Abstract:
Silicon possesses an indirect band-gap, which limits its use in some photonic applications. A phonon generator is included in a silicon-based device, which promotes electron-hole recombination and so allows silicon to emit photons efficiently. Phonons may be generated by optical or electrical stimulation or as a result energy relaxation of hot-electrons.

Description:
FIELD OF THE INVENTION 
     The present invention relates to photonic devices. 
     BACKGROUND OF THE INVENTION 
     Silicon is the material of choice for the vast majority of microelectronic applications. Silicon is inexpensive and silicon-based processing techniques are well established. However, silicon possesses an indirect band gap and so some optical processes, such as photon emission, require the assistance of a phonon of a suitable wavevector, thus severely limiting efficiency of these processes. As a result, the use of silicon in some photonic applications, such as the manufacture of light emitting diodes, semiconductor lasers and optical modulators, is severely limited. 
     Phonon induced luminescence in compound semiconductors is disclosed by K. F. Renk in “Non-equilibrium Phonons in Non-metallic Crystals”, Eisenmenger and Kaplyanskii Eds., North-Holland, 1986). These compound semiconductors, however, exhibit lattice polarisation and strong lattice coupling and are already efficient photon generators. 
     Attempts to make silicon an efficient photon generator are disclosed in “Silicon Based Optoelectronic Materials”, Tischler et al. Eds., Material Research Society Proc., 298 (1993). The methods disclosed make use of quantum confinement and Si:SiGe heterostructures. However, these methods have not been particularly successful and devices based on these methods certainly do not lend themselves to conventional silicon-based processing techniques. 
     SUMMARY OF THE INVENTION 
     With a view to overcoming this difficulty, the present invention provides a photon emission device comprising a region of relatively low-efficiency photon emission material and a phonon generator operable to supply phonons to said region of relatively low-efficiency photon emission material so as to make it emit photons with a relatively high efficiency. 
     Said phonon generator may comprise an input structure to receive electromagnetic energy so as to produce phonons. Said phonon generator may comprise a converter to convert electrical excitations into lattice excitations. Said phonon generator may comprise an electrode to apply an electric field so as to produce phonons. 
     Said photon generator may comprise a fabricated device arranged on a substrate. 
     Said region of relatively low-efficiency photon emission material may comprise indirect band gap semiconductor material, such as silicon. 
     Said phonon generator may comprise a local lattice polarizer, such as a doped, compensated semiconductor, or a first layer of semiconductor doped with n-type impurities and a second layer of semiconductor doped with p-type semiconductor. Said first layer and said second layer may be separated by 1-5 nm. 
     Said phonon generator further may include an electric field generator to stimulate phonon generation. Said electric field may be provided by an electrode disposed at an interface with said local lattice polarizer. 
     Said electric field generator may comprise an electrode and an insulator, wherein said insulator may be disposed between said local lattice polarizer and said electrode. 
     Said phonon generator may further comprise an electron-hole pair generator, for example in response to a pulse of electromagnetic radiation. Said pulse of electromagnetic radiation may be of duration less than 100 fs. Said electromagnetic radiation may have an energy above the value of band gap of said doped, compensated semiconductor material. 
     Said electron-hole pair generator may comprise an electric pulse and said pulse may have duration less than 50 ps and a pulse height of the order of a few volts. 
     Said phonon generator may comprise a hot-electron injector. Said hot-electron injector may comprise an electrode and a layer of insulator. 
     Said device may further comprise a hot-electron thermalizer so as to create phonons. 
     Said electrode may comprise a metal, which may be aluminium. 
     Said insulator may comprise silicon dioxide or silicon nitride and may have a thickness less than 20 nm. 
     An advantage of the invention is that it allows the integration of photonic technology with conventional silicon-based logic circuitry and memory. This is particularly beneficial to applications in telecommunications and computing. 
