A slot waveguide modulator

Abstract

This document describes a slot waveguide modulator that utilizes both silicon photonics and electro-optic materials to achieve high bandwidth, high efficiency and low optical loss. The slot waveguide modulator has an electro-optic layer that is sandwiched between two mode stopper layers, where the mode stopper layers are formed in waveguide layers having a refractive index that is higher than the refractive index of the electro-optic layer.

Description

A SLOT WAVEGUIDE MODULATORCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to Singapore patent application no. 10202400812S which was filed on 21 March 2024, the contents of which are hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] This application relates to a slot waveguide modulator that utilizes both silicon photonics and electro-optic materials to achieve high bandwidth, high efficiency and low loss.BACKGROUND

[0003] Optical modulators play a pivotal role in applications such as telecommunications, data centers, and high-performance computing and as such, are essential components in highspeed and reliable data communication systems. Presently, demand for data transmission continues to grow, fueled by the increasing adoption of emerging technologies such as the Internet of Things (loT), autonomous vehicles, and artificial intelligence (Al) which require high-speed and reliable communication infrastructure to support their data-intensive operations. This increasing demand underscores the need for the development of high-speed, high-efficiency optical modulators to ensure seamless operation and maximize the potential of these technologies.

[0004] Furthermore, the advancement of optical modulators with high efficiency, low insertion loss, and broad bandwidth is not only critical for meeting current industry requirements but also for addressing the escalating challenges associated with future data transmission needs.

[0005] The two main types of optical modulators known to those skilled in the art are silicon photonic modulators based on the frcc-carricr dispersion effect and electro-optic (EO) modulators that utilize the Pockels effect. Both technologies have been extensively researched over the past decades with each offering distinct advantages and limitations.

[0006] Silicon photonic modulators leverage well-established Complementary Mctal- Oxide-Semiconductor (CMOS) manufacturing infrastructure. This allows such modulators to be cost-effectively mass produced and seamlessly integrated with silicon photonic integrated circuits (PICs) and potentially with electrical circuits. While silicon photonic modulators have demonstrated promising performance, achieving an optimal balance of key performance metrics such as efficiency, bandwidth, and insertion loss simultaneously remains a significant problem faced by those skilled in the ait.

[0007] Conversely, electro-optic (EO) modulators inherently offer higher modulation bandwidth as compared to silicon photonic modulators. However, the efficiency of such EO modulators is often compromised by design trade-offs, particularly regarding electrode spacing, which affects the strength of the applied electric field. Additionally, EO modulators tend to be more expensive to manufacture and exhibit a lower fabrication yield compared to silicon photonic modulators, which benefit from 300mm wafer-scale fabrication.

[0008] Another major challenge associated with EO modulators is the integration and packaging process of such EO modulators. The additional coupling and packaging required for interfacing EO modulators with silicon PICs introduce further complexity, potentially affecting the system link budget and overall performance.

[0009] As a result, those skilled in the art are constantly looking for a modulator that can take advantage of the benefits of silicon photonic modulators and EO modulators without the drawbacks mentioned above, allowing the modulator to achieve high bandwidth, high efficiency and low loss.SUMMARY

[0010] In one aspect, the present application discloses a slot waveguide modulator. The modulator comprises a first layer having a first refractive index being disposed on a substrate, a first mode stopper layer formed adjacent to the first layer and a second mode stopper layer formed adjacent to the first layer such that the first layer is positioned between the first and second mode stopper layers, wherein the first and second mode stopper layers are formed on the substrate. An electro-optic layer having a second refractive index is disposed on the first layer, and the first and second mode stopper layers. A second layer having the first refractiveindex is disposed on the electro-optic layer, and a third mode stopper layer is formed adjacent to the second layer and a fourth mode stopper layer is formed adjacent to the second layer such that the second layer is positioned between the third and fourth mode stopper layers, wherein the third and fourth mode stopper layers are formed on the electro-optic layer. In embodiments of this aspect, the second refractive index is lower than the first refractive index such that optical field confinement can be achieved within the electro-optic layer, and the first layer is electrically connected to a first electrode, and the second layer is electrically connected to a second electrode such that an electric field modulates a refractive index of the electro-optic layer when a potential difference exists between the first and second electrodes.

[0011] Tn another embodiment of the one aspect, the modulator further includes an input mode coupler coupled to an input end of the electro-optic layer, and an output mode coupler coupled to an output end of the electro-optic layer. In embodiments of this aspect, the input mode coupler is configured to convert a mode of optical signals received from a bus waveguide and to couple the converted optical signals into the electro-optic layer, and the output mode coupler is configured to receive modulated optical signals from the electro-optic layer.

