Design and fabrication method of micro-ring electro-optic modulator based on surface plasmon

Abstract

A kind of micro-ring electro-optic modulator based on surface plasmon and preparation method thereof, comprising: silicon oxide layer and lithium niobate layer, wherein: the upper surface of lithium niobate layer is provided with under-coupled silicon race track micro-ring and straight waveguide constituting resonance ring, the two ends of straight waveguide are input end and output end respectively, lithium niobate layer is provided with lithium niobate sharp cone mode conversion structure for converting optical wave into surface plasmon wave, lithium niobate ridge waveguide and a pair of gold electrode, vertical coupling structure for light to propagate upwards or downwards is arranged between lithium niobate layer and silicon layer.The present application utilizes silicon micro-ring with surface plasmon waveguide, enhances the local action on optical field, significantly improves the modulation efficiency of modulator, realizes large bandwidth.At the same time, over-coupled micro-ring is designed, only specific wavelength light is coupled into micro-ring, high-speed modulation is carried out, and the great transmission loss caused by only surface plasmon transmission is reduced.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of semiconductor photoelectric devices, in particular to a design and preparation method of a micro-ring electro-optical modulator based on surface plasmons. BACKGROUND

[0002] As a core device of optical computing, optical communication and other technologies, an electro-optical modulator needs to meet the requirements of large bandwidth, small size and low power consumption. SUMMARY

[0003] The application provides a design and preparation method of a micro-ring electro-optical modulator based on surface plasmons, which is used for solving the problems of large area, low voltage, inability to load high-speed electrical signals and low modulation efficiency of an existing electro-optical modulator based on a thin film lithium niobate.

[0004] The application is achieved by the following technical scheme:

[0005] The application relates to a micro-ring electro-optical modulator based on surface plasmons, which comprises a silicon oxide layer and a lithium niobate layer.

[0006] The lithium niobate ridge waveguide is located between the pair of gold electrodes.

[0007] The silicon racetrack micro-ring is in a closed structure or an open structure.

[0008] The silicon racetrack micro-ring and the straight waveguide in the resonant ring are in an over-coupling state, and the distance and the coupling length are optimized by using a particle swarm algorithm.

[0009] The length of the silicon triangular waveguide, the length, the width and the width of the lithium niobate sharp cone mode conversion structure are also optimized by using a particle swarm algorithm.

[0010] Technical effects

[0011] Compared with the existing lithium niobate electro-optical modulator, the application introduces the surface plasmon waveguide into the over-coupling micro-ring for the first time by using the vertical coupling mode, and realizes the lithium niobate electro-optical micro-ring modulator based on the surface plasmon, which is super-compact, has large bandwidth, low loss and high modulation efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a schematic diagram of the structure of the present application;

[0013] In the figure: straight waveguide input end 1, silicon racetrack micro-ring 2, gold electrode 3, silicon oxide 4, lithium niobate 5, output end 6, surface plasmonic waveguide 7, surface plasmonic waveguide cross section 8, lithium niobate ridge waveguide 9, vertical coupling structure 10, sharp taper mode conversion structure 11;

[0014] Figure 2 is a detailed diagram of the sharp taper mode conversion structure of the present application;

[0015] Figure 3 is a diagram of the cross-sectional electric field distribution of the surface plasmonic waveguide of the present application;

[0016] Figure 4 is a schematic diagram of the vertical coupling structure of the present application, and a schematic diagram of its related variant scheme;

[0017] Figure 5 is a schematic diagram of the manufacturing process steps of the present application;

[0018] Figure 6 is a schematic diagram of the transmission spectrum of the output end of the present application;

[0019] Figure 7 is a schematic diagram of the optical modulation amplitude of the present application compared with the conventional surface plasmon;

[0020] Figure 8 is a schematic diagram of the electric field distribution of the micro-ring electro-optical modulator of the present application from the input end to different regions of the output end;

[0021] Figure 9 is a schematic diagram of the calculation results of the coupling parameters between the micro-ring and the straight waveguide of the present application;

[0022] Figure 10 is a schematic diagram of the calculation results of the key parameters of the vertical coupling structure of the present application;

[0023] Figure 11 is a flowchart of the preparation method of the present application;

[0024] In the figure: step 1 in the dashed box is the manufacturing step of the silicon racetrack micro-ring, and the subsequent steps 2 to 7 are the cross-sectional schematic diagrams of the racetrack ring region at each step and the cross-sectional schematic diagrams of the surface plasmonic region at each step;

[0025] Figure 12 is the transmission spectrum curve of the surface plasmonic micro-ring modulator of the present application and the resonance peak shift transmission spectrum thereof with applied voltage;

[0026] Figure 13For surface plasmonic waveguide region SEM image. DETAILED DESCRIPTION

[0027] As Figure 1 shown, the embodiment relates to a lithium niobate thin film electro-optic modulator based on surface plasmon effect, comprising: a silicon oxide layer 4 and a lithium niobate layer 5, wherein: the upper surface of the lithium niobate 5 is respectively provided with an under-coupled silicon racetrack micro-ring 2 and a straight waveguide, the lithium niobate layer 5 is provided with a surface plasmonic waveguide 7 composed of a lithium niobate ridge waveguide 9 and an electrode 3, both ends of the lithium niobate ridge waveguide are provided with a tapered mode conversion structure 11 in the form of a trapezoid in plan view, and both ends of the straight waveguide are respectively an input end 1 and an output end 6.

