Surface emitting laser

Abstract

A surface emitting laser for example, a photonic crystal surface emitting laser (PCSEL) 14 (Figure 2a) comprises an active layer within which radiation is generated and coupled into a photonic crystal layer 2a (see Figure 2b) is provided. The photonic crystal layer 2a comprises a first array of scattering centres 7 configured to define two or more fundamental lattice vectors 8a, 8b (see Figure 2b) of the photonic crystal layer 2a and to redirect a portion of optically coupled light out of the device plane to form an output field of the surface emitting laser 14. A reflector 13 bordering the photonic crystal layer acts to reduce in plane leakage of the optically coupled light from the photonic crystal layer. The relative orientation of the two or more fundamental lattice vectors 8a,8b and the reflector 13 acts to cross couple the optically coupled light between different fundamental lattice vectors of the photonic crystal layer 2a. The resulting output beam produced by the surface emitting laser exhibits a high quality gaussian profile.

Description

The present invention relates to the field of surface emitting laser devices and methods for manufacturing and operation of these devices. In particular, the present invention relates to a photonic crystal surface emitting laser (PCSEL) device. Semiconductor laser devices are solid-state lasers based on semiconductor gain media, where optical amplification is usually achieved through stimulated recombination of charge carriers. Most semiconductor laser devices are laser diodes based on a semiconductor gain medium, which is pumped with an electrical current in a region where n-doped and p-doped semiconductor materials meet. As the photon energy of a laser diode is close to the bandgap energy, compositions with different bandgap energies allow for different emission wavelengths. There is a great variety of semiconductor laser devices, spanning wide parameter regions and many different application areas. Fabry Perot (FP) lasers are the original, semiconductor laser device technology. In these devices the laser feedback and emission are both in-plane, and the gain reflection is produced by facet mirrors such that the generated output light comes out of an end of the laser. For this reason, FP lasers are often referred to as edge emitting lasers (EEL). An alternative type of EEL laser is a Distributed Feedback Laser (DFB). DFB lasers also have in-plane feedback and emission, but in these devices the gain reflection is produced by the employment of a grating structure. Another type of known semiconductor laser device technology is Vertical Cavity Surface Emitting Lasers (VCSELs). The laser resonator of a VSCEL consists of two distributed Bragg reflector (DBR) mirrors, with an active region arranged between the DBR mirrors. Within a VSCEL, both the laser feedback and emission are both out of plane, where the laser output light emits from a top surface of the laser. Photonic crystal surface emitting lasers (PCSELs) are a newer class of semiconductor laser device. PCSELs have been found to have beneficial properties including coherent oscillation, and low divergences of emitted light. PCSELs are also the only semiconductor laser design that employs in-plane feedback and out of plane, surface emission. A simplified PCSEL structure, as generally depicted by reference numeral 1, is presented in Figure 1. In particular, Figure 1 (a) presents an exploded perspective view of the PCSEL 1, while Figure 1(b) presents a top view of a photonic crystal layer 2 of the PCSEL. Axes are provided within Figure 1 to assist the following description of the operation of the PCSEL 1. The PCSEL 1 can be seen to comprise a semiconductor substrate 3 upon which the other layers of the PCSEL 1 are formed. The other layers of the PCSEL 1, as illustrated in Figure 1, are the lower cladding layer 4, the active layer 5, the photonic crystal layer 2, and the upper cladding layer 6. As will be appreciated by a person skilled in the art, one or more further intermediate layers may also be arranged within the PCSEL 1, as required. The purpose of the lower 4 and upper 6 cladding layers is to confine the light generated in the active layer 5. The lower 4 and upper 6 cladding layers may comprise first and second graded cladding layers, where the incorporation of graded cladding layers is found to provide for better electrical conduction through the PCSEL 1 and for better optical confinement within the active layer 5. The active layer 5 of the PCSEL may contain quantum wells and or quantum dots. For example, it may contain one or more of InGaAs / GaAs quantum wells, InAs / GaAs quantum dots, GaAs / AIGaAs quantum wells, InGaAsP quantum wells and AllnGaAsP quantum wells, although many other active layer designs are known to those in the art. The photonic crystal layer 2, as depicted in Figure 1(b), comprises a semiconductor material, such as InGaP. However, the photonic crystal layer 2 may comprise an alternative material, such as InGaAsP or any other suitable semiconductor material. As shown in Figure 1(b), the photonic crystal layer 2 comprises an array of scattering centres 7 formed by patterning the semiconductor material with periodic regions having a second refractive index which is different from the first refractive index of the semiconductor material (commonly referred to in the art as “atoms”), thus forming a periodic lattice of scattering centres 7 within the photonic crystal layer 2. The scattering centres 7 (or atoms) having the second refractive index, different from the semiconductor material, may be provided by air gaps or voids. Alternatively, the scattering centres 7 (or atoms) may be filled by overgrowth with a suitable filler material having a second refractive index (e.g., GaAs). The lattice structure formed by the scattering centres 7 of the photonic crystal layer 2 define two fundamental lattice vectors 8a and 8b of the photonic crystal layer 2 that are substantially perpendicular. The photonic crystal layer 2 is optically coupled to the active layer 5 causing Bragg diffraction of the optically coupled light within the photonic crystal layer 2, which in turn causes the optically coupled light to primarily resonate in the photonic crystal layer 2 along the two substantially perpendicular fundamental lattice vectors 8a and 8b at a particular wavelength determined by the periodicity, or lattice constant, of the photonic crystal layer 2. A portion of light is also directed out of the plane the photonic crystal layer 2 by the lattice structure. Electrical contacts 9a, 9b are located on the external surfaces of the PCSEL 1. The first electrical contact 9a is located on the output surface 10 of the PCSEL 1 and the second electrical contact 9b is provided adjacent to the lower cladding layer 4. As a result of the above-described structure, when an electrical current is provided between the first 9a and second 9b electrical contacts, the PCSEL 1 begins to lase and an out output field 11 is emitted from the output surface 10. As shown in Figure 1, the first electrical contact 9a is square shaped, with an aperture 12, through which output light 11 of the PCSEL 1 is extracted when the PCSEL 1 is lasing. However, the first electrical contact 9a may be any appropriate shape comprising a central aperture through which the output field 11 can be emitted. As shown in Figure 1 (b), reflectors 13 may be located at the edges of the PCSEL 1 to reflect light generated by the PCSEL 1 and thus contain this generated light within the photonic crystal layer 2. The reflectors 13 are typically arranged to reflect back along the x and the y axis of the PCSEL 1 which coincide with the two fundamental lattice vectors 8a and 8b of the photonic crystal layer 2, as presented within Figure 1. The reflectors 13 thus act to improve confinement and minimise light leakage from the PCSEL 1 by redirecting the light that would otherwise exit the PCSEL 1 in a direction parallel to the plane of the photonic crystal layer 2 i.e. the x-y plane. Desirable qualities of the PCSEL 1, for most applications, include achieving a high output beam quality, minimising thresholds, while maximising the overall device efficiency. A low threshold enables lasing with minimal power, while a high efficiency ensures a significant portion of the input