     According to the present invention there is also provided a method of operating a phonon emission device comprising a region of relatively low-efficiency photon emission material said method comprising supplying phonons to said region of relatively low-efficiency photon emission material so as to make it emit phonons with a relatively high-efficiency. 
     According to the present invention there is still further provided a modulator comprising a region of optically transparent material and a phonon generator operable to supply phonons to said region of optically transparent material so as to increase photon absorption therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawing in which: 
     FIG. 1 is an electron dispersion relation for GaAs; 
     FIG. 2 is an electron dispersion relation for silicon; 
     FIG. 3 is a cross-sectional view of an optically excited phonon generator; 
     FIG. 4 is a cross-sectional view of an electrically excited phonon generator; 
     FIGS. 5 (A-E) shows the fabrication sequence of a semiconductor laser comprising an optically excited phonon generator of a first type; 
     FIGS. 6 (A-E) shows the fabrication sequence of a semiconductor laser comprising an optically excited phonon generator of another configuration of a second type; 
     FIGS. 7 a  and  b  shown doping structures suitable for creating local lattice polarisations; 
     FIG. 8 is a phonon dispersion relation for silicon; 
     FIG. 9 is a cross-sectional view of a semiconductor laser incorporating a phonon generator; 
     FIG. 10 is a cross-sectional view of a semiconductor homojunction laser incorporating a phonon generator; 
     FIG. 11 is a perspective view of an array of semiconductor lasers; 
     FIG. 12 is a perspective view of a modulator and 
     FIG. 13 is a schematic of an array of modulators. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, gallium arsenide (GaAs) exhibits a direct band gap. This means that the lowest point of the conduction band  1  occurs at the same electron wavevector, k, as the highest point of the valence band  2 . An electron-hole pair may be created by the promotion of an electron  3  from the valence band to the conduction band by the absorption of a sufficiently energetic photon  4 . During this process electron momentum is conserved. For photon emission, this process is reversed and an electron-hole pair recombines. 
     Referring to FIG. 2, silicon (Si) exhibits an indirect band gap. Here, the lowest point of the conduction band  5  occurs at a different electron wavevector from the highest point of the valence band  6 . An electron-hole pair is created by the promotion of an electron  7  from the valence band maximum to the conduction band minimum by the absorption of a sufficiently energetic photon  8  and the interaction of a phonon  9  having a frequency of 4.49 THz. However, for photon emission, a phonon is required to provide the missing momentum necessary for the electron-hole pair to recombine. Said change in momentum is a significant fraction of the Brillouin zone and suitable phonons of high wavevector (short wavelength and high energy) are not usually thermally activated. Therefore, silicon is not ordinarily an efficient photon emitter at room temperature, although it may be used as a good photon absorber, if photons have energies in excess of the band gap energy E g , where E g =1.12 eV. The invention provides a way of using an indirect band gap material such as silicon as an efficient photo-emitter. 
     In accordance with the invention, phonons are produced, using a phonon generator, and supplied to low efficiency materials such as Si, to cause it to emit photons with high efficiency. Some embodiments of phonon generators suitable for this purpose will now be described. 
     Phonon Generation by Optical Stimulation 
     Referring to FIG. 3, illustrates a structure which can be used as a coherent first embodiment of phonon generator based on an optically excited metal-oxide-semiconductor device  10 . 
     Using p-type silicon as a substrate  11 , a layer of compensated silicon (Si)  12  is grown by low-pressure chemical vapour deposition in a manner well known per se. The compensated Si layer  12  is 200 nm thick and is doped with boron (B) and arsenic (As) to a concentration of N B =N As =10 20  cm −3 . A silicon dioxide (SiO) tunnel barrier  13  is grown by dry oxidation at 900° C. The thickness of the SiO 2  barrier  13  is about 5 nm. Finally, an aluminium (Al) surface gate  14  is sputtered in a manner well known per se. The thickness of the Al layer  14  is 50 nm. It will be appreciated that the surface gate  14  may be patterned using conventional lithographic and dry etching techniques. 
     Operation of the coherent phonon generator  10  will now be described. 
     A gate voltage V g =5V is applied to the surface gate  14 , with the compensated Si layer  12  grounded, to create an electric field at the surface of the compensated Si layer  12 . A frequency-doubled Ti-Saphire laser (not shown) is used to generate a pulse  15  having a duration of about 50 fs and a wavelength of about 400-600 nm, which is directed onto the surface of the device  10 . Electron-hole pairs are generated in the silicon and are separated by the applied field, which creates an electric dipole. The dipole interacts with local lattice polarisations, created by closely spaced p- and n-type impurities, to produce optic phonons. These optic phonons decay through Klemens and Vallée channels into high energy, high wavevector phonons, for example transverse acoustic phonons. 
     Phonon Generation by Electrical Stimulation 
     Referring again to FIG. 3, the device structure  10  can also be used as a second embodiment of phonon generator based on an electrically excited metal-oxide-semiconductor device  10  and does not use optical stimulation. 
     The phonon generator is fabricated in the same manner as previously described. 
     Operation of the phonon generator  10  will now be described. 
     A gate voltage V g =5V is applied to the surface gate  14 , with the compensated Si layer  12  grounded, to create an electric field at the surface of the compensated Si layer  17 . A 50 ps electric pulse of V g =5V is applied to the surface gate  21 . The leading edge of the pulse provides a dipole impulse to excite the lattice. The dipole interacts with local lattice polarisations to produce optic phonons. These optic phonons decay through Klemens and Vallée channels into high energy, high wavevector phonons, for example transverse acoustic phonons. 
     FIG. 4, illustrates a third embodiment of phonon generator based on an electrically excited metal-oxide-semiconductor device  16 . 
     Using p-type Si as a substrate  17 , a layer of compensated Si  18  is grown by low-pressure chemical vapour deposition in a manner well known per se. The compensated Si layer  18  is 200 nm thick and is doped with B and As to a concentration of N B =N As =10 20 cm −3 . A SiO 2  tunnel barrier  19  is grown by dry oxidation at 900° C. The thickness of the SiO 2  barrier  19  is about 5 nm. Finally, an Al surface gate  20  is sputtered in a manner well known per se. The thickness of the Al layer  20  is 50 nm. The surface gate  20  is patterned using conventional lithographic and dry etching techniques to form two field gates  20   a ,  20   b  separated by 5 μm. A ‘T’-shaped Al pulse gate  21 , which is 2 μm wide at its base, 10 μm at its top and 200 nm tall, is fabricated in a manner well known per se. 
     Operation of the coherent phonon generator  16  will now be described. 
     A field gate voltage V g =5V is applied to the field gates  20   a ,  20   b , with the compensated Si layer  17  grounded, to create an electric field at the surface of the compensated Si layer  17 . A 50 ps electric pulse of V g =−5V is applied to the pulse gate  21 . The leading edge of the ultrafast electric pulse provides a dipole impulse to excite the lattice. The dipole interacts with local lattice polarisations to produce optic phonons. These optic phonons decay through Klemens and Vallée channels into high energy, high wavevector phonons, for example transverse acoustic phonons. 
     It will be appreciated that longer electrical pulses may be used, although this will favour generation of incoherent, rather than coherent, phonons. 
     Semiconductor Lasers Comprising Phonon Generators 
     A first embodiment of a device according to the present invention is shown in FIG.  5  and comprises a semiconductor laser that includes a Si laser cavity  22  and a phonon generator  23  that uses optical stimulation as described with reference to FIG. 3, to stimulate photo-emission in the laser cavity  22 . 
