Abstract:
A device comprising a metal layer on a crystalline silicon substrate, and a waveguide that has a refractive index greater than that of the crystalline silicon, wherein the waveguide is arranged to couple light to a surface plasmon mode at an interface between the silicon substrate and the metal layer when a waveguide mode is phase matched to the surface plasmon mode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. 371 and claims the benefit of PCT Application No. PCT/GB2014/051844 having an international filing date of 16 Jun. 2014, which designated the United States, which PCT application claimed the benefit of Great Britain Application No. 1313592.6 filed 30 Jul. 2013, the disclosure of each of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an optical modulator, and in particular a plasmon based optical modulator. The modulator of the invention may be used for optical interconnects in, for example, computers. 
     BACKGROUND OF THE INVENTION 
     Current computers are increasingly limited by power consumption and heat dissipation issues, roughly clamping the clock rate of the Central Processing Unit (CPU) at ˜3-4 GHz since around 2004. Most of this power (50-80%) is consumed in interconnects, i.e. the metal traces that move information around and on/off the chip. As these wires have become thinner and data-rates faster the overall energy efficiency has plummeted. This is one of the biggest challenges facing the computer industry today. 
     Optical interconnects are seen as the solution to this problem. Electrical data generated from electronic circuitry is encoded into a beam of light using an electro-optic modulator, transmitted via an optical cable/waveguide and converted back into electrical data using photo-detectors at the receiving end. Unlike an electrical wire, the limit on data transmission in an optical waveguide can be as high as 100 Tbit/s and data transfer at high bit rates is much more energy efficient. 
     In silicon based optical interconnects, there are primarily two modulation techniques employed: (a) interference based modulation, typically using Mach-Zehnder Interferometer (MZI) type modulators, and (b) resonance based modulation, typically using ring resonators.  FIG. 1  shows a top view and a cross sectional view of (a) a MZI based modulator and (b) a ring resonator based modulator. In both cases, a thick oxide cladding (˜2 μm) is required to guide light in top silicon layer 
     In MZI modulators, optical modulation is achieved by changing the refractive index of one of the arms. This creates a phase difference between the two arms. By modulating this phase difference constructive and destructive interference can be achieved. This gives rise to intensity modulation at the output of the modulator. In ring resonator based modulators, the basic principle is to tune the ring in and out of resonance by changing its refractive index. In both these approaches, a material incompatibility arises when combining electronics and optics on the same silicon platform. This is because conventional optical waveguiding techniques require a lower cladding generally in the form of a thick buried oxide layer to guide light (as shown in  FIG. 1 ). When integrated with electrical circuits such a layer traps heat in the electrical components, for example the transistors, and so reduces the integration density. This is unacceptable to the electronics industry. As a result, true integration of optical components with CMOS circuitry is not possible with conventional optical interconnect techniques. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a device comprising a metal layer on a crystalline silicon substrate, and a waveguiding layer that has a refractive index greater than that of the crystalline silicon, wherein the waveguide is arranged to couple light to a surface plasmon mode at an interface between the silicon substrate and the metal when a waveguide mode is phase matched to the surface plasmon mode. The waveguide may be an amorphous silicon waveguide. A dielectric layer may be provided between the metal layer and the waveguide. 
     Surface plasmons are coherent electron oscillations at the interface between any two materials where the real part of the dielectric function changes sign across the interface (e.g. a metal-dielectric interface). A surface plasmon mode can be guided by a single interface between a metal and a dielectric. To excite a surface plasmon mode at a metal-dielectric interface a dielectric waveguide can be used. Light can couple from the waveguide to the surface plasmon mode. 
     By using surface plasmon modes to guide light between bulk silicon and a metal layer the need for any bottom cladding layer can be avoided. This allows a simple implementation of the device in an electro-optic circuit allowing front-end integration with minimal changes to electrical device manufacturing processes currently in use (for example CMOS). 
     The device may comprise means of modulating the refractive index of the crystalline silicon substrate to change a phase matching condition between the surface plasmon mode and waveguide mode to modulate light carried by the amorphous silicon waveguide. 
     The means of modulating may be operable to accumulate or deplete carrier density in the crystalline silicon. 
     The means for modulating may comprise an electrical device formed in or on the crystalline silicon substrate. The electrical device may comprise a pin junction. The electrical device may comprise a Schottky junction at the metal-silicon substrate interface. 
     A thin oxide layer may be provided between the crystalline silicon substrate and the metal to form an electrical device comprising a MOS capacitor. 
     The waveguide may be created by ion-implantation of partially amorphous silicon. 
     According to another aspect of the invention, there is provided an electrical circuit comprising at least one optical interconnect that includes at least one device according to the first aspect of the invention. 
