Patent Description:
A second aspect of the present invention relates to a method for obtaining the electro-optical modulator of the first aspect of the invention.

An electro-optical modulators comprising the features of the preamble of claim <NUM> is known in the art, for example in document <CIT>.

That known electro-optical modulator provide, in theory, good results regarding its optical transmission, which is not the case regarding its operating optical bandwidth, which is clearly improvable.

It is, therefore, necessary to provide an alternative to the state of the art which covers the gaps found therein, by providing an electro-optical modulator with an optimized arrangement for achieving a large optical bandwidth and a high optical modulation efficiency.

To that end, the present invention relates to an electro-optical modulator as claimed in claim <NUM>.

For a preferred embodiment, particularly when there is only one waveguide, each of the top and bottom graphene sheets meets the above stated requirements regarding their extension along the X-direction above part of the width of the optical waveguide or completely above the whole width of the optical waveguide and beyond.

For an embodiment, at least one of the top and bottom graphene sheets extends along the X-direction from a <NUM>% to a <NUM>% above the optical waveguide width, or completely above the optical waveguide width and beyond through a further projecting portion with a length of up to a <NUM>% of the optical waveguide width.

For an implementation of said embodiment, at least one of the top and bottom graphene sheets extend along the X-direction from a <NUM>% to a <NUM>% above the optical waveguide width, or completely above the optical waveguide width and beyond through a further projecting portion with a length of up to a <NUM>% of the optical waveguide width.

According to a preferred embodiment, the top and bottom graphene sheets extend along the X-direction the same length.

Alternatively, the top and bottom graphene sheets have different lengths.

For an embodiment, the semiconductor substrate is covered by a thin layer called cladding, made of an oxide material, such as SiO<NUM>.

For an embodiment of the electro-optical modulator of the first aspect of the present invention:.

For three different implementations of that embodiment:.

For different implementations (which can be combined with any of the above described three implementations) of the above mentioned embodiment, the top and bottom sheets of the second dielectric material which sandwich the top graphene sheet, contact the second electrode and extend, along the X-direction, just above and just below, respectively, the top graphene, or up to the first electrode, or up to any intermediate point between them.

Also for different implementations (which can be combined with any of the above described implementations) of the above mentioned embodiment, the top sheet of the second dielectric material covering the upper face of the bottom graphene sheet or a top and a bottom sheets of the second dielectric material sandwiching the bottom graphene sheet, contact the first electrode and extend, along the X-direction, just above and just below, respectively, the bottom graphene, or up to the second electrode, or up to any intermediate point between them.

According to an embodiment, regarding the different sheets of the second dielectric material:.

According to an embodiment, the total dielectric thickness, including the thickness of the sheet of a first dielectric material and the thickness of the second dielectric material, ranges from <NUM> to <NUM>.

For an embodiment, the thickness of the sheet of a first dielectric material ranges from a <NUM>% to a <NUM>% with respect to the thickness of the second dielectric material.

For some embodiments, the first dielectric material is at least one of the following materials, or a combination thereof: HfO<NUM>, SiO<NUM>, Si<NUM>N<NUM>, Al<NUM>O<NUM>, ZrO<NUM>, TiO<NUM>, TiN, HfSiO<NUM>, ZrSiO<NUM>, Calcium Copper Titanate, Barium Titanate, Strontium Titanate, Barium Strontium Titanate, Polystyrene, Polypropylene, Polyiamide, Polyethylene, and Polytetrafluoroethylene.

With respect to the second dielectric material, for some embodiments is at least one of the following 2D layered materials, or a combination thereof: hBN, MoTe<NUM>, WSe<NUM>, WS<NUM>, graphene, MoS<NUM>, MoSe<NUM>, WS2, WSe<NUM>, black phosphorus, and SnS<NUM>.

For some embodiments, the electro-optical modulator of the present invention only comprises one waveguide.

However, for other embodiments, the electro-optical modulator of the present invention comprises two waveguides.

Particularly, for one of those other embodiments, the electro-optical modulator according to the first aspect of the present invention implements a Mach-Zehnder interferometer based arrangement, wherein the above mentioned optical waveguide is a first optical waveguide branch, and the modulator further comprises:.

Several different implementations of said embodiment for implementing a Mach-Zehnder interferometer based arrangement are possible, some of which will be described below.

Particularly, for one of said implementations, the electro-optical modulator further comprises:.

wherein at least one of the further top and further bottom graphene sheets extends along said X-direction:.

For a variant of the above described implementation:.

For another variant of said implementation of the Mach-Zehnder interferometer based arrangement embodiment:.

For another variant of the implementation of the Mach-Zehnder interferometer based arrangement embodiment, the electro-optical modulator further comprises a fourth electrode located between the second and third electrodes along the X-direction, wherein the upper face of the second optical waveguide branch is located at, below or above a location between said fourth and third electrodes, along the X-direction; and wherein:.

