustrated schematically in Fig. 4, which shows an optical device comprising a body 400, a plurality of concave cavities 402 containing a low-refractive index material, a plurality of light-emitting regions 404 from which micro-jets 406 are emitted, and a continuous electrically conducting layer 408 disposed over the cavities 402 and light-emitting regions 404. In Figs, l to 4, optical devices are illustrated in two-dimensional cross-sections. As described above, the cavities can take various forms in three dimensions. Figure 5 schematically illustrates an optical device according to an embodiment of the present invention. The optical device includes a body 500, in the surface of which are formed elongate trough-like cavities 502 defining elongate light-emitting regions 504 between the troughs 502. The cavities 502 are elongated in a direction parallel to the surface. In another embodiment, as shown in Fig. 6, an optical device can comprise an array of cavities 602 configured in a two-dimensional array on the surface of the body 600, so as to define a plurality of localised light-emitting regions 604 between the cavities 602 within the array. In the embodiment of Fig. 6 a close-packed array is shown, but in other embodiments the cavities could be arranged in a different manner, for example as a square grid array.

Referring now to Figure 7, an optical device disposed in front of a photovoltaic element is schematically illustrated, according to an embodiment of the present invention. The optical device is similar to the one shown in Fig. 3, and includes a body 700, a plurality of cavities 702, a plurality of light-emitting regions 704 arranged to emit micro-jets 706, and an electrically conducting layer 708 over each light-emitting region 704. The optical device is disposed in front of a photovoltaic element 710 for converting electromagnetic radiation to an electrical current. By 'in front of, it is meant that during operation of the photovoltaic element, light passes through the optical device before entering the photovoltaic element.

As shown in Fig. 7, the electrically conducting layer 708 disposed over the light- emitting regions 704 forms a front electrode of the photovoltaic element 710. A rear electrode 712 is disposed on the opposite side of the photovoltaic element 710, which can be formed from any suitable semiconductor material. Although the rear electrode 712 is formed as a continuous film in the present embodiment, in other embodiments an array of discrete rear electrodes could be provided. In the present embodiment the bulk of the photovoltaic element 710 is formed from p-doped silicon and an n-doped layer is formed in the surface adjacent to the front electrodes. When a photon enters the photovoltaic element and is absorbed by the semiconductor material, an electron- hole pair is generated. The electrons and holes migrate to the front and rear electrodes, causing an electrical current to flow through an external load connected across the front and rear electrodes. In the embodiment of Fig. 7, the front electrodes of the photovoltaic element are formed as elongate bar-shaped electrodes. Accordingly, elongate trough-like cavities such as the ones shown in Fig. 5 can be used to define a plurality of elongate light- emitting regions, which can be aligned with the elongate front electrodes.

In a conventional photovoltaic cell, light can enter the semiconductor layer at any point along the front surface. When an electron-hole pair is generated far from a pair of electrodes, the electron-hole pair can recombine without being separated and pulled to the separate electrodes. This results in energy being wasted as heat rather than being converted to electrical energy, reducing the overall efficiency. However, by combining the photovoltaic cell with an optical device for producing micro-jets of light, as in the present embodiment, light hitting the front of the optical device can be focussed onto the semiconductor regions directly between a pair of electrodes. This ensures that a higher percentage of electron-hole pairs are dissociated and successfully captured at the electrodes, improving the device efficiency, particularly when excitons have low diffusion lengths.

Although in the embodiment of Fig. 7 a silicon-based photovoltaic cell is used, in other embodiments the device can be combined with other types of photovoltaic cells. For example, the photovoltaic cell could be an organic photovoltaic cell comprising p- and n-doped polymer instead of silicon.

Referring now to Fig. 8, an optical device disposed in front of a charge coupled device is schematically illustrated, according to an embodiment of the present invention. As with Fig. 7, the optical device includes a body 800, a plurality of cavities 802, a plurality of light-emitting regions 804 arranged to emit micro-jets, and an electrically conducting layer 808 over each light-emitting region 804. The optical device is disposed in front of a charge-coupled device (CCD) such that the electrically conducting layer 808 forms an electrode of the CCD. Again, by 'in front of, it is meant that during use light passes through the optical device before entering the CCD.

