Patent Description:
In recent years, a plethora of applications based on photonic integrated circuits (PICs) have emerged including data centre communications, coherent telecommunications, filters, supercontinuum generation, spectroscopy, biosensing, quantum optics and microwave photonics. With the increasing interest in the emerging photonic circuits, a successful photonic platform requires electro-optic modulators.

<CIT> describes a MOS optical modulator. <CIT> also describes a grating coupler which has a plurality of projections separated from each other in an optical waveguide direction and a slab portion formed between any two of the projections adjacent to each other and formed integrally with them; the MOS (metal oxide semiconductor) optical modulator has a projection extending in the optical waveguide direction and slab portions formed on both sides of the projection, respectively, and formed integrally therewith. The projection of the grating coupler and the MOS optical modulator is formed of a first semiconductor layer, a second insulating layer, and a second semiconductor layer stacked successively on a first insulating layer, while the grating coupler and the MOS optical modulator each have a slab portion formed of the first semiconductor layer.

<CIT> discloses a MOS capacitor-type optical modulator and method of fabricating a MOS capacitor- type optical modulator, wherein the MOS capacitor-type optical modulator has a MOS capacitor region which comprises an insulator formed of an epitaxially grown crystalline rare earth oxide (REO).

<CIT> describes a ring optical modulator which includes an SOI (silicon-on-insulator) substrate, including at least first and second top silicon layers, and a silicon-based ring resonator formed on the SOI substrate. The silicon-based ring resonator includes first and second top silicon layers, a thin dielectric gate layer disposed between the top silicon layers, first and second electric contacts, and first rib-type waveguide and ring-shape rib-type waveguide formed on the second top silicon layer. The thin dielectric layer includes a first side in contact with the first top silicon layer and a second side in contact with the second top silicon layer. With electric signals applied on the electric contacts, free carriers accumulate, deplete or invert within the top silicon layers on the first and second sides of the thin dielectric gate layer beneath the ring-shape rib-type waveguide, simultaneously, and a refractive index of the ring-shape rib-type waveguide confining optical fields is modulated.

<CIT> describes a device which includes a first semiconductor layer disposed on a substrate, a dielectric layer disposed between the first semiconductor layer and a second semiconductor layer dissimilar from the first semiconductor layer. A capacitor is formed of at least a portion of the first semiconductor layer, the dielectric layer, and the second semiconductor layer, and is to be included in an optical waveguide resonator.

According to an aspect of the present invention, there is provided an electro-optic modulator for a waveguide. The electro-optic modulator comprises a first semiconductor layer, a second semiconductor layer, a dielectric layer interposed between the second semiconductor layer and the first semiconductor layer and a coupling layer for coupling a guided mode of the waveguide to at least one of the first semiconductor layer and the second semiconductor layer. When the guided mode is coupled to at least one of the first semiconductor layer and the second semiconductor layer, the coupling layer is disposed between the first semiconductor layer and the waveguide such that a centre of the guided mode does not overlap with the electro-optic modulator. The electro-optic modulator is configured to induce a modulation on the guided mode of the waveguide by changing a refractive index in response to a voltage applied between the first semiconductor layer and the second semiconductor layer.

In some implementations, the first semiconductor layer and the second semiconductor layer comprise opposite types of doping to each other such that when the first semiconductor layer exhibits an n-type behaviour, the second semiconductor layer exhibits a p-type behaviour and when the first semiconductor layer exhibits a p-type behaviour, the second semiconductor layer exhibits an n-type behaviour.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise a non-degenerate semiconductor.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise a III-V semiconductor.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise one or more of silicon, germanium and SiGe alloys.

In some implementations, a structure of at least one of the first semiconductor layer and the second semiconductor layer is one of the following: amorphous, hydrogenated amorphous, polycrystalline, nanocrystalline or crystalline.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise an alloy including one or more of NiSi, Ni2Si, NiSi2, TiSi, TiSi2, CoSi, Co2Si, PtSi or germanide thin films.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise an intrinsic semiconductor.

In some implementations, at least one of the first semiconductor layer and the second semiconductor layer comprise a semi-metallic layer.

In some implementations, a thickness of the dielectric layer is between <NUM> and <NUM>.

In some implementations, a thickness of the semiconductor layer and the second semiconductor layer is less than <NUM>.

In some implementations, the coupling layer comprises one or more of silicon dioxide (SiO2), silicon oxynitride (SiON), aluminium oxide (Al2O3), aluminium nitride (AlN), Benzocyclobutene (BCB), poly methyl methacrylate (PMMA) or Parylene.

In some implementations, the dielectric layer comprises one or more of silicon dioxide (SiO2), silicon oxynitride (SiON), Benzocyclobutene (BCB), poly methyl methacrylate (PMMA) or Parylene.

In some implementations, there is provided a device including a waveguide including a core and the electro-optic modulator described hereinbefore. A cross-section of the core is of a polygonal shape at at least one position along the propagation direction of the guided mode and at the at least one position, the electro-optic modulator is formed to be substantially parallel to and to cover a side of the cross-section of the core.

In some implementations, at the at least one position, the electro-optic modulator is formed to be substantially parallel to and to cover two or more consecutive sides of the cross-section of the core.

In some implementations, an extent of the first semiconductor layer along the propagation direction of the guided mode is larger than an extent of the dielectric layer along the same direction. The extent of the dielectric layer along the propagation direction of the guided mode is larger than an extent of the second semiconductor layer in the same direction.

In some implementations, there is provided a Mach-Zehnder modulator including an input port, an output port, a first splitter and a second splitter connected respectively to the input port and the output port and configured to split a guided mode of light received respectively from the input port and the output port into two guided modes of light, and a first arm and a second arm disposed between and connecting the first splitter and the second splitter such that a Mach-Zehnder interferometer is formed. The first arm comprises a waveguide and a first electro-optic modulator, being the electro-optic modulator described hereinbefore. The second arm comprises a waveguide and a second electro-optic modulator, being the electro-optic modulator described hereinbefore.

In some implementations, there is provided a ring resonator including a bus waveguide comprising an input port and an output port, a ring resonator coupled to the bus waveguide. The ring resonator comprises a waveguide and the electro-optic modulator described hereinbefore.

According to another aspect of the present invention, there is provided a method of fabricating an electro-optic modulator described hereinbefore on a waveguide, the method comprising: depositing a coupling layer on the waveguide; depositing a first semiconductor layer on the coupling layer; depositing a dielectric layer on the first semiconductor layer; and depositing a second semiconductor layer on the dielectric layer.

Certain embodiments of the present invention will now be described, by way of examples, with reference to the accompanying drawings, in which:.

An electro-optic modulator is an optical device in which physical properties of a beam of light or a guided mode of light are modulated in response to an electric signal. Examples of physical properties of light include phase, amplitude or polarization.

The electro-optic effect, on which the electro-optic modulator operates are based, includes the electro-refractive effect and the electro-absorptive effect.

Phase modulation of light may be achieved via the electro-refractive effect of a material, in which in response to an electric field, the real part of the refractive index of the material changes. When a light beam is passing through a material with an electro-optic property, the path length the light beam experiences changes with the strength of the electric field applied to the material. The modulation of polarization can also be achieved via the electro-refractive effect, for example, with a non-centrosymmetric material.

The amplitude modulation of light may be achieved via the electro-absorptive effect of a material, in which in response to an electric field, the imaginary part of the refractive index of the material changes. When a light beam is passing through a material with an electro-optic property, the attenuation the light beam experiences changes with the strength of the electric field applied to the material.

The electro-refractive effect and the electro-absorptive effect as described above are inherent properties of materials. State of the art electro-optic modulators operating via the electro-optic effect use materials such as PZT (Lead zirconate titanate), BaTiO3 (Barium titanate), and LiNbO3 (Lithium niobate).

The electro-refractive effect and the electro-absorptive effect as defined hereinbefore are known to be relatively weak in pure silicon at the communication wavelengths <NUM> and <NUM>. The modulation of light in silicon devices may be achieved via the plasma dispersion effect, in which the change in the density of charges leads to the effective changes in the real and imaginary part of the refractive index.

The charge density within silicon devices can be manipulated with an electric field via carrier injection, accumulation or depletion. Therefore, the modulation of phase and amplitude of light can be achieved with silicon devices via the plasma dispersion effect.

In this specification, the term 'electro-optic modulator' will be understood to encompass all of the mechanisms discussed hereinbefore, namely the electro-optic effect and the plasma dispersion effect, and other possible mechanisms to induce the modulation of phase, amplitude or polarization of light in response to an electric field. Therefore, the term "electro-optic modulator" in this specification is used to mean any optical device in which a beam of light or a guided mode of light are modulated in response to an electric signal.

