CN115933274A - Terahertz wave generator and preparation method thereof - Google Patents
Terahertz wave generator and preparation method thereof Download PDFInfo
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Abstract
The embodiment of the application relates to the field of optical communication, and provides a terahertz wave generator and a preparation method thereof. When the first signal light and the second signal light irradiate the cross section of the terahertz wave generator, the first signal light and the second signal light generate terahertz waves after difference frequency, and the terahertz waves are transmitted out along the surface of the second waves. The terahertz wave generator can generate terahertz waves according to existing signal light, is simple in structure and easy to achieve, is high in conversion rate of the existing signal light, and can generate more effective terahertz waves.
Description
Technical Field
The application relates to the technical field of optical communication, in particular to a terahertz wave generator and a preparation method thereof.
Background
Terahertz waves, a wave having a frequency range between 0.1HZ and 10THZ, can be applied in many different fields, such as terahertz spectroscopy, biomedical diagnosis, material identification, 6G high-speed communications, and the like. In the whole spectrum, the terahertz wave is the only wave which can simultaneously meet the ultra-wide bandwidth and high data rate of optical fiber communication and the wireless transmission capability of microwave communication.
Therefore, in order to realize further research on THz waves, an on-chip terahertz wave generating device is needed.
Disclosure of Invention
The embodiment of the application provides a terahertz wave generator and a preparation method thereof, the terahertz wave generator can generate terahertz waves through the existing signal light conversion, the conversion effect is high, and more effective terahertz waves can be generated.
A first aspect of embodiments of the present application provides a terahertz wave generator, including:
the terahertz wave generating device comprises a substrate and at least one terahertz wave generating structure, wherein the substrate is used for supporting the at least one terahertz wave generating structure;
the terahertz wave generating structure comprises a first waveguide and a second waveguide made of a metal material, wherein the second waveguide comprises at least one first periodic structure and at least one second periodic structure; the first waveguide is arranged between the first periodic structure and the second periodic structure, and gaps which are periodically distributed along the direction of the first waveguide are arranged on one sides of the first periodic structure and the second periodic structure, which pass through the first waveguide;
under the condition that first signal light and second signal light irradiate the cross section of a terahertz wave generator, the first signal light and the second signal light propagate in the first waveguide, and generate terahertz waves after carrying out difference frequency in the process of propagation, wherein the terahertz waves are transmitted along the surface of the first periodic structure and the surface of the second periodic structure.
A second aspect of embodiments of the present application provides a method of manufacturing a terahertz wave generator, the method including:
determining a first material for preparing a first waveguide and a second material for preparing a second waveguide;
determining the wavelength of the first signal light and the wavelength of the second signal light according to the frequency of the terahertz wave;
determining a parameter of the first waveguide and a parameter of the second waveguide according to a first condition that a difference value of a first propagation constant of the first signal light and a second propagation constant of the second signal light is equal to a third propagation constant of the terahertz wave; the terahertz waves are generated by the first signal light and the second signal light through a difference frequency;
preparing the first waveguide on a substrate according to the parameters of the first waveguide based on the first material;
and preparing the second waveguide on the substrate according to the parameters of the second waveguide based on the second material.
The technical scheme provided by the embodiment of the application can at least achieve the following beneficial effects:
the terahertz wave generator comprises a substrate and at least one terahertz wave generating structure, the substrate is used for supporting the at least one terahertz wave generating structure, the terahertz wave generating structure comprises a first waveguide and a second waveguide made of metal materials, the second waveguide comprises at least one first periodic structure and at least one second periodic structure, the first waveguide is arranged between the first periodic structure and the second periodic structure, and gaps which are periodically distributed along the direction of the first waveguide are arranged on one sides of the first periodic structure and one side of the second periodic structure, which are close to the first waveguide, respectively. When the first signal light and the second signal light irradiate the cross section of the terahertz wave generator, the first signal light and the second signal light are transmitted in the first waveguide, and generate terahertz waves after difference frequency is carried out in the transmission process, wherein the terahertz waves are mainly transmitted along the surface of the first periodic structure facing the first waveguide and the surface of the second periodic structure facing the first waveguide. The terahertz wave generator can generate terahertz waves according to existing signal light, is simple in structure and easy to achieve, is high in conversion rate of the existing signal light, and can generate more effective terahertz waves.
