A second harmonic converter and a preparation method thereof and a second harmonic conversion system

Abstract

The present invention provides a second harmonic converter and a preparation method thereof, and a second harmonic conversion system. The second harmonic converter comprises an absorber, and the absorber comprises a substrate layer, a bottom metal layer, a dielectric layer and a metal resonator layer; the material of the metal resonator layer is molybdenum and has a chiral structure; through the second harmonic converter of the refractory metal Mo plasmon chiral structure and the preparation method thereof, the conversion of the second harmonic of polarized light is realized, a nonlinear chiral effect is generated, and good high temperature stability and mechanical stability are achieved.

Description

Technical Field

[0001] The present invention relates to the field of micro-nano optical technology, and in particular to a second harmonic converter and a preparation method thereof, as well as a second harmonic conversion system. Background Art

[0002] Chirality is a basic property of nature and plays an important role in chemistry, biology, medicine, and materials science. Due to the non-centrosymmetry of the molecular structure, chiral structures may have good nonlinear optical activity, such as the generation of second and third harmonics, circularly polarized luminescence, optical sensing and detection, and other unique physical properties related to chiral molecular structures. Since its discovery, the surface plasmon effect has received widespread attention and rapid development. The surface plasmon effect is essentially the behavior of collective coherent oscillations of free electrons at the interface, which makes the electromagnetic field localized in a range far smaller than the wavelength scale, and various interactions between light and matter are greatly enhanced, including chirality, nonlinear optical effects, photoacoustic effects, etc.

[0003] Nonlinear optics mainly studies the phenomena and related applications caused by the interaction between strong light and nonlinear dielectric materials, including optical harmonic generation, ultrafast optical switching and optical imaging. Although nonlinear optical effects have been widely used in photonic devices, only when the intensity of the external electric field is large enough will a relatively obvious nonlinear effect occur. The inherent nonlinear optical response of the material is very weak, and its size is large, making it difficult to apply it to the field of integrated micro-nano optoelectronic devices. For this reason, the light-matter interaction effect enhanced by plasmons is particularly important in improving the nonlinear conversion efficiency of weak light. If the nonlinear material also has chiral properties, a nonlinear chiral effect will occur.

[0004] Most plasmonic metasurfaces are based on precious metals, such as gold (Au) and silver (Ag), because they have excellent ability to generate surface plasmon resonance. However, due to the photothermal effect or the excitation of high-intensity lasers, the ultra-high temperature generated by a large amount of heat energy can cause severe deformation of its structure, leading to severe degradation of its optical properties. Therefore, the photothermal stability of plasmonic structures of conventional materials is poor.

[0005] In view of this, the present invention provides a second harmonic converter and a preparation method thereof and a second harmonic conversion system. Through the second harmonic converter of the refractory metal Mo plasmon chiral structure and the preparation method thereof, the conversion of the second harmonic of polarized light is realized, a nonlinear chiral effect is generated, and good high temperature stability and mechanical stability are achieved. Summary of the invention

[0006] The object of the present invention is to provide a second harmonic converter, which includes an absorber, and the absorber includes a substrate layer, a bottom metal layer, a dielectric layer and a metal resonator layer; the material of the metal resonator layer is molybdenum and has a chiral structure.

[0007] Furthermore, the material of the substrate layer is a silicon wafer or a glass wafer.

[0008] Furthermore, the material of the bottom metal layer is molybdenum.

[0009] Furthermore, the material of the dielectric layer is aluminum oxide or silicon dioxide.

[0010] Furthermore, the chiral structures include "Z", "L", "S" and "E" shapes.

[0011] Furthermore, the dielectric layer is arranged on one side of the bottom metal layer, and the substrate layer is arranged on the other side; the metal resonator layer is arranged on a side of the dielectric layer away from the bottom metal layer; and the thickness of the metal resonator layer is 55 nm.

