CN111272881A - Laser ultrasonic system and method for non-contact detection of thermal diffusivity of nano-film - Google Patents

Abstract

The invention discloses a laser ultrasonic system and a method for detecting thermal diffusivity of a nano-film in a non-contact manner. The invention further obtains the thermal diffusivity of the nano film by analyzing the heterodyne signal carrying the micro-vibration information detected by the detector.

Description

Laser ultrasonic system and method for non-contact detection of thermal diffusivity of nano-film

technical field

The invention belongs to the field of laser ultrasonic non-contact detection, in particular to a laser ultrasonic system and method for non-contact detection of thermal diffusivity of nano-films.

Background technique

Nano film materials have the characteristics of impact resistance, wear resistance, thermal breakdown resistance and so on. They are commonly used as conducting and dielectric elements in microelectronics, often as reflective or polarizing elements in integrated optics, and have many other applications in the aerospace, biotechnology, automotive, photography, and disk industries. Many traditional spectroscopic and scanning detection techniques can determine the chemical, morphological, and other properties of nanofilms. However, there are not many suitable techniques for measuring thermal diffusivity of thin films, in part because nanomaterials can be thin or small and easily damaged.

Laser ultrasonic nondestructive testing can detect the relevant physical properties or structural properties of the sample in time without compromising various properties of the sample, such as the sound velocity in the detection material, the thermal diffusivity and mass diffusion in the liquid or solid material. coefficient, temperature measurement of gas species, carrier bipolar diffusion coefficient and non-equilibrium carrier lifetime and carrier mobility of semiconductor materials, etc. Due to its non-destructive, fast and accurate characteristics, photoacoustic detection technology has been applied to the measurement of thermal diffusivity of nano-film materials.

The excited sample surface produces micro-vibration caused by surface ultrasound, and the excited part is periodically fluctuated and distributed. And because it is excited by pulsed laser, the excited micro-vibration is an unstable "transient grating". The probe light/reference light incident on the sample surface is reflected or diffracted by the sample, and finally received by the signal receiving module of the system. The relationship between the detected signal and the thermal diffusivity is obtained by analyzing the variation law of the intensity of the heterodyne signals diffracted by the acoustic wave with time.

However, it is difficult in practice to obtain the thermal diffusivity of thin films by directly detecting the light intensity information diffracted by the sample. Because the micro-vibration caused by the excited ultrasonic signal may be very small, the detected diffracted light intensity information may be very weak, which is not enough to analyze the relevant information of the ultrasonic signal, so it is necessary to combine the heterodyne detection technology. The optical heterodyne technology couples the directly measured detection light signal with the reference light, and the detector obtains the coupled signal, and its signal size can be dozens of times that of the signal detected by the direct detection method, so the optical The differential detection method is very sensitive to small vibrations. However, it is difficult to adjust the phase relationship between the two beams to meet the coupling requirements. Common heterodyne systems use the rotation of the glass plate to control the optical path of one of the beams, and thus the phase difference between the two beams. However, this method is relatively rough and cannot accurately meet the requirements of precise phase matching between the two beams.

To sum up, although the photoacoustic nondestructive testing technology can detect the thermal diffusivity of thin films nondestructively, rapidly and accurately. However, if the thermal diffusivity of the film is analyzed by directly detecting the light intensity information diffracted by the sample, the micro-vibration caused by the excited ultrasonic signal may be small, which is not enough to analyze the relevant information of the ultrasonic signal and the thermal conductivity of the film. diffusivity. However, if a common heterodyne detection system with parallel plates is added, the adjustment method is rough and inaccurate, and the precise phase matching requirements between the two beams cannot be accurately completed, and the precise measurement of the thermal diffusivity of the thin film cannot be achieved.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films.

In order to achieve the above purpose, the present invention adopts the following technical scheme: a laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films, comprising an excitation light emission module, a detection light emission module, a dichroic mirror, a phase mask, and a shutter , a first achromatic lens, a phase adjuster, a second achromatic lens, a moving sample stage, and a signal receiving module, and are set to:

The excitation light emitted by the excitation light emission module is transmitted through the dichroic mirror and then focused on the phase mask. The phase mask divides the excitation light into two beams, and the two excitation lights pass through the shutter, the first achromatic lens, and the second The chromatic aberration lens focuses on the sample surface set on the moving sample stage;

The detection light emitted by the detection light emission module is reflected by the dichroic mirror and then focused on the phase mask. The phase mask divides the detection light into two beams, one of which is used as the detection light to pass through the shutter, the first achromatic lens, the phase The regulator and the second achromatic lens are focused on the sample surface set on the moving sample stage, and the other beam is used as a reference light to be focused on the moving sample stage through the shutter, the first achromatic lens, and the second achromatic lens in turn. the sample surface;

The two beams of excitation light, probe light and reference light overlap at the same position of the sample;

The signal receiving module is used to receive the heterodyne signal reflected and diffracted by the sample.

