Optical second harmonic measuring system and attached sample table system thereof

Abstract

The invention provides an optical second harmonic measurement system and a sample stage system thereof, the optical second harmonic measurement system comprises an optical characterization system and the sample stage system, the sample stage system comprises: a housing which is a sealed tetragonal structure and comprises four side surfaces; the transparent window is positioned at the top of the shell; the sample table is positioned at the bottom of the shell, is positioned below the transparent window and is used for placing a sample; the liquid nitrogen inlet, the nitrogen outlet and the heating opening are formed in the first side face of the shell, and the liquid nitrogen inlet comprises an inlet pipe extending into the sample table; the nitrogen outlet comprises a discharge pipe extending out of the interior of the sample table; and the heating port comprises a lead connected to a resistance wire inside the sample table.

Description

Technical Field

[0001] The invention relates to the technical field of optical testing, and in particular to a multi-field wide temperature range optical second harmonic measurement system. Background Art

[0002] When a beam of light with a frequency of w hits a medium with broken spatial inversion symmetry, the reflected light contains both light with a frequency of w and light with frequencies of 2w, 3w, etc. The light with a frequency of 2w is called the optical second harmonic. The intensity of the optical second harmonic depends on the microstructure and properties of the material being measured, such as the crystal system to which the material belongs, the degree of spatial inversion symmetry breaking, and the interaction between polarization domains inside the material. Therefore, the optical second harmonic measurement technology can reveal the microstructure and symmetry inside the material and its changes with external field and time, and is very sensitive to the microstructure and phase change of the material.

[0003] The current optical second harmonic measurement technology can ensure that the measured sample can be measured by heating and cooling under a relatively high vacuum state. However, there are still three main problems with the current optical second harmonic measurement system: 1. Some of the current measurement systems achieve measurement in the low temperature range below room temperature by cooling, and some achieve measurement in the high temperature range above room temperature by heating. It is impossible to achieve continuous measurement in a wide temperature range from low temperature to high temperature. Therefore, the existing system cannot reveal the continuous microstructural changes of multi-phase change materials and the interaction between ferroelectric domains or iron ordering within them; 2. The current measurement system cannot introduce the regulation of other physical fields, such as gas fields and electric fields, and cannot reveal the changes in the internal microstructure and polarization intensity of the material under the action of specific gas fields and electric fields; 3. The current measurement system cannot perform long-term continuous measurement at high temperatures. Long-term heating at temperatures above 900K will cause damage to the sample stage and unstable temperature, reducing the reusability of the sample stage and the reliability of the system. More seriously, it will affect the airtightness of the system, causing leakage in the system and destroying the vacuum degree of the system. Summary of the invention

[0004] Based on the above problems of the prior art, the present invention provides an optical second harmonic measurement system with a wide temperature range and capable of applying a variety of external fields, and the sample stage system includes:

[0005] The housing is a sealed cubic structure including four sides;

[0006] a transparent window located at the top of the housing;

[0007] a sample stage located on the bottom of the housing, the sample stage being located below the transparent window and used for placing a sample;

[0008] A liquid nitrogen inlet, a nitrogen outlet, and a heating port are located on the first side of the housing, the liquid nitrogen inlet includes an inlet tube extending into the interior of the sample stage; the nitrogen outlet includes an exhaust tube extending from the interior of the sample stage; and the heating port includes a wire connected to a resistance wire inside the sample stage.

[0009] In one embodiment, the sample stage system further comprises:

[0010] at least two electrical holes located on a third side surface of the housing opposite to the first side surface;

[0011] At least two probes located in the housing and connected to the at least two power-through holes respectively through wires;

[0012] Wherein, the at least two probes are used to connect to electrodes on the sample.

[0013] In one embodiment, the sample stage system further comprises:

[0014] The magnetic steel ring located around the sample stage is used to absorb and fix the at least two probes.

[0015] In one embodiment, the housing further comprises:

[0016] A first vent hole and a second vent hole are respectively located on opposite fourth and second sides of the housing.

[0017] In one embodiment, the first vent hole and the second vent hole are respectively connected to a molecular pump for pumping gas to achieve a vacuum environment.

[0018] In one embodiment, the first vent hole is connected to a gas bottle for inflation, and the second vent hole is used to discharge foreign gas molecules to achieve a gas environment.