[0012] The object of the present invention is to provide a method for preparing any of the above-mentioned second harmonic converters, comprising: growing a molybdenum layer on a substrate layer by a DC magnetron sputtering method to obtain a bottom metal layer; depositing a layer on the bottom metal layer by electron beam evaporation; A metal is deposited on the dielectric layer to obtain a dielectric layer; a layer of photoresist is deposited on the dielectric layer, and the photoresist is exposed and developed using an electron beam lithography process to obtain a photoresist layer with a chiral structure in the cavity; a layer of molybdenum metal is deposited on the photoresist layer by electron beam evaporation, and the photoresist layer is peeled off to obtain a metal resonator layer with a chiral structure.

[0013] Furthermore, the method of growing a molybdenum layer on the substrate layer by using a DC magnetron sputtering method includes: drying the chamber with industrial-grade nitrogen, and using molybdenum as a target material; before depositing the thin film, pre-sputtering the molybdenum target in an argon atmosphere for a preset sputtering time; adjusting the pressure of the sputtering chamber when it is vacuum to a preset vacuum pressure; adjusting the pressure of the sputtering chamber when it is working to a preset working pressure; introducing pure argon gas into the chamber at a preset flow rate; the deposition temperature is room temperature, the deposition time is a preset deposition time, and the deposition power is a preset deposition power.

[0014] Furthermore, the preset spraying time is 20 minutes; the preset vacuum pressure is Torr; the preset working pressure is Torr; the preset flow rate is 2.0 SCCM; the preset deposition time is 60 min; the preset deposition power is 100 w.

[0015] The object of the present invention is to provide a second harmonic conversion system based on the second harmonic converter described in any one of the above items, comprising a laser, the second harmonic converter and a coupling optical fiber; the laser is used to emit laser light of a required wavelength as a light source; the light source generates chiral light through a combination of a polarizer and a quarter-wave plate; the second harmonic converter is irradiated by the chiral light and generates a nonlinear chiral effect, reflecting a second harmonic; the coupling optical fiber is used to transmit the second harmonic.

[0016] The technical solution of the embodiment of the present invention has at least the following advantages and beneficial effects:

[0017] The second harmonic converter provided by the present invention has a bottom metal layer, a dielectric layer and a refractory metal molybdenum resonance layer that resonate with each other, has a strong light absorption capability, can realize the conversion of the second harmonic of chiral light, and has good high-temperature stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 An exemplary schematic diagram of a second harmonic converter provided by the present invention;

[0019] Figure 2 Three views of the second harmonic converter provided by the present invention, a is a top view, b is a side view, and c is another side view;

[0020] Figure 3 A schematic diagram of a chiral structure provided by the present invention;

[0021] Figure 4a An absorption diagram of the linear chiral effect of the second harmonic converter with a "Z"-shaped chiral structure provided by the present invention;

[0022] Figure 4b An absorption diagram of the linear chiral effect of the second harmonic converter with an "L"-shaped chiral structure provided by the present invention;

[0023] Figure 4c An absorption diagram of the linear chiral effect of the second harmonic converter with an "S"-shaped chiral structure provided by the present invention;

[0024] Figure 4d An absorption diagram of the linear chiral effect of the second harmonic converter having an "E"-shaped chiral structure provided by the present invention;

[0025] Figure 5a A schematic diagram of the second harmonic conversion efficiency of the second harmonic converter provided by the present invention for left-handed light;

[0026] Figure 5b A schematic diagram of the second harmonic conversion efficiency of the second harmonic converter provided by the present invention for right-handed light;

[0027] Figure 6 A result diagram of the nonlinear chiral effect of the second harmonic converter provided by the present invention;

[0028] Figure 7 A schematic flow chart of a method for preparing a second harmonic converter provided by the present invention;

[0029] Figure 8 Schematic diagram of the second harmonic generation principle;

[0030] Icon: 1-bottom metal layer, 2-dielectric layer, 3-metal resonator layer, 4-substrate layer, 5-photoresist layer. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0032] The present invention selects refractory metal as the main material of the absorber. The metal with high melting point and stability, combined with the metasurface energy, makes the material show excellent optical absorption, photothermal stability and mechanical stability.

