WO2022179491A1

WO2022179491A1 - Metamaterial-based terahertz second harmonic generation device and generation method

Info

Publication number: WO2022179491A1
Application number: PCT/CN2022/077243
Authority: WO
Inventors: 文永正; 王陈; 周济
Original assignee: 清华大学
Priority date: 2021-02-23
Filing date: 2022-02-22
Publication date: 2022-09-01
Also published as: CN113009746A

Abstract

A metamaterial-based terahertz second harmonic generation device and generation method. The device comprises a metamaterial. The metamaterial comprises: a substrate (3), and a single resonant unit disposed on the substrate (3) or a plurality of resonant units disposed on the substrate (3) in an array form; the resonance unit comprises a field enhancement structure (1) and a coupling structure (2), and the coupling structure (2) is located at a position where a local magnetic field and/or local electric field of the field enhancement structure (1) is enhanced. The present invention has a compact structure and a high degree of design freedom, can generate terahertz second harmonics at room temperature, and is compatible with existing micro- and nano-fabrication processes.

Description

Metamaterial-based terahertz second harmonic generation device and generation method

This application claims the priority of Chinese Patent Application No. 202110200308.0 filed on February 23, 2021. The disclosure of the above Chinese patent application is hereby incorporated by reference in its entirety as a part of this application.

technical field

The embodiments of the present disclosure belong to the technical field of nonlinear terahertz, and in particular, relate to a metamaterial-based terahertz second harmonic generation device and generation method.

Background technique

Terahertz waves are located between microwaves and infrared light waves. They have broad application prospects in medical imaging, wireless communication systems, non-destructive testing, chemical identification, and sub-millimeter astronomy. Terahertz technology will greatly accelerate manufacturing safety, public health , biomedical, defense, communications, and quality inspection and other wavelength-restricted areas of technology development. At present, the generation and processing technology of terahertz waves is still in its infancy, and it is difficult for conventional devices for microwave and infrared light waves to generate and modulate electromagnetic waves in the terahertz frequency range, so new devices and technologies need to be developed.

Second harmonic generation refers to the phenomenon that nonlinear materials radiate frequency-doubled light whose frequency is twice that of the fundamental frequency wave used for excitation under fundamental frequency light excitation. It is the earliest discovered and widely used phenomenon in nonlinear optical effects. It plays a key role in important fields such as the development of new waveband laser sources. However, due to the particularity of terahertz waves, it is difficult to generate the second harmonic of terahertz waves at present. Only a few materials such as antiferromagnetic crystals and superconductors can be generated at certain special frequencies, and most of them require Low temperature (liquid nitrogen, liquid helium temperature) environment, so it is difficult to meet the development and application requirements of terahertz light source and its chip-level integration.

As a new type of artificial dielectric material, metamaterials mainly depend on artificial structure rather than material composition, so they have the advantages of high design freedom, compact structure, and easy integration. Many natural materials have been produced in the fields of linear optics and terahertz. Special physical properties that are difficult to achieve. These advantages also allow metamaterials to provide new ideas and approaches for terahertz second harmonic generation.

SUMMARY OF THE INVENTION

According to an embodiment of the present disclosure, a metamaterial-based terahertz second harmonic generation device is provided, including a metamaterial, wherein the metamaterial includes: a substrate, a single resonant unit disposed on the substrate or an array of A plurality of resonance units arranged on a substrate in the form; the resonance units include a field enhancement structure and a coupling structure, and the coupling structure is in a position where the local magnetic field of the field enhancement structure is enhanced and/or the local electric field is enhanced Location.

For example, at the location where the local magnetic field of the reinforcing structure is enhanced, the ratio of the magnetic field strength to the magnetic field intensity of the fundamental frequency terahertz wave incident on the device is greater than 1; and the local electric field of the reinforcing structure is enhanced At the position of , the ratio of the electric field strength to the electric field strength of the fundamental frequency terahertz wave is greater than 1.

For example, the size of the field enhancement structure is smaller than the wavelength of the fundamental frequency terahertz wave incident on the device.

For example, the size of the field enhancement structure is less than or equal to three quarters of the wavelength of the fundamental frequency terahertz wave.

For example, the resonant unit has an asymmetric structure in at least one direction.

For example, the field enhancement structure is a ring-shaped structure with openings.