     The device is fabricated as follows. Using p-type silicon as a substrate  24 , a layer of intrinsic Si  25  is grown by low-pressure chemical vapour deposition in a manner well known per se as shown in FIG. 5 a . The intrinsic Si layer  25  is 400 nm thick and has a background doping concentration of N i ≦10 15 cm −3 . The surface of the intrinsic Si layer  25  is patterned using conventional optical lithography techniques to open a window 5×5 μm in optical resist. A succession of ion beam implantations using As + and B + ions are used to produce a highly doped (N B =N As =10 20 cm −3 ), fully compensated region  26  in the window region, as shown in FIG. 5 b . The resist is removed and the surface of the intrinsic Si layer  25  is again patterned using conventional optical lithography techniques to open a window 20×20 μm in optical resist. A low-energy ion beam implantation using As + ions is used to produce a shallow n-type well  27 . The substrate is annealed at 1000° C. to remove lattice damage to and activate the implants. 
     A mesa is defined by depositing and patterning optical resist on the surface of the intrinsic  25  and compensated  26  layers and by using a CF 4 /O 2  reactive ion etch. A SiO 2  tunnel barrier  28  is grown by dry oxidation at 900° C. across the whole surface. The thickness of the SiO 2  barrier  28  is about 5 nm. An Al layer  29  is then sputtered in a manner well known per se to produce the configuration shown in FIG. 5 c . The thickness of the Al layer  29  is 50 nm. A window in optical resist is opened over the intrinsic region  25  and the Al layer  29  and the SiO 2  tunnel barrier  28  are dry etched using BCl 3 /Cl 2  and CHF 3  respectively to leave a surface gate  30  over the compensated region  26  as shown in FIG. 5 d.    
     Finally, 200 nm of Al:Si (99:1) alloy is deposited on the surface of the mesa and the substrate and patterned using conventional optical lithographic techniques and BCl 3 /Cl 2  RIE to form laser ohmic contacts  31   a ,  31   b  shown in FIG. 5 e . It will be understood that the semiconductor laser has a cavity which extends between opposite ends of the mesa thus formed such that the side edges  25   a, b  of the mesa define semi-reflective ends facets for the cavity. Cavity  22  is primarily filled with intrinsic silicon which does not normally exhibit a lasing action because it is an indirect bandgap material. However, the phonon generator  23  produces phonons which allow photons to be produced in the laser cavity when a suitable bias voltage is applied to the laser ohmic contract  31   a ,  31   b . This will now be described in more detail. 
     Operation of the semiconductor laser will now be described. 
     The laser is primed by applying a bias of V=1.5V between the ohmic contacts  31   a ,  31   b  of the laser, to produce a ready supply of electron and holes within the intrinsic Si layer  25  that are ready to recombine. However, in the absence of phonons, the electrons and holes do not recombine radiatively and so the laser does not emit light. 
     The phonon generator is primed by applying a gate voltage V g =5V to the surface gate  30 , with the compensated Si layer  26  grounded, to create an electric field at the surface of the compensated Si layer  26 . 
     To trigger phonon generation, a frequency-doubled Ti-Saphire laser is used to generate a train of light pulses  32  that is directed onto the surface of the phonon generator  23 . Each pulse has a duration of 50 fs and a wavelength of 400-600 nm and the train length has a duration, T, where T≧50 fs. Electron-hole pairs are generated in the compensated Si layer  26  and are separated by the applied field, which creates an electric dipole. The dipole interacts with local lattice polarisations to produce optic phonons. These optic phonons decay into high energy, high wavevector phonons. This process generates a pulsed supply of phonons that bathes the laser cavity. This increases the efficiency of photon emission and enables the intrisic Si  25  to emit photons. 
     A lasing action occurs within the cavity  22  between the end facets  25   a, b , with the result that light is emitted from the cavity as shown by arrow P in FIG. 5 e . The duration of the pulse of emitted light is approximately T. The wavelength of the emitted light P is 1.1 μm. 
     Referring to FIG. 6, a second embodiment of the present invention is a semiconductor laser comprising a laser cavity  33  and a phonon generator  34  based on optical stimulation according to the principles previously described with reference to FIG.  4 . 