[0033] like Figure 1 As shown, the absorber of the second harmonic converter provided by the present invention comprises a substrate layer 4, a bottom metal layer 1, a dielectric layer 2 and a metal resonator layer 3; the material of the metal resonator layer is molybdenum and has a chiral structure. Specifically, the dielectric layer is arranged on one side of the bottom metal layer, and the substrate layer is arranged on the other side; the metal resonator layer is arranged on the side of the dielectric layer away from the bottom metal layer; the thickness h3 of the metal resonator layer is 55nm.

[0034] The material of the substrate layer is a silicon wafer or a glass wafer. The material of the bottom metal layer is molybdenum. The material of the dielectric layer is aluminum oxide or silicon dioxide. Figure 3 As shown, the chiral structure includes the shapes of "Z", "L", "S" and "E". The circular dichroism spectrum of the chiral structure of the "Z" shape is as follows Figure 4a As shown; the circular dichroism spectrum of the "L"-shaped chiral structure is shown Figure 4b As shown; the circular dichroism spectrum of the "S"-shaped chiral structure is as follows Figure 4c As shown; the circular dichroism spectrum of the "E"-shaped chiral structure is shown Figure 4d shown.

[0035] like Figure 7As shown, the method for preparing a second harmonic converter includes preparing a refractory plasma chiral nanostructure by electron beam lithography, specifically:

[0036] Step I, growing a molybdenum layer on the substrate layer by DC magnetron sputtering to obtain a bottom metal layer. Growing a molybdenum layer on the substrate layer by DC magnetron sputtering includes: using industrial grade nitrogen ( ) Dry the chamber, and molybdenum is used as the target material; before depositing the film, the molybdenum (Mo) target is pre-sputtered in an argon (Ar) atmosphere for a preset sputtering time to ensure an oxide-free environment in the chamber. The preset sputtering time refers to the preset sputtering time of the target material, for example, the preset sputtering time can be 20 minutes. The pressure of the sputtering chamber when it is vacuumed is adjusted to the preset vacuum pressure. The preset vacuum pressure refers to the preset pressure in the vacuum chamber, for example, the preset vacuum pressure can be Torr. The pressure in the sputtering chamber is adjusted to the preset working pressure. The preset working pressure refers to the preset pressure in the sputtering chamber when sputtering the molybdenum layer. For example, the preset working pressure is Torr. Pure argon gas (99.99%) is introduced into the chamber at a preset flow rate. The preset flow rate refers to the preset speed for introducing the gas into the chamber, for example, the preset flow rate is 2.0 SCCM. The deposition temperature is room temperature, the deposition time is the preset deposition time, and the deposition power is the preset deposition time. The preset deposition time and the preset deposition power refer to the preset time and power for depositing the thin film, respectively. For example, the preset deposition time is 60min; the preset deposition power is 100w. The preset sputtering time, preset vacuum pressure, preset working pressure, preset flow rate, preset deposition time and preset deposition time can be specifically set according to experimental or production requirements.

[0037] Step II, depositing a layer of Metal, to obtain a dielectric layer.

[0038] Step III: depositing a layer of photoresist on the dielectric layer.

[0039] Step IV, exposing and developing the photoresist in a shape pattern by electron beam lithography to obtain a photoresist layer with a chiral structure in the cavity.

[0040] Step V, depositing a layer of molybdenum metal on the photoresist layer by electron beam evaporation.

[0041] Step VI, stripping the photoresist layer to obtain a metal resonator layer with a chiral structure. For example, using acetone solvent to dissolve the photoresist, and then stripping to form a chiral molybdenum nanostructure with dimensions of h1=200nm, h2=140nm, h3=55nm, w=100nm, s=70nm and l=360nm, such as Figure 1 and Figure 2 As shown. Among them, h1 is the thickness of the bottom metal layer, h2 is the thickness of the electrolyte layer, h3 is the thickness of the metal resonator layer, and l, s and w are the three dimensions of the "Z"-shaped chiral structure.

[0042] The present invention provides a second harmonic conversion system based on the above-mentioned two-ceramic harmonic converter, comprising a laser, the second harmonic converter and a coupling optical fiber, which can be coupled to devices such as a cold charge coupled device (CCD) camera. The laser is used to emit laser light of a desired wavelength as a light source; the light source generates chiral light through a combination of a polarizer and a quarter wave plate; the second harmonic converter is irradiated by the chiral light and generates a nonlinear chiral effect, reflecting the second harmonic; the coupling optical fiber is used to transmit the second harmonic.

[0043] The second harmonic generation principle of the second harmonic converter of the present invention is as follows:

[0044] When the frequencies of the two incident photons are the same, they are When the frequency of the photons generated by the medium This process is called optical frequency doubling, also known as optical second harmonic generation. The essence of the generation of second harmonics is the superposition and increase and decrease of photons, such as Figure 8 The optical properties of metals and heavily doped semiconductors are generally dominated by free carriers. The nonlinear optical response of charge carriers in metals can be described by a hydrodynamic model.

[0045] Electron fluid density and the electron velocity field Satisfies the Euler equation,

[0046] (1)

[0047] and the continuity equation, ,in , is the effective electron mass is the electron collision rate, is the frequency, is the electric field vector, is the magnetic field vector; is the electron fluid density, is the electron speed, represents the spatial coordinates, Represents time, and is often used to describe and analyze the movement and distribution of electrons in space. Combining the continuity equation with the equation, and expanding all fields in a perturbative manner, the free electron polarization intensity With polarization current Satisfies the nonhomogeneous system of equations:

[0048]

[0049] in and are the polarization vectors of the fundamental field and the second harmonic field respectively; is a constant used to describe the range of the electron cloud; represents a nonlinear source; represents the equilibrium charge density of the base field in the absence of any external field; represents the charge of an electron; represents the fundamental electric field; represents the second harmonic electric field; Indicates the vibration frequency.

[0050] The nonlinear source is expressed as:

[0051] (4)

[0052] Equation 4 reflects the second harmonic contribution from the surface and interior, where represents the fundamental electric field; Represents the fundamental magnetic field.

[0053] The linear nonlocal terms can be safely ignored in the equations because they do not affect the generation of harmonics. The second harmonic conversion efficiency strongly depends on The behavior on the metal surface is only along The derivative in the direction perpendicular to the metal boundary is effective. Select the metal boundary 0, the electron pressure can be ignored ( );vector The parallel component Almost a constant. , the normal component Can be written as

[0054] (5)

[0055] in, represents a function that changes rapidly from 0 to 1; from this, the surface polarization equation for second harmonic generation is derived as:

[0056] (6)

[0057] As mentioned before, the second harmonic pressure term Ignored.

[0058] We use equation (2) to convert the electric field Expressed as The function of , which expresses the effective nonlinear surface current density:

[0059] (7)

[0060] in, represents the effective nonlinear surface current density; Indicates direction Unit vector of direction; represents the electron density unit vector; the following values ​​of the parameters are used: = , and .

[0061] The conversion efficiency of the second harmonic is expressed as: = , is the beam cross section, For light intensity, is the output power, is the input power.

[0062] The chirality generation principle of the second harmonic converter described in the present invention is as follows:

[0063] Circular dichroism is a commonly used parameter to characterize chiral nanostructures and is given by:

[0064] (9)

[0065] in Refers to the absorption rate of left-handed light. Refers to the absorption rate of right-handed light.

[0066] Taking into account the nonlinear SHG-CD, we express the formula as:

[0067] (10)

[0068] in and Represent the intensities of left-handed light and right-handed light respectively.