Preferably, the light-emitting emitting module includes a 532nm pulsed laser, an attenuator and a cylindrical lens, the 532nm pulsed laser, the attenuator and the cylindrical lens are located on the same optical axis, and the cylindrical lens focuses the excitation light on the phase mask.

Preferably, the detection light emission module includes an 830 nm continuous laser and a spherical mirror, and the 830 nm continuous laser and the spherical mirror are located on the same optical axis.

Preferably, the signal receiving module includes a focusing spherical lens and a photoelectric signal receiver:

The focusing spherical lens is used to couple the probe light and the reference light carrying the sample information excited by the excitation light into the photodetector to obtain the heterodyne signal.

Preferably, the detection light reflected by the dichroic mirror and the excitation light transmitted by the dichroic mirror converge at the same point on the phase mask, and the detection light and the excitation light form a fixed angle to ensure that the phase adjuster only allows the detection light to pass through, without blocking the excitation light.

Preferably, the heterodyne signal received by the signal receiving module is:

Among them, _q is the wavelength of the excited acoustic wave, _IS is the light intensity of the probe light after reflection by the sample, IR is the light intensity of the reference light after diffraction by the sample, λ is the wavelength of the excitation light, and u(t) is the microvibration correlation of the sample surface. function.

Preferably, the microvibration correlation function of the sample surface is specifically:

u(t)=U ₀ sin(qx)

In the formula, U ₀ is the amplitude of the ultrasonic displacement, and q is the wavelength of the excited acoustic wave;

The reference light intensity after diffraction by the sample is specifically:

In the formula, I ₀ is the light intensity of the reference light when it is not diffracted by the sample.

The present invention also proposes a non-contact method for detecting the thermal diffusivity of the nano-film, specifically:

Build a laser ultrasound system;

Obtain the heterodyne signal after reflection and diffraction by the sample;

The thermal expansion rate of the sample is obtained by solving the heterodyne signal.

Preferably, the specific method for calculating the thermal expansion rate of the sample according to the heterodyne signal is:

Substitute the expression of the ultrasonic displacement amplitude U ₀ and the expression of the acoustic wavelength into the heterodyne signal to obtain the thermal diffusivity D of the film, where the ultrasonic displacement amplitude U ₀ is expressed as:

where d is the linear thermal expansion coefficient, E ₀ is the average energy density of the excitation light, ρ is the material density, c is the specific heat, and D is the thermal diffusivity;

The expression of the excited acoustic wavelength q is:

where θ is the diffraction angle obtained after the excitation light is diffracted by the phase mask, λ is the wavelength of the excitation light, and Λ is the grating constant of the thermal grating generated by the excitation light excited the sample;

Preferably, the obtained thermal diffusivity is:

where τ is the relaxation time at the excitation acoustic wavelength, and Λ is the relaxation time at the excitation acoustic wavelength.

Compared with the prior art, the present invention has the following significant advantages:

Based on the principle of photoacoustic excitation and detection, the photoacoustic nondestructive testing technology is combined with the heterodyne detection technology, and the phase regulator and other related components are added to the system to make the detected photoacoustic signal stronger and the relevant information contained in it. Therefore, the thermal diffusivity of the analyzed nano-film is more accurate; the invention can realize non-destructive, accurate and fast measurement of the nano-film material without contacting the sample.

Description of drawings

Figure 1 is a schematic structural diagram of a laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films.

FIG. 2 is a schematic top plan view of a part of the structure of a laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to the present invention.

FIG. 3 is a schematic diagram of the simulation situation of the excitation light and the probe light/reference light focusing on the focal spot at the sample in the present invention.

FIG. 4 is a schematic diagram showing the reflection and diffraction of the probe light and the excitation light respectively after passing through the film excited by the excitation light.

Detailed ways

As shown in Figures 1 and 2, a laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films includes an excitation light emission module, a detection light emission module, a dichroic mirror 4, a phase mask 7, a shutter 8, The first achromatic lens 9, the phase adjuster 10, the second achromatic lens 11, the moving sample stage and the signal receiving module are set as:

The excitation light emitted by the excitation light emission module is transmitted through the dichroic mirror 4 and then focused on the phase mask 7. The phase mask 7 divides the excitation light into two beams, and the two excitation lights pass through the shutter 8 and the first achromatic lens in turn. 9. The second achromatic lens 11 is focused on the sample surface arranged on the moving sample stage;

The detection light emitted by the detection light emission module is reflected by the dichroic mirror 4 and then focused on the phase mask 7. The phase mask 7 divides the detection light into two beams. The chromatic lens 9, the phase adjuster 10, and the second achromatic lens 11 are focused on the surface of the sample set on the moving sample stage, and the other beam is used as a reference light through the shutter 8, the first achromatic lens 9, and the second achromatic lens in turn. The lens 11 focuses on the sample surface arranged on the moving sample stage;

The two beams of excitation light, probe light and reference light overlap at the same position of the sample to meet the requirements of spot size and energy.

The signal receiving module is used to receive the heterodyne signal reflected and diffracted by the sample.

Specifically, the phase mask 7 is located on the focal plane of the cylindrical mirror. When the excitation light is transