An electro-optical modulator

Abstract

The disclosure proposes an integrated electro-optical modulator, and a corresponding method of fabricating and operating the electro-optical modulator. The electro-optical modulator comprises: a cladding layer; a waveguide core embedded in the cladding layer; a ferroelectric layer; at least two electrodes for modulating an optical wave as it passes through the waveguide core; and at least three conductor blocks, wherein a first conductor block is electrically connected to a first electrode, and wherein a second conductor block is electrically connected to a second electrode. The at least two electrodes are configured to generate an electric-field via the first conductor block and the second conductor block across at least a part of the ferroelectric layer.

Description

[0001] AN ELECTRO-OPTICAL MODULATOR

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to an electro-optical modulator. The disclosure proposes an electro- optical modulator, a corresponding method of fabricating the electro-optical modulator, and a corresponding method of operating the electro-optical modulator. The electro-optical modulator comprises a ferroelectric layer, a waveguide core, and electrodes, and is arranged in a new kind of way.

[0004] BACKGROUND

[0005] An electro-optical modulator is a device which converts an electrical signal, for example, a stream of 0V and IV signals encoding digital information, into the optical domain, e.g., by turning light on or off depending on an input voltage. Electro-optical modulation is an essential element of the modem telecommunication systems, playing a critical role in the conversion and transmission of electrical signals into optical signals. The most widely employed mechanism for this process is the electro-optic effect, commonly known as the Pockels effect.

[0006] However, silicon, the standard material for integrated photonics, does not exhibit the Pockels effect due to its centrosymmetric structure. This necessitates the integration of other materials to enable electro-optic modulation. One such promising candidate is Barium Titanate (BTO), which has one of the highest Pockels coefficients, making it a promising choice for on-chip modulation.

[0007] Losses in electro-optical modulators can significantly impede their performance and, consequently, the overall energy efficacy of the integrated optical systems they are a part of.

[0008] For example, losses in electro-optical modulators can result in the attenuation of the optical signal. Thus, the optical signal may not reach its intended destination with the required power levels, necessitating the use of additional amplifiers or repeaters, which in turn increases the complexity and cost of the system. Furthermore, losses can lead to a degradation in the signal quality. Maintaining signal integrity is paramount, as the demand for high data rates is ever-increasing. Losses in the modulator can introduce noise and distortions, which can result in a higher bit error rate, making the transmitted data less reliable.

[0009] In BTO modulators (as with any modulator dependent on the Pockels effect), the level of modulation increases as the electrical field intensifies, since the refractive index of the material linearly varies with respect to the applied electric field. The proximity of the non-optically transparent electrodes, classically metal for high frequency modulator electrodes, directly influences this effect, as closer electrodes facilitate a higher field strength under the same voltage. Thus, a straightforward method to boost modulation efficiency is to decrease the distance between metal electrodes. However, enhancing the electrical field by bringing the metals closer to the waveguide comes with a trade-off: the optical loss scales up exponentially, given the highly absorptive nature of conventional electrodes such as metal.

[0010] Generally, conventional electro-optical modulators that comprise metal electrodes that are close to each other have extremely high optical losses. Further, the fabrication complexity is increased as metals must be etched with extreme high precision.

[0011] Moreover, depending on the crystallographic orientation of the modulating material (in the case of BTO, along the c-axis or a-axis), the direction of the applied electric field required to achieve optimal modulation needs to be adjusted. For instance, c-axis-oriented BTO can be poled effectively using an applied electric field in the vertical direction (referred to as out-of-plane field), while a-axis-oriented BTO requires the electric field to be applied in the horizontal direction (referred to as in-plane field). Complications arise, however, when both orientations (c-type and a-type) coexist within a single layer.

[0012] SUMMARY

[0013] In view of the above, an objective of this disclosure is to improve the modulation efficiency and / or flexibility of an electro-optical modulator. Another objective is to improve the control over the direction of the electric field applied to the electro-optical modulator. Another objective is to reduce losses in the electro-optical modulator. In particular, an objective is to increase the electrical field across the ferroelectric material without increasing, or while only slightly increasing or reducing the optical loss.

[0014] These and other objectives are achieved by this disclosure as described in the enclosed independent claims. Advantageous implementations are further defined in the dependent claims.

[0015] A first aspect of this disclosure provides an integrated electro-optical modulator comprising: a cladding layer; a waveguide core embedded in the cladding layer; a ferroelectric layer, wherein the waveguide core is separated from the ferroelectric layer by the cladding layer; at least three electrodes comprising a first electrode, a second electrode, and a third electrode; and at least three conductor blocks comprising a first conductor block electrically connected to the first electrode, a second conductor block electrically connected to the second electrode, and a third conductor block electrically connected to the third electrode; wherein the at least three electrodes are configured to receive an electrical signal thereby generating an electric-field via the at least three conductor blocks across at least a part of the ferroelectric layer, wherein the electric-field induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer thereby modulating an optical wave as it passes through the waveguide core, wherein the extension direction of the waveguide core is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer is provided above the waveguide core in the vertical direction, wherein at least one conductor block of the at least three conductor blocks is above the ferroelectric layer in the vertical direction and at least one conductor block of the at least three conductor blocks is below the ferroelectric layer in the vertical direction, wherein at least one conductor block of the at least three conductor blocks extends to a different horizontal location than another conductor block of the at least three conductor blocks, wherein the electric-field comprises a component in the vertical direction and a component in the horizontal direction.

[0016] The electro-optical modulator may be configurable to apply the electric-field in multiple different orientations, for example, any orientation may be possible. This may be useful for many applications including but not limited to modulating optical waves with an arbitrary polarization, poling of the ferroelectric layer with an arbitrary crystalline orientation, for providing a polarization rotator, and for periodically poling the ferroelectric layer for second harmonic generation and entangled photon generation. The electro-optical modulator may comprise a controller configured to control the at least three electrodes, for example, by providing the electrical signal.

[0017] The electric-field can be tuned and / or controlled to point to any direction, for example, based on different electrical signals being received by the at last three electrodes. For example, the electro- optical modulator may be configured to generate a first version of the electric field in a first direction across the ferroelectric layer in a first timeslot and a second version of the electric-field in a second direction across the ferroelectric layer in a second timeslot.

[0018] For example, the controller may be configured to provide a first electrical signal in a first timeslot and a second electrical signal in a second timeslot to modify the direction of the electric-field.

[0019] The electric-field may or may not be spatially non-uniform across the ferroelectric layer.

[0020] The mechanical deformation of the ferroelectric layer may be based on the piezoelectric effect. The changes of the refractive index of the ferroelectric layer may be based on the electro-optical effect.

[0021] At least one of the first conductor block, the second conductor block, the third conductor block, and the initial layer may be provided directly on the ferroelectric layer. For example, first conductor block, the second conductor block, and the third conductor block may be provided directly on the ferroelectric layer.

[0022] At least one of the first conductor block, the second conductor block, the third conductor block, and the insulating layer may be provided above the ferroelectric layer. Alternatively or additionally, at least one of the first conductor block, the second conductor block, the third conductor block, and the insulating layer may be provided below the ferroelectric layer.

[0023] The cladding layer may comprise one or more cladding sub-layers. The cladding layer may be made of multiple materials. The cladding layer may be for providing electrical isolation and / or optical wave confinement inside the waveguide core. At least one of: the first conductor, the second conductor block, the third conductor block, and the insulating layer may be partially or fully embedded in the cladding layer.

[0024] The waveguide core, for example, the optical wave in the waveguide core, may be optically coupled to the ferroelectric layer.

[0025] One skilled in the art will recognize that the electro-topical modulator may further comprise a fourth electrode and / or a fourth conductor block that may respectively be configured analogously to, for example, the first electrode or the first conductor block, respectively. For example, the at least three electrodes may be at least four electrodes and comprise the fourth electrode. For example, the at least three conductor blocks may be at least four conductor blocks and comprise the fourth conductor block.

[0026] The term “electrical signal” may refer to the collective effect of a plurality voltages applied to the at least three electrodes that contribute to the modulation process.

[0027] The electrical signal may be synchronized signal, comprising multiple electrical sub-signals or voltage signals that are synchronized with each other. Thus, the modulation of the optical wave can be efficiently controlled. For example, the controller may be configured to synchronize the electrical sub-signals, before said electrical sub-signals are provided to the at least three electrodes.

[0028] The electrical signal may comprise a modulation signal component, for example, a radio frequency signal, used for modulation and a direct current signal used for poling the ferroelectric layer. The at least three electrodes and at least three conductor blocks may be configured to decouple the modulation of the optical wave from the poling of the ferroelectric layer.

[0029] The electro-optical modulator may or may not be one of: an optical isolator, an optical circulator, a micro-electromechanical system (MEMS) device, a phase shifter, and a polarization rotator.

[0030] For example, the at least one conductor block of the at least three conductor blocks and the another conductor block of the at least three conductor blocks may be one of: horizontally translated in relation to each other, not horizontally identically aligned, horizontally distanced from each other, and one extending farther in the horizontal direction than the other. Different conductor blocks may or may not be provided in the same processing step deposition layer.