Lithium niobate modulators have been, for a while, the gold standard for modulators in terms of electrical bandwidth and extinction ratio, even if they suffer from several limiting factors such as large footprints and insertion losses of several dBs. In the past few years, various electro-optic modulators in an integrated form have been demonstrated using different materials, such as in Si, InP and thin-film LiNbO<NUM>, and have proven to be appealing for high symbol rate modulation. Although all these integrated concepts offer a wide modulation bandwidth larger than <NUM>, they suffer from different drawbacks, such as complex fabrication processes (3D Si doping), large propagation losses (InP-on-Si, <NUM> dB/cm), or from large foot-print (larger than <NUM> across), hindering the integration with other photonic components. Integration of light sources with these modulators can be realized by means of hybrid integration, for example, by integrating a III-V light source with a SOI chip or by monolithic integration but all the explored solutions have faced industrial challenges.

One solution to address these issues is to use a Fabry-Perot etalon to minimize the size of the device, and thus enhancing the maximum operating speed. This solution however has been obtained by engineering multiple quantum well structures in III-V materials, which requires expensive processing for the fabrication of such devices with the required quality. In addition, the structure cannot be laterally integrated, which is a further limitation that impedes its implementation for on-chip photonics.

As an alternative solution to the above includes a solution based on carrier injection, namely to form a p/n junction around a Si waveguide such that a local modulation of the complex index of refraction is obtained by electrically biasing said p/n junction. Another solution exploiting carrier depletion in p-i-n junctions has been proposed and demonstrated in literature. Unfortunately, such approaches are prone to large optical losses in Si on the order of 3dB/cm at <NUM>, high electrical RF losses due to the large currents flow, considerable heat dissipation and large footprint.

This specification relates to an electro-optic modulator which may address some of these issues.

<FIG> is a schematic that illustrates an exemplary embodiment of an electro-optic modulator <NUM>.

<FIG> shows a cross-section view of a waveguide including a core <NUM> and the electro-optic modulator, or the EO modulator, <NUM>. The core <NUM> and the EO modulator <NUM> are embedded within or at least in contact with a surrounding material <NUM>.

The waveguide may be formed by the core <NUM> and a cladding, where at least part of the cladding may be formed by the surrounding material <NUM>.

The core <NUM> has a different refractive index than the surrounding material <NUM> such that it supports a guided mode <NUM>. Equivalently, the region around the centre of the guided mode <NUM> with a different refractive index than the surrounding material <NUM> may be regarded as the core <NUM>. The shape of the cross-section the core <NUM> in the yz-plane is shown to be a square shape in the example of <FIG>. However, the shape of the cross-section of the core <NUM> is not limited to a square shape. In some implementations, the shape of the cross-section of the core <NUM> may be of a polygonal shape.

The guided mode <NUM> may reside in the core <NUM> and the cladding. In some implementations, one or more surface of the core <NUM> may be in contact with the cladding. In some implementations, the core <NUM> may be wholly embedded within a cladding. As long as the core <NUM> supports the guided mode <NUM> to propagate along the core <NUM>, the configuration of the waveguide is not limited to these examples.

In some implementations, the surrounding material <NUM> around the core <NUM> forming the cladding of the waveguide may be uniform throughout the transverse profile of the mode <NUM>.

In some implementations, the surrounding material <NUM> around the core <NUM> forming the cladding of the waveguide may include one or more further interfaces at which the refractive index changes such that the transverse profile of the guided mode <NUM> may extend beyond that one or more interfaces within the surrounding material <NUM>.

In some implementations, the core <NUM> may be embedded in the surrounding material <NUM> without any of the surfaces exposed to air or vacuum.

In some implementations, at least one surface of the core <NUM> may be exposed to air or vacuum. For example, for a rib waveguide formed with the core <NUM>, at least one surface is completely exposed to air or vacuum without being covered with the surrounding material <NUM>. In this case, the cladding may be formed by the surrounding material <NUM> in contact with the core <NUM> and the space around the exposed part of the core <NUM>, which may be air or vacuum.

The guided mode <NUM> travels in the x-direction, namely in a perpendicular direction to the cross section of the core <NUM>, guided by the waveguide formed by the core <NUM> and the cladding material around the core formed within the surrounding material <NUM>.

The transverse mode profile, namely the power or intensity distribution of the guided mode <NUM> in the yz-plane, parallel to the cross section of the core <NUM>, resides substantially within the cross-section of the core <NUM> but extends beyond the boundary defined by the cross-section of the core <NUM> in the yz-plane.

In some implementations, the transverse mode profile of the guided mode <NUM> may have the highest intensity in the centre of the core <NUM>. In some implementations, the centre of the core <NUM> and the centre of the guided mode <NUM> may coincide and have the highest intensity within the transverse mode profile of the guided mode <NUM> in the yz-plane.

The overall shape of the guided mode <NUM> is illustrated with a dotted line as shown in <FIG>. The dotted line depicting the guided mode <NUM> in <FIG> is a visual guide only to illustrate the approximate extent of the guided mode <NUM>. For example, the dotted line <NUM> may represent a line following the same intensity, such as <NUM>/e<NUM> of the peak intensity at the centre of the guided mode <NUM> for the surrounding material <NUM> with a uniform refractive index around the core <NUM>. As shown in the dotted line representing the guided mode <NUM> in <FIG>, at least a fraction of the power of the guided mode <NUM> may reside outside the core <NUM>.

In some implementations, the fraction of power residing outside the core <NUM> may comprise an evanescent field on the surface of the core.

The EO modulator <NUM> is configured to induce attenuation and/or phase shift on the guided mode <NUM>. This is achieved by the change of the refractive index induced in the EO modulator <NUM> in response to an electric field applied to the EO modulator.

In some implementations, the EO modulator <NUM> is configured to induce a phase shift on the guided mode <NUM>. This is achieved by the change of the real part of the refractive index induced in the EO modulator <NUM> in response to an electric field applied to the EO modulator.

In some implementations, the EO modulator <NUM> is configured to induce attenuation on the guided mode <NUM>. This is achieved by the change of the imaginary part of the refractive index induced in the EO modulator <NUM> in response to an electric field applied to the EO modulator.

In some implementations, the EO modulator <NUM> may be positioned away from the centre of the guided mode <NUM>.

In some implementations, the EO modulator <NUM> may be placed outside the core <NUM> in case the core <NUM> has a different refractive index compared to the surrounding material <NUM>.

In some implementations, when the core <NUM> does not exhibit a step-like refractive index change with respect to the surrounding material <NUM> and the refractive index changes gradually towards the centre of the guided mode <NUM> such that the boundary of the core <NUM> is not clearly defined in the yz-plane parallel to the cross-section of the core <NUM>, the EO modulator <NUM> may be placed such that the extent of the EO modulator <NUM> does not overlap the centre of the guided mode <NUM>. In this case, as will be discussed later, the position of the EO modulator <NUM> may be determined to provide a balance between the depth of modulation and the degree of loss.

Also in the later examples, although the core <NUM> and the surrounding material may be shown to have a definite boundary for explaining the concept, in practice, the change of the refractive index at these boundaries may not be stepwise and be gradual varying around the interface. It is understood that also in these cases the EO modulator <NUM> may be placed such that the extent of the EO modulator <NUM> does not overlap the centre of the guided mode <NUM>.

The balance between the depth of modulation and the degree of loss may be specific to each application. In other words, the depth of modulation and the degree of loss may be determined for each application. For example, a tolerable degree of loss may be specified for a certain application. Then the position of the EO modulator <NUM> may be determined to maximise the operation bandwidth within the specified tolerable optical loss.

In some implementations, the waveguide may be designed such that the modification of the effective refractive index of the waveguide by the EO modulator <NUM> is taken into consideration. For example, the transverse extent of the guided mode <NUM> with respect to the position and the material composition of the EO modulator <NUM> may be determined a priori at the design stage.

In some implementations, the EO modulator <NUM> may be disposed within the cladding forming part of the surrounding material <NUM> which embeds the core <NUM>.

In some implementations, the EO modulator <NUM> may be disposed outside an interface formed within the surrounding material <NUM> but close enough to the core <NUM> to induce phase modulation of the guided mode <NUM>. For example, an interface formed within the surrounding material <NUM> may support an evanescent field as part of the guided mode <NUM> and the EO modulator <NUM> may be positioned such that the EO modulator <NUM> interacts with the evanescent field of the guided mode <NUM>. For another example, the guided mode <NUM> may extend beyond an interface formed within the surrounding material <NUM>. The fraction of power of the guided mode <NUM> beyond that interface may be significant enough such that the EO modulator <NUM> positioned beyond that interface can impart phase modulation or amplitude modulation to the guided mode <NUM>.

In some implementations, the refractive index of the material forming the EO modulator <NUM> may be different from the refractive index of the surrounding material <NUM> with which the EO modulator <NUM> is in direct contact. This may be the case irrespective of the voltage or the electric field applied to the EO modulator <NUM>. In other words, within the operation range of the voltage of the EO modulator <NUM>, the refractive index of the EO modulator <NUM> may be different from the refractive index of the surrounding material <NUM>.