For example, the coupling structure is located within and/or outside the area enclosed by the annular structure.

For example, the field enhancement structure is a block-like structure that is structurally continuous in a first direction and a second direction perpendicular to each other.

For example, the constituent material of the field enhancement structure is a material that resonates with the fundamental frequency terahertz wave incident on the device, and is selected from any one of the following: conductor metal, semiconductor and dielectric material.

For example, the coupling structure is provided on the substrate to be distinct from the substrate or the coupling structure is integral with the substrate.

For example, the coupling structures of the plurality of resonance units are separated from each other or continuous with each other.

For example, the constituent material of the coupling structure is a material that generates carriers, and is selected from any one of the following: semiconductor materials, semi-metal materials, two-dimensional materials, and conductor metals.

For example, the material of the substrate is a material with low loss to terahertz waves, and is selected from any one of the following: a semiconductor material, a dielectric material and a polymer material.

For example, the resonance unit is composed of two parts, a field enhancement structure and a coupling structure.

For example, the arrays are regularly arranged or randomly arranged.

According to an embodiment of the present disclosure, there is also provided a method for generating a terahertz second harmonic based on metamaterials, using the device as described above to generate a second terahertz harmonic, and the method includes: a fundamental frequency terahertz wave incident On the device, the field enhancement structure resonates with the fundamental frequency terahertz wave to generate a resonant current or a resonant electric field, thereby causing enhancement of the local magnetic field and/or the local electric field, and the coupling structure is in the field enhancement structure At the position where the local magnetic field is enhanced and/or the local electric field is enhanced, its carriers are driven by the magnetic field force to do anharmonic vibration, and then radiate the second terahertz harmonic at room temperature.

For example, the method further includes: adjusting a parameter of the metamaterial in the device, the parameter of the metamaterial including a period parameter of a plurality of the resonant units, a geometric parameter of the resonant unit, and a dielectric constant of the substrate .

For example, the geometric parameters of the resonance unit include: geometric parameters of the field enhancement structure, geometric parameters of the coupling structure, and relative positions between the field enhancement structure and the coupling structure.

For example, by changing the parameters of the metamaterial in the device, the tunable range of the second terahertz harmonic is 0.1-30 THz.

Description of drawings

In order to illustrate the technical solutions of the embodiments of the present disclosure more clearly, the accompanying drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present disclosure, rather than limit the present disclosure.

FIG. 1( a ) is a schematic diagram of a resonant unit and a substrate of a metamaterial in Example 1 according to an embodiment of the present disclosure;

Fig. 1(b) is a schematic diagram of a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 1(a) in a plane with a period of 70 μm×70 μm;

2(a) is a schematic diagram of a resonant unit and a substrate of a metamaterial in Example 2 according to an embodiment of the present disclosure;

Fig. 2(b) is a schematic diagram of a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 2(a) in a plane with a period of 37 μm×37 μm;

3(a) is a schematic diagram of a resonant unit and a substrate of a metamaterial in Example 3 according to an embodiment of the present disclosure;

Fig. 3(b) is a schematic diagram of a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 3(a) in a plane with a period of 1600 nm × 1600 nm.

4(a) is a schematic diagram of a resonant unit and a substrate of a metamaterial in Example 4 according to an embodiment of the present disclosure;

Fig. 4(b) is a schematic diagram of a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 4(a) in a plane with a period of 40 μm×40 μm.

5(a) is a schematic diagram of a resonant unit and a substrate of a metamaterial in Example 5 according to an embodiment of the present disclosure;

Fig. 5(b) is a schematic diagram of the metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 5(a) in a plane with a period of 95 μm×45 μm.

6(a)-6(e) are transmission spectra, reflection spectra, and absorption spectra of metamaterials in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively, according to embodiments of the present disclosure.

7(a)-7(e) are current distributions when the metamaterial is resonated in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively, according to embodiments of the present disclosure.

FIGS. 8(a)-8(e) are the y-direction raw and bandpass filtered time-domain transmission spectra of metamaterials in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively, according to embodiments of the present disclosure .

9( a )- FIG. 9( e ) are the frequency-domain transmission spectra of the metamaterial in the x-direction in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively, according to embodiments of the present disclosure.

10(a)-10(e) are frequency domain transmission spectra of metamaterials in the y-direction in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively, according to embodiments of the present disclosure.