power is converted to useful light output, allowing for increased power output. Therefore, the development of a PCSEL that can improve these attributes are important for the use of these devices for practical applications. For some PCSEL devices of the design presented in Figure 1, it has been found that the far field emission profile of the output field 11 takes the form of a cross or an ellipse rather than a desired gaussian emission profile. This is an undesirable effect on the output beam quality which can restrict the commercial applications of the PCSEL 1. Furthermore, the mode diameter of the output field 11 generated by the PCSEL 1 is an important parameter in applications where specific spatial characteristics of the beam are required. The mode diameter of the output field 11 varies depending on the specific design of the PCSEL 1 and its operating conditions. Typically, the mode diameter of the PCSELs 1 output field 11 is generally in the range of 100’s of micrometres, this value being constrained by the dimensions of the device. If the dimensions of the PCSEL 1 are reduced to correspondingly reduce the mode diameter of the output field 11 then there is an associated reduction in power and efficiency of the PCSEL 1. Therefore, to ensure sufficient coupling strength for maintaining high power and efficiency there is a lower limit to the dimensions that can be adopted for the PCSEL 1. An area where the lower limit of the mode diameter of the PCSELs 1 output field 10 is found to be problematic is in the fields of silicon photonics or optical fibres. Silicon photonic gratings and optical fibres often require significantly smaller mode field diameters, typically in the range of tens of micrometres. As such, additional optical elements and beam shaping components are necessary at the output of a PCSEL 1 to achieve coupling into these devices. The inclusion of these further components has a number of disadvantages, including variations in the direction of the output beam and optical losses, while resulting in a more complicated and larger laser system. Summary of Invention It is therefore an object of an embodiment of the present invention to provide an alternative photonic crystal surface emitting laser (PCSEL) to those known in the art. It is a further object of embodiments of the present invention to provide a surface emitting laser that obviates or mitigates one or more drawbacks or disadvantages of the prior art. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the present invention there is provided a surface emitting laser comprising: a photonic crystal layer defining a device plane of the surface emitting laser; an active layer within which radiation is generated; the photonic crystal layer being optically coupled to the active layer to provide optically coupled light within the photonic crystal layer; the photonic crystal layer having a first array of scattering centres configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of optically coupled light out of the device plane to form an output field of the surface emitting laser; a reflector bordering the photonic crystal layer to reduce in plane leakage of the optically coupled light from the photonic crystal layer; wherein the relative orientation of the two or more fundamental lattice vectors and the reflector cross couples the optically coupled light between different fundamental lattice vectors of the photonic crystal layer. For the above surface emitting laser, the positioning of the scattering centres and the reflector influences the light optically coupled into the photonic crystal layer from the active layer to produce cross coupling between different in-plane modes of the photonic crystal layer. The interaction of the optically coupled light with the reflector can be thought of as acting to create one or more ring cavities for the optically coupled light propagating within the photonic crystal layer. The direction of propagation for the optically coupled light within the photonic crystal layer changes when the light is incident on the reflector. Similar to conventional surface emitting lasers, the reflector also enhances light confinement, preventing optical leakage from the surface emitting laser, resulting in lower thresholds and higher efficiencies. Moreover, for the above surface emitting laser the reflector also assists in promoting two-dimensional coupling of modes between lattice vectors of the photonic crystal layer, thereby preventing the formation of linear standing waves along one or more of the fundamental lattice vectors of the photonic crystal. Consequently, the resulting output beam demonstrates a higher beam quality compared to surface emitting lasers known in the art. In particular, the direction of light propagation within the plane of the photonic crystal layer prevents the development of linear cavities within the photonic crystal layer. Such linear cavities, if present, could cause a far-field emission profile resembling a cross or an ellipse. However, the relative orientation of the two or more fundamental lattice vectors and the reflector ensures that the optically coupled light is coupled between different fundamental lattice vectors of the photonic crystal layer, preventing the appearance of a cross, or an ellipse, in the far-field output. Preferably the photonic crystal layer comprises a first region and a second region. Preferably the first region comprises the first array of scattering centres. Preferably the first region comprises an area that is less than half of an area of the photonic crystal layer. Preferably the second region is configured to confine coupled light within the photonic crystal layer. According to a second aspect of the present invention there is provided a surface emitting laser comprising: a photonic crystal layer defining a device plane of the surface emitting laser; an active layer within which radiation is generated; the photonic crystal layer being optically coupled to the active layer to provide optically coupled light within the photonic crystal layer; the photonic crystal layer having a first region within which is located a first array of scattering centres configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of the optically coupled light out of the device plane to form an output field of the surface emitting laser and a second region configured to confine coupled light within the photonic crystal layer, wherein the first region comprises an area that is less than half of an area of the photonic crystal layer. The introduction of the first and second regions within the photonic crystal layer of the surface emitting laser addresses a challenge encountered by surface emitting lasers known in the art, namely the reduction of mode field diameter of the output field. For most applications, to generate an output field with sufficient power, the area of the surface emitting laser needs to be sufficiently large to produce the required high-power output field. However, for surface emitting lasers known in the art, the mode field diameter of the output field is directly related to the area of the in-plane extent of the photonic crystal layer. The above surface emitting laser overcomes this limitation by introducing a first region and a second region in the photonic crystal layer. The first region efficiently controls the output coupling, selecting the output beam size and ensuring precise output coupling of the emitted beam. The second region of the photonic crystal layer facilitates an increase in coupling strength within the photonic crystal layer. This design is particularly useful for applications where the size of the photonic crystal layer results in mode field diameters that might otherwise be unsuitable and or require further beam shaping. By providing the capability to adjust the mode field diameter of the output field, without effecting the overall output power, it is possible to directly meet the requirements of various applications. Preferably the surface emitting laser further comprises a reflector bordering the photonic crystal layer. The reflector is arranged to reduce in plane leakage of the optically coupled light from the photonic crystal layer. Preferably the relative orientation of the two or more fundamental lattice vectors of the photonic crystal layer and the reflector cross couples the