Drawings
Fig. 1 is a schematic structural diagram of a terahertz wave generating structure according to an exemplary embodiment of the present application;
FIG. 2 is a schematic structural diagram of a first periodic structure and a second periodic structure according to an exemplary embodiment of the present application;
fig. 3 is a schematic structural diagram of another terahertz wave generating structure shown in an exemplary embodiment of the present application;
FIG. 4 is a side view of a terahertz wave generating structure shown in an exemplary embodiment of the present application;
FIG. 5 is a graph illustrating propagation constants and mode field distributions for a communications band in accordance with an exemplary embodiment of the present application;
FIG. 6 is a dispersion curve and its corresponding mode profile for a second waveguide shown in an exemplary embodiment of the present application;
fig. 7 (a) is a graph showing a relationship between a terahertz wave conversion efficiency and a propagation distance under a condition that phase matching is satisfied according to an exemplary embodiment of the present application;
FIG. 7 (b) is a graph illustrating the change in overlap integral factor and effective mode area under the condition of satisfying phase matching according to an exemplary embodiment of the present application;
fig. 7 (c) is a transmission spectrum of a terahertz wave shown in an exemplary embodiment of the present application;
FIG. 8 illustrates an effective nonlinear coefficient d according to an exemplary embodiment of the present application eff And TE 1 E of mode z A component distribution;
FIG. 9 (a) is a diagram illustrating another variation of overlap integral factor and effective mode area under the condition of satisfying phase matching according to an exemplary embodiment of the present application;
fig. 9 (b) is a diagram illustrating another relationship between the conversion efficiency of the terahertz wave with the propagation distance under the condition that the phase matching is satisfied according to an exemplary embodiment of the present application;
fig. 9 (c) is a transmission spectrum of another terahertz wave shown in an exemplary embodiment of the present application;
fig. 10 is a method for manufacturing a terahertz wave generator provided by the present application.
Reference numerals:
1000. a terahertz wave generator; 10. a substrate; 20. a terahertz wave generating structure; 30. a protective structure;
101. a substrate; 102. a buffer layer; 201. a first waveguide; 202. a second waveguide;
2021. a first periodic structure; 2022. a second periodic structure.
Detailed Description
Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the application, as detailed in the appended claims.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in this application and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this application belongs. The use of "first," "second," and similar terms in the description and in the claims does not indicate any order, quantity, or importance, but rather is used to distinguish one element from another. Also, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one. "plurality" or "a number" means two or more. Unless otherwise indicated, "front", "rear", "lower" and/or "upper" and the like are for convenience of description and are not limited to one position or one spatial orientation. The word "comprising" or "comprises", and the like, means that the element or item listed as preceding "comprising" or "includes" covers the element or item listed as following "comprising" or "includes" and its equivalents, and does not exclude other elements or items. The terms "connected" or "coupled" and the like are not restricted to physical or mechanical connections, but may include electrical connections, whether direct or indirect.
Terahertz waves, which are waves with a frequency range between 0.1HZ and 10THZ, can be applied in many different fields, such as terahertz spectroscopy, biomedical diagnostics, material identification, 6G high-speed communications, etc. In the whole frequency spectrum, the terahertz wave is the only wave which can simultaneously meet the ultra-wide bandwidth and high data rate of optical fiber communication and the wireless transmission capability of microwave communication.
However, an effective THz source generating device is not available at present, so the present application provides a terahertz wave generator 1000, and the terahertz wave generator 1000 can generate terahertz waves after performing difference frequency according to existing signal light, has high conversion efficiency, and can generate more effective terahertz waves.
Next, a structure of terahertz wave generation provided by the present application will be described.
As shown in fig. 1, fig. 1 is a schematic structural diagram of a terahertz-wave generator 1000 provided by the present application, wherein the terahertz-wave generator 1000 includes a substrate 10 and at least one terahertz-wave generating structure 20.
The substrate 10 is used to support at least one terahertz-wave generating structure 20.
Continuing to be shown in fig. 1, the base 10 includes a substrate 101 and a buffer layer 102.
The shape of the substrate 101 may be a rectangular parallelepiped, a cube, a cylinder, a prism, or the like, which is not limited in this application. The substrate 101 may be a clean single crystal wafer with a specific crystal plane and appropriate electrical, optical and mechanical properties for growing epitaxial layers, which can support and improve thin film properties. The material of the substrate 101 is, for example, silicon (Si), the length and the width of the substrate 101 may be greater than or equal to the length or the width of the buffer layer 102, the thickness of the substrate 101 is greater than the thickness of the buffer layer 102, and the thickness of the substrate 101 is generally set to be more than two hundred micrometers, for example, may be set to be 300 micrometers. The dimension between the first waveguide and the second waveguide is in the micrometer range, and the movement requires the support of the base, so long as it can support other structures disposed thereon, and therefore, the application is not particularly limited with respect to the material and thickness of the substrate 101.
