COMMUNICATIONS APPARATUS

Abstract: A optical communications apparatus has an optical transmitter (T) with a signal source (10) for supplying a data signal comprising data to be transmitted, a photo-emitter (40) for emitting photons, a spin injector (30) supplying spin-polarised charge carriers for causing the photo-emitter to emit photons having a polarisation determined by the spin-polarisation of te charge carriers supplied by the spin injector (30), and a controller (20) for controlling the spin injector to cause the spin polarisation of the charge carriers supplied by the spin polarisation of the charge carriers supplied by the spin injector (30) to be controlled in accordance with data to be transmitted such that, in operation, the photo-emitter (40) provides an optical signal in which the polarisation of the photons emitted by the photo-emitter is modulated on the basis of data supplied by the signal sources; and a receiver (R) with a polarisation of photons received from the photo-emitter (40) and a data extractor (24) for extracting data transmitted in the optical signal provided by the photo-emitter on the basis of the photon polarisations determined by the polarisation detector.

Description: COMMUNICATIONS APPARATUS This invention relates to communications apparatus, in particular communications apparatus for enabling communications using an optical communications path. There is considerable interest in optical communications for telecommunications and data communications purposes, especially for the final part of a communications link from a communications network provider such as a telecommunications network provider to and within the premises of an end user such as a business. These optical communications systems may be free-space or optical fibre based and generally use a time (phase/frequency) and/or amplitude modulation scheme for transmitting data. There is, however, an on-going requirement for greater bandwidth or information encoding modulation in such communication systems. Another possible modulation scheme is polarisation modulation. Polarisation modulation has the advantage that manipulating the polarisation does not diminish the signal power and should not limit the available signal bandwidth and modulation depth. In addition, in comparison to amplitude modulation, polarisation modulation should have reduced signal transmission errors because noise is less likely to cause signal detection errors . There are a number of different types of optical polarisation modulators currently available. These polarisation modulators modulate optical polarisation using physical effects such as electro-optic effects wherein, for example, birefringence is induced or modified in a liquid (Kerr effect) or solid (Pockels effect) or mechanical effects such as photo-elasticity where stressing by compression or stretching of a solid transparent to the light to be transmitted induces birefringence. Existing polarisation devices are, however, relatively bulky and generally require the use of macroscopic electro-optic crystals. Furthermore, because of difficulties in maintaining polarisation in optical fibres and with birefringence in free space optical communications, such optical polarisation modulators have generally only been used to enable transmission of data by amplitude modulation. A first aspect of the present invention provides a transmitter for optical communications apparatus wherein the transmitter comprises a photo-emitting device, a spin injector for injecting spin polarised carriers into the photo-emitting device to cause the photo-emitting device to emit photons that are either left or right circularly polarised, dependent upon the polarisation of the injected spin current, and control means for controlling the spin injector to modulate or change the polarisation of the charge carriers injected into the photo-emitting device in accordance with the signal to be transmitted. In an embodiment, the spin injector comprises a three- dimensionally quantum confined region arranged to operate in the Coulomb blockage regime coupled by tunnel barriers to input and output Fermi leads whereby application of a magnetic field causes Zeeman splitting in the quantum confined region and any Zeeman splitting in the Fermi leads is such that only a fraction, if any, of the current in the Fermi leads is spin polarised but conduction through the quantum confined region occurs primarily by sequential tunnelling of electrons having only one spin polarisation. The spin polarisation of an electron in the quantum confined region and thus the spin polarisation of the charge carriers provided by the spin injector may be controlled by controlling at least one of an applied magnetic field and voltages applied to the spin injector. In another embodiment, the spin injector comprises two spin injector devices each comprising a ferromagnetic heterostructure controlled by respective local magnetic fields to emit spin polarised charge carriers of opposite polarity and the control means comprises first and second control means for controlling operation of the first and second spin polarisation devices so that either one or both of the spin injector devices emit spin polarised charge carriers at the same time. The optical signal emitted by the photo-emitting device may also be amplitude modulated by controlling the current of charge carriers supplied by the spin injector. The optical signal may also be phase modulated in addition to polarisation modulation. For example, where as in the above described embodiment, two separate spin injector devices are provided, then the relative timings of operation of the two spin injector devices may also be used to transmit information in the optical signal. In another aspect the present invention provides a spin injector for use in a transmitter for an optical communications apparatus wherein the spin polarisation of charge carriers emitted by the spin polariser is controlled by a gate voltage. In an embodiment, the spin injector may be a layer of magnetic semiconductor which exhibits hole-induced ferromagnetism that can be controlled by field-effect, for example by use of an insulated gate with negative gate voltage increasing hole concentration and enhancing the ferromagnetic interaction. In another aspect the present invention provides a method of enabling control over the polarisation of photons emitted by a vertical cavity surface emitting laser (VCSEL) which comprises using a spin injector to drive the VCSEL so that the injected current is spin-polarised thereby controlling the polarisation of photons emitted by the VCSEL. This should enable the problems of undesired polarisation switching that have been observed in VCSELs (see for example, the paper entitled "Polarisation Switching in VCSELs: Experiments and Theory" by Danckaert et al published in the 18 th congress of the International Commission for Optics:Optics for the next Millennium, Proceedings of the SPIE Volume 3749 at pages 302 to 303, 1999 and other similar articles. In one aspect, the present invention provides a receiver for a communications apparatus, wherein the receiver comprises a spin injector device operated in reverse so that the current through the spin injector device is determined by the polarisation of incident photons. In another aspect, the present invention provides a magnetic sensor wherein the magnetic sensor comprises a spin injector that provides charge carriers of one or the other spin polarity dependent upon the polarity of a magnetic field to which the spin injector is subjected and means for sensing the polarisation of the charge carriers emitted by the spin injector to determine the polarity of the magnetic field to which the spin injector is subjected. The spin injector may have any of the forms described above and, in an embodiment, the spin injector may be associated with soft magnetic pole pieces to enhance its sensitivity. In an embodiment, a transmitter for use in optical communications apparatus may comprises a two dimensional array of photo-emitting devices each comprising a spin injector and a photo-emitter that emits right or left circularly polarised photons, dependent upon the spin polarisation of the charge carrier injected into the photo-emitter by the spin injector. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 shows a very diagrammatic functional block diagram of communication apparatus embodying the invention; Figures 2 and 3 show very diagrammatic functional different diagrammatic views of part of a transmitter of the communication apparatus shown in Figure 1; Figure 4 shows a functional block diagram of one example of a receiver for use in the communication apparatus shown in Figure 1; Figure 5 shows a functional block diagram of another example of a receiver for use in the communication apparatus shown in Figure 1; Figure 6 shows a very diagrammatic cross-sectional view through part of a semiconductor body to illustrate one example of a photoemissive device, comprising a spin injector and a photoemitter, for use in the transmitter of the communication apparatus shown in Figure 1; Figure 7 shows a very diagrammatic cross-sectional view through part of a semiconductor body to illustrate another photoemissive device for use in the transmitter of the communication apparatus shown in Figure 1; Figure 8 shows a cross-sectional view of part of a semiconductor body illustrating another example of a photoemissive device for use in the transmitter of the communication apparatus shown in Figure 1; Figure 9 shows a functional diagram of a two-dimensional array that may be used in the transmitter of the communication apparatus shown in Figure 1; Figure 10 shows a very diagrammatic functional block diagram of another example of communication apparatus embodying the invention; Figure 11 shows a very diagrammatic functional block diagram of another example of communication apparatus embodying the invention; Figures 12a and 12b show schematic sectional views through parts of examples of photoemissive devices suitable for use in the communication apparatus shown in Figure 11; Figure 13 shows a very diagrammatic cross-sectional view of part of a semiconductor body to illustrate another example of a photo-emissive device, similar to that shown in Figure 6, for use in the transmitter of the communication apparatus shown in Figure 1; Figure 14 shows a diagrammatic representation of part of the spin injector shown in Figure 12b to illustrate a modification thereof: Figure 15 shows a very diagrammatic top-plan view of the spin injector shown in Figure 6 to illustrate the addition of soft magnetic pole pieces for focussing a magnetic field; and Figures 16 and 17 show very diagrammatic functional diagrams of a memory device embodying the invention. Referring now to the drawings, Figure 1 shows a very diagrammatic functional block diagram of communication apparatus embodying the present invention. As shown in Figure 1, the communication apparatus comprises a transmitter T and a receiver R. The transmitter T comprises a signal source 10 that provides, for example, from an original analogue signal or a digital data signal, digital electrical signals in which the data to be transmitted is represented as zeros and ones in conventional manner. The