In embodiments such as the one shown in Fig. 8, any conventional structure can be used for the CCD itself. In the present embodiment, the CCD is formed from silicon and includes a plurality of n-doped channels 810 and a p-doped layer 812. A plurality of potential barriers 814 are formed in the p-doped layer 812, and are configured to define a plurality of potential wells 816. Each potential well 816 acts as a single pixel of the CCD, and the potential barriers 816 prevent photogenerated electrons within one pixel migrating to adjacent pixels.

The cavities in the optical device of Fig. 8 are arranged in a two-dimensional array corresponding to the layout of pixels in the CCD. That is, the cavities are arranged to define a separate light-emitting region for each pixel in the CCD. As shown in Fig. 8, the light-emitting regions 804 are situated in proximity to the centre of a pixel, that is, the centre of a potential well 816 in the silicon layer. This arrangement can improve the light-collecting efficiency of the CCD, by ensuring that light near the periphery of a pixel which would normally hit a potential barrier 814 is instead directed towards the potential well 816 within the pixel, meaning that a higher current will be generated for the same amount of light hitting a pixel.

Embodiments of the invention have been described in which cavities are symmetrically arranged about one or more light-emitting regions, with the result that light is emitted in a direction normal to the surface of the optical device. However, the invention is not limited to this arrangement. Figure 9 schematically illustrates an optical device 900 comprising asymmetric cavities 902, according to an embodiment of the present invention. As shown in Fig. 9, when a plurality of cavities 902a, 902b are formed to be asymmetric about a light-emitting region 904, so as to cause light to be emitted from said light-emitting region 904 at an angle inclined to the normal of the surface at said light-emitting region 904. Specifically, when a smaller cavity 902b is disposed adjacent to a larger cavity 902a, the light-emitting region defined between the asymmetric cavities 902a, 902b emits a micro-jet that is tilted away from the normal, towards the smaller of the two cavities 902a, 902b. The direction of the micro-jet can be accurately controlled by varying the relative shapes and dimensions of the cavities.

Fig. 10 schematically illustrates an optical device disposed in front of a waveguide, according to an embodiment of the present invention. The optical device includes two surface cavities 1002 configured to define a light-emitting region 1004 between the cavities 1002. In the present embodiment the device is disposed between an optical fibre 1010 and a waveguide 1020 patterned on a substrate 1022, although in other embodiments the device could be used to guide electromagnetic radiation from any suitable source into a waveguide. As shown in Fig. 10, the light-emitting region 1004 is aligned with the input end of the waveguide 1020, such that during use the electromagnetic radiation enters the body 1000 of the device and is guided into the waveguide 1020. The optical device receives light across the width of the optical fibre core 1010, and concentrates the light into a micro-jet that can be accurately aligned with the end of the waveguide 1020. This arrangement ensures that a high proportion of light from the optical fibre 1010 is coupled into the waveguide 1020.

Embodiments of the invention have been described in which micro-jets of lights are emitted from light-emitting regions defined by cavities formed in a surface. Various fabrication techniques can be used to form the cavities. For example, the cavities can be formed directly in a substrate by using focussed ion beam (FIB) milling, or by using conventional lithography. As a further example, the cavities could be formed using a reflow technique, for instance by exposing strips of a photoresist layer, etching, and heating the remaining islands of photoresist to allow the material to reflow and form concave troughs between the islands.

A further embodiment of the present invention is illustrated in Fig. 11. In this embodiment, light-guiding devices are integrated into an apparatus for selecting small- scale particles from a fluid flow and steering the particles to an imaging chamber.

Figure 11 schematically illustrates the apparatus for selecting and imaging small-scale particles, according to the present embodiment. The left-hand diagram in Fig. 11 shows the apparatus 1100 in plan view, and the right-hand diagram shows part of the apparatus 1100 in cross section.