     Using p-type silicon as a substrate  35 , a layer of intrinsic Si  36  is grown by low-pressure chemical vapour deposition in a manner well known per se as shown in FIG. 6 a . The intrinsic Si layer  36  is 400 mn thick and has a background doping concentration of N i ≦10 15 cm −3 . The surface of the intrinsic Si layer  36  is patterned using conventional optical lithography techniques to open a window 5×5 μm in optical resist. A succession of ion beam implantations using As + and B + ions are used produce a highly doped (N B =N As =10 20 cm −3 ), fully compensated region  37  in the window region, as shown in FIG. 6 b . The resist is removed and the surface of the intrinsic Si layer  36  is again patterned using conventional optical lithography techniques to open a window 20×20 μm in optical resist. A low-energy ion beam implantation using As + ions is used to produce a shallow n-type well  38 . The substrate is annealed at 1000° C. to remove lattice damage to and activate the implant. 
     A mesa is defined by depositing and patterning optical resist on the surface of the intrinsic  36  and compensated  37  layers and by using a CF 4 /O 2  reactive ion etch. A SiO 2  tunnel barrier  39  is grown by dry oxidation at 900° C. across the whole surface. The thickness of the SiO 2  barrier  39  is about 5 nm. An Al layer  40  is then sputtered in a manner well known per se to produce the configuration shown in FIG. 6 c . The thickness of the Al layer  40  is 50 nm. A window in optical resist is opened over the intrinsic region  35  and the Al layer  40  and the SiO 2  tunnel barrier  39  are dry etched using BCl 3 /Cl 2  and CHF 3  respectively to leave a patterned surface gate  41  over the compensated region  36  as shown in FIG. 6 d.    
     Finally, 200 nm of Al:Si (99:1) alloy is deposited on the surface of the mesa and the substrate and patterned using conventional optical lithographic techniques and BCl 3 /Cl 2  RIE to form laser ohmic contacts  42   a ,  42   b  shown in FIG. 6 e.    
     Operation of the semiconductor laser will now be described. 
     The laser is primed by applying a bias of V=1.5V between the ohmic contacts  42   a ,  42   b  of the laser, to produce a ready supply of electron and holes within the intrinsic Si layer  36  that are ready to recombine However, in the absence of phonons, the electrons and holes do not recombine and so the laser does not emit light. 
     The phonon generator is primed by applying a gate voltage V g =5V to the patterned surface gate  41 , with the compensated Si layer  37  grounded, to create an electric field at the surface of the compensated Si layer  37 . 
     To trigger phonon generation, a frequency-doubled Ti-Saphire laser is used to generate a train of light pulses  43  that is directed onto the surface of the phonon generator  41 . Each pulse has a duration of 50 fs and a wavelength of 400-600 nm and the train length has a duration, T, where T≧50 fs. Electron-hole pairs are generated in the compensated Si layer  37  and are separated by the applied field, which creates an electric dipole. The dipole interacts with local lattice polarisations to produce optic phonons. These optic phonons decay into high energy, high wavevector phonons. This process generates a pulsed supply of phonons that bathes the laser cavity. This increases the efficiency of photon emission and enables the intrinsic Si  36  to emit photons. 
     A lasing action occurs within the cavity  33  between the end facets  36   a, b , with the result that light is emitted from the cavity as shown by arrow P in FIG. 6 e . The duration of the pulse of emitted light is approximately T. The wavelength of the emitted light P is 1.1 μm. 
     It will be appreciated that the optically excited phonon generators described in the first and second embodiments of the present invention may be replaced by an electrically stimulated phonon generator as previously described with reference to FIG.  3 . 
     The embodiments above use heavily doped, fully compensated Si to produce local lattice polarisation. Referring to FIGS. 7 a  and  7   b , alternative doping structures may be used for the regions  26 ,  37  in FIGS. 5 and 6 respectively. 
     A first alternative doping structure  44  comprises growing alternate layers of n-type  45  and p-type  46  Si, doped with As and B respectively, although other n- and p-type impurities may be used. The alternate layers are 2nm thick and the doping concentration are N AS =N B =1×10 20 cm −3 . 