[0031] In a further implementation form of the first aspect, the first conductor block is provided on, for example, directly provided on, the ferroelectric layer, or on a first buffer layer that is provided on, for example, directly provided on, the ferroelectric layer, wherein the second conductor block is provided on, for example, directly provided on, the ferroelectric layer, or on a second buffer layer that is provided on, for example, directly provided on, the ferroelectric layer.

[0032] The electro-optical modulator may or may not comprise the first buffer layer. The electro-optical modulator may or may not comprise the second buffer layer.

[0033] In a further implementation form of the first aspect, the at least three conductor blocks are each a non-metal layer and / or an optically transparent layer.

[0034] Thus, the at least three conductor blocks can be provided closely above or below the ferroelectric layer to improve the modulation efficiency.

[0035] In a further implementation form of the first aspect, the at least three electrodes are arranged to not generate an electric-field via the at least three conductor blocks across any other ferroelectric layer of the electro-optical modulator.

[0036] In a further implementation form of the first aspect, a ratio of the component in the horizontal direction of the electric-field and the component in the vertical direction of the electric-field is in a range of 0.01 to 100 across the ferroelectric layer.

[0037] For example, the ratio may be in a range of one of: 0.02 to 50, 0.05 to 20, 0.1 to 10, and 0.2 to 5.

[0038] In a further implementation form of the first aspect, the electrical signal comprises a first voltage signal, a second voltage signal, and a third voltage signal, wherein the first electrode is configured to receive the first voltage signal, the second electrode is configured to receive the second voltage signal, and the third electrode is configured to receive the third voltage signal, wherein the first voltage signal, the second voltage signal, and the third voltage signal are direct current voltage signals that are simultaneously received by respectively the first electrode, the second electrode, and the third electrode, and / or wherein the first voltage signal, the second voltage signal, and the third voltage signal are alternating current voltage signals that have the same frequency and respectively a phase-difference to each other.

[0039] Thus, said voltage signals may be synchronized to increase modulation efficiency and improve the control over the electric-field.

[0040] In a further implementation form of the first aspect, the second conductor block is electrically isolated from the first conductor block and from the third conductor block, and wherein the third conductor block is electrically isolated from the first conductor block.

[0041] In a further implementation form of the first aspect, the ferroelectric layer has a Pockels tensor containing at least one non-zero, for example, non- vanishing, element rij where i^j.

[0042] In a further implementation form of the first aspect, the cladding layer provides electrical insulation between the first conductor block and the second conductor block; or wherein the electro-optical modulator further comprises an insulating layer provided on the ferroelectric layer or on a third buffer layer that is provided on the ferroelectric layer, wherein the insulating layer provides electrical insulation between the first conductor block and the second conductor block.

[0043] In a further implementation form of the first aspect, the ferroelectric layer is arranged between the waveguide core and at least one of the first conductor block, the second conductor block, the third conductor block, and the insulating layer if present.

[0044] In a further implementation form of the first aspect, the electric-field generated via the first conductor block and the second conductor block extends predominantly, for example, only, in the horizontal direction.

[0045] In a further implementation form of the first aspect, the electric-field is further generated via the third conductor block to provide an additional component of the electric field that extends predominately, for example, only, in the vertical direction. In a further implementation form of the first aspect, the first conductor block is a p-doped semiconductor block and the second conductor block is a n-doped semiconductor block.

[0046] In a further implementation form of the first aspect, a distance between the first conductor block and the second conductor block is 500 nm to 1500 nm.

[0047] In a further implementation form of the first aspect, at least one of: the distance between the ferroelectric layer and the waveguide core is 5 nm to 300 nm, the ferroelectric layer has a thickness of 20 nm to 2000 nm, the first conductor block has a thickness of 20 nm to 300 nm, and the second conductor block has a thickness of 20 nm to 300 nm.

[0048] In a further implementation form of the first aspect, at least one of: the ferroelectric layer comprises barium titanate, the waveguide core comprises silicon nitride, and each buffer layer may be a lattice buffer layer.

[0049] Each buffer layer may comprise at least one of Germanium (Ge), Tin-zinc-oxide (SZO), and strontium titanate (STO).

[0050] In a further implementation form of the first aspect, the ferroelectric layer is configured to generate an acoustic wave based on the piezoelectric effect induced by the electric-field, wherein the acoustic wave modulates the optical wave as it passes through the waveguide core.

[0051] The electrical signal may be a radio frequency (RF) signal. The frequency of the signal may be matched to the resonance frequency of the transducer.

[0052] The acoustic wave may be a bulk acoustic wave or a surface acoustic wave.

[0053] The type of the acoustic wave may be adaptable. For example, two different electrical signals received by the at least three electrodes may generate respectively a surface acoustic wave and a bulk acoustic wave. The electro-optical modulator may be an optical isolator / circulator. For example, the optical wave may be coupled from a first mode into another mode only in the propagation direction of the optical wave.

[0054] Each conductor block of the at least two conducting layers may significantly increase the generated surface acoustic waves, for example, when said conductor block is grounded.

[0055] In a further implementation form of the first aspect, the ferroelectric layer is configured to generate two or more acoustic waves having different frequencies, due to the at least three conductor blocks having different resonance frequencies.

[0056] The ferroelectric layer may be configured to generate each acoustic wave of the two or more acoustic waves based on the piezoelectric effect induced by the electric-field.

[0057] Each acoustic wave of the two or more acoustic waves may or may not modulate the optical wave as it passes through the waveguide core.

[0058] The acoustic wave may be formed based on the two or more acoustic waves.

[0059] In a further implementation form of the first aspect, the two or more acoustic waves are configured to interfere to set a propagation direction of the acoustic wave.

[0060] Thus, the propagation direction of the acoustic wave may be adaptable.

[0061] The propagation direction of the acoustic wave may be set such that the acoustic wave is a surface acoustic wave or a bulk acoustic wave. For example, during a first timeslot the acoustic wave may be a bulk acoustic wave and during a second timeslot the acoustic wave may be a surface acoustic wave.

[0062] In a further implementation form of the first aspect, the at least three electrodes are configured to induce independent actuations in orthogonal directions in the ferroelectric layer based on the electric field generated via the at least three conductor blocks, to modulate the optical wave as it passes through the waveguide core based on said independent actuations. For example, a cantilever may be suspended in the waveguide core and / or the cladding layer, wherein the electro optical modulator may be configured to cause a displacement of the cantilever based on the independent actuation of the ferroelectric layer to modulate the optical wave as it passes through the waveguide core.

[0063] The least three electrodes may be configured to control stress gradients in the ferroelectric layer based on the electric field generated via the at least three conductor blocks.

[0064] The electro-optical modulator may be a MEMS device.

[0065] A MEMS device may be fabricated based on an additional undercut step to suspend the ferroelectric layer and enable its movement as a result of the applied voltage / electrical signal.

[0066] In a further implementation form of the first aspect, the electro-optical modulator is a microelectromechanical system array and comprises a plurality of individual devices patterned on the ferroelectric layer, wherein the at least three electrodes are configured to individually control the plurality of individual devices based on the electric-field generated via the at least three conductor blocks.

[0067] The plurality of individual devices may for example, be MEMS mirrors in an optical phased array.

[0068] In a further implementation form of the first aspect, the electro-topical modulator is configured to determine and / or modify, for example, pole, a c-axis and / or an a-axis of the ferroelectric layer.

[0069] The electrical signal may comprise a modulation signal component, for example, a radio frequency signal, used for modulation and a direct current signal used for poling the ferroelectric layer. The at least three electrodes and at least three conductor blocks may be configured to decouple the modulation of the optical wave from the poling of the ferroelectric layer.

[0070] A second aspect of this disclosure provides a method of operating an integrated electro-optical modulator, wherein the electro-optical modulator comprises: a cladding layer; a waveguide core embedded in the cladding layer; a ferroelectric layer, wherein the waveguide core is separated from the ferroelectric layer by the cladding layer; at least three electrodes comprising a first electrode, a second electrode, and a third electrode; and at least three conductor blocks comprising a first conductor block electrically connected to the first electrode, a second conductor block electrically connected to the second electrode, and a third conductor block electrically connected to the third electrode; wherein the method comprises receiving an electrical signal with the at least three electrodes thereby generating an electric-field via the at least three conductor blocks across at least a part of the ferroelectric layer, wherein the electric-field induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer thereby modulating an optical wave as it passes through the waveguide core, wherein the extension direction of the waveguide core is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer is provided above the waveguide core in the vertical direction, wherein at least one conductor block of the at least three conductor blocks is above the ferroelectric layer in the vertical direction and at least one conductor block of the at least three conductor blocks is below the ferroelectric layer in the vertical direction, wherein at least one conductor block of the at least three conductor blocks extends to a different horizontal location than another conductor block of the at least three conductor blocks, and wherein the electric-field comprises a component in the vertical direction and a component in the horizontal direction.

[0071] The method of the second aspect may have implementation forms that correspond to the implementation forms of the electro-optical modulator of the first aspect. The method of the second aspect and its implementation forms achieve the advantages and effects described above for the electro-optical modulator of the first aspect and its respective implementation forms.

[0072] A third aspect of this disclosure provides a method of fabricating an integrated electro-optical modulator, the method comprising: forming a cladding layer; forming a waveguide core embedded in the cladding layer; forming a ferroelectric layer, wherein the waveguide core is separated from the ferroelectric layer by the cladding layer; forming at least three electrodes comprising a first electrode, a second electrode, and a third electrode; and forming at least three conductor blocks comprising a first conductor block electrically connected to the first electrode, a second conductor block electrically connected to the second electrode, and a third conductor block electrically connected to the third electrode; wherein the at least three electrodes are configured to receive an electrical signal thereby generating an electric-field via the at least three conductor blocks across at least a part of the ferroelectric layer, wherein the electric-field induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer thereby modulating an optical wave as it passes through the waveguide core, wherein at least one conductor block of the at least three conductor blocks is above the ferroelectric layer in the vertical direction and at least one conductor block of the at least three conductor blocks is below the ferroelectric layer in the vertical direction, wherein at least one conductor block of the at least three conductor blocks extends to a different horizontal location than another conductor block of the at least three conductor blocks, and wherein the extension direction of the waveguide core is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer is provided above the waveguide core in the vertical direction, wherein the electric-field comprises a component in the vertical direction and a component in the horizontal direction.

[0073] In a further implementation form of the third aspect, the first conductor block is provided on the ferroelectric layer, or on a first buffer layer that is provided on the ferroelectric layer and included in the electro-optical modulator, wherein the second conductor block is provided on the ferroelectric layer, or on a second buffer layer that is provided on the ferroelectric layer.

[0074] The third conductor block may be provided on one of: the ferroelectric layer, the first buffer layer, the second buffer layer, a third buffer layer that is provided on the ferroelectric layer.

[0075] In a further implementation form of the third aspect, the method further comprises poling or repoling the ferroelectric layer by using the electric-field.

[0076] An electro-optical modulator comprising at least three electrodes can enable the re-poling the ferroelectric material, and can thus enable tunable performance. For example, this may be enabled by the at least three conductor blocks being provided in different layers. For example, the least two conductor blocks may form a double layer of optically transparent electrodes. Having two different conductor blocks may enable the re-programming of both the c-axis and the a-axis of the ferroelectric layer, for example, BTO.

[0077] In a further implementation form of the third aspect, the method further comprises: providing a wafer, wherein the wafer comprises a substrate and a first cladding layer; and providing the ferroelectric layer on the first cladding layer; providing a second cladding layer on at least one of: the first conductor block, the second conductor block, the third conductor block, to embed said layer or layers and the ferroelectric layer in the cladding layer formed by the first cladding layer and the second cladding layer; providing the at least three electrodes.

[0078] In a further implementation form of the third aspect, providing the ferroelectric layer comprises: bonding a ferroelectric wafer or dies to the first cladding layer, wherein the ferroelectric wafer comprises a ferroelectric layer and one or more other layers, and wherein the ferroelectric layer is bonded to the first cladding layer, wherein forming at least on of: the first conductor block, the second conductor block, and the third conductor block comprises: removing the one or more other layers; and providing at least one of: the first conductor block, the second conductor block, and the third conductor block on the ferroelectric layer or on the buffer layer; or patterning the one or more other layers to form at least one of: the first conductor block, the second conductor block, and the third conductor block on the ferroelectric layer or on the buffer layer.

[0079] In a further implementation form of the third aspect, one of: the method further comprises providing an insulating layer on the ferroelectric layer or on the buffer layer, wherein the insulating layer provides electrical insulation between at least two of: the first conductor block, the second conductor block, and the third conductor block; patterning the one or more other layers further forms an insulating layer, wherein the insulating layer provides electrical insulation between at least two of the first conductor block, the second conductor block, and the third conductor block; and the second cladding layer provides electrical insulation between at least two of: the first conductor block, the second conductor block, and the third conductor block.

[0080] In a further implementation form of the third aspect, the one or more other layers comprise the buffer layer and an initial layer provided on the buffer layer, wherein the initial layer is a semiconductor block, wherein the method further comprises at least two of: doping a first section of the initial layer to form the first conductor block, doping a second section of the initial layer to form the second conductor block, doping a third section of the initial layer to form the third conductor block; wherein the method further comprises and patterning the initial layer, the buffer layer, and the ferroelectric layer.

[0081] For example, patterning the buffer layer may comprise grinding and / or Chemical Mechanical Planarization (CMP) and / or etching. The ferroelectric wafer may comprise: the initial layer, the buffer layer below the initial layer, the ferroelectric layer below the buffer layer.

[0082] The method may further comprise fabricating the ferroelectric wafer by providing the initial layer, providing the buffer layer below the initial layer, growing the ferroelectric layer below the buffer layer.

[0083] The method of the third aspect may have implementation forms that correspond to the implementation forms of the electro-optical modulator of the first aspect. The method of the third aspect and its implementation forms achieve the advantages and effects described above for the electro-optical modulator of the first aspect and its respective implementation forms.

[0084] A fourth aspect of this disclosure provides an integrated electro-optical modulator comprising: a cladding layer; a waveguide core embedded in the cladding layer; a ferroelectric layer, wherein the waveguide core is separated from the ferroelectric layer by the cladding layer; at least two electrodes comprising a first electrode and a second electrode; a first conductor layer provided on the ferroelectric layer or on a first buffer layer that is provided on the ferroelectric layer, wherein the first conductor layer is electrically connected to the first electrode; and a second conductor layer provided on the ferroelectric layer or on a second buffer layer that is provided on the ferroelectric layer, wherein the second conductor layer is electrically connected to the second electrode; wherein the at least two electrodes are configured to receive an electrical signal thereby generating an electric-field via the first conductor layer and the second conductor layer across at least a part of the ferroelectric layer, wherein the electric-field changes a refractive index of the ferroelectric layer thereby modulating an optical wave as it passes through the waveguide core.

[0085] The electro-optical modulator may comprise a controller configured to control the at least two electrodes, for example, by providing the electrical signal.

[0086] At least one of the first conductor layer, the second conductor layer, and the initial layer may be provided directly on the ferroelectric layer.

[0087] The ferroelectric layer may be provided above the waveguide core. At least one of the first conductor layer, the second conductor layer, and the insulating layer may be provided above the ferroelectric layer. Alternatively or additionally, at least one of the first conductor layer, the second conductor layer, and the insulating layer may be provided below the ferroelectric layer.

[0088] The cladding layer may comprise one or more cladding sub-layers. The cladding layer may be made of multiple materials. The cladding layer may be for providing electrical isolation and / or optical wave confinement inside the waveguide core.

[0089] At least one of: the first conductor, the second conductor layer, and the insulating layer may be partially or fully embedded in the cladding layer.

[0090] The waveguide core, for example, the optical wave in the waveguide core, may be optically coupled to the ferroelectric layer.

[0091] In an implementation form of the fourth aspect, the second conductor layer is electrically isolated from the first conductor layer.

[0092] In a further implementation form of the fourth aspect, the ferroelectric layer has a Pockels tensor containing at least one non -vanishing, for example, non-zero, element rij where i^j.

[0093] In a further implementation form of the fourth aspect, the cladding layer provides electrical insulation between the first conductor layer and the second conductor layer; or wherein the electro- optical modulator further comprises an insulating layer provided on the ferroelectric layer or on a third buffer layer that is provided on the ferroelectric layer, wherein the insulating layer provides electrical insulation between the first conductor layer and the second conductor layer.

[0094] The insulating layer may be used to define the area where the E-field is generated.

[0095] In a further implementation form of the fourth aspect, the ferroelectric layer is arranged between the waveguide core and at least one of: the first conductor layer, the second conductor layer, and the insulating layer if present. In a further implementation form of the fourth aspect, the extension direction of the waveguide core is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer is provided above the waveguide core in the vertical direction, wherein the electric-field generated by the at least two electrodes comprises a component in the vertical direction and / or a component in the horizontal direction.

[0096] In a further implementation form of the fourth aspect, the electric-field generated via the first conductor layer and the second conductor layer extends predominantly, for example, only, in the horizontal direction.

[0097] In a further implementation form of the fourth aspect, the at least two electrodes are at least three electrodes further comprising a third electrode, wherein the electro-optical modulator further comprises a third conductor layer electrically connected to the third electrode, wherein the electricfield is further generated via the third conductor layer to provide an additional component of the electric field that extends predominately in the vertical direction.

[0098] In a further implementation form of the fourth aspect, the first conductor layer is a p-doped semiconductor layer and the second conductor layer is a n-doped semiconductor layer.

[0099] In a further implementation form of the fourth aspect, a distance between the first conductor layer and the second conductor layer is 500 nm to 1500 nm.

[0100] In a further implementation form of the fourth aspect, at least one of: the distance between the ferroelectric layer and the waveguide core is 5 nm to 300 nm, the ferroelectric layer has a thickness of 20 nm to 2000 nm, the first conductor layer has a thickness of 20 nm to 300 nm, and the second conductor layer has a thickness of 20 nm to 300 nm.

[0101] In a further implementation form of the fourth aspect, at least one of: the ferroelectric layer comprises barium titanate, the waveguide core comprises silicon nitride, and each buffer layer may be a lattice buffer layer.

