Electro-optic modulator

Abstract

An electro-optic modulator comprising a semiconductor material, such as designed for use in modulating light by exploiting the electro-optic MKA-effect, which is equipped with an index of refraction that is dependent on an electric field applied to the modulator, characterized in that the semiconductor material is provided as a ferroelectric, ferrielectric, or antiferroelectric Van der Waals material with a layered crystal structure that breaks inversion symmetry, wherein the semiconductor material is selected for having an energy bandgap (Eg) in the range of 0.5 eV to 7 eV at 20 °C.

Description

[0001]Electro-optic modulator The invention relates to an electro-optic modulator comprising a semiconductor or insulator material which is equipped with an index of refraction that is dependent on an electric field applied to the modulator. Electro-optic modulators are used in photonic devices and instruments, and can be used to modulate information on light beams from a wide variety of sources, in particular, lasers. The variable index of refraction that is dependent on the electric field is also known as the Pockel’s effect, when the variability of the index is predominantly large and linear. Examples of useful crystals that exhibit a linear electro-optic effect include GaAs, CdTe (cubic crystals), KDP (Potassium Dihydrogen Phosphate), ADP (Ammonium dihydrogen phosphate - tetragonal), LiNbO3 and LiTaO3 (trigonal). Because the Pockel’s coefficients are different for the ordinary and extraordinary polarizations of the lightbeam that is modulated, the electro-optic modulator functions as a voltage controlled variable wave retarder. There is a continuous quest for an improved electro-optic modulator, which brings better results in terms of • high data transmission speed, which is crucial for high- speed Internet, 5G networks, and data centers • low power consumption, providing energy savings and increased battery life in portable devices • signal quality to minimize transmission errors • compact size and increasing the possibility to integrate with other semiconductors • wide bandwidth for support of advanced communications protocols • high stability and reliability improving the quality of communications • small latency to improve speed in switching between states. Such an improved electro-optic modulator is particularly desirable in fields like telecommunications, data communications and photonics, but is also relevant to construction and aerospace (i.e. windows and lenses). One of the desirable aspects is to have a electro-optic modulator of which the refraction index is more sensitive to the electric field that is applied to the modulator. The current prior art electro-optic modulator employs LiNbO3, which shows the best-known sensitivity up-to-date. It is an object of the invention to in particular improve the sensitivity of the refraction index in relation to the applied electric field to the modulator. To surpass the state of the art, this invention aims to rely on a newly discovered electro- optic effect which extends beyond the known Pockels and Kerr effects. The Pockels effect is a directionally-dependent linear variation in the refractive index of an optical medium that occurs in response to the application of an electric field. The Kerr effect is a weak, third-order non-linear variation in the refractive index of an optical medium that is wavelength independent an also occurs in response to the application of an electric field. The current invention instead aims to rely on a previously unknown, seemingly non-linear, effect which we will coin as the MKA-effect or el Mrabet, van der Kolk, Ali effect. According to an aspect of the invention, an electro-optic modulator may be provided comprising a semiconductor material which is equipped with an index of refraction that is dependent on an electric field applied to the modulator. The semiconductor material is provided as a ferroelectric, ferrielectric, or antiferroelectric Van der Waals material with a layered crystal structure that breaks inversion symmetry. The semiconductor material is selected for having an energy bandgap (Eg) in the range of 0.5 eV to 7 eV at 20 °C. The electro-optic modulator may be designed, such as means of its thickness, to exhibit a 10-3- 10-1refractive index point change per volt. Wherever refractive index is referred to, also separate from these examples, the effect pertains to at least one of in-plane or out-of-plane refraction. For the sake of completeness, a layered structure here refers to a material composed of individual layers stacked upon each other, forming an overall three-dimensional (3D) crystal lattice. Each of these layers is