Detailed ways

In order to make the objectives, technical solutions and advantages of the embodiments of the present disclosure more clear, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some, but not all, embodiments of the present disclosure. Based on the described embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present disclosure.

Unless otherwise specified, the raw materials involved in the embodiments of the present disclosure can be obtained from open commercial sources.

Embodiments of the present disclosure provide a metamaterial-based terahertz second harmonic generation device, including a metamaterial; the metamaterial includes: a substrate, a single resonant unit disposed on the substrate or disposed on the substrate in an array form A plurality of resonant units on the resonator unit; the resonant unit includes a field enhancement structure and a coupling structure, and the coupling structure is located at a position where the local magnetic field of the field enhancement structure is enhanced and/or a position where the local electric field is enhanced.

As mentioned above, a plurality of resonant units are arranged on the substrate in an array; in this case, the array may be arranged regularly or randomly.

For example, the resonance unit is composed of two parts, the field enhancement structure and the coupling structure, that is, the resonance unit does not include other components except the enhancement structure and the coupling structure, so that the terahertz second harmonic generation device structure according to the embodiment of the present disclosure Simpler and more compact.

When the field-enhancing structure resonates with the fundamental frequency terahertz wave incident on the THz second harmonic generating device, a resonant current or a resonant electric field is generated, which in turn causes a significant enhancement of the local magnetic field and/or the local electric field, the strength of which is comparable to that of the local electric field. Compared with the incident fundamental frequency terahertz wave, the electromagnetic field strength is increased by several times to thousands of times, depending on the structural design. The coupling structure is in the position where the local magnetic field of the field-enhancing structure is enhanced and/or the local electric field is enhanced, and the magnetic field force on its carriers is:

where ω is the angular frequency of the fundamental terahertz wave, q is the carrier charge,

is the mobility of free electrons in the Drude model, E(ω) and B(ω) are the vector amplitudes of the local electric and magnetic fields, respectively, t is the time, and cc is the complex conjugate. Equation (1) shows that due to the action of the local electric field and/or the local magnetic field, the carriers in the coupled structure can be driven by the magnetic field force to do anharmonic vibration, and then radiate the double-frequency terahertz harmonics, and the second-order polarization can be Expressed as:

where ε ₀ is the vacuum permittivity, ω _P , μ _e0 and γ are the plasma frequency, DC mobility and electron collision rate of the constituent materials of the coupled structure,

is the unit vector in the direction perpendicular to the local electric field and local magnetic field. For example, by changing the geometric parameters of the field-enhancing structure and the coupling structure, as well as the relative positions of the two, the characteristics of the generated THz second harmonic can be designed on demand, including the frequency, bandwidth, polarization state, Phase and Intensity, etc.

It can be seen from equation (1) that the magnetic field force F _B is positively correlated with the electric field and the magnetic field strength, and the magnetic field enhancement or the electric field enhancement or both the electromagnetic and electric field enhancement can obtain a significant magnetic field force, so the coupling structure is in the field enhancement structure. where the local magnetic field is enhanced and/or the local electric field is enhanced. That is, the coupling structure is in a position where the local magnetic field of the reinforcement structure is enhanced, or the local electric field of the reinforcement structure is intensified, or both the local magnetic field and the local electric field of the reinforcement structure are intensified.

For example, at locations where the local magnetic field of the reinforcing structure is enhanced, the ratio of the magnetic field strength to the magnetic field strength of the incident fundamental frequency terahertz wave is greater than 1. For example, the ratio of the electric field strength to the electric field strength of the incident fundamental frequency terahertz wave is greater than 1 at the location where the local electric field of the reinforcing structure is enhanced.

For example, fundamental frequency terahertz waves incident on the reinforcement structure can be generated by means of optical rectification, back-wave oscillators, quantum cascade lasers, free electron lasers, photoconductive antennas, etc. Using the terahertz second harmonic generation device according to the embodiment of the present disclosure to generate the second terahertz harmonic can conveniently widen the frequency range of the existing terahertz wave, and obtain a higher frequency terahertz wave, which is compared with the existing terahertz wave. Hertzian waves enable higher-rate communications and higher-resolution imaging.

For example, the size of the field-enhancing structure is smaller than the wavelength of the fundamental frequency terahertz wave, and is a geometry with sub-wavelength dimensions to meet the resonance requirements. For example, the size of the field enhancement structure is less than or equal to three quarters of the wavelength of the incident fundamental frequency terahertz wave to better meet the resonance requirements.

For example, the resonant unit is asymmetrical in at least one direction to better generate the second terahertz harmonic. For example, in this case, the polarization direction of the incident fundamental frequency terahertz wave is perpendicular or parallel to the at least one direction, so as to better generate the second harmonic of terahertz. However, the embodiment of the present disclosure is not limited thereto, and the incident fundamental frequency terahertz wave may also be circularly polarized light or polarized light whose polarization direction is at any angle to the at least one direction.

For example, the field-enhancing structure is a ring-shaped structure with openings for better terahertz second harmonic generation. For example, the field enhancement structure may have one opening or multiple openings, which is not limited in this embodiment of the present disclosure. For example, the ring structure may be a ring shape of any shape, such as a square ring, a circular ring, and a polygonal ring, which is not limited in this embodiment of the present disclosure. For example, the coupling structure is located in the area enclosed by the ring structure, so that the structure of the resonance unit is more compact. However, the embodiment of the present disclosure is not limited thereto, and the coupling structure may also have a portion located outside the area enclosed by the ring structure. For example, the entire coupling structure is located outside the area enclosed by the ring coupling structure. For example, a part of the coupling structure is located within the area enclosed by the ring structure, and a part is located outside the area enclosed by the ring structure.

For example, the field enhancement structure is a block-like structure that is structurally continuous in a first direction and a second direction perpendicular to each other. For example, the field enhancement structure is a strip-like structure; in this case, for example, the coupling structure is arranged parallel to the enhancement structure.

For example, the constituent material of the field enhancement structure needs to meet the basic requirements of metamaterial resonance for the material, that is, the constituent material of the field enhancement structure is a material that resonates with the incident fundamental frequency terahertz wave, and its material selection includes gold, silver, Conductive metals such as copper and aluminum, doped or intrinsic semiconductors such as silicon, germanium, and gallium arsenide, and dielectric materials such as titanium dioxide, barium titanate, aluminum oxide, and silicon nitride.

For example, the coupling structures of the plurality of resonance units are independent from each other or continuous with each other.

For example, the constituent material of the coupling structure is a material that generates carriers in a certain way; the carrier generation methods include but are not limited to element doping, impact ionization, photoexcitation, intrinsic excitation, thermal excitation, or high-energy charged particle excitation, etc. ; The material selection includes doped or intrinsic semiconductor materials such as silicon, germanium, gallium arsenide and indium antimonide, semi-metallic materials such as bismuth, two-dimensional materials such as graphene and molybdenum disulfide, gold, silver, copper, aluminum Equal conductor metals.

For example, the substrate is a material with low loss to terahertz waves, which can be semiconductor materials such as silicon, germanium, gallium arsenide, indium phosphide, etc., or dielectric materials such as quartz and sapphire, or polyimide ( Polyimide), polydimethylsiloxane (PDMS), parylene (Parylene) and other polymer materials, the specific selection depends on the operating frequency and application scenarios.

Embodiments of the present disclosure also provide a method for generating second terahertz harmonics based on metamaterials, using the device as described above to generate second terahertz harmonics. The method includes: the fundamental frequency terahertz wave is incident on the device as described above, and the field enhancement structure resonates with the fundamental frequency terahertz wave to generate a resonant current or a resonant electric field, thereby causing the enhancement of the local magnetic field and/or the local electric field , the coupling structure is in the position where the local magnetic field and/or the local electric field of the field-enhancing structure is enhanced, and its carriers are driven by the magnetic field force to do anharmonic vibration, and then radiate the second terahertz harmonic at room temperature.

For example, the method further includes: adjusting parameters of the metamaterial in the device, the parameters of the metamaterial including a period parameter of the plurality of resonance units, a geometric parameter of the resonance unit, and a dielectric constant of the substrate. Thereby, the adjustment of the frequency of the generated terahertz second harmonic is realized.

For example, by changing the parameters of the metamaterial in the device, the tunable range of the second terahertz harmonic generated is 0.1-30 THz.

The embodiments of the present disclosure have at least the following beneficial effects:

(1) The embodiments of the present disclosure provide a metamaterial-based terahertz second harmonic generation device and method, which has an ultra-high degree of design freedom, and can design metamaterial structures according to actual needs to achieve terahertz second harmonic generation. generation of waves;

(2) Compared with the existing terahertz second harmonic generation technology, the terahertz second harmonic generation device provided by the present disclosure can work at room temperature without low-temperature equipment, which greatly simplifies the overall terahertz optics The complexity and working conditions of the system;

(3) The terahertz second harmonic generation device and method provided by the embodiments of the present disclosure mainly rely on the structural coupling of metamaterials, and there is no strict requirement on the composition materials as a whole, so it is possible to reduce the processing time by selecting materials that are easier to process. Difficulty and cost; it can be compatible with the existing micro-nano processing technology to achieve chip-level integration; it can also select different material components to prepare metamaterials according to the actual situation to meet the application needs of different scenarios.

Hereinafter, four examples according to embodiments of the present disclosure will be described to further explain the metamaterial-based THz second harmonic generation device and the method for generating second harmonics in the THz frequency band using the device.

Example 1

Example 1 provides a device and method for generating second harmonics in the terahertz frequency band (0.7THz) using metamaterials, which are shown in Figures 1(a) and 1(b).

Mark 1 in Fig. 1(a) refers to the field enhancement structure, such as a double-slit square ring made of gold, with an outer side length of 46 μm and a ring width of 6 μm. The median lines are all 10 μm and the thickness is 300 nm.

Mark 2 in Fig. 1(a) refers to the coupling structure, for example, the material is n-type doped silicon (conductivity is 3037S/m, mobility is 379cm^2/(V*s)), size is 31μm×15.5μm, The coupling structure is located inside the double-slit square ring, 1.5 μm away from its inner edge, and the thickness is 300 nm.

Mark 3 in Fig. 1(a) refers to a substrate with low loss to the terahertz band, for example, the material is high-resistance silicon (resistivity 10000Ω·cm), and the thickness is 10 μm.

Fig. 1(b) is a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 1(a) in a plane with a period of 70 μm × 70 μm.

Figure 6(a) is the response curve of the metamaterial array in Example 1 in the frequency domain. With the normal incidence of the x-polarized plane wave (ie, the fundamental frequency terahertz wave), the metamaterial resonates at 0.7 THz.

Figure 7(a) is the current distribution of the metamaterial resonant unit in Example 1 when it resonates. The gold double split square ring generates a ring current due to the resonance. The ring current further causes the enhancement of the local magnetic field and electric field. In the part where the magnetic field and electric field are enhanced, the non-harmonic vibration occurs under the driving of the magnetic field force.

Figure 8(a) is the time-domain transmission spectrum in the y-direction obtained by the finite element analysis method of the metamaterial array in Example 1 and the second-harmonic time-domain spectrum after bandpass filtering with a center frequency of 1.4 THz.

Using the finite element analysis method (the software used is COMSOL Multiphysics), the time-domain simulation of the metamaterial is used to obtain the original transmission spectrum, and then the second harmonic is obtained by band-pass filtering the original transmission spectrum.

In the example of the present disclosure, when the peak electric field intensity of the incident fundamental frequency terahertz wave is 10 ⁷ V/m, the obtained second harmonic peak intensity is 1123 V/m.

Figures 9(a) and 10(a) are the frequency domain spectra obtained by Fourier transform of the time-domain transmission spectra of the metamaterial array in the x and y directions in Example 1. The simulation results are consistent with the theory, and there is no two-way transmission in the x direction. Sub-harmonic generation, a clear second-harmonic generation is observed in the y-direction.

Example 2

Example 2 provides a device and method for generating second harmonics in the terahertz frequency band (1 THz) using metamaterials, which are shown in Figures 2(a) and 2(b).

Mark 1 in Fig. 2(a) refers to the field enhancement structure, such as a single open ring composed of gold, with an outer diameter of 25 μm, a ring width of 4 μm, an opening width of 4 μm, and a thickness of 300 nm.

Mark 2 in Figure 2(a) refers to the coupling structure, for example, the material is n-type doped silicon (conductivity is 3037S/m, mobility is 379cm^2/(V*s)), the diameter is 14μm, and the distance from the gold single The inner edge of the open ring is 1.5 μm, and the thickness is 300 nm.