[0012] In another embodiment of the one aspect, a push-pull Mach Zehnder modulator is disclosed. The Mach Zehnder modulator includes an input waveguide for receiving an optical signal, a Y-branch splitter coupled to the input waveguide for splitting the optical signal received by the input waveguide into a slot waveguide modulator configured according to the one aspect of this embodiment and a branch waveguide, whereby an input mode coupler is provided between the Y-branch splitter and the slot waveguide modulator to convert a mode of the optical signal and couple the converted optical signal into the electro-optic layer of the slot waveguide modulator, a recombiner for recombining modulated optical signals received from electro-optic layer of the slot waveguide modulator with optical signals received from the branch waveguide, and an output waveguide for emitting the recombined optical signals.

[0013] In another aspect, the present application discloses a method for forming a slot waveguide modulator. The disclosed method comprises the steps of forming a first layer having a first refractive index on a substrate, forming a first mode stopper layer adjacent to the first layer and forming a second mode stopper layer adjacent to the first layer such that the first layer is positioned between the first and second mode stopper layers, wherein the first andsecond mode stopper layers arc formed on the substrate. The method then includes the step of forming an electro-optic layer having a second refractive index on the first layer, and the first and second mode stopper layers, forming a second layer having the first refractive index on the electro-optic layer, forming a third mode stopper layer adjacent to the second layer and forming a fourth mode stopper layer adjacent to the second layer such that the second layer is positioned between the first and second mode stopper layers, wherein the third and fourth mode stopper layers are formed on the electro-optic layer. In embodiments of this another aspect, the second refractive index is lower than the first refractive index such that optical field confinement can be achieved within the electro-optic layer. The first layer is electrically connected to a first electrode, and the second layer is electrically connected to a second electrode such that an electric field modulates a refractive index of the electro-optic layer when a potential difference exists between the first and second electrodes.

[0014] In another embodiment of this another aspect, the disclosed method further comprises the steps of coupling an input mode coupler to an input end of the clcctro-optic layer and coupling an output mode coupler to an output end of the electro-optic layer. In embodiments of this another aspect, the input mode coupler is configured to convert a mode of optical signals received from a bus waveguide and to couple the converted optical signals into the electro-optic layer, and the output mode coupler is configured to receive modulated optical signals from the electro-optic layer.

[0015] In yet another embodiment of this another aspect, a method for forming a push-pull Mach Zehnder modulator is disclosed. The disclosed method comprises the steps of providing an input waveguide for receiving an optical signal, coupling a Y-branch splitter to the input waveguide for splitting the optical signal received by the input waveguide into a slot waveguide modulator formed according to the method disclosed in accordance with embodiments of this another aspect, and into a branch waveguide, whereby an input mode coupler is formed between the Y-branch splitter and the slot waveguide modulator to convert a mode of the optical signal and to couple the converted optical signal into the clcctro-optic layer of the slot waveguide modulator, providing a recombiner for recombining modulated optical signals received from electro-optic layer of the slot waveguide modulator with optical signals received from the branch waveguide, and coupling an output waveguide to the recombiner for emitting the recombined optical signals.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various embodiments of the present disclosure are described below with reference to the following drawings:Figure 1A illustrates a cross-sectional view of an embodiment of a slot waveguide modulator in accordance with embodiments of the present disclosure;Figure IB illustrates a cross-sectional view of another embodiment of the slot waveguide modulator in accordance with embodiments of the present disclosure;Figure 2A illustrates a cross-sectional view of another embodiment of a slot waveguide modulator with electrodes in accordance with embodiments of the present disclosure;Figure 2B illustrates a cross-sectional view of yet another embodiment of a slot waveguide modulator with electrodes in accordance with embodiments of the present disclosure;Figure 3 illustrates a perspective view of an embodiment of a slot waveguide modulator with input and output mode couplers being coupled to the slot waveguide modulator in accordance with embodiments of the present disclosure;Figure 4 illustrates a cross-sectional view of yet another embodiment of a slot waveguide modulator in accordance with embodiments of the present disclosure;Figure 5 illustrates a top view of the slot waveguide modulator shown in Figure 4;Figure 6 illustrates a schematic and corresponding simulation result of a mode converter including a waveguide and a mode coupler in accordance with embodiments of the disclosure; Figure 7 illustrates the simulation result of an optical mode profile and an electrical field profile in a slot waveguide modulator in accordance with embodiments of the disclosure;Figure 8a illustrates a plot showing the simulated efficiency of the slot waveguide modulator in accordance with embodiments of the disclosure;Figure 8b illustrates a plot showing the simulated loss of the slot waveguide modulator in accordance with embodiments of the disclosure;Figure 8c illustrates a plot showing the simulated bandwidth of the slot waveguide modulator in accordance with embodiments of the disclosure; andFigure 9 illustrates a flowchart that sets out the process or method for forming the slot waveguide modulator in accordance with embodiments of the disclosure.DETAILED DESCRIPTION

[0017] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0018] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements. The term “and / or” includes any and all combinations of one or more of the associated listed items.