typically a two-dimensional (2D) plane where atoms are strongly bonded together, but the bonding between the layers is weaker. The thickness of the material determines the refractive index point change. The optical effect is believed to become pronounced for the materials having a thickness of 1 - 300 nm. This particular range is relevant for photonic applications. Most preferably, the material can be chosen to have a thickness in the range of 10 - 200 nm. The material here would then be allowed to be purposed as a waveguide. In the latter range, the material, such as demonstrated for CIPS, is believed to exhibit an improved refractive index point chang of 0.001 - 0.5 per volt for incident wavelengths within the range of 200-1700 nm, such as at least one of incident wavelength of 210.6, 632.8, 1550 and 1688.3 nm. The newly found effect, wherein the refractive index changes as function of thickness and wavelength, has been demonstrated in the thickness range of 14.09 nm to 160.89 nm for CIPS, i.e. CuInP2S6. While CuInP₂S₆ is the primary stoichiometry, Cu:In:P2S6 = 1:1:1, there are other stable variations in CIPS, such as (1- x):(1+y):1, wherein x,y = 0 – 1, preferably 0 – 0,1. It stands to reason that x,y = 1 may be excluded from the former range. The person skilled in the art will recognize the latter range as deviations within the same structural framework rather than distinct stable compounds. It is also alternatively possible to achieve the above ratios through substitutional or intercalation doping, as long as one does so without substantially disrupting the lattice. It is expected that materials which share the aforementioned properties of CIPS also exhibit this behavior. Using these materials we can improve the sensitivity of the electro-optic modulator which paves the way for improvement on other issues as mentioned above. The electro-optic modulator of the invention is therefore provided with the features of one or more of the appended claims. Optionally, the electro-object modulator of the invention is characterized in that the modulator substantially comprises CIPS. It is found that in comparison with the prior art, the electro-optic modulator of the invention has a sensitivity which is several orders of magnitude higher than the prior art electro- optic modulator. The invented electro-optic modulator can exhibit a 10-1index point change per volt, whereas the prior art modulator sensitivity is 10-4index point change per volt. The improved results are in particular achievable when the CIPS material comprises a thickness in the range from ~200 nm to 10 nm. Even beyond CIPS. The accompanying drawings, which is incorporated into and forms a part of the specification, illustrates results achieved with an embodiment of the present invention and, together with the description, serves to explain the principles of the invention. The drawings are only for the purpose of illustrating and is not to be construed as limiting the invention. In the drawings, graphs are shown of the refractive index(n) versus thickness at incident wavelengths of 210.6,632.8, 1550 and 1688.3 nm. Also shown are the refractive index, extinction coefficient, complex dielectric function and dielectric loss for completeness. Data is thus available for the range between 210.6 nm and 1688.3 nm showing the afore described MKA-effect in CIPS. The relevant Figures are 1, 3, 5 and 7. Figure 21 shows the refractive index(n) versus thickness at an incident wavelength of 300 nm. The figures show a significant change in the refractive index of the ordinary optical axis in comparison with the extraordinary optical axis of the electro-optic modulator of the invention, at all applied wavelengths employing the material CIPS for the electro-optic modulator. The change in the refractive index can be clearly seen across a film thickness range from 160.89 nm to 14.09 nm. This particular range is relevant for photonic applications. The figures also show the extinction coefficient κ at the before mentioned wavelength as a function of thickness (in nanometers) for a material. The extinction coefficient κ, is related to the material's ability to absorb light at this wavelength, and it is shown for both in-plane (squares) and out-of-plane (circles) orientations. The relevant Figures are 2, 4, 6 and 8. The figures further show the dielectric loss (represented as ε2 / ε1at the before mentioned wavelengths as a function of thickness (in nanometers) for the material. The dielectric loss is plotted separately for in-plane (squares) and out-of-plane (circles) measurements. The relevant Figures are 11, 14, 17 and 20. Lastly, the figures show both the real and imaginary part of the dielectric function ε1and ε2respectively