When the refractive index of the material forming the EO modulator <NUM> is different from the index of the surrounding material <NUM> in contact, the guided mode <NUM> may be scattered due to this index mismatch, which leads to attenuation of the guided mode <NUM>.

When the refractive index of the material forming the EO modulator <NUM> has a imaginary part at the operating wavelength, the inherent dissipation or absorption of the EO modulator <NUM> leads to the attenuation of the guided mode <NUM>.

As the EO modulator <NUM> is disposed closer to the core <NUM> or the centre of the guided mode <NUM>, the degree of scattering or attenuation will be larger and loss is higher because the EO modulator <NUM> is placed in a position of higher intensity within the transverse mode profile of the guided mode <NUM>. However, the degree of modulation per applied voltage or the modulation depth is also higher because the EO modulator may be placed in a position of higher intensity within the transverse mode profile of the guided mode <NUM>.

At a given position of the EO modulator <NUM> and a given area in the yz-plane of the EO modulator <NUM>, both the modulation depth and the degree of scattering or attenuation, therefore loss, are proportional to the extent of the EO modulator <NUM> along the length of the waveguide, namely in x-direction.

A balance between loss, due to scattering or absorption, and the modulation depth can be found by adjusting one or more of the following parameters: the position of the EO modulator <NUM> with respect to the guided mode <NUM> in the transverse yz-plane parallel to the cross-section of the core <NUM>, the extent of the EO modulator <NUM> in x-direction perpendicular to the cross-section of the core <NUM>, the volume of the material used for the EO modulator <NUM>, the shape of the cross-section of the EO modulator <NUM> in the transverse yz-plane, and the refractive index of the material used for the EO modulator <NUM>.

In some implementations, the loss due to scattering or absorption may be reduced by reducing the volume of the material used for the EO modulator <NUM>, such as silicon or any electro-optic material. For example, if the EO modulator <NUM> is in the form of a thin film, the thickness of the film may be rendered as thin as possible as long as the efficiency of modulation is maintained. This aspect will be discussed in more detail in <FIG>.

In some implementations, the loss due to scattering may be reduced by choosing the material used for the EO modulator <NUM> with a refractive index closer to the refractive index of the surrounding material <NUM> at an operating wavelength.

The parameters to adjust for the balance between the loss and the depth of modulation are not limited to these. The balance between the loss and the depth of modulation may be determined for a desired operation based on the design parameters such as required depth of modulation or specification on the degree of loss per length.

Since modulation is more efficient if the EO modulator <NUM> is located closer to the centre of the guided mode <NUM>, in some examples of the prior art, the material of the core <NUM> itself is made with a material with an electro-optic effect, such as lithium niobate such that index modulation occurs in the middle of the mode and throughout the majority of the transverse area of the mode. In some other examples of prior art, the electro-optic modulator is arranged with silicon waveguides such that the plasma dispersion effect, or the change in the concentration of charge is induced in the middle of a guided mode or within the core of the silicon waveguide. However, in these cases, the design of the electro-optic modulator may have to be specific to the material of the waveguide.

The concept of the embodiment shown in <FIG> may be applied to a core <NUM> of the waveguide constructed with any material as long as the fabrication of the EO modulator <NUM> is compatible with the surrounding material <NUM>. For example, the material of core <NUM> can be a centrosymmetric material with a negligible electro-optic effect or a material where the generation of plasma-dispersion is not feasible. The choice of the material of the core <NUM> of the waveguide may not have to relate to the electro-optic capability or modulation capability of the EO modulator <NUM>. For example, the core <NUM> of the waveguide may be constructed with Si<NUM>N<NUM> (silicon nitride).

Silicon nitride does not exhibit any appreciable electro-optic effect and does not accommodate the generation of plasma-dispersion. However, the silicon nitride as the core <NUM> may allow for a low-loss propagation of the guided mode <NUM>.

In the rest of the specification, the concept will be further described by way of examples comprising a waveguide with a Si<NUM>N<NUM> (silicon nitride) as the core <NUM> and a capacitor structure made with semiconductor layers as the EO modulator <NUM>.

<FIG> is a schematic that illustrates an exemplary embodiment of an electro-optic modulator with references to <FIG>.

The electro-optic modulator <NUM>, or EO modulator <NUM> is positioned in the vicinity of a waveguide including a core <NUM> embedded in a surrounding material <NUM>. The refractive index of the surrounding material <NUM> is lower than the refractive index of the core <NUM>. In particular, the EO modulator <NUM> is positioned such that amplitude modulation and/or phase modulation can be imparted on the guided mode <NUM> supported by the waveguide.

In the example of <FIG>, the cross-section of the core <NUM> has a rectangular form and is positioned near an interface <NUM> formed by the end surface of the surrounding material <NUM>. The core <NUM> is embedded in the surrounding material <NUM> such that three sides of the cross-section of the core <NUM> is embedded within the surrounding material <NUM> and one of the sides of the cross-section of the core <NUM>, a first side <NUM>, is parallel to or flush with the interface <NUM> of the surrounding material <NUM>. Therefore, the interfaces formed by the surrounding material <NUM> and the first side <NUM> of the core <NUM> form one planar surface in the x-y plane. The transverse profile of the guided mode <NUM> of the waveguide therefore may extend beyond the plane formed by the first side <NUM> and the interface <NUM> in the positive z-direction.

The thickness of the core <NUM>, in z-direction, perpendicular to the interface <NUM> and the first side <NUM>, may be between <NUM> and <NUM>. The examples of the material of the core <NUM> may include silicon nitride (SixNy) with different stoichiometry and hydrogenated silicon nitride. The core <NUM> may be deposited using one or more of PECVD, sputtering or LPCVD techniques.

The examples of the material of the surrounding material <NUM> include one or more of silicon dioxide (SiO<NUM>), silicon oxynitride (SiON) or aluminium oxide (Al<NUM>O<NUM>). The surrounding material <NUM> may be deposited using one or more of PECVD, LPCVD or via the thermal oxidation of a silicon substrate.

A guided mode <NUM> travels in the x-direction, perpendicular to the cross section of the core <NUM>, guided by the waveguide formed by the core <NUM> and the surrounding material <NUM>.

The EO modulator <NUM> includes a first semiconductor layer <NUM>, a second semiconductor layer <NUM> and a dielectric layer <NUM> in between the semiconductor layer <NUM> and the second semiconductor layer <NUM>. The first semiconductor layer <NUM> is positioned closer to the core <NUM> than the second semiconductor layer <NUM>.

In some implementations, the first semiconductor layer <NUM> may be a silicon layer.

In some implementations, the second semiconductor layer <NUM> may be a silicon layer.

In some implementations, the second semiconductor layer <NUM> may comprise a germanium layer.

In some implementations, the second semiconductor layer <NUM> may comprise a graphene layer.

In some implementations, the first semiconductor layer <NUM> may be deposited with LPCVD, PECVD, sputtering, PVD or ALD techniques with doping levels less than <NUM>×<NUM><NUM> at/cm<NUM>.

In some implementations, the second semiconductor layer <NUM> may be deposited with LPCVD, PECVD, MOCVD, MBE, evaporation, sputtering or ALD techniques with doping levels less than <NUM>×<NUM><NUM> at/cm<NUM>.

In some implementations, the second semiconductor layer <NUM> may be low-doped p-type polycrystalline germanium or low-doped n-type silicon, which behave as non-degenerate semiconductors. When the first semiconductor layer <NUM> is n-type silicon, the second semiconductor layer <NUM> may be low-doped, non-degenerate p-type polysilicon or polycrystalline germanium. When the first semiconductor layer <NUM> is p-type silicon, the second semiconductor layer <NUM> may be low-doped n-type polysilicon. By heavily-doping and low-doping a semiconductor layer, the semiconductor layer is rendered to be degenerate and non-degenerate silicon layers, respectively. Non-degenerate semiconductor layer does not produce "free-charge bands" at the conduction or at the valence band of a semiconductor. These free charges are responsible for the metallic behaviour and optical losses.

In particular, when a highly doped or a metal-semiconductor alloy layer is regarded as metallic, the EO modulator <NUM> may correspond to a conventionally defined MOS capacitor. The MOS capacitor typically comprises a capacitor-like structure including a layer which behaves as a metal, which exhibits no appreciable band gap and a semiconductor material which exhibits a finite band gap. The MOS capacitor allows for bending of the band structure, which leads to charge inversion and accumulation.

In some implementations, the second semiconductor layer <NUM> comprises a degenerate semiconductor layer. The use of degenerate semiconductor layer may alleviate issues which may arise from using metallic layer as the second semiconductor layer, such as losses at the interface due to plasmonic effects.

In some other implementations, the second semiconductor layer <NUM> comprises a semi-metallic layer or 2D material, which is a single layer material with a 2D-like electronic behaviour with high carrier mobility, such as graphene or silicene. These semi-metallic layer may comprise group IV elements. The use of such a semi-metallic layer for the second semiconductor layer <NUM> may alleviate issues which may arise from using metallic layers as the second semiconductive layer <NUM>, such as losses at the interface due to thickness.