digital output of the signal source 10 is supplied to a modulator 20 that, as will be described in greater detail below, controls the polarisation of photons output by a photo- emissive device 21. The photo-emissive device 21 consists of a spin injector 30 that provides charge carriers, in this example, electrons, having one or the other spin polarity (spin state up or spin state down) dependent upon the control signals applied by the modulator 20. The spin- polarised charge carriers supplied by the spin injector 30 are injected into a photo-emitter 40 in the form of a PIN diode that, as explained in the Letter to Nature by Fiederling et al at pages 787 to 789 of volume 402 of Nature published on 16 December 1999 and by Ohno et al at pages 790 to 792 of volume 402 of Nature published on 16 December 1999, in response emits circularly polarised photons with the direction of polarisation (right or left) being dependent on the spin polarisation (up or down) of the injected spin polarised current. As reported by D Awaschalom and J Kikkawa in Physics Today Volume 52 at page 33, 1999 and in Nature Volume 397 at page 139 (1999), surprisingly, the electrons maintain spin coherence over macroscopic distances (typically 100 micrometres) and spin dephasing times of a hundreds nanoseconds have been observed. Accordingly, the spin polarised electrons generated in the spin injector can maintain spin coherence for a sufficiently long time that the current injected into the photo-emitter is still spin-polarised. In this embodiment, the circularly polarised (left or right) photons are emitted into free space to be received by a receiver R. The receiver R has a polarisation detector 22 that detects the polarisation (right or left circularly polarised) of the received photons and supplies this information to a signal extractor 24 that, in conventional manner extracts the data carried by the signal from this information. As will be appreciated by those skilled in the optical communications art, the modulator 20 and photo emissive device 21 shown in Figure 1 replace the switched laser or LED of a conventional optical communications arrangement. The manner in which the series of digital zeros and ones representing the data may be represented by the optical signal supplied by the photo-emissive device may vary. In a simple scheme, right circularly polarised photons may represent a digital one while left circularly polarised photons may represent a digital zero or vice versa. More sophisticated modulation schemes can be envisaged. Furthermore, in addition to modulating the polarisation, amplitude modulation may be effected by allowing the modulator to control the current through the spin injector and so the number of spin-polarised electrons injected into the photo-emitter which will, in turn, control the number of photons emitted by the photo- emitter 40. Furthermore, pulse width or phase modulation schemes may be used in addition to the polarisation modulation. Where such additional modulation schemes are also used, then, as is well known in the communications art, the signal extractor 24 will include appropriate conventional demodulation circuitry for recovering the data from the modulated signal. An example of a photo-emissive device 21 suitable for use in the transmitter T shown in Figure 1 will now be described with reference to Figures 2 and 3 which show very schematic functional block diagram of the photoemissive device 21 from different view points. In the interest of clarity some components shown in Figure 2 are omitted from Figure 3 and vice versa. In this example, the spin injector 30 comprises a three- dimensionally quantum confined region in the form of a quantum dot 2a in a quantum dot region 2 coupled by tunnel barriers TBl and TB2 to respective input and output Fermi leads 3 and 4 formed by highly doped semiconductor regions such that the Fermi leads 3 and 4 have a g-factor smaller than that of the quantum dot region 2. A DC magnetic field generator ("DC FIELD GEN" in Figure 3) 5 is provided for producing a magnetic field in the plane of the layers of the spin injector 30 (perpendicular to the paper in Figure 2) for inducing Zeeman splitting in the quantum dot 2a. The input and output Fermi leads 3 and 4 are coupled to respective contacts LI and L2 between which a voltage source VS is coupled. An annular gate G contacts the quantum dot region 2 and is coupled to a gate voltage source GV (Figure 3). The quantum dot 2a is configured to operate in the Coulomb blockade regime which is discussed in detail in, for example, section 2.1 at pages 114 to 118 of the chapter entitled "Electron transport in quantum dots" by Kouwenhoven et al in the review text book entitled "Mesoscopic Electron Transport" edited by Sohn, Kouwenhoven, and Schδn (ISBN No. 0-7923-4737-4). The Coulomb blockade regime requires that: K B T « ΔE , e 2 /C ( 1 ) where K B is Boltzmann's constant, T is temperature, ΔE is the difference between energy levels in a quantum dot, e is the electron charge, C is the capacitance of the quantum dot, and the " , " means that the inequality is satisfied for both components on the right hand side, that is: K B T « ΔE and K B T « e 2 /C so that tunnelling of an electron onto the quantum dot is only possible when the additional energy e/C is applied by, for example, means of a voltage applied to the gate G. As shown in Figure 3, the photo-emitter 40 of the transmitter T is coupled to contact L2 to receive the spin filtered current from the spin injector 30. The manner in which this spin injector 30 generates spin-polarised electrons for injection into the photo- emitter 40 will now be briefly explained. Further details can be found in International Application No. GBOO/03422, the whole contents of which are hereby incorporated by reference. Thus, in operation of the spin injector 30, if necessary, a voltage is applied to the annular gate G to provide further confinement to define a quantum dot 2a within the quantum dot region 2 and a voltage V 12 is applied by the voltage source VS between the contacts LI and L2. This voltage is, in the Coulomb blockade regime with one electron 20 in the uppermost ground state of the quantum dot 2a, related to the chemical potentials μ λ and μ 2 of the Fermi leads 3 and 4 as follows: V lf2 = (μι-μ 2 )/e (2) A DC magnetic field B dc is applied, using the DC field generator 5 to, as is well known in the art, lift of the spin degeneracy of the energy levels in the quantum dot and cause Zeeman splitting between the spin state up and spin state down levels where the Zeeman splitting Δ z is given by: Δ Z =|E1- El|=μ B |gB| (3) where Etand Elrepresent the energy of the up and down spin states, g is the g-factor of the material, μ B is the Bohr magneton and B is the applied DC magnetic field. The spin injector 30 is configured such that the Zeeman splitting resulting from the applied DC magnetic field is sufficiently large that the Zeeman splitting in the quantum dot 2a is larger than Δμ and larger than KT. The Zeeman splitting in the Fermi leads 3 and 4 is very small compared to the Fermi energy in the leads so that both spin polarisations (up and down) are equally available for transport through the quantum dot 2a. This is the standard situation for typical quantum dot experiments as described in the textbook "Mesoscopic Electron Transport" mentioned above with the Fermi energy in the leads being typically a few hundred degrees Kelvin while the Zeeman splitting is of the order of a few Kelvin at most (for DC magnetic fields of a few Tesla at most). The voltages applied to the spin injector 30 cause the quantum dot 2a to contain an odd number, N, of electrons with the uppermost electron in the ground state having, in this example, spin state up. When the magnetic field B dc is applied, Zeeman splitting Δ z occurs in the ground E s and triplet energy states, thus lifting the spin degeneracy. In this example, the direction of the DC magnetic field B dc is taken to be such that the electron in spin state up has a lower energy than would an electron in spin state down. It will, of course, be appreciated that both the singlet E s and triplet energy levels are split. In order for a current to flow through the spin injector 30, an electron must tunnel from the Fermi lead 3 onto the quantum dot 2a and an electron must tunnel off the quantum dot 2a onto the Fermi lead 4. Because the quantum dot 2 is in the Coulomb blockade regime, the charge on the dot is quantised in units of electron charge. Also, because the tunnelling coupling is weak, tunnelling between the Fermi leads 3 and 4 and the quantum dot 2a can be described as a perturbation enabling the standard master equation approach to be used to calculate the current in the stationery limit. Evaluation of tunnelling rates shows that two regimes of transport can naturally be distinguished, namely a sequential tunnelling regime where the number of electrons N on the quantum dot 2a fluctuates between N and N±l and which is a first order transition which must obey energy conservation and a cotunnelling regime which is of higher order and thus provides a much smaller contribution to the current and in which an electron effectively tunnels directly from one Fermi lead 3 to the other Fermi lead 4 via a virtual state on the quantum dot 2a with the only allowed processes being second order transitions with the initial and final electron number on the quantum dot 2a being the same, namely N. In order for the spin injector 30 to be in the sequential tunnelling regime, the voltage applied to the gate G is tuned so that the chemical potential μ x of the input Fermi lead 3 is comparable to or greater than the energy of the next unoccupied level of the quantum dot 2a while the chemical potential μ 2 of the output Fermi lead 4 is less than or comparable to the energy of that level namely: When E s >μ 1 and E s >μ 2 the spin injector 30 is in the co- tunnelling regime where two possible virtual states, a triplet and a singlet state, can be occupied. If N is even then the ground state contains a topmost singlet state with E s <μ ! and E s <μ 2 . The device is configured and operated such that: E τ+ - E s , Δ 2 > Δμ, K B T where Δμ = |μ x _μ 2 | (5) where the " , " again means that the inequality is satisfied for both components on each side of the inequality that is : E τ+ -E s > Δμ , E τ+ -E s > K B T, Δ z > Δμ and Δ z > K B T The g-factor, magnetic field applied bias Δμ and temperature K a l^ o are such that only ground state transitions are allowed by energy conservation. Therefore, because the ground state of the quantum dot 2a already contains an electron of spin-up state, a spin- up state electron on the input Fermi lead 3 cannot tunnel into the ground state and accordingly the only possibility is for that electron to tunnel into an excited triplet state. This is, however, forbidden by energy conservation. In contrast, an electron with spin state down and an energy equal to the additional Coulomb charging energy e 2 /C can tunnel onto the dot 2a, thus obeying Fermi's Golden rule. Thus, only an electron in a spin-down state can tunnel onto the dot 2a in the sequential tunnelling regime if the uppermost electron on the dot is in the spin-up state. Therefore, in the Coulomb blockade regime the quantum