     A second alternative doping structure  47  comprises growing Si in which are inserting alternate n-type  48  and p-type  49  δ 8 -doped doped layers in Si  50 , comprising As and P respectively, although other n- and p-type impurities may be used. The δ-doped layers are spaced 2 nm apart and the sheet doping concentration are N 2D:As =N 2D:B =2×10 13 cm −2 . 
     Phonon Generation by Hot-Electron Relaxation 
     FIG. 8 illustrates a phonon dispersion relationship for silicon for Transverse Acoustic (TA), Longitudinal Acoustic (LA), Transverse Optic (TO) and Longitudinal Optic (LO) phonons. Phonon wavenumber as a proportion of the Brillouin zone is plotted along the abscissa and phonon energy is plotted along the ordinate. The TA phonon dispersion relation is relatively flat at high values of wavevector. In other words, there is little change in energy over a large wavevector range. If electrons are injected into the active region of a phonon generator with a kinetic energy similar to or slightly higher than the flat region of the TA phonon curve, corresponding to about 20 meV above the conduction band edge, the electrons will loose energy by TA phonon scattering. Phonon-phonon scattering will then generate the phonons of the correct wavevector for optical excitation and the excitation will itself increase the coupling of that wavevector. 
     Referring to FIG. 9, a third embodiment of the present invention is a semiconductor laser comprising a cavity  51  and a phonon generator  52  based on hot electron relaxation. 
     Using &lt;100&gt; orientated p-type silicon as a substrate  53 , an intrinsic layer Si  54  is epitaxially grown by low-pressure chemical vapour deposition in a manner well known per se. The intrinsic Si layer  54  is 400 nm thick and has a background doping concentration of N I ≦10 15 cm −3 . A SiO 2  tunnel barrier  55  is grown by dry oxidation at 800° C. The thickness of the SiO 2  barrier  55  is about 3 nm. An Al contact  56  is then sputtered in a manner well known per se. The thickness of the Al layer  56  is 200 nm. 
     The surface is patterned using conventional optical lithographic techniques and a portion of the Al contact  56  and the underlying SiO 2  barrier  55  are removed by dry etching. The portion that is not removed forms the hot-electron injector of the phonon generator  52 . 
     A shallow B + ion implant is used to define a p-type ohmic contact region. A layer of Al:Si (99:1%) is sputtered and patterned using conventional lithographic and etching techniques to form the p-type contact  57  to the intrinsic Si layer  54 . Using a CF 4 /O 2  dry etch a laser cavity is defined with optically flat and parallel facing ends. 
     Operation of the semiconductor laser will now be described. 
     A negative bias is applied to the Al layer  56  with respect to the contact  57  and typically, V=2V. Electrons are injected from the Al contact  56  into the intrinsic Si layer  54 . Hot electrons tunnel through the SiO 2  barrier  55  and are thermalised by generating phonons. The thermalised electrons combine with holes injected by the contact  57 , emitting light. The light is thus emitted into cavity  51  so as to stimulate a laser action, with the result that light is in the direction of arrow P cavity as shown by arrow P. 
     Referring to FIG. 10, a fourth embodiment of the present invention is a homojunction semiconductor laser comprising a cavity  58  and a phonon generator  59  based on hot electron relaxation. 
     Using &lt;100&gt; orientated p-type silicon as a substrate  60 , a layer of p-type Si  61  is epitaxially grown by low-pressure chemical vapour deposition in a manner well known per se. The p-type Si layer  61  is 200 nm thick and is doped with B to a concentration of N B =10 18 cm −3 . A layer of n-type Si  62  is epitaxially grown. The n-type Si layer  62  is 200 nm thick and is doped with As to a concentration of N As =10 18 cm −3 . A SiO 2  tunnel barrier  63  is grown by dry oxidation at 800° C. The thickness of the SiO 2  barrier  63  is about 3 nm. Finally, an Al contact  64  is sputtered in a manner well known per se. The thickness of the Al layer  56  is 200 nm. 