In some implementations, when the first semiconductor layer <NUM> is a silicon layer and the second semiconductor layer <NUM> is a silicon layer, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are doped with opposite type of doping. For example, when the second semiconductor layer <NUM> is p-doped, the first semiconductor layer <NUM> is n-doped, and vice versa.

In some implementations, when the first semiconductor layer <NUM> is a silicon layer and the second semiconductor layer <NUM> is a germanium layer, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be both non-degenerate semiconductive layers.

In some implementations, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> comprise a group IV semiconductor.

In some implementations, when the first semiconductor layer <NUM> and the second semiconductor layer <NUM> comprise a group IV semiconductor and when the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are doped with opposite type of doping, doping level may be such that the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are non-degenerate semiconductors. For example, the level of concentration to be non-degenerate may be around <NUM><NUM>, although it depends on the exact dopant species and on the microstructure of the semiconductor (e.g. single crystal, poly-crystalline, amorphous) and on the material of the semicondcutor (e.g. Si, Ge or III-V structures).

In some implementations, when the first semiconductor layer <NUM> or the second semiconductor layer <NUM> are intrinsic semiconductor layers, the resistivity and the Schottky barrier height of one or more of the first semiconductor layer <NUM> or the second semiconductor layer <NUM> to electrical contacts may be adjusted by alloying with transition metals, forming NiSi, Ni<NUM>Si, NiSi<NUM>, TiSi, TiSi<NUM>, CoSi, Co<NUM>Si, PtSi, Ni<NUM>Ge<NUM>, NiGe or other transition metal-group IV semiconductor thin film portions.

In some implementations, the material for the first semiconductor layer <NUM> and the second semiconductor layer <NUM>, may be group IV semiconductors such as amorphous Si, hydrogenated amorphous silicon, polycrystalline silicon or crystalline silicon.

In some implementations, the material for the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be group IV semiconductors such as Germanium and SiGe alloys.

In some implementations, the material for the semiconductor layer <NUM> and the second semiconductor layer <NUM> may be III-V semiconductors such as GaAs, AlGaAs, InGaP.

In some implementations, the thickness, in z-direction, perpendicular to the interface <NUM> and the first side <NUM>, of the semiconductor layer <NUM> and the second semiconductor layer <NUM> may be between <NUM> to <NUM>.

In some implementations, one or both of the semiconductor layer <NUM> and the second semiconductor layer <NUM>, when they comprise a group IV semiconductor, may be treated with thermal recrystallisation techniques.

A first distance <NUM> is defined to be a distance between the first side <NUM> of the core <NUM> and one of the two surfaces of the first semiconductor layer <NUM> which is closer to the core <NUM>. The first distance <NUM> represents the size of the gap formed between the semiconductor layer <NUM> and the core <NUM>.

The first distance <NUM> may be smaller than <NUM>.

The gap between the first semiconductor layer <NUM> and the core <NUM> may be filled with one or more of silicon dioxide (SiO<NUM>), silicon oxynitride (SiON), aluminium oxide (Al<NUM>O<NUM>), or aluminium nitride (AlN). The layer forming the gap between the first semiconductor layer <NUM> and the core <NUM> may be deposited using one or more of PECVD, LPCVD, atomic layer deposition (ALD) or by thermal oxidation of silicon or aluminium substrate, or by nitridation of silicon or aluminium substrate.

By controlling the first distance <NUM>, the position of the EO modulator <NUM> with respect to the guided mode <NUM> and with the core <NUM> may be determined. Therefore, the first distance <NUM> is one of the parameters that relate to the balance between the loss and the depth of modulation. The layer providing the first distance <NUM> will be called a coupling layer in the later examples.

A second distance <NUM> is defined to be a distance between the semiconductor layer <NUM> and the second semiconductor layer <NUM>. The second distance <NUM> is the distance between one of the two surfaces of the first semiconductor layer <NUM> which is further from the core <NUM> and one of the two surfaces of the second semiconductor layer <NUM> closer to the core <NUM>. The second distance <NUM> represents the size of the gap formed between the silicon layer <NUM> and the second semiconductor layer <NUM>. The second distance <NUM> corresponds to the thickness of the dielectric layer <NUM> forming part of the EO modulator <NUM>.

The second distance may be between <NUM> and <NUM>.

In some implementation, the gap between the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may comprise one or more of silicon dioxide (SiO<NUM>), silicon oxynitride (SiON). The gap between the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be deposited by oxidation of silicon substrate or by atomic layer deposition (ALD).

In some implementations, the gap between the first semiconductor layer <NUM> and the second semiconductor layer <NUM>, or the gap between the core <NUM> and the semiconductor layer <NUM> may comprise one or more of Benzocyclobutene (BCB), poly methyl methacrylate (PMMA) or parylene, in case the waveguide and the EO modulator <NUM> can be fabricated within below <NUM>.

The first semiconductor layer <NUM> and the second semiconductor layer <NUM> are respectively connected to electrical contacts. In some implementations, one or both of the respective electrical contacts may form Shottky junctions. In some implementations, the position of the electrical contacts may be positioned further than <NUM> from the closest surface of the core <NUM> in the x- or y-direction such that optical loss is reduced due to these electrical contacts scattering the guided mode <NUM>.

In some implementations, the electrical contacts comprise an alloy and the composition of the alloyed contact is such that its refractive index is lower than the refractive index of the core <NUM>, thereby allowing the electrical contacts to be positioned closer than <NUM> from the closest surface of the core <NUM> in the x- or y- direction while minimising optical losses.

The first semiconductor layer <NUM>, the second semiconductor layer <NUM> and the dielectric layer <NUM> form a capacitor structure in which the silicon layer <NUM> and the second semiconductor layer <NUM> are separated by the dielectric layer <NUM>.

In some implementations, the refractive index of the semiconductor layer <NUM> may change when an electric field is applied via the two electrical contacts made with the first semiconductor layer <NUM> and the second semiconductor layer <NUM>. For example, the carrier may accumulate in the first semiconductor layer <NUM> near the dielectric layer <NUM>.

Alternatively, in some implementations, the first semiconductor layer <NUM> may comprise an ultrathin silicon layer such that when an electric field is applied via the two electrical contacts made with the semiconductor layer <NUM> and the second semiconductor layer <NUM>, the carrier may accumulate near the dielectric layer <NUM> such that the semiconductor layer is fully depleted or fully accumulated. In some implementations, the refractive index of both the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may change when an electric field is applied via the two electrical contacts made with the semiconductor layer <NUM> and the second semiconductor layer <NUM>.

In some implementations, the refractive index of the second semiconductor layer <NUM> may change when an electric field is applied via the two electrical contacts made with the semiconductor layer <NUM> and the second semiconductor layer <NUM>.

An overlap length <NUM> is defined to be the lateral width in y-direction, over which the semiconductor layer <NUM> and the second semiconductor layer <NUM> overlap to form the capacitor structure.

For example, as shown in <FIG>, the first semiconductor layer <NUM> may extend in the negative y-direction and the second semiconductor layer <NUM> may extend in the positive y-direction to allow electrical contacts to be formed with a sufficient distance from the core <NUM>. The first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be arranged to overlap, when viewed down into the xy-plane, directly over the core <NUM> above the first side <NUM>. This is such that as discussed hereinbefore, the EO modulator <NUM> interacts with the guided mode <NUM> extending beyond the first side <NUM> of the core.

In <FIG>, the transverse mode of the guided mode <NUM> is represented as a dotted line. As discussed above, this line is merely a guide to the eye indicating that at least part of the power of the guided mode <NUM> resides outside the core <NUM> and above the first side <NUM> such that it couples at least to the first semiconductor layer <NUM>.

In the example of <FIG>, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are embedded in the same material as the dielectric layer <NUM>. The gap between the core <NUM> and the first semiconductor layer <NUM> and the gap between the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are filled with the material for the dielectric layer <NUM>. The second semiconductor layer <NUM> is covered with the material for the dielectric layer <NUM> such that neither the first semiconductor layer <NUM> nor the second semiconductor layer <NUM> are exposed to air or vacuum.

<FIG> and <FIG> show graphs representing the results of a numerical simulation with references to <FIG> and <FIG>.

A numerical simulation was performed for the configuration of the EO modulator <NUM> shown in the example in <FIG>. The following parameters were fixed for performing the simulation: the material of the dielectric layer <NUM> was set to be silicon dioxide. The width of the core <NUM> in y-direction was set to be <NUM> and the thickness of the core in z-direction was set to be <NUM>. The first distance <NUM> was set to be <NUM>, and the second distance <NUM> was set to be <NUM>. The wavelength of the simulation was at <NUM>. The first distance <NUM> and the second distance <NUM> may be determined by the minimum thickness of the layer which can be fabricated reliably and reproducibly during the manufacturing process.

