Lithium niobate film on-chip polarizing structure using plasma surface waves

Abstract

The invention belongs to the field of on-chip lithium niobate film optical waveguide devices, and provides a lithium niobate film on-chip polarization structure using plasma surface waves, which comprises a silicon substrate, a silicon dioxide substrate, a lithium niobate ridge waveguide and a silicon dioxide buffer layer which are sequentially stacked from bottom to top, the input end of the lithium niobate ridge waveguide is provided with a metal coating. According to the invention, the metal coating is attached to the surface of the lithium niobate ridge waveguide, so that a TM surface mode is formed. The effective refractive index imaginary part of the TM surface mode is larger than that of a common mode, so that the attenuation of the TM surface mode along with the transmission distance is larger, and the bias selection function is realized.

Description

Technical Field

[0001] The present invention belongs to the field of lithium niobate thin film optical waveguide devices on a chip, and particularly relates to a polarization structure on a lithium niobate thin film chip using surface plasmon waves. Background Art

[0002] In electromagnetic compatibility design and rectification, electric field measurement plays a crucial role. Accurately and quickly obtaining electric field information can bring convenience to equipment research and development, design, rectification, and use. Traditional electric field sensors use antennas as receiving ends, and the detected near-field signals are input into receiving devices such as spectrum analyzers, receivers, and ADCs through RF transmission lines for detection, so as to obtain electric field information. This detection method has high sensitivity and strong versatility. However, due to problems such as narrow bandwidth, large volume, and strong interference existing in the antenna itself, and factors such as losses caused by feeders and RF transmission lines during long-distance transmission and disturbances in the surrounding electromagnetic environment, this measurement method has defects such as low frequency flatness, large error in measurement results (amplitude), and poor measurement repeatability.

[0003] With the maturity and commercialization of lithium niobate thin film technology, electric field measurement sensors have entered a new stage of development. The refractive index difference between the core layer and the substrate of the lithium niobate thin film optical waveguide can be about 0.7, which greatly improves the light beam confinement ability of the optical waveguide, reduces the waveguide width to less than 1 μm, and at the same time can significantly reduce the electrode spacing, improving the electrode gain and sensitivity. Moreover, due to the improvement of the electrode gain, the half-wave electric field of the sensor can be significantly reduced, so as to achieve a predetermined modulation depth with a shorter waveguide structure, expanding the working bandwidth of the sensor. However, since two polarization modes can be transmitted in the lithium niobate thin film optical waveguide and the equivalent refractive indices of the two polarization modes are different, the corresponding linear operating points are different, which will lead to a reduction in the working efficiency of the sensor. Summary of the Invention

[0004] Aiming at the deficiencies in the prior art, the present invention proposes a polarization structure on a lithium niobate thin film chip using surface plasmon waves. In this method, a partial metal cladding is covered on the surface of the lithium niobate ridge waveguide of the lithium niobate thin film, so as to form a TM surface plasmon mode that can be transmitted, and further achieve TE mode polarization selection.

[0005] The technical solution of the present invention is specifically as follows:

[0006] A polarization structure on a lithium niobate thin film chip using surface plasmon waves, comprising a silicon substrate, a silicon dioxide substrate, a lithium niobate ridge waveguide, and a silicon dioxide buffer layer stacked in sequence from bottom to top;

[0007] A metal cladding is provided at the input end of the lithium niobate ridge waveguide.

[0008] Preferably, the width of the metal cladding is equal to that of the lithium niobate ridge waveguide.

[0009] Preferably, the height of the metal cladding is 20 nm, the length is 50 μm, and the floating range does not exceed 1 dB.

[0010] Preferably, the material of the metal cladding is gold.

[0011] Preferably, the thickness of the silicon substrate is 0.5 mm, the thickness of the silicon dioxide substrate is 4.7 μm, the thickness of the lithium niobate ridge waveguide is 0.4 μm, the width is 1 μm, and the thickness of the silicon dioxide buffer layer is 1 μm.