     The surface is patterned using conventional optical lithographic techniques and a portion of the Al contact  64  and the underlying SiO 2  barrier  63  are removed by dry etching. The portion that is not removed forms the hot-electron injector of the phonon generator  59 . 
     A layer of Al:Si (99:1%) is sputtered and patterned using conventional lithographic and etching techniques to form an ohmic contact  65  to the n-type Si layer  62 . An ohmic contact (not shown) is made to the reverse side of the p-type substrate  60  in a manner well known per se. Using a CF 4 /O 2  dry etch a laser cavity  58  is defined with optically flat and parallel facing ends. 
     Operation of the semiconductor laser will now be described. 
     The laser is primed by forward biasing the p-n junction with a bias voltage, V b , of 2V between the ohmic contact  65  to the n-type Si layer  62  and the ohmic contact (not shown) to the p-type substrate  60 . 
     To generate phonons, electrons are injected from the Al contact  64  into the n-type Si layer  62  by applying to the Al contact  64  an injection bias, V inj , of −0.5V. Hot electrons tunnel through the SiO 2  barrier  63  and are thermalised by generating phonons. These phonons promote electron-hole recombination at the p-n junction with the result that photons of energy 1.12 eV are emitted into cavity  58 , stimulating laser action such that laser light leaves the cavity in the direction of arrow P. 
     FIG. 11 illustrates a fifth embodiment of the present invention that comprises an array of semiconductor lasers. 
     The array of lasers comprises first and second lasers  66   a ,  66   b  as shown in FIGS. 9 or 10 above. The first and second lasers  66   a ,  66   b  share the same aluminium metallization  67  from which hot electrons are injected. However, the first and second lasers  66   a ,  66   b  have separate electrical contacts  68   a ,  68   b  corresponding to contacts  57  or  65  in FIGS. 9 or  10  for switching on and off each laser independently of each other. Such an arrangement can be used to convert an electrical data bus into an optical databus, with the optical signals being fed into optical waveguides. 
     FIG. 12 illustrates a sixth embodiment of the present invention that comprises an optical modulator. 
     Incoming light pulses pass down an optical waveguide  69  comprising silicon. The light pulses serve as the optical stimulation for the generation of phonons in a phonon generator  70  as shown in FIG. 9 above. An electrical pulse of V=2V is applied between the gate  71  of phonon generator  70  and the contact  72 . 
     Different arrangements may be used for the phonon generator and for the cavity of the semiconductor laser. 
     Referring to FIG. 13, the arrangement as shown in FIG. 12 may be incorporated into switching circuits. Optical inputs, carried by micro-fabricated waveguides  73   a ,  73   b ,  73   c ,  73   d , are switched by electric inputs, conveyed by metal tracks  74   a ,  74   b ,  74   c ,  74   d . Outputs are delivered to waveguides or optical fibres through optical couplings  75   a ,  75   b ,  75   c ,  75   d . In this example, the outputs are sequentially delayed by having different phonon path lengths between the phonon generation region and the electrical injection region. 
     It will be appreciated that many modifications may be made to the embodiments above. For instance, any indirect band gap semiconductor may benefit from phonon injection including, but not limited to, Germanium (Ge), Aluminium Antimonide (AlSb), Gallium Phosphide (GaP), Silicon Carbide (SiC) and ternary alloys containing these binary compositions. The device need not necessarily have a laser cavity. The device may be a light emitting diode. 
     A phonon generator may also be used to change the absorption coefficient in Si. Silicon is transparent in the infrared. Supplying phonons will effectively reduce the band-gap energy and so promote photon absorption. This may be used to fabricate a modulator. 
     As used herein, “optical processes” is intended to include processes involving visible and non-visible radiation, and includes infrared and ultraviolet radiation.