     According to yet another aspect of the invention there is provided a method for forming an amorphous silicon waveguide comprising using ion implantation to distort crystalline silicon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of the invention will now be described by way of example and with reference to the accompanying drawings, of which: 
         FIG. 2( a )  is a cross section through a surface plasmon based coupler; 
         FIG. 2( b )  is a simulation of magnetic field distribution under phase matching condition for the structure of  FIG. 2( a ) ; 
         FIG. 2( c )  shows dispersion relations for the structure of  FIG. 2( a ) ; 
         FIG. 2( d )  shows simulated transmission spectra for different refractive indices of the bulk silicon layer of the coupler of  FIG. 2( a ) ; 
         FIG. 3  is shows an electro-optic modulator based on (a) a MOS capacitor, (b) a p-i-n diode, and (c) a Schottky diode beside a MOS field effect transistor; 
         FIG. 4  shows ellipsometry measurements of amorphous silicon before and after implanatation; 
         FIG. 5  is a cross section through a surface plasmon based coupler butt coupled to a germanium photodiode; 
         FIG. 6  is a 3D view of a CMOS chip with on-chip optical interlink, and 
         FIG. 7  is a top view of a CMOS chip with an on-chip and off-chip optical interlink. 
     
    
    
     DETAILED DESCRIPTION 
     The invention uses surface plasmon modes to guide light using a single interface between a metal and a dielectric. To excite a surface plasmon mode at a metal-dielectric interface, a dielectric waveguide is used. The waveguide has a higher refractive index than the bulk substrate, where the surface plasmon mode exists. 
       FIG. 2( a )  shows a cross sectional view of a surface plasmon coupling structure  10 . This has a bulk silicon substrate  12 , and a thin metal layer  14 , for example aluminium, on the bulk substrate  12 . On the metal  14  is a low refractive index buffer layer  16 . Between an upper cladding layer  18  and the buffer  16  is an amorphous silicon waveguide  20 . This has a higher refractive index than the bulk silicon substrate  12 . The thickness of the metal layer  14  is typically in the range 5 to 100 nm, for example in the range 30 to 40 nm. The thickness of the buffer layer  16  can be in the range 1 to 500 nm, for example 200 nm. However, this may vary considerably depending on the application. The waveguide  20  has a thickness in the range 100 nm to 1 micron. 
     Light is launched at an input port of the amorphous silicon waveguide  20 . Under phase matching conditions, when the waveguide mode has the same k-vector as the surface plasmon mode, light transfers from the waveguide mode to the lossy surface plasmon mode at the metal-dielectric interface. This results a dip in transmission spectrum at the output of the waveguide. 
     When the refractive index of the dielectric  12  under the metal  14  changes (Δn), the dispersion relation of the surface plasmon mode shifts (Δω=ω×Δn/n). However, as the dip in transmission is given by the phase matching conditions, the intersection point changes in both frequency and k-vector resulting in a frequency shift ΔΩ of the transmission dip that can be much larger than Δω. This sensitivity value can be above 10,000/RIU, which is ideal for modulators based on the weak electro-optic effects of silicon. It also provides a broad bandwidth that is useful for many applications. 
       FIG. 2( b )  shows a simulated magnetic field distribution under phase matching condition for the structure of  FIG. 2( a ) .  FIG. 2( c )  shows dispersion relations for the structure of  FIG. 2( a ) . These show extremely high sensitivity of the coupling between waveguide mode and surface plasmon mode. The red curve gives a portion of the dispersion curve of the surface plasmon mode. The red dashed curve shows the curve after a change in refractive index of the bottom silicon layer. The blue curve shows the dispersion relation of the waveguide mode.  FIG. 2( d )  shows simulated transmission spectra at the amorphous silicon waveguide output for different refractive index of the bulk silicon layer. 
       FIG. 3  shows various electrical devices integrated with the optical structure  10  of  FIG. 2( a )  on a conventional CMOS chip. 
       FIG. 3( a )  is an electro-optic device that uses a metal-oxide-silicon interface of a typical MOS capacitor for supporting a surface plasmon mode. The electro-optic device of  FIG. 3( a )  has two heavily doped n-type regions  22  formed in bulk p-type silicon  24 . Metal contacts  26  are electrically coupled with the heavily doped n-type regions  22 . Between the two n-type regions  22  is a p-type region  27  on which a thin layer  28  of low refractive index material is formed. On top of this, a thin metal layer  30  is deposited to form the metal-oxide-silicon interface. On the metal layer  30  is a buffer oxide layer  32  on top of which is an amorphous silicon waveguide  34 . To allow voltage to be applied to the metal layer an electric contact  36  is provided. 
     The evanescent tails of the waveguide mode and the surface plasmon mode overlap in the buffer oxide layer. When the phase matching condition is achieved, light is transferred from top waveguide to the surface plasmon mode and a dip in transmission spectrum at the output is observed. With an applied voltage at the gate of the MOS capacitor, carriers will be depleted or accumulated under the metal, giving rise to a change in refractive index and a consequent shift in the dip in transmission spectrum. This allows very high speed modulation, which can also be very efficient due to the low resistance and capacitance of this configuration. The use of a surface plasmon mode to control the optical intensity in the top waveguide allows monolithic integration of an electro-optic modulator on a conventional CMOS chip. 