[0028] The lithium niobate ridge waveguide 9 is located between a pair of gold electrodes 3.

[0029] As Figure 4 shown, the silicon racetrack micro-ring 2 is in a closed structure or an open structure.

[0030] As Figure 4 b and Figure 4 d shown, when the silicon racetrack micro-ring 2 is in a closed structure, the upper silicon racetrack ring waveguide and the lithium niobate tapered mode conversion structure 11 form a vertical coupling structure 10, which can realize that part of the light waves in the ring enter the surface plasmonic waveguide downward and part of the light propagates in the micro-ring.

[0031] As Figure 4 a and Figure 4 c shown, when the silicon racetrack micro-ring 2 is in an open structure, the end of the silicon racetrack micro-ring 2 is provided with a triangular silicon waveguide 12, and the corresponding position of the lithium niobate ridge waveguide 9 is a lithium niobate tapered mode conversion structure 11, and the triangular silicon waveguide 12 and the lithium niobate tapered mode conversion structure 11 form a vertical coupling structure 10 to improve the efficiency of converting light waves into surface plasmonic waves.

[0032] The input end 1 of the straight waveguide receives light of the C band, part of the wavelength light is coupled into the silicon racetrack micro-ring 2, the light waves in the ring enter the surface plasmonic waveguide downward through the vertical coupling structure 10 and are converted into surface plasmonic waves. The surface plasmonic waves propagate along the interface between the lithium niobate ridge waveguide 9 and the electrode. At this time, the signal to be propagated can be converted into an electrical signal form and directly loaded onto the metal electrode. The electrical signal will be converted into a resonant light wave peak shift through the electro-optic effect of lithium niobate, and finally the signal modulation is realized.

[0033] As Figure 5 and Figure 11 shown, the embodiment relates to an implementation method based on the above-mentioned electro-optic modulator, which deposits single crystal silicon on a lithium niobate insulating silicon oxide platform (LNOI) to process an electro-optic modulator, specifically including:

[0034] Step 1) fabrication of a silicon racetrack micro-ring as shown in Figure 5 a and b: a 300 nm-thick silicon film with a refractive index of 3.4 was deposited on a lithium niobate film by PECVD, and a 300 nm-thick silicon racetrack micro-ring with an opening was obtained by ICP etching. The micro-ring structure is shown in Figure 4 a and b.

[0035] The lithium niobate layer of the lithium niobate film has a thickness of 600 nm and a refractive index of 2.2, the buried oxide layer has a thickness of 300 nm and a refractive index of 1.44, and the upper cladding layer is air with a refractive index of 1.

[0036] Step 2) fabrication of a lithium niobate ridge waveguide as shown in Figure 4 c: a photoresist pattern was first prepared on the surface of the device by electron beam exposure, and then the lithium niobate layer was etched by ICP etching to a depth of 300 nm. The cross-section is shown in Figure 8 Step 3.

[0037] The lithium niobate ridge waveguide 9 has a length of 7.5 μm and a width of 80 nm.

[0038] Step 3) fabrication of a metal electrode as shown in Figure 5 d: gold was deposited in the etched pit of the lithium niobate layer by electron beam evaporation to ensure that the gold electrode is connected to the narrow lithium niobate ridge waveguide, and the thickness of the deposited gold is 300 nm.

[0039] The smoothness of the sidewalls is very high due to the transmission of surface plasmon waves, Figure 5 The upper and lower electrodes in d are made in two steps according to the standard lift-off process. First, a single-sided electrode is made to avoid the problems of uneven metal falling off in the middle surface plasmon waveguide area, misalignment of the electrode and the waveguide, and failure to maintain close connection between the electrode and the waveguide when the electrodes are prepared at the same time. Figure 13 a and b respectively show SEM images of one-step and two-step electrode preparation.

[0040] According to the experimental method, a laser with a wavelength of 1550 nm enters the prepared modulator by grating coupling from the laser, and the output spectrum is connected to a fiber spectrometer to scan the transmission spectrum. The results are shown in Figure 6 a and b. Figure 12 a and b.