for the previously mentioned wavelengths as a function of thickness (in nanometers) for the material, with separate measurements for in-plane (squares) and out-of-plane (circles) directions. The relevant Figures are 9, 10, 12, 13, 15, 16, 18 and 19. The dielectric function itself is a complex number, typically written as: ε=ε1+iε2 where: • ε1 (the real part) represents the material's ability to store electric energy (its dispersive properties). • ε2 (the imaginary part) represents the losses in the material due to absorption. It is believed that the data on CIPS supports the presence of the effect in similarly layered Thiophosphate structures CuCrP₂S₆, AgVP₂S₆, AgCrP₂S₆, and CuVP₂S₆. Since these materials have comparable bonding, spacing, and stacking patterns, electro-optical effects observed in CIPS provide a foundational understanding of similar effects in these materials. CIPS data is further believed to support same electro-optical effects in MnPS₃, VPS₃, CrPS₃, CoPS₃, NiPS₃, FePS₃, TiPS₃, and CuPS₃ due to the shared layered crystal structure. Oxyhalides, MOX₂, where M = Nb, Ta, or V; X = Cl, Br or I, namely exhibit layered structures sufficiently similar to thiophosphates. These are all ferroelectric, ferrielectric, or antiferroelectric Van der Waals materials having a layered crystal structure that breaks inversion symmetry, wherein the semiconductor material is selected for having an energy bandgap (Eg) in the range of 0.5 eV to 7 eV20 at °C. In closing it is posited that the possible 10-3- 10-1refractive index point change per volt at selected thicknesses far exceeds the Kerr effect. Kerr pertains to electric dipole alignment from an external field. The invention is showing an electro-optical effect in materials which are known to already have electric dipole ordering for all of the thicknesses explored herein. The Kerr effect also does not or is not known to change with thickness in any way similar to the observed changing index of refraction. Finally, the Kerr effect is wavelength independent, since it is only driven by the internal induced dipole moment alignment. It is “on” or “off” for nearly all electromagnetic radiation below a material’s band gap size. The present invention demonstrates wavelength dependence. The MKA-effect is seemingly far larger for shorter wavelengths approaching the band-gap size and it decreases as wavelength increases. Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the invention, the invention is not restricted to this particular embodiment which can be varied in many ways without departing from the invention. The discussed exemplary embodiment shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claim to this exemplary embodiment. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exemplary embodiment. While the person skilled in the art will know how to arrive at an electro-optic modulator, a representative embodiment of the electro-optic modulator is now described. The semiconductor material, such as CIPS, is provided as a thin film on a substrate such as sapphire or quartz using standard semiconductor deposition techniques - e.g. exfoliation / transfer methods known for Van der Waals crystals. Alternatives techniques may be found in chemical vapor deposition, or molecular beam epitaxy. Electrodes are fabricated from conductive oxides such as indium tin oxide (ITO) or alternative conductive layers such as Au or Pt, with typical thicknesses between 10 nm and 200 nm, deposited by sputtering or evaporation. In terms of geometry and architecture, electrodes may be positioned not only above and below the active CIPS film but also laterally on the left and right sides. This arrangement allows the application of both out-of-plane and in-plane electric fields across the film, thereby providing greater flexibility in modulating the optical properties. In one implementation, the electrode separation in the out-of-plane configuration is defined by the thickness of the CIPS layer (for instance, 10–200 nm). The modulator may further be configured to operate in a waveguide geometry, where the guided optical mode interacts with the active CIPS region under the applied electric field. Standard lithographic techniques are used to define both electrodes and optical confinement. These fabrication steps are within the common general knowledge of the skilled person in integrated photonics and semiconductor device engineering, and allow the realization of the electro-optic modulators described herein. As an alternative architecture, the electro-optic modulator may be realized in the form of a microring resonator. In such an embodiment, the CIPS film is patterned into or coupled with a microring waveguide structure fabricated