[0019] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0020] As used herein, “comprising” means including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0021] As used herein, “consisting of’ means including, and limited to, whatever follows the phrase “consisting of’. Thus, use of the phrase “consisting of’ indicates that the listed elements arc required or mandatory, and that no other elements may be present.

[0022] It should be noted that although the terms first, second and third are used herein to describe various elements, these elements should not be limited by these terms as these terms are meant to only distinguish one element from another element. Thus, the first element described herein could be termed as a second clement without departing from this disclosure.

[0023] In the context of various embodiments, the term “surround” means to enclose something completely to form a barrier around it Thus, the use of the term “surround” indicates that something is on all sides of another thing.

[0024] In the context of various embodiments, the term “disposed on" relates to the placement or deposition of one material or layer onto the surface of another and may involve one or more types of deposition techniques.

[0025] In the context of various embodiments, the tenn “around” or “adjacent” means to be in the proximity or location of something and does not necessarily mean that two objects have to be in contact.

[0026] In the context of various embodiments, the directional terms mentioned herein, such as “above” and “below” or “upper” and “lower” refer to directions as described with reference to the drawings. Therefore, the directional terms arc only used for illustration and arc not meant to limit the present disclosure.

[0027] As used herein, a “layer” refers to a material portion including a region having a particular thickness. The layer may extend over the entirety of the structure or may cover only part of the structure as defined in the description. For example, a layer may be located between two horizontal planes; may be located between, or at, a top surface and a bottom surface of the structure. The layer may also extend horizontally, vertically, and / or along the surface of the structure.

[0028] Additionally, for the sake of brevity, extensive explanations of conventional techniques of fabricating semiconductor devices and integrated circuits are not described in detail herein. The tasks and processes described herein may also be integrated into a more comprehensive procedure with extra steps of features that are not elaborated upon in this document. Specifically, certain processes of fabricating semiconductor devices arc well known to one skilled in the art hence, such processes will be omitted entirely.

[0029] A slot waveguide modulator that integrates a modulation component into a silicon photonic platform thereby combining the advantages of silicon (Si) and electro-optic (EO)materials is disclosed in this document. The modulation component comprises two key sections: a mode converter section, which facilitates efficient coupling of light into and out of the modulation region, and a Scmiconductor-Oxidc-Scmiconductor Capacitor (SOSCAP) structured slot waveguide modulation section, designed to enable high-speed, high-efficiency optical phase modulation.

[0030] Figure 1A illustrates a cross-sectional view of a slot waveguide modulator in accordance with embodiments of the present disclosure. In particular, slot waveguide modulator 100 is built upon substrate 102, which serves as the foundational layer for slot waveguide modulator 100. In embodiments of the disclosure, substrate 102 may comprise silicon, silicon dioxide or any other similar type of dielectric. Waveguide layer 104 is then formed on substrate 102, followed by mode stopper layer 106a being formed on substrate 102 adjacent to waveguide layer 104 and mode stopper layer 106b being formed on substrate 102 adjacent to waveguide layer 104 such that waveguide layer 104 is positioned between mode stopper layers 106a and 106b. Electro-optic (EO) layer 108 is then formed on waveguide layer 104 and mode stopper layers 106a and 106b. Waveguide layer 112 is then formed on EO layer 108, directly above waveguide layer 104. Mode stopper layer 110a is subsequently formed on EO layer 108, adjacent to waveguide layer 112, and mode stopper layer 110b is formed on EO layer 108, adjacent to waveguide layer 112 such that waveguide layer 112 is positioned between mode stopper layers 110a and 110b. It should be noted that a part of EO layer 108 is sandwiched between waveguide layers 104 and 112 as illustrated in Figure 1 A.

[0031] In embodiments of the disclosure, waveguide layers 104 and 1 12 may comprise of materials that have a higher refractive index compared to the refractive index of the materials used in EO layer 108, thereby forming a slot waveguide region where the optical field is strongly confined within EO layer 108. This high-index / low-index / high-index configuration enables the generation of a slot mode, ensuring that the majority of the optical power remains within EO layer 108. In embodiments of the disclosure, waveguide layers 104 and 112 may comprise, but arc not limited to, polycrystallinc silicon (Poly-Si) or silicon (Si) which may have a refractive index of about 3.45 for a wavelength of 1550 nanometers, while EO layer 108 may comprise ferroelectric materials such as lithium niobate (LiNbOs), barium titanate (BTO) and scandium aluminum nitride (ScAlN) which each may have a refractive of about 2.2, 2.4 and 2.1, respectively for a wavelength of 1550 nanometers. Waveguide layers 104 and 112may each have a thickness that is less or equal to 120 nanometers, and EO layer 108 may have a thickness that is less or equal to 100 nanometers.