The buffer layer 102 is disposed on the upper surface of the substrate 101. The buffer layer 102 may be disposed on the upper surface of the substrate 101 by a bonding process, a pressing process, or the like, which is not limited in this application. The shape of the buffer layer 102 may be a rectangular parallelepiped, a cube, a cylinder, a prism, or the like, and the shape of the buffer layer 102 may be the same as that of the substrate 101 or may be different from that of the substrate 101. The buffer layer 102 can prevent the first signal light, the second signal light and the terahertz wave from leaking to the substrate 101, thereby improving the first signal light and the second signal lightThe conversion efficiency of the signal light into the terahertz wave is improved, so that more terahertz waves are generated. The buffer layer 102 may be made of, for example, silicon dioxide (SiO) 2 ) And the like. The buffer layer 102 is disposed on the substrate 101, so the length and width of the buffer layer 102 may be smaller than those of the substrate 101, or may be equal to those of the substrate 101. And since the buffer layer 102 is disposed between the first waveguide 201 and the substrate 101, which functions to prevent leakage of light, the thickness of the buffer layer 102 needs to be greater than the wavelength of the second signal light. It is to be noted here that the wavelength of the first signal light is smaller than the wavelength of the second signal light.
For example, the substrate 10 may be provided by a commercial manufacturer, and the thickness and width of the entire substrate 10 may be determined by the terahertz waves, so that the substrate 10 can better support the at least one terahertz wave generating structure 20. The upper surface of the substrate 10 may be provided with one terahertz wave generating structure 20, two terahertz wave generating structures 20, three terahertz wave generating structures 20, and the like, which is not limited in this application.
The substrate 10 is used to support at least one terahertz-wave generating structure 20, and the terahertz-wave generating structure 20 is described below, and as shown in fig. 1 in continuation, the terahertz-wave generating structure 20 includes a first waveguide 201 and a second waveguide 202, the first waveguide 201 being disposed on the buffer layer 102, and the second waveguide 202 being disposed on the first waveguide 201.
The material of the first waveguide 201 may be, for example, gallium phosphide (GaP), gallium arsenide (GaAs), gallium selenide (GaSe), lanthanide (LN), etc., and the material of the first waveguide 201 is not specifically limited in the present application, as long as the material of the first waveguide 201 satisfies the second-order nonlinear tensor χ (2) Not zero.
Arranging a substrate film required by a first waveguide on the buffer layer 102, and preparing the first waveguide on the buffer layer 102 through a waveguide etching process; a second waveguide is fabricated on the buffer layer 102 by means of metallization. If the thickness of the substrate film is equal to the desired cross-sectional height h of the first waveguide e Then, when the waveguide is etched, the substrate film is directly penetrated, and then the second waveguide formed by the metal coating is directly positioned on the buffer layer 102The above step (1); but generally a substrate film is used having a thickness greater than the desired first waveguide cross-sectional height h e Then, after the substrate film is etched to form the first waveguide, a layer of substrate may remain on the buffer layer, and then the second waveguide formed by the metal plating film is directly located on the remaining substrate.
In the process of preparing the first waveguide 201, if the first waveguide 201 is etched on the upper surface of the buffer layer 102 by etching, it is difficult to vertically etch in the etching process in many cases, so that the protrusion 2011 with different widths of the upper base and the lower base is formed. However, if the substrate of the first waveguide 201 is thin, the first waveguide 201 may also be etched to form the first waveguide 201 having the same width of the upper bottom and the lower bottom, the shape of the first waveguide 201 may be, for example, any one of a ridge structure, a trapezoid structure, or a rectangle structure, and after the first waveguide 201 sets the parameters thereof to the preset parameters, the difference between the propagation constant of the first signal light and the propagation constant of the second signal light can be equal to the propagation constant of the terahertz wave. After the substrate 10 is completely set, the parameters of the first waveguide 201 may be continuously set according to the preset first condition and the thickness of the substrate 10, so that the laser can etch the first waveguide 201 according to the preset parameters when performing laser etching. Wherein the first condition is that a difference value of the first propagation constant of the first signal light and the propagation constant of the second signal light is equal to a third propagation constant of the terahertz wave. It should be noted here that the first signal light is generally referred to as pump light, and the second signal light is generally referred to as signal light.