The apparatus 1100 comprises a main flow channel 1101, an imaging chamber 1102 spaced apart from the main flow channel 1101, and a side channel 1103 connecting the main flow channel 1101 to the imaging chamber 1102. The side channel 1103 is in fluid communication with both the main flow channel 1101 and the imaging chamber 1102.

A light guiding structure 1104 similar to one of the linear light-emitting ridges shown in Fig. 5 is formed in the base of the main flow channel 1101, the side flow channel 1103 and the imaging chamber 1102. The path of the light guiding structure 1104 initially follows the main flow channel 1101 and then follows the side channel 1103 to the imaging chamber 1102. The light guiding structure 1104 emits a focussed microjet 1105 that extends along the main flow chamber 1105, down the side channel 1103 and into the imaging chamber 1102.

When the apparatus 1100 is filled with fluid, and a fluid flow passes along the main flow channel 1101, a differential pressure between the main flow channel 1101 and the side channel 1103 can be employed to cause suspended particles to preferentially follow the path of the main flow channel 1101, rather than diverting into the side channel 1103. At the same time, the focussed microjet 1105 created by the light guiding structure 1104 in the base of the channels can be used to capture small-scale particles 1106 and control the path of the particles by an optical tweezing effect.

Particles of sufficiently small scale to be affected by the optical tweezing effect can be selectively steered into the side channel 1103 and towards the imaging chamber 1102. When the particle reaches the imaging chamber 1102, a horizontal microjet 1107 formed by another light guiding structure 1108 in a side wall of the imaging chamber 1102 can be used to trap the particle below an imaging lens, for example a microlens 1109. Another horizontal microjet may be formed by a light guiding structure opposite the light guiding structure 1108. Particles are polarised by the light of the microjet, and are attracted to a high intensity region of the microjet situated near the emission point. Therefore by disposing two light guiding structures 1108 such that microjets intersect, a trapping region can formed at the centre of the imaging chamber 1102.

Further still, a vertical microjet may be formed from a light guiding structure in the floor or in the roof of the imaging chamber 1102. In other words, two microjets are arranged to be perpendicular to each other. Having at least two intersecting microjets, either parallel or perpendicular to each other, enables particles to be steered towards a particular region of the trap.

This enables particles in the fluid to be selected for, and then observed under the micro- lens 1109, in a fluid. The apparatus of the present embodiment can, for example, be used as a biological analysis suite.

While Fig. 11 shows the fluid leaving the imaging chamber 1102, in some embodiments this does not occur and the fluid is collected in the imaging chamber 1102. In these embodiments, the light guiding structure 1108 and the horizontal microjet 1107 formed by that light guiding structure are not required. In other embodiments, the light guiding structure 1104 emits a focussed microjet 1105 down the main channel 1101 only. The microjet 1105 is used to guide particles within the fluid flow from one side of the channel to the other. In further related

embodiments, the imaging chamber 1102 is in the path of the main channel 1101. In these embodiments, a second light guiding structure is disposed in the side wall of the imaging chamber 1102, perpendicular to the light guiding structure 1104, so as to trap the small scale particles as explained above. Alternatively, instead of the imaging chamber 1102, the second light guiding structure is disposed in the wall of the channel 1101 so as to trap small scale particles in pockets in the wall or floor of the channel 1101. This may be advantageous when separating unwanted particles from a fluid flow, for example to decontaminate water.

In further embodiments, the main flow channel 1101 is divided into two or more sub channels. Here, a light guiding structure in the floor of the main channel 1101 is used to switch (or redirect) the direction of flow of particles of a particular size down a predetermined one of the sub-channels. The sub-channels may also include a light guiding structure in their respective floors. For example, for a flow of fluid containing particle types A, B and C, the apparatus 1100 is configured to direct particles A into a first sub channel a, direct particles B into a second sub channel b, and direct particles C into a third sub channel c. Each sub-channel may have an imaging chamber 1102, as described above, in its path. Alternatively, each sub-channel may be provided with at least one light guiding structure for forming a horizontal microjet to trap the respective particles. Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the

accompanying claims.