[0032] As can be seen from Figure 1 A, mode stopper layers 106a, 106b, 110a and 110b are formed adjacent to the higher-index waveguide layers 104 and 112, such that parts of the lower- index EO layer 108 arc sandwiched between mode stopper layers 106a, 106b, 110a and 110b to prevent optical mode leakage from EO layer 108 into the surrounding silicon regions. In embodiments of the disclosure, mode stopper layers 106a, 106b, 110a and 110b may be formed using photonic crystal structures, a slab waveguide layer or a subwavelength grating waveguide layer. These mode stopper layers assist to maintain the slot mode by confining the optical field within EO layer 108 and this reduces optical loss while stabilizing the profile of the mode.

[0033] In embodiments of the disclosure, mode stopper layers 106a, 106b, 110a and 110b may be designed to be electrically conductive by either using a photonic crystal having a periodic structure of heavily doped semiconductor materials, such as n-type or p-type silicon or heavily doped semiconductor mode stoppers. The periodic arrangement of such a photonic structure creates a photonic bandgap, effectively confining the optical mode within the slot waveguide and preventing mode leakage into adjacent waveguide layers 104 and 112. Simultaneously, the higher doping concentration of the photonic crystal structure or the semiconductor mode stoppers ensure that it is conductive, allowing mode stopper layers 106a, 106b, 110a and 110b to act as part of the electrode configuration in slot waveguide modulator 100.

[0034] It should be noted that metal electrodes may not be placed too close to the waveguide in which optical signals arc being transmitted and / or modulated, specifically near EO layer 108, as metal electrodes tend to inherently introduce high optical losses, which can degrade the transmission efficiency of the optical signals. Conversely, if the electrodes are positioned too far away, this results in the electric field dissipating into surrounding insulating materials, e.g. SiO2, effectively reducing the modulation efficiency of slot waveguide modulator 100. To address this challenge and to ensure that slot waveguide modulator 100 is able to achieve high efficiency, waveguide layers 104 and 112 are doped to be conductive, allowing waveguide layers 104 and 112 to function as part of the electrode structure while maintaining direct contact with EO layer 108. Through this approach, the effective spacing between the conductive layersmay be reduced to sub- 120 nanometers, thereby significantly enhancing the electric field strength within the EO material, which directly improves modulation efficiency. The detailed steps for making waveguide layers 104 and 112 electrically conductive through the introduction of dopants are omitted for brevity as such steps are known to those skilled in the art.

[0035] In embodiments of the disclosure, cladding layers 114a and 114b may be formed on cither side of EO layer 108 as illustrated in Figure IB. Cladding layers 114a and 114b, positioned on either side of EO layer 108, work in conjunction with mode stopper layers 106a, 106b, 110a and 110b to enhance optical confinement by preventing light leakage from EO layer 108 into surrounding regions. Cladding layers 1 14a and 1 14b may comprise of low-refractive- index materials (relative to EO layer 108), such as silicon dioxide (SiO2), silicon oxynitride (SiON), and aluminum oxide (AI2O3). Specifically, cladding layer 114a may be disposed on mode stopper layer 106a adjacent to EO layer 108 such that cladding layer 114a is positioned between mode stopper layer 106a and mode stopper layer 110a. Similarly, cladding layer 114b may be formed on mode stopper layer 106b adjacent to EO layer 108 such that cladding layer 114b is positioned between mode stopper layer 106b and mode stopper layer 110b.

[0036] A cross-sectional view of a slot waveguide modulator in which metal electrodes are provided or formed on waveguide layers 104 and 112 in accordance with embodiments of the disclosure is illustrated in Figure 2A. As can be seen, metal electrode 202 is formed on waveguide layer 112 while metal electrode 204 is formed on waveguide layer 104. It can be seen that the electrode 204 is formed on a part of first layer 104 that is away from a part of first layer 104 that is positioned between mode stopper layers 106a and 106b. Similarly, electrode 202 is formed on a part of second layer 1 12 that is away from a part of second layer 1 12 that is positioned between mode stopper layers 110a and 110b. In embodiments of the disclosure, metal electrodes 202 and 204 may comprise bulk electrodes made of gold (Au), silver (Ag), aluminum (Al) or any similar metal that has high electrical conductivity.

