CN117030660A - Device for measuring electro-optic coefficient of ferroelectric film - Google Patents

Abstract

The invention provides a device for measuring the electro-optical coefficient of a ferroelectric film, which includes a sample placement module, an incident module, an electric field module and a dual optical path interference module. The dual optical path interference module is used to generate linearly polarized light after penetrating the ferroelectric film to be measured. The phase difference between the two polarized lights is used to output the electro-optical coefficient component corresponding to the ferroelectric film to be measured. The dual optical path interference module includes a birefringent interference unit and a Mach‑Zenhder interference unit. The birefringent interference unit is used to measure the ferroelectric film to be measured. The electro-optical coefficient component in the ferroelectric film to be measured can be separated using birefringence interference method, and the Mach-Zenhder interference unit is used to measure the electro-optical coefficient component in the ferroelectric film to be measured that cannot be separated using birefringence interference method; the present invention comprehensively utilizes the two interference results to Different electro-optical coefficient components are measured.

Description

A device for measuring the electro-optical coefficient of ferroelectric films

Technical field

The invention relates to the technical field of photoelectric measurement, and in particular to a device for testing ferroelectric thin film materials.

Background technique

Ferroelectric materials include lithium niobate, barium titanate, lead lanthanum zirconate titanate, etc. Ferroelectric films are widely used in high-speed electro-optical modulators, tunable filters and other fields. These thin film materials have multiple non-zero electro-optical coefficient components. Depending on the crystal orientation, the direction of the modulated electric field, the polarization state of light and the propagation direction, the electro-optical coefficients that play a role are different. During the development of thin film materials and device preparation, it is necessary to Electro-optical coefficients are measured to guide the development of materials and devices.

At present, the birefringence of crystals is usually used to measure the electro-optical coefficient of ferroelectric film materials. This method mainly places a quarter-wave plate behind the ferroelectric film sample, and adjusts the birefringence phase difference by rotating the quarter-wave plate. The birefringent output light is adjusted to linearly polarized light. A sinusoidal modulation voltage is applied to the ferroelectric thin film material. The phase difference is modulated. The azimuth angle of the output linearly polarized light will change according to a sinusoidal rule. The azimuth angle is detected by the analyzer, using The lock-in amplifier detects the signal and analyzes the relationship between the azimuth angle and the modulation voltage amplitude to obtain the electro-optical coefficient.

However, in the existing solution, the quarter-wave plate adjusts the birefringent light into linearly polarized light. Under the action of the modulation voltage, the azimuth angle of the linearly polarized light will change. The detection azimuth angle is measured as the modulation voltage changes. Electro-optical coefficient. This method has the following three problems: The first problem is that under the action of the modulation voltage, the polarization state of the light will become elliptically polarized light, which can only be approximately considered to be linearly polarized light, so a certain measurement error will occur; The second problem is that after the quarter-wave plate adjusts the birefringent light into linearly polarized light, the operating point is not at the orthogonal bias point, and the linearity and sensitivity of the electro-optical modulation response are low. These two problems lead to large measurement errors and low sensitivity in the existing solution; the third problem is that it is impossible to separate the electro-optical coefficient component that simultaneously acts on the phase change in birefringence, such as the electro-optical coefficient component of the barium strontium titanate film. r ₁₃ and r ₃₃ .

Contents of the invention

The purpose of the present invention is to provide a device for testing ferroelectric thin film materials to improve the technical problem of low detection sensitivity of the prior art devices for testing ferroelectric thin film materials. The present invention can measure more than the prior art. The electro-optical coefficient component of .

In order to solve the above technical problems, the present invention provides a device for measuring the electro-optical coefficient of a ferroelectric film, which includes a sample placement module, an incident module, an electric field module and a dual optical path interference module. The sample placement module is used to place the ferroelectric film to be measured, and the incident module is used to place the ferroelectric film to be measured. The module is used to provide linearly polarized light to illuminate the ferroelectric film to be measured. The electric field module is used to provide an electric field to the ferroelectric film to be measured. The dual optical path interference module is used to generate two polarizations based on the linearly polarized light penetrating the ferroelectric film to be measured. The phase difference between the lights is used to output the electro-optical coefficient component corresponding to the ferroelectric film to be measured;

Among them, the dual optical path interference module includes a birefringence interference unit and a Mach-Zenhder interference unit. The birefringence interference unit is used to measure the electro-optical coefficient component that can be separated using the birefringence interference method in the ferroelectric film to be measured. The Mach-Zenhder interference unit is used to Measure the electro-optical coefficient component in the ferroelectric film to be measured that cannot be separated using birefringence interferometry.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the sample placement module is a rotatable sample stage, and the rotatable sample stage is used to place the ferroelectric film to be measured.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the incident module includes a coaxially arranged laser, a half-wave plate and a polarizer. The laser is used to provide a light source in the visible band or infrared band, and the polarizer is used to The light emitted by the light source is converted into linearly polarized light. The half-wave plate is used to adjust the polarization direction of the light emitted by the light source to maximize the intensity of the light emitted by the polarizer.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the electric field module includes a signal generator, a voltage amplifier, a first electrode and a second electrode. The signal generator is used to provide a first voltage signal, and the voltage amplifier is used to The first voltage signal is amplified into a second voltage signal, and the first electrode and the second electrode are respectively attached to two ends of the ferroelectric film to be measured to transmit the second voltage signal to the ferroelectric film to be measured.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the first electrode and the second electrode are made of gold, and the distance between the first electrode and the second electrode ranges from 5 μm to 10 μm.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the birefringence interference unit includes a phase compensator, an analyzer and a first detector arranged coaxially in sequence, and the phase compensator is arranged coaxially with the incident unit. The first detector is used to detect the intensity of the received interference signal;