The parameter of the first waveguide 201 may be a cross-sectional dimension including the first waveguide 201. The cross-sectional dimension of the first waveguide 201 may be obtained by simulation from the wavelength of the first signal light, the wavelength of the second signal light, the frequency of the first signal light, the frequency of the second signal light, the frequency of the terahertz wave, a first propagation constant, a second propagation constant, a third propagation constant, and the like.
Since the terahertz wave irradiates the cross section of the terahertz wave generator 1000 through the first signal light and the second signal light, the first signal light and the second signal light propagate in the first waveguide 201 and are generated after difference frequency is performed in the process of propagation, that is, the terahertz wave is obtained by conversion according to the first signal light and the second signal light, wherein the expression of the conversion efficiency is as follows:
wherein, P p And P s Input powers, P, of the first signal light and the second signal light, respectively THz Is the output power of the terahertz wave, L is the length of the waveguide, Z 0 And c are respectively the impedance and the speed of light in vacuum, ω THz And alpha THz Respectively the frequency and attenuation coefficient of the terahertz wave, d eff Is the effective nonlinear coefficient, ζ and A eff The overlap integral factor and the effective mode area, respectively, and y represents the propagation distance of the wavelength of the terahertz wave in the waveguide.
The overlap integral factor is embodied as:
E j is an intrinsic field of the terahertz wave, the first signal light and the second signal light,
is formed by d 33 The reduced nonlinear coefficient tensor, ω p And ω s The frequencies of the first signal light and the second signal light, respectively. And x, y and z are corresponding coordinates in a coordinate system.
The expression of the effective mode area is A eff ＝(A opt1 A opt2 A THz ) 1/3 Wherein:
as can be seen from the above expression, in order to improve the conversion efficiency of the first signal light and the second signal light into the terahertz wave, the effective mode area a needs to be reduced eff Increase the overlap productDividing factor zeta, selecting larger nonlinear coefficient d eff And reducing the loss alpha to the terahertz wave THz . And the difference value of the first propagation constant of the first signal light and the second propagation constant of the second signal light is equal to the third propagation constant of the terahertz wave, so that the terahertz wave can be generated at any position by the first signal light and the second signal light with the maximum conversion efficiency. If the difference value between the first propagation constant of the first signal light and the second propagation constant of the second signal light is not equal to the third propagation constant of the terahertz wave, within a certain propagation range, the energy of the terahertz wave generated by the difference frequency of the first signal light and the second signal light is transmitted to the first signal light and the second signal light in reverse, so that the conversion efficiency of the terahertz wave is reduced. Therefore, determining the parameters of the waveguide according to the first condition and the length of the substrate 10 can improve the conversion efficiency of the first signal light and the second signal light, and generate more effective terahertz waves.
Illustratively, a lithium niobate single crystal thin film (LNOI) is selected as the substrate film of the first waveguide 201, and EBL (Electron Beam Lithography System) photoresist is deposited on the LNOI thin film, followed by photolithography and development, followed by direct etching of the LN, and finally photoresist removal and cleaning to complete the processing of the first waveguide 201.
If the material of first waveguide 201 is Periodically Poled Lithium Niobate (PPLN), periodic poling is required after the LN waveguide etch is completed.
Further, the terahertz wave generating structure 20 further includes a second waveguide 202, the second waveguide 202 includes at least one periodic structure and at least one second periodic structure 2022, and the material of the first periodic structure 2021 and the second periodic structure 2022 may be a metal material such as gold, silver, aluminum, copper, and the like. For example, if the first periodic structure 2021 and the second periodic structure 2022 are the same, and the gap is a semi-rectangular structure with a hollow interior, as shown in fig. 2, which is a schematic structural diagram of one of the first periodic structures 2021 and one of the second periodic structures 2022, the shape of the first periodic structure 2021 may be the same as or different from that of the second periodic structure 2022, which is not limited in this application. The first periodic structure 2021 is disposed on the upper surface of the buffer layer 102, and the second periodic structure 2022 is also disposed on the upper surface of the buffer layer 102, wherein the connection manner between the first periodic structure 2021 and the buffer layer 102 and the connection manner between the second periodic structure 2022 and the buffer layer 102 may be, for example, bonding, pressing, etching, and the like, which is not limited herein.
