Low-loss mid-infrared electro-optical modulator based on graphene and ITO and preparation method of low-loss mid-infrared electro-optical modulator

Abstract

The invention relates to the technical field of metasurface electro-optical devices, in particular to a low-loss mid-infrared electro-optical modulator based on graphene and ITO and a preparation method thereof, SiO2, Al2O3, graphene, TiO2 and ITO are sequentially arranged in the electro-optical modulator from bottom to top, and Si is embedded in SiO2 to serve as a central waveguide. Single-layer graphene and ITO with the thickness of 30 nm are placed in a staggered mode to form a heterojunction, change of carriers in the Si waveguide is caused, and then modulation of the change of effective refraction in the Si waveguide is achieved; besides, the graphene and the ITO form a parallel-plate capacitor, the switching energy of the modulator is reduced, high modulation efficiency is achieved, and finally, metal electrodes are placed on the graphene and the ITO respectively, so that voltage directly controls heterojunction. According to experimental verification, a high extinction ratio is achieved under low insertion loss, and multi-rate modulation can be achieved.

Description

Technical Field

[0001] This invention relates to the field of metasurface electro-optic devices, specifically to a low-loss mid-infrared electro-optic modulator based on graphene and ITO and its fabrication method. Background Technology

[0002] Electro-optic modulators, as key components in optical communication, have attracted extensive research. Based on wavelength, electro-optic modulators are classified into near-infrared modulators, mid-infrared modulators, and far-infrared modulators. Currently, most research on electro-optic modulators focuses on the near-infrared band, especially the 1550nm band. The mid-infrared band is used in fiber optic communication, gas detection, and biomedicine. Mid-infrared electro-optic modulators utilize Si and Ge as waveguide materials, forming MIR electro-optic Ge modulators or MIR electro-optic Si modulators. Ferroelectric oxides are used to fabricate MIR modulators, such as lithium niobate and barium titanate. Additionally, two-dimensional materials, such as graphene and black scale, are used to fabricate MIR electro-optic modulators. For mid-infrared electro-optic modulators, some researchers have studied the modulation depth, with the maximum modulation depth approaching 100%. Currently, the maximum extinction ratio of mid-infrared electro-optic modulators based on Ge and Si materials is around 35dB, but the insertion loss also increases by nearly 3dB. Materials using LN exhibit a higher extinction ratio, reaching around 40dB, and an insertion loss as low as 0.45dB. However, LN is more expensive than Si. Regarding modulation rates, most modulators operate at a single modulation rate, and these rates rarely reach 100Gbps. Summary of the Invention

[0003] The purpose of this invention is to provide a low-loss mid-infrared electro-optic modulator based on graphene and ITO and its preparation method, which combines graphene and ITO to achieve a low-loss, high extinction ratio, and multi-rate modulator in the mid-infrared.

[0004] To achieve the above objectives, this invention provides a low-loss mid-infrared electro-optic modulator based on graphene and ITO, which is formed by combining multiple semiconductor materials. From bottom to top, SiO2, Al2O3, graphene, TiO2, and ITO are arranged sequentially. Si is embedded in SiO2 as a central waveguide, and the top surface of Si is close to Al2O3. The three layers of graphene, TiO2, and ITO have the same width and are not fully covered. The TiO2 and ITO layers are partially suspended above the graphene, and part of the graphene layer is exposed to the air. Au metal electrodes are also placed on the exposed ITO and graphene layers.

[0005] The dimensions of each layer in the low-loss mid-infrared electro-optic modulator based on graphene and ITO are as follows:

[0006] The SiO2 layer is 690 μm long, 5000 nm wide, and 750 nm high;

[0007] The Si layer is 690 μm long, 1800 nm wide, and 500 nm high;

[0008] The Al2O3 layer is 690 μm long, 5000 nm wide, and 30 nm high;

[0009] The graphene layer is 690 μm long and 3400 nm wide, and is a single layer.

[0010] The TiO2 layer is 690 μm long, 3400 nm wide, and 140 nm high;

[0011] The ITO layer is 690um long, 3400nm wide, and 30nm high;

[0012] The Au layer is 690 μm long, 400 nm wide, and 70 nm high.

[0013] This invention also provides a method for fabricating a low-loss mid-infrared electro-optic modulator based on graphene and ITO, comprising the following steps:

[0014] Step 1: Select an SOI wafer and form the lower silicon strip structure through EBL photolithography and etching;

[0015] Step 2: Lay SiO2 on both sides of the silicon strip;

[0016] Step 3: Lay an Al2O3 thin film;

[0017] Step 4: Generate a single-layer graphene capping sheet and lay it on top of the Al2O3 film. Use 0.5% HF to laterally etch and form an overhanging step at the edge of the graphene.