<FIG> shows a graph <NUM> that presents the simulation result of evaluating Lπ as the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are varied. Lπ is defined to be a length of the EO modulator, in x-direction along the core <NUM>, required to achieve a phase shift of π when the voltage applied to the EO modulator <NUM> is 1V.

The graph <NUM> shows Lπ, in the y-axis <NUM> as a function of the thickness in the x-axis <NUM>. In this simulation, the overlap length <NUM> was set to be <NUM>, wider than the width of the core <NUM>, <NUM>. It was assumed that both the semiconductor layer <NUM> and the second semiconductor layer <NUM> were silicon layers and the thickness of these layers have been varied simultaneously from <NUM> to <NUM>.

The graph <NUM> shows that Lπ decreases rapidly as the thickness increases from <NUM> to <NUM>. From <NUM> to <NUM> the decrease of Lπ is not as rapid. As the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> increases, the more power of the guided mode <NUM> interacts with the EO modulator <NUM>, the less length of the EO modulator <NUM> is required to achieve the same degree of phase shift. In other words, the effective refractive index change per applied voltage increases as the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> to produce the same degree of phase shift.

As the thickness increases, the loss due to scattering or absorption also increases. The loss was also evaluated from the simulation and the loss increased more or less linearly from <NUM> dB to <NUM> dB from <NUM> to <NUM> thickness. This suggests that at this position of the EO modulator <NUM> defined by the first distance <NUM>, and within this range of thickness, the loss is proportional to the volume of the material of the EO modulator <NUM>.

The capacitance of the EO modulator <NUM>, which relates to the bandwidth of the operation, was also evaluated from the simulation. The capacitance did not vary with the thickness and was largely dependent on the length of the device and varied from <NUM> fF to <NUM> fF for <NUM> to <NUM> length of the EO modulator <NUM>, respectively.

Therefore, according to the graph <NUM>, for the given geometry of the waveguide, the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be chosen to be around <NUM> thickness such that an optimum operation may be obtained considering the modulation depth and the loss.

<FIG> shows a graph <NUM> that presents the simulation result of evaluating Lπ as the overlap length <NUM> of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> is varied.

The graph <NUM> shows Lπ in the y-axis <NUM> as a function of the overlap length <NUM> in the x-axis <NUM>. In this simulation, it was assumed that the first semiconductor layer <NUM> and the second semiconductor layer <NUM> were silicon layers and their thickness is <NUM>. The overlap length <NUM> was varied from <NUM> to <NUM>.

The graph <NUM> shows that Lπ decreases rapidly as the overlap length <NUM> increases up to about <NUM>. As the overlap length <NUM> increases further, the decrease of Lπ is not as rapid.

When the lateral extent, in y-axis, of the capacitor formed by the first semiconductor layer <NUM> and the second semiconductor layer <NUM> is smaller than the width of the core <NUM>, <NUM>, the efficiency of modulation decreases, therefore the total length of the EO modulator <NUM> may be made longer to achieve the same degree of phase modulation.

In this specific configuration and dimensions of the EO modulator <NUM> simulated under <FIG>, when the lateral extent, in y-axis, of the capacitor formed by the first semiconductor layer <NUM> and the second semiconductor layer <NUM> exceeds the width of the core <NUM>, it may not significantly increase the degree of modulation. This may be attributed to the fact that the guided mode <NUM> of the core <NUM> may be largely confined around the cross-section of the core <NUM> and as the overlap length <NUM> increases beyond the extent to cover the guided mode <NUM>, the efficiency of modulation therefore does not increase significantly. Such behaviour may change when, for example, the EO modulator <NUM> is used for a waveguide where the guided mode <NUM> is less tightly confined.

The loss due to the EO modulator <NUM> was also evaluated with the simulation. The loss also follows a similar trend as the graph <NUM>, namely that the loss decreases rapidly from <NUM> to <NUM> overlap length <NUM> but does not decrease significantly further from <NUM> overlap length <NUM>. In this specific configuration and dimensions of the EO modulator <NUM> simulated under <FIG>, this may also be attributed to the fact that the guided mode <NUM> of the core <NUM> may be largely confined around the cross-section of the core <NUM> and as the overlap length <NUM> increases beyond the extent to cover the guided mode <NUM>, the loss therefore does not increase significantly.

The capacitance of the EO modulator <NUM> was also evaluated with the simulation. The capacitance was minimised around the overlap length of <NUM> to be around <NUM> fF, and increased rapidly and monotonically as the overlap length increased upto <NUM>, to be around <NUM> fF. The capacitance of the EO modulator at <NUM> overlap length was around <NUM> fF. In this specific configuration and dimensions of the EO modulator <NUM> simulated under <FIG>, the decrease of capacitance with the change of the overlap length <NUM> from <NUM> to <NUM> can mainly be attributed to the decrease of the required length of the EO modulator <NUM>, Lπ. The rapid and monotonous increase of the capacitance with the change of the overlap length <NUM> from <NUM> may be attributed to the linear increase of the size of the capacitor.

The design of the EO modulator <NUM> may be determined also considering the capacitance because the bandwidth of operation of the EO modulator <NUM> depends highly on the capacitance. According to the simulation of the specific configuration and dimensions of the EO modulator <NUM> simulated under <FIG>, the capacitance is minimised around <NUM> overlap length <NUM>.

Therefore, according to the graph <NUM>, the balance between the depth of modulation, loss and the bandwidth of operation may be provided at the overlap length <NUM> of around <NUM>, which corresponds to the width of the core <NUM>.

In some implementations, the lateral extent of the capacitor formed by the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be rendered comparable to the width of the core <NUM> to reduce loss.

Although not shown here as a graph, a simulation was performed to evaluate the effect of the second distance <NUM>, the distance between the first semiconductor layer <NUM> and the second semiconductor layer <NUM>. In this simulation, the thickness of the semiconductor layer <NUM> and the second semiconductor layer <NUM> were set at <NUM> and the overlap length was set to be <NUM>. When the second distance <NUM> was varied from <NUM> to <NUM>, Lπ varied from <NUM> to <NUM>, largely monotonically and loss varied from <NUM> dB to <NUM>. 5dB, also largely monotonically. As expected, the simulation showed that the second distance <NUM> may be made as small as possible to reduce optical loss and to enhance the depth of modulation. The second distance <NUM> may be determined by the minimum thickness of the layer fabricated reliably and reproducibly during manufacturing.

The design may be initially targeted to achieve a phase shift of π with <NUM> V applied to the EO modulator <NUM>. Then the design parameters may be varied to optimise the operation of the EO modulator <NUM>. For example, to reduce the optical loss arising from the EO modulator, the first distance <NUM> may be changed or the thickness of the first semiconductor layer <NUM> and/or the second semiconductor layer <NUM> may be changed. The resulting voltage required to achieve a phase shift of π may range from <NUM> V to <NUM> V.

The electro-optic modulator <NUM>, or EO modulator <NUM> is positioned in the vicinity of a waveguide including a core <NUM> embedded in a surrounding material <NUM>. The refractive index of the surrounding material <NUM> is lower than the refractive index of the core <NUM>. In particular, the EO modulator <NUM> is positioned such that amplitude modulation and/or phase modulation can be imparted on the guided mode <NUM> supported by the waveguide. As discussed above, the dotted line representing the transverse mode profile of the guided mode <NUM> is only a guide to the eye.

In the example of <FIG>, the cross-section of the core <NUM> has a rectangular form and is positioned near an interface <NUM> formed by the end surface of the surrounding material <NUM>. The core <NUM> is embedded in the surrounding material <NUM> such that three sides of the cross-section of the core <NUM> is embedded within and in contact with the surrounding material <NUM> and one of the sides of the cross-section of the core <NUM>, a first side <NUM>, is parallel to or flush with the interface <NUM> of the surrounding material <NUM>. Therefore, the interface <NUM> formed by the surrounding material <NUM> and the first side <NUM> of the core <NUM> forms one planar surface in the x-y plane. The transverse profile of the guided mode <NUM> of the waveguide therefore may extend beyond the plane formed by the first side <NUM> and the interface <NUM> in the positive z-direction.

The thickness of the core <NUM>, in z-direction, namely perpendicular to interface <NUM> and the first side <NUM>, may be between <NUM> and <NUM>. The examples of the material of the core <NUM> are as discussed for the core <NUM> presented in the example of <FIG>. The core <NUM> can be deposited in the same fashion as the core <NUM> presented in the example of <FIG>.

The examples of the material of the surrounding material <NUM> are as discussed for the surrounding material <NUM> presented in the example of <FIG>.

A guided mode <NUM> travels in the x-direction, perpendicular to the cross section of the core <NUM>, guided by the waveguide formed by the core <NUM> and a cladding formed within the surrounding material <NUM>.