The first periodic structure 2021 includes a gap, the line segment forming the gap being any of a U-shaped arc, a semi-circular arc, a three-quarters circular arc, a rectangular line segment, a T-shaped line segment, a saw-tooth line segment, etc., and may even be an irregular curve, a broken line, etc.
The second periodic structure 2022 also includes a gap, the line segment forming the gap being any of a U-shaped arc, a semi-circular arc, a three-quarter circular arc, a rectangular line segment, a T-shaped line segment, a saw-tooth line segment, etc., and may even be an irregular curve, a broken line, etc.
The first periodic structure 2021 and the second periodic structure 2022 are disposed on both sides of the first waveguide 201.
The distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 are both within a first distance threshold range.
Then, the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 may be the same distance, that is, the first waveguide 201 is located at a position between the first periodic structure 2021 and the second periodic structure 2022, or the first periodic structure 2021 and the second periodic structure 2022 are symmetrically disposed at both sides of the first waveguide 201, such an arrangement enables the generated terahertz waves to propagate at the surface of the second waveguide, and enables the conversion efficiency of the first signal light and the second signal light to be highest.
Of course, the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 may be different, which is not limited in the present application.
The first periodic structure 2021 and the second periodic structure 2022 are provided with the above-described notches toward the side of the first waveguide 201. In the exemplary diagrams shown in fig. 1 and 3, the straight lines with arrows represent the first signal light and the second signal light, and the directions of the arrows represent the directions in which the first signal light and the second signal light are transmitted to the terahertz wave generator, that is, in the case where the terahertz wave needs to be generated, the first signal light and the second signal light are emitted to the cross section of the terahertz wave generator. In the case where the first signal light and the second signal light irradiate the cross section of the terahertz wave generator 1000, the first signal light and the second signal light propagate in the first waveguide 201 and generate a terahertz wave after undergoing a difference frequency in the propagation process, and the terahertz wave is mainly transmitted along the surface of the first periodic structure facing the first waveguide and the surface of the second periodic structure facing the first waveguide, so that the smaller the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201, the smaller the mode field of the terahertz wave can be compressed to a sub-wavelength scale, thereby improving the conversion efficiency of the first signal light and the second signal light.
The nonlinear process in the hybrid waveguide of the second waveguide 202 and the first waveguide 201 can be described by the following equation:
wherein A is j ,n j (j = THz, p, s) are the normalized amplitudes of the THz, pump and signal waves and their corresponding effective refractive indices, respectively, Z 0 And c is the impedance and speed of light, ω, respectively, in vacuum THz And alpha THz Respectively the frequency and attenuation coefficient of the THz wave, d eff Is the effective nonlinear coefficient, ζ and A eff Respectively overlap integral factor and effective mode area, Δ β = β p -β s -β THz Is the amount of phase mismatch between three wavelengths, beta p Is a propagation constant of the first signal light, beta s Y represents a propagation distance of the THz wavelength in the waveguide as a propagation constant of the second signal light.
Therefore, when the second waveguide 202 is fabricated, the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 are determined according to the above principle, thereby compressing the field distribution of the terahertz wave to a sub-wavelength scale, that is, the characteristic scale of the mode field of the terahertz wave is smaller than the wavelength of the terahertz wave. Therefore, field distribution of the terahertz waves can be more concentrated, the effective mode area can be reduced, the overlapping integral factor can be increased, and the conversion efficiency of the first signal light and the second signal light can be further improved. The terahertz wave propagates on the second waveguide in the form of a surface wave, and transmission loss can be reduced.
It should be noted here that, theoretically, the smaller the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201, the higher the conversion efficiency, but in order to avoid causing a large loss, the distance between the first periodic structure 2021 and the second periodic structure 2022 needs to be larger than the width of the first waveguide 201 sandwiched therebetween and cannot be too close to the first waveguide 201.
In addition, since the terahertz wave dispersion curve of the second waveguide 202 is distributed in the form of energy bands, it is characterized in that: in the second waveguide 202, the group velocity of the optical wave tends to zero as the propagation constant increases. The group velocity going to zero means: in the second waveguide 202, the light has an increasing locality, so that a dispersion curve has a band gap appearing at the boundary of the brillouin zone. The dispersion curve has slow light effect and field enhancement effect at the position close to the Brillouin zone boundary, and the light field locality of the dispersion curve at the position close to the Brillouin zone boundary can make the mode field of the terahertz wave smaller, so that the nonlinear effective mode area can be further reduced, the weight superposition integral factor can be increased, and the conversion efficiency can be further improved.