Among them, the phase compensator is used to adjust the working point where the two polarized lights generated in the dual optical path interference module interfere to an orthogonal bias point.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the Mach-Zenhder interference unit includes a first optical path unit and a second optical path unit. The first optical path unit includes a first beam splitter, a phase detector, and a first beam splitter that are coaxially arranged in sequence. a compensator, an analyzer, a second beam splitter and a first detector; the second optical path unit includes a first beam splitter, a first reflecting mirror, a second reflecting mirror, a second beam splitter and a first detector.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the first reflector and the second reflector are also provided with a reference sample stage. The reference sample stage is used to place a reference film sample. The reference film sample is quartz glass. To compensate for the optical path difference between the first optical path unit and the second optical path unit.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, the device also includes a second detector, a lock-in amplifier and a computer control system. The lock-in amplifier is electrically connected to the first detector, and the computer control system is connected to the phase lock-in amplifier. Amplifier electrical connections;

Among them, the second detector is used to detect the light intensity of the partial beam where birefringent interference does not occur when the birefringent interference unit is working normally, the lock-in amplifier is used to detect the interference signal received by the first detector, and the computer control system is used to detect the light intensity according to the interference signal received by the first detector. The interference signal outputs the electro-optical coefficient component corresponding to the ferroelectric film to be measured.

In the device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention, when the ferroelectric film to be measured is a barium strontium titanate film, a birefringent interference unit is used to measure the electro-optical coefficient component r ₄₂ of the barium strontium titanate film, using The Mach-Zenhder interference unit measures the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ of the barium strontium titanate film respectively.

The beneficial effects of the present invention are: different from the prior art, the present invention provides a device for measuring the electro-optical coefficient of ferroelectric films, which includes a sample placement module, an incident module, an electric field module and a dual optical path interference module. The sample placement module is used for Place the ferroelectric film to be measured, the incident module is used to provide linearly polarized light to illuminate the ferroelectric film to be measured, the electric field module is used to provide an electric field to the ferroelectric film to be measured, and the dual optical path interference module is used to penetrate the ferroelectric film to be measured according to the linearly polarized light. The phase difference between the two polarized lights generated after the electric film is used to output the electro-optical coefficient component corresponding to the ferroelectric film to be measured. Among them, the dual optical path interference module includes a birefringent interference unit and a Mach-Zenhder interference unit. The birefringent interference unit is used The Mach-Zenhder interference unit is used to measure the electro-optical coefficient component that cannot be separated using birefringence interference in the ferroelectric film to be measured. The Mach-Zenhder interference unit is used to measure the electro-optical coefficient component that cannot be separated using birefringence interference in the ferroelectric film to be measured. The invention provides The device for measuring the electro-optical coefficient of ferroelectric films introduces the Mach-Zenhder interference unit structure on the basis of the birefringent interference unit structure, in which the birefringent interference unit is used to measure the electro-optical coefficient components in the ferroelectric film to be measured that can be separated using the birefringent interference method. At the same time, a Mach-Zenhder interference unit is used to measure the electro-optical coefficient component in the ferroelectric film to be measured that cannot be separated using birefringence interference method, so that the two interference results can be combined to measure different electro-optical coefficient components, thereby greatly improving the accuracy of measuring ferroelectricity. The detection sensitivity and accuracy of the thin film electro-optical coefficient device thus saves detection costs.

Description of the drawings

Figure 1 is a schematic structural diagram of a device for measuring the electro-optical coefficient of a ferroelectric film provided by an embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

Please refer to FIG. 1 , which is a schematic structural diagram of a device 100 for measuring the electro-optical coefficient of a ferroelectric film according to an embodiment of the present invention. Specifically, the device 100 for measuring the electro-optical coefficient of a ferroelectric film includes a sample placement module, an incident module, an electric field module and a dual optical path interference module. The sample placement module is used to place the ferroelectric film 105 to be measured, and the incident module is used to provide linearly polarized light. The ferroelectric film 105 to be measured is irradiated. The electric field module is used to provide an electric field to the ferroelectric film 105 to be measured. The dual optical path interference module is used to determine the phase between the two polarized lights generated after the linearly polarized light penetrates the ferroelectric film 105 to be measured. The difference is used to output the electro-optical coefficient component corresponding to the ferroelectric film 105 to be measured;

Among them, the dual optical path interference module includes a birefringence interference unit and a Mach-Zenhder interference unit. The birefringence interference unit is used to measure the electro-optical coefficient component that can be separated using the birefringence interference method in the ferroelectric film 105 to be measured. The Mach-Zenhder interference unit is used The electro-optical coefficient component that cannot be separated using the birefringence interference method is used in measuring the ferroelectric thin film 105 to be measured.

In the embodiment of the present invention, the sample placement module is a rotatable sample stage 104, and the rotatable sample stage 104 is used to place the ferroelectric film 105 to be measured.

In the embodiment of the present invention, the Mach-Zenhder interference unit has a Mach-Zenhder switching structure. The principle of the Mach-Zenhder switching structure is realized by relying on the coherence of light. That is, the same light source can be used through two different path, obtain two mutually equivalent beams, and then bring them together so that the pattern changes caused by the phase difference can be seen.

In the embodiment of the present invention, the incident module includes a coaxially arranged laser 101, a half-wave plate 102 and a polarizer 103. The laser 101 is used to provide a light source in the visible band or the infrared band, and the polarizer 103 is used to emit light from the light source. The light is converted into linearly polarized light, which can adjust the azimuth angle of the linearly polarized light. The half-wave plate 102 is used to adjust the polarization direction of the light emitted by the light source to maximize the intensity of the light emitted from the polarizer 103 .