[0012] Preferably, the lithium niobate thin film on-chip polarization structure can transmit the TE component in the input light, attenuate the TM component, and the isolation degree is 36 dB.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] In the present invention, a TM surface mode is formed by attaching a metal cladding to the surface of the lithium niobate ridge waveguide. The imaginary part of the equivalent refractive index of the TM surface mode is larger than that of the ordinary mode, so its attenuation with the transmission distance is also larger, thereby realizing the polarization selection function. Description of the Drawings

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. By referring to the drawings, the features and advantages of the present invention can be more clearly understood. The drawings are schematic and should not be construed as limiting the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0016] Figure 1 It is a cross-sectional schematic diagram of the lithium niobate thin film on-chip polarization structure using the surface plasma wave of the present invention.

[0017] Figure 2 It is a schematic diagram of the lithium niobate ridge waveguide and the metal cladding in the present invention.

[0018] Figure 3 It is the polarization effect diagram of the lithium niobate thin film on-chip polarization structure of the present invention.

[0019] 1 - silicon substrate, 2 - silicon dioxide substrate, 3 - lithium niobate ridge waveguide, 4 - silicon dioxide buffer layer, 5 - metal cladding. Detailed Embodiments

[0020] In order to more clearly understand the above objects, features, and advantages of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0021] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0022] The working principle of the lithium niobate thin film on-chip polarization structure utilizing the surface plasmon of the present invention is as follows:

[0023] 1. Metal optical frequency characteristics

[0024] As a conductive medium, the dielectric constant of a metal exhibits different characteristics at different frequency bands, which is mainly due to the non-negligible conductivity of the conductive medium. Assuming that the conductivity of the conductive medium is σ, the complex dielectric constant of the conductive medium is

[0025]

[0026] where ε is the real part of the dielectric constant, equivalent to the dielectric constant of the medium, ε 0 is the vacuum dielectric constant, ω is the angular frequency of the electromagnetic wave, and j is the imaginary unit. Correspondingly, the refractive index of the conductive medium is satisfies

[0027] The Helmholtz equation of the conductive medium is

[0028]

[0029] where E is the electric field strength, is the Laplace operator, and k 0 is the wave number in vacuum.

[0030] Assuming that the electromagnetic wave propagates in the z direction in the conductive medium, then there is

[0031]

[0032] where E(z) is the electric field strength varying with the position z, and E 0 is the initial amplitude of the electric field strength.

[0033] Let

[0034]

[0035] where n is the real part of the refractive index of the conductive medium.

[0036] Then

[0037] E(z) = E 0 exp(-αz)exp[j(ωt - k 0 nz)] (5)

[0038] It can be seen that the electromagnetic wave propagating in the conductive medium will attenuate with the increase of the propagation distance. The attenuation coefficient is α, which is related to the imaginary part of the refractive index and determines the attenuation of the amplitude; while the real part is related to the phase of the electromagnetic wave propagation and determines the phase velocity of the electromagnetic wave propagation.

[0039] The relationship between the conductivity and the angular frequency of the electric field in the conductive medium is

[0040]

[0041] where N is the charge density, e is the electron charge, m is the electron mass, and γ is the attenuation constant of the electron motion, and its typical value is 10 14 . From the above formula, the following conclusions can be obtained:

[0042] (1) When ω << γ, σ is a real number, and the conductivity of the metal is independent of the frequency.

[0043] (2) When ω is comparable to γ, the conductivity of the metal is frequency-dependent.

[0044] (3) When ω >> γ, the conductivity of the metal is close to a pure imaginary number and its value is extremely small. The metal exhibits the characteristics of a dielectric, that is, it is transparent to electromagnetic waves. The ray flaw detector makes use of this characteristic of high-frequency electromagnetic waves.

[0045] Substituting Equation (6) into Equation (1) gives

[0046]

[0047] where is the plasma oscillation frequency formed by the conductive medium under the action of the electric field, which is about 10 16 Hz magnitude. Considering that in the optical frequency range, ε ≈ 1 usually, then the real part of the dielectric constant is negative at this time, and the imaginary part of the dielectric constant is also negative. Generally, the magnitude of the imaginary part of the dielectric constant in the optical frequency range is much smaller than the real part.

[0048] 2. Plasma surface wave

[0049] When certain conditions are met, electromagnetic waves can be transmitted at the interface between the metal and the dielectric, and its amplitude decays exponentially with the distance from the interface, which is called the surface plasma wave (SPW).

[0050] Assume that the plasma surface wave propagates along the z-axis direction, the x = 0 surface is the dielectric interface, the part where x < 0 is the dielectric region with a dielectric constant of ε 1 , and the part where x > 0 is the metal region with a dielectric constant of ε 2 . The propagation is unrestricted in the y direction, that is, the partial derivative with respect to y Let the electric field strengths of the plasma surface wave in the two regions be

[0051]

[0052] where represents the electric field strength of the plasma surface wave in the dielectric region, and E 1 represents the amplitude of this electric field strength, represents the electric field strength of the plasma surface wave in the metal region, and E 2 represents the amplitude of this electric field strength, t represents time, z represents the position in the z-axis direction, and α 1 represents the attenuation coefficient of the plasma surface wave in the dielectric region, and α 2 represents the attenuation coefficient of the plasma surface wave in the metal region, β is the phase constant, and α 1 , α 2 , β > 0.

[0053] Since

[0054]

[0055] where k is the wave number, is the electric field strength vector.

[0056] Substituting Equation (8) into (9) gives

[0057]

[0058] Also, since then there is

[0059]

[0060] where is the gradient operator, · represents the dot product, which is used here to calculate the dot product between the gradient operator and the electric field vector, that is, the divergence, and E ix , E iy , E iz respectively represent the components of the electric field strength in the x-axis, y-axis, and z-axis directions.

[0061] From Equations (11) and (8), we get

[0062]

[0063] In addition, according to the requirements of the electric field boundary conditions for tangential continuity and normal discontinuity, it can be obtained that the electric field components on both sides of the interface also satisfy

[0064]

[0065] Equations (12) and (13) are the electric field boundary conditions satisfied by the plasma surface wave. Similarly, the magnetic field boundary conditions satisfied by the plasma surface wave can be obtained as

[0066]

[0067] and

[0068]

[0069] where H ix , H iy , H iz respectively represent the components of the magnetic field intensity on the x-axis, y-axis, and z-axis.

[0070] According to Maxwell's equations

[0071]

[0072] where represents the magnetic field intensity, μ 0 is the magnetic permeability in vacuum, i, j, k are the coordinate unit vectors in three-dimensional space, represents the curl of the calculated vector.

[0073] It can be obtained that

[0074]

[0075] Substituting Equation (8) into the above equation, it can be obtained that

[0076]

[0077] Substituting Equation (12) into the above equation and considering (10), it can be obtained that

[0078]

[0079] Substituting Equations (13) and (15) into Equation (19), it can be obtained that

[0080]

[0081] Since α 1 , α 2 >0, then ε 1 , ε 2 have opposite signs, and E 1y = E 2y = 0. Substituting into Equation (19), it can be known that H1x = H 2x = H 1z = H 2z = 0, so this surface plasma wave is a TM wave.

[0082] From the above analysis, it can be seen that the surface plasma wave has the following characteristics:

[0083] (1) The surface plasma wave can only be a TM wave.

[0084] (2) The surface plasma wave can only exist when the dielectric constants on both sides of the dielectric interface have opposite signs. From the previous analysis, it can be seen that in the optical frequency band, the dielectric constant of metal is negative. Therefore, a metal layer deposited on the dielectric surface can form a surface plasma wave.

