Patent Publication Number: US-11050219-B2

Title: Laser device and method for its operation

Description:
BACKGROUND TO THE INVENTION 
     Field of the Invention 
     The present invention relates to a laser device and a method for its operation. It has particular applicability to a laser device based on a photonic crystal surface emitting laser (PCSEL). 
     Related Art 
     Photonic crystal surface emitting lasers (PCSELs) are a new class of semiconductor light emitter. PCSELs have been found to have beneficial properties including coherent oscillation, and low divergences of emitted light. 
     PCSELs work using an active light emitting layer and a photonic crystal layer. These two layers are arranged near or adjacent to each other as part of a layered structure. The photonic crystal is made of a material having a first refractive index, which is patterned with discrete regions having a second refractive index different from the first refractive index arranged in a periodic lattice structure, thus forming the photonic crystal. The regions having the second refractive index can be holes, or can be filled with another material. The lattice causes Bragg diffraction in the photonic crystal, which in turn causes light to resonate in the photonic crystal at a particular wavelength determined by the periodicity/lattice constant of the photonic crystal. 
     When a voltage is applied across a PCSEL, the active layer emits light. Light leaks out of the active layer and into the photonic crystal. Light in the photonic crystal having a wavelength coincident with the lattice constant of the photonic crystal resonates in the photonic crystal, and is amplified. This amplified light then leaves the PCSEL through an emission surface. 
     Gelleta et al. (2015) [15] discloses the results of modelling investigations on the effect of external reflection on the TE mode of PCSELs. The work reported in Gelleta et al (2015) suggests that the use of external reflection may be of interest for improving mode stability in cavities of modest (about 100 μm 2 ) areas. However, no experimental work was presented to support such effects. 
     SUMMARY OF THE INVENTION 
     Previous work by the inventors&#39; research group has been published as WO 2016/027105 [20], in which a laser structure is disclosed having a first PCSEL element and a second PCSEL element coupled via a coupling region. The coupling region is electrically controllable in order to provide coherent coupling between the first PCSEL and the second PCSEL. 
     The present inventors have realised that existing laser devices based on PCSELs may be susceptible of further modification and/or improvement, for example to improve the controllability of the laser output, in terms of intensity, beam divergence, beam shape, etc. The present invention has accordingly been devised. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems. Further advantages of preferred embodiments of the present invention are discussed below. 
     In a general aspect, the present invention modifies known PCSEL devices by providing controllable and/or selective reflection of laterally propagating light back into the PCSEL. 
     In a first preferred aspect, the present invention provides a laser device having a photonic crystal surface emitting laser (PCSEL) element, there being provided at least at a first lateral side of the PCSEL element a reflector arranged to reflect back into the PCSEL element at least a portion of light travelling out of the PCSEL element through the first lateral side of the PCSEL element, there being interposed between the first lateral side of the PCSEL element and the reflector an electrically controllable light-transmission region configured to control the transmission of light from the PCSEL element to the reflector, based on an electrical input. 
     In a second preferred aspect, the present invention provides a method for operating a laser device, the laser device having a photonic crystal surface emitting laser (PCSEL) element, the method including the step of controlling the reflection of light, travelling out of the PCSEL element through a first lateral side of the PCSEL element, back into the PCSEL element, by causing an electrical input to control the transmission of light through a light-transmission region interposed between the first lateral side of the PCSEL element and a reflector. 
     Accordingly, feedback inside the PCSEL element from light reflected back into the PCSEL element, can be controlled based on the electrical input. The inventors have found that controlling feedback in this way enables the lasing power output and beam shape of the laser device to be controlled electronically. Advantageously, the lasing power output and beam shape can be controlled, if required, without direct modulation of the PCSEL element itself. 