dot 2a blocks tunnelling of spin-up state electrons from the Fermi lead 3 when the uppermost electron on the quantum dot is a spin-up state electron. This process is fundamentally different to previously introduced spin blockade effects such as described by the Wiedmann et al in Physics review letters volume 74, 1995 at page 984 because they occur for non-spin polarised currents and vanish with increasing magnetic fields, in contrast to the effect being discussed here. Once the spin-down state electron has tunnelled onto the quantum dot 2a then the spin-down state electron can tunnel from the quantum dot 2a onto the lower chemical potential μ 2 output Fermi lead 4 and the spin-up state electron on the quantum dot returns to its previous energy level. Because the Zeeman splitting Δ z > Δμ,K B T, tunnelling of the spin-up state electron from the quantum dot 2a onto the output Fermi lead 4 is prohibited because otherwise an excited spin would be left on the quantum dot 2a, violating energy conservation. Above or below a sequential tunnelling resonance tuned by the gate voltage, that is when E s is greater than the chemical potentials of the input and output Fermi leads 3 and 4 or is less than the chemical potentials of the input and output Fermi leads 3 and 4, the spin injector 30 is in the cotunnelling regime where tunnelling can only occur directly from one lead to the other via a virtual state on the quantum dot. In both cases, that is above and below the resonance, the efficiency (that is the ratio of the required spin-down current to the unwanted spin-up current) can be made very large by tuning the gate voltage and/or the chemical potentials μ 1 and μ 2 back to resonance such that the device eventually returns to the sequential tunnelling regime. The spin filtering is very efficient provided that Δ 2 > Δμ,K B T. It will be appreciated that inelastic processes and processes where the quantum dot 2a is not in the ground state are suppressed by the Zeeman energy. As can be seen from the above, the spin state of the electrons output from the quantum dot 2a is determined by the spin state of the uppermost electron present on the quantum dot 2a. The spin state of the uppermost electron on the quantum dot 2a is controlled by the modulator 20. As illustrated diagrammatically in Figure 3, the modulator 20 may control the DC magnetic field generator 5 as to control magnetic field polarity. Reversing the polarity of the magnetic field B dc causes a reversal in the Zeeman splitting so that, where N is odd, the uppermost ground state electron will be spin state down and the filtered current will be spin state up while where N is even the filtered current will be spin state down. Thus, by reversing the polarity of the magnetic field B dc from that described above, the modulator 20 can cause the uppermost electron on the quantum dot 2a to be in the spin-down state (rather than the spin-up state) so that tunnelling through the quantum dot 2a of a spin-up state electron but not a spin-down state electron is allowed, thereby reversing the polarization of the spin polarised electrons output by the spin injector. Thus, the modulator 20 can change the polarisation of the photons output by the photo-emitter 40 from left to right circularly polarised and vice versa by reversing the polarity of the magnetic field produced by the magnetic field generator. The spin polarisation of the spin-filtered current can also be controlled by the modulator 20 tuning the gate voltage and/or chemical potentials of the Fermi leads (i.e. the voltage supplied by the voltage source VS) to change the number of electrons N on the quantum dot 2a from odd to even with, for the magnetic field polarisations discussed above, the filtered current being a spin state down electron current where N is odd and the filtered current being a spin-up state current where N is even. Examples of receivers R that may be used in a communications apparatus shown in Figure 1 will now be described with reference to Figures 4 and 5. In the receiver R shown in Figure 4, the polarisation detector 22 comprises two photodetectors 41a and 41b provided for detecting photons of the wavelength or band of wavelengths emitted by the photo-emissive device 21. Each of the photodetectors 41a and 41b is associated with a polarising filter PF1 and PF2 with the polarising filter PF1 being arranged to transmit only left circularly polarised photons and the polarising filter PF2 being arranged to transmit only right circularly polarised photons. Thus, each of the photo detectors 41a and 41b only provides an output to the signal extractor 24 when it receives the photons of the correct circular polarisation. The signal extractor 24 can thus determine from the signals received from the photo detectors 41a and 41b the data transmitted by the transmitter T in accordance with the modulation scheme adopted for the communication. Figure 5 shows a very diagrammatic representation of any other example of a receiver R suitable for use in the communication apparatus shown in Figure 1. In this example, the polarisation detector 23 has the same structure as the spin injector described above with reference to Figures 2 and 3 except that a part of the contact Ll is transparent to the received photons P or has a window or aperture 3a for enabling the received photons P to be incident on a surface 3a of the Fermi lead 3. The incident photons result in generation of electrons of the corresponding spin polarity. A voltage/current detector 25 is coupled across the contact Ll and L2. In this embodiment, the voltage/current detector 25 applies a voltage between the contact Ll and L2 and senses the current flowing between the contacts Ll and L2. This current will be dependent on the spin state of the uppermost electron on the quantum dot 2a with, as described above, when the uppermost ground state electron on the quantum dot 2a is in spin state up, only electrons having spin state down being transmitted through the quantum dot 2a. The polarisation detector 23 shown in Figure 5 will thus provide a current output that is dependent on whether the received photons are left or right circularly polarised. Two such polarisation detectors 23 may be provided with one of the quantum dots 2a being tuned to have its uppermost ground state electron in the spin-up state and the other to have its uppermost ground state in the spin- down state so that a current flows through one of the devices when the received photons are right circularly polarised and through the other of the two devices when the received photons are left circularly polarised. The output of each of the voltage/current detectors 25 will then be supplied to the signal extractor 24 which, as described above, can then recover the transmitted data. In the above described arrangement, the voltage/current detector 25 applies a voltage between the contact Ll and L2 and the current is detected. As another possibility, the voltage/current detector 25 may apply a small biasing current between the contacts Ll and L2 and the voltage between these contacts may be detected with, in this case, the voltage being dependent on whether the incident photons are right or left circularly polarised. Each of the receivers R described above enables the data to be recovered from the polarisation modulated signal supplied by the photo-emissive device 21. The spin injector 30 may be manufactured using standard quantum dot fabrication techniques such as, for example MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapour deposition) and lithographic techniques such as x-ray or electron beam lithography. The photoemitter 40 will generally be integrated with the spin injector 30. Figure 6 shows a very diagrammatic cross-sectional view of the part of a semiconductor body 10 to illustrate one way of manufacturing a photoemissive device 21 In this example, the spin injector 30 is formed on top of the photoemitter 40. The photoemitter 40 comprises a p-conductivity type Gallium Arsenide layer 43 on which is provided a p-conductivity Aluminum Gallium Arsenide (AlGaAs) layer 44 followed by an intrinsic (that is not- intentionally doped) Gallium Arsenide layer 45 and an n-conductivity type Aluminium Gallium Arsenide layer 46. An electrode 47 contacts the p-conductivity type Gallium Arsenide layer 43. The photo-emitter 40 is therefore formed as a PIN diode. The spin injector 30 is then formed by growing alternate layers of, in this example, III-V semiconductors. The Fermi leads 3 and 4 may be formed of Gallium Arsenide which has a g-factor of about -0.44 while the quantum dot region 2 may be formed of Indium Gallium Arsenide which has a g-factor of 15. As another possibility, the quantum dot region 2 may also be formed of Gallium Arsenide. However, the use of Indium Arsenide to form the quantum dot region 2 has the advantage that the g-factor of the quantum dot region 2 will be much higher so that when the spin injector 30 is subjected to a DC magnetic field, the Zeeman splitting in the quantum dot will be about 30 times greater than for Gallium Arsenide. This has the advantage that operating temperatures can be about 30 times higher when Indium Arsenide rather than Gallium Arsenide is used to form the quantum dot region 2. A further increase in Zeeman splitting, and thus operating temperature, for the same magnetic field can be achieved by using a magnetic semiconductor to form the quantum dot region 2. For example, the material used may be n-conductivity type Beryllium Manganese Zinc Selenide alloy (Be x Mn y Zn 1 _ x _ y Se) having a composition such that the II-VI material is lattice-matched to the III-V material forming the quantum dot region 2 which is, in this example, n-conductivity type Gallium Arsenide (GaAs). Typically the II-VI alloy may have the composition Be 0 . o7 Mn 0 . o3 Zn 0 . 