[0085] (3) From equations (10) and (20), we can obtain Since ε 1 + ε 2 < 0, then That is n 1 represents the refractive index of the dielectric. From the above analysis, it can be seen that the real part of the effective refractive index of the surface plasma mode is greater than the refractive index of the dielectric in contact with the metal, while the effective refractive index of the guided mode in a general dielectric waveguide is less than the refractive index of the core layer.

[0086] 3. Polarization selection structure of metal-clad waveguide

[0087] According to the above analysis, attaching metal to the waveguide surface can form a TM surface mode. The imaginary part of the effective refractive index of the TM surface mode is relatively large compared with that of the ordinary mode. Therefore, its attenuation with the transmission distance is also relatively large. Based on this characteristic, the polarization selection function can be realized.

[0088] Next, a specific example is used to verify the above principle.

[0089] Example 1

[0090] The present invention provides a polarization selection structure on a lithium niobate thin film chip, as Figure 1-2 shown. The polarization selection structure on the lithium niobate thin film chip includes the following structures from bottom to top:

[0091] A silicon substrate with a thickness of 0.5 mm, a silica substrate with a thickness of 4.7 μm, a lithium niobate ridge waveguide with a thickness of 0.4 μm and a width of 1 μm, and a silica buffer layer with a thickness of 1 μm. A metal cladding with a height of 20 nm, a width of 1 μm, and a length of 50 μm is covered on the lithium niobate ridge waveguide. The metal cladding is located at a position close to the input end on the lithium niobate ridge waveguide.

[0092] Figure 3Shows the polarization effect of the in-plane polarization structure of the lithium niobate thin film of the present invention. When a Gaussian beam is input into the waveguide, when the input Gaussian beam is in the TE mode, the TE mode can be transmitted (see Figure 3 (a)); when the input light is in the TM mode, the TM mode will be cut off (see Figure 3 (b)). Thus, it can be seen that the polarization structure of the present invention realizes the polarization selection function.

[0093] In the present invention, unless otherwise clearly specified and defined, terms such as "installation", "connection", "connection", "fixation" and the like shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0094] In the present invention, unless otherwise clearly specified and defined, the first feature being "above" or "below" the second feature may include the direct contact between the first and second features, or may include the situation where the first and second features are not in direct contact but in contact through other features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes that the first feature is directly above and obliquely above the second feature, or merely means that the horizontal height of the first feature is higher than that of the second feature. The first feature being "below", "under" and "beneath" the second feature includes that the first feature is directly below and obliquely below the second feature, or merely means that the horizontal height of the first feature is lower than that of the second feature.

[0095] In the present invention, the terms "first", "second", "third", "fourth" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. The term "plurality" means two or more, unless otherwise clearly defined.

[0096] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lithium niobate thin film on-chip polarization structure using plasma surface waves, characterized in that: It includes a silicon substrate, a silicon dioxide substrate, a lithium niobate ridge waveguide and a silicon dioxide buffer layer which are stacked in sequence from bottom to top; The input end of the lithium niobate ridge waveguide is provided with a metal coating.

2. The lithium niobate thin film on-chip polarization structure according to claim 1, characterized in that: The width of the metal cladding layer is equal to that of the lithium niobate ridge waveguide.

3. The lithium niobate thin film on-chip polarization structure according to claim 2, characterized in that: The metal coating has a height of 20 nm and a length of 50 μm.

4. The lithium niobate thin film on-chip polarization structure according to claim 1, characterized in that: The material of the metal coating is gold.

5. The lithium niobate thin film on-chip polarization structure according to claim 1, characterized in that: The thickness of the silicon substrate is 0.5 mm, the thickness of the silicon dioxide substrate is 4.7 μm, the thickness of the lithium niobate ridge waveguide is 0.4 μm and the width is 1 μm, and the thickness of the silicon dioxide buffer layer is 1 μm.

6. The lithium niobate thin film on-chip polarization structure according to claim 1, characterized in that: The polarizing structure on the lithium niobate thin film can transmit the TE component of the input light and attenuate the TM component, with an isolation of 36dB.

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