[0018] Step 5: Align and transfer the TIO2 layer using microbridges, so that 52% of the TIO2 layer overlaps on the top surface of the graphene eaves, while the remaining 48% of its width extends out of the eaves and is suspended in the air;

[0019] Step 6: ITO deposition using ALD or RF sputtering;

[0020] Step 7: Lay Au metal.

[0021] Optionally, in step 3, an Al2O3 film is deposited using ALD technology, specifically using trimethylaluminum and water as precursors, and reacted and deposited layer by layer at 100–300°C.

[0022] Optionally, in step 4, copper-catalyzed CVD is used to grow a 2.346 × 10⁻⁶ spore size on a 25 µm Cu(111) foil.5 The process involved a single-layer graphene layer of cm², operating at 1040°C with CH4 / H2 = 15 / 30 sccm for 30 min. Specifically, a 100 nm PMMA 950 kb support layer was spin-coated, followed by Cu etching at room temperature with 0.2 M (NH4)2S2O8 for 3 h. The graphene was then rinsed three times with deionized water, vacuum dried at 80°C for 30 min, and PMMA removed with acetone at 50°C for 2 h. Finally, the graphene was rinsed with IPA and dried with N2.

[0023] This invention provides a low-loss mid-infrared electro-optic modulator based on graphene and ITO and its fabrication method. The electro-optic modulator is arranged from bottom to top as SiO2, Al2O3, graphene, TiO2, and ITO, with Si embedded in SiO2 as the central waveguide. This invention forms a heterojunction by alternately placing single-layer graphene and 30nm thick ITO, causing changes in charge carriers in the Si waveguide, thereby achieving modulation through changes in effective refraction in the Si waveguide. Furthermore, graphene and ITO form a parallel-plate capacitor, reducing the switching energy of the modulator and achieving high modulation efficiency. Finally, metal electrodes are placed on the graphene and ITO respectively to allow direct voltage control of the heterojunction. Experimental results show that this invention achieves a high extinction ratio with low insertion loss and supports multi-rate modulation. At a voltage of 7V and a modulation rate of 40Gbps, the insertion loss is 0.23dB and the extinction ratio is 25.50dB. Meanwhile, by changing the light intensity, the modulation rate can be switched between 40Gbps and 210Gbps, and the extinction ratio can reach more than 22dB. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the structure of a low-loss mid-infrared electro-optic modulator based on graphene and ITO according to the present invention.

[0026] Figure 2 This is a schematic diagram of the fabrication process of a low-loss mid-infrared electro-optic modulator based on graphene and ITO according to the present invention.

[0027] Figure 3 This is a schematic diagram of modulation rate and extinction ratio in a specific embodiment of the present invention.

[0028] Figure 4 This is a schematic diagram of the transmission spectrum at different voltages in a specific embodiment of the present invention.

[0029] Figure 5 This is a schematic diagram of the extinction ratio at different modulation rates in a specific embodiment of the present invention. Detailed Implementation

[0030] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0031] This invention provides a low-loss mid-infrared electro-optic modulator based on graphene and ITO, which is formed by combining multiple semiconductor materials. From bottom to top, SiO2, Al2O3, graphene, TiO2, and ITO are arranged in sequence. Si is embedded in SiO2 as a central waveguide, and the top surface of Si is close to Al2O3. The three layers of graphene, TiO2, and ITO have the same width and are not fully covered. The TiO2 and ITO layers are partially suspended above the graphene, and part of the graphene layer is exposed to the air. Au metal electrodes are also placed on the exposed ITO and graphene layers.

[0032] The dimensions of each layer in the low-loss mid-infrared electro-optic modulator based on graphene and ITO are as follows:

[0033] The SiO2 layer is 690 μm long, 5000 nm wide, and 750 nm high;

[0034] The Si layer is 690 nm long, 1800 nm wide, and 500 nm high;

[0035] The Al2O3 layer is 690 μm long, 5000 nm wide, and 30 nm high;

[0036] The graphene layer is 690 μm long and 3400 nm wide, and is a single layer.

[0037] The TiO2 layer is 690 μm long, 3400 nm wide, and 140 nm high;

[0038] The ITO layer is 690um long, 3400nm wide, and 30nm high;

[0039] The Au layer is 690 μm long, 400 nm wide, and 70 nm high.