9 Se. The use of such a II-VI magnetic material would increase the operating temperature of the spin filtering device again by a factor of 10 which would allow operation at room temperature where the applied magnetic field has a magnitude of a few Tesla and the quantum dot has dimensions of 50 nanometers or less. The layers deposited on top of the PIN diode 40 are patterned using standard vertical quantum dot defining lithographic techniques to define a column C providing first and second n-conductivity type III-V regions defining the Fermi Leads 3 and 4 separated by, as explained above, either another III-V or a II-VI region defining the quantum dot region 2. As can be seen from Figure 5, the etching process does not continue all the way through the bottom of the lowermost layer deposited onto the PIN diode, rather a portion 3a of that layer remains . A chromium layer 12 is then deposited to enable ohmic contact to the Fermi lead 3 followed by an insulating material (generally silicon dioxide) layer 13 followed by a further metallic layer (generally chromium) 14 and a further insulating layer 15. Vias VI and V2 are etched through these layers in known manner and metallisation (again generally chromium) deposited to define the first contact Ll contacting the chromium layer 12, the gate G contacting the chromium layer 14 and the contact L2 contacting the Fermi lead 4. The entire structure is patterned using standard lithographic techniques to define the spin injector 30 as a MESA on top of the PIN diode 40 so that an area SA of the uppermost surface of the n-conductivity type Aluminium Gallium Arsenide layer 46 of the PIN diode structure is exposed from which photons P can be emitted during use. Figure 7 shows a very diagrammatic cross-sectional view through part of a semiconductor body 10a to illustrate another example of a photoemissive device 21. In this example, the semiconductor body 10a comprises a substrate 11, generally an intrinsic (that is not intentionally doped) Gallium Arsenide (GaAs) substrate 11, on to which may be grown, as is well known in the MBE and MOCVD art, a Gallium Arsenide |Aluminium Gallium Arsenide superlattice structure SL to provide a clean relatively defect free surface S. In this example, the spin injector 30 has the same structure as shown in Figure 6. However, the spin injector 30 is formed on top of the superlattice SL rather than on top of the PIN diode 40. Using standard lithographic, etching and epitaxial techniques, the PIN diode 40' is also formed on the superlattic SL laterally spaced from the spin filtering device. The PIN diode 40' consists of a p-conductivity Aluminium Gallium Arsenide layer 43' followed by a p-conductivity type Gallium Arsenide layer 44', an intrinsic Gallium Arsenide layer 45' and an n-conductivity type Aluminium Gallium Arsenide layer 46' providing a surface SA' from which photons P may be emitted during use. The layers 44', 45' and 46' are etched to expose a contact region CO of the layer 43' and insulating material 16 and metallisation deposited and lithographically defined to define an electrode 47' contacting the p-conductivity type Aluminium Gallium Arsenide layer 43' and an electrode 48 coupling the contact L2 to the n-conductivity type Aluminum Gallium Arsenide layer 46'. It will, of course, be appreciated that Figures 6 and 7 are very schematic and that the relative dimensions are not in proportion. In particular, layer thicknesses have been relative exaggerated in these figures and generally the area SA or SA' of the photoemissive surface of the PIN diode 40 or 40' will be much larger than shown. Using standard lithographic techniques, the column C may have a circular cross section with a diameter of about 0.5 micrometers which, as will be understood by the person skilled in the art, is not sufficiently small to define a quantum dot within the quantum dot region 2. However, as well known by person skilled in the art, confinement in the lateral direction in Figure 6 or 7 may be achieved by applying a voltage to the gate G to define within the quantum dot region 2 a quantum dot having a diameter of about 50 nanometers. The quantum dot is required to operate in the quantum Coulomb blockade regime. For the materials and quantum dot dimensions given above, then the quantised level spacing and Coulomb charging energy will be of the order of 1 meV (milli electron volt) so that, in this example, the device should be operated at liquid Helium temperatures (a few Kelvin) using conventional low temperature generation techniques. It will, however, be appreciated by those skilled in the art that scaling down the size of the quantum dot by a factor of 10 will raise the necessary temperature by a factor of 100 thus allowing operation at room temperature. The DC magnetic field generator 5 is required to generate a homogenous DC magnetic field of the order of at most a few Tesla. The DC magnetic field generator 5 may, therefore, consist of a commercially available nanomagnet. As other possibilities, the DC magnetic field generator 5 may be provided in a manner similar to that used for IBM magnetic storage discs or dysprosium dots may be used. Where nanomagnets or dysprosium dots are used, then these may be integrated with the device 1. As another possibility, a superconducting magnet may be used as the DC magnetic field generator 5. Dependent upon the g-factor of the quantum dot region 2, the magnetic fields required may be provided by, for example, permanent magnets or even electrically driven solenoids. As will be appreciated by those skilled in the art, a number of photoemissive devices 21 comprising a spin injector 30 and a photoemitter 40, 40a, having, for example, the structure shown in Figure 6 or 7 may be integrated on the same substrate and coupled so as to provide a two-dimensional array of photo-emissive devices 21 in which the polarisation of the photons produced by an individual photo-emissive device can be controlled by selecting N to be odd or even or by locally reversing the magnetic field as described above. Figure 8 shows a cross-sectional view of part of a semiconductor body 10 in which a two-dimensional array of photo-emissive devices 21 is formed and Figure 9 shows a block diagram of the array 30 to illustrate addressing of individual photo-emissive devices. Figure 8 shows a cross-sectional view of part of the semiconductor body carrying a single photo-emissive device 21 comprising a spin injector 30 and photo-emitter 40. It will, of course, be appreciated that the semiconductor body will carry a two-dimensional array of such devices extending in rows and columns in the x and y direction shown in Figure 9 and coupled by row and column conductors Rl, R2, R3 and Cl to C3 as shown schematically in Figure 9. Although Figure 9 shows a three by three array MA of photo-emissive devices 21, generally the memory will consist of many more such devices . As can be seen from Figure 8, the spin injector 30 have the same structure as shown in Figure 6. However, in this case, the photo-emitter 40 is formed on top of the contact L2 rather than vice versa and between the spin injectors 30 and the substrate 11 is sandwiched a current grid arrangement consisting of a parallel spaced-apart first conductors 200 extending in the y direction in Figure 8 and, separated by an insulating layer 201 from the first conductors, parallel spaced-apart of second conductors 202 extending in the x direction in Figure 8 (that is into the plane of the paper) so that the first and second conductors 200 and 202 are mutually perpendicular. The second conductors 200 are separated from the memory devices 1 by an insulating layer 203. The insulating layers may, as is known in the art, comprise silicon dioxide or intrinsic Gallium Arsenide. The mutually perpendicular first and second conductors 200 and 202 are arranged such that each spin injector 30 is situated over a crossover point CP between a pair of first and second conductors 201 and 202. As shown in Figure 9, the contacts Ll of the Fermi input leads 3 of spin injectors of rows of photo-emissive devices 21 are coupled via respective row conductors Rl to R3 to an input shift register 100 having an input 101 for enabling addressing of the row conductors in turn. The contacts L2 of each of the Fermi output leads 4 in a column are coupled to a corresponding column conductor Cl to C3. In this example, all of the gates G are driven at the same voltage and are coupled via a common electrode CE to a gate drive circuit (GD) 104 as is well known in the art. Each of the first current conductors 201 is coupled to a first drive circuit 204 while each of the second conductors 202 is coupled to a second drive circuit 205. As can be seen in Figure 9, the pitch of the first and second current conductors 201 and 202 is smaller than that of the row and column conductors such that each photo-emissive device 21 is uniquely associated with a pair of first and second conductors 201 and 202. Thus, for example, the photo-emissive device 21 2f2 (where the 22 indicates that the photo-emissive device is in the second row and the second column) is coupled to the fifth first conductor 201 5 and the fifth second conductor 202 5 . In the embodiment shown by Figures 8 and 9, the current grid first and second drive circuits 204 and 205 are used to adjust the applied magnetic field locally for a particular spin injector of a selected photo-emissive device. This is achieved by supplying to the first and second conductors 201 and 202 uniquely associated with the selected photo-emissive device (for example, first conductor 201 5 and second conductor 202 5 for the device 1 22 ) currents which individually are insufficient but which together provide a magnetic field sufficient to cause Zeeman splitting at the spin injector 30 22 of the required polarity. The currents supplied by the first and second drive circuits 204 and 205 to the first and second conductors 201 and 202 will be such that the polarity of the local DC magnetic field at each spin injector can be either one direction or the other so that the associated photoemitter emits either left or right circularly polarised photons . A separate DC magnetic field generator may be provided to generate a constant DC magnetic field over the entire array and the current grid first and second driving circuits 204 and 205 may then be used to modify the DC magnetic field locally to provide the required Zeeman splitting. The spin injectors 30 in a column can be accessed sequentially by addressing the two conductors in sequence in known manner so that, each time a spin injector 30 is addressed, the associated photo-emitter 40 emits photons which are circularly polarised in a direction dependent upon the spin state of the uppermost odd electron on the corresponding quantum dot. As another possibility, multiplexing drive arrangements may be used for the first and second drive circuits 204 and 205 so that only one conductor 201 per row and one conductor 202 per column is required. The photo-emissive devices 30 may be driven in sequence so that the output of each photo-emitter device represents a different part of the signal being transmitted with the polarisation of the photons be modulated as described above. As another possibility, the first and second drive circuits may be arranged to drive all of the photo-emissive devices 21 in synchronism so that the arrays simply enables a greater photon output for detection by the receiver R. In the above described embodiments, the transmitter and receiver form a direct line of site communication apparatus. The present invention may, however, also be applied in a retroreflective system where the transmitted beam is returned, possibly altered or modulated by the receiver, to the originating transmitter for detection. This may be achieved by, for example, replacing the receiver R shown in Figure 1 with a retroreflector such as a multiple quantum well device changing the reflectance or a passive retroreflector such as a mirror or corner cube and providing the components of the receiver R within the transmitter in Figure 1 , and taking account in the data extraction of the fact that the polarisation of the circularly polarised photons will be rotated by 180° upon reflection. Figure 10 shows a functional block diagram similar to Figure 1 to illustrate another example of a retroreflective type of communication apparatus. In this example, the transmitter T is in the form of a transmitter receiver T/R and includes a polarisation detector 22 and signal extractor 24 in addition to the transmitter components shown in Figure 1. In this case, however, instead of simply providing a retroreflector, the receiver R shown in Figure 1 is replaced by a receiver transmitter R/T which