[0102] Each buffer layer may comprise at least one of Germanium (Ge), Tin-zinc-oxide (SZO), and strontium titanate (STO). A fifth aspect of this disclosure provides a method of fabricating an integrated electro-optical modulator, the method comprising: forming a cladding layer; forming a waveguide core embedded in the cladding layer; forming a ferroelectric layer, wherein the waveguide core is separated from the ferroelectric layer by the cladding layer; forming at least two electrodes comprising a first electrode and a second electrode; forming a first conductor layer provided on the ferroelectric layer or on a buffer layer that is provided on the ferroelectric layer, wherein the first conductor layer is electrically connected to the first electrode; and forming a second conductor layer provided on the ferroelectric layer or on the buffer layer, wherein the second conductor layer is electrically connected to the second electrode; wherein the at least two electrodes are configured to receive an electrical signal thereby generating an electric-field via the first conductor layer and the second conductor layer across at least a part of the ferroelectric layer, wherein the electric-field changes a refractive index of the ferroelectric layer thereby modulating an optical wave as it passes through the waveguide core.

[0103] In an implementation form of the fifth aspect, the extension direction of the waveguide core is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer is formed above the waveguide core in the vertical direction, wherein the electric-field generated by the at least two electrodes comprises a component in the vertical direction and / or a component in the horizontal direction.

[0104] In a further implementation form of the fifth aspect, the method further comprises: providing a wafer, wherein the wafer comprises a substrate and a first cladding layer; and providing the ferroelectric layer on the first cladding layer; providing a second cladding layer on the first conductor layer and the second conductor layer, to embed the first conductor layer, the second conductor layer, and the ferroelectric layer in the cladding layer formed by the first cladding layer and the second cladding layer; providing the at least two electrodes.

[0105] In a further implementation form of the fifth aspect, providing the ferroelectric layer comprises: bonding a ferroelectric wafer or dies to the first cladding layer, wherein the ferroelectric wafer comprises a ferroelectric layer and one or more other layers, and wherein the ferroelectric layer is bonded to the first cladding layer, wherein forming the first conductor layer, and the second conductor layer comprises: removing the one or more other layers; and providing a first conductor layer and a second conductor layer on the ferroelectric layer or on the buffer layer; or patterning the one or more other layers to form the first conductor layer and the second conductor layer on the ferroelectric layer or on the buffer layer.

[0106] In a further implementation form of the fifth aspect, one of: the method further comprises providing an insulating layer on the ferroelectric layer or on the buffer layer, wherein the insulating layer provides electrical insulation between the first conductor layer and the second conductor layer; patterning the one or more other layers further forms an insulating layer, wherein the insulating layer provides electrical insulation between the first conductor layer and the second conductor layer; and the second cladding layer provides electrical insulation between the first conductor layer and the second conductor layer.

[0107] In a further implementation form of the fifth aspect, the one or more other layers comprise the buffer layer and an initial layer provided on the buffer layer, wherein the initial layer is a semiconductor layer, wherein the method further comprises: doping a first section of the initial layer to form the first conductor layer, doping a second section of the initial layer to form the second conductor layer, and patterning the initial layer, the buffer layer, and the ferroelectric layer.

[0108] For example, patterning the buffer layer may comprise grinding and / or Chemical Mechanical Planarization and / or etching.

[0109] The ferroelectric wafer may comprise: the initial layer, the buffer layer below the initial layer, the ferroelectric layer below the buffer layer.

[0110] The method may further comprise fabricating the ferroelectric wafer by providing the initial layer, providing the buffer layer below the initial layer, growing the ferroelectric layer below the buffer layer.

[0111] The method of the fifth aspect may have implementation forms that correspond to the implementation forms of the electro-optical modulator of the fourth aspect. The method of the fifth aspect and its implementation forms achieve the advantages and effects described above for the electro-optical modulator of the fourth aspect and its respective implementation forms. In this disclosure, the c-axis refers to a crystallographic axis that is perpendicular to the basal plane (for example, the plane formed by the a-axis and, where applicable, the b-axis) of a crystal lattice. In the context of layered materials, such as graphite or transition metal di chalcogenides, the c-axis corresponds to the direction of stacking of the atomic layers, which is typically associated with weaker interlayer interactions (e.g., van der Waals forces).

[0112] In this disclosure, the a-axis refers to one of the crystallographic axes lying within the basal plane of the crystal lattice. In hexagonal or trigonal systems, the a-axis is one of the equivalent axes defining the in-plane directions of atomic arrangements within a single layer of the material.

[0113] Further, in this disclosure, a first element and a second element are considered to be different element, if not explicitly mentioned otherwise. For example, a first section and a second section are considered to be different sections. In another example, a first timeslot and a second timeslot are considered to be different timeslots.

[0114] Further, in this disclosure, “approximately” can mean ..w ithin a certain range comprising the nominal value“ . That range may be +- 10 %, +-5% or +-2 % of the nominal value.

[0115] Notably, in this disclosure, forming or providing a layer "on" another layer may mean growing / depositing these layers one upon the other. Thus, surfaces of these layers may be in contact. Forming a layer "above" or “below” another layer may mean that this layer is formed after the other layer, but there may be formed one or more layers in between.

[0116] Further, in this disclosure, the phrase “above” when referring to a physical location may refer to a vertical direction, wherein the extension direction of the waveguide core is perpendicular to the vertical direction and a horizontal direction.

[0117] BRIEF DESCRIPTION OF DRAWINGS

[0118] The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

[0119] FIG. 1 shows an electro-optical modulator according to this disclosure. FIG. 2 shows an exemplary electro-optical modulator comprising four electrodes according to this disclosure

[0120] FIG. 3 shows an exemplary electro-optical modulator according to this disclosure.

[0121] FIG. 4 shows the generation of acoustic waves with a ferroelectric layer.

[0122] FIG. 5 shows an electro-optical modulator for generating SAWs according to this disclosure.

[0123] FIG. 6 shows an exemplary MEMS device according to this disclosure.

[0124] FIG. 7 shows a method according to this disclosure.

[0125] FIG. 8 shows a method according to this disclosure.

[0126] FIG. 9 shows an electro-optical modulator according to this disclosure.

[0127] FIG. 10 shows a cross section of an exemplary electro-optical modulator according to this disclosure.

[0128] FIG. I la shows an optical mode according to this disclosure.

[0129] FIG. 1 lb shows an E-field inside an electro-optical modulator according to this disclosure.

[0130] FIG. 12 shows a cross section of an exemplary electro-optic modulator according to this disclosure.

[0131] FIG. 13a shows an optical mode according to this disclosure.

[0132] FIG. 13b shows an E-field inside an electro-optical modulator according to this disclosure. FIG. 14 shows steps for fabricating an electric-optic modulator according to this disclosure.

[0133] FIG. 15 shows steps of a first process flow for fabricating an electric-optic modulator according to this disclosure.

[0134] FIG. 16 shows steps of a first process flow for fabricating an electric-optic modulator according to this disclosure.

[0135] FIG. 17 shows steps of a first process flow for fabricating an electric-optic modulator according to this disclosure.

[0136] FIG. 18 shows steps of a second process flow for fabricating an electric-optic modulator according to this disclosure.

[0137] FIG. 19 shows steps of a second process flow for fabricating an electric-optic modulator according to this disclosure.

[0138] FIG. 20 shows steps of a second process flow for fabricating an electric-optic modulator according to this disclosure.

[0139] FIG. 21 shows a method according to this disclosure.

[0140] DETAILED DESCRIPTION OF EMBODIMENTS

[0141] FIG. 1 shows an electro-optical modulator 100 according to this disclosure.

[0142] The integrated electro-optical modulator 100 comprises: a cladding layer 101; a waveguide core 104 embedded in the cladding layer 101; a ferroelectric layer 102, wherein the waveguide core

[0143] 104 is separated from the ferroelectric layer 102 by the cladding layer 101; at least three electrodes

[0144] 105 comprising a first electrode 105a, a second electrode 105b, and a third electrode 105c; and at least three conductor blocks 111 comprising a first conductor block 108, a second conductor block 109, and a third conductor block 110.. The first conductor block 108 is electrically connected to the first electrode 105a, the second conductor block 109 is electrically connected to the second electrode 105b, and the third electrode 105c is electrically connected to the third conductor block 110, which is indicated by the dashed- dotted arrows in FIG. 1.

[0145] The at least three electrodes 105 are configured to receive an electrical signal thereby generating an electric-field 107 via the at least three conductor blocks 111 across at least a part of the ferroelectric layer 102, wherein the electric-field 107 induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer 102 thereby modulating an optical wave 106 as it passes through the waveguide core 104. The electric-field 107 comprises a component in a vertical direction and a component in a horizontal direction.

[0146] The extension direction of the waveguide core 104 is perpendicular to the vertical direction and the horizontal direction. The ferroelectric layer 102 is provided above the waveguide core 104 in the vertical direction. At least one conductor block 109 of the at least three conductor blocks 111 is above the ferroelectric layer 102 in the vertical direction and at least one conductor block 108 of the at least three conductor blocks 111 is below the ferroelectric layer 102 in the vertical direction. At least one conductor block 108 of the at least three conductor blocks 111 extends to a different horizontal location than another conductor block 109 of the at least three conductor blocks 111.