In the embodiment of the present invention, the electric field module includes a signal generator 118, a voltage amplifier 117, a first electrode 106 and a second electrode 107. The signal generator 118 is used to provide a first voltage signal, and the voltage amplifier 117 is used to convert the first voltage The signal is amplified into a second voltage signal, and the first electrode 106 and the second electrode 107 are respectively attached to both ends of the ferroelectric film 105 to be measured to transmit the second voltage signal to the ferroelectric film 105 to be measured.

Specifically, the material of the first electrode 106 and the second electrode 107 is gold, and the distance between the first electrode 106 and the second electrode 107 ranges from 5 μm to 10 μm. The larger the distance between the first electrode 106 and the second electrode 107 , The higher the electric field intensity input to the ferroelectric film 105 to be measured.

In the embodiment of the present invention, the birefringence interference unit includes a phase compensator 108, an analyzer 109 and a first detector 116 arranged coaxially in sequence. The phase compensator 108 is arranged coaxially with the incident unit. The first detector 116 is To detect the received interference signal strength;

Among them, the phase compensator 108 is used to adjust the working point where the two polarized lights generated in the dual optical path interference module interfere to an orthogonal bias point.

In the embodiment of the present invention, the Mach-Zenhder interference unit includes a first optical path unit and a second optical path unit. The first optical path unit includes a first beam splitter 110, a phase compensator 108, an analyzer 109, which are coaxially arranged in sequence. The second beam splitter 114 and the first detector 116 , the second optical path unit includes the first beam splitter 110 , the first reflecting mirror 111 , the second reflecting mirror 113 , the second beam splitter 114 and the first detector 116 .

Specifically, the first reflector 111 and the second reflector 113 are also provided with a reference sample stage. The reference sample stage is used to place the reference film sample 112. The material of the reference film sample 112 is quartz glass. The thickness of the reference film sample 112 is the same as that to be measured. The thickness of the ferroelectric film 105 is similar, and the reference film sample 112 is used to compensate for the optical path difference between the first optical path unit and the second optical path unit.

Furthermore, the ferroelectric film 105 to be measured is provided by the user, and the thickness of different samples may be slightly different; the reference film sample 112 is used to adjust the interference optical path difference of the optical instrument, and the thickness is generally fixed or several types can be provided.

In the embodiment of the present invention, the device 100 for measuring the electro-optical coefficient of a ferroelectric film includes a second detector 115, a lock-in amplifier 119, and a computer control system 120. The lock-in amplifier 119 is electrically connected to the first detector 116, and the computer control system 120 electrically connected to the lock-in amplifier 119;

Among them, the second detector 115 is used to detect the light intensity of the partial beam where birefringence interference does not occur when the birefringent interference unit is operating normally, and the lock-in amplifier 119 is used to detect the interference signal received by the first detector 116. The computer controls The system 120 is used to output the electro-optical coefficient component corresponding to the ferroelectric film 105 to be measured according to the interference signal.

Specifically, when the ferroelectric film 105 to be measured is a barium strontium titanate film, a birefringence interference unit is used to measure the electro-optical coefficient component r ₄₂ of the barium strontium titanate film, and a Mach-Zenhder interference unit is used to measure the electro-optical coefficient component r 42 of the barium strontium titanate film respectively. The electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ .

Specifically, the specific process of measuring the electro-optic coefficient component of a ferroelectric film using the device 100 for measuring the electro-optic coefficient of a ferroelectric film is as follows:

Before measurement, determine the crystal orientation of the ferroelectric film material and the electro-optical coefficient component of the ferroelectric film 105 to be measured, and determine whether the second reflector 113 needs to be removed.

When measuring, first set the azimuth angle of the polarizer 103 according to the measurement requirements, read the interference signal detected by the first detector 116, and rotate the half-wave plate 102 to maximize the signal value;

Then place the sample to be measured, select normal incidence or oblique incidence according to the measurement requirements, and connect the driving signal line to the modulation electrode;

Afterwards, adjust the interference phase difference, read the signal value of the first detector 116, record the maximum value and the minimum value, and then set the phase difference at the intermediate value, which is the orthogonal bias point; where, the birefringence interference unit During normal operation (removing the second reflector 113), the interference phase difference is adjusted only through the phase compensator 108; during normal operation of the Mach-Zenhder interference unit (double beam measurement, without removing the second reflector 113), the phase compensator is used 108 and the reference film sample 112 to adjust the interference phase difference.

Finally, a sinusoidal modulation voltage is applied, the interference output signal is detected through the lock-in amplifier 119, the amplitude of the sinusoidal modulation voltage is changed, multiple data are recorded, and the electro-optical coefficient measurement value is obtained by analyzing the data.

Specifically, the specific working principle of using the above-mentioned device 100 for measuring the electro-optical coefficient of a ferroelectric film to measure the electro-optical coefficient component of a ferroelectric film is as follows:

The laser 101, the half-wave plate 102, and the polarizer 103 form an incident module, which can adjust the azimuth angle of the incident polarized light. After the light passes through the first beam splitter 110, a beam of light is incident on the sample to be measured. The sample is placed on the rotatable sample stage 104. The phase compensator 108 and the analyzer 109 form a birefringent interference light path for detecting the light. Measurement of the separated electro-optical coefficient components; at this time, the second reflecting mirror 113 is removed under the control of the computer control system 120, and the optical loop composed of the reflecting mirror is disconnected. This optical path does not participate in interference, and the electro-optical coefficient is measured using the birefringence interference method. component, the optical power reaching the second detector 115 is used for internal optical power detection and system fault diagnosis.