optically coupled light between different fundamental lattice vectors of the photonic crystal layer. Optionally, the photonic crystal layer comprises a square shaped area. In this embodiment the relative orientation of the two or more fundamental lattice vectors and the reflector is preferably such that significant reflection levels from the reflector occurs at a 45 degree angle of incidence i.e reflection levels greater than 50%. Most preferably there is no, or minimal reflection, back along the angle of incidence. Alternatively, the photonic crystal layer comprises a triangle shaped area. In a further alternative, the photonic crystal layer comprises a hexagonal shaped area. In these embodiments the relative orientation of the two or more fundamental lattice vectors and the reflector is preferably such that significant reflection from the reflector occurs at a 30 degree angle of incidence i.e. reflection levels greater than 50%. Most preferably there is no, or minimal reflection, back along the angle of incidence. The active layer may generate TE mode radiation. Alternatively, the active layer may generate TM mode radiation. The refractive index of the photonic crystal layer and the refractive index of the reflectors may differ by an amount greater than or equal to 0.4. Most preferably the first region comprises an area that is less than a third of the area of the photonic crystal layer. Optionally, the first region comprises an area that is less than a quarter of the area of the photonic crystal layer. In further alternative embodiments, the first region comprises an area that is less than a fifth, or a tenth, or a one hundredth or a thousandth of the area of the photonic crystal layer. The first area may be as small as a ten thousandth of the area of the photonic crystal layer. Preferably the reflector comprises distributed a Bragg reflector. Alternatively, the reflector comprises a photonic crystal reflector comprising an array of scattering centres. Preferably the photonic crystal reflector is a first order photonic crystal grating. Preferably the photonic crystal reflector is a first order M point reflector. Optionally the photonic crystal reflector is a third order M point reflector. The design of the photonic crystal reflector determines the efficiency with which the optically coupled light within the photonic crystal layer is reflected. Preferably the surface emitting laser further comprises two or more reflectors bordering the photonic crystal layer. The second region of the photonic crystal layer may comprise one or more arrays of scattering centres forming a first order grating. The first region may comprise a second order photonic crystal. Preferably the surface emitting laser further comprises a first and second electrical contact located on opposite sides of the active layer and the photonic crystal layer. Most preferably the first electrical contact comprises an aperture which defines an output surface of the surface emitting laser device. The aperture size may be between 1 pm and 1mm. Optionally, the aperture size is between 1 pm and 500 pm. Alternatively, the aperture size is between 1 pm and 60 pm. In a further alternative the aperture size is between 1 pm and 30 pm. Optionally the first array of scattering centres comprises two interspersed regular arrays of scattering centres. The first and second regular array of scattering centres may comprise rectangular shapes. Preferably the rectangular shaped scattering centres of the first regular array are rotated relative to the rectangular shaped scattering centres of the second regular array. Most preferably, the rectangular shaped scattering centres of the first regular array are rotated by 90 degrees relative to the rectangular shaped scattering centres of the second regular array. Preferably the reflectors are redacted with the same periodicity as the two interspersed regular arrays. The second region may also comprise two interspersed regular arrays. The above configuration comprising interspersed arrays of scattering centres encourages in plane reflections of light propagating within the photonic crystal layer, thus enhancing the cross coupling of the optically coupled light between different fundamental lattice vectors of the photonic crystal layer. Preferably the photonic crystal layer further comprises one or more phase shift regions. Optionally the one or more phase shift regions comprises quarter wave phase shift regions. Preferably the quarter wave phase shift regions are located along the two or more fundamental lattice vectors of the photonic crystal layer. Preferably the quarter wave shift regions are provided by a quarter-wavelength spacing located within the photonic crystal layer. In the absence of quarter wave shift region pairs of non-degenerate standing wave modes may be present within the photonic crystal layer. Such non-degenerate modes significantly compromise the performance of the surface emitting laser, impacting outcoupling efficiency, device stability, and mode selection. Incorporating quarter wave shift regions restores degeneracy to the standing wave modes. Most preferably the reflectors extend into the photonic crystal layer along one of the two or more fundamental lattice vectors. This results in the two or more fundamental lattice vectors having different lengths. A substantially square photonic crystal layer may lead to the generation of two degenerate polarisation modes in the surface emitting laser, resulting in undesirable switching noise and potential instability, particularly when one linear polarisation dominates. Modifying the photonic crystal layer to have fundamental lattice vectors of different lengths creates a difference in mode gain for the polarisation modes associated with each axis, favouring higher gain along the longer fundamental lattice vectors. This allows for precise control and selection of the polarisation mode, addressing the challenges posed by degenerate polarisation modes in a substantially square photonic crystal layer. Preferably the surface emitting laser further comprises a non-absorbing reflector region at the boundary between the photonic crystal layer and the reflectors. Preferably, the nonabsorbing reflector region is formed by a Quantum Well Intermixing (QWI) technique. The photonic crystal layer may further comprise a plurality of regions, with alternate regions comprising: an array of scattering centres configured to redirect optically coupled light out of the device plane to form an output field of the surface emitting laser and; a region configured to confine coupled light within the photonic crystal layer. By incorporating further regions within the photonic crystal layer in this manner, it is possible to create two or more output fields for the surface emitting laser. Preferably the surface emitting laser further comprises a mirror located to redirect light that is coupled out of the plane of the photonic crystal layer and into the surface emitting laser back towards the output surface. Preferably the one or more arrays of scattering centres comprise voids formed within the semiconductor layer. Most preferably the voids are infilled with a second semiconductor material. Optionally the second semiconductor material comprises Indium Phosphide (InP) and a Tin, Sulphur or Indium dopant material. According to a third aspect of the present invention there is provided a method of manufacturing a surface emitting laser, the method comprising: -providing a photonic crystal layer defining a device plane of the surface emitting laser -providing an active layer within which radiation is generated; -optically coupling the photonic crystal layer to the active layer to provide optically coupled light within the photonic crystal layer; -locating a first array of scattering centres within the photonic crystal layer configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of the optically coupled light out of the device plane to form an output field of the surface emitting laser; -providing a reflector that borders the photonic crystal layer to reduce in plane leakage of the optically coupled light from the photonic crystal layer, and -arranging the relative orientation of the two or more fundamental lattice vectors and the reflector to cross couple the optically coupled light between different fundamental lattice vectors