Mark 3 in Figure 2(a) refers to a substrate with low loss to the terahertz band, for example, the material is high-resistance silicon (resistivity 10000Ω·cm), and the thickness is 10 μm.

Fig. 2(b) is a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 2(a) in a plane with a period of 37 μm × 37 μm.

Figure 6(b) is the response curve of the metamaterial array in Example 2 in the frequency domain. The metamaterial resonates at 1 THz with the normal incidence of the x-polarized plane wave (ie, the fundamental frequency terahertz wave).

Figure 7(b) shows the current distribution of the metamaterial resonant unit in Example 2 when it resonates. The principle is the same as that in Example 1. The carriers in the doped silicon are driven by the magnetic field force to generate an anharmonic vibration.

Figure 8(b) is the time-domain transmission spectrum in the y-direction obtained by the finite element analysis method (same as Example 1) of the metamaterial array in Example 2 and the second harmonic time-domain spectrum after bandpass filtering with a center frequency of 2THz . In the example of the present disclosure, when the peak electric field intensity of the incident fundamental frequency terahertz wave is 10 ⁷ V/m, the obtained second harmonic peak intensity is 835 V/m.

Figures 9(b) and 10(b) are the frequency domain spectra obtained by Fourier transform of the time-domain transmission spectra of the metamaterial array in the x and y directions in Example 2. The simulation results are consistent with the theory, and there is no two-way transmission in the x-direction. Sub-harmonic generation, a clear second-harmonic generation is observed in the y-direction.

Example 3

Example 3 provides a device and method for generating second harmonics in the higher terahertz frequency band (30 THz) using metamaterials as shown in Figures 3(a) and 3(b).

Mark 1 in Figure 3(a) refers to the field enhancement structure, such as a single open square ring composed of aluminum, the outer side length is 980nm, the ring width is 200nm, the opening width is 100nm, the opening is at the center position, and the thickness is 100nm.

Mark 2 in Fig. 3(a) refers to the coupling structure. For example, the material is bismuth (conductivity is 2.2×10 ⁵ S/m, mobility is 0.11m^2/(V*s)), and the size is 480nm×480nm. The distance from the inner edge of the aluminum single open square ring is 50nm, and the thickness is 100nm.

Mark 3 in Fig. 3(a) refers to a substrate with low loss in the higher terahertz band, for example, the material is silicon dioxide (dielectric constant is 4.82+0.026i), and the thickness is 500 nm.

Fig. 3(b) is a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Fig. 3(a) in a plane with a period of 1600 nm × 1600 nm.

Figure 6(c) is the response curve of the metamaterial array in Example 3 in the frequency domain, with the normal incidence of the x-polarized plane wave (ie, the fundamental frequency terahertz wave), the metamaterial resonates at 30 THz.

Figure 7(c) shows the current distribution of the metamaterial resonant unit in Example 3 when it resonates. The principle is the same as that in Example 1. The carriers in bismuth are driven by the magnetic field force to generate an anharmonic vibration.

Figure 8(c) is the time-domain transmission spectrum in the y-direction obtained by the finite element analysis method (same as Example 1) of the metamaterial array in Example 3 and the second harmonic time-domain spectrum after bandpass filtering with a center frequency of 60THz . In the example of the present disclosure, when the peak electric field intensity of the incident terahertz wave is 10 ⁸ V/m, the obtained peak intensity of the second harmonic wave is 2490 V/m.

Fig. 9(c) and Fig. 10(c) are the frequency-domain spectra obtained by Fourier transform of the time-domain transmission spectra of the metamaterial array in the x and y directions in Example 3. The simulation results are consistent with the theory, and there is no two-way transmission in the x direction. Sub-harmonic generation, a clear second-harmonic generation is observed in the y-direction.

Example 4

Example 4 provides a device and method for generating second harmonics in the terahertz frequency band (1 THz) using metamaterials, which are shown in Figures 4(a) and 4(b).

Mark 1 in Fig. 4(a) refers to the field enhancement structure, such as a single open ring composed of gold, with an outer diameter of 23 μm, a ring width of 4 μm, an opening width of 3.5 μm, and a thickness of 300 nm.

Mark 2 in Figure 4(a) refers to the coupling structure, for example, the material is n-type doped silicon (conductivity is 479S/m, mobility is 996cm^2/(V*s)), the side length is 40um, and the thickness is 300nm.