[0037] A cross-sectional view of another embodiment of the slot waveguide modulator in which parts of mode stoppers 126 and 120 extend from substrate 102 through waveguide layers 104 and 112 and EO layer 108 is illustrated in Figure 2B. This arrangement assists in the further suppression of mode leakage into the surrounding high-index regions, i.e., waveguide layers104 and 112, while maintaining a strong slot mode within EO layer 108. Further, the presence of these extensions ensures that the optical field remains concentrated within the slot region, thereby maximizing the electro-optic interaction necessary for efficient modulation.

[0038] Figure 3 illustrates a perspective view of a slot waveguide modulator with integrated mode converters to illustrate the flow of optical signals through the modulation section in accordance with embodiments of the disclosure. As can be seen, optical signals 301 are introduced into the system through mode converter 302 which facilitates the transition of optical signals transmitted along abus waveguide to slot waveguide modulator 100. After being modulated by slot waveguide modulator 100, modulated optical signals 305 exits through mode converter 304. In embodiments of the disclosure, mode converters 302 and 304 may each comprise a waveguide and a mode coupler.

[0039] Mode converters 302 and 304 act as the optical interfaces between the slot waveguide modulator 100 and the silicon / silicon nitride (Si / SiN) bus waveguides (not shown) that may be used within a larger optical platform. Mode converters 302 and 304 are designed to be butt coupled to the corresponding ends of slot waveguide modulator 100 to ensure direct and efficient mode conversion, i.e., to the input or output ends of EO layer 108. In particular, to achieve efficient operation, mode converter 302 is provided at the input of slot waveguide modulator 100 to selectively excite the desired slot mode, while mode stopper layers 106a, 106b, 110a and 110b are used to suppress unintended modes and maintain stable propagation over long distances along the slot waveguide.

[0040] As an example, a mode coupler provided within mode converter 302 may be configured to convert the incoming optical mode from a TE (transverse electric) polarization in the bus waveguide to a TM (transverse magnetic) mode, which is required for exciting the slot mode inside EO layer 108 of slot waveguide modulator 100. This polarization conversion ensures that the majority of the optical field is confined within EO layer 108, thereby maximizing electro-optic interaction and enhancing modulation efficiency. A mode coupler within mode converter 304 is then configured to perform the inverse function of the mode coupler provided within mode converter 302 which is to convert the outgoing optical mode from the TM-polarized slot mode back into a TE-polarized mode that can efficiently propagatethrough the Si / SiN bus waveguides of the platform. The detailed workings of mode converters 302 and 304 are omitted for brevity as it is known to those skilled in the art.

[0041] Given their interdependent functions, mode converters 302 and 304 and mode stopper layers 106a, 106b, 110a and 110b may be co-designed and directly integrated into the multi-layered structure of slot waveguide modulator 100 to ensure that the slot waveguide is able to support the desired optical mode throughout the entire modulation section. This integration is essential for achieving high modulation efficiency while minimizing optical losses caused by mode mismatches or unintended mode coupling.

[0042] Additionally, mode converters 302 and 304 play a crucial role in ensuring seamless compatibility with existing silicon photonics platforms to enable low-loss optical coupling between a conventional silicon waveguide and slot waveguide modulation 100.

[0043] To further optimize the performance of slot waveguide modulator 100, it is proposed that waveguide layers 104 and 112 be extended laterally from the slot waveguide region to form electrical contacts with the metal routing and pads. A cross-sectional view of such an exemplary embodiment is illustrated in Figure 4 while a top view of this embodiment is illustrated in Figure 5. As can be seen, Figure 4 illustrates the electrical configuration of slot waveguide modulator 100 with emphasis on the placement of electrodes 402 and 404 away from the slot waveguide region, and the application of a potential difference between electrodes 402 and 404 through the use of radio frequency (RF) source 401 for generating clcctro-optic modulation of optical signals in EO layer 108.

[0044] Specifically, electrodes 402 and 404 are electrically connected to waveguide layers 112 and 104, respectively. These electrodes are used to apply an external voltage supplied by RF source 401 across EO layer 108, i.e., a potential difference across EO layer 108, which induces a change in the refractive index of EO layer 108 via the electro-optic effect. The placement of electrodes 402 and 404 on the respective conductive layers of slot waveguide modulator 100 ensures that an electric field is effectively concentrated across EO layer 108 while minimizing the optical losses introduced by these electrodes. By adjusting the voltage applied by power source or RF source 401, the refractive index of EO layer 108 may be dynamically tuned, modulating the optical signal propagating through the waveguide.

[0045] Figure 6 illustrates a schematic representation and simulation results of various sections that make up a mode converter which is designed to transition an optical mode from a standard waveguide into a slot waveguide. Figure 6 is generally divided into three sections: input waveguide mode 607, coupler transition mode 608, and final slot waveguide mode 609. A schematic representation of the formation of the physical structures which show the gradual tapering of the waveguide leading into the slot waveguide region to facilitate efficient mode conversion is illustrated as steps 611, 612, 613, 614 and 615.