The present invention also relates, in a second aspect, to a method for obtaining an electro-optical modulator as claimed in claim <NUM>.

In the following some preferred embodiments of the invention will be described with reference to the enclosed figures. They are provided only for illustration purposes without however limiting the scope of the invention.

In the present section some working embodiments of the electro-optical modulator of the first aspect of the present invention will be described with reference to the Figures.

Specifically, in <FIG> and <FIG> embodiments for which the modulator has only one waveguide are disclosed, while in <FIG> an embodiment for which the modulator has two waveguides is disclosed, particularly to implement different Mach-Zehnder interferometer based arrangements.

As shown in <FIG> and <FIG>, for the embodiment there illustrated, the electro-optical modulator comprises:.

For the illustrated embodiment, each of the top Gu and bottom Gb graphene sheets extends along the X-direction completely above the whole width of the optical waveguide W, and beyond through a respective further projecting portion, identified with the referrals pu and pb, respectively.

Each of those projecting portions pu, pb has a length, along the X direction, of up to <NUM>% of the optical waveguide W width, identified with the referral "a". In <FIG>, those projecting portions pu, pb seem to be longer, but that's just a schematic representation which cannot be used to extract a teaching concerning a dimension from a measurement therein. Indeed, projecting portions pu and pb have a length of up to <NUM>% the width "a".

Alternatively, for non-illustrated embodiments, each of the top Gu and bottom Gb graphene sheets extends along the X-direction above only part of the width of the optical waveguide W, particularly above a part which ranges from <NUM>% to <NUM>% of the width "a" of the optical waveguide W.

In the present document, and particularly in the diagrams of <FIG> and <FIG>, the terms "graphene broadening" has been used to refer to the above described extension along the X-direction of the top Gu and bottom Gb graphene sheets, although in this case in absolute terms (i.e. not by means of percentages), for a width "a" for the optical waveguide W of <NUM>.

Particularly, positive values for the "graphene broadening" correspond to values of the above mentioned length of each of the projecting portions pu and pb. A waveform with a "graphene broadening" of <NUM> (i.e. for a projecting portion with a length of <NUM>% the width "a") is shown which is not part of the present invention, but depicted only for comparison purposes with the rest of waveforms.

The "graphene broadening" negative values refer to the above described embodiment for which each of the top Gu and bottom Gb graphene sheets extends along the X-direction above only part ranging from <NUM>% to <NUM>% of the width a of the optical waveguide W. Particularly, each negative value correspond to a length value, measured along the X-direction, of the distance existing between the free edge of the corresponding graphene sheet (top Gu or bottom Gb) and the edge of the waveguide W towards which the graphene sheet is extending but does not arrive to. In other words, for a width "a" of <NUM>, a "graphene broadening" of -<NUM> corresponds with a part of a <NUM>% of the width "a" along which the corresponding graphene sheet extend along the X-direction.

By comparing the results of the diagrams of <FIG> and <FIG>, one can see how slightly varying the "graphene broadening" has a great impact in the increase of the operating optical bandwidth while the optical transmission is only slightly reduced.

The present inventors have discovered that this is due to the fact that the region where the top Gu and bottom Gb graphene sheets overlap has a capacitance C that is directly proportional to its dimensions. The bandwidth of the modulator is defined as BW = <NUM>/ 2πRC. The smaller the overlap between the top Gu and bottom Gb graphene sheets, the lower the C, thus increasing the bandwidth (this is directly seen in <FIG>). From the other side, a lower C, i.e. a lower overlapped region, will have a worst transmission (as seen <FIG>).

The diagrams of <FIG> and <FIG>, and also those of <FIG>, <FIG>, <FIG> and <FIG>, have been obtained from numerical simulations for an electro-optical modulator for which the first dielectric material is HfO<NUM> and the second electro-optical material is hBN. However, similar results can be obtained for other types of dielectric materials, such as for those cited in a previous section of this document.

For the embodiment illustrated in <FIG> and <FIG>:.

The diagrams of <FIG> and <FIG> respectively show the effect of the top sheet M4 thickness and of the bottom sheet M1 thickness (including a zero-thickness M1 sheet case). The results illustrated in those diagrams show that, for the M4 top sheet, the thicker the better, and that for the M1 bottom sheet, the thinner the better.

The influence of the thickness of the sheet DS of a first dielectric material is shown in <FIG>, for which the first dielectric material is HfO<NUM>. The total thickness of the dielectric sheets (all of them, the ones of the first dielectric material and the ones of the second dielectric material) has been kept constant, so that when the thickness of HfO<NUM> has been increased, the thickness of the M2 and/or M3 sheets has been proportionally reduced. The results show how for the DS sheet, the higher percentage the better.