of the photonic crystal layer. Embodiments of the third aspect of the present invention may comprise features to implement the preferred or optional features of the first and second aspects of the present invention or vice versa. According to a fourth aspect of the present invention there is provided a method for manufacturing a surface emitting laser, the method comprising: -providing a photonic crystal layer defining a device plane of the surface emitting laser -providing an active layer within which radiation is generated; -optically coupling the photonic crystal layer to the active layer to provide optically coupled light within the photonic crystal layer -proving a first and second region within the photonic crystal layer; -locating a first array of scattering centres within the first region configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of the optically coupled light out of the device plane to form an output field of the surface emitting laser; and -configuring the second region to confine optically coupled light within the photonic crystal layer, and -arranging the first region to comprise an area that is less than half of an area of the photonic crystal layer. Embodiments of the fourth aspect of the present invention may comprise features to implement the preferred or optional features of the first, second and third aspects of the present invention or vice versa. Brief Description of Drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: Figure 1 presents (a) an exploded perspective view of a photonic crystal surface emitting laser; and (b) a top view of a photonic crystal layer of the photonic crystal surface emitting laser, as known in the art; Figures 2 presents (a) an exploded perspective view of a photonic crystal surface emitting laser; and (b) a top view of a photonic crystal layer of the photonic crystal surface emitting laser, in accordance with an embodiment of the present invention; Figure 3 (a) and (b) present top views of alternative photonic crystal layers for the photonic crystal surface emitting laser in accordance with an embodiment of the present invention; Figures 4 presents (a) an exploded perspective view of a photonic crystal surface emitting laser; and (b) a top view of a photonic crystal layer of the photonic crystal surface emitting laser, in accordance with an alternative embodiment of the present invention; Figure 5 presents (a) an exploded perspective view of a photonic crystal surface emitting laser; and (b) a top view of a photonic crystal layer of the photonic crystal surface emitting laser in accordance with an alternative embodiment of the present invention; Figure 6 presents (a) an exploded perspective view of a photonic crystal surface emitting laser; and (b) a top view of a photonic crystal layer of the photonic crystal surface emitting laser in accordance with an alternative embodiment of the present invention; Figure 7 (a) and (b) present top views of alternative photonic crystal layers for the photonic crystal surface emitting laser in accordance with an embodiment of the present invention; Figure 8 (a) and (b) present top views of further alternative photonic crystal layers for the photonic crystal surface emitting laser in accordance with an embodiment of the present invention; Figure 9 presents top view of a yet further alternative photonic crystal layer for the photonic crystal surface emitting laser in accordance with an embodiment of the present invention; Figure 10 presents a top view of a yet further alternative photonic crystal layer for the photonic crystal surface emitting laser in accordance with an embodiment of the present invention; Figure 11 presents a side view of a photonic crystal surface emitting laser in accordance with an alternative embodiment of the present invention; and Figure 12 presents a side view of a photonic crystal surface emitting laser in accordance with an alternative embodiment of the present invention. In the description which follows, like parts are marked throughout the specification and drawings with the same reference numerals. The drawings are not necessarily to scale, and the proportions of certain parts have been exaggerated to better illustrate details and features of embodiments of the invention. Detailed Description Figure 2(a) presents an exploded perspective view of a PCSEL 14 in accordance with an embodiment of the present invention, while Figure 2(b) illustrates a top view of the photonic crystal layer 2a of the PCSEL 14. Analysis of the PCSEL 1 of the type presented in Figure 1 has led the applicant to conclude that the undesirable effect of the far field emission profile of the output field 11 taking the form of a cross, or ellipse, results from a combination of factors. The first is lower than expected scattering levels across the photonic crystal layer 2 by the scattering centres 7. The second factor resides in the fact that the reflectors 13 are orientated to reflect light back along the fundamental lattice vectors 8a and 8b of the photonic crystal layer 2 which coincide with the x and y axes of the device. The overall result is the generation of two independent orthogonal linear modes within the photonic crystal layer 2, one coinciding with each of the fundamental lattice vectors 8a and 8b the photonic crystal layer 2. The main difference between the PCSEL 1 of Figure 1, as is known in the art, and the PCSEL 14 of Figure 2, is that the reflectors 13 have a different relative orientation to the in plane fundamental lattice vectors 8a and 8b of the photonic crystal layer 2a, as defined by the scattering centres 7. More specifically, in Figure 2, the reflectors 13 are oriented to provide significant reflection levels at an angle of incidence of 45 degrees to the reflectors 13 and no, or minimal reflection back along the direction of incidence. This has the effect that when light is incident on the reflectors 13, it is reflected in such a manner that the propagation angle is changed by 90 degrees. As such, the relative orientation of the fundamental lattice vectors 8a and 8b and the reflectors 13 acts to cross couple light optically coupled into photonic crystal layer 2a from the active layer 5 between different inplane modes of the photonic crystal layer 2a. The interaction of the optically coupled light with the reflectors 13 can be thought of as acting to create one or more ring cavities for the optically coupled light propagating within the plane of the photonic crystal layer 2a. Note that in Figure 2, the reflectors 13 are distributed Bragg reflectors (DBR) the configuration of which is selected depending on the desired wavelength of the output field 11 of the PCSEL 14. As previously discussed, the reflectors 13 act to confine the light generated within the photonic crystal layer 2a, while also assisting in enhancing the 2D coupling across the modes of the photonic crystal layer 2a, as described above. In particular, the relative rotation between the reflectors 13 and the fundamental lattice vectors 8a and 8b, defined by the scattering centres 7, enhances the 2D coupling and so prevents linear standing wave formation from the reflectors 13 at opposite sides of the photonic crystal layer 2a. In contrast, in Figure 1 (b), the angle of the reflectors 13 relative to the fundamental lattice vectors 8a and 8b of the photonic crystal layer 2 effectively act to create two linear cavities within the PCSEL 1, where independent lasing occurs along the fundamental lattice vectors 8a and 8b. A drawback of this known configuration is its potential to induce linear standing wave formation between reflectors 13 at opposite sides of the photonic crystal layer 2, where the configuration supports undesirable modes (side-to-side modes) within the photonic crystal layer 2. These effects can impact the beam quality of the output field 11 and therefore are undesirable in applications where a high-quality, stable laser beam is essential. The configuration depicted in Figure 2(b) leads to an improvement in the beam quality of the output field, along with more control over the spatial distribution of modes in the laser cavity. By changing the relative angle between the reflectors 13 and the fundamental lattice vectors 8a and 8b defined by the scattering centres 7, light can couple into the modes of the orthogonal fundamental lattice vectors 8a and 