Based on the above determination principle, the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 can be determined.
Then, the parameters due to the second waveguide 202 include the cross-sectional size, the notch size, and the period length of the notch distribution of the first periodic structure, and the cross-sectional size, the notch size, and the period length of the notch distribution of the second periodic structure. The specific determination steps are as follows:
1. firstly, the frequency of the first signal light and the frequency of the second signal light are subjected to difference to obtain the frequency of the terahertz wave;
2. determining the wavelengths of the first signal light and the second signal light according to the frequency of the terahertz wave;
3. the difference between a first propagation constant of the first signal light in the first waveguide and a second propagation constant of the second signal light in the first waveguide is equal to a third propagation constant of the terahertz wave on the surface of the second waveguide;
4. inputting the frequency of the first signal light, the frequency of the second signal light, the frequency of the terahertz wave, the wavelength of the first signal light, the wavelength of the second signal light, an etching process, a metal coating process and the like, and the relationship among the first propagation constant, the second propagation constant and the third propagation into simulation software for simulation, and obtaining a first waveguide parameter and each parameter of the second waveguide which accord with the relationship among the three propagation constants according to a simulation result.
In addition, after the second waveguide 202 is arranged, in order to protect the terahertz-wave generator 1000 to prolong the service life thereof, a protection structure 30 may be arranged on the upper surface of the terahertz-wave generating structure 20 to protect the terahertz-wave generator 1000.
The following describes, by way of example, a process for manufacturing the terahertz-wave generating structure 20 provided in the present application:
example 1: as shown in fig. 3, it is now required to generate a terahertz wave with a frequency of 0.42THz, the wavelength of the first signal light is 1684.6nm, and the wavelength of the second signal light is 1688.6nm. The first waveguide 201 is made of LN, and the hollow structure of the second waveguide 202 is zigzag.
In order to utilize the maximum nonlinear coefficient d of LN 33 LN is selected as x-cut type, and the modes of the first signal light and the second signal light are both TE 00 The fundamental mode, i.e., the main component of the electric field, is primarily along the optical axis (z-axis) of the lithium niobate crystal. During the actual processing of the LN waveguide, the etching tilt angle of the first waveguide 201 is mainly determined by the processing process, and as shown in fig. 4, the etching angle θ is 60 °. Basic junction of the first waveguide 201After the structure is determined, the propagation constant beta of the terahertz wave is obtained through simulation THz The approximate range of (1). As the parameters shown in fig. 4, the geometric parameters of the first signal light and the second signal light can be adjusted as follows: width w of the first waveguide 201 0 Etching depth h e LN unetched thickness h LN . The three geometric parameters, propagation constant beta p And beta p The larger the corresponding beta p -β s The larger, the more beta can be achieved within a suitable range p -β s ＝β THz . The parameters of the LN first waveguide 201 that satisfy this case here are: LN unetched thickness h LN 600nm, etch depth h e 300nm, width of the top of the waveguide w 0 And was 1.4 μm. Below the LN layer is the SiO2 buffer layer 102 thickness
4.7 μm with an overlying SiO2 covering the protective layer thickness>
2 μm, and a thickness h of the Si substrate 101 at the lowermost part Si Is 500 μm. The propagation constant of the optical wave in the nonlinear dielectric thin film waveguide and the corresponding mode field distribution under the parameters are obtained through simulation calculation (figure 5).
FIG. 6 is a diagram showing the dispersion curve of the terahertz waves and the mode distribution of each dispersion of the second waveguide 202, where the eigenmode of the dispersion curve corresponding to the solid line is referred to as TE 0 The mode of the dispersion curve corresponding to the dotted line is called TE 1 Their main components of the electric field are all along the optical axis direction (z-direction) of the LN. The terahertz wave z dispersion curve is changed by adjusting the parameters shown in FIG. 4, for example, the fixed period p is not changed, and the width p of the sawtooth is increased 2 Increasing the depth d of the groove, decreasing the spacing g and increasing the metal thickness h Au The dispersion curve in fig. 6 can be shifted upward, that is, the propagation constant of the terahertz wave increases. The adjustment of 0.42THz to be generated is at TE 0 The parameter for determining the metallic portion of the metamaterial waveguide shown in fig. 4 was determined to be p =145 μm, p, by the fact that the propagation constant in the vicinity of the brillouin zone boundary of the dispersion curve satisfies the phase matching 1 ＝40μm，p 2 ＝105μm，w＝13μm，d＝2μm，g＝7μm，h Au =0.8 μm. At this time, the 0.42 terahertz wave satisfies phase matching with the first signal light 1684.6nm and the second signal light 1688.6nm.