consists of a polarisation detector 22 which may have the form described above with reference to Figure 4 or Figure 5 and a signal measurer 24a that retrieves the transmitter data from the output of the polarisation detector. The output of the signal measurer 24a is supplied to a modulator 20' that, in the same manner as described above, controls a spin injector 30' that injects spin polarised charge carriers into a photo-emitter 40'. Photons P' emitted by the photo-emitter 40' are detected by the polarisation detector 22 of the transmitter receiver T/R and the transmitted data is extracted by the signal extractor 24 as described above. The spin injectors 30 and 30', photo-emitters 40 and 40' and polarisation detector 22 may have the same form as described above. The signal measurer 24a and modulator 20' may simply cause the photo-emissive device comprising the spin injector 30' and photo-emitter 40' to return the signal received by the polarisation detector 22 of the receiver transmitter R/T without any modification. As another possibility, the signal measurer 24a and modulator 20' may modify the signal by, for example, phase or amplitude modulating it. As another possibility, the photo-emitter 40' may be arranged to emit photons at a different frequency from the photo-emitter 40. In this case, of course, the polarisation detector 22 of the transmitter receiver T/R will be adapted to detect photons of the frequency emitted by the photo-emitter 40'. As described above, the spin injector may be fabricated so that the dimensions of the quantum dot are about 50 nanometres which requires operation at very low temperature (typically liquid Helium temperature, that is a few Kelvin). However, the operating temperature increases dramatically with reduction in the quantum dot size so that, for example, a scaling down of the dot size by a factor of 10 would increase the temperature required by a factor of 100 allowing room temperature operation. As will be appreciated by those skilled in the art, Cadmium Selenide (CdSe) nanocrystals have been fabricated with a dot diameter of 6 nanometres and a Coulomb charging energy of 30 millielectron volts. Couloumb blockade behaviour has also been seen at high temperature in C 60 molecules and in carbon nanotubes (see for example the aforementioned text book entitled "Mesoscopic Electron Transport" by Sohn et al ) . Indeed, there is no fundamental physical discontinuity between a quantum dot and a large molecule or even atom and thus the present invention may also be applied to such quantum-confined nanostructures or regions. Where size constraints make contact difficult then, in the embodiments described above, scanning tunnelling electron microscope (STM) techniques may be used to address and read the state of a spin injector. For example, where necessary, addressing and read out of may be achieved using microscopic arrays of scanning tunnelling tips that are atomically sharp so that operation at a molecular or even atomic level may be possible. The requirement specified above that: Δ z > K B T (4) can be relaxed if desired for the following reasons. Thus, if: Δ z < K B T (<level spacing) (5) then the two lowest spin states (spin-up and spin-down) on the quantum dot become almost equally populated and it can easily be shown that the ratio of occupation probabilities for spin-up and spin-down is: 1 + Δ 2 / K B T (6) which, consequentially, represents the ratio of spin-up to spin-down current in the output Fermi lead 4. Thus the spin filtering effect becomes very small for: Δ z / K B T « 1 ( 7 ) However, it may still be possible to detect this imbalance between the spin-up and spin-down currents using PIN diodes and photodetectors as described above given that Ohno et al were able to detect an imbalance of up and down currents of a few percent as reported in the above mentioned Letter to Nature. Accordingly, an imbalance in the spin-up/spin-down current should be detectable with K B T being up to 50 times larger than the Zeeman splitting Δ z on the quantum dot. Using for the quantum dot a material having a g-factor of 15 such as Indium Arsenide as described above and appropriately selecting the quantum dot dimensions should therefore easily enable operating temperatures approaching room temperatures to be achieved. Although the above described embodiments show vertical spin injectors and PIN diodes, they may be produced in lateral configuration and on semiconducting or insulating substrates. For example, a lateral spin injectors may be provided by confining a two-dimensional electron gas using electrodes as described with reference to, for example, Figure 1.3 in the chapter entitled "Electron Transport in Quantum Dots" in the aforementioned textbook by Sohn et al . In the above described embodiments, the spin injector 30 consists of a single switchable spin injector and the spin state of the spin polarised electrons provided by the spin injector is controlled by the modulator 20. Figure 11 shows a diagrammatic functional block diagram of another example of communication apparatus in accordance with the present invention that differs from that described above in that the photo-emissive device 21 has first and second spin injectors 30a and 30b each associated with a corresponding photo-emitter 40' and 40". In this case, the modulator 20 is replaced by a controller 20a and first and second modulators 20b and 20c which control the first and second spin injectors 30a and 30b respectively. One of the first and second spin injectors 30a and 30b is arranged to provide spin-up state electrons while the other is arranged to provide spin-down state electrons. In this case, the controller 20a controls actuation of the first and second modulators 20b and 20c in accordance with the data to be transmitted and the modulation scheme being adopted so that, in a simple case where a digital zero is to be represented by left circularly polarised photons and a digital one by right circularly polarised photons, then the controller 20a activates the first and second modulators 20b and 20c appropriately so that when left circularly polarised photons are required, only the spin injector 30a or 30b that causes its associated photo-emitter to emit left circularly polarised photons is activated and so on. In this case, the polarisation detector 22 will detect the overall polarisation with there being three possible different polarisation results, an overall polarisation of zero when both photo- emitters are active and left or right circularly polarised photons when only one of the photo-emitters is active. The first and second modulators 20b and 20c may also enable amplitude modulation by, as discussed above, controlling the current through the photo-emitters and, in addition, data may be transmitted by phase modulation by, for example, controlling the relative timing between the outputs of the two photo-emitters 40' and 40". The signal extractor 24 will, of course, be arranged to extract information in accordance with the modulation scheme determined for the communication which, for any particular instance of the apparatus, may include amplitude and phase (relative to timing) modulation as well as polarisation modulation. The first and second spin injectors 30a and 30b may have the structure described above with, in this case, the spin state of the uppermost ground state electron on the quantum dot being fixed so that the quantum dot of one of the first and second spin injectors allows only spin- up state electrons to be injected into the photo-emitter and the quantum dot of the other first and second spin ejectors allows only spin-down state electrons to be injected into the associated photo-emitter 40". As another example, the spin injectors 30a and 30b may be in the form of ferromagnetic semiconductor heterostructures as described, in for example, the paper entitled "Electrical Spin Ejection in a Ferromagnetic Semiconductor Heterostructure" by Ohno et al published as a Letter to Nature in Nature Volume 402, 16 December 1999 at pages 790 to 792 or as described in another Letter to Nature entitled "Injection and Detection of a Spin-Polarised Current in a Light-Emitting Diode" by Fiederling et al published in Nature Volume 402, 16 December 1999 at pages 787 to 789. Figure 12a shows a cross-sectional view of a semiconductor body 100 illustrating schematically an example of a photo-emissive device 30" using the principles described in the Letter to Nature by Fiederling et al. In this case, the device structure comprises a PIN diode consisting of a p-conductivity type gallium arsenide layer 101 followed by a p-conductivity type aluminium gallium arsenide layer 101a, an intrinsic (not intentionally doped) gallium arsenide layer 103 and an n-conductivity type aluminium gallium arsenide layer 104. On top of this PIN diode is provided a magnetic semiconductor material in the form a relatively lightly n-conductivity type magnetic semiconductor layer 105 followed by a more highly n-conductivity type magnetic semiconductor layer 106. Ohmic contacts 102a and 102b are provided on the layers 101 and 106 and the magnetic semiconductor material layers 105 and 106 are selectively etched to define a MESA on top of the PIN diode so that photons P can be emitted from the surface SA. In this example, the magnetic semiconductor material is a quaternary II, VI magnetic semiconductor material: Be x Mn y Zn ! _ x _ y Se ( 1) The magnetic semiconductor material functions as a spin aligning material when the device is placed within a magnetic field B and serves, as explained in the Fiederling paper, to inject spin polarised electrons into the gallium aluminium/aluminum gallium arsenide light emitting diode which, in response, ejects circularly polarised photons. As described in the Fiederling paper, the spin polarisation of the electrons and thus the circularly polarisation of the emitted photons can be controlled by reversing the magnetic field. Accordingly, the first and second spin injectors shown in Figure 11 may both have the structure shown in Figure 12a but be associated with nanomagnetics or dysprosium dots or the like that generate different polarity magnetic fields.for the two different spin injectors. The two spin injectors may be formed on the same substrate provided that the separation of the two spin injectors is such that one does not lie within the near field of the magnetic field generator of the other. Typically, this can be achieved if the separation between the two spin injectors is comparable to their respective footprints (cross- sectional area) . Figure 12b illustrates schematically an example of a spin injector of the type described by Ohno et al in the aforementioned letters to Nature. In this case, the ferromagnetic heterostructure is again based on gallium arsenide and consists of an n-conductivity type gallium arsenide substrate 201 on which is provided an n-conductivity type gallium arsenide buffer layer 202 followed by an intrinsic gallium arsenide layer 203, an intrinsic indium gallium arsenide layer 204 and an intrinsic gallium arsenide spacer layer 205 on top of which is provided the ferromagnetic material in the form, in this case, of a p-conductivity type layer of gallium manganese arsenide 206. Respective contacts 207a and 207b are provided