Mark 3 in FIG. 4( a ) refers to a substrate with low loss to the terahertz band, for example, the material is high-resistance silicon (resistivity 10000Ω·cm), and the thickness is 20 μm.

Figure 4(b) is a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Figure 4(a) in a plane with a period of 40 μm×40 μm.

For example, in Example 1, Example 2, and Example 3, the coupling structures of the plurality of resonance units are separated from each other. For example, in Example 4, the coupling structures of the plurality of resonance units are continuous with each other, and in this case, the coupling structures have both a portion located inside the annular field enhancement structure and a portion located outside the annular field enhancement structure.

In Example 1, Example 2 and Example 3, the coupling structure 2 is located in the area enclosed by the annular field enhancement structure 1, respectively. However, it should be noted that the coupling structures 2 may also be respectively located outside the regions enclosed by the annular field enhancement structures 1 .

Fig. 6(d) is the response curve of the metamaterial array in Example 4 in the frequency domain, with the normal incidence of the x-polarized plane wave (ie, the fundamental frequency terahertz wave), the metamaterial resonates at 1 THz.

Figure 7(d) shows the current distribution of the metamaterial resonant unit in Example 4 when it resonates. The principle is the same as that in Example 1. The carriers in the doped silicon are driven by the magnetic field force to generate an anharmonic vibration.

Figure 8(d) is the time-domain transmission spectrum in the y-direction obtained by the finite element analysis method (same as Example 1) of the metamaterial array in Example 4 and the second harmonic time-domain spectrum after bandpass filtering with a center frequency of 2THz . In the example of the present disclosure, when the peak electric field intensity of the incident terahertz wave is 5×10 ⁷ V/m, the obtained peak intensity of the second harmonic wave is 1049 V/m.

Figures 9(d) and 10(d) are the frequency domain spectra obtained by Fourier transform of the time-domain transmission spectra of the metamaterial array in the x and y directions in Example 4. The simulation results are consistent with the theory, and there is no second harmonic in the x-direction. wave generation, and a clear second harmonic generation is observed in the y-direction.

Example 5

Example 5 provides a device and method for generating second harmonics in the terahertz frequency band (1 THz) using metamaterials, which are shown in Figures 5(a) and 5(b).

Mark 1 in Fig. 5(a) refers to the field enhancement structure, such as a rectangular strip made of gold, with a length of 79 µm, a width of 4 µm and a thickness of 300 nm.

Mark 2 in Figure 5(a) refers to the coupling structure, for example, the material is n-type doped silicon (conductivity 3037S/m, mobility 379cm^2/(V*s)), length 79μm, width 10μm , 1.5 μm from the long side of the gold rectangular strip with a thickness of 300 nm.

Mark 3 in Fig. 5(a) refers to a substrate with low loss to the terahertz band, for example, the material is high-resistance silicon (resistivity 10000Ω·cm), and the thickness is 15 μm.

Figure 5(b) is a metamaterial array obtained by periodically expanding the metamaterial resonant unit shown in Figure 5(a) in a plane with a period of 95 μm×45 μm.

Figure 6(e) is the response curve of the metamaterial array in Example 5 in the frequency domain, with the normal incidence of the x-polarized plane wave (ie, the fundamental frequency terahertz wave), the metamaterial resonates at 1 THz.

Figure 7(e) is the current distribution of the metamaterial resonant unit in Example 5 when the resonant unit is resonated. The gold rectangular strips are enhanced by the local magnetic field and electric field due to the electrical resonance, and the carriers in the doped silicon are in the areas where the magnetic field and electric field are enhanced. , the non-harmonic vibration occurs under the driving of magnetic field force.

Figure 8(e) is the time-domain transmission spectrum in the y-direction obtained by the finite element analysis method (same as Example 1) of the metamaterial array in Example 5 and the second harmonic time-domain spectrum after bandpass filtering with a center frequency of 2THz . In the example of the present disclosure, when the peak electric field intensity of the incident fundamental frequency terahertz wave is 5×10 ⁷ V/m, the obtained second harmonic peak intensity is 3233 V/m.

Figures 9(e) and 10(e) are the frequency domain spectra obtained by Fourier transform of the time-domain transmission spectra of the metamaterial array in the x and y directions in Example 5. The simulation results are consistent with the theory, and there is no two-way transmission in the x-direction. Sub-harmonic generation, a clear second-harmonic generation is observed in the y-direction.