[0019] In one embodiment, the sample stage system further comprises:

[0020] a temperature measuring component located inside the sample stage; and

[0021] The temperature measurement port located on the first side of the housing includes a wire connected to a temperature measuring component inside the sample stage to measure the temperature inside the sample stage.

[0022] In one embodiment, the housing further comprises:

[0023] At least one group of cooling ports includes a first cooling port, a second cooling port, and a pipeline for circulating cold water connecting the first cooling port and the second cooling port.

[0024] The present invention also provides an optical second harmonic measurement system, comprising:

[0025] The sample stage system as described above; and

[0026] The optical characterization system is used to focus the required light onto the sample in the sample stage system and output the optical second harmonic signal received from the sample.

[0027] In one embodiment, the optical characterization system includes:

[0028] A half wave plate, used for receiving laser light from an external light source and changing the polarization direction of the laser light;

[0029] A first polarizer, used for receiving the laser from the half wave plate and generating linearly polarized laser;

[0030] A first filter, used for receiving the laser from the first polarizer and filtering out stray light other than the wavelength of the measuring laser;

[0031] A first focusing lens, used for receiving the laser from the first filter and focusing it onto the sample in the sample stage system;

[0032] A second focusing lens, used for receiving the optical second harmonic signal from the sample and focusing it;

[0033] a second polarizer, used for receiving the optical second harmonic signal from the second focusing lens, and generating a component of the optical second harmonic signal parallel to the reflection surface and a component perpendicular to the reflection surface;

[0034] a second filter, for receiving the optical second harmonic signal from the second polarizer, and filtering out stray light other than the doubled frequency light; and

[0035] The second focusing lens is used to receive the optical second harmonic signal from the second filter and output it for measurement.

[0036] The optical second harmonic measurement system and the sample stage system of the present invention realize continuous optical second harmonic signal measurement from liquid nitrogen temperature to high temperature (900K), and can reveal the in-situ real-time evolution of the polarization intensity of micro-regions inside non-centrosymmetric materials and the coupling intensity between ferroelectric domains in a wide temperature range, covering the phase transition interval of most polarized materials, and avoiding the error caused by the segmented measurement of the existing measurement system. At the same time, the present invention also couples multiple physical fields (gas field and electric field, etc.) into the second harmonic measurement system, providing a research means for revealing the interaction between the external physical field (gas molecules and moving electrons) and the internal polarization displacement of the material and the influence on the coupling intensity between ferroelectric domains. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1FIG. 4 is a schematic diagram of an optical second harmonic measurement system according to an embodiment of the present invention.

[0038] Figure 2 A top view of a sample stage system according to one embodiment of the present invention is shown.

[0039] Figure 3 A top view of a sample stage system according to another embodiment of the present invention is shown.

[0040] Figure 4 A schematic diagram of a sample stage according to an embodiment of the present invention is shown.

[0041] Figure 5 The variation of the second harmonic intensity of BaTiO3 and BiFeO3 thin film samples under vacuum with temperature is shown.

[0042] Figure 6 The figure shows the continuous change of the second harmonic intensity of BaTiO3 thin film samples with temperature in vacuum and 1atm O2 environment.

[0043] Figure 7 The figure shows the continuous change of the second harmonic intensity of the BaTiO3 thin film sample with temperature when no current is passed and when 1uA current is passed.

[0044] Figure 8 The polarization dependence of BaTiO3 thin film samples at two temperatures, 300 and 900K, is shown. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention will be further described in detail below through specific embodiments in conjunction with the accompanying drawings. It should be noted that the embodiments given in the present invention are only for illustration and do not limit the protection scope of the present invention.

[0046] Figure 1 1 shows a schematic diagram of a second harmonic measurement system according to an embodiment of the present invention. The measurement system 100 includes an optical characterization system 120 and a sample stage system 200. The optical characterization system 120 includes a half wave plate 101, a polarizer 102, a filter 103, a focusing lens 104, a focusing lens 106, a polarizer 107, a filter 108, and a focusing lens 109. The sample stage system 200 includes a housing 201, a sample stage 203, and a molecular pump 250. The detailed structure of the sample stage system 200 will be described below in conjunction with Figure 2 Provide detailed explanation.