When another beam of light after passing through the first beam splitter 110 enters the loop composed of the first reflector 111 and the second reflector 113, the two beams of light perform double-beam interference at the second beam splitter 114. This At this time, a Mach-Zenhder interferometer structure is formed. By reasonably arranging the polarization state, the crystal orientation of the ferroelectric film 105 to be measured, and the modulation voltage between the first electrode 106 and the second electrode 107, a single ordinary light or extraordinary light is divided into Two beams of light participate in the interference and are used to measure the electro-optical coefficients that cannot be separated by birefringence interferometry. Since the detectable signal is usually very weak, a lock-in amplifier 119 is required for detection.

Specifically, the linearly polarized light after passing through the polarizer 103 is incident on the sample to be measured, and the half-wave plate 102 is rotated to maximize the intensity of the light emitted from the polarizer 103 . When birefringence interferometry is used, due to birefringence, the polarized light becomes elliptically polarized light after passing through the sample to be measured. The phase compensator 108 is adjusted to compensate the phases of the two birefringent lights, and the phases are compensated to the orthogonal bias. state to obtain high sensitivity and linearity. Generally, the azimuth angle of the polarizer 103 is set to 45°. At this time, the light behind the phase compensator 108 is circularly polarized light. The amplitudes of the two polarized lights participating in the interference are equal, which can provide good interference contrast. At this time, the transmission axes of the analyzer 109 and the polarizer 103 are perpendicular to each other, and the two polarized lights passing through the analyzer 109 interfere. In addition, the sample stage in the sample placement module can be rotated to accurately adjust the incident angle. When the crystal orientation of the ferroelectric film 105 to be measured is [001], the sample stage on which the ferroelectric film 105 is placed is rotated, and the light is incident obliquely on the ferroelectric film 105 to be measured, forming an angle with the optical axis, thus producing a double refraction.

Furthermore, when double-beam interference measurement is used, the azimuth angle of the polarizer 103 is adjusted according to the electro-optical coefficient component to be measured, so that only ordinary light or extraordinary light is transmitted in the ferroelectric film 105 to be measured, without birefringence and polarization analysis. The transmission axes of the polarizer 109 and the polarizer 103 are parallel to each other. The phase compensator 108 and the reference ferroelectric film 105 to be measured are adjusted to compensate the phases of the two light beams to an orthogonal bias state, and the two light beams interfere at the second beam splitter 114 . Among them, the path difference between the two light beams in the Mach-Zenhder interference unit is relatively large. It is difficult to complete the phase compensation only through the phase compensator 108 in the first optical path unit. It is necessary to place the reference film sample 112 in the second optical path unit at the same time. The optical path difference between the first optical path unit and the second optical path unit is compensated. Specifically, when a voltage is applied between the first electrode 106 and the second electrode 107, the ordinary light refractive index or the extraordinary light refractive index will change, and the phase difference of the interference will also change, so that the interference light intensity will change. . By choosing to apply voltages between the first electrode 106 and the second electrode 107 in different directions, and adjusting the polarization state and incident light direction, different electro-optical coefficient components can be measured.

Further, in order to detect weak signals, a sine wave modulation voltage is usually applied between the first electrode 106 and the second electrode 107, and then the intensity of the interference signal received by the first detector 116 is detected through the lock-in amplifier 119.

The technical solution of the present application will now be described with reference to specific embodiments.

1. Electro-optical effect analysis:

Take BTO material (barium strontium titanate thin film material) as an example to measure its electro-optical coefficient component. By searching for information, we can know that BTO material is a negative uniaxial crystal when no voltage is applied. When natural light enters the crystal of BTO material from one direction, it will form two polarized lights. The light that vibrates perpendicular to the optical axis is called ordinary light, which is parallel. The light that vibrates in the plane formed by the incident direction and the optical axis is called extraordinary light e-light;

Among them, the refractive index (n _o ) of BTO material for ordinary light is greater than the refractive index (ne ₎ of BTO material for extraordinary light, n _o =2.437, n _e =2.365.

Specifically, by searching the linear electro-optical coefficient matrix manual, it can be seen that the electro-optical coefficient of the BTO material has three non-zero independent variables (r ₁₃ , r ₃₃ and r ₄₂ ).

According to the refractive index ellipsoid theory, the z-axis is usually taken as the optical axis direction; at this time, the refractive index ellipsoid of the BTO material is a regular ellipsoid, and the ellipsoid equation is:

Since the electro-optical coefficient of BTO material has three non-zero independent variables, under the action of external power supply, the variation coefficient of the refractive index ellipsoid (second-order symmetry tensor) will change, which is expressed as follows:

Among them, Δβ ₁ to _{Δβ 6} are the variation coefficients of the refractive index ellipsoid of the BTO material under the action of an external power source, _E The voltage on, E _z is the voltage in the z-axis direction of the refractive index ellipsoid.

Substituting formula (2) into formula (1), after applying voltage, the inductive refractive index ellipsoid equation of BTO material can be expressed as:

It can be seen from formula (3) that the effect of applying voltage in the x-axis direction of the refractive index ellipsoid is the same as that of applying voltage in the y-axis direction of the refractive index ellipsoid. They can be interchanged with each other. The effects of Ex and Ey are exactly the same. , so only two cases of applying E _x and E _z need to be analyzed.