     In a third preferred aspect, the present invention provides a laser device having a PCSEL element, wherein respective reflectors are provided at least at a first lateral side of the PCSEL element and at a second lateral side of the PCSEL element, each respectively arranged to reflect back into the PCSEL element at least a portion of light travelling out of the PCSEL element through the respective lateral sides of the PCSEL element, wherein the respective reflectors are wavelength-selective. 
     In a fourth preferred aspect, the present invention provides a method of operating a laser device, the laser device having a photonic crystal surface emitting laser (PCSEL) element, wherein respective wavelength-selective reflectors are provided at least at a first lateral side of the PCSEL element and at a second lateral side of the PCSEL element, the method including the step of emitting light from the PCSEL element through the respective lateral sides of the PCSEL element, the reflectors each respectively being arranged to reflect at least a portion of the light travelling out of the PCSEL element through the respective lateral sides of the PCSEL element, back into the PCSEL element. 
     As will be understood, a wavelength-selective reflector reflects light depending on its wavelength, typically within a predefined wavelength range about a selected wavelength. Such reflectors can conveniently be formed within the device architecture, e.g. formed by epitaxial and/or lithographic techniques. They therefore provide a convenient format for reflectors which enable feedback of in-plane light back into the PCSEL element. Advantageously, the wavelength-selective reflector may only reflect light at a wavelength corresponding to the resonant frequency of light inside the photonic crystal of the PCSEL element. In other words, the reflector can be made only to reflect light at the lasing frequency of the PCSEL element. 
     Preferably, the wavelength-selective reflector may reflect substantially all light within the predetermined wavelength range that is incident upon it. Moreover, it may preferably reflect substantially all light corresponding to the lasing frequency of the PCSEL element that is incident upon it. 
     Accordingly, losses of light at the lasing frequency through the reflector can be minimised, and feedback effects in the PCSEL element can be maximised. 
     The first, second, third and/or fourth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features. 
     The reflector of the first and second aspects may also be a wavelength-selective reflector. 
     The reflector (and wavelength-selective reflector) may be located in the plane of the PCSEL element. 
     The electrically controllable light-transmission region may be located in the plane of the PCSEL element. 
     The electrically controllable light-transmission region may be switchable by the electrical input between a transmission state in which substantially all light is transmitted through it to be reflected by a corresponding reflector, and an extinction state in which substantially no light is transmitted through it, thereby controlling the transmission of light from the PCSEL element to the reflector. 
     In practice, the transmission may be tunable through a continuous range of optical states, between the extinction state and the transmission state. In this respect, the tuning can be thought of as analog, rather than binary. 
     Accordingly, when the electrically controllable light-transmission region is in the extinction state, and preventing light from being transmitted to the corresponding reflector, light will not be reflected back into the PCSEL element by the reflector at the corresponding lateral side, and there will be no feedback from that edge of the device. On the other hand, when the electrically controllable light-transmission region is in the transmission state, and allowing light to reach the corresponding reflector, light will be reflected back into the PCSEL element, resulting in feedback from that edge of the device. When the electrically controllable light-transmission region is in an optical state between the extinction state and the transmission state, an intermediate fraction of light will be reflected back into the device by the reflector at the corresponding lateral side. 
     In the first and second embodiments there may be provided at a second lateral side of the PCSEL element a second reflector arranged to reflect back into the PCSEL element at least a portion of light travelling out of the PCSEL element through the second lateral side of the PCSEL element. Moreover, the second reflector may be provided in addition to the (first) reflector at the first lateral side of the PCSEL element. 
     There may also be, interposed between the second lateral side of the PCSEL element and the second reflector, a second electrically controllable light-transmission region configured to control the transmission of light from the PCSEL element to the second reflector, based on an electrical input. Moreover, the second electrically controllable light-transmission region may be provided in addition to the (first) electrically controllable light-transmission region interposed between the first reflector and the first lateral side. 
     The first and second lateral sides may be adjacent to each other. 