[0047] The half wave plate 101 is used to receive laser light from an external light source and can change the polarization direction of the laser light. In one embodiment, the laser wavelength is 800nm. The polarizer 102 is used to receive the laser light from the half wave plate 101 and generate a linearly polarized laser light with a fixed polarization direction. The light intensity of the laser light can be changed by the combination of the half wave plate 101 and the polarizer 102. In one embodiment, the polarizer 102 can be controlled by a rotating motor, and the combination of the polarizer 102 and the half wave plate 101 can change the polarization of the incident light without changing the intensity of the incident light. The polarization angle φ can be freely adjusted from 0 to 360°. The filter 103 is used to filter out stray light other than a specific measurement wavelength. The focusing lens 104 is used to receive the laser light from the filter 103 and focus it on the sample 202. Among them, the linearly polarized laser light is focused by the focusing lens 104 to increase the light intensity, and the focused light spot hits the surface of the sample 202 and generates an optical second harmonic signal. The focusing lens 106 is used to receive the optical second harmonic signal from the sample 202 and focus it to the polarizer 107. The polarizer 107 is used to generate the p component parallel to the reflection surface and the s component perpendicular to the reflection surface of the optical second harmonic. In one embodiment, the polarizer 107 can be controlled by a rotating motor, which can change the polarization of the reflected light, and the polarization angle φ can be freely adjusted from 0 to 360°. The filter 108 is used to filter out stray light other than the doubled frequency light. The focusing lens 109 is used to receive the light from the filter 108, and output it to the photon counter after focusing to detect the optical second harmonic signal.

[0048] Figure 2 A top view of a sample stage system according to an embodiment of the present invention is shown. The sample stage system 200 includes a housing 201, and the housing 201 may be a tetrahedral (e.g., a cube) structure, the interior of which may form a sealed cavity. In one embodiment, the housing 201 is made of an aluminum alloy. A transparent window 215 is provided on the top surface of the housing 201, and the transparent window 215 is detachable so that a user can easily replace the sample 202. In one embodiment, a sealing rubber ring is provided around the transparent window 215 to facilitate sealing. The transparent window 215 is made of a material with low optical loss. In one embodiment, the transparent window 215 is made of glass. A sample stage 203 is provided on the bottom surface of the housing 201 for placing the sample 202 to be measured. The specific structure of the sample stage 203 will be described below in conjunction with the description of the embodiment of the present invention. Figure 4 The light from the optical characterization system 120 can be incident on the sample 202 through the transparent window 215 , or the light from the sample 202 can be received through the transparent window 215 .

[0049] The sample stage 203 has a magnetic steel ring 204 around the bottom, which can absorb and fix the probes 213 and 214. The sample 202 may include electrodes, and the probes 213 and 214 can be connected to the electrodes of the sample 202 respectively to provide an electric field for the sample 202. The probes 213 and 214 are connected to the power holes 211 and 212 on the third side C3 of the housing 201 through wires (not shown in the figure). The power holes 211 and 212 can be connected to an external power supply to power the probes 213 and 214. In one embodiment, more probes may be included, and accordingly, more power holes may be included, for example, Figure 3 Four power holes 230 are shown in FIG. In one embodiment, the power holes 211 and 212 can be implemented as a two-pin Lemon interface. When an electric field is applied, the internal polarization of the sample 202 can be enhanced or reversed; when a current is applied, mobile electrons will flow through the sample 202; both of the above situations will change the interaction between adjacent ferroelectric domains inside the material. Therefore, the sample stage system of the present invention can achieve the comparison of the in-situ evolution of the second-order nonlinear optical response of the sample 202 before and after the application of the electric field over time, providing evidence for studying the interaction between the electric field / current and the ferroelectric domain.

[0050] The second side surface C2 and the fourth side surface C4 of the housing 201 respectively include a first vent hole 209 and a second vent hole 210. The first vent hole 209 and the second vent hole 210 are used for gas exchange to provide a desired gas environment, such as a nitrogen environment, an oxygen environment, and a high vacuum environment.

[0051] In one embodiment, the first vent 209 and the second vent 210 are connected to a molecular pump respectively, and the molecular pump group is used to pump air from the first vent 209 and the second vent 210 to achieve a vacuum environment of the sample stage system 200. A high vacuum environment can prevent the sample surface from freezing due to water vapor, thereby affecting the reliability of the measurement result. The present invention can use the molecular pump group to pump vacuum from the first vent 209 and the second vent 210 to achieve a high vacuum environment, and the ultimate vacuum degree can be better than 10 -4 Pa.