In one embodiment, when E _x ≠ 0 and E _y =E _z =0, formula (3) can be expressed as:

Comparing formula (4) with formula (1), the zx cross term appears, indicating that one main axis of the induced refractive index ellipsoid coincides with the y-axis of the original refractive index ellipsoid, and the other two main axis directions can be obtained by rotating around the y-axis (the original After the refractive index ellipsoid is rotated θ in the xoz plane, the induced refractive index ellipsoid can be obtained):

Among them, when E _x >0, it is counterclockwise rotation; when E _x <0, it is clockwise rotation; the angle of rotation θ satisfies the following formula:

Let y=0, substitute into formula (4), and obtain the refractive index ellipse equation in the xoz plane as follows:

Among them, under normal circumstances, the rotation angle θ is very small, and the lengths of the three main axes of the induced refractive index ellipsoid also change. The refractive index changes in the three directions are as follows:

Substituting formula (5) into formula (7), as follows:

It can be seen from formula (8) that after an external voltage is applied, the change in refractive index is proportional to the square of the voltage.

In another embodiment, when E _z ≠0 and E _x =E _y =0, formula (3) can be expressed as:

At this time, it can be seen from formula (9) that compared with when no voltage is applied, the induced refractive index ellipsoid does not rotate relative to the original refractive index ellipsoid, and is still a positive ellipsoid, but the length of the main axis of the induced refractive index ellipsoid is Changes have occurred (when an electric field is applied in the z-axis direction of the original refractive index ellipsoid, both the ordinary light refractive index and the extraordinary light refractive index change), as follows:

n′ _x =n′ _y =n′ _o ,n′ _z =n′ _e (10);

At this time, the film surface is the xoz plane, the vertical optical axis of light passes through the BTO film, the birefringence occurs in the x and z directions, and the inductive refractive index change is as follows:

It can be seen from formula (11) that after a voltage is applied in the z-axis direction of the original refractive index ellipsoid, the refractive index change is proportional to the voltage.

2. Measurement plan analysis:

In the first embodiment of the present invention, when the z-axis (optical axis) of the original refractive index ellipsoid is perpendicular to the surface of the BTO film to be measured, a voltage can only be applied in the x-axis direction of the original refractive index ellipsoid, and the film The surface is an xoy plane, and light passes through the BTO film along the optical axis. At this time, let z=0. After substituting into formula (4), the inductive refractive index ellipsoid equation is as follows:

It can be seen from formula (12) that there is no birefringence (ne _e does not exist), which is equivalent to the optical axis z on the original refractive index ellipsoid turning an angle to z′, z′ is the new crystal axis, but Instead of the optical axis, the BTO crystal becomes a biaxial crystal. The z direction is an optical axis C1 of the biaxial crystal. That is to say, the angle between the optical axis and z′ is θ. The direction of the optical axis C1 is always in the z direction regardless of the electric field. , the other optical axis is on the other side of z′.

In order to obtain birefringence, the light can be transmitted along 45° between xoz (achieved by rotating the sample stage on which the BTO material to be measured is placed), and the polarization direction is 45° with the xoz plane (achieved by adjusting the polarizer 103).

Correspondingly, when the birefringence interference unit is working, when the light in two polarization directions passes through the BTO sample of length L, the phase difference δ _E produced is (λ is the wavelength, E _x is the original refractive index ellipsoid on the x-axis voltage applied in the direction):

At this time, the phase change is proportional to the voltage applied in the x-axis direction of the original refractive index ellipsoid. Therefore, the electro-optical coefficient component r ₄₂ of the BTO material can be obtained through the birefringence interference unit and combined with formula (13).

In the second embodiment of the present invention, when the z-axis (optical axis) of the original refractive index ellipsoid is on the surface of the BTO film to be measured, an additional element can be added in the x-axis direction or z-axis direction of the original refractive index ellipsoid. Voltage.

(a) When voltage E _x is applied in the x-axis direction of the original refractive index ellipsoid:

The film surface is an xoz plane, and the vertical optical axis of light passes through the BTO film, which is related to r _42. The birefringence occurs in the x and z directions, and the inductive refractive index change is shown in formula (8):

Correspondingly, between the light along the two polarization directions that pass through the BTO film, according to formula (8), the phase difference generated by E _x can be obtained as (not counting the static phase difference, ignoring the rotation in the xoz plane):

At this time, the phase change is proportional to the square of the voltage applied by the original refractive index ellipsoid in the x-axis direction. Therefore, the electro-optical coefficient component r ₄₂ of the BTO material can be obtained through the birefringence interference unit and combined with formula (14).

(b) When voltage E _z is applied in the z-axis direction of the original refractive index ellipsoid:

The film surface is an xoz plane, the vertical optical axis of light passes through the BTO film, the birefringence occurs in the x and z directions, and the inductive refractive index change is as shown in formula (11):

Correspondingly, between the light along the two polarization directions that pass through the BTO film, according to formula (11), the phase difference generated by E _z can be obtained as (not counting the static phase difference, ignoring the rotation in the xoz plane):

At this time, the phase change is proportional to the voltage applied in the z-axis direction of the original refractive index ellipsoid. The inductive refractive index change is related to r ₁₃ and r _33. The birefringence interference method cannot separate r ₁₃ and r ₃₃ . (According to formula (15), it is impossible to solve two unknown quantities in one equation). Therefore, the electro-optical coefficient components r ₁₃ and r ₃₃ of the BTO material cannot be measured through the birefringence interference unit.

3. Use the device 100 for measuring the electro-optical coefficient of ferroelectric thin films of the present invention to analyze the specific measurement plan.

The Mach-Zenhder interference unit is used to measure two electro-optical coefficients that cannot be separated by birefringence interference method, such as the electro-optical coefficient components r ₁₃ and r ₃₃ of BTO material. At this time, a Mach-Zenhder interferometer structure is formed. The second reflector 113 can be moved away under the control of the computer control system 120. When measuring other electro-optical coefficients, such as r ₄₂ of the electro-optical coefficient component of the BTO material, the optical path composed of the second reflector 113 is disconnected and does not participate. Interference, at this time the light can reach the second detector 115 for internal optical power detection and system fault diagnosis. The following takes BTO crystal film as an example to analyze the research plan.