     The inventors have found that feedback from two adjacent lateral sides of a PCSEL element can be of particular use for controlling the beam shape from the PCSEL, and can be used to select a particular lasing mode of the PCSEL (this is discussed in more detail below). It is advantageous to be able to control the feedback from two adjacent lateral sides individually, as will become clear in the detailed description (below). 
     Respective reflectors may be provided at first, second and third lateral sides of the PCSEL element, each respectively arranged to reflect back into the PCSEL element at least a portion of the light travelling out of the PCSEL element through the respective lateral sides of the PCSEL element. 
     Alternatively, respective reflectors may be provided at first, second, third and fourth (i.e. preferably all) lateral sides of the PCSEL element, each respectively arranged to reflect back into the PCSEL element at least a portion of the light travelling out of the PCSEL element through the respective lateral sides of the PCSEL element. 
     In the case of the first and second aspects, there may be an electrically controllable light-transmission region interposed between each respective reflector and lateral side of the laser device. 
     Each electrically controllable light-transmission region may be independently controlled by a respective electrical input. 
     Accordingly, reflection at the respective lateral sides of the PCSEL element can be independently controlled. This provides a high degree of electrical control over the feedback, and hence power output and beam shape, of the PCSEL element. 
     The reflector may be a distributed Bragg reflector (DBR). 
     At least one electrically controllably light-transmission region may be segmented, each segment configured to control the transmission of a respective portion of light from the PCSEL element to the reflector. Preferably, each electrically controllable light-transmission region may be segmented. In particular, each electrically controllable light-transmission region may be split into at least two segments. In practice, any number of segments may be used. For example, 2, 3, 4, 5 or 6 segments could be used. 
     Each segment of each electrically controllable light-transmission region may be independently controlled by a respective electrical input. In particular, each segment may be individually controlled to be in either a transmission state or an extinction state. 
     The inventors consider that such a segmented controllable light-transmission region can provide further control over the lasing power output and beam shape from a PCSEL element. They also consider that such a segmented controllable light-transmission region could be used for beam steering, by changing the position in the plane of the PCSEL element at which the lasing output is centred. 
     Further optional features of the invention are set out below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows a perspective view of a PCSEL element. 
         FIG. 2  shows a cross-sectional view of the PCSEL element of  FIG. 1 . 
         FIG. 3  shows a plan view of a photonic crystal for use in the PCSEL element of  FIGS. 1 and 2 . 
         FIG. 4  is a graph of normalised frequency (y-axis) as a function of wave vector (x-axis), showing the photonic band structure of the photonic crystal of  FIG. 3 . 
         FIGS. 5A to 5D  show the modulus of the magnetic field distribution in the PCSEL element of  FIG. 3 , for the frequencies A, B, C and D shown in  FIG. 4 . 
         FIG. 6  shows a graph of lasing power (y-axis) as a function of current (x-axis) for PCSELs with different numbers of reflection surfaces. 
         FIG. 7  shows the orientation of the 0° and 90° directions of  FIG. 8 . 
         FIGS. 8A to 8C  are graphs of normalized beam intensity (y-axis) as a function of angle (x-axis) for PCSELs operated at 1.1× threshold current, and with different numbers of reflection surfaces 
         FIGS. 9A to 9C  show graphs of beam intensity (y-axis) as a function of emission wavelength for PCSELs operated at a range of currents, and with different numbers of reflection surfaces. 
         FIG. 10  shows a plan view of a laser device according to an embodiment of the present invention. 
         FIG. 11  shows a cross-sectional view of the laser device of  FIG. 10 , viewed along line AA-AA. 
         FIG. 12  shows a plan view of a laser device according to another embodiment of the present invention. 
         FIG. 13  shows examples of centred lasing emission points (A, B, C) for the laser device of  FIG. 12 . 