[0040] For detailed structure, please refer to [link / reference]. Figure 1Figure 1(a) shows a 3D schematic diagram of the structure, and Figure 1(b) shows a cross-sectional view. Specifically, the low-loss mid-infrared electro-optic modulator based on graphene and ITO consists of a central Si waveguide (W1=1800nm, H6=500nm) embedded in SiO2 (W2=5000nm, H5750nm). This structure forms a heterojunction by interleaving monolayer graphene and ITO (H2=30nm), causing changes in charge carriers in the Si waveguide, thereby achieving modulation through changes in the effective refraction of the Si waveguide. Simulation results show that the effective refractive index of monolayer graphene covering the entire Al2O3 (H4=30nm) is the same as that covering the boundary of the Si waveguide; therefore, it is only necessary to cover the Si waveguide. In this configuration, graphene and ITO form a parallel-plate capacitor, reducing the switching energy of the modulator and achieving high modulation efficiency. Titanium dioxide (TiO2) is an insulating material with a high dielectric constant, serving as the high dielectric in this context. To achieve a high extinction ratio and low insertion loss, TiO2 (H3=140) is added as an insulating layer between graphene and ITO. Metal electrodes are placed on the graphene and ITO respectively, allowing for direct voltage control of the heterojunction. The Al2O3 layer acts as a passivating oxide, providing a protective layer to prevent damage to the Si waveguide.

[0041] Furthermore, the present invention also provides a method for fabricating a low-loss mid-infrared electro-optic modulator based on graphene and ITO, which includes the following steps:

[0042] Step 1: Select an SOI wafer and form the lower silicon strip structure through EBL photolithography and etching;

[0043] Step 2: Lay SiO2 on both sides of the silicon strip;

[0044] Step 3: Lay an Al2O3 thin film;

[0045] Step 4: Generate a single-layer graphene capping sheet and lay it on top of the Al2O3 film. Use 0.5% HF to laterally etch and form an overhanging step at the edge of the graphene.

[0046] Step 5: Align and transfer the TIO2 layer using microbridges, so that 52% of the TIO2 layer overlaps on the top surface of the graphene eaves, while the remaining 48% of its width extends out of the eaves and is suspended in the air;

[0047] Step 6: ITO deposition using ALD or RF sputtering;

[0048] Step 7: Lay Au metal.

[0049] Different patterning techniques are employed using high-resolution photolithography combined with selective etching. TiO2 and ITO layers can be deposited via RF magnetron sputtering or atomic layer deposition. The carrier concentration of the deposited ITO can be controlled by adjusting the oxygen concentration in the deposited device. For detailed fabrication process flow, please refer to [link to specific details]. Figure 2 Starting with SOI wafers, the lower silicon strips can be formed through EBL photolithography and etching. Figure 2 (a) Display. The figure shows the other steps. Figure 2 (bc) Lay SiO2 on both sides of the silicon strip. Figure 2 (d) shows that the Al2O3 was deposited using atomic layer deposition (ALD) technology, with trimethylaluminum (TMA) and water (H2O) as precursors, and was deposited layer by layer at 100–300°C to form a uniform and dense Al2O3 film with controllable thickness. Figure 2 (ef) Monolayer graphene transfer was performed on a 25µm Cu(111) foil using copper-catalyzed CVD, resulting in a thickness of 2.346 × 10⁻⁶. 5 A single-layer graphene sheet was prepared at 1040°C (CH4 / H2 = 15 / 30 sccm, 30 min). A 100 nm PMMA 950 kb support layer was spin-coated, followed by Cu etching at room temperature with 0.2 M (NH4)2S2O8 for 3 h. The sheet was then rinsed three times with deionized water, vacuum dried at 80°C for 30 min, and then acetone removed at 50°C for 2 h. After IPA rinsing, the sheet was dried with N2 to obtain a clean, wrinkle-free single-layer graphene cover. Lateral etching with 0.5% HF was used to create "eaves"-like suspended steps at the graphene edges, ensuring 100% contact between the graphene and the underlying layer. Subsequently, TiO2 was transferred using "microbridges," with 52% overlapping the top surface of the graphene eaves and the remaining 48% extending beyond the eaves and suspended in the air. Figure 2 (g) ITO deposition using ALD or RF sputtering (the doping level of ITO should be controlled by adjusting the oxygen concentration). Figure 2 (h) Finally, lay the metal.

[0050] This invention is further validated and illustrated with experimental data; please refer to [link / reference]. Figures 3 to 5 :

[0051] A modulation rate of 40 Gbps was achieved in the mid-infrared band at 3.5 μm, with an insertion loss of 0.23 dB and an extinction ratio of 25.50 dB. Various modulation rates can be achieved by changing the incident light intensity. For example... Figure 3 As shown, an extinction ratio of over 22dB can be achieved at speeds of 40Gbps-210Gbps.