8b so that the far field output field 11 exhibits the desired gaussian emission profile rather than the cross profile, or elliptical profile, often found with devices of the type presented in Figure 1. Note that the angle of incidence for the optically coupled light propagating along the fundamental lattice vectors 8a and 8b within the photonic crystal layer 2a and the reflectors 13 need not necessarily be at 45 degrees as depicted in Figure 2(a). As will be appreciated by a person skilled in the art, the relative angle between the reflectors 13 and the fundamental lattice vectors 8a and 8b of the photonic crystal layer 2a may be at any suitable, non-perpendicular angle, that results in cross coupling of light between different modes of the photonic crystal layer 2a, thus essentially forming one or more ring cavities for light propagating within the photonic crystal layer 2a. Such, alternative embodiments are provided by the photonic crystal layers 2b and 2c presented in Figures 3(a) and 3(b), respectively. In Figure 3(a) the photonic crystal layer 2b has the form of an equilateral triangle while in Figure 3(b) the photonic crystal layer 2c has the form of regular hexagon. Reflectors 13 are again located around the perimeter of the photonic crystal layers 2b and 2c. In these embodiments, the reflection of the optical coupled light propagating along the fundamental lattice vectors 8 within the photonic crystal layer 2b or 2c from the reflectors 13 is set to be maximised at an angle of incidence of 30 degrees. Theoretical analysis of the PCSEL 14 of the type presented in Figure 2 has led the applicant to realise that there is an issue with the operation of the of the PCSEL 14 when the active layer 5 is configured to generate TE mode radiation and the refractive index of the photonic crystal layer 2a is of a similar value to that of the reflectors 13. With this arrangement, Brewster’s angle for the light optically coupled within the photonic crystal layer 2a is calculated to be around 45 degrees. As is known to those skilled in the art, TE modes have a p-type polarisation i.e. its electric field is perpendicular to Its direction of propagation and lies parallel to the plane of incidence between the photonic crystal layer 2a and the reflectors 13. The result is that instead of providing significant reflection levels at the angle of incidence of 45 degrees to the reflectors 13, the Brewster effect causes the p-type polarised light to propagate straight into the reflectors 13. There are a number of solutions to this problematic effect that can be incorporated within the PCSEL 14. One solution is to employ an active layer 5 that is configured to generate TM mode radiation. As is known to those skilled in the art, TM modes have an s-type polarisation i.e. its magnetic field is perpendicular to its direction of propagation and its electric field is perpendicular to the plane of incidence between the photonic crystal layer 2a and the reflectors 13. With this arrangement the light optically coupled within the photonic crystal layer 2a is unaffected by the Brewster effect and so significant reflection levels of the light optically coupled within the photonic crystal layer 2a can be achieved. An alternative solution to this problem resides in the design of the reflectors 13. It is found that by designing the reflectors 13 to introduce a An >0.4 between the refractive indices of the photonic crystal layer 2a and the reflectors 13 is sufficient to avoid the condition of Brewster’s angle for TE mode radiation coupled into the photonic crystal layer 2a from the active layer 5. As a result, significant reflection levels of such light at 45 degrees can be achieved. It will be appreciated by the skilled reader that the alternative photonic crystal layers 2b and 2c presented in Figures 3(a) and 3(b), respectively, will also avoid this above mentioned problematic issue when the active layer 5 is configured to generate TE mode radiation and the refractive index of the photonic crystal layers 2b or 2c is of a similar value to that of the reflectors 13. The reason for this is that the light optically coupled within the photonic crystal layer 2b or 2c has an angle of incidence with the reflectors 13 that is around 30 degrees, some 15 degrees off the calculated Brewster’s angle value of 45 degrees. A photonic crystal surface emitting laser (PCSEL) device 15 in accordance with an alternative embodiment of the present invention, and its method of production, will now be described with reference to Figure 4. In particular, Figure 4(a) presents an exploded perspective view of the PCSEL 15 and Figure 4(b) presents a top view of the photonic crystal layer 2d of the PCSEL 15. The PCSEL 15 of Figure 4(a) comprises the same semiconductor layer arrangement as the PCSEL 1 presented in Figure 1(a). However, the photonic crystal layer 2d of the PCSEL 15, as detailed in Figure 4(b), comprises an alternative structure. Firstly, the photonic crystal layer 2d is seen to comprise a first region 16 and a second region 17. In Figure 4(b) the first region 16 is located in a central location of the photonic crystal layer 2d, and is a circular area comprising a second order photonic crystal. The first region 16 is surrounded by the second region 17, wherein, in the present embodiment, the second region 17 is devoid of scattering centres. The output coupling for the PCSEL 15 is provided by the first region 16, as only the second order photonic crystal, having the array of scattering centres 7, will be capable of generating a useful laser emission. The optically pumped region 18 of the PCSEL 15 covers the both the first 16 and second 17 regions, where any light generated within the first 16 and second 17 regions is outcoupled via the first region 16 only. As such the first region 16 effectively selects the beam size of the output field 11 and ensures precise output coupling of the emitted output field 11 while the second region 17 of the photonic crystal layer 2d facilitates an increase in coupling strength within the photonic crystal layer 2d. The first region 16 comprises an area that is less than half of an area of the photonic crystal layer 2d. It will be appreciated by the skilled reader that the area of the first region 16 could be decreased further such that it is less than a third, or a quarter, or a fifth, or a tenth, or a one hundredth or a thousandth of the area of the photonic crystal layer 2d. It is anticipated that a photonic crystal layer 2d having a first area 16 as small as a ten thousandth of the area of the photonic crystal layer 2d could be incorporated into the PCSEL 15. The aperture 12 of the PCSEL 15 is aligned with the first region 16 of the photonic crystal layer 2d, with the size of the aperture 12 corresponding to the size of the first region 16. The size of the aperture is typically in the range between 1 pm and 1 mm. Note that while the first region 16 of the photonic crystal layer 2d is depicted as a circularly shaped region, it could alternatively adopt any other geometry or shape. Furthermore, it will be appreciated by the skilled reader that, in alternative embodiments, the first region 16 does not need to be located centrally within the photonic crystal layer 2d but may instead be located in a different location of the photonic crystal layer 2d e.g. closer to the edge of the photonic crystal layer 2d. The circular scattering centres 7 of the photonic crystal layer 2d in the first region 16 may alternatively comprise different regular or irregular geometric shapes (e.g. triangular, oval, diamond, square or chevron shapes). Furthermore, the lattice shape of the first region 16 may comprise alternative regular or irregular lattice structures (e.g. triangular, hexagonal or Kagome) having different lattice constants (a). As with the PCSEL 1 of Figure 1 (b), reflectors 13 border the photonic crystal layer 2d of the of the PCSEL 15, as shown in Figure 4(b). More specifically, in Figure 4(b) the reflectors 13 border the second region 17 of the photonic crystal layer 2d. The reflectors 13 again act to confine the light generated within the photonic crystal layer 2d. The optically pumped region 18 of the PCSEL 15 may extend into the area comprising the reflectors 13. As will be appreciated by a person skilled in the art, the optically pumped region 18 may instead only be