on a substrate. Electrodes are positioned so as to apply either an in-plane or out-of-plane electric field to the ring, thereby tuning the effective refractive index of the guided mode. The applied field shifts the resonance frequency of the microring, allowing high- sensitivity modulation of light intensity or phase at specific wavelengths. This architecture provides compact device footprints and enhanced modulation efficiency due to the resonant field enhancement in the microring cavity. Separately from the above electrode separation may be defined by the thickness of the CIPS layer (for instance, 10-200 nm). The modulator is further configured to operate in a waveguide geometry, where the guided optical mode interacts with the active CIPS region under the applied electric field. Standard lithographic techniques are used to define both electrodes and optical confinement. These fabrication steps are within the common general knowledge of the skilled person in integrated photonics and semiconductor device engineering, and allow the realization of the electro-optic modulators described herein. Separately from the above the electro-optic modulator according to any of the embodiments mentioned throughout the description may be designed such that the semiconductor material is provided as a film, having a thickness between 10-200 nm, deposited on a substrate and arranged between a pair of electrodes formed from conductive oxides or metallic films, such that application of a voltage across the electrodes modulates the refractive index of the semiconductor material. Further separately from the above, but also applicable to every embodiment discussed throughout the description: The electro- optic modulator according to any embodiment discussed for modulating along an electro-optic effect characterized by a thickness- and wavelength-dependent non-linear variation of the refractive index in response to the applied electric field, thereby causing the sensitivity of the refractive index to exceed that achievable with the Pockels or Kerr effects. Another way of looking at it would be having the electro-optic modulator according to any embodiment configured for use in modulating light by exploiting an electro-optic effect (also called the MKA-effect), wherein the effect is characterized by a thickness- and wavelength-dependent non-linear variation of the refractive index in response to the applied electric field, thereby causing the sensitivity of the refractive index to exceed that achievable with the Pockels and Kerr effects. The MKA effect appears to arise from a ferro-ionic coupling mechanism, in which the spontaneous ferroelectric polarization of the material interacts with incident light and is dynamically modulated by the mobility of ionic species (e.g. Cu(I) cations in (CIPS) CuInP₂S₆), leading to a thickness- and wavelength- dependent, non-linear, and unusually large variation of the refractive index under an applied electric field. For CIPS the MKA effect appears between 20 and 170 nm approximately. This thickness may be different in different materials also mentioned in the application. Looking at CIPS separately: The MKA effect occurs at room temperature (315 K) in ferro-ionic 2D CIPS where coupling between Cu(I)-driven polarization and ionic mobility causes a nonlinear, thickness- and wavelength- dependent change of refractive index far exceeding Pockels and Kerr effect. The effect occurs for a thickness window: t ≈ 22– 170 nm (“anomalous” regime), with notable features near 80-100 nm - and t~22–50 nm. The largest δn appears near ~280 nm and peak birefringence near ~340 nm. UV / blue photons couple more strongly to interband transitions affected by polarization / ionic configuration, so index is more sensitive there. At longer wavelengths (e.g., telecom), effects persist but are smaller because dispersion moves you away from those sensitive transitions. As such, also separately from the above – and separately from CIPS, a use of the electro-optic modulator would preferably pertain to UV and blue light, that is to say 200-600 nm, most preferably 210-450 nm. For completeness’s sake a series of mutually different electro- optic modulator 1.1-1.10 according to the invention are shown in Figures 22-31. Figure 22 shows a relatively simple modulator 1.1 comprising top electrodes TE on top of a semiconductor material film SC according to the invention, preferably CIPS. Figure 23 shows an alternative modulator 1.2 comprising all features of modulator 1.1 as well as bottom electrodes BE embedded within the semiconductor material film, or below said film. Figure 24 shows yet another and more complex modulator 1.3 comprising all features of modulator 1.2 as well