         FIG. 14  is a cross-sectional view along line BB-BB of  FIG. 13 , showing a beam steering effect when the output of the laser device is directed through a lens. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION 
     PCSELs have been shown to give continuous wave operation at room temperature [1], [2]. By controlling the design, i.e. periodic structure, of the photonic crystal layer in a PCSEL, it is possible to control lasing properties such as beam shape [3] and polarisation [4]. It is also possible to achieve beam steering in this way [5]. PCSELs can be fabricated through wafer fusion [2], or by epitaxial growth [6], [7], [8], [9], [10]. 
     By fabricating arrays of individual PCSEL elements, high lasing power outputs have been achieved [6], [11], [12], [13]. Changes to the photonic crystal layer in particular have been used to increase lasing power output [14]. The highest power output achieved from a single PCSEL element do date was achieved using epitaxial growth, and a controlled photonic crystal design [6]. 
     Gelletta et al. (2015) [15] investigated the effect of external reflection from a single lateral side of a PCSEL element on the lasing properties of a PCSEL. By varying the phase of the external reflector, they were able to show changes to threshold gain (lasing threshold), and to band frequency. The change to the lasing threshold was dependent on the phase of the reflection surface, and the nature of the change was different for the different resonant frequency modes of the photonic crystal in the PCSEL. There was no investigation into the effect that varying the phase of the reflection surface across the photonic crystal region would have. The work reported in Gelletta et al. (2015) [15] was not based on experimental measurement but instead was based on modelling and analysis. 
     Chassagneux et al. (2009) [16] showed that including facets (reflection surfaces) at a lateral side of a THz PCSEL with a hexagonal photonic crystal lattice allowed whispering gallery frequency modes and frequency modes, or frequency modes of the photonic crystal, to be alternately selected. 
     As discussed in more detail below, the present inventors have found that by including a reflection surface to the edge of a PCSEL element, lasing threshold and far-field beam shape of the PCSEL element can be changed. The refection surface can be considered as having a random phase with respect to the photonic crystal layer. The reflection phase varies across the PCSEL element. 
       FIGS. 1 and 2  respectively show a perspective view and a cross-sectional view of a PCSEL element. The PCSEL element of  FIGS. 1 and 2  can be grown by metalorganic vapour phase epitaxy (MOVPE) onto a GaAs substrate, for example. 
     The PCSEL element of  FIGS. 1 and 2  has an n-AlGaAs lower cladding layer  102 , InGaAs active layer  104 , p-InGa etch stop layer  106 , InGaP photonic crystal layer  108 , p-AlGaAs upper cladding layer  110 , p + GaAs cap layer  112 , and ring electrode  114 . Typically a contact electrode would also be provided adjacent to the lower cladding layer  102 . As shown in  FIG. 1 , the contact electrode is annular, with an aperture through which light can be extracted when the PCSEL is lasing. 
     The active layer 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/AlGaAs quantum wells, and AlInGaAsP quantum wells. The active layer may be an undoped GaAs/InGaAs triple quantum well core containing 38 nm InGaAs quantum wells with 20 nm GaAs barriers. 
     The photonic crystal is formed by patterning the InGaP layer with periodic regions having a second refractive index which is different from the first refractive index of the InGaP, thus forming the periodic lattice structure of the photonic crystal. The patterning is done using electron beam lithography (EBL). In particular, holes are patterned in a SiO 2  hard mask, and the pattern is transferred into the InGaP by CH 4 /H 2 /O 2  reactive ion etching (RIE). The regions having the second refractive index InGaP may be left as air gaps/voids, or can alternatively be filled by overgrowth with a suitable filler material having a second refractive index, such as GaAs. 
     The photonic crystal has a footprint size in the plane of the PCSEL element of 150 μm×150 μm. As will be understood, different embodiments may use different dimensions. 
     The photonic crystal may alternatively, or additionally, comprise one or more of AlGaAs, GaInAs, GaInAsP, AlInGaAs, and AlInGaN. 