[0046] The simulated plot relating to waveguide mode 602 shows the initial optical mode propagating through a conventional Si / SiN waveguide. As can be seen, the optical field is tightly confined in a single high-index region, representing a transverse electric (TE) mode typically found in standard waveguides.

[0047] As the light passes through the mode coupler, the optical field splits into two lobes, indicating a transition from TE to TM polarization. This can be seen from the simulated plot relating to coupler transition mode 604. This stage acts to ensure that proper mode matching is achieved between the bus waveguide and the slot waveguide.

[0048] In the final stage, based on the simulated plot relating to slot waveguide mode 606, it can be seen that the optical mode has been successfully coupled into the slot waveguide, where the optical field is confined between the two high-index layers. This demonstrates that the majority of the optical power is concentrated within the low-index EO layer, maximizing the electro-optic interaction essential for modulation.

[0049] Figure 7 illustrates the heat map simulation results of both the optical mode profile and the electrical field distribution within a slot waveguide modulator. Heat map 702 illustrates the optical mode profile, while heat map 704 illustrates the electrical field confinement within the structure. These simulations demonstrate how the slot waveguide modulator designed in accordance with embodiments of this disclosure enhanced both optical and electrical confinement within the slot.

[0050] Heat map simulation 702 shows that the optical field is strongly confined within the low-index slot region between two high-index layers. The high-intensity region 703 in the center indicates that the majority of the optical power is concentrated within the slot, ensuring strong light-matter interaction with the EO layer. The surrounding low-intensity regions represent areas where the optical field is significantly weaker, minimizing losses into the surrounding structure.

[0051] Heat map simulation 704 illustrates the distribution of the applied electric field across the slot waveguide. Region 705 indicates strong electric field intensity, which is concentrated within the slot region, aligning with the optical mode confinement in heat map simulation 702. This confirms that the electric field effectively overlaps with the optical field, a critical factor for achieving high electro-optic modulation efficiency. The gradual intensity drop outside the slot suggests minimal field leakage, ensuring efficient electrical control over the modulation process.

[0052] Figures 8a, 8b and 8c illustrate the simulation results of a slot waveguide modulator designed in accordance with embodiments of the disclosure. Figure 8a illustrates the simulated variation in relative phase shift of the optical signal as a function of applied voltage, referenced to a zero-bias condition. The results indicate a linear relationship between the phase shift and the applied voltage, confirming that the modulation efficiency is directly proportional to the potential difference across the electrodes of the slot waveguide modulator. A higher voltage results in a greater phase shift, demonstrating the effective operation of the electro-optic effect within the slot waveguide.

[0053] Figure 8b illustrates the simulated optical loss as a function of applied voltage. The results show that optical loss decreases as the applied voltage increases, suggesting that the electro-optic tuning effect enhances light confinement within the slot waveguide while reducing scattering losses. Meanwhile, Figure 8c presents the simulated bandwidth as a function of bias voltage. The results indicate that the modulator bandwidth increases with applied voltage, reaching values approaching 100 GHz at a bias voltage of approximately 4 Volts.

[0054] A process for forming a slot waveguide modulator in accordance with embodiments of the disclosure is illustrated in Figure 9.[0055 J Process 900 begins at step 902 by forming a first layer having a first refractive index on a substrate. Process 900 then forms a first mode stopper layer adjacent to the first layer and forms a second mode stopper layer adjacent to the first layer such that the first layer is positioned between the first and second mode stopper layers and this takes place at step 904. At step 906, process 900 then forms an electro-optic (EO) layer having a second refractive index on the first layer and the first and second mode stopper layers. Process 900 then forms a second layer having the first refractive index on the EO layer at step 908. At step 910, process 900 forms a third mode stopper layer adjacent to the second layer and forms a fourth mode stopper layer adjacent to the second layer such that the second layer is positioned between the third and fourth mode stopper layers. In accordance with embodiments of the disclosure, the second refractive index is lower than the first refractive index such that optical field confinement can be achieved within the EO layer. Process 900 then electrically connects the first layer to a first electrode, and the second layer to a second electrode at step 912 such that an electric field modulates a refractive index of the EO layer when a potential difference exists between the first and second electrodes.

[0056] In other embodiments, process 900 then couples an input mode coupler to an input end of the electro-optic layer and process 900 couples an output mode coupler to an output end of the electro-optic layer. In this embodiment, the input mode coupler is configured to convert a mode of optical signals received from a bus waveguide and to couple the converted optical signals into the electro-optic layer, and the output mode coupler is configured to receive modulated optical signals from the electro-optic layer.