The EO modulator <NUM> includes a first semiconductor layer <NUM>, a second semiconductor layer <NUM>, an dielectric layer <NUM> and a coupling layer <NUM>. The first semiconductor layer <NUM> is positioned closer to the core <NUM> than the second semiconductor layer <NUM>. The dielectric layer <NUM> is disposed between the first semiconductor layer <NUM> and the second semiconductor layer <NUM>. The coupling layer <NUM> is disposed between the core <NUM> and the first semiconductor layer <NUM>. The coupling layer <NUM> is for coupling the guided mode <NUM> of the waveguide at least to the first semiconductor layer <NUM>. The coupling layer <NUM> may be configured such that when the EO modulator <NUM> is coupled to the waveguide, the coupling layer <NUM> is disposed between the core <NUM> and the first semiconductor layer <NUM>.

The first semiconductor layer <NUM> and the second semiconductor layer <NUM> can be deposited and treated as discussed for the first semiconductor layer <NUM> and the second semiconductor layer <NUM> in the example of <FIG>.

The material composition, the doping, the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are as discussed for the first semiconductor layer <NUM> and the second semiconductor layer <NUM> in the example of <FIG>.

The thickness of the coupling layer <NUM> in z-direction may be smaller than <NUM>.

In some implementations, the coupling layer <NUM> may comprise one or more of silicon dioxide (SiO<NUM>), silicon oxynitride (SiON), aluminium oxide (Al<NUM>O<NUM>), or aluminium nitride (AlN).

In some implementations, the coupling layer <NUM> may be deposited using one or more of PECVD, LPCVD, atomic layer deposition (ALD) or by thermal oxidation of silicon or aluminium substrate, or by nitridation of silicon or aluminium substrate.

By controlling the thickness of the coupling layer <NUM>, the position of the EO modulator <NUM> with respect to the guided mode <NUM> and with the core <NUM> may be determined. Therefore, the thickness of the coupling layer <NUM> is one of the parameters that relate to the optimum between the loss and the depth of modulation.

The thickness of the dielectric layer <NUM> in z-direction may be between <NUM> and <NUM>.

In some implementations, the dielectric layer <NUM> may comprise one or more of silicon dioxide (SiO<NUM>), silicon oxynitride (SiON).

In some implementations, the dielectric layer <NUM> may be deposited by oxidation of silicon substrate or by atomic layer deposition (ALD).

In some implementations, the dielectric layer <NUM> and/or the coupling layer <NUM> may comprise one or more of Benzocyclobutene (BCB), poly methyl methacrylate (PMMA) or parylene, in case the waveguide and the EO modulator <NUM> can be fabricated within below <NUM>.

The first semiconductor layer <NUM> and the second semiconductor layer <NUM> are respectively connected to electrical contacts as described in the example of <FIG>.

The material examples for the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are as discussed in the example of <FIG>. In the example of <FIG>, the first semiconductor layer <NUM> is deposited on the coupling layer <NUM> such that the first semiconductor layer <NUM> covers the first side <NUM> of the core <NUM> viewed down on the xy-plane. As discussed hereinbefore in <FIG>, the degree to which the first semiconductor layer <NUM> covers the first side <NUM> of the core <NUM> relates to the overlap length <NUM> and the overlap length <NUM> may be determined depending on how tightly confined the guided mode <NUM> is. In some implementations, the first semiconductor layer <NUM> may be deposited to fully cover the core <NUM>. In some implementations, the first semiconductor layer <NUM> may be deposited to partially cover the core <NUM>. Similar concept applies to the second semiconductor layer <NUM> and the dielectric layer <NUM>.

For example, if the guided mode <NUM> supported by the waveguide formed with the core <NUM> and the cladding formed within the surrounding material <NUM> is only loosely confined because the refractive index difference between the core <NUM> and the surrounding material <NUM> is small, the semiconductor layer <NUM> may be deposited such that it fully covers the first side <NUM> of the core <NUM> and further beyond the core <NUM> in positive y-direction such that the EO modulator <NUM> interacts efficiently with the guided mode <NUM>.

For another example, if the guided mode <NUM> supported by the waveguide formed with the core <NUM> and the cladding formed within the surrounding material <NUM> is tightly confined because the refractive index difference between the core <NUM> and the surrounding material <NUM> is large such as in the case of silicon nitride as a core and silicon dioxide as a cladding, the first semiconductor layer <NUM> may be deposited such that the semiconductor layer <NUM> terminates around the area defined by the first side <NUM>, as shown in <FIG> and as suggested by the simulation result of <FIG>.

Starting from the area near the first side <NUM> of the core <NUM>, the semiconductor layer <NUM> extends into the negative y-direction, in the transverse plane and parallel to the plane of the substrate, such that the electrical contacts can be formed on the semiconductor layer <NUM>. The electrical contact may be positioned at least <NUM> from the center of the core <NUM>.

The dielectric layer <NUM> is disposed on top of the first semiconductor layer <NUM> such that the dielectric layer <NUM> covers the core <NUM>, viewed down on the xy-plane, with part of the first semiconductor layer <NUM> between the core <NUM> and the dielectric layer <NUM>.

The dielectric layer <NUM> may be deposited to extend into the positive y-direction, in the transverse plane and parallel to the plane of the substrate, starting from the area near the core <NUM>, in the opposite direction to the first semiconductor layer <NUM>. The second semiconductor layer <NUM>, starting from the area near the core <NUM>, extends into positive y-direction such that electrical contact can be formed on the second semiconductor layer <NUM>. The electrical contact may be positioned at least <NUM> from the center of the core <NUM>.

The second semiconductor layer <NUM> can be deposited on the dielectric layer <NUM>. The second semiconductor layer <NUM> may be deposited such that viewed down into xy-plane, the second semiconductor layer <NUM> covers the first side <NUM> of the core <NUM>. As discussed hereinbefore, the degree to which the dielectric layer <NUM> covers the core may be determined depending on the waveguide formed by the core <NUM> and the surrounding material <NUM>. In particular, the second semiconductor layer <NUM> may be deposited such that it does not extend beyond in the negative y-direction the position where the dielectric layer <NUM> terminates. Since the second semiconductor layer <NUM> forms a capacitor structure with the first semiconductor layer <NUM>, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> must not be in any electrical contact with each other.

Since the dielectric layer <NUM> is formed in two different heights in z-direction, namely on top of the semiconductor layer <NUM> near the first side <NUM> of the core <NUM> and on top of the coupling layer <NUM> away from the core <NUM> into the positive y-direction, the second semiconductor layer <NUM>, deposited on top of the dielectric layer <NUM>, largely follows the profile of the dielectric layer <NUM> and lies in two different heights in z-direction.

In the example of <FIG>, the first semiconductor layer <NUM>, the second semiconductor layer <NUM> are deposited to form a capacitor type structure with the dielectric layer <NUM> between them. The capacitor type structure wholly covers the lateral extent, in y-direction, of the core <NUM>. The first semiconductor layer <NUM> and the second semiconductor layer <NUM> are arranged to overlap, when viewed down into the xy-plane, directly over the core <NUM> above the first side <NUM>. This is such that the EO modulator <NUM> interacts with the guided mode <NUM> extending beyond the first side <NUM> of the core.

The electro-optic modulator <NUM>, or EO modulator <NUM>, is positioned in the vicinity of a waveguide including a core <NUM> in contact with a surrounding material <NUM>. The refractive index of the surrounding material <NUM> is lower than the refractive index of the core <NUM>. The EO modulator <NUM> is positioned such that amplitude modulation and/or phase modulation can be imparted on the guided mode <NUM> supported by the waveguide.

In the example of <FIG>, the cross-section of the core <NUM> has a rectangular form and is positioned near an interface <NUM> formed by the end surface of the surrounding material <NUM>. The core <NUM> is fabricated such that one of the sides of the cross-section of the core <NUM>, a first side <NUM>, is parallel to or flush with the interface <NUM> of the surrounding material <NUM>. Therefore, the interfaces formed by the surrounding material <NUM> and the first side <NUM> of the core <NUM> form one planar surface in the x-y plane.

In some implementations, in a region without the EO modulator <NUM>, the core <NUM> and the surrounding material <NUM> may form a rib waveguide where the three sides of the cross-section of the core <NUM> which is not in contact with the surrounding material <NUM> may be exposed to air or vacuum.

Alternatively, in some implementations, in a region without the EO modulator <NUM>, the core <NUM> may be embedded in the surrounding material <NUM> on all four sides of the cross-section of the core <NUM> such that the waveguide is formed with the surrounding material <NUM> as the cladding.

In both cases, the transverse profile of the guided mode <NUM> of the waveguide therefore may extend beyond the sides of the cross-section of the core <NUM>.

The thickness of the core <NUM>, in z-direction, namely perpendicular to the plane of the substrate on which the EO modulator <NUM> and the waveguide are deposited, may be between <NUM> and <NUM>. The examples of the material of the core <NUM> are as described for the core <NUM>, <NUM>, <NUM> in the examples of <FIG>, <FIG> and <FIG>. The core <NUM> can be deposited in the same fashion as the core <NUM> presented in the example of <FIG>.