     An SEM image of the periodic lattice  300  of the photonic crystal of the PCSEL element is shown in  FIG. 3 . The light regions  302  are InGaP having the first refractive index discussed above, and the dark regions  304  are filled with GaAs having the second refractive index discussed above. As can be clearly seen from  FIG. 3 , the dark GaAs regions having the second refractive index each have a triangular shape, and are collectively arranged in a square lattice. 
     This type of photonic crystal is found to be of particular use for generating high lasing power outputs. As the skilled person will understand, other photonic crystal structures as are known in the art could equally be used. For example, the regions having the second refractive index may have square, circular, or other shapes, and may be collectively arranged in a square, hexagonal, or other lattice arrangement. 
     The PCSEL element having the above structure has been shown to operate at room temperature under continuous wave condition. 
       FIG. 4  shows the theoretical photonic band structure about the gamma point (Γ) of the InGaP/GaAs photonic structure of  FIG. 3 .  FIG. 4  was obtained by computer modelling, by solving the definite frequency eigenstates of Maxwell&#39;s equations. The simulation was run using MIT photonic bands (MPB) [18]. The frequency (x-axis) in  FIG. 4  is normalized by multiplying frequency by a/c, where a=lattice constant, and c=speed of light. Γ is the point at which the group velocity of the light in the photonic crystal is zero, and hence represents the point at which resonance occurs in the photonic crystal. Hence,  FIG. 4  shows the four frequencies (A, B, C and D) at which light can resonate in a photonic crystal of  FIG. 3 . 
       FIGS. 5A, 5B, 5C and 5D  show the theoretical modulus of the magnetic field distribution of transverse-electric (TE) polarized waves in the photonic crystal of  FIG. 3 , for the four resonant frequencies A, B, C and D (respectively) shown by  FIG. 4 . 
     In the PCSEL described above, A and B lasing modes were observed in practice. 
     The periodic lattice causes Bragg diffraction of light travelling in the photonic crystal to occur. This Bragg diffraction in turn causes light of a specific frequency to resonate, and hence become amplified, in the photonic crystal. Resonance occurs when the group velocity of light propagating in the photonic crystal approaches zero. The frequency at which light resonates in the crystal will depend on the periodicity/lattice constant of the photonic crystal. This resonant frequency will be the lasing frequency of the PCSEL element. 
     When a voltage is applied across the PCSEL element, the active layer will emit light. This light enters the photonic crystal, and resonates at a given frequency as discussed above to produce laser light. This laser light is then extracted through the emission surface of the PCSEL element (upper cladding layer  110 ). 
     Not all light produced by the laser will be extracted through the laser emission surface. Some light travelling in the plane of the PCSEL element will be transmitted through the lateral sides of the PCSEL element. 
     The inventors have found that by reflecting light back into a PCSEL element from a lateral side of the PCSEL element, thereby changing the feedback inside the PCSEL element, the lasing power and beam shape of the PCSEL element can be changed. 
     To demonstrate this effect, the inventors used a PCSEL element having a 1.5 μm Al 0.4 Ga 0.6 As lower cladding layer, 3 μm quantum well active region (8 nm In 0.2 Ga 0.8 As quantum wells separated by 20 nm GaAs layers), 40 nm p-In 0.48 Ga 0.52  etch stop layer, 150 nm In 0.48 Ga 0.52 P photonic crystal layer (patterned using EBL, with GaAs used to fill the holes in the photonic crystal), 1.5 μm p-Al 0.4 Ga 0.6 As upper cladding layer, and a 400 nm p + GaAs cap layer. The photonic crystal has the same lattice structure as is shown in  FIG. 3 , and has a square footprint in the plane of the device of 150 μm×150 μm. An annular contact electrode with an aperture size of 52 μm was formed on the top cap layer. 