[0057] In other embodiments, process 900 forms a first cladding layer on the first mode stopper layer adjacent to the electro-optic layer such that the first cladding layer is positioned between the first mode stopper layer and the third mode stopper layer; and forms a second cladding layer on the second mode stopper layer adjacent to the electro-optic layer such that the second cladding layer is positioned between the second mode stopper layer and the fourth mode stopper layer.

[0058] Mach Zehnder Modulator (MZM) configuration

[0059] In embodiments of the disclosure, the performance of the slot waveguide modulator may be improved by employing the slot waveguide modulator in a push-pull Mach-Zehnder Modulator (MZM) configuration. The MZM is based on the well-established Mach-Zehnder Interferometer (MZI) structure, where an incoming optical signal is split into two distinct paths — one directed towaid a slot waveguide modulator and the other toward a branch waveguide. In embodiments of the disclosure, the slot waveguide modulator may be utilized in both of these paths. The signals from both paths are then recombined after modulation. In operation, an input waveguide of the MZM first receives the optical signal, which is then split into two equal parts by a Y-branch splitter. One part of the signal is directed to the slot waveguide modulator designed in accordance with embodiments of the disclosure, where an input mode coupler is used to convert the mode of the optical signal, ensuring efficient coupling into the electro-optic (EO) layer of the slot waveguide modulator. Within the slot waveguide modulator, an electric field applied across the EO layer induces a phase shift in the optical signal via the electro-optic effect, modifying its phase relative to the signal traveling through the branch waveguide.

[0060] After modulation, both optical signals — one from the slot waveguide modulator and the other from the branch waveguide — arc recombined using a rccombincr. This push-pull configuration ensures that the two signals interfere constructively or destructively, depending on the applied voltage, effectively converting the phase modulation into intensity modulation at the output waveguide. This structure allows for high-speed, low-power optical modulation, as the push-pull drive enhances efficiency and reduces power consumption. By leveraging the strong optical confinement of the slot waveguide and the push-pull operation of the Mach- Zehnder architecture, this design effectively doubles the modulation efficiency, as the applied electric fields induce opposing phase shifts in each arm, enhancing the overall modulation depth.

[0061] Due to this high-efficiency design, the length of the MZM can be significantly reduced, leading to lower insertion loss and higher bandwidth. The shorter device footprint not only improves integration within silicon photonics platforms but also enables high-speed modulation while maintaining low power consumption.

[0062] Numerous other changes, substitutions, variations, and modifications may be ascertained by the skilled in the art and it is intended that the present application encompass all such changes, substitutions, variations, and modifications as falling within the scope of the appended claims.

Claims

CLAIMS1. A slot waveguide modulator comprising: a first layer having a first refractive index being disposed on a substrate; a first mode stopper layer formed on the substrate adjacent to the first layer and a second mode stopper layer formed on the substrate adjacent to the first layer such that the first layer is positioned between the first and second mode stopper layers; an electro-optic layer having a second refractive index being disposed on the first layer, and the first and second mode stopper layers; a second layer having the first refractive index being disposed on the electro-optic layer; a third mode stopper layer formed on the electro-optic layer adjacent to the second layer and a fourth mode stopper layer formed on the electro-optic layer adjacent to the second layer such that the second layer is positioned between the third and fourth mode stopper layers, wherein the second refractive index is lower than the first refractive index such that optical field confinement can be achieved within the electro-optic layer, wherein the first layer is electrically connected to a first electrode, and the second layer is electrically connected to a second electrode such that an electric field modulates a refractive index of the electro-optic layer when a potential difference exists between the first and second electrodes.

2. The slot waveguide modulator according to claim 1 further comprising: an input mode coupler coupled to an input end of the electro-optic layer; and an output mode coupler coupled to an output end of the electro-optic layer, wherein the input mode coupler is configured to convert a mode of optical signals received from a bus waveguide and to couple the converted optical signals into the electrooptic layer, and wherein the output mode coupler is configured to receive modulated optical signals from the electro-optic layer.

3. The slot waveguide modulator according to claims 1 or 2, wherein the first, second, third and fourth mode stopper layers comprise a photonic crystal layer, a slab waveguide layer or a sub wavelength grating waveguide layer.

4. The slot waveguide modulator according to any one of claims 1 to 3, wherein the electro-optic layer comprises lithium niobate (I.iNBoi), scandium aluminum nitride (ScAlN), barium titanate (BTO).

5. The slot waveguide modulator according to any one of claims 1 to 4, wherein the first and second layers comprise a doped semiconductor material configured to provide electrical conductivity.

6. The slot waveguide modulator according to claim 5, wherein the semiconductor material comprises polycrystalline silicon or silicon.

7. The slot waveguide modulator according to any one of claims 1 to 6, wherein the electro-optic layer has a thickness less or equal to lOOnm.