[0052] In another embodiment, the first vent hole 209 can be connected to a gas bottle (e.g., an oxygen bottle, a nitrogen bottle, etc.) to inflate the cavity of the housing 201, and the second vent hole 210 is used to discharge the impurity molecules in the cavity to achieve other gas environments of the sample stage system 200. The gas environment generally includes a protective gas (nitrogen, etc.) and a reactive gas (oxygen, etc.) to protect the sample or react with the sample (extracting oxygen or supplementing oxygen). The sample stage system in the present invention can achieve any desired gas environment.

[0053] The first side of the housing 201 includes a liquid nitrogen inlet 205, a nitrogen outlet 206, a heating port 207, and a temperature measurement port 208. The liquid nitrogen inlet 205 includes an inlet pipe 216 extending into the interior of the sample stage 203, and the nitrogen outlet 206 includes an exhaust pipe 217 extending from the interior of the sample stage 203. Liquid nitrogen enters the interior of the sample stage 203 from the liquid nitrogen inlet 205 via the inlet pipe 216, and after cooling the sample stage 203, the nitrogen is discharged from the nitrogen outlet 206 via the exhaust pipe 217. The heating port 207 includes a wire 218 connected to a heating resistance wire inside the sample stage 203, and the wire 218 can be connected to an external power supply to heat the sample stage 203. The temperature measurement port 208 includes a wire 219 connected to a temperature measuring component inside the sample stage 203, which is used to measure the temperature inside the sample stage 203. In the present invention, liquid nitrogen is input from the liquid nitrogen inlet 205, nitrogen is discharged from the nitrogen outlet 206, and the temperature is reduced by taking away heat through the vaporization of liquid nitrogen, and the temperature is increased by heating the resistance wire, so that the sample stage 203 can achieve a continuous temperature range of 77-900K, which covers the phase change range of most polarized materials. The temperature maintenance and the control of the heating and cooling rate are well known in the art and will not be described in detail here. For example, it can be achieved by using a Linkam temperature control system and similar equipment.

[0054] Figure 3 A top view of a sample stage system according to another embodiment of the present invention is shown. It includes four power holes 230 and four cooling ports 241-244. A pipeline for circulating cold water is connected between cooling port 241 and cooling port 242 to prevent the housing from overheating. A pipeline for circulating cold water is also connected between cooling port 243 and cooling port 244 to prevent the housing from overheating. In one embodiment, more or fewer groups of cooling ports may also be used. In one embodiment, the housing can be fixed to a fixing device such as an optical frame by screws. Figure 3 The rest of the Figure 2 The same is not repeated here.

[0055] Figure 4 Schematic diagram of a sample stage according to one embodiment of the present invention is shown. For clarity, Figure 4 and Figure 2 The same reference numerals are used. Figure 4 The heating resistor wire 219 inside the sample stage 203 is shown in the figure, which is wrapped around the inside of the sample stage 203. The sample stage can be made of a material with good thermal conductivity. In one embodiment, the sample stage is made of silver. In one embodiment, the sample stage is silver-welded and sealed to improve the stability and durability of the sample stage at high temperatures. In one embodiment, the sample is fixed to the sample stage with heat-conductive silver glue. Preferably, the sample stage 203 is hollow.

[0056] The effectiveness of the second harmonic measurement system of the present invention is described below through specific embodiments.

[0057] First embodiment:

[0058] This embodiment measures and compares the similarities and differences in the second harmonic signal changes with temperature in a vacuum environment between a BaTiO3 thin film sample with multiple phase transition points grown on a SrTiO3 substrate and a BiFeO3 thin film sample without a ferroelectric-paraelectric phase transition in the temperature range of 77-900K, so as to verify the effectiveness of the second harmonic measurement system of the present invention.

[0059] In this embodiment, the sample is first fixed on the sample stage with heat-conducting silver glue, and then the sample chamber is evacuated to 10 -4 Pa, and then the sample was cooled to 77 K and stabilized for ten minutes to ensure that the sample temperature was uniform and stable. Finally, the sample was slowly heated to 900 K at a heating rate of 1 K / min, and the data was measured at the same time.