located within the first 16 and second 17 regions of the photonic crystal layer 2d. By incorporating a smaller embedded first region 16 within the photonic crystal layer 2d, it allows for the PCSEL 15 to provide an output field 11 with a reduced mode field diameter, when compared to PCSELs known in the art (see Figure 1(a)). As the overall dimensions of the PCSEL 15 are not reduced in size, the smaller mode field diameter does not act to reduce the output power or power efficiency of the PCSEL 15. The small mode field diameter is useful for applications which require a particular size for the output field 11 of the PCSEL 15. For example, when coupling the output field 11 into optical fibres or silicon photonic gratings. The reduced mode field diameter of the output field 11 means fewer optical components are required when coupling the output field 11 into such devices. Figures 5(a) presents an exploded perspective view of a photonic crystal surface emitting laser 19 in accordance with an embodiment of the present invention. Figure 5(b) shows a top view of the photonic crystal layer 2e of the photonic crystal surface emitting laser 19. The PCSEL 19 again comprises a similar semiconductor layer arrangement as the PCSEL 15 presented in Figure 4. Furthermore, as shown in Figure 5(b), the photonic crystal layer 2e again comprises both a first region 16 and a second region 17. However, in the presently described embodiment, both the first region 16 and the second region 17 of the photonic crystal layer 2e comprise an array of scattering centres 7, a first array 7a located in the first region 16 and a second array 7b located within the second region 17. The first array of scattering centres 7a provides a second order photonic crystal structure, while the second array of scattering centres 7b provides a first order grating. Only the second order photonic crystal structure within the first region 16 of the photonic crystal layer 2e will create laser emission i.e. will redirect light that has optically coupled into the photonic crystal layer 2e from the active layer 5, out of the plane of the photonic crystal layer 2e, to form the output field 11 of the PCSEL 19. However, both the first 16 and second region 17 can be optically coupled to the active layer 5, where any light generated in the second region 17 is then outcoupled via the first region 16 of the photonic crystal layer 2e. Note that the scattering centres 7a and 7b of the first region 16 and or second regions 17 may comprise different regular or irregular geometric shapes. Additionally, the geometry of the first region 16 and or second region 17 of the photonic crystal layer 2e may adopt any other geometry or shape. In alternative embodiments, the first region 16 may be located in a different location of the photonic crystal layer 2e e.g. closer to the edge of the photonic crystal layer 2e. Figure 6(a) presents an exploded perspective view of a PCSEL 20 and Figure 6(b) illustrates a top view of the photonic crystal layer 2f of the PCSEL 20, in accordance with an alternative embodiment of the present invention. In particular, the PCSEL 20 of Figure 6 is effectively a combination of the PCSELs 14 and 19 illustrated in Figure 2 and Figure 5, respectively. The photonic crystal layer 2f of Figure 6 is shown to comprise a first region 16, where the first region 16 is a second order photonic crystal. The first region 16 is surrounded by a second region 17, where the second region 17 comprises a first order photonic crystal. Additionally, the photonic crystal layer 2f is bordered by reflectors 13 which are configured provide significant reflection levels at an angle of incidence of 45 degrees to the reflectors 13 and no, or minimal reflection back along the direction of incidence . The relative orientation of the reflectors 13 to the fundamental lattice vectors 8a and 8b, as defined by the scattering centres 7 of the photonic crystal layer 2f, again results in cross coupling of light optically coupled into photonic crystal layer 2f from the active layer 5 between different in-plane modes of the photonic crystal layer 2f. The interaction of the optically coupled light with the reflectors 13 can alternatively be thought of as acting to create one or more ring cavities for the optically coupled light propagating within the photonic crystal layer 2f. The structure of the photonic crystal layer 2f results in a PCSEL 20 which combines the advantageous properties of the previous embodiments, as described with reference to Figures 2 and 5. Namely, the PCSEL 20 provides enhanced confinement of the light and enhanced 2D coupling between different modes of the photonic crystal layer 2f, a high power output and high quality output beam 11. Moreover, the mode field diameter of the output beam 11 is determined by the size of the first region 16 within the photonic crystal layer 2f. This feature simplifies the coupling process with various devices, enhancing the compatibility of the PCSEL 20 for use within different applications i.e. with additional silicon optics. Figure 7 presents top views of alternative photonic crystal layer 2g and 2h for the photonic crystal surface emitting laser 20 of Figure 6(a). The photonic crystal layers 2g and 2h of Figure 6(a) and (b), respectively, also comprises reflectors 13 that act to confine light generated within the photonic crystal layer 2g and 2h. However, the reflectors 13 are provided in the form of photonic crystal reflectors rather than the distributed Bragg reflectors of Figure 6. The photonic crystal reflectors 13 border the photonic crystal layer 2g and 2h, and are set to reflect at a suitable angle to the fundamental lattice vectors 8a and 8b of the first and second regions to ensure cross coupling of light optically coupled into photonic crystal layer 2g or 2h from the active layer 5 between different in-plane modes of the photonic crystal layer 2g or 2h. In Figure 7, the photonic crystal reflectors 13 are designed to have a period that provides for efficient reflection of light having an angle of incidence around 45 degrees while providing for no, or minimal reflection back along the direction of incidence. A different lattice structure for the photonic crystal reflectors 13 is illustrated in Figures 7(a) and (b). By careful selection of the period of the lattice structure, and or M point reflection order, it is possible to control the reflection strength of the photonic crystal reflectors 13 and maintain mode-matching at the boundary between the photonic crystal layer 2g or 2h and the photonic crystal reflectors 13. As will be appreciated by the skilled reader, the scattering centres 7 of the photonic crystal reflectors 13 can assume various shapes, provided they are orientated at an angle that provides reflection back into the photonic crystal layer 2g and 2h. Examples of permissible shapes for scattering centres 7 of the photonic crystal reflectors 13 include, but are not restricted to, circles, dashes, double lattice patterns, triangles, ovals, and more. Furthermore, the photonic crystal reflectors 13 may comprise a single lattice or double lattice structure. Figure 8 (a) and (b) present alternative top views of a photonic crystal layer 2i for the photonic crystal surface emitting laser 20 of Figure 6(a). The photonic crystal layer 2i of Figure 8(a) and (b) illustrates an alternative orientation for the scattering centres 7a and 7b within the photonic crystal layer 2i. The scattering centres 7a and 7b can be seen to be arranged within two interspersed regular arrays. In the first array, the scattering centres 7a comprise rectangular shapes, the longer sides of which are orientated in parallel with the x-axis of the photonic crystal surface emitting laser 20. In the second array, the scattering centres 7b again comprise the same rectangular shapes. However, in contrast to the first array, the longer sides of the rectangular shaped scattering centres 7b are orientated in parallel with the y-axis of the photonic crystal surface emitting laser 20. This arrangement of the scattering centres 7a and 7b results in a plurality of substantially square cells, the majority of which are bordered by the scattering centres 7a and 7b. This configuration encourages 90 degree in plane reflections of light propagating within the photonic crystal layer 2i thus ensuring cross coupling