on the layers 201 and 206. Again, in this case, the ferromagnetic semiconductor layer 206 acts as a spin polariser injecting spin polarised electrons into the PIN diode which generates circularly polarised photons that are either left or right circularly polarised dependent upon the direction of the magnetic field. Again, reversing the magnetic field reverses the spin state of the electrons ejected into the PIN diode and thus the polarisation of the circularly polarised photons emitted by the device. Other forms of ferromagnetic heterostructurs may be used as the first and second spin ejectors with the spin polarisation of spin injection being controlled by the respective local magnetic field. Further examples of ferromagnetic semiconductor heterostructures are described in, for example, a paper by Xu et al entitled "Ferromagnetic Metal Semiconductor Heterostructures for Magneto-electronic Devices" published in Sensors and Actuators, Volume 81 (2000) at pages 258 to 262 and WO 97/41606. In the examples described above, with reference to Figures 12a and 12b, the spin polarisation of the charge carriers provided by the spin injector is controlled by the magnetic field polarisation. A further Letter to Nature by Ohno et al entitled "Electric Field Control of Ferromagnetism" Published in Nature, Volume 408, 21/28 December 2000 at pages 944 to 946 suggests that electric field control can be achieved by using as the ferromagnetic semiconductor material a material that exhibits hole induced ferromagnetism such as indium manganese arsenide (In, Mn)As and coupling this to an insulated gate electrode. For example, this may be achieved with the photo-emissive device structure shown in Figure 12b by providing an annular insulated gate IG structure surrounding the ferromagnetic layer 206 and coupling the conductive layer of the insulated gate to a gate voltage source GV controllable by the associated modulator 20b or 20c so as to control the ferromagnetism within the ferromagnetic layer as described in the Ohno et al letter published in Nature, Volume 408 mentioned above. Such an electric field controllable ferromagnetic layer may be used as the input Fermi lead for the spin injector described above with reference to Figures 2 and 3. Figure 13 shows a cross sectional view similar to Figure 6 of a modification of the photo-emissive device shown in Figure 6 where the Fermi lead 4 is formed of a ferromagnetic material exhibiting hole-induced ferromagnetism as described in the Ohno et al paper and a vertical insulated gate structure consisting of a gate insulator 300 and a conductive layer 301 is coupled to an insulated gate contact IG enabling electric field control of ferromagnetism within the Fermi lead 4. The magnetic phase change from paramagnetic to ferromagnetic produced by using the insulated gate to increase the hole concentration as described in Ohno et al has, in the device structure shown in Figure 13, the effect of changing the energy levels in the quantum dot 2a, thereby altering the spin state of the uppermost ground electron on the quantum dot 2a and thereby controlling the spin state of the electrons injected into the photo-emitter. The polarisation of the electrons emitted by the spin ejector can thus be controlled by controlling the voltage applied to the insulated gate. The above described embodiments enables provision of communication apparatus using polarisation modulation where the polarisation modulation is effected by spin polarisation of injected electrons so that the polarisation of the photons is controlled by the spin polarisation of the current injected by the spin injector. The modulation rate of the photon polarisation is the same as that can be achieved for modulation of the spin polarised current which should be in excess of tens of Gigahertz (GHz). Although modulation rates of tens of Gigahertz are available from standard modulation schemes there is a requirement for greater bandwidth and, because the spin injectors described above are not restricted by issues of electron inertia and device capacitance they should offer much higher bandwidth, anticipated to be of the order of tens of Gigahertz per Tezla which is greater than is available from existing optical communication schemes. Moreover, simultaneous use of amplitude, time and polarisation modulation may allow for a three level coding scheme and significant further improvement in bandwidth. In addition, use of combined amplitude, phase and polarisation modulation offers the possibility of new and more powerful coding schemes including the possibility of harmonic and spectual aspects. In addition, in contrast to conventional polarisation modulators, the spin control polarisation modulators described above, being solid state, lend themselves to small volumes or footprints, low cost and easy electronic integration. In addition, arrays of photo-emissive devices as described above with reference to Figures 8 and 9 may be used to generate multiple optical signals or beams with different switching polarisation allowing the possibility of additional functionality by use of optical interference techniques. The communications apparatus shown in, for example, Figure 10 may be used in point to point retro-reflective free space communications schemes such as those described in WO 98/35328 or WO 00/48338 which both offer high data bandwidth of line-of-sight point-to-point links combined with the ability to deliver high bandwidth to a large number of simultaneous users. Each of the multiple end points or receivers may, as described with reference to Figure 10 above enable duplex operation by modulating a return light beam. In the embodiments described above the optical communication between the transmitter and the receiver or the transmitter/receiver and receiver/transmitter is through a free space. The present invention may, however, also be applied where the communication between the receiver and transmitter is via an optical fibre coupling using, as described at page 344 of the text book entitle "Opto Electronic Devices" by S Desmond Smith published by Prentice Hall (ISBN 0-13-143769-0), polarisation-preserving optical fibres are used for the optical coupling. As will be appreciated, various combinations and permutations of simplex and duplex communications systems can be produced using the photo-emissive devices described above together with other, conventional, optical communications systems . The present invention may also be used for quantum cryptographic applications by use of two such photo-emissive devices with appropriate use of variable (quarter) wave plates to control the polarisation and combine the beams for entanglement purposes. In the above described embodiments, the photo-emitter is a PIN diode. The PIN diode may be a laser diode. As another example, the photo-emitter may be a vertical cavity surface emitting laser (VCSEL) as described in, for example, EP-A-0491502 and as produced by, for example, Honeywell Sensing and Control with the fact that the electrons injected into the VCSEL via the spin injector are spin polarised serving to stablise or control the polarisation of the photons emitted by the VCSEL. In another aspect, the present invention provides a VCSEL wherein the polarisation of emitted light is controlled by using as the current source for the VCSEL a spin injector as described above. As described above, the spin polarisation of the electrons provided by the spin injectors may be controlled by controlling a magnetic field. Such a spin injector may therefore be used as a magnetic field detector with the output of the spin injector being coupled to a spin polarisation detector. In this case, in the examples shown in Figures 2 and 3 the DC field generator 5 may be omitted so that, in the absence of a magnetic field, no Zeeman splitting occurs and accordingly spin polarisation does not occur while if a magnetic field of sufficient strength of one or the other polarity is present, then a current consisting of electrons of one or other spin polarisation will be generated. In this case, the polarisation detector 23 may consist of, for example, a photo-emitter 40 as described above and the polarisation detector 23 shown in Figure 4 with the signal extractor 24 in this case operating to determine from the outputs of the photo detectors 41a and 41b whether the spin injector is providing a polarised current and if so, the polarisation state, thereby enabling a determination to be made as to whether a magnetic field is present and, if so, its polarisation. Such magnetic sensors may be provided with focussing soft magnetic pole pieces to focus a uniform applied magnetic field in order to increase the sensor sensitivity. These soft magnetic pole pieces may be provided by depositing a permalloy onto the surface of the structure shown in Figure 6 and patterning the layer using conventional photo-lithographic and etching techniques to produce a metalisation pattern similar to that shown very schematically in Figure 15 where the frusto-conical shapes numbered 400 represent the soft magnetic pole pieces. The spin injectors described above that utilise a ferromagnetic heterostructure may also, for similar reasons, be used to enable detection of a magnetic field, with again, the existence of a magnetic field and its polarisation being determined in dependence upon whether the current from the spin injector is polarised and if so, the spin polarisation state. The photo-emissive devices described above may be used to enable generation of high frequency signals by operating the spin injector close to threshold thereby using the associated non-linearlity to operate the device as a mixer providing sum and difference frequencies when modulated by a second input parameter for example a magnetic or electric field. The use of an electric field as described above with reference to Figure 14 to control ferromagnetism within a layer may, in addition to being useful in the spin injectors described above, also be used in a spin memory device as described in International Application No. GB00/03416. Figures 16 and 17 are very functional schematic diagrams similar to Figures 2 and 3 illustrating such as memory device. Although superficially this memory device appears similar to the spin injector described above, it differs remarkably in its operation. The reason for this is that, in contrast to the spin injector described above with reference to Figures 2 and 3, in the memory device shown in Figures 16 and 17, as explained in International Application No. GB00/03416, the quantum dot region 2 and Fermi leads 3 and 4 are formed of material such that the Fermi leads 3 and 4 have a g-factor considerably greater than that of the quantum dot region being formed, for example, of magnetic II-IV semi-conductor materials while the quantum dot region is formed of a III-IV semiconductor material. This difference in g-factor means that, as described in the aforementioned International Application, when a DC magnetic field is applied by the DC field generator 5 shown in Figure 17 the Zeeman splitting in the Fermi leads 3 and 4 is very much greater than in the quantum dot to the extent that any electron current in the Fermi leads 3 and 4 is spin polarised so that if, for example, the current in the Fermi leads