     The lasing power, beam shape and emission spectra were first measured for the pristine PCSEL element (i.e. without any reflection surfaces). Next, the substrate on which the PCSEL element was grown was cleaved along a first lateral side of the PCSEL element, so as to introduce a first reflection surface at the first lateral side of the PCSEL element. Lasing power and far-field beam shape were measured. Then, the substrate was cleaved along a second lateral side of the PCSEL element, adjacent to the first lateral side, so as to introduce a second reflection surface adjacent to the first reflection surface of the PCSEL element. Lasing power and far-field beam shape were measured again. The results are shown in  FIGS. 6, 8A to 8C, and 9A to 9C . 
     The inventors found that adding at least one reflection surface resulted in a single lasing mode (A or B) being selected. 
       FIG. 6  is a graph of lasing power (y-axis) as a function of applied current (x-axis). The lasing power from 0-200 mA was measured. As can be seen from the graph of  FIG. 6 , the lasing threshold of the pristine PCSEL element was 112 mA, and the lasing power at 200 mA was about 1×10 −5  a.u. (arbitrary units). 
     The lasing power of the PCSEL element with a single reflection surface was then measured from 0-200 mA. As can be seen from the graph of  FIG. 6 , the single reflection surface reduced the lasing threshold to 88 mA. The lasing power at 200 mA was increased to about 2×10 −5  a.u. 
     The lasing power of the PCSEL element with two reflection surfaces was then measured from 0-200 mA. As can be seen from the graph of  FIG. 6 , the lasing power and lasing threshold were not particularly altered by adding the second reflection surface compared with having only one reflection surface. 
     The experimental results of  FIG. 6  show that adding a reflection surface at a lateral side of a PCSEL element increases lasing power, and reduces the lasing threshold. 
       FIG. 7  shows the location of the first cleave/reflection surface  702  and the second cleave/reflection surface  703  relative to the PCSEL element. 
       FIGS. 8A to 8C  show beam distribution graphs of normalised intensity against angle for far-field measurements of light emitted through the emission surface of the PCSEL element of  FIGS. 1 and 2 , at an operating current of 1.1× threshold current, and with different numbers of reflection surfaces. 
     In  FIGS. 8A to 8C , the 0° line is beam intensity as a function of angle parallel to the reflection surface. That is, the 0° line shows how beam intensity varies as a function of angle along line  704  in  FIG. 7  (where line  702  is the reflection surface). The 90° line shows how beam intensity as a function of angle perpendicular to the reflection surface. That is, the 90° line shows how beam intensity varies as a function of angle along line  706  in  FIG. 7 . The zero degrees point on the x-axis in  FIGS. 8A to 8C  corresponds to a viewing position along an axis passing through the point at which the lines  704  and  706  of  FIG. 7  intersect, the axis being perpendicular to the plane of the PCSEL element. 
       FIG. 8A  is the far-field beam distribution for the PCSEL element with no reflection surfaces. As can be seen from the figure, the beam shape is approximately circular, and has an angular spread of approximately ±2.5°. 
       FIG. 8B  is the far-field beam distribution for a PCSEL element with a single reflection surface. As can be seen from  FIG. 8B , a single reflection surface has the effect of widening the beam in the direction parallel to the reflection surface, and concentrating the beam in the direction perpendicular to the reflection surface. The angular spread in the direction perpendicular to the reflection surface was decreased slightly, to about ±2°, whilst the angular spread in the direction parallel to the reflection surface was increased to about ±10°. 
       FIG. 8C  is the far-field beam distribution for a PCSEL element with a single reflection surface. As can be seen from this figure, the addition of a second reflection surface adjacent to the first reflection surface has the effect of concentrating the beam in the direction parallel to the first reflection surface, such that the beam is concentrated in both directions when compared with the PCSEL element without any reflection surfaces, with an angular spread of approximately ±1°. 
     Hence, the addition of a single reflection surface at a lateral side of the PCSEL element creates a beam with an oval shape. The addition of two reflection surfaces, at adjacent lateral sides of the PCSEL element, has the effect of reducing the angular spread of the beam when compared with the PCSEL element with no reflection surfaces. 