between different in- plane modes of the photonic crystal layer 2i. Distributed Bragg reflectors 13, blazed at 45 degrees to the x or y axis of the photonic crystal surface emitting laser 20 are located around the perimeter of the photonic crystal layer 2i. This again ensures that light propagating within the photonic crystal layer 2i is reflected through 90 degrees to further assist the cross coupling between different in-plane modes of the photonic crystal layer 2i. However, in similar, manner to the embodiment of Figure 7, the reflectors 13 may alternatively comprise photonic crystal reflectors. Figure 8(b) shows a further enhancement to the structure depicted in Figure 8(a), where the distributed Bragg reflectors 13 are redacted in locations with the same periodicity as the scattering centres 7a and 7b. It is further noted that the elements of alternative perimeter distributed Bragg reflectors 13 are offset by half the periodicity of the scattering centres 7a and 7b. This arrangement ensures that the electric field of the light propagating within the photonic crystal layer 2i is continuous at the transition between the scattering centres 7a and 7b and the reflectors 13. Figure 9 presents an alternative top view of a photonic crystal layer 2j for the photonic crystal surface emitting laser 20 of Figure 6(a). The photonic crystal layer 2i of Figure 9 also comprises a first region 16 and a second region 17, with reflectors 13 surrounding the second region 17. The photonic crystal layer 2j further comprises phase shift regions 21 that are incorporated into the structure of the photonic crystal layer 2j. In the presently described embodiment, the phase shift regions 21 are diagonal quarter wave phase shift regions located parallel to the fundamental lattice vectors 8a and 8b, of the photonic crystal layer 2j. Such quarter-wave shift (QWS) regions 21 may be introduced by altering the structure of the photonic crystal layer 2j to introduce a quarter-wavelength phase shift. In the presently described embodiment, the QWS regions 21 are provided by introducing a quarterwavelength spacing in the photonic crystal layer 2j. Incorporating diagonal quarter wave shift (QWS) regions 21 within both the first region 16 and the second region 17 of the photonic crystal layer 2j improves the performance of the PCSEL 20. In the absence of such QWS regions 21 pairs of non-degenerate standing wave modes may be present within the photonic crystal layer 2j, where the field nodes are either coincident with the scattering regions 7 of the photonic crystal, or the field nodes are located in between the scattering regions 7. The wave modes are non-degenerate due to the difference in effective index of the scattering regions 7 and the areas between the scattering regions 7. Such non-degenerate modes significantly compromise the performance of PCSEL 20, impacting outcoupling efficiency, device stability, and mode selection. However, the introduction of the QWS regions 21 of Figure 9 modifies the device structure to restore degeneracy to the standing wave patterns, enhancing the overall performance of the PCSEL 20. The implementation of diagonal QWS regions may include adjustments in partial steps or distributed arrangements. It will be appreciated by the skilled reader that in alternative embodiments the phase shift regions 21 may take a different form and or may be designed to introduce a different phase shift as required by the design of the PCSEL. For example, in the photonic crystal layer 2b and 2c of Figure 3(a) and (b), respectively, the phase shift regions should be designed to introduce a third wave shift, rather than a quarter wave shift. The substantially square photonic crystal layers described with reference to the embodiment of Figure 9 leads to the generation of two degenerate polarisation modes in the PCSEL 20, which can result in undesirable switching noise and potential instability within the output field 11 of the device, particularly when one linear polarisation dominates. To mitigate this issue, Figure 10 presents a further alternative top view of a photonic crystal layer 2k for the photonic crystal surface emitting laser 20 of Figure 6(a). The photonic crystal layer 2k has a structure similar to that depicted in Figure 9, as it features a first region 16 and a second region 17, reflectors 13 surrounding the second region 17, and diagonal quarter wave shift (QWS) regions 21. In contrast to the embodiment presented in Figure 9, the reflectors 13 extend into the second region 17 along one of the fundamental lattice vectors 8b resulting in the area of the photonic crystal layer 2k being an irregular hexagon rather than a square. Modifying the photonic crystal layer 2k to have fundamental lattice vectors 8a and 8b of different lengths creates a difference in mode gain for the two polarisation modes, favouring higher gain along the longer propagation axes 8a. Therefore, the photonic crystal layer 2k allows for precise control and selection of the polarisation mode, addressing the challenges posed by degenerate polarisation modes in the substantially square photonic crystal layers described above. In the presently described embodiment the output beam 11 will be polarised parallel to the longer propagation axes 8a. It will be appreciated by the skilled reader that further modifications to the area of the photonic crystal layer 2i may be made by further altering the location of the reflectors 13 thus allowing control of the polarisation of the output beam 11. Figure 11 presents a side view of a PCSEL 22 in accordance with an alternative embodiment of the present invention. The PCSEL 22 incorporates many of the same features of the PCSELs described above with reference to Figures 2 to 10. For example, the PCSEL 22 can be seen to comprise an photonic crystal layer 2I optically coupled to an active region 5. The photonic crystal layer 2I again comprises a first region 16, where the first region 16 is a second order photonic crystal; a second region 17 which surrounds the first region 16, where the second region 17 comprises a first order photonic crystal and reflectors 13 (not explicitly shown in Figure 11) located around the perimeter of the second region 17, the reflectors 13 again being orientated at 45 degrees to fundamental lattice vectors 8a and 8b of the photonic crystal layer 2I. In the embodiment presented in Figure 11 it can be seen that a dielectric clad region or mirror 23 is located with the first region 16 in order to redirect light that is coupled out of the plane of the photonic crystal layer 2I and into the PCSEL 22 back towards the output surface 10. As a result, the dielectric clad or mirror 23 acts to increase the power of the output field 11 generated by the operation of the PCSEL 22. The PCSEL 22 also further comprises a non-absorbing reflector region 24 at the boundary between the photonic crystal layer 2I and the reflectors 13. The non-absorbing reflector region may be formed by employing a Quantum Well Intermixing (QWI) technique. QWI is a technique known in the art where selected areas of a semiconductor device are modified to form the desired non-absorbing reflector, see for example Semiconductor Science Technology volume 8(6), pages 1136-1155 (1993) paper by J. H. Marsh entitled “Quantum Well Intermixing”. In summary, QWI can be achieved through a post-growth annealing processes where capping layers are selectively applied to regions or the semiconductor device before the device is heated to a high temperature (e.g. 700 to 1000°C) for several minutes. A number of different capping layers, of varying thickness, have been used during these annealing processes, including titanium dioxide (TiO2), aluminium (Al) and silicon dioxide (SiO2). The difference in thermal expansion coefficients between the capped and non-capped regions of the semiconductor device results in a relative shift of the absorption profiles between these regions which results in the desired non-absorbing reflection region 24. Figure 12 presents a side view of a PCSEL 25 in accordance with an alternative embodiment of the present invention. The PCSEL 25 is similar to the PCSEL 22 described above with reference to Figure 11 however, instead of the PCSEL 22 comprising a single first region 16 and dielectric clad or mirror 23 combination, PCSEL 25 instead comprises six