[0052] Figure 4The transmission spectra are shown at different voltages. 4(a) is the transmission spectrum at 7V, and 4(b) is the transmission spectrum at 0-8V. Figure 4 (a) It can be seen that at a voltage of 7V, the modulation depth can reach 30.8dB and the insertion loss is 0.23dB, achieving low insertion loss. Figure 4 (b) It is shown that under the condition of increased voltage, the modulation depth shifts to the left but remains at around 3.5 μm, which proves that modulation can be achieved at the 3.5 μm waveguide.

[0053] from Figure 5 As can be seen, the extinction ratios at modulation rates of 40Gbps (5(a), 100Gbps), 150Gbps, and 210Gbps (5(d)) show that eye diagrams can be achieved at different modulation rates, with an extinction ratio of 25.50dB at 40Gbps. Figure 4 , 5 It can achieve a high extinction ratio with low loss. The extinction ratio is 41.13dB at 100Gbps, 25.55dB at 150Gbps, and 29.52dB at 210Gbps.

[0054] The above description discloses only one or more preferred embodiments of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A low-loss mid-infrared electro-optic modulator based on graphene and ITO, characterized in that, It is formed by combining multiple semiconductor materials, arranged from bottom to top as SiO2, Al2O3, graphene, TiO2, and ITO. Si is embedded in SiO2 as a central waveguide, and the top surface of Si is close to Al2O3. The graphene, TiO2, and ITO layers have the same width and are not fully covered. The TiO2 and ITO layers are partially suspended above the graphene, and part of the graphene layer is exposed to the air. Au metal electrodes are also placed on the exposed ITO and graphene layers.

2. The low-loss mid-infrared electro-optic modulator based on graphene and ITO as described in claim 1, characterized in that, The dimensions of each layer in the low-loss mid-infrared electro-optic modulator based on graphene and ITO are as follows: The SiO2 layer is 690 μm long, 5000 nm wide, and 750 nm high; The Si layer is 690 μm long, 1800 nm wide, and 500 nm high; The Al2O3 layer is 690 μm long, 5000 nm wide, and 30 nm high; The graphene layer is 690 μm long and 3400 nm wide, and is a single layer. The TiO2 layer is 690 μm long, 3400 nm wide, and 140 nm high; The ITO layer is 690um long, 3400nm wide, and 30nm high; The Au layer is 690 μm long, 400 nm wide, and 70 nm high.

3. A method for fabricating a low-loss mid-infrared electro-optic modulator based on graphene and ITO, used to fabricate the low-loss mid-infrared electro-optic modulator based on graphene and ITO as described in any one of claims 1 or 2, characterized in that, Includes the following steps: Step 1: Select an SOI wafer and form the lower silicon strip structure through EBL photolithography and etching; Step 2: Lay SiO2 on both sides of the silicon strip; Step 3: Lay an Al2O3 thin film; Step 4: Generate a single-layer graphene capping sheet and lay it on top of the Al2O3 film. Use 0.5% HF to laterally etch and form an overhanging step at the edge of the graphene. Step 5: Align and transfer the TIO2 layer using microbridges, so that 52% of the TIO2 layer overlaps on the top surface of the graphene eaves, while the remaining 48% of its width extends out of the eaves and is suspended in the air; Step 6: ITO deposition using ALD or RF sputtering; Step 7: Lay Au metal.

4. The method for fabricating a low-loss mid-infrared electro-optic modulator based on graphene and ITO as described in claim 3, characterized in that, In step 3, Al2O3 thin films are deposited using ALD technology, specifically using trimethylaluminum and water as precursors, and reacted and deposited layer by layer at 100–300°C.

5. The method for fabricating a low-loss mid-infrared electro-optic modulator based on graphene and ITO as described in claim 3, characterized in that, In step 4, copper-catalyzed CVD was used to grow 2.346 × 10⁻⁶ spores on a 25 µm Cu(111) foil. 5 The process involved a single-layer graphene layer of cm², operating at 1040°C with CH4 / H2 = 15 / 30 sccm for 30 min. Specifically, this involved spin-coating a 100 nm PMMA 950 kb support layer, followed by Cu etching at room temperature with 0.2 M (NH4)2S2O8 for 3 h, rinsing with deionized water three times, vacuum drying at 80°C for 30 min, removing PMMA with acetone at 50°C for 2 h, rinsing with IPA, and then drying with N2.

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