8. The slot waveguide modulator according to claim 1 further comprising: a first cladding layer formed on the first mode stopper layer adjacent to the electro-optic layer such that the first cladding layer is positioned between the first mode stopper layer and the third mode stopper layer; and a second cladding layer formed on the second mode stopper layer adjacent to the electro-optic layer such that the second cladding layer is positioned between the second mode stopper layer and the fourth mode stopper layer.

9. The slot waveguide modulator according to claim 1, wherein the first electrode is disposed on another part of the first layer that is away from a part of the first layer that is positioned between the first and second mode stopper layers and the second electrode is disposed on another part of the second layer that is away from a part of the second layer that is positioned between the third and fourth mode stopper layers.

10. A push-pull Mach Zehnder modulator comprising: an input waveguide for receiving an optical signal; a Y-branch splitter coupled to the input waveguide for splitting the optical signal received by the input waveguide into a slot waveguide modulator according to claim 1 and a branch waveguide, whereby an input mode coupler is provided between the Y-branch splitter and the slot waveguide modulator to convert a mode of the optical signal and couple the converted optical signal into the electro-optic layer of the slot waveguide modulator;a recombiner for recombining modulated optical signals received from electro-optic layer of the slot waveguide modulator with optical signals received from the branch waveguide; and an output waveguide for emitting the recombined optical signals.1 1. A method for forming a slot waveguide modulator, the method comprising: forming a first layer having a first refractive index on a substrate; forming a first mode stopper layer on the substrate adjacent to the first layer and forming a second mode stopper layer on the substrate adjacent to the first layer such that the first layer is positioned between the first and second mode stopper layers; forming an electro-optic layer having a second refractive index on the first layer, and the first and second mode stopper layers; forming a second layer having the first refractive index on the electro-optic layer; forming a third mode stopper layer on the electro-optic layer adjacent to the second layer and forming a fourth mode stopper layer on the electro-optic layer adjacent to the second layer such that the second layer is positioned between the third and fourth mode stopper layers, wherein the second refractive index is lower than the first refractive index such that optical field confinement can be achieved within the electro-optic layer, wherein the first layer is electrically connected to a first electrode, and the second layer is electrically connected to a second electrode such that an electric field modulates a refractive index of the electro-optic layer when a potential difference exists between the first and second electrodes.

12. The method according to claim 11 further comprising: coupling an input mode coupler to an input end of the electro-optic layer; and coupling an output mode coupler to an output end of the electro-optic layer, wherein the input mode coupler is configured to convert a mode of optical signals received from a bus waveguide and to couple the converted optical signals into the electrooptic layer, and wherein the output mode coupler is configured to receive modulated optical signals from the electro-optic layer.

13. The method according to claims 11 or 12, wherein the first, second, third and fourth mode stopper layers comprise a photonic crystal layer, a slab waveguide layer or a subwavelength grating waveguide layer.

14. The method according to any one of claims 11 to 13, wherein the electro-optic layer comprises lithium niobate (LiNBo;), scandium aluminum nitride (ScAlN), barium titanate (BTO).

15. The method according to any one of claims 11 to 14, wherein the first and second layers comprise a doped semiconductor material configured to provide electrical conductivity.

16. The method according to claim 15, wherein the semiconductor material comprises polycrystalline silicon or silicon.

17. The method according to any one of claims 11 to 16, wherein the electro-optic layer is formed with a thickness of less or equal to lOOnm.

18. The method according to claim 11 further comprising the steps of: forming a first cladding layer on the first mode stopper layer adjacent to the electrooptic layer such that the first cladding layer is positioned between the first mode stopper layer and the third mode stopper layer; and forming a second cladding layer on the second mode stopper layer adjacent to the electro-optic layer such that the second cladding layer is positioned between the second mode stopper layer and the fourth mode stopper layer.

19. The method according to claim 11 , wherein the first electrode is formed on another part of the first layer that is away from a part of the first layer that is positioned between the first and second mode stopper layers and the second electrode is disposed on another part of the second layer that is away from a part of the second layer that is positioned between the third and fourth mode stopper layers.

20. A method for forming a push-pull Mach Zehnder modulator comprising: providing an input waveguide for receiving an optical signal; coupling a Y-branch splitter to the input waveguide for splitting the optical signal received by the input waveguide into a slot waveguide modulator formed according to themethod in claim 11 and into a branch waveguide, whereby an input mode coupler is formed between the Y-branch splitter and the slot waveguide modulator to convert a mode of the optical signal and to couple the converted optical signal into the electro-optic layer of the slot waveguide modulator; providing a recombiner for recombining modulated optical signals received from electro-optic layer of the slot waveguide modulator with optical signals received from the branch waveguide; and coupling an output waveguide to the recombiner for emitting the recombined optical signals.

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