[0069] (11)

[0070] in, represents the left-handed polarized electric field; represents the right-handed polarized electric field; represents the x-component of the electric field; represents the y component of the electric field; Table transposition. Under the electric field excitation of left-handed and right-handed light, the second harmonics generated by the chiral structure have different amplitudes, ellipticities and elliptical orientation angles, resulting in nonlinear CD.

[0071] Example 1

[0072] The second harmonic converter based on the refractory metal plasmon chiral structure provided by the present invention has strong light absorption capability, can realize the conversion of chiral light second harmonic, and has good high temperature stability. The present invention is further described below in conjunction with embodiments.

[0073] The second harmonic converter of the present invention is prepared by combining Figure 7 The instructions are as follows:

[0074] 1) Providing a substrate layer, wherein the material of the substrate layer is a silicon wafer;

[0075] 2) 10 nm Ti, 200 nm Mo and 140 nm aluminum oxide were sequentially deposited on the silicon wafer by electron beam evaporation at a rate of 1.4 / s, wherein Ti was an adhesion layer.

[0076] 3) Then, a photoresist diluted with 1:1 ether was coated on the substrate at a speed of 3000 rpm, the spin coating time was 50 s, and the substrate was baked at 180°C for 3 minutes.

[0077] 4) The nanostructure was determined by electron beam lithography, treated with hexanoic acid ester for 3 min, and then treated with MIBK / IPA-8:1 for 1 min (development).

[0078] 5) Use industrial grade nitrogen ( ) was dried and Mo was used as the target material and pre-sputtered in Ar atmosphere for about 20 min.

[0079] 6) The working pressure was 2.4×10−2 Torr. Pure Ar (99.99%) was introduced into the chamber at a flow rate of 2.0 SCCM. The deposition temperature was RT, the deposition time was 60 min, and the deposition power was 100 W.

[0080] 7) Use acetone solvent to dissolve the photoresist.

[0081] 8) Soft bake and cure the film.

[0082] Packaged as an absorber, it serves as a component for reflecting the second harmonic during the harmonic conversion process.

[0083] The second harmonic converter of the present invention is coupled with a CCD camera to form a second harmonic conversion system. The specific optical path module includes a laser source, a modulator, a second harmonic converter, a coupling optical fiber, a cold charge coupled device (CCD) camera and a spectrometer. The implementation method is as follows:

[0084] 1) The self-built laser light source produces a Gaussian pulse train with a transformation limit of 30 fs and an average power of about 20 mW.

[0085] 2) Flexible wavelength tunability within the desired wavelength range via spatial light modulator.

[0086] 3) Create circularly polarized light by combining a polarizer and a quarter wave plate. Check the quality of the CPL with an additional polarizer.

[0087] 4) The excitation light is focused to the second harmonic converter of the present invention as part of the reflected second harmonic in the conversion process.

[0088] 5) The second harmonic signal is generated at vertical incidence and received and recorded by a cooled charge-coupled device (CCD) camera connected to the spectrometer. Figure 5a , Figure 5b and Figure 6 shown.

[0089] Example 2

[0090] The absorber preparation method of the second harmonic converter of the present invention is as follows:

[0091] 1) Providing a substrate layer, wherein the substrate layer is made of a glass sheet;

[0092] 2) 200 nm Mo, 140 nm aluminum oxide and 55 nm Mo were deposited on the silicon wafer by atomic layer deposition (ALD). All Mo films were deposited on Si (100) wafers by physical vapor deposition (PVD) using a Mo target with a purity of 5N.

[0093] 3) Before PVD, grow 100nm thick thermal To provide electrical insulation, or use / Deposition by atomic layer deposition .

[0094] 4) In the ASM A412 vertical furnace, or / Post-deposition annealing of 300mm wafers at temperatures up to .

[0095] 5) The designed chiral patterns were mask-free milled in the top Mo layer using a FIB system (FEI Helios Nanolab 600).