<FIG> shows how the order of the different dielectric sheets within the top and bottom graphene sheets (only M3, M2 and DS have been taken into account) has no influence in the optical transmission. This is due to the fact that, for the simulations, M1 and M2 are the same materials and what matters is their total thickness.

The influence of the thickness of dielectric material within the top and bottom graphene sheets (M3+M2+DS) is shown in <FIG>, showing that the smaller the thickness the better. The relative thickness (percentage) of each dielectric sheet has been kept constant.

The influence of the thickness of the whole dielectric material (M1+M2+M3+M4+DS) is shown in <FIG>, showing that the higher the thickness, the better. The relative thickness (percentage) of each dielectric sheet has been kept constant.

<FIG> schematically show different variants and/or implementations of an embodiment for which the electro-optical modulator according to the first aspect of present invention implements a Mach-Zehnder interferometer based arrangement, wherein the optical waveguide W is a first optical waveguide branch W1, and the modulator further comprises:.

For the variant shown in <FIG>, the second optical waveguide branch W2 is left uncovered, just embedded within the substrate S. Therefore, in use, i.e. when a voltage V1 is applied between the first E1 and the second E2 electrodes, only the phase of the light travelling through the core of the first optical waveguide branch W1 is changed.

For the rest of variant, i.e. for those of <FIG>, in use, the phase of the light travelling through the cores of both optical waveguide branches, W1 and W2, is changed, and the electro-optical modulator further comprises:.

One or both of the further top Guf and further bottom Gbf graphene sheets extends along the X-direction:.

Specifically, for the variant illustrated by <FIG> and <FIG>:.

Alternatively, for a non-illustrated embodiment, each of the top Gu and bottom Gb graphene sheets extends along the X-direction above only part of the width of the second optical waveguide branch W2, particularly above a part which ranges from <NUM>% to <NUM>% of the width of the second optical waveguide branch W2.

The differences between the variants of <FIG> and <FIG> are just the electrodes to which the terminals of voltage source V1 is applied, as voltage source V2 is applied to the same electrodes for both figures. For <FIG>, voltage V1 is applied to the second electrode E2 while the first E1 electrode is connected to ground, and for <FIG> the inverse connection is made.

For the variant illustrated by <FIG>, the electro-optical modulator further comprises a fourth electrode E4 located between the second E2 and third E3 electrodes along the X-direction, wherein the upper face of the second optical waveguide branch W2 is located below a location between the fourth E4 and third electrodes E3, along the X-direction; and wherein:.

For the variant of <FIG>, the voltage sources V1 and V2 are connected the same as for that of <FIG>.

As shown in <FIG>, for the variant there illustrated:.

For the variant of <FIG>, three voltages are applied, namely V1, V2 and V3, each to a respective of the first E1, second E2 and third E3 electrodes.

As state above, <FIG> are schematic, which means, for example, that those elements depicted floating are not floating but really bent.

The operation of the modulators of <FIG> is the already known associated to Mach-Zehnder interferometer based arrangements together with the one already explained above for the modulator of <FIG> and <FIG>, i.e. it benefits from the same effects explained above with respect to the "graphene broadening".

Claim 1:
An electro-optical modulator, comprising:
- a semiconductor substrate (S);
- at least first (E1) and second electrodes (E2) distanced from each other along an X-direction;
- an optical waveguide (W) embedded within and/or arranged on said semiconductor substrate (S), an upper face of said optical waveguide (W) being located at, below or above a location between said first (E1) and second (E2) electrodes, along said X-direction, wherein said optical waveguide (W) longitudinally extends along at least a direction transversal to said X-direction;
- a bottom graphene sheet (Gb) arranged and extending along said X-direction over at least a portion of the upper face of the semiconductor substrate (S), with a first end electrically connected to said first electrode (E1) and without the bottom graphene sheet (Gb) reaching the second electrode (E2);
- a sheet (DS) of a first dielectric material extending over said bottom graphene sheet (Gb);
- a top graphene sheet (Gu) arranged and extending along said X-direction over at least a portion of said sheet (DS) of a first dielectric material, with a first end electrically connected to said second electrode (E2) and without the top graphene sheet (Gu) reaching the first electrode (E1); and characterized in that
- a second dielectric material, which is a two-dimensional material with at least dielectric properties different to the dielectric properties of said first dielectric material, covering:
- upper and bottom faces of said top graphene sheet (Gu), and/or
- an upper face or both upper and bottom faces of said bottom graphene sheet (Gb);
wherein at least one of said top (Gu) and bottom (Gb) graphene sheets extends along said X-direction:
- above part of the width of the optical waveguide (W), wherein said part ranges from <NUM>% to <NUM>% of the width of the optical waveguide (W); or
- completely above the whole width of the optical waveguide (W), and beyond through a respective further projecting portion with a length, along the X direction, of up to <NUM>% of the optical waveguide (W) width.