as lateral electrodes LE. Figure 25 shows an alternative modulator 1.4 comprising all features of modulator 1.1. Modulator 1.4 differs in that the semiconductor material film comprises undulations in between the top electrode TE. Additionally, the semiconductor material film extends on top of an insulating substrate film (IF), such as SiO2, which itself is provided on top of a conducting substrate CS, such as a doped Si. Figure 26 shows a modulator 1.5. Differences between modulator 1.5 and 1.4 will be discussed hereinbelow. In this example the semiconductor material film SC is provided directly on top of the insulating substrate IF . In this example bottom electrodes BE are embedded in the insulating substrate IF below the semiconductor material film SC and, preferably directly, below a top electrode. Figure 27 shows a modulator 1.6. Differences between modulator 1.6 and 1.5 will be discussed hereinbelow. In this example lateral electrodes LE are provided connecting to both the insulating substrate IF as well as the semiconductor material film SC. Figure 28 shows a modulator 1.7. Differences between modulator 1.7 and 1.4 will be discussed hereinbelow. Instead of undulations in the semiconductor material film SC, waveguides WG, such as comprising of silicon–carbide SiC, are provided on top of the semiconductor material film SC. These extend between the top electrodes TE. In this example the semiconductor material film SC is provided on top of an insulating film IF, such as SiO2, which is provided on top of a conducting substrate CS, such as doped Si. Figure 29 shows a modulator 1.8 according to modulator 1.7 with the addition of bottom electrodes BE embedded in the insulating substrate IF underneath the semiconductor material film SC, preferably directly, below the top electrodes TE. Figure 30 shows a modulator 1.9 according to modulator 1.8 with the addition of lateral electrodes LE connecting to both the insulating substrate IF as well as the semiconductor material film SC. Figure 31 an electro-optic modulator 1.10 realized in the form of a microring resonator. In this example there is provided an insulating film IF, which may be provided on top of a conducting film (not shown, but customary). On top of the IF there is provided an annular wave guide and a linear wave guide (each indicated with WG). Within the area circumscribed by the annular wave guide there is provided a bottom electrode BE on top of the insulating film IF. The bottom electrode is preferably disc- shaped. A semiconductor material film SC is provided such that it is laid on top of the ring shaped wave guide and bottom electrode. Lateral electrodes LE are provided both on top opposite lateral ends of the semiconductor material film SC connecting both to the material film and the insulating material IF. Finally a top electrode TE is provided on top of the semiconductor material film SC directly above the bottom electrode BE. The waveguide’s material may comprise silicon carbide. Concludingly, there is described herein an electro-optic modulator comprising a semiconductor material, for use in modulating light by exploiting the electro-optic MKA-effect, which is equipped with an index of refraction that is dependent on an electric field applied to the modulator, characterized in that the semiconductor material is provided as a ferroelectric, ferrielectric, or antiferroelectric Van der Waals material with a layered crystal structure that breaks inversion symmetry, wherein the semiconductor material is selected for having an energy bandgap (Eg) in the range of 0.5 eV to 7 eV at 20 °C. The MKA-effect being a thickness- and wavelength-dependent non- linear variation of the refractive index, in response to the applied electric field, thereby in use causing the sensitivity of the refractive index to exceed that achievable with the Pockels and Kerr effects. One may also separately from the above understand that a semiconductor material, such as CIPS or any other material mentioned as an alternative, may be selected preferably for use in modulating light by exploiting the electro-optic MKA-effect within a material thickness, such as in the range of 10-200 nm, and for wave lengths, such as in the range of 210-450 nm, wherein the MKA-effect is larger than the Pockels and Kerr effect for said same thickness range and wavelength range. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference. Unless specifically stated as being “essential” above, none of the various components or the interrelationship thereof are essential to the operation of the invention. Rather, desirable results can be achieved by substituting various components and / or reconfiguration of their relationships with one another.