[0060] Figure 5 The figure shows the variation of the second harmonic intensity of BaTiO3 and BiFeO3 thin film samples under vacuum with temperature. Figure 5 It can be seen that the second harmonic intensity of the BaTiO3 film changes steeply around 201, 280 and 495K. These three temperatures correspond to the three phase transition temperatures of the BaTiO3 bulk material at -90, 5 and 130°C, respectively, which indicates that the symmetry of the BaTiO3 film has undergone a transformation from rhombus-orthogonal-tetragonal-cubic. The second harmonic intensity of the BiFeO3 film changes slowly and continuously with temperature, which suggests that there is no ferroelectric-paraelectric phase transition of BiFeO3 in this temperature range. The above samples show similar phase transition characteristics that can be revealed by other means in the second harmonic measurement system of the present invention, verifying that the measurement system can be effectively used for continuous measurement in a wide temperature range.

[0061] Second embodiment:

[0062] This example measures and compares the second-order response of a BaTiO3 thin film sample grown on a SrTiO3 substrate in a vacuum and in a pure oxygen atmosphere over a temperature range of 77–900K, and verifies the ability of the system to apply a gas field.

[0063] In this embodiment, the sample is first fixed on the sample stage, and then pure oxygen is introduced from the first vent and impurity molecules are discharged from the second vent to maintain a pure oxygen (O2) atmosphere of one atmosphere (1atm) in the sample chamber. The sample is then cooled to 77K and stabilized for ten minutes. Finally, the sample is slowly heated to 900K at a heating rate of 1K / min and data is measured.

[0064] Figure 6The second harmonic intensity of BaTiO3 thin film samples in vacuum and 1atm O2 environment changes continuously with temperature. By comparing the two, it can be seen that the pure oxygen environment will reduce the second harmonic intensity of BaTiO3 as a whole, but it does not significantly change the phase transition temperature of BaTiO3 at low temperatures (201 and 280K) and will effectively affect the transition process from tetragonal to cubic phase (495K). It can be seen that the gas atmosphere can effectively regulate the polarization intensity and the interaction between adjacent electric domains. The second harmonic measurement system of the present invention can effectively adjust the gas environment.

[0065] Third embodiment:

[0066] This embodiment measures and compares the second harmonic signal variation with temperature of a BaTiO3 thin film sample grown on a SrTiO3 substrate in the temperature range of 77-900K with and without current, and verifies the ability of the system to apply an electric field.

[0067] In this embodiment, the sample is first fixed and the sample chamber is evacuated to 10 -4 Pa, and then cool the sample to 77 K and stabilize for ten minutes. A Keithley 2450 current source was used to apply a constant current of 1 uA diagonally to the sample through a two-pin Lemon electrical interface, and the sample was slowly heated to 900 K at a rate of 1 K / min and the data was measured.

[0068] Figure 7 The figure shows the continuous change of the second harmonic intensity of the BaTiO3 thin film sample with temperature when no current is passed and when 1uA current is passed. By comparing the two, it can be seen that the current environment can enhance the second harmonic signal of BaTiO3 and affect the second-order nonlinear response of the material in the high temperature range. It can be seen that the mobile electrons introduced by the external current can effectively change the polarization intensity inside the material and the coupling between adjacent polarization domains. The second harmonic measurement system of the present invention can effectively introduce an electrical field.

[0069] Fourth embodiment:

[0070] This example measures and compares the dependence of BaTiO3 thin film samples grown on SrTiO3 substrates on the polarization of the incident laser at two temperatures, 300 and 900K, and verifies the system's ability to perform long-term measurements at high temperatures.

[0071] In this embodiment, the sample is first fixed and the sample chamber is evacuated to 10 -4 Pa, and then after stabilizing at room temperature for ten minutes, change the polarization angle φ of the incident light for measurement. Similarly, raise the temperature of the sample to 900K and stabilize it for ten minutes for the above measurement.

[0072] Figure 8The polarization dependence characteristics of the BaTiO3 thin film sample at two temperatures of 300 and 900K are shown. Through fitting, it is found that BaTiO3 remains in a typical tetragonal phase at 300K and has a strong second-order response. At 900K, BaTiO3 is no longer sensitive to the polarization of the incident light and its secondary response intensity is almost 0, and its structure shows obvious centrosymmetric characteristics (cubic phase). The above experiment stays at 900K for more than 4 hours, proving that the measurement system designed by the present invention can effectively realize long-term uninterrupted measurement at high temperatures.