[0041] Compared with traditional thin-film lithium niobate modulators, the device size is reduced from millimeter level to micron level. The modulator occupies an area of 10 μm x 20 μm, which is reduced by two orders of magnitude. The modulation efficiency is improved by two orders of magnitude to about 0.09 Vcm, and the simulation calculation shows that the 3dB bandwidth is greater than 1 THz.

[0042] The electro-optical modulator is simulated by using Lumerical FDTD simulation software, and the light field simulation of the surface plasmon region is as shown in Figure 3 As shown in the figure, the light is converted into surface plasmon body waves and conducted on the gold electrode side wall, greatly enhancing the action of the electric field and the light field.

[0043] By using the Monte Carlo method simulation, the modulation amplitude comparison results of the surface plasmon micro-ring electro-optical modulator and the prior art surface plasmon Mach-Zehnder modulator are as shown in Figure 6 Compared with the general Mach-Zehnder structure, the light intensity modulation amplitude of the racetrack ring surface plasmon modulator is about doubled, which means that the driving voltage is reduced by three times, and thus the modulation efficiency is increased by 7 times, and the power consumption is reduced to one-seventh. Compared with the prior art surface plasmon modulator, the modulation power consumption of the present application is significantly reduced, and the modulation efficiency is greatly improved.

[0044] Compared with the prior art, the over-coupled micro-ring and vertical coupling structure of the device design combines the ultra-small size lithium niobate surface plasmon waveguide and the micro-ring to realize the ultra-compact area, low power consumption and ultra-large bandwidth electro-optical micro-ring modulator.

[0045] The above specific embodiments can be adjusted in different ways by those skilled in the art without departing from the principles and purposes of the present application, the protection scope of the present application is subject to the claims and is not limited by the above specific embodiments, and each implementation scheme within the scope is subject to the constraints of the present application.

Claims

1. A surface plasmon based micro-ring electro-optic modulator, characterized in that, The application relates to an optical modulator, which comprises: a silicon layer and a lithium niobate layer, wherein: a silicon race track micro ring and a straight waveguide constituting a resonance ring are arranged on the upper surface of the lithium niobate layer; a lithium niobate sharp cone mode conversion structure, a lithium niobate ridge waveguide and a pair of gold electrodes are arranged in the lithium niobate layer; the two ends of the straight waveguide are respectively an input end and an output end; and the lithium niobate ridge waveguide is located between the pair of gold electrodes; the silicon race track micro ring is in a closed structure or an open structure and is in an over-coupling state between the silicon race track micro ring and the straight waveguide; the silicon race track micro ring is in an open structure and has an uncoupling side opening; the end of the silicon race track micro ring is provided with a triangular silicon waveguide; the lithium niobate ridge waveguide at the corresponding position is the lithium niobate sharp cone mode conversion structure; and the triangular silicon waveguide and the lithium niobate sharp cone mode conversion structure constitute a vertical coupling structure to improve the conversion efficiency of light waves into surface plasmon waves in the lithium niobate layer; the silicon race track micro ring is in a closed structure; part of the light waves in the ring enter the surface plasmon waveguide downward; and part of the light waves propagate in the ring; the narrow waveguide in the ring and the lithium niobate sharp cone mode conversion structure constitute a vertical coupling structure.

2. The surface plasmon-based micro-ring electro-optic modulator of claim 1, wherein, The modulator receives C-band light through the input end of the straight waveguide; part of the wavelength light is coupled into the silicon race track micro ring; the light waves in the ring enter the surface plasmon waveguide downward through the vertical coupling structure and are converted into surface plasmon waves; the surface plasmon waves propagate along the interface between the lithium niobate ridge waveguide and the electrodes; at this time, the signal to be propagated can be converted into an electric signal form and directly loaded onto the metal electrode; the electric signal is converted into a peak position shift of the resonant light wave through the electro-optic effect of lithium niobate to realize modulation.

3. The surface plasmon-based micro-ring electro-optic modulator of claim 1, wherein, The distance, coupling length, length, width and width of the silicon race track micro ring and the straight waveguide, the length, width and width of the triangular silicon waveguide and the length, width and width of the lithium niobate sharp cone mode conversion structure are optimized through a particle swarm algorithm.

4. A method of implementing the electro-optic modulator of any one of claims 1-3, wherein, The application relates to an optical modulator, which comprises: step 1) depositing a silicon film on a lithium niobate film through PECVD and etching the silicon film through ICP to obtain a race track micro ring with an opening; step 2) preparing a lithium niobate ridge waveguide: after preparing a photoetching pattern on the surface of the device through electron beam exposure, etching the lithium niobate layer through ICP; step 3) preparing a metal electrode: depositing gold in the etching pit of the lithium niobate layer through electron beam evaporation to ensure that the gold electrode is connected with the narrow lithium niobate ridge waveguide.

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