According to the coupled wave equation, the conversion efficiency gamma is close to the maximum value of 1.51 multiplied by 10 when the terahertz wave propagates to 3cm in the first channel and the second channel -4 W -1 [ FIG. 7 (a) ]]. The conversion efficiency of the present application is high because the overlap integration factor has a larger overlap integration factor and a smaller effective mode area at the hollow-out portions of the first periodic structure 2021 and the second periodic structure 2022 [ fig. 7 (b) ]]. The wavelength of the first signal light was set to 1688.6nm, and the wavelength of the second signal light was changed around 1684.6nm, whereby a transmission spectrum of THz was obtained [ FIG. 7 (c) ]]. The THz has high conversion efficiency in the range of 0.14-0.44THz, so the THz has the property of broadband transmission spectrum.
Example 2: in addition, the first higher-order mode TE of the second waveguide 202 is selected 1 The efficient generation of terahertz waves can also be realized. And the distance between the first periodic structure 2021 and the first waveguide 201 and the distance between the second periodic structure 2022 and the first waveguide 201 can be further reduced under the condition that phase matching is achieved, which increases the overlap integration factor and reduces the effective mode area, so that the nonlinear conversion efficiency is further improved. But because of TE 1 Main component E of electric field z (y) are antisymmetrically distributed along the propagation direction, and the overlap integration factor ζ is also antisymmetrically distributed. In order to make the first signal light, the second signal light, and the terahertz wave satisfy the phase matching condition, a material of the first waveguide 201 may be selected to be Periodically Poled Lithium Niobate (PPLN). The length of the PPLN is twice the periodic structure of the second waveguide 202, i.e.' A p And =2p. The hybrid structure at this time is shown in fig. 8. Effective nonlinear coefficient d after polarization of lithium niobate eff Antisymmetric distribution:
zeta multiplied by the overlap integral factor ensures that the integral terms in the effective mode area expression are symmetrically distributed, so that the generated terahertz waves are continuously accumulated.
Terahertz waves with the frequency of 0.42THz need to be generated, and the first propagation constant, the second propagation constant and the third propagation constant still need to meet the first condition; the wavelength of the first signal light is 1583.2nm, and the wavelength of the second signal light is 1586.8nm. Let the etch depth h of LN e At 250nm, LN poling period Λ =300 μm, the duty cycle of the poling is 0.5, while the other parameters of the LN remain the same as those of example 1. Changing the geometry of the second waveguide 202 allows the desired generation of 0.42THz to occur in the TE 1 The dispersion curve is band-edge and phase matching conditions are satisfied. The parameters of the second waveguide metal part at this time were determined to be p =150 μm, p1=25 μm, p2=125 μm, w =15 μm, d =3 μm, g =2 μm, with a thickness of 800nm remaining unchanged. As shown in FIG. 9 (a), the effective mode area of the portion of the sawtooth in the metamaterial waveguide was reduced to 19 μm by calculation 2 And the overlap integration factor increased to 0.81. The corresponding conversion efficiency is improved by one order of magnitude, and when the terahertz wave is transmitted to 6cm, the terahertz wave is as high as 1.32 multiplied by 10 -3 W -1 [ FIG. 9 (b)]. As can be seen from fig. 9 (c), as the propagation length increases, the bandwidth of the terahertz wave transmission spectrum decreases. Here, the terahertz wave transmission spectrum does not exhibit a broadband characteristic because the second dispersion curve of the terahertz wave changes faster than the first dispersion curve, and the amount of phase mismatch increases faster.
In another embodiment, as shown in fig. 10, the present application also provides a terahertz wave generator manufacturing method, including the steps of:
step S1001, determining a first material for preparing a first waveguide and a second material for preparing a second waveguide;
step S1002, determining a wavelength of the first signal light and a wavelength of the second signal light according to a frequency of the terahertz wave;
step S1003 of determining a parameter of the first waveguide and a parameter of the second waveguide according to a first condition that a difference between a first propagation constant of the first signal light and a second propagation constant of the second signal light is equal to a third propagation constant of the terahertz wave; the terahertz wave is generated by the first signal light and the second signal light through the difference frequency;
step S1004, a first waveguide is prepared on the substrate according to parameters of the first waveguide based on the first material.