       FIG. 3( b )  shows an electro-optic modulator based on a p-i-n silicon diode. This has a heavily doped n-type region and a heavily doped p-type region in bulk p-type silicon. Metal contacts are electrically coupled with the heavily doped n-type and p-type regions  38  and  40  respectively. Between the n-type and p-type regions  38  and  40  is a region of p-type bulk silicon  42  on which a thin metal layer  44  is deposited. Above the metal layer  44 , there may be a buffer oxide layer  46  on top of which is the waveguiding layer  48 . To allow voltage to be applied to the metal layer  44  an electric contact  50  is provided. In this configuration, modulation in refractive index under the metal layer  44  is achieved by changing the carrier density in the intrinsic region of the p-i-n diode by applying a forward bias. 
     As before, the evanescent tails of the waveguide mode and the surface plasmon mode overlap (when there is a buffer oxide layer  46 , the overlap occurs in the buffer  46 ). When the phase matching condition is achieved, light is transferred from top waveguide  48  to the surface plasmon mode and a dip in transmission spectrum at the output is observed. By varying the forward bias applied to the diode, carriers will be depleted or accumulated under the metal  44 , giving rise to a change in refractive index and a consequent shift in the dip in transmission spectrum. 
       FIG. 3( c )  shows an electro-optic modulator based on a Schottky barrier at a metal-silicon interface. The modulator is formed on a heavily doped p-type region  50  in bulk silicon. Two metal contacts  52  are electrically coupled with the heavily doped p-type region  50  between which a metal layer  54  is deposited. As before, on the metal layer  54  is a buffer layer  56  on top of which is an amorphous silicon waveguide  58 . To allow voltage to be applied to the metal layer  54  an electric contact  60  is provided. By applying voltage modulation at the metal layer  54 , the depletion region under the metal layer  54  can be modified, giving rise to a modulation in the refractive index of the bulk silicon under the metal layer  54 . 
     The electro-optic device of the present invention can be made using standard processing techniques. In one embodiment, the top waveguide comprises a high refractive index amorphous silicon waveguiding layer. The amorphous silicon may be deposited using Chemical Vapour Deposition, a standard processing tool. As deposited, such material is rarely 100% amorphous having refractive indices in the 3.7-3.8 range. By implanting this layer with high energy silicon ions (160 keV with a dose of 2e15 per cm 2 , it can be fully amorphized allowing the realization of the required higher (3.95) refractive index.  FIG. 4  shows ellipsometry measurements of amorphous silicon before and after implanatation. Very low propagation losses have been demonstrated for amorphous silicon [Electronics Letters 41, 1377-1379 (2005), the contents of which are incorporated herein by reference] making them ideal for the realisation photonic integrated circuits. In another embodiment, a silicon germanium or germanium layer may be deposited to provide the top waveguide. This layer may exhibit refractive indices of 4-4.5 in the wavelength range of interest. By adjusting the silicon fraction, the absorption coefficient may be adjusted to give low losses at the telecoms wavelengths. 
     The invention allows optics and traditional electronics to be integrated thus allowing on-chip optical interconnects without compromising the integration density of the electronics. Due to the extremely sensitive coupling mechanism, this design operates at low power despite the weak electro-optic properties of silicon that are essential for data communications. This technique could also be used to couple light into low capacitance all silicon-photodiodes similar to those described in Nano Letters 11, 2219-2224 (2011), the contents of which are incorporated herein by reference, thus completing the optical link. Alternatively, conventional Germanium Photodiodes [Optics Express 20, 1096-1101 (2012), the contents of which are incorporated herein by reference] may be used. Integrating these is straightforward, for example, the germanium may be grown direct on the silicon substrate and the top waveguide butt coupled or evanescently coupled to the germanium, see  FIG. 5 . 
       FIG. 6  shows a 3D view of a section of a CMOS chip with on-chip optical interconnects based of the modulator design discussed above. For clarity, in  FIG. 6  only local electrical interconnects are shown and intermediate and global interconnects are not shown. The optical components are distributed in two layers. All active optical components, such as modulators and detectors are on a bulk silicon substrate layer along with all electronic components, whereas all passive components such as waveguides, directional couplers, splitters, thermo-optic switches etc. are in an α:Si access layer. 
     Light from a high power off-chip laser source is coupled into the passive network. This can be done using, for example, butt-coupling techniques (as shown in  FIG. 6 ) or grating couplers. Cheap high power (500-1000 mW) laser sources are now available. Using suitable multiplexors many channels for on-chip communications may be provided (for example thousands each carrying 100 uW). A traditional drawback of these lasers is their large spectal bandwidth (10s nm) making them unsuitable for many systems. The wide bandwidth of the modulator described here is ideally suited to working with such a light source providing a frame work for on-chip optical networks. Due to the relatively high refractive index of α:Si in comparison to silicon, light is mainly confined in the passive network and coupled to the bulk silicon at the modulator or detector regions whenever the phase matching condition is satisfied. The use of a surface plasmon mode to guide light in a bulk silicon substrate allows the full potential of optical components as well as electrical circuits to be realised on the same platform. 
       FIG. 7  shows a top view of an α:Si passive network. This design is suitable for both on- and off-chip interconnects. 
     A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the scope of the invention. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitations. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.