is spin state up polarised while the uppermost ground state electron on the quantum dot is also spin state up, then no sequential tunnelling can occur through the dot. Application of an AC pulse perpendicular to the DC magnetic field using the AC field generator 6 shown in Figure 16 at a frequency ω with an energy equal to the Zeeman splitting within the quantum dot in accordance with electron spin resonance techniques results in a Rabi spin flop whereby the spin state of the ground state electron changes from the spin state up to the higher energy spin state down enabling sequential tunnelling, resulting in a current that can be detected by the current sensor CS. The spin state of the uppermost ground state electron on the quantum dot can thus be determined by the current sensor in response to the application of the AC pulse enabling use of this device structure as a memory. The device structure described in International Patent Application No. GB00/03416 may be modified in a manner similar to that described above with reference to Figure 13 by providing the input Fermi leads 3 of a semiconductor material that exhibits hole induced ferromagnetism and providing, as illustrated very diagrammatically in Figures 16 and 17, an insulated gate electrode GA and an auxiliary gate voltage control 600 to enable a magnetic phase change, from paramagnetic to ferromagnetic, to be induced in the Fermi lead 4 by applying the appropriate voltage to the insulated gate electrode GA. As discussed above, this magnetisation of the layer 3 changes the energy levels in the quantum dot 2a and can be used to cause a change in the spin state of the uppermost ground state electron on the quantum dot so that whether the uppermost electron is a spin state up or spin state down electron can be determined by the voltage applied to the gate electrode GA. This enables the state of the memory device (that is whether it represents a zero or a one) to be controlled by the voltage applied by the auxiliary gate voltage control 600. As described in International Patent Application No. GB00/03416, two dimensional arrays of such memory devices may be provided with the spin state of the uppermost electron on each quantum dot being controlled by the voltage applied to the corresponding gate electrode GA. Electric field control of the memory device is thus provided. In the above described embodiments, the semiconductor materials mentioned are generally III-V materials. Other semiconductor materials such as silicon or germanium may be used to form for, for example, a photo-emitter and/or a spin injector. Where in the above embodiments ferromagnetic semiconductors are referred to, it may also be possible to use ferromagnetic metals. The spin injectors and spin memory devices described above also have an additional advantage that they may be employed in place of silicon-on-insulator devices for dynamic random access memory in environments that require radiation hard devices .

Claims: CLAIMS : 1. A optical communications apparatus, comprising: optical signal transmitting means comprising: signal supplying means for supplying a data signal comprising data to be transmitted; photo-emitting means for emitting photons; spin injection means for supplying spin-polarised charge carriers for causing the photo-emitting means to emit photons having a polarisation determined by the spin-polarisation of the charge carriers supplied by the spin injections means; and control means for controlling the spin injection means to cause the spin polarisation of the charge carriers supplied by the spin injection means to be controlled in accordance with data to be transmitted such that, in operation, the photo-emitting means provides an optical signal in which the polarisation of the photons emitted by the photo-emitting means is modulated on the basis of the data signal supplied by the signal supplying means; and receiving means comprising: optical signal polarisation detecting means for determining the polarisation of photons received from the photo-emitting means; and data extracting means for extracting data transmitted in the optical signal provided by the photo- emitting means on the basis of the photon polarisations determined by the polarisation detecting means . 2. Apparatus according to claim 1, wherein the spin injection means comprises a spin injector having: a spin filter having an input region for carrying a current, an output region for carrying a current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby charge carriers can only pass from the input region to the output region by tunnelling through the quantum region; and Zeeman splitting means for causing Zeeman splitting in the spin filter such that the. eeman splitting in the input and output regions is less than the Fermi energy such that, in operation, the spin filter outputs a tunnelling current predominantly of one spin polarity. 3. Apparatus according to claim 1, wherein the spin injection means comprises a spin injector having: a spin filter having an input region for carrying an electron current, an output region for carrying an electron current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby electrons can only pass from the input region to the output region by tunnelling through the quantum region; and Zeeman splitting means for causing Zeeman splitting in the spin filter, the quantum region and input and output regions being formed such that the Zeeman splitting in the input and output regions is less than the Fermi energy such that, in operation, the spin filter outputs a tunnelling current predominantly of one spin polarity. 4. Apparatus according to claim 2 or 3 , wherein the Zeeman splitting means comprises means for applying a DC magnetic field to the quantum region. 5. Apparatus according to claim 2, 3 or 4, wherein the control means is operable to control the predominant spin polarity of the tunnelling current. 6. Apparatus according to claim 5, wherein the quantum region has a gate and the control means comprises means for controlling at least one of a gate voltage and a voltage between the input and output regions to control whether the quantum dot contains an odd or even number of electrons. 7. Apparatus according to claim 5 or 6, wherein the control means is arranged to control the direction of Zeeman splitting. 8. Apparatus according to claim 2 or 3, wherein the Zeeman splitting means comprises means for applying a DC magnetic field to the spin filter, and the control means are operable to control the predominant spin polarity of the tunnelling current by controlling the polarity of the DC magnetic field. 9. Apparatus according to any one of claims 2 to 8, wherein the spin injection means comprises two such spin injectors and the control means is operable to control operation of each of the spin injectors so that one spin injector provides photons of one polarity and the other spin injector provides photons of a different polarity. 10. Apparatus according to claim 1, wherein the spin injection means comprises at least one spin injector having a ferromagnetic region that provides charge carriers of a spin polarity determined by a magnetic field to which the ferromagnetic region is subject and the control means is operable to control that magnetic field. 11. Apparatus according to claim 1, wherein the spin injection means comprises two spin injectors one arranged to provide charge carriers of one polarity and the other arranged to provide charge carriers of the opposite polarity and the control means is operable to control actuation of the two spin injectors. 12. Apparatus according to claim 11, wherein each of the spin injectors has a ferromagnetic region that provides charge carriers of a spin polarity determined by a magnetic field to which the ferromagnetic region is subject and the two spin injectors are subject to opposite polarity magnetic fields . 13. Apparatus according to claim 1, wherein the spin injection means comprises at least one spin injector having a semiconductor region that exhibits hole-induced ferromagnetism and means for controlling the hole concentration in the semiconductor region. 14. Apparatus according to claim 13, wherein the hole concentration controlling means comprises a field effect electrode. 15. Apparatus according to claim 13, wherein the hole concentration controlling means comprises an insulated gate electrode. 16. Apparatus according to claim 13, 14 or 15, wherein the spin injection means comprises two such spin injectors and the control, means is operable to control the hole concentration controlling means of each spin injector. 17. Apparatus according to claim 11 or 12 or 16, wherein the control means is operable to cause one, the other, both or neither of the spin injectors to be operable at the same time. 18. Apparatus according to any one of claims 2 to 17, wherein the photo-remitting means comprises a respective photo-emitter associated with the or each spin injector. 19. Apparatus according to any one of the preceding claims, wherein the polarisation detecting means of the receiving means comprises spin injecting means arranged to operate in reverse so as to produce a spin polarised current with the spin polarisation being dependent on the polarisation of photons received by the spin injecting means . 20. Apparatus according to claim 19, wherein the spin injections means of the receiving means comprises at least one spin injector in accordance with any one of claims 2 to 19. 21. Apparatus according to any one of the preceding claims, wherein the polarisation detecting means of the receiving means comprises two photodetectors each operable to detect only photons of a particular polarity. 22. Apparatus according to any one of the preceding claims, wherein the photon polarisation is either left or right circularly polarised. 23. Apparatus according to any one of the preceding claims, wherein the photo-emitting means comprises at least one photodiode. 24. Apparatus according to any one of the preceding claims, wherein the photo-emitting means comprises at least one vertical cavity surface emitting layer. 25. Apparatus according to any one of the preceding claims, wherein the optical signal transmitting and receiving means are operable to communicate via at least one of free space and polarisation retaining optical fibre means. 26. Apparatus according to any one of claims 1 to 25, wherein the optical signal receiving means is operable to provide a return optical signal and the optical signal transmitting means also includes receiving means. 27. Apparatus according to claim 26, wherein the optical signal receiving means includes transmitting means as claimed in any one of claims 1 to 25. 28. A transmitter apparatus for optical communications apparatus, comprising: signal supplying means for supplying a data signal comprising data to be transmitted; photo-emitting means for emitting photons; spin injection means for supplying spin-polarised charge carriers for causing the photo-emitting means to emit photons having a polarisation determined by the spin-polarisation of the charge carriers supplied by the spin injections means; and control means for controlling the spin injection means to cause the spin polarisation of the charge carriers supplied by the spin injection means to be controlled in accordance with data to be transmitted such that, in operation, the photo-emitting means provides an optical signal in which the polarisation of the photons emitted by the photo-emitting means is modulated on the basis of the data signal supplied by the signal supplying means . 