Claims

CLAIMS 1. An electro-optic modulator comprising a semiconductor material, such as for use in modulating light by exploiting the electro-optic MKA-effect, which is equipped with an index of refraction that is dependent on an electric field applied to the modulator, characterized in that the semiconductor material is provided as a ferroelectric, ferrielectric, or antiferroelectric Van der Waals material with a layered crystal structure that breaks inversion symmetry, wherein the semiconductor material is selected for having an energy bandgap (Eg) in the range of 0.5 eV to 7 eV at 20 °C.

2. The electro-optic modulator of claim 1, wherein the semiconductor material substantially comprises a crystal belonging to the ABP₂X₆ family, where A is at least one metal, such as a transition metal element, that has an oxidation state of +1 (M⁺), where B is at least one metal, such as a main group metal element, that has an oxidation state of +3 (M³⁺), and X is a chalcogen selected from the group consisting of Sulphur (S), Tellurium (Te) and Selenium (Se).

3. The electro-optic modulator of claim 2, wherein the semiconductor material is selected from the group consisting of CuInP₂S₆, CuCrP₂S₆, AgVP₂S₆, AgCrP₂S₆, and CuVP₂S₆.

4. The electro-optic modulator of claim 1, wherein the material is CuInP₂S₆, wherein Copper and Indium have a ratio in the range of 0,8:1 – 1:0,8, such as comprising substitutional or intercalation doping without significantly disrupting the lattice.

5. The electro-optic modulator of claim 1, wherein the semiconductor material substantially comprises a crystal belonging to the AP₂X₆ family, where A is at least one metal, such as a transition metal element, that has an oxidation state of +4 (M4⁺) and X is a chalcogen selected from the group consisting of Sulphur (S), Tellurium (Te) and Selenium (Se).

6. The electro-optic modulator of claim 5, wherein the semiconductor material is selected from the group consisting of SnP2Te6, SnP2S6, SnP2Se6,GeP2S6, GeP2Te6 and GeP2Se6.

7. The electro-optic modulator of claim 1, wherein the semiconductor material substantially comprises a crystal belonging to the APX₃ family, where A is at least one metal having an oxidation state of +2 (M²⁺) or a combination of metals having an oxidation state of +1 (M1⁺) and +3 (M³⁺), and wherein X is a chalcogen selected from Sulphur (S), Tellurium (Te) and Selenium (Se).

8. The electro-optic modulator of claim 7, wherein the semiconductor material is selected from the group consisting of MnPS3, VPS3, CrPS3, CoPS3, NiPS3, FePS3, TiPS3, and CuPS3.

9. The electro-optic modulator of claim 1, wherein the semiconductor material substantially comprises a crystal belonging to the AOX₂ family, where A is a transition metal cation selected from the group consisting of niobium (Nb), tantalum (Ta), and vanadium (V), and X is a Halide selected from Chlorine (Cl), Bromine (Br), or Iodine (I).

10. The electro-optic modulator of claim 9, wherein the semiconductor material is selected from the group consisting of NbOCl₂, NbOBr₂, NbOI₂, TaOCl₂, TaOBr₂, TaOI₂, VOCl₂, VOBr₂, and VOI₂.

11. The electro-optic modulator of any one of the preceding claims, wherein the material comprises a thickness in the range from 300 nm to 1 nm, preferably 200 nm to 10 nm.

12. The electro-optic modulator according to any one of claims 1-11, characterized in that the variability of the index of refraction occurs with incident light having a wavelength in the range of 210,6 nm - 1688,3 nm, such as at least one of 210.6, 300, 632.8, 1550 and 1688.3 nm.

13. An electro-optic modulator comprising a semiconductor material which is equipped with an index of refraction that is dependent on an electric field applied to the modulator, characterized in that the electro-opticmodulator substantially comprises CIPS, i.e. CuInP2S6.

14. The electro-optic modulator according to claim 13, wherein the CIPS material comprises a thickness in the range from 160.89 nm to 14.09 nm.

15. The electro-optic modulator according to claim 13 or 14, wherein the variability of the index of refraction occurs with incident light having a wavelength of approximately 300 nm.

16. Use of the electro-optic modulator according to any one of claims 1–15 for modulating light by exploiting the electro- optic MKA-effect, such as for light with a wavelength of 200- 600 nm, preferably 210-450 nm, wherein the MKA-effect is characterized by a thickness- and wavelength-dependent non- linear variation of the refractive index in response to the applied electric field, said variation being substantially larger in magnitude than the Kerr effect and distinct from the linear response of the Pockels effect by its non-linearity.

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