The examples of the material of the surrounding material <NUM> are as discussed for the surrounding material <NUM>, <NUM>, <NUM> presented in the example of <FIG>, <FIG> and <FIG>. A guided mode <NUM> travels in the x-direction, perpendicular to the cross section of the core <NUM>, guided by the waveguide formed by the core <NUM> and a cladding formed within the surrounding material <NUM>.

The EO modulator <NUM> includes a first semiconductor layer <NUM>, a second semiconductor layer <NUM>, a dielectric layer <NUM> and a coupling layer <NUM>. The first semiconductor layer <NUM> is positioned closer to the core <NUM> than the second semiconductor layer <NUM>. The dielectric layer <NUM> is positioned between the first semiconductor layer <NUM> and the second semiconductor layer <NUM>. The coupling layer <NUM> is positioned between the core <NUM> and the semiconductor layer <NUM>. The coupling layer <NUM> is for coupling the guided mode <NUM> of the waveguide at least to the semiconductor layer <NUM>. The coupling layer <NUM> may be configured such that when the EO modulator <NUM> is coupled to the waveguide, the coupling layer <NUM> is disposed between the core <NUM> and the silicon layer <NUM>.

The first semiconductor layer <NUM> and the second semiconductor layer <NUM> can be deposited and treated as discussed for the first semiconductor layer <NUM>, <NUM> and the second semiconductor layer <NUM>, <NUM> in the examples of <FIG> and <FIG>.

The material composition, the doping, the thickness of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are as discussed for the semiconductor layer <NUM>, <NUM> and the second semiconductor layer <NUM>, <NUM> in the example of <FIG> and <FIG>.

The examples of the material composition of the coupling layer <NUM> are as discussed for the coupling layer <NUM> in the example of <FIG>.

The coupling layer <NUM> can be deposited in the same fashion as the coupling layer <NUM> as discussed in the example of <FIG>.

The examples of the material composition of the dielectric layer <NUM> are as discussed for the dielectric layer <NUM> in the example of <FIG>.

The dielectric layer <NUM> may be deposited in the same fashion as the dielectric layer <NUM> in the example of <FIG>.

The operating principle and configuration of the EO modulator <NUM>, including the electrical contacts and a capacitor-like structure formed by the first semiconductor layer <NUM>, the second semiconductor layer <NUM> and the dielectric layer <NUM>, are as discussed for the EO modulator <NUM> in the example of <FIG>.

In the example of <FIG>, the coupling layer <NUM> is deposited on the three sides of the cross-section of the core <NUM> which are not in contact with the surrounding material <NUM>. The first semiconductor layer <NUM> is disposed on the coupling layer <NUM> such that the first semiconductor layer <NUM> covers the three sides of the cross-section of the core <NUM>, largely following the profile of the coupling layer <NUM>. In the case of EO modulator <NUM>, <NUM> in <FIG> and <FIG>, the first semiconductor layer <NUM>, <NUM> is parallel to the first side <NUM>, <NUM> of the core <NUM>, <NUM>, covering the area around the first side <NUM>, <NUM>. In the example of <FIG>, the first semiconductor layer <NUM> is deposited such that it covers the three sides of the cross-section of the core <NUM>. This may provide more efficient modulation of the guided mode <NUM> compared to the EO modulators <NUM>, <NUM> shown in <FIG> and <FIG> where only one side of the cross-section of the core <NUM>, <NUM> is covered with the semiconductor layer <NUM>, <NUM>. Also, the modulation is insensitive to the polarisation of the guided mode <NUM> within the waveguide. In contrast, in the example of <FIG>, the guided mode <NUM> may be modulated when the guided mode <NUM> is a TE mode.

Starting from the area near the first side <NUM> of the core <NUM>, the first semiconductor layer <NUM> extends into the negative y-direction such that the electrical contacts can be formed on the first semiconductor layer <NUM>. The electrical contact may be positioned at least <NUM> from the center of the core <NUM>.

The dielectric layer <NUM> is disposed on top of the first semiconductor layer <NUM>. The profile of the dielectric layer <NUM> in yz-plane may follow the profile of the first semiconductor layer <NUM> in yz-plane.

In some implementations, the dielectric layer <NUM> may be deposited to extend into the positive y-direction starting from the area near the core <NUM>.

The second semiconductor layer <NUM> can be subsequently deposited on the dielectric layer <NUM>. The profile of the second semiconductor layer <NUM> in yz-plane may follow the profile of the dielectric layer <NUM> in yz-plane. In particular, the second semiconductor layer <NUM> may be deposited such that it does not extend beyond in the negative y-direction the position where the dielectric layer <NUM> terminates. Since the second semiconductor layer <NUM> forms a capacitor structure with the first semiconductor layer <NUM>, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> must not be in any electrical contact with each other. The extent to which the dielectric layer <NUM> covers the core <NUM> may be determined also in view of this aspect.

In some implementations, the second semiconductor layer <NUM>, starting from the area near the core <NUM>, may extend into positive y-direction such that electrical contact can be formed on the second semiconductor layer <NUM>. The electrical contact may be positioned at least <NUM> from the core <NUM>.

In the example of <FIG>, the first semiconductor layer <NUM>, the second semiconductor layer <NUM> are deposited to form a capacitor type structure with the dielectric layer <NUM> between them. The capacitor type structure wholly covers the three sides of the cross-section of the core <NUM> in yz-plane. This is such that the EO modulator <NUM> interacts with the guided mode <NUM> extending beyond the three sides of the cross-section of the core.

<FIG> shows the EO modulator <NUM> disposed on the waveguide including the core <NUM>. In particular, <FIG> shows the extent of the EO modulator <NUM> in x-direction, perpendicular to the cross section of the core <NUM> which is also the direction of the propagation of the guided wave <NUM>.

The surrounding material <NUM> in xy-plane under the first surface <NUM> of the core <NUM> as discussed in <FIG>.

In some implementations, the waveguide including the core <NUM> may be a rib waveguide such that the core <NUM> is only in contact with the planar surface of the surrounding material <NUM> in xy-plane. The core <NUM> may be exposed to vacuum or air where EO modulator <NUM> is not present.

In some implementations, the waveguide and the EO modulator <NUM> are embedded in the surrounding material <NUM>. This arrangement can be achieved by depositing the surrounding material <NUM> after depositing EO modulator <NUM> such that neither the core <NUM> nor the EO modulator <NUM> are exposed to air or vacuum.

In some implementations, the coupling layer <NUM> may be deposited on the core <NUM>, on three sides which are not in contact with the surrounding material <NUM> throughout the core <NUM>. Therefore, the outermost surfaces of the core <NUM> exposed to air or vacuum may comprise the coupling layer <NUM>.

In some implementations, the coupling layer <NUM> may be deposited on the core <NUM> only where the first semiconductor layer <NUM> is present in between the core <NUM> and the first semiconductor layer <NUM>.

In some implementations, the extent or in x-direction or the length of the first semiconductor layer <NUM>, the dielectric layer <NUM>, and the second semiconductor layer <NUM> are such that the first semiconductor layer <NUM> is the longest and the second semiconductor layer <NUM> is the shortest. This is to ensure that the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may not be accidentally electrically shorted during fabrication. The efficiency of modulation or modulation depth, or Lπ may be dependent on the extent of the second semiconductor layer <NUM> in x-direction.

Although the example of <FIG> and <FIG> relate to the EO modulator <NUM> covering three sides of the core <NUM>, this concept can be extended to covering four sides or all of the sides of the core <NUM>, <NUM>, <NUM> forming the waveguide to increase efficiency of modulation.

<FIG> is a schematic that illustrates an exemplary embodiment of a Mach-Zehnder modulator.

<FIG> shows a top view of the Mach-Zehnder modulator, or MZ modulator <NUM>. The MZ modulator <NUM> includes a photonic Mach-Zehnder interferometer on which an EO modulator <NUM> is disposed.

The EO modulator <NUM> may be one of the examples of the EO modulator <NUM>, <NUM>, <NUM>, <NUM> described in <FIG>, <FIG>, <FIG>, <FIG> and <FIG>.

The MZ modulator <NUM> includes a waveguide formed with a core <NUM> and a cladding formed within the surrounding material, which is not visible in <FIG>. The waveguide in the example of <FIG> is as described in <FIG>, <FIG>, <FIG>, <FIG> and <FIG> hereinbefore. For example, as shown in <FIG>, the cross-section of the core <NUM>, <NUM> in yz-plane is in a rectangular shape and three sides of the cross-section of the core is embedded in the surrounding material <NUM>, <NUM>.

The MZ modulator <NUM> includes an input port <NUM> and an output port <NUM>. In some implementations, the input port <NUM> and the output port <NUM> may be connected to other components in the same substrate via the same type of waveguide including the core <NUM>. In some implementations, the input port <NUM> and the output port <NUM> may be configured such that other components can be connected to the MZ modulator <NUM>. As long as the guided mode <NUM> can be interfaced via the input port <NUM> and the output port <NUM>, the implementation of the input port <NUM> and the output port <NUM> is not limited to these examples.