(a) Use a birefringent interference unit to measure the electro-optical coefficient component r ₄₂ :

Assume that the azimuth angle of the polarizer 103 is θi (the angle with the horizontal plane), and the analyzer 109 is perpendicular to the transmission axis of the polarizer 103, then the received light intensity is:

Among them, I is the light intensity received by the first detector 116 after applying a voltage, I ₀ is the light intensity received by the first detector 116 when no voltage is applied, and δ is the difference between the two polarized lights participating in the interference. phase difference. After applying a voltage to the BTO film, the refractive index ellipsoid will change, causing the phase difference δ to change. The change in δ measurement can be used to calculate the electro-optical coefficient.

By applying voltages in different directions and using different polarized light, different electro-optical coefficient components can be measured. In order to improve the detection accuracy, a modulation voltage with a frequency of ω can be applied to the first electrode 106 and the second electrode 107. Assume that the voltage is:

E＝E ₀ cosωt (17);

According to the previous analysis, the electro-optical coefficient component r ₄₂ plays a role alone when the voltage _Ex is applied. When the voltage _Ex is applied in the x-axis direction of the original refractive index ellipsoid:

According to formula (13) and formula (14), it can be seen that the phase difference is proportional to the first power or the square of the voltage. Here, formula (14) is used as an example to analyze, and the frequency component needs to be measured. At this time, the phase difference generated by the voltage can be expressed as:

δ _E =δ _e0 cos ² ωt (18);

Among them, δ _E0 is the amplitude of the cosine phase, which can be obtained according to the phase difference expression of formula (14). When calculating, |δ _E0 | can be substituted. According to formula (14), we get:

At this time, the total phase difference can be written as:

δ=δ ₀ +δ _E0 cos ² ωt (20);

Among them, δ ₀ is the static phase difference, including the phase difference generated by the devices in the optical path and the phase difference generated by the DC bias. Then according to formula (16) we can know:

Taking δ ₀ =π/2, θ _i =π/4, and approximately considering cosδ _E ≈1, sinδ _E =δ _E , we can get:

Using the double angle function for transformation, we can get:

Among them, the first two items in the formula (23) are DC components, and the third item is the frequency doubled component. The frequency doubled component is measured by the lock-in amplifier 119, and the electro-optical coefficient component r ₄₂ is calculated based on the formula (19).

(b) The Mach-Zenhder interference unit is used to achieve independent measurement of the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ :

Under this scheme, the phase difference is proportional to the modulation voltage, and the detected light intensity is:

I=I ₀ +I ₀ cosδ (24);

The phase difference generated can be expressed as:

δ _E =δ _E0 cosωt (25);

At this time, the total phase difference can be written as:

δ=δ ₀ +δ _E0 cosωt (26);

Assuming that the light intensity in the two arms is equal, according to formula (25) and formula (26), we can know:

δ=δ ₀ +δ _E =δ ₀ +δ _E0 cosωt (27);

When measuring the electro-optical coefficient component r ₁₃ , ordinary light (o light) is used, and the polarization direction is perpendicular to the optical axis, and we get:

When measuring the electro-optical coefficient component r ₃₃ , using extraordinary light (e light) with the polarization direction parallel to the optical axis, we get:

Taking δ ₀ =π/2, it is approximately considered that cosδ _E ≈1, sinδ _E =δ _E , and can be obtained from formula (24) and formula (27):

I＝I ₀ -I ₀ δ _E0 cosωt (30);

Formula (30) is similar to formula (23), in which the first term is the DC component and the second term is the fundamental frequency component. The fundamental frequency component is measured through the lock-in amplifier 119 to obtain δ _E0 , thus combining formula (28) or formula ( 29) Calculate the electro-optical coefficient component r ₁₃ or r ₃₃ .

Therefore, the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ of the BTO crystal cannot be separated using the birefringence interference method. Only an equivalent electro-optical coefficient can be obtained. In order to separate them, the aforementioned Mach-Zenhder interference unit can be used. Select o-light and e-light transmission respectively, and conduct separate measurements on the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ .

Generally, the modulated output signal is a weak signal, and a sinusoidal modulation voltage with a frequency ω needs to be applied between the first electrode 106 and the second electrode 107, and the interference output light intensity is detected by the lock-in amplifier 119. When measuring r ₄₂ , the frequency doubled component is detected. When measuring the electro-optical coefficient component r ₁₃ and electro-optical coefficient component r ₃₃ , the fundamental frequency component is detected. The corresponding electro-optical coefficient is calculated based on the detection results.

Correspondingly, the method of measuring other ferroelectric thin film materials is similar. PLZT (lead lanthanum zirconate titanate thin film material) and BTO belong to the 4mm point group in the crystal, have the same independent electro-optical coefficients, and the same measurement processing plan. The LN (lithium niobate) film belongs to the 3m point group asymmetric crystal and has 8 non-zero electro-optical coefficient components, 4 of which are independent of each other, namely r ₁₃ , r ₃₃ , r ₂₂ and r ₄₂ . The processing method when measuring r ₁₃ and r ₃₃ is similar to the BTO film. Apply the electric field E _z and detect the fundamental frequency component; when measuring r ₂₂ and r ₄₂ , apply the electric field E _y , detect the fundamental frequency component when measuring r ₂₂ , and detect the fundamental frequency component when measuring r ₄₂ . Detect frequency doubled components.