combined first region 16a to 16f and dielectric clad regions or mirrors 23a to 23f. As a result, the output field of the PCSEL 25 comprises six corresponding output fields 11 a to 11f. As will be appreciated by the skilled reader, the PCSEL 25 is not limited to six output fields 11 a to 11 f but instead can comprise multiple output fields, each corresponding to a combined first region 16 and dielectric clad region or mirror 23. The multiple dielectric clad regions or mirrors 23 may be arranged in a regular or an irregular array, as required for the particular application of the PCSEL 25. A surface emitting laser comprising an active layer within which radiation is generated and coupled into a photonic crystal layer is provided. The photonic crystal layer comprises a first array of scattering centres configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of optically coupled light out of the device plane to form an output field of the surface emitting laser. A reflector bordering the photonic crystal layer acts to reduce in plane leakage of the optically coupled light from the photonic crystal layer. The relative orientation of the two or more fundamental lattice vectors and the reflector acts to cross couple the optically coupled light between different fundamental lattice vectors of the photonic crystal layer. The resulting output beam produced by the surface emitting laser exhibits a high quality gaussian profile. Throughout the specification, unless the context demands otherwise, the term “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, reference to any prior art in the description should not be taken as an indication that the prior art forms part of the common general knowledge. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further 1 modifications or improvements may be incorporated without departing from the scope of 2 the invention as defined by the appended claims. 3

Claims

1) A surface emitting laser comprising:a photonic crystal layer defining a device plane of the surface emitting laser;an active layer within which radiation is generated;the photonic crystal layer being optically coupled to the active layer to provide optically coupled light within the photonic crystal layer;the photonic crystal layer having a first array of scattering centres configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of optically coupled light out of the device plane to form an output field of the surface emitting laser;a reflector bordering the photonic crystal layer to reduce in plane leakage of the optically coupled light from the photonic crystal layer;wherein the relative orientation of the two or more fundamental lattice vectors and the reflector cross couples the optically coupled light between different fundamental lattice vectors of the photonic crystal layer.2) A surface emitting laser as claimed in claim 1 wherein the photonic crystal layer comprises a first region and a second region.3) A surface emitting laser as claimed in claim 2 wherein the first region comprises the first array of scattering centres.4) A surface emitting laser as claimed in either of claims 2 or 3 wherein the first region comprises an area that is less than half of an area of the photonic crystal layer.5) A surface emitting laser as claimed in any of claims 2 to 4 wherein the second region is configured to confine coupled light within the photonic crystal layer.6) A surface emitting laser as claimed in any of the preceding claims wherein the photonic crystal layer comprises a square shaped area, a triangle shaped area or a hexagonal shaped area.7) A surface emitting laser as claimed in any of the preceding claims wherein the active layer generates TE mode radiation.8) A surface emitting laser as claimed in any of claims 1 to 6 wherein the active layer generates TM mode radiation.9) A surface emitting laser as claimed in any of the preceding claims wherein a refractive index of the photonic crystal layer and a refractive index of the reflectors differ by an amount greater than or equal to 0.4.10) A surface emitting laser as claimed in any of claims 4 to 9 wherein the first region comprises an area that is less than a third, or a quarter, or a fifth, or a tenth or a one hundredth or a thousandth or a ten thousandth of the area of the photonic crystal layer.11) A surface emitting laser as claimed in any of the preceding claims wherein the reflector comprises a distributed Bragg reflector.12) A surface emitting laser as claimed in any of claims 1 to 10 wherein the reflector comprises a photonic crystal reflector comprising an array of scattering centres.13) A surface emitting laser as claimed in claim 12 wherein the photonic crystal reflector is a first order photonic crystal grating.14) A surface emitting laser as claimed in any of the preceding claims wherein the surface emitting laser further comprises two or more reflectors bordering the photonic crystal layer.15) A surface emitting laser as claimed in any of claims 2 to 14 wherein the second region of the photonic crystal layer comprises one or more arrays of scattering centres forming a first order grating.16) A surface emitting laser as claimed in any of claims 2 to 15 wherein the first region comprises a second order photonic crystal.17) A surface emitting laser as claimed in any of the preceding claims wherein the surface emitting laser further comprises a first and second electrical contact locatedon opposite sides of the active layer and the photonic crystal layer, the first electrical contact comprising an aperture which defines an output surface of the surface emitting laser device.18) A surface emitting laser as claimed in claim 17 wherein the aperture size is between 1 pm and 1mm, or is between 1 pm and 500 pm, or is between 1 pm and 60 pm, or is between 1 pm and 30 pm.19) A surface emitting laser as claimed in any of the preceding claims wherein the first array of scattering centres comprises two interspersed regular arrays of rectangular shaped scattering centres wherein the rectangular shaped scattering centres of the first regular array are rotated relative to the rectangular shaped scattering centres of the second regular array.20) A surface emitting laser as claimed in claim 19 wherein the reflectors are redacted with the same periodicity as the two interspersed regular arrays.21) A surface emitting laser as claimed in any of the preceding claims wherein the photonic crystal layer further comprises one or more phase shift regions.22) A surface emitting laser as claimed in any of the preceding claims wherein the reflectors extend into the photonic crystal layer along one of the two or more fundamental lattice vectors to provide the two or more fundamental lattice vectors with different lengths.23) A surface emitting laser as claimed in any of the preceding claims wherein the surface emitting laser further comprises a non-absorbing reflector region at the boundary between the photonic crystal layer and the reflectors.24) A surface emitting laser as claimed in any of the preceding claims wherein the surface emitting laser further comprises a mirror located to redirect light that is coupled out of the plane of the photonic crystal layer and into the surface emitting laser back towards an output surface.25) A method of manufacturing a surface emitting laser, the method comprising:-providing a photonic crystal layer defining a device plane of the surface emitting laser-providing an active layer within which radiation is generated;-optically coupling the photonic crystal layer to the active layer to provide optically coupled light within the photonic crystal layer;-locating a first array of scattering centres within the photonic crystal layer configured to define two or more fundamental lattice vectors of the photonic crystal layer and to redirect a portion of the optically coupled light out of the device plane to form an output field of the surface emitting laser;-providing a reflector that borders the photonic crystal layer to reduce in plane leakage of the optically coupled light from the photonic crystal layer, and-arranging the relative orientation of the two or more fundamental lattice vectors and the reflector to cross couple the optically coupled light between different fundamental lattice vectors of the photonic crystal layer.

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