[0147] This disclosure may be based on the integration of materials with different ferroelectric domains, for example, an electro-optic material like barium titanate (BTO), into an integrated photonic platform. The Pockels effect of these ferroelectric materials can be used to modify the index of refraction of the material which can be modulated by an electric field 107. Alternatively or additionally, the piezoelectric effect of these ferroelectric materials induced by an electric field can be used to mechanically deform the material.

[0148] Some ferroelectric materials, for example, BTO, are known to have a high electro-optic (Pockels) effect which makes it suitable for electro-optical modulation. The strength of Pockels effect is however dependent on the crystalline orientation of the ferroelectric medium as well as light polarization with regards to the direction of the applied electric field 107. Transparent ferroelectric materials have different crystal orientations which can slightly vary from wafer to wafer or position to position on the same wafer. Therefore, it is generally difficult to assess the optimum direction of the applied field to harness the maximum Pockels effect.

[0149] In order to maximize the electric field 107 and, consequently, the modulation, the voltage may be brought as close as possible to the waveguide, i.e., metal lines may be brought extremely close to the optical mode. However, this can result in a dramatic increase in optical loss when metals are brought too close to the waveguide.

[0150] Silicon as a material may be used to enhance the strength of the effect, by pulling a large part of the optical mode into the bulk of the thin film ferroelectric material.

[0151] An alternative for BTO as a ferroelectric material may be, for example, Lithium Niobate (LNO), or Lead Zirconate Titanate (PZT).

[0152] To establish the silicon, a Silicon On-Insulator (SOI) wafer may be used instead of a silicon wafer. Since the silicon layer may be patterned before bonding, if it is sufficiently thinned and doped, it can also function as a bottom electrode that is provided below the ferroelectric layer 102.

[0153] Multiple electrode configurations may be used to orient the electrical field 107 in all spatial directions. This allows to modulate both a Transverse Electric (TE) mode and a Transverse Magnetic (TM) mode, and to potentially deal with semi-random directionality of crystals, as it may be easy to change the electrical field 107 direction.

[0154] Adding more than two electrical contacts, e.g., three or four electrodes 105 connected to at least three conductor blocks 111, to a ferroelectric active layer 102, for example, BTO, such that at least one conductor block is placed above the ferroelectric layer 102 and at least one conductor block is placed below the ferroelectric layer 102, offers considerable advantages across various integrated photonic applications. These benefits arise because multiple electrodes 105 provide finer control over the electric field distribution, often enabling enhanced performance, different functionalities, and improved design flexibility.

[0155] An electro-topical modulator 100 comprising at least three electrodes 105 may provide the following advantages over electro-topical modulators comprising only two electrodes. The versatility of the platform allows for application of the electric field 107 in any desired dimension.

[0156] FIG. 2 shows an exemplary electro-optical modulator 100 comprising four electrodes 105 according to this disclosure. In this example, two conductor blocks 108, 109 that are electrically connected to the electrodes 105a, 105b are provided on the ferroelectric layer 102 from above and two other conductor blocks 110 that are electrically connected to other electrodes 105c are below the ferroelectric layer 102.

[0157] Altering the direction of the electric field 107 may be particularly advantageous in order to maximize the modulation of both light polarizations. Moreover, BTO, along with similar ferroelectric materials, is composed of numerous electric domains with varying directions. Although the orientation of these domains is vital for maximizing electro-optical modulation, they are challenging to ascertain prior to measurement.

[0158] Therefore, the ability to apply the electric field 107 in any desired dimension is a beneficial feature.

[0159] This versatility can also facilitate the realization of devices beyond modulators, such as polarization rotators.

[0160] FIG. 3 shows another exemplary electro-optical modulator 100 according to this disclosure.

[0161] The exemplary electro-optical modulator 100 comprises a plurality of electrodes 105. The plurality of electrodes 105 may be configured to apply an electric field 107 across the ferroelectric layer 102 in one or more directions, for example, all three dimensions, based on the electrodes 105 and conductor blocks 111 configurations. In this example, three exemplary electric fields 107 are shown, one in a vertical direction and two in a horizontal direction. The electrical field 107 between the first conductor block 108 and the second conductor block 109 is applied across a significantly smaller area to increase its strength. Additionally, FIG. 3 shows a transition of an optical wave 106 from the waveguide core 104 to another waveguide core 104 and exemplary dimensions and materials of the electro-optical modulator 100.

[0162] The exemplary electrode scheme shown in FIGs. 2 and 3 allow a (near-) full flexibility over direction of the applied electric-field 107. Based on such capabilities, enhanced electro-optical modulators can be provided that, for example, allow for at least one of: dynamic field tuning, universal poling, and increasing the modulation bandwidth, which are explained below.

[0163] Dynamic field tuning: With three or more electrodes 105, it becomes possible to create spatially non-uniform electric fields across the ferroelectric layer 102. This allows precise control of the electro-optical response, such as tuning the phase shift or optimizing overlap between the optical mode and the modulated refractive index region.

[0164] Universal poling: Both c-axis and c-axis type of a ferroelectric layer, for example, BTO, can be poled. This in turn enhances the modulation strength.

[0165] Increasing the modulation bandwidth: When more than two electrodes are used, it becomes possible to decouple an RF signal, which may be used for modulation, from a DC signal, which may be used for poling the ferroelectric layer. This can allow specific sections of the modulator to be selectively poled, thereby enhancing a high-frequency response of the device. As a result, the electro-optical bandwidth of the modulators can be significantly increased.

[0166] Additionally or alternatively, improved integrated circulators or isolators using bulk or surface acoustic waves can be provided.

[0167] For example, FIG. 4c shows an optical isolator 100 or optical circulator 100 according to this disclosure. Generating, e.g., surface acoustic waves (SAW), can be used in order to break reciprocity and eventually, an optical isolator (or circulator) can be achieved. Such devices rely on an acoustic transducer, for example, the ferroelectric layer 102, which converts an electrical signal, for example, an RF input signal, to an acoustic signal, which travels across the device. The travelling acoustic wave may interact with, for example, modulate, the optical wave 106 inside the waveguide core 104. For example, this interaction can result in coupling the guided mode into another mode, but only in the propagation direction of the optical wave 106. This conversion can be used to implement an isolator or a circulator 100, as shown in FIG. 4. The third conductor block 110 is not shown in FIG. 4.

[0168] An electro-optical modulator 100 may allow for creation of both SAW and BAW. SAW and bulk acoustic waves (BAW) may each use a different type of transducers to be generated. FIG. 4a shows the cross section and top view of an interdigitated transducer (IDT) generating SAWs. FIG. 4b shows a bulk acoustic transducer. FIG. 4c shows an electro-optical modulator 100 generating both BAW and SAW with the same or different frequencies. An electrode configuration including at least three electrodes 105 and at least three conductor blocks 102 allows for formation of both types of waveguides 104 on the same platform.

[0169] The resonance frequencies of a conductor block 109 above and a conductor block below 108 the ferroelectric layer 102 can be tailored to generate acoustic waves with different propagation directions.

[0170] An electro-optical modulator 100 may enable to form stacked interdigitated transducers (IDT).

[0171] For example, FIG. 5 shows a stacked 4-electrode 105 IDT for generating SAWs. The resonance frequencies can be separately tailored. It is possible to sense and send acoustic waves simultaneously. It also allows for generation of larger orders of acoustic waves, controllable by tuning the phase difference between the RF signals which are applied to the top and bottom IDTs. The third conductor block 110 is not shown in FIG. 5.

[0172] For example, if the at least three electrodes 105 include four electrodes 105, two IDTs may be stacked on only one layer of the ferroelectric layer 102. This configuration may allow for simultaneously generating two different frequencies. Further, non-linear acoustic operations such as frequency adders can be implemented.

[0173] Additionally or alternatively, enhanced SAW can be generated. Adding an additional conductor block 108, 109, 110, e.g., when it is grounded, can significantly increase the generated surface acoustic waves. This means, that an applied RF voltage could be smaller, in order to achieve the same effect, which means reduction of power consumption. Additionally or alternatively, frequency multiplexing can be enabled. The combination of SAW and BAW enables the utilization of their distinct frequency ranges for complementary functionalities, for example, in frequency multiplexing applications. SAWs typically operate at lower frequencies, ranging from the MHz to the low GHz range. This makes them well-suited for processes requiring slower modulation or precise, localized control of optical or acoustic interactions near the surface of a material. On the other hand, BAWs can propagate at significantly higher frequencies, often reaching into the GHz range or beyond, making them ideal for applications demanding faster modulation speeds or for exciting higher-order acoustic modes that interact with more complex photonic systems.

[0174] By combining these frequency differences, integrated devices can operate with multi -frequency acoustic fields. For example, SAWs can be employed for coarse adjustments or to enable mode selection, while BAWs can provide fine-tuning or high-speed modulation of the same system. This dual -frequency operation enables additional functionality for optical or RF -photonic systems, allowing for the simultaneous processing of multiple signals or the implementation of frequency- multiplexed modulation schemes. Such systems could process multiple wavelengths or RF channels in parallel, improving the efficiency and versatility of photonic circuits. The interplay of these wave types thus enables more sophisticated functionality that neither SAWs nor BAWs could achieve alone.