[0073] The second harmonic measurement system and the sample stage system of the present invention realize continuous second harmonic signal measurement from liquid nitrogen temperature to high temperature zone, and can reveal the in-situ real-time evolution of the polarization intensity of micro-regions inside non-centrosymmetric materials and the coupling intensity between ferroelectric domains in a wide temperature range, covering the phase change interval of most polarized materials, and avoiding the error caused by the segmented measurement of the existing measurement system. At the same time, the present invention also couples multiple physical fields (gas field and electric field, etc.) into the second harmonic measurement system, providing a research means for revealing the interaction between the external physical field (gas molecules and moving electrons) and the internal polarization displacement of the material and the influence on the coupling intensity between ferroelectric domains.

[0074] Although the present invention has been described through preferred embodiments, the present invention is not limited to the embodiments described herein but includes various changes and modifications that may be made without departing from the scope of the present invention.

Claims

1. A sample stage system, comprising: The housing is a sealed cubic structure including four sides; a transparent window located at the top of the housing; a sample stage located on the bottom of the housing, the sample stage being located below the transparent window and used for placing a sample; A liquid nitrogen inlet, a nitrogen outlet, and a heating port are located on the first side of the housing, the liquid nitrogen inlet includes an inlet tube extending into the interior of the sample stage; the nitrogen outlet includes an exhaust tube extending from the interior of the sample stage; and the heating port includes a wire connected to a resistance wire inside the sample stage.

2. The sample stage system according to claim 1, wherein: The sample stage system also includes: at least two electrical holes located on a third side surface of the housing opposite to the first side surface; At least two probes located in the housing and connected to the at least two power-through holes respectively through wires; Wherein, the at least two probes are used to connect to electrodes on the sample.

3. The sample stage system according to claim 2, wherein: The sample stage system also includes: The magnetic steel ring located around the sample stage is used to absorb and fix the at least two probes.

4. The sample stage system according to claim 1, wherein: The housing also includes: A first vent hole and a second vent hole are respectively located on opposite fourth and second sides of the housing.

5. The sample stage system according to claim 4, wherein: The first vent hole and the second vent hole are respectively connected to a molecular pump for pumping gas to achieve a vacuum environment.

6. The sample stage system according to claim 4, wherein: The first vent hole is connected to a gas bottle for inflation, and the second vent hole is used to discharge impurity molecules to achieve a gas environment.

7. The sample stage system according to claim 1, wherein: The sample stage system also includes: a temperature measuring component located inside the sample stage; and The temperature measurement port located on the first side of the housing includes a wire connected to a temperature measuring component inside the sample stage to measure the temperature inside the sample stage.

8. The sample stage system according to claim 1, wherein: The housing also includes: At least one group of cooling ports includes a first cooling port, a second cooling port, and a pipeline for circulating cold water connecting the first cooling port and the second cooling port.

9. An optical second harmonic measurement system, comprising: The sample stage system according to any one of claims 1 to 8; and The optical characterization system is used to focus the required light onto the sample in the sample stage system and output the optical second harmonic signal received from the sample.

10. The sample stage system according to claim 9, wherein: The optical characterization system shown includes: A half wave plate, used for receiving laser light from an external light source and changing the polarization direction of the laser light; A first polarizer, used for receiving the laser from the half wave plate and generating linearly polarized laser; A first filter, used for receiving the laser from the first polarizer and filtering out stray light other than the wavelength of the measuring laser; A first focusing lens, used for receiving the laser from the first filter and focusing it onto the sample in the sample stage system; A second focusing lens, used for receiving the optical second harmonic signal from the sample and focusing it; a second polarizer, used for receiving the optical second harmonic signal from the second focusing lens, and generating a component of the optical second harmonic signal parallel to the reflection surface and a component perpendicular to the reflection surface; a second filter, for receiving the optical second harmonic signal from the second polarizer, and filtering out stray light other than the doubled frequency light; and The second focusing lens is used to receive the optical second harmonic signal from the second filter and output it for measurement.

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