     Based on the above findings, the inventors have developed a laser device in which lasing power and beam shape can be changed electronically. Such a laser device is shown in  FIG. 10 . 
       FIGS. 9A to 9C  show graphs of beam intensity as a function of emission wavelength for the PCSEL element of  FIGS. 1 and 2 , when operated at a range of currents, and with different numbers of reflection surfaces. 
       FIG. 9A  shows beam intensity as a function of emission wavelength for the pristine PCSEL element with no reflection surfaces. As can be seen from  FIG. 9A , the pristine PCSEL element has a tendency to have two spectral maxima at lower current values. 
       FIG. 9B  shows beam intensity as a function of emission wavelength for the PCSEL element with a first reflection surface. As can be seen from  FIG. 9B , the addition of a single reflection surface results in a single spectral emission peak, which is centred at approximately 63.5 nm. 
       FIG. 9C  shows beam intensity as a function of emission wavelength for the PCSEL element with a second reflection surface adjacent to the first reflection surface. As can be seen from  FIG. 9C , the PCSEL element still has a single spectral emission peak. The emission maxima can be seen to increase as a function of current. The inventors consider that this increased maxima is a result heating effects caused by current confinement in the PCSEL element with two cleaves. 
     The laser device  500  of  FIG. 10  includes a PCSEL element  502 . This PCSEL element  502  in this embodiment has the same structure as the PCSEL element  100  of  FIGS. 1 and 2 . A controllable light-transmission region  504  and a DBR reflector  506  are formed at each lateral side of the PCSEL element  502 . The controllable light-transmission region at each lateral side is interposed between the PCSEL element and the respective DBR reflector. This forms the laser device. 
     Each DBR reflector  506  is configured to reflect light which is incident upon it and travelling in a direction substantially parallel to the plane of the PCSEL element  502 . The DBR reflectors exist in the plane of the PCSEL element. In this way, light travelling out of the PCSEL element  502  can be reflected back into the PCSEL element  502  by a DBR reflector  506 , thereby altering the feedback within the PCSEL element. 
     The optical properties of each controllable light-transmission region  504  are controllable by an electrical input. In particular, by controlling a current supplied to a given controllable light-transmission region  504 , the optical properties of the controllable light-transmission region  504  can be controlled. In particular, the optical properties of the controllable light-transmission region can be controlled to be anywhere between a transmission state, in which substantially all light is transmitted, and an extinction state, in which substantially no light is transmitted. Preferably, it is switched between the transmission and extinction states. Moreover, the controllable light-transmission region is controlled by passing a current through it. When this current exceeds a threshold value, the contact is essentially transparent, allowing light to pass through it. This is the transmission state. When the current is below the threshold value, the contact no longer allows light to pass through it. This is the extinction state. 
     When controlled to be in the transmission state, the controllable light-transmission region  504  will transmit light travelling out of the PCSEL element  502  in the plane of the PCSEL element. This light will then be reflected by the corresponding DBR reflector  506 , and back into the PCSEL element  502 . Conversely, when the controllable light-transmission region is controlled to be in the extinction state, light will not be reflected back into the PCSEL element  502 . Accordingly, the feedback from each lateral side of the PCSEL element can be electronically controlled. 
     As has been shown in relation to  FIGS. 6, 8A to 8C and 9A to 9C  above, by selectively controlling whether light is reflected back from specific lateral sides of the PCSEL element, the power output and beam shape of the laser device can be altered. Hence, the laser device  500  enables the lasing power and beam shape to be altered electronically. 
     For example, by controlling the controllable light-transmission regions  504  so a first controllable light-transmission region is in a transmission state and the other three are in an extinction state, the lasing threshold can be reduced, and the lasing power at a given operating current above the lasing threshold can be increased. Furthermore, the beam can be made to have an oval shape. By controlling two adjacent controllable light-transmission regions  504  to be in the transmission state, and controlling the other two to be in the extinction state, the laser beam from the laser device  500  can be concentrated. 