[0175] The electro-optical modulator 100 may comprise or form a MEMS structure.

[0176] Incorporating more than two electrodes 105 into MEMS structures based on a ferroelectric layer 102 offers significant advantages by enabling precise control over stress, displacement, and dynamic tunability. MEMS devices fabricated on a ferroelectric layer 102, for example, BTO, may exploit its piezoelectric or ferroelectric properties for actuation, sensing, or dynamic reconfiguration. With a multi -el ectrode design, various functionalities and improvements become feasible.

[0177] A MEMS 100 device may be fabricated based on an additional undercut step to suspend the ferroelectric layer 102 and enable its movement as a result of the applied voltage / electrical signal. FIG. 6a shows using a different elements of a piezoelectric tensor of the ferroelectric layer 102 by applying an electric field in different directions. For example, a vertical electric-field applied in different directions may result in a displacement or strain in opposite directions.

[0178] FIG. 6b shows that a complex shape of the suspended structure can be engineered by placing electrodes 105 and applying voltages. For example, this may be based on predominantly d31 and d33 elements of piezoelectricity.

[0179] FIG. 6c shows an example structure 100, where an engineered displacement of a suspended cantilever can result in optical wave 106 modulation inside the waveguide core 104.

[0180] In the context of the piezoelectric tensor, dss and dsi are coefficients describing how mechanical strain is generated in a material due to an applied electric field. dss: Describes the strain in the same direction as the applied electric field (longitudinal effect). dsi: Describes the strain perpendicular to the applied electric field (transverse effect).

[0181] For example, d33 determines how much the piezoelectric materials expands / contracts in the same direction as the applied electric field, while d31 determines how much the material expands / contracts in the transverse direction of the applied electric field.

[0182] An electro-optical modulator 100 may enable multi-directional actuation. By applying voltages to three or more electrodes 105, a highly localized or multi -directional actuation can be provided. For example, with three electrodes 105, independent actuation in orthogonal directions can be achieved, allowing complex movements such as bending, twisting, or coupled displacements in multiple dimensions. For example, this provides advantages for devices such as tunable optical mirrors, lenses, or phase shifters where precise and multi -axis control is required.

[0183] An electro-optical modulator 100 may enable stress gradient control. Multi-electrode configurations enable the application of spatially varying electric fields 107, which can induce controlled stress gradients within the ferroelectric layer 102. For example, MEMS devices that rely on mechanical deformation, for example, as resonators or switches, may be improved with this capability. By fine-tuning the stress profile, the mechanical properties of the MEMS device, such as resonant frequency or deflection amplitude, can be dynamically adjusted. For example, FIG. 5 shows how a stress gradient induced by asymmetric voltage application across four electrodes 105 may alter the curvature of a ferroelectric cantilever 102.

[0184] An electro-optical modulator 100 may be a reconfigurable MEMS array. In MEMS arrays, where multiple devices are patterned on a single ferroelectric layer 102, three or more electrodes 105 per device can enable independent control of individual elements. This independence allows for the dynamic reconfiguration of the array, such as selectively tuning the positions or resonances of individual MEMS mirrors in an optical phased array. Additionally, this can enhance reliability by providing redundancy. For example, if one electrode 105a fails, the remaining electrodes 105 can partially compensate for its function.

[0185] FIG. 7 shows a method 300 according to this disclosure. The method 300 is a method of operating an integrated electro-optical modulator 100, for example, the electro-optical modulator 100 shown in FIG. 1.

[0186] The integrated electro-optical modulator 100 comprises: a cladding layer 101; a waveguide core 104 embedded in the cladding layer 101; a ferroelectric layer 102, wherein the waveguide core

[0187] 104 is separated from the ferroelectric layer 102 by the cladding layer 101; at least three electrodes

[0188] 105 comprising a first electrode 105a, a second electrode 105b, and a third electrode 105c; and at least three conductor blocks 111 comprising a first conductor block 108, a second conductor block 109, and a third conductor block 110.

[0189] The first conductor block 108 is electrically connected to the first electrode 105a, the second conductor block 109 is electrically connected to the second electrode 105b, and the third electrode 105c is electrically connected to the third conductor block 110.

[0190] The method 300 comprises a step 301 of receiving an electrical signal with the at least three electrodes 105. The method 300 further comprises a step 302 of thereby generating an electricfield 107 via the at least three conductor blocks 110 across at least a part of the ferroelectric layer 102. The method 300 further comprises a step 303 of the electric-field 107 inducing a mechanical deformation and / or changing a refractive index of the ferroelectric layer 102. The method 300 further comprises a step 304 of thereby modulating an optical wave 106 as it passes through the waveguide core 104.

[0191] The electric-field 107 comprises a component in a vertical direction and a component in a horizontal direction. The extension direction of the waveguide core 104 is perpendicular to the vertical direction and the horizontal direction. The ferroelectric layer 102 is provided above the waveguide core 104 in the vertical direction. At least one conductor block 109 of the at least three conductor blocks 111 is above the ferroelectric layer 102 in the vertical direction and at least one conductor block 108 of the at least three conductor blocks I l l is below the ferroelectric layer 102 in the vertical direction. At least one conductor block 108 of the at least three conductor blocks 111 extends to a different horizontal location than another conductor block 109 of the at least three conductor blocks 111.

[0192] FIG. 8 shows a method 400 according to this disclosure. The method 200 is a method of fabricating an integrated electro-optical modulator 100, for example, the electro-optical modulator 100 shown in FIG. 1.

[0193] The manufacturing method of the electro-optical modulator 100 according to this disclosure may be directly compatible with standard integrated photonics used in integrated photonics.

[0194] The method 400 comprises a step 401 of forming a cladding layer 101. Further, the method 400 comprises a step 402 of forming a waveguide core 104 embedded in the cladding layer 101. Further, the method 400 comprises a step 403 of forming a ferroelectric layer 102, wherein the waveguide core 104 is separated from the ferroelectric layer 102 by the cladding layer 101. Further, the method 400 comprises a step 404 of forming at least three electrodes 105 comprising a first electrode 105a, a second electrode 105b, and a third electrode 105c.

[0195] Further, the method 400 comprises a step 405 of forming at least three conductor blocks 111 comprising a first conductor block 108 electrically connected to the first electrode 105a, a second conductor block 109 electrically connected to the second electrode 105b, and a third conductor block electrically connected to the third electrode 105b. Generally, the at least three electrodes 105 are configured to receive an electrical signal thereby generating an electric-field 107 via the at least three conductor blocks 111 across at least a part of the ferroelectric layer 102, wherein the electric-field 107 induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer 102 thereby modulating an optical wave 106 as it passes through the waveguide core 104.

[0196] The electric-field 107 comprises a component in a vertical direction and a component in a horizontal direction. The extension direction of the waveguide core 104 is perpendicular to the vertical direction and the horizontal direction. The ferroelectric layer 102 is provided above the waveguide core 104 in the vertical direction. At least one conductor block 109 of the at least three conductor blocks 111 is above the ferroelectric layer 102 in the vertical direction and at least one conductor block 108 of the at least three conductor blocks I l l is below the ferroelectric layer 102 in the vertical direction. At least one conductor block 108 of the at least three conductor blocks 111 extends to a different horizontal location than another conductor block 109 of the at least three conductor blocks 111.

[0197] FIG. 9 shows an electro-optical modulator 100 according to this disclosure.

[0198] The integrated electro-optical modulator 100 comprises: a cladding layer 101; a waveguide core 104 embedded in the cladding layer 101; a ferroelectric layer 102, wherein the waveguide core

[0199] 104 is separated from the ferroelectric layer 102 by the cladding layer 101; at least two electrodes

[0200] 105 comprising a first electrode 105a and a second electrode 105b; a first conductor layer 108; and a second conductor layer 109 provided on the ferroelectric layer 102 or on a second buffer layer 103b that is provided on the ferroelectric layer 102.

[0201] The first conductor layer 108 is provided on the ferroelectric layer 102 or on a first buffer layer 103a that is provided on the ferroelectric layer 102, which is indicated by the dashed line surrounding the optional first buffer layer 103a in FIG. 9. The second conductor layer 109 is provided on the ferroelectric layer 102 or on a second buffer layer 103b that is provided on the ferroelectric layer 102, which is indicated by the dashed line surrounding the optional second buffer layer 103b in FIG. 9. The first buffer and the second buffer layer may or may not be the same layer. The first conductor layer 108 is electrically connected to the first electrode 105a and the second conductor layer 109 is electrically connected to the second electrode 105b, which is indicated by the dashed-dotted arrows in FIG. 9.

[0202] The at least two electrodes 105 are configured to receive an electrical signal thereby generating an electric-field 107 via the first conductor layer 108 and the second conductor layer 109 across at least a part of the ferroelectric layer 102, wherein the electric-field 107 changes a refractive index of the ferroelectric layer 102 thereby modulating an optical wave 106 as it passes through the waveguide core 104.