Step S1005, preparing a second waveguide on the substrate according to the parameters of the second waveguide based on the second material.
The preparation method is described above for the terahertz wave structure, and the description thereof is omitted here.
It is understood that a person skilled in the art can combine, split, recombine and the like the embodiments of the present application to obtain other embodiments on the basis of several embodiments provided by the present application, and the embodiments do not depart from the scope of the present application.
The above embodiments are only intended to be specific embodiments of the present application, and are not intended to limit the scope of the embodiments of the present application, and any modifications, equivalent substitutions, improvements, and the like made on the basis of the technical solutions of the embodiments of the present application should be included in the scope of the embodiments of the present application.
Claims (10)
1. A terahertz-wave generator, characterized by comprising:
the terahertz wave generating device comprises a substrate and at least one terahertz wave generating structure, wherein the substrate is used for supporting the at least one terahertz wave generating structure;
the terahertz wave generation structure comprises a first waveguide and a second waveguide made of a metal material, wherein the second waveguide comprises at least one first periodic structure and at least one second periodic structure; the first waveguide is arranged between the first periodic structure and the second periodic structure, and notches which are periodically distributed along the direction of the first waveguide are arranged on one sides of the first periodic structure and the second periodic structure, which are close to the first waveguide;
under the condition that first signal light and second signal light irradiate the cross section of a terahertz wave generator, the first signal light and the second signal light propagate in the first waveguide, and generate terahertz waves after carrying out difference frequency in the process of propagation, wherein the terahertz waves are transmitted along the surface of the first periodic structure and the surface of the second periodic structure.
2. The terahertz-wave generator according to claim 1,
the first periodic structure and the second periodic structure are symmetric about a first waveguide;
a distance between the first periodic structure and the first waveguide and a distance between the second periodic structure and the first waveguide are both within a first distance threshold range, the first distance threshold being greater than a cross-sectional width of the first waveguide.
3. The terahertz-wave generator according to claim 1, wherein a parameter of the first waveguide satisfies a first preset range;
the parameters of the second waveguide satisfy a second preset range, and the first preset range and the second preset range are used for enabling the propagation constant of the first signal light, the propagation constant of the second signal light, and the propagation constant of the terahertz wave to satisfy phase matching.
4. The terahertz-wave generator according to claim 1, wherein the base includes a substrate and a buffer layer, the buffer layer being disposed on an upper surface of the substrate, the first waveguide being disposed on an upper surface of the buffer layer,
the size of the substrate is greater than or equal to the size of the buffer layer,
the buffer layer has a thickness greater than a wavelength of the second signal light.
5. The terahertz-wave generator according to claim 1, further comprising a protection structure provided on an upper surface of the terahertz-wave generating structure.
6. The terahertz-wave generator according to claim 3,
the parameters of the first waveguide include: a cross-sectional dimension of the first waveguide;
the parameters of the second waveguide include: a cross-sectional dimension, a gap dimension, and a length of the first periodic structure,
and a cross-sectional dimension, a notch dimension, and a period length of a notch distribution of the second periodic structure.
7. A preparation method of a terahertz wave generator is characterized by comprising the following steps:
determining a first material for preparing a first waveguide and a second material for preparing a second waveguide;
determining the wavelength of the first signal light and the wavelength of the second signal light according to the frequency of the terahertz wave, wherein the difference between the frequency of the first signal light and the frequency of the second signal light is equal to the frequency of the terahertz wave;
determining a parameter of the first waveguide and a parameter of the second waveguide according to a first condition that a difference value of a first propagation constant of the first signal light and a second propagation constant of the second signal light is equal to a third propagation constant of the terahertz wave; the terahertz wave is generated by the first signal light and the second signal light through a difference frequency;
preparing the first waveguide on a substrate according to the parameters of the first waveguide based on the first material;
and preparing the second waveguide on the substrate according to the parameters of the second waveguide based on the second material.
8. The production method according to claim 7, wherein the terahertz wave is located in the vicinity of a Brillouin zone boundary of the terahertz wave dispersion curve of the second waveguide.
9. The method of claim 7, wherein the first waveguide is a periodically poled waveguide and the periodic length of the first waveguide is twice the periodic length of the second waveguide.
10. The production method according to claim 9, wherein the terahertz wave is located in the vicinity of a brillouin zone boundary of a terahertz wave high-order dispersion curve of the second waveguide.
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