The metamaterials involved in the examples of the present disclosure are prepared by, for example, micro-nano processing methods, including ultraviolet exposure, electron beam exposure, ion implantation, and electron beam evaporation, and the like.

The above descriptions are only exemplary embodiments of the present disclosure, and are not intended to limit the protection scope of the present disclosure, which is determined by the appended claims.

Claims

A metamaterial-based terahertz second harmonic generation device, comprising metamaterials, wherein,

The metamaterial includes: a substrate, a single resonant unit arranged on the substrate or a plurality of resonant units arranged on the substrate in an array;

The resonance unit includes a field enhancement structure and a coupling structure, and the coupling structure is located at a position where the local magnetic field of the field enhancement structure is enhanced and/or a position where the local electric field is enhanced.
The device of claim 1, wherein,

At the location where the localized magnetic field of the enhancement structure is enhanced, the ratio of the magnetic field strength to the magnetic field strength of the fundamental frequency terahertz wave incident on the device is greater than 1; and

The ratio of the electric field strength to the electric field strength of the fundamental frequency terahertz wave is greater than 1 at the location where the local electric field of the reinforcing structure is enhanced.
A device according to claim 1 or 2, wherein the size of the field enhancement structure is smaller than the wavelength of the fundamental frequency terahertz wave incident on the device.
4. The device of claim 3, wherein the size of the field enhancement structure is less than or equal to three quarters of the wavelength of the fundamental frequency terahertz wave.
The device according to any one of claims 1-4, wherein the resonance unit is an asymmetric structure in at least one direction.
The device of any one of claims 1-5, wherein the field enhancement structure is a ring-shaped structure with an opening.
The device of claim 6, wherein the coupling structure is located within and/or outside the area enclosed by the annular structure.
The device according to any one of claims 1-5, wherein the field enhancement structure is a bulk structure that is structurally continuous in a first direction and a second direction perpendicular to each other.
The device according to any one of claims 1-8, wherein the constituent material of the field enhancement structure is a material that resonates with the fundamental frequency terahertz wave incident on the device, and is selected from any one of the following Species: conductor metals, semiconductors and dielectric materials.
9. The device of any one of claims 1-9, wherein the coupling structure is provided on the substrate to be distinct from the substrate or the coupling structure is integral with the substrate.
The device according to any one of claims 1-10, wherein the coupling structures of the plurality of resonance units are separated from each other or continuous with each other.
The device according to any one of claims 1-11, wherein the constituent material of the coupling structure is a material that generates carriers, and is selected from any one of the following: semiconductor materials, semi-metal materials, two-dimensional materials and conductor metals.
The device according to any one of claims 1-12, wherein the material of the substrate is a material with low loss to terahertz waves, and is selected from any one of the following: semiconductor materials, dielectric materials and polymers Material.
The device according to any one of claims 1-13, wherein the resonance unit is composed of two parts, a field enhancement structure and a coupling structure.
The device of any one of claims 1-14, wherein the arrays are arranged regularly or randomly.
A method for generating terahertz second harmonics based on metamaterials, using the device according to any one of claims 1-15 to generate terahertz second harmonics, the method comprising:

The fundamental frequency terahertz wave is incident on the device, and the field enhancement structure resonates with the fundamental frequency terahertz wave to generate a resonant current or a resonant electric field, thereby causing the enhancement of the local magnetic field and/or the local electric field, the said The coupling structure is located at a position where the local magnetic field of the field-enhancing structure is enhanced and/or the local electric field is enhanced, and its carriers are driven by the magnetic field force to perform anharmonic vibration, thereby radiating the second terahertz harmonic at room temperature.
17. The method of claim 16, further comprising: adjusting a parameter of a metamaterial in the device, the parameter of the metamaterial including a period parameter of a plurality of the resonant cells, a geometric parameter of the resonant cell, and the substrate dielectric constant.
The method according to claim 17, wherein the geometric parameters of the resonance unit include: geometric parameters of the field enhancement structure, geometric parameters of the coupling structure, and the gap between the field enhancement structure and the coupling structure relative position.
The method of claim 16, wherein by changing the parameters of the metamaterial in the device, the adjustable range of the second terahertz harmonic is 0.1-30 THz.