[0203] FIG. 10 shows a cross section of an exemplary electro-optical modulator 100 according to this disclosure.

[0204] The cross section may be perpendicular to the extension direction of the waveguide core 104.

[0205] FIG. 10 shows a first and a second conductor layer 108, 109, for example, doped silicon, above a ferroelectric layer 102, for example, BTO, and waveguide core 104, for example, Silicon Nitride (SiN). The first and second conductor layer 109 may be used as electrodes or as an extension of the electrodes 105a, 105b. In comparison with metal electrodes, doped silicon may have 4 to 7 orders of magnitude less optical loss, with exact value depending on the doping level of silicon as well as the type of the metal used.

[0206] The first conductor layer 108 may be separated, in an area above the waveguide core 104, from the second conductor layer 109 by a cladding layer 101. Thus, an electric-field 107 can be efficiently and closely applied across the ferroelectric layer 102.

[0207] FIG. I la shows an optical mode according to this disclosure.

[0208] The exemplary mode was calculated at 1550 nm.

[0209] FIG. 1 lb shows an E-field 107 inside an electro-optical modulator 100 according to this disclosure. FIG. I la shows the calculated optical mode inside the modulator device. In this exemplary simulation, the distance between the two silicon electrodes was set to 1 pm. This gives an approximately 10-times improvement in terms of electrical field 107 enhancement compared to metal electrodes which are typically approximately 10 pm apart to avoid excessive optical loss. The calculated electrical field 107 shown in FIG. 3b in x-direction (electrical field 107 orientation as shown in FIG. 10) is based on 10V applied voltage. A high modulation efficiency of 0.36 V.cm, and only 1.8 dB loss can be achieved to achieve a full extinction (7t-phase shift).

[0210] The calculations associated with FIGs. I la and 1 lb are based on an electro-optical modulator 100 as shown in FIG. 10.

[0211] FIG. 12 shows a cross section of an exemplary electro-optic modulator according to this disclosure.

[0212] For example, the waveguide core 104 may be a SiN layer.

[0213] The first conductor layer 108 may be separated, in an area above the waveguide core 104, from the second conductor layer 109 by an insulting layer. Thus, an electric-field 107 can be efficiently and closely applied across the ferroelectric layer 102.

[0214] The exemplary electro-optic modulator shown FIG. 12 is, for example, the electro-optic modulator shown FIG. 10 additionally comprising the insulting layer.

[0215] FIG. 13a shows an optical mode according to this disclosure.

[0216] FIG. 13b shows a shows an E-field 107 inside an electro-optical modulator 100 according to this disclosure.

[0217] The modulation efficiency of the electro-optical modulator 100 according to this disclosure may be improved compared to conventional electro-optical modulators 100. For example, the figure- of-merit VTPL of the electro-optical modulator 100 may be 0.24 Vcm for an intrinsic region width of 1 pm, and a 1.7 dB loss for the device length of Ln = 240 pm. The calculations associated with FIGs. 13a and 13b are based on an electro-optical modulator 100 as shown in FIG. 12.

[0218] The electro-optic modulator may have at least one of the following advantages: 1) an increased mode overlap with the ferroelectric layer 102, 2) up to three modulation effects can be combined: plasma dispersion effect (carriers in Si), Kerr effect (intrinsic region in Si), and Pockels effect (in ferroelectric layer 102), and 3) post-fabrication control over the intrinsic width may be enabled, which may allow for trading off the bandwidth and modulation depth using reverse DC bias.

[0219] FIGs. 14 to 17 show a first process flow of fabricating an exemplary electric-optic modulator according to this disclosure.

[0220] FIGs. 14, 18, 19, and 20 show a second process flow of fabricating an exemplary electric-optic modulator according to this disclosure.

[0221] Further, FIGs. 14 to 20 show exemplary materials that may be used for the electric-optic modulator. Alternatively, other materials may be used.

[0222] FIG. 14 shows steps for fabricating an electric-optic modulator according to this disclosure.

[0223] FIG. 14a shows providing a SOI wafer, performing Si patterning (+deep etch), performing oxide deposition, and performing planarization.

[0224] FIG. 14b shows performing low pressure chemical vapor deposition (LPCVD) SiN growth, for example, at approximately 800°C, and planarization.

[0225] FIG. 14c shows BTO waferbonding.

[0226] For example, to generate a ferroelectric wafer, a ferroelectric layer 102 may be grown on a buffer layer 103, which may be provided on an initial layer 111, for example, a silicon substrate.

[0227] FIG. 15 shows steps of the first process flow for fabricating an electric-optic modulator according to this disclosure. FIG. 15a shows removal of at least a part of the ferroelectric wafer.

[0228] The removal may comprise at least one of: buffer layer removal, grinding, chemical mechanical planarization (CMP), and annealing.

[0229] FIG. 15b shows BTO patterning, for example, by ion beam etching and milling.

[0230] FIG. 15c shows oxide deposition and BTO planarization.

[0231] FIG. 16 shows steps of the first process flow for fabricating an electric-optic modulator according to this disclosure.

[0232] FIG. 16a shows an intermediate structure for fabricating an electric-optic modulator. For example, the intermediate structure as shown in FIG. 15c.

[0233] FIG. 16b shows a-Si deposition, for example, at 400-450°C.

[0234] FIG. 16c shows silicon patterning.

[0235] FIG. 16d shows performing ion implantations.

[0236] FIG. 16e shows oxide deposition and CMP.

[0237] FIG. 16f shows Ge window etching, Ge epitaxial growth, CMP, and doping, for example, at 550 °C.

[0238] FIG. 17 shows steps of the first process flow for fabricating an electric-optic modulator according to this disclosure.

[0239] FIG. 17a shows via definition.

[0240] FIG. 17b shows metal deposition, and interconnect formation. FIG. 18 shows steps of the second process flow for fabricating an electric-optic modulator according to this disclosure. The steps shown in FIG. 18 may be performed after the steps shown in FIG. 14.

[0241] FIG. 18a shows removal of at least a part of the ferroelectric wafer.

[0242] The removal may comprise at least one of: grinding, and CMP.

[0243] FIG. 18b shows performing ion implantation.

[0244] Further, rapid thermal annealing, for example, at approximately 1050°C and for 5s, may be performed.

[0245] FIG. 18c shows silicon patterning, buffer layer removal, and BTO annealing.

[0246] FIG. 19 shows steps of the second process flow for fabricating an electric-optic modulator according to this disclosure.

[0247] FIG. 19a shows buffer layer removal and BTO patterning.

[0248] FIG. 19b shows oxide deposition and CMP.

[0249] FIG. 19c shows Ge window etching, Ge epitaxial growth, CMP, and doping, for example, at 550 °C.

[0250] FIG. 20 shows steps of the second process flow for fabricating an electric-optic modulator according to this disclosure.

[0251] FIG. 20a shows via definition.

[0252] FIG. 20b shows metal deposition, and interconnect formation. The first and / or the second process flow may form a general process flow of realizing a photonic platform including, for example, Si / SiN / BTO.

[0253] The manufacturing method of the electro-optical modulator 100 according to this disclosure may be directly compatible with standard integrated photonics used in integrated photonics.

[0254] FIG. 21 shows a method 200 according to this disclosure. The method 200 is a method of fabricating an integrated electro-optical modulator 100, for example, the electro-optical modulator 100 shown in FIG. 1. The method 200 comprises a step 201 of forming a cladding layer 101. Further, the method 200 comprises a step 202 of forming a waveguide core 104 embedded in the cladding layer 101. Further, the method 200 comprises a step 203 of forming a ferroelectric layer 102, wherein the waveguide core 104 is separated from the ferroelectric layer 102 by the cladding layer 101. Further, the method 200 comprises a step 204 of forming at least two electrodes 105 comprising a first electrode 105a and a second electrode 105b. Further, the method 200 comprises a step 205 of forming a first conductor layer 108 provided on the ferroelectric layer 102 or on a buffer layer 103 that is provided on the ferroelectric layer 102, wherein the first conductor layer

[0255] 108 is electrically connected to the first electrode 105a. Further, the method 200 comprises a step 206 of forming a second conductor layer 109 provided on the ferroelectric layer 102 or on the buffer layer 103, wherein the second conductor layer 109 is electrically connected to the second electrode 105b.

[0256] Generally, the at least two electrodes 105 are configured to receive an electrical signal thereby generating an electric-field 107 via the first conductor layer 108 and the second conductor layer

[0257] 109 across at least a part of the ferroelectric layer 102, wherein the electric-field 107 changes a refractive index of the ferroelectric layer 102 thereby modulating an optical wave 106 as it passes through the waveguide core 104.

[0258] The controller may be a processor.

[0259] Generally, the processor may be configured to perform, conduct or initiate the various operations of the electro-optic modulator 100 described herein. The processor may comprise hardware and / or may be controlled by software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The electro-optic modulator 100 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor, in particular under control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor, causes the various operations of the electro-optic modulator 100 to be performed. In one embodiment, the electro-optic modulator 100 may comprises one or more processors and a non-transitory memory connected to the one or more processors. The non- transitory memory may carry executable program code which, when executed by the one or more processors, causes the electro-optic modulator 100 to perform, conduct or initiate the operations or methods described herein.