At present, the mainstream methods for measuring the electro-optical coefficient of ferroelectric films generally include the following two methods:

Option 1: Place a quarter-wave plate behind the ferroelectric film sample, adjust the birefringence phase difference by rotating the quarter-wave plate, adjust the birefringent output light into linearly polarized light, and apply it on the ferroelectric film material Sinusoidal modulation voltage, the phase difference is modulated, and the azimuth angle of the output linearly polarized light will change according to the sinusoidal law. The azimuth angle is detected by the analyzer 109, the lock-in amplifier 119 is used to detect the signal, and the changes in the azimuth angle and modulation voltage amplitude are analyzed. relationship, the electro-optical coefficient can be obtained.

Option 2: Based on Option 1, a magneto-optical modulator is placed behind the quarter-wave plate. The magneto-optical modulator is based on the Faraday optical rotation effect and can rotate the polarization state of light. In this scheme, a DC voltage is applied to the ferroelectric thin film material, and a sinusoidal modulation voltage is applied to the magneto-optical modulator. The DC voltage determines the azimuth angle of the linearly polarized light output by the quarter-wave plate and its change. This azimuth angle is further determined by Transferred to the modulation frequency of the magneto-optical modulator, it can be detected by the lock-in amplifier 119. By changing the DC voltage and analyzing the change of azimuth angle with DC voltage, the electro-optical coefficient can be obtained.

However, the above two solutions have the following shortcomings:

(1) The existing solution can only measure ferroelectric films with [100] and [010] crystal orientations. Since the existing solution uses vertical incidence, for films with [001] crystal orientation, light is transmitted along the optical axis, there is no birefringence, and there is no interference output, so it cannot be measured.

(2) The existing solution can only measure a small number of electro-optical coefficients. Since the existing solution uses the birefringence of the crystal, some electro-optical coefficients (two or more) will determine the birefringence interference effect at the same time, and they cannot be separated, specifically:

When a voltage is applied to a ferroelectric thin film sample, the ordinary light refractive index and extraordinary light refractive index of the material will change due to the electro-optical effect. Birefringence can be used to detect the changes in these refractive indexes. However, due to the complexity of the electro-optical coefficient matrix of ferroelectric materials, in some cases, the changes in the material's ordinary light refractive index and extraordinary light refractive index are associated with multiple different electro-optical coefficient components. In this case, they cannot be separated through birefringence interference. separated (for example, the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ of the barium titanate film cannot be separated).

(3) The existing scheme has large measurement errors and low sensitivity. The existing solution uses a quarter-wave plate to adjust the birefringent light into linearly polarized light. Under the action of the modulation voltage, the azimuth angle of the linearly polarized light will change. The electro-optical coefficient is measured by detecting the change of the azimuth angle with the modulation voltage. . There are two problems with this method. One problem is that under the action of the modulation voltage, the polarization state of the light will become elliptically polarized light, which can only be approximately regarded as linearly polarized light, so a certain measurement error will occur; the other problem is that Yes, after the quarter-wave plate adjusts the birefringent light to linearly polarized light, the operating point is not at the orthogonal bias point, and the linearity and sensitivity of the electro-optical modulation response are low. These two problems lead to large measurement errors and low sensitivity in existing solutions.

(4) The existing scheme has the problem of complex structure. Solution 2 uses a magneto-optical modulator, which increases the complexity of the system structure, makes system debugging, calibration, measurement operations, and data processing more complex, and also increases the source of system errors.

In view of the above-mentioned problems existing in the prior art, the present invention uses dual-beam interference to solve this problem. That is, the Mach-Zenhder double-beam interferometer structure is introduced on the basis of birefringence interference, and the two interference results are comprehensively used to measure different electro-optical coefficients.

Compared with existing technical solutions, the technical solution of the present invention has the following advantages:

(1) The electro-optical coefficient measurement method of ferroelectric thin films of the present invention can measure thin film materials with different lattice orientations, including [100], [010] and [001] crystal orientations. In the present invention, a rotatable sample stage is used. For the [001] oriented film, the light is obliquely incident to produce a birefringence effect, and the birefringence interference result is determined according to the incident angle.

(2) The method for measuring electro-optical coefficients of ferroelectric films of the present invention can measure more electro-optical coefficients. Due to the complexity of the crystal electro-optical coefficient matrix, some electro-optical coefficients (two or more) will determine the birefringence interference effect at the same time, and they cannot be separated by adjusting the polarization state, incident light, and modulation voltage direction. The invention introduces the Mach-Zenhder interferometer structure on the basis of birefringence interference, and comprehensively utilizes the two interference results to measure different electro-optical coefficients.

(3) The electro-optical coefficient measurement method of ferroelectric thin films of the present invention has higher sensitivity and accuracy. The present invention uses a phase compensator 108 in the optical path to set the interference working point at an orthogonal bias point. There is a linear relationship between the system output and the modulation voltage, and the response is the most sensitive.

Different from the existing technology, the present invention provides a device 100 for measuring the electro-optical coefficient of a ferroelectric film, which includes a sample placement module, an incident module, an electric field module and a dual optical path interference module. The sample placement module is used to place the ferroelectric film to be measured. 105. The incident module is used to provide linearly polarized light to illuminate the ferroelectric film 105 to be measured. The electric field module is used to provide an electric field to the ferroelectric film 105 to be measured. The dual optical path interference module is used to penetrate the ferroelectric film 105 to be measured according to the linearly polarized light. The phase difference between the two polarized lights generated is used to output the electro-optical coefficient component corresponding to the ferroelectric film 105 to be measured. The dual optical path interference module includes a birefringent interference unit and a Mach-Zenhder interference unit. The birefringent interference unit is used for The electro-optical coefficient component in the ferroelectric film 105 to be measured can be separated using the birefringence interference method, and the Mach-Zenhder interference unit is used to measure the electro-optical coefficient component in the ferroelectric film 105 to be measured that cannot be separated using the birefringence interference method; the invention provides The device 100 for measuring the electro-optical coefficient of a ferroelectric film introduces a Mach-Zenhder interference unit structure based on the birefringence interference unit structure, in which the birefringence interference unit is used to measure the ferroelectric film to be measured 105 that can be separated using the birefringence interference method. For the electro-optical coefficient component, a Mach-Zenhder interference unit is used to measure the electro-optical coefficient component in the ferroelectric film 105 to be measured that cannot be separated using the birefringence interference method, so that the two interference results can be combined to measure different electro-optical coefficient components, thereby greatly improving the electro-optical coefficient component. The detection sensitivity and accuracy of the device 100 for measuring the electro-optical coefficient of the ferroelectric film are improved, thereby saving detection costs.