To form a Mach-Zehnder interferometer, between the input port <NUM> and the output port <NUM>, the waveguide splits into a first arm <NUM> and a second arm <NUM>. The guided mode <NUM> input in the input port <NUM> may be split into the first arm <NUM> and the second arm <NUM> such that the guided mode <NUM> launches into the first arm <NUM> and the second arm <NUM> with substantially equal power. In some implementations, the splitting of the optical mode may be achieved via a multi-mode interference (MMI) device.

In one of the arms, in this example in the second arm <NUM>, the EO modulator <NUM> is integrated into the MZ modulator <NUM>. The EO modulator <NUM> is deposited on the waveguide forming the first arm <NUM>, for example, as described in <FIG>.

The coupling layer <NUM> is deposited above the core <NUM> and the surrounding material around that part of the core <NUM>. The first semiconductor layer <NUM> is deposited on the coupling layer <NUM>. The dielectric layer, also not visible in <FIG>, is deposited on the first semiconductor layer <NUM>. The second semiconductor layer <NUM> is deposited on the oxide layer to form the EO modulator <NUM>, for example, the EO modulator <NUM> described in <FIG>. The coupling layer <NUM> is for coupling the guided mode <NUM> of the waveguide at least to the first semiconductor layer <NUM>. The coupling layer <NUM> may be configured such that when the EO modulator <NUM> is coupled to the waveguide, the coupling layer <NUM> is disposed between the core <NUM> and the semiconductor layer <NUM>.

In some implementations, the length of the second arm <NUM> in x-direction is arranged to be comparable to or longer than Lπ of the EO modulator <NUM>. By applying a voltage within the possible operating voltage range of the EO modulator <NUM>, the guided mode <NUM> travelling in the second arm <NUM> can acquire π phase shift when Vπ is applied to the EO modulator. Since the guided mode <NUM> which entered in the first arm <NUM> did not acquire any additional phase shift, when the guided modes <NUM> in the first arm <NUM> and the second arm <NUM> recombine, a destructive interference may occur at the output port <NUM> when Vπ is applied to the EO modulator <NUM>. This provides amplitude modulation of the guided mode <NUM>.

<FIG> shows a top view of the Mach-Zehnder modulator <NUM> or MZ modulator. The description of the MZ modulator <NUM> of <FIG> applies also to the amplitude modulator <NUM> in that it includes a photonic Mach-Zehnder interferometer embedding the EO modulator <NUM>, <NUM>, <NUM>, <NUM> described in <FIG>, <FIG>, <FIG>, <FIG> and <FIG>, except that the amplitude modulator <NUM> in this example includes two EO modulators, a first EO modulator <NUM> and a second EO modulator <NUM>, in the first arm <NUM> and the second arm <NUM>, respectively.

In some implementations, the two EO modulators <NUM>, <NUM> may operate in a push-pull configuration. The first EO modulator <NUM> and the second EO modulator <NUM> may be driven with the same voltage source or driven with two voltage source which are phase-locked with each other such that the phase shift with the same amplitude but with an opposite sign are generated in the first EO modulator <NUM> and the second EO modulator <NUM>. The length of the arms <NUM>, <NUM> can be half the length of the MZ modulator <NUM> of <FIG> in case π phase shift is required for amplitude modulation. Due to shorter lengths of the first arm <NUM> and the second arm <NUM>, the overall loss may be reduced.

<FIG> is a schematic that illustrates an exemplary embodiment of a ring resonator modulator.

<FIG> shows a top view of the ring resonator modulator <NUM>. The ring resonator modulator <NUM> includes a ring resonator <NUM> on which an EO modulator <NUM> is disposed and a bus waveguide <NUM>. For example, the EO modulator <NUM> may be the EO modulator <NUM> described in <FIG> and <FIG>. However, the configuration of the EO modulator <NUM> is not limited to the example of the <FIG> and <FIG>. Any one of the EO modulators <NUM>, <NUM>, <NUM>, <NUM> described in the earlier examples may be applied to the ring resonator modulator <NUM>.

The ring resonator <NUM> and the bus waveguide <NUM> include a waveguide formed with a core <NUM> and a cladding formed within the surrounding material which is not visible in <FIG>. For example, the waveguide is as described in <FIG> hereinbefore: the cross-section of the core <NUM> in yz-plane is in a rectangular shape and at least one side of the cross-section of the core is in contact with the surrounding material <NUM>. However, the ring resonator modulator <NUM> can also be constructed with any geometry of the waveguide described in earlier examples.

The ring resonator <NUM> and the bus waveguide <NUM> are coupled to each other at a coupling region <NUM>, which is demarcated with a dotted line in <FIG>. In some implementations, the ring resonator <NUM> and the bus waveguide <NUM> may be coupled to each other by being in proximity such that the guided mode <NUM> are evanescently coupled. In this case, the distance between the ring resonator <NUM> and the bus waveguide <NUM> may be one of the parameters that determine the degree of coupling between the ring resonator <NUM> and the bus waveguide <NUM>.

The EO modulator <NUM> may be disposed on the waveguide forming the ring resonator <NUM>. The EO modulator <NUM> may partially overlap the circumference of the ring resonator <NUM>. <FIG> shows that the EO modulator <NUM> covers approximately half of the circumference. However, the extent to which the EO modulator <NUM> covers the ring resonator <NUM> is not limited to this example. For example, the EO modulators <NUM> may cover almost the entire circumference except the coupling area.

In some embodiments, the EO modulator <NUM> may cover the coupling area <NUM> in addition to all or part of the circumference of the ring resonator <NUM>.

The coupling layer <NUM> is deposited above the core <NUM> and the surrounding material around that part of the core <NUM>. The first semiconductor layer <NUM> is deposited on the coupling layer <NUM>. The dielectric layer <NUM> is deposited on the first semiconductor layer <NUM>. The second semiconductor layer <NUM> is deposited on the dielectric layer <NUM> to form the EO modulator <NUM>.

The ring resonator modulator <NUM> includes an input port <NUM> and an output port <NUM> on each side of the bus waveguide <NUM>. In some implementation, the input port <NUM> and the output port <NUM> may be connected to other components in the same substrate via the same type of waveguide including the core <NUM>. In some implementations, the input port <NUM> and the output port <NUM> may be connectorised such that other components can be connected to the ring resonator modulator <NUM>. As long as the guided mode <NUM> can be interfaced via the input port <NUM> and the output port <NUM>, the implementation of the input port <NUM> and the output port <NUM> is not limited to these examples.

In response to an electric field or voltage applied to the EO modulator <NUM>, the EO modulator <NUM> can introduce phase shift to the guided mode <NUM> travelling in the ring resonator <NUM>, thereby shifting the resonance of the ring resonator <NUM>. This leads to the frequency shift of transmission spectrum between the input port <NUM> and the output port <NUM>.

<FIG> is a flowchart that illustrates an exemplary method of fabricating an electro-optic modulator.

In particular, <FIG> shows a method of forming an EO modulator <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> on a waveguide formed including a core <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

At step <NUM>, a coupling layer <NUM>, <NUM>, <NUM>, <NUM> is deposited on the waveguide. In some implementations, the coupling layer <NUM>, <NUM>, <NUM>, <NUM> may be deposited such that the coupling layer <NUM>, <NUM>, <NUM>, <NUM> is directly in contact with at least one surface of the core <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The examples of the deposition methods of the coupling layer <NUM>, <NUM>, <NUM>, <NUM> are described in the earlier examples.

At step <NUM>, a first semiconductor layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM> is deposited on the coupling layer <NUM>, <NUM>, <NUM>, <NUM>. The examples of the deposition methods of the first semiconductor layer <NUM>, <NUM>, <NUM>, <NUM> are described in the earlier examples.

At step <NUM>, an dielectric layer <NUM>, <NUM>, <NUM>, <NUM>, , is deposited on the first semiconductor layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The examples of the deposition methods of the dielectric layer <NUM>, <NUM>, <NUM>, <NUM> are described in the earlier examples.

Claim 1:
An electro-optic modulator (<NUM>; <NUM>; <NUM>; <NUM>) for a waveguide, comprising:
a first semiconductor layer (<NUM>; <NUM>; <NUM>);
a second semiconductor layer (<NUM>; <NUM>; <NUM>);
a dielectric layer (<NUM>; <NUM>; <NUM>) interposed between the second semiconductor layer and the first semiconductor layer; and
a coupling layer (<NUM>; <NUM>; <NUM>) for coupling a guided mode (<NUM>) of the waveguide to at least one of the first semiconductor layer and the second semiconductor layer,
wherein, when the guided mode is coupled to at least one of the first semiconductor layer and the second semiconductor layer, the coupling layer is disposed between the first semiconductor layer and the waveguide such that a centre of the guided mode does not overlap with the electro-optic modulator, and
wherein the electro-optic modulator is configured to induce a modulation on the guided mode of the waveguide by changing a refractive index in response to a voltage applied between the first semiconductor layer and the second semiconductor layer.