[0260] In the claims as well as in the description of this disclosure, the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

Claims1. An integrated electro-optical modulator (100) comprising: a cladding layer (101); a waveguide core (104) embedded in the cladding layer (101); a ferroelectric layer (102), wherein the waveguide core (104) is separated from the ferroelectric layer (102) by the cladding layer (101); at least three electrodes (105) comprising a first electrode (105a), a second electrode (105b), and a third electrode (105c); and at least three conductor blocks (111) comprising a first conductor block (108) electrically connected to the first electrode (105a), a second conductor block (109) electrically connected to the second electrode (105b), and a third conductor block (110) electrically connected to the third electrode (105c); wherein the at least three electrodes (105) are configured to receive an electrical signal thereby generating an electric-field (107) via the at least three conductor blocks (111) across at least a part of the ferroelectric layer (102), wherein the electric-field (107) induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer (102) thereby modulating an optical wave (106) as it passes through the waveguide core (104), wherein the extension direction of the waveguide core (104) is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer (102) is provided above the waveguide core (104) in the vertical direction, wherein at least one conductor block (109) of the at least three conductor blocks (111) is above the ferroelectric layer (102) in the vertical direction and at least one conductor block (108) of the at least three conductor blocks (111) is below the ferroelectric layer (102) in the vertical direction, wherein at least one conductor block (108) of the at least three conductor blocks (111) extends to a different horizontal location than another conductor block (109) of the at least three conductor blocks (111), and wherein the electric-field (107) comprises a component in the vertical direction and a component in the horizontal direction.

2. The electro-optical modulator (100) according to claim 1, wherein the first conductor block (108) is provided on the ferroelectric layer (102), or on a first buffer layer (103a) that is provided on the ferroelectric layer (102), wherein the second conductor block (109) is provided on the ferroelectric layer (102), or on a second buffer layer (103b) that is provided on the ferroelectric layer (102).

3. The electro-optical modulator (100) according to claim 1 or 2, wherein the at least three conductor blocks (111) are each a non-metal layer and / or an optically transparent layer.

4. The electro-optical modulator (100) according to any one of the preceding claims, wherein the at least three electrodes (105) are arranged to not generate an electric-field via the at least three conductor blocks (111) across any other ferroelectric layer of the electro- optical modulator (100).

5. The electro-optical modulator (100) according to any one of the preceding claims, wherein a ratio of the component in the horizontal direction of the electric-field (107) and the component in the vertical direction of the electric-field (107) is in a range of 0.01 to 100 across the ferroelectric layer (102).

6. The electro-optical modulator (100) according to any one of the preceding claims, wherein the electrical signal comprises a first voltage signal, a second voltage signal, and a third voltage signal, wherein the first electrode (105a) is configured to receive the first voltage signal, the second electrode (105b) is configured to receive the second voltage signal, and the third electrode (105c) is configured to receive the third voltage signal, wherein the first voltage signal, the second voltage signal, and the third voltage signal are direct current voltage signals that are simultaneously received by respectively the first electrode (105a), the second electrode (105b), and the third electrode (105c), and / or wherein the first voltage signal, the second voltage signal, and the third voltage signal are alternating current voltage signals that have the same frequency and respectively a phasedifference to each other.

7. The electro-optical modulator (100) according to any one of the preceding claims, wherein the second conductor block (109) is electrically isolated from the first conductor block (108) and from the third conductor block (110), and wherein the third conductor block (110) is electrically isolated from the first conductor block (108).

8. The electro-optical modulator (100) according to any one of the preceding claims, wherein the ferroelectric layer (102) has a Pockels tensor containing at least one non-zero element nj where i^j.

9. The electro-optical modulator (100) according to any one of the preceding claims, wherein the cladding layer (101) provides electrical insulation between the first conductor block (108) and the second conductor block (109); or wherein the electro-optical modulator (100) further comprises an insulating layer provided on the ferroelectric layer (102) or on a third buffer layer (103) that is provided on the ferroelectric layer (102), wherein the insulating layer provides electrical insulation between the first conductor block (108) and the second conductor block (109).

10. The electro-optical modulator (100) according to any one of the preceding claims, wherein the ferroelectric layer (102) is arranged between the waveguide core (104) and at least one of: the first conductor block (108), the second conductor block (109), and the insulating layer if present.

11. The electro-optical modulator (100) according to any one of the preceding claims, wherein the electric-field (107) generated via the first conductor block (108) and the second conductor block (109) extends predominantly in the horizontal direction.

12. The electro-optical modulator (100) according to any one of the preceding claims, wherein the electric-field (107) is further generated via the third conductor block to provide an additional component of the electric field (107) that extends predominately in the vertical direction.

13. The electro-optical modulator (100) according to any one of the preceding claims,wherein the ferroelectric layer (102) is configured to generate an acoustic wave based on the piezoelectric effect induced by the electric-field, wherein the acoustic wave modulates the optical wave (106) as it passes through the waveguide core (104).

14. The electro-optical modulator (100) according to claim 13, wherein the ferroelectric layer (102) is configured to generate two or more acoustic waves having different frequencies, due to the at least three conductor blocks (111) having different resonance frequencies.

15. The electro-optical modulator (100) according to claim 14, wherein the two or more acoustic waves are configured to interfere to set a propagation direction of the acoustic wave.

16. The electro-optical modulator (100) according to any one of the preceding claims, wherein the at least three electrodes (105) are configured to induce independent actuations in orthogonal directions in the ferroelectric layer (102) based on the electric field (107) generated via the at least three conductor blocks (111), to modulate the optical wave (106) as it passes through the waveguide core (104) based on said independent actuations.

17. The electro-optical modulator (100) according to any one of the preceding claims, wherein the electro-optical modulator (100) is a micro-electromechanical system array and comprises a plurality of individual devices patterned on the ferroelectric layer (102), wherein the at least three electrodes (105) are configured to individually control the plurality of individual devices based on the electric-field (107) generated via the at least three conductor blocks (111).

18. A method of operating an integrated electro-optical modulator (100), wherein the electro- optical modulator (100) comprises: a cladding layer (101); a waveguide core (104) embedded in the cladding layer (101); a ferroelectric layer (102), wherein the waveguide core (104) is separated from the ferroelectric layer (102) by the cladding layer (101); at least three electrodes (105) comprising a first electrode (105a), a second electrode (105b), and a third electrode (105c); andat least three conductor blocks (111) comprising a first conductor block (108) electrically connected to the first electrode (105a), a second conductor block (109) electrically connected to the second electrode (105b), and a third conductor block (110) electrically connected to the third electrode (105c); wherein the method comprises receiving an electrical signal with the at least three electrodes (105) thereby generating an electric-field (107) via the at least three conductor blocks (111) across at least a part of the ferroelectric layer (102), wherein the electric-field (107) induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer (102) thereby modulating an optical wave (106) as it passes through the waveguide core (104), wherein the extension direction of the waveguide core (104) is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer (102) is provided above the waveguide core (104) in the vertical direction, wherein at least one conductor block (109) of the at least three conductor blocks (111) is above the ferroelectric layer (102) in the vertical direction and at least one conductor block (108) of the at least three conductor blocks (111) is below the ferroelectric layer (102) in the vertical direction, wherein at least one conductor block (108) of the at least three conductor blocks (111) extends to a different horizontal location than another conductor block (109) of the at least three conductor blocks (111), and wherein the electric-field (107) comprises a component in the vertical direction and a component in the horizontal direction.

19. A method of fabricating an integrated electro-optical modulator (100), the method comprising: forming a cladding layer (101); forming a waveguide core (104) embedded in the cladding layer (101); forming a ferroelectric layer (102), wherein the waveguide core (104) is separated from the ferroelectric layer (102) by the cladding layer (101); forming at least three electrodes (105) comprising a first electrode (105a), a second electrode (105b), and a third electrode (105c); and forming at least three conductor blocks (111) comprising a first conductor block (108) electrically connected to the first electrode (105a), a second conductor block (109) electricallyconnected to the second electrode (105b), and a third conductor block (110) electrically connected to the third electrode (105c); wherein the at least three electrodes (105) are configured to receive an electrical signal thereby generating an electric-field (107) via the at least three conductor blocks (111) across at least a part of the ferroelectric layer (102), wherein the electric-field (107) induces a mechanical deformation and / or changes a refractive index of the ferroelectric layer (102) thereby modulating an optical wave (106) as it passes through the waveguide core (104), wherein the extension direction of the waveguide core (104) is perpendicular to a vertical direction and a horizontal direction, wherein the ferroelectric layer (102) is provided above the waveguide core (104) in the vertical direction, wherein at least one conductor block (109) of the at least three conductor blocks (111) is above the ferroelectric layer (102) in the vertical direction and at least one conductor block (108) of the at least three conductor blocks (111) is below the ferroelectric layer (102) in the vertical direction, wherein at least one conductor block (108) of the at least three conductor blocks (111) extends to a different horizontal location than another conductor block (109) of the at least three conductor blocks (111), and wherein the electric-field (107) comprises a component in the vertical direction and a component in the horizontal direction.

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