29. Apparatus according to claim 28, wherein the spin injection means comprises a spin injector having: a spin filter having an input region for carrying a current, an output region for carrying a current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby charge carriers can only pass from the input region to the output region by tunnelling through the quantum region; and Zeeman splitting means for causing Zeeman splitting in the spin filter such that the Zeeman splitting in the input and output regions is less than the Fermi energy such that, in operation, the spin filter outputs a tunnelling current predominantly of one spin polarity. 30. Apparatus according to claim 28, wherein the spin injection means comprises a spin injector having: a spin filter having an input region for carrying an electron current, an output region for carrying an electron current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby electrons can only pass from the input region to the output region by tunnelling through the quantum region; and Zeeman splitting means for causing Zeeman splitting in the spin filter, the quantum region and input and output regions being formed such that the Zeeman splitting in the input and output regions is less than the Fermi energy such that, in operation, the spin filter outputs a tunnelling current predominantly of one spin polarity. 31. Apparatus according to claim 29 or 30, wherein the Zeeman splitting means comprises means for applying a DC magnetic field to the quantum region. 32. Apparatus according to claim 29, 30 or 31, wherein the control means is operable to control the predominant spin polarity of the tunnelling current. 33. Apparatus according to claim 32, wherein the quantum region has a gate and the control means comprises means for controlling at least one of a gate voltage and a voltage between the input and output regions to control whether the quantum dot contains an odd or even number of electrons. 34. Apparatus according to claim 32 or 33, wherein the control means is arranged to control the direction of Zeeman splitting. 35. Apparatus according to claim 29 or 30, wherein the Zeeman splitting means comprises means for applying a DC magnetic field to the spin filter, and the control means are operable to control the predominant spin polarity of the tunnelling current by controlling the polarity of the DC magnetic field. 36. Apparatus according to any one of claims 29 to 35, wherein the spin injection means comprises two such spin injectors and the control means is operable to control operation of each of the spin injectors so that one spin injector provides photons of one polarity and the other spin injector provides photons of a different polarity. 37. Apparatus according to claim 28, wherein the spin injection means comprises at least one spin injector having a ferromagnetic region that provides charge carriers of a spin polarity determined by a magnetic field to which the ferromagnetic region is subject and the control means is operable to control that magnetic field. 38. Apparatus according to claim 28, wherein the spin injection means comprises two spin injectors one arranged to provide charge carriers of one polarity and the other arranged to provide charge carriers of the opposite polarity and the control means is operable to control actuation of the two spin injectors. 39. Apparatus according to claim 38, wherein each of the spin injectors has a ferromagnetic region that provides charge carriers of a spin polarity determined by a magnetic field to which the ferromagnetic region structure is subject and the two spin injectors are subject to opposite polarity magnetic fields. 40. Apparatus according to claim 28, wherein the spin injection means comprises at least one spin injector having a semiconductor region that exhibits hole-induced ferromagnetism and means for controlling the hole concentration in the semiconductor region. 41. Apparatus according to claim 40, wherein the hole concentration controlling means comprises a field effect electrode. 42. Apparatus according to claim 40, wherein the hole concentration controlling means comprises an insulated gate electrode. 43. Apparatus according to claim 40, 41 or 42, wherein the spin injection means comprises two such spin injectors and the control means is operable to control the hole concentration controlling means of each spin injector. 44. Apparatus according to claim 38 or 39 or 43, wherein the control means is operable to cause one, the other, both or neither of the spin injectors to be operable at the same time. 45. Apparatus according to any one of claims 29 to 44, wherein the photo-emitting means comprises a respective photo-emitter associated with the or each spin injector. 46. Apparatus according to claims 26 to 45, wherein the photon polarisation is either left or right circularly polarised. 47. Apparatus according to claims 28 to 46, wherein the photo-emitting means comprises at least one photodiode. 48. Apparatus according to claims 28 to 47, wherein the photo-emitting means comprises at least one vertical cavity surface emitting layer. 49. A receiver apparatus for an optical communications apparatus, comprising: optical signal polarisation detecting means for determining the polarisation of photons received from photo-emitting means; and data extracting means for extracting data transmitted in the optical signal provided by the photo- emitting means on the basis of the photon polarisations determined by the polarisation detecting means. 50. Apparatus according to claim 49, wherein the polarisation detecting means of the receiving means comprises spin injecting means arranged to operate in reverse so as to produce a spin polarised current with the spin polarisation being dependent on the polarisation of photons received by the spin injecting means. 51. Apparatus according to claim 50, wherein the spin injection means of the receiving means comprises at least one spin injector in accordance with any one of claims 2 to 19. 52. Apparatus according to claim 49, wherein the polarisation detecting means of the receiving means comprises two photodetectors each operable to detect only photons of a particular polarity. 53. Apparatus according to any one of claims 49 to 52, wherein the photon polarisation is either left or right circularly polarised. 54. Apparatus according to any one of claims 49 to 53, also including transmitting means as claimed in any one of claims 2 to 25. 55. A memory comprising: a memory device having an input region for carrying a current, an output region for carrying a current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby charge carriers can only pass from the input region to the output region by tunnelling through the quantum region; Zeeman splitting means for controlling Zeeman splitting in the memory device such that the input and output regions are spin polarised whereby conduction through the input and output regions is by charge carriers of one spin polarity; and control means for controlling the spin polarisation of an uppermost charge carrier in the quantum region to control the tunnelling current through the memory device, wherein the input region comprises a semiconductor region that exhibits hole-induced ferromagnetism and means are provided for controlling the hole concentration in said semiconductor to control the energy levels in the quantum region. 56. A memory comprising: a memory device having an input region for carrying an electron current, an output region for carrying an electron current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby electrons can only pass from the input region to the output region by tunnelling through the quantum region; Zeeman splitting means for controlling Zeeman splitting in the memory device such that the input and output regions are spin polarised whereby conduction through the input and output regions is by electrons of one spin polarity; and control means for controlling the spin polarisation of an uppermost electron in the quantum region to control the tunnelling electron current through the memory device, wherein the input region comprises a semiconductor region that exhibits hole-induced ferromagnetism and means are provided for controlling the hole concentration in said semiconductor to control the energy levels in the quantum region. 57. Apparatus according to claim 55 or 56, wherein the hole concentration controlling means comprises a field effect electrode. 58. Apparatus according to claim 55 or 56, wherein the hole concentration controlling means comprises an insulated gate electrode. 59. A memory according to any one of claims 55 to 58, wherein the Zeeman splitting means comprises means for applying a DC magnetic field. 60. A memory according to any one of claims 55 to 58, wherein the control means comprises means for applying an AC magnetic field pulse for causing a Rabi spin flop in the quantum region when the Zeeman splitting in the quantum region is such that a resonance with the AC magnetic field pulse occurs. 61. A memory according to any one of claims 55 to 60, further comprising adjustment means for adjusting the Zeeman splitting within the quantum region. 62. A memory according to claim 61, wherein the adjustment means comprises means for applying a gate voltage to the quantum region to adjust the energy levels within the quantum region. 63. A magnetic field sensor comprising: a spin filter having an input region for carrying a current, an output region for carrying a current, and a three-dimensionally confined quantum region arranged to operate in the Coulomb blockade regime and separating the input and output regions whereby charge carriers can only pass from the input region to the output region by tunnelling through the quantum region, whereby magnetic field induced Zeeman splitting of the energy levels in the quantum region enables a tunnelling current predominantly of one spin polarity with the spin polarity being dependent on the magnetic field polarity; and means for determining the presence of a magnetic field from the spin polarity of the tunnelling current. 64. A sensor according to claim 63, wherein magnetic pole pieces are provided for concentrating a magnetic field at the spin filter. 65. A method of controlling the polarisation of photons emitted by a vertical cavity surface emitting laser, which method comprises driving the laser by injecting spin polarised charge carriers. 66. A method according to claim 65, which comprises using a spin injection means having the features set out in any one of claims 2 to 15 to inject the spin polarised charge carriers.

Inventors: DOYLE RICHARD ALAN, EDGE GORDON MALCOLM, TIMSON DANIEL, DOYLE, RICHARD, ALAN, EDGE, GORDON, MALCOLM, TIMSON, DANIEL
Applicants: DOYLE RICHARD ALAN, EDGE GORDON MALCOLM, SCIENT GENERICS LTD, TIMSON DANIEL, DOYLE, RICHARD, ALAN, EDGE, GORDON, MALCOLM, SCIENTIFIC GENERICS LIMITED, TIMSON, DANIEL
Application year: 2001
Main classification: H04B10
IPC codes: G02F2, H01L33, H01S5, H04B10