[0096] 6) The vacuum pump cleans the sputtered particles and packages them into absorbers, which serve as components for reflecting the second harmonic during the harmonic conversion process.

[0097] The second harmonic conversion system of the present invention is implemented as follows:

[0098] 1) Optical absorption and CD spectra were obtained using a broadband white light source and coupled to a grating spectrometer (Horiba, iHR320). The data were processed as Figure 4a , 4b , 4c and 4d.

[0099] 2) Generate CPL using a polarizer combined with a quarter wave plate. Check the quality of the CPL using an additional polarizer.

[0100] 3) Directed by a lens with a focal length of 250 mm, it is focused on the second harmonic converter of the present invention as the part that reflects the second harmonic during the conversion process.

[0101] The second harmonic signal is generated at normal incidence and is received and recorded by a cooled charge-coupled device (CCD) camera connected to the spectrometer.

[0102] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A second harmonic converter, comprising an absorber, characterized in that: The absorber includes a substrate layer, a bottom metal layer, a dielectric layer and a metal resonator layer; the material of the metal resonator layer is molybdenum and has a chiral structure; the dielectric layer is arranged on one side of the bottom metal layer, and the substrate layer is arranged on the other side; the metal resonator layer is arranged on the side of the dielectric layer away from the bottom metal layer.

2. The second harmonic converter according to claim 1, characterized in that: The material of the substrate layer is a silicon wafer or a glass wafer.

3. The second harmonic converter according to claim 1, characterized in that: The material of the bottom metal layer is molybdenum.

4. The second harmonic converter according to claim 1, characterized in that: The material of the dielectric layer is aluminum oxide or silicon dioxide.

5. The second harmonic converter according to claim 1, characterized in that: The chiral structures include "Z", "L", "S" and "E" shapes.

6. The second harmonic converter according to claim 1, characterized in that: The thickness of the metal resonator layer is 55 nm.

7. A method for preparing a second harmonic converter according to any one of claims 1 to 6, comprising: Growing a molybdenum layer on the substrate layer by using a DC magnetron sputtering method to obtain a bottom metal layer; A layer is deposited on the bottom metal layer by electron beam evaporation metal, to obtain a dielectric layer; Depositing a layer of photoresist on the dielectric layer, and exposing and developing the photoresist in an electron beam lithography process to obtain a photoresist layer with a chiral structure in the cavity; A layer of molybdenum metal is deposited on the photoresist layer by electron beam evaporation, and the photoresist layer is peeled off to obtain a metal resonator layer with a chiral structure.

8. The method for preparing a second harmonic converter according to claim 7, characterized in that: The method of growing a molybdenum layer on the substrate layer by using a DC magnetron sputtering method comprises: The chamber was dried with industrial grade nitrogen, and molybdenum was used as the target material; Before depositing the film, the molybdenum target was pre-sputtered in an argon atmosphere for a preset sputtering time; Adjusting the vacuum pressure of the sputtering chamber to a preset vacuum pressure; Adjust the working pressure of the sputtering chamber to the preset working pressure; Pure argon gas is introduced into the chamber at a preset flow rate; The deposition temperature is room temperature, the deposition time is a preset deposition time, and the deposition power is a preset deposition power.

9. The method for preparing a second harmonic converter according to claim 8, characterized in that: The preset spraying time is 20 minutes; the preset vacuum pressure is Torr; the preset working pressure is Torr; the preset flow rate is 2.0 SCCM; the preset deposition time is 60 min; the preset deposition power is 100 w.

10. A second harmonic conversion system based on the second harmonic converter according to any one of claims 1 to 6, characterized in that: It includes a laser, a second harmonic converter and a coupling optical fiber; the laser is used to emit laser light of a required wavelength as a light source; the light source generates chiral light through a combination of a polarizer and a quarter-wave plate; the second harmonic converter is irradiated by the chiral light and generates a nonlinear chiral effect, reflecting the second harmonic; the coupling optical fiber is used to transmit the second harmonic.

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