It should be noted that the above embodiments all belong to the same inventive concept, and the descriptions of each embodiment have different emphases. If the descriptions in individual embodiments are not exhaustive, please refer to the descriptions in other embodiments.

The above embodiments only express the implementation of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the patent scope of the invention. It should be noted that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the scope of protection of the patent of the present invention should be determined by the appended claims.

Claims (10)

1. A device for measuring the electro-optical coefficient of a ferroelectric film, characterized in that it includes a sample placement module, an incident module, an electric field module and a dual optical path interference module, the sample placement module is used to place the ferroelectric film to be measured, the incident The module is used to provide linearly polarized light to illuminate the ferroelectric film to be measured, the electric field module is used to provide an electric field to the ferroelectric film to be measured, and the dual optical path interference module is used to penetrate according to the linearly polarized light. The phase difference between the two polarized lights generated after the ferroelectric film to be measured is used to output the electro-optical coefficient component corresponding to the ferroelectric film to be measured; Wherein, the dual optical path interference module includes a birefringence interference unit and a Mach-Zenhder interference unit. The birefringence interference unit is used to measure the electro-optical coefficient component in the ferroelectric film to be measured that can be separated using the birefringence interference method, so The Mach-Zenhder interference unit is used to measure the electro-optical coefficient component in the ferroelectric film to be measured that cannot be separated using the birefringence interference method. 2. The device for measuring electro-optical coefficients of ferroelectric films according to claim 1, characterized in that the sample placement module is a rotatable sample stage, and the rotatable sample stage is used to place the ferroelectric film to be measured. film. 3. The device for measuring the electro-optical coefficient of ferroelectric films according to claim 2, wherein the incident module includes a coaxially arranged laser, a half-wave plate and a polarizer, and the laser is used to provide visible waveband or A light source in the infrared band, the polarizer is used to convert the light emitted by the light source into linearly polarized light, and the half-wave plate is used to adjust the polarization direction of the light emitted by the light source so that the output of the polarizer The intensity of the incident light is maximum. 4. The device for measuring the electro-optical coefficient of a ferroelectric film according to claim 3, wherein the electric field module includes a signal generator, a voltage amplifier, a first electrode and a second electrode, and the signal generator is used to provide a third electrode. a voltage signal, the voltage amplifier is used to amplify the first voltage signal into a second voltage signal, the first electrode and the second electrode are respectively attached to both ends of the ferroelectric film to be measured, to transmit the second voltage signal to the ferroelectric film to be measured. 5. The device for measuring the electro-optical coefficient of a ferroelectric film according to claim 4, wherein the first electrode and the second electrode are made of gold, and the first electrode and the second electrode are made of gold. The spacing range is 5μm～10μm. 6. The device for measuring electro-optical coefficients of ferroelectric thin films according to claim 4, wherein the birefringence interference unit includes a phase compensator, an analyzer and a first detector arranged coaxially in sequence, and the phase The compensator is coaxially arranged with the incident unit, and the first detector is used to detect the intensity of the received interference signal; Wherein, the phase compensator is used to adjust the working point where the two polarized lights generated in the dual optical path interference module interfere to an orthogonal bias point. 7. The device for measuring electro-optical coefficients of ferroelectric thin films according to claim 6, characterized in that the Mach-Zenhder interference unit includes a first optical path unit and a second optical path unit, and the first optical path unit includes sequentially coaxial The first beam splitter, the phase compensator, the analyzer, the second beam splitter and the first detector are provided, and the second optical path unit includes the first beam splitter, the first a reflecting mirror, a second reflecting mirror, the second beam splitter and the first detector. 8. The device for measuring the electro-optical coefficient of ferroelectric thin films according to claim 7, wherein the first reflector and the second reflector are further provided with a reference sample stage, and the reference sample stage is used to place a reference Thin film sample, the reference thin film sample is quartz glass, used to compensate for the optical path difference between the first optical path unit and the second optical path unit. 9. The device for measuring electro-optical coefficients of ferroelectric thin films according to claim 8, characterized in that the device further includes a second detector, a lock-in amplifier and a computer control system, the lock-in amplifier and the first The detector is electrically connected, and the computer control system is electrically connected to the lock-in amplifier; Wherein, the second detector is used to detect the light intensity of a part of the light beam where birefringence interference does not occur when the birefringence interference unit is operating normally, and the lock-in amplifier is used to detect the interference received by the first detector. signal, and the computer control system is configured to output the electro-optical coefficient component corresponding to the ferroelectric film to be measured according to the interference signal. 10. The device for measuring the electro-optical coefficient of a ferroelectric film according to claim 9, characterized in that when the ferroelectric film to be measured is a strontium barium titanate film, the birefringence interference unit is used to measure the titanate. The electro-optical coefficient component r ₄₂ of the strontium barium titanate film is measured using the Mach-Zenhder interference unit to respectively measure the electro-optical coefficient component r ₁₃ and the electro-optical coefficient component r ₃₃ of the strontium barium titanate film.