       FIG. 11  shows the laser device  500  viewed along line AA-AA. As can be seen from  FIG. 11 , the PCSEL element  502  has the same structure as the PCSEL element from  FIGS. 1 and 2 . 
     The controllable light-transmission regions  504  have the same layered structure as the PCSEL element. Accordingly, the controllable light-transmission regions  504  have substantially the same refractive index as the PCSEL element  502 , thereby preventing reflection from occurring at the interface between the PCSEL element  502  and the controllable light-transmission regions  504 . However, the InGaP layer of the controllable light-transmission regions  504  (corresponding to the photonic crystal layer  108  of the PCSEL element) is not patterned with periodic regions having a different refractive index from the InGaP. This layer is therefore not a photonic crystal in the controllable light-transmission regions  504 . The controllable light-transmission regions  504  have their own electrodes  600 , which are electrically isolated from the PCSEL element electrodes  114 . 
     As such, the controllable light-transmission regions  504  can be controlled electronically, independently of each other, and independently of the PCSEL element  114 . 
     The PCSEL element  502 , controllable light-transmission regions  504  and DBR reflector  506  all exist in the same plane, so as to selectively reflect light travelling out of the PCSEL element in the plane of the laser device, based on a current supplied to the optically tunable region(s)  504 . 
     In some embodiments, several laser devices may be grown on a single substrate. This substrate will then be cleaved after the fabrication process, thereby providing a plurality of individual laser devices, each laser device containing a single PCSEL element. 
       FIG. 12  shows a modification to the laser device of  FIGS. 10 and 11 . In this laser device  800 , each controllable light-transmission region  802  is segmented into a plurality of optically tunable segments  804 . Each segments  804  may be controlled independently from the other segments. Accordingly, each individual segment can be independently controlled to be in either a transmission state or an extinction state. As such, the feedback from a given lateral side can be modulated. For example, by having a single segment at a lateral side in the transmission state, and the other segments in the extinction state, the lateral side can be made to provide weak feedback to the PCSEL element. 
     This ability to fine-tune the feedback at each lateral side can enable the emission intensity of the laser device to be fine-tuned, with a higher level of control than by uniformly controlling feedback from an entire lateral side, as in the embodiment of  FIGS. 10 and 11 . 
     Furthermore, by spatially modulating the feedback from each lateral side, the beam shape can more accurately be controlled. Hence, segmented electrodes can be used to steer the laser beam, as well as adjust its shape and angular divergence. 
     The beam steering effect is illustrated in  FIGS. 13 and 14 . 
     By individually controlling the segments  804 , the laser output of the PCSEL device  800  can be centred at different positions in the plane of the photonic crystal.  FIG. 13  illustrates three different exemplary positions (A, B and C) at which the laser output of the laser device  800  can be centred, by individually controlling the segments  804 . These positions are intended to be illustrative, and non-limiting. 
       FIG. 14  shows a cross-section along line BB-BB of  FIG. 13 , and a path that laser light emitted from points A, B and C would take if allowed to pass through a lens  900  focused at B. The light from these three points is transmitted and refracted by the lens  900 .  FIG. 14  shows the path of light from point A to point A′, point B to B′, and point C to C′, points A′, B′ and C′ being in plane  902 . Hence, by controlling the laser output to be centred at one of points A, B or C, beam steering can be achieved. 
     The inventors have also found that this fine-tuning of the lasing output can be used to compensate for variations in device performance brought about by manufacturing issues for each individual PCSEL element. For example, deleterious effects on the lasing output caused by inaccuracies in the photonic crystal structure, heatsinking, and other manufacturing issues, can be compensated for by fine-tuning the feedback using the segmented contacts, e.g. to achieve higher single mode lasing power outputs. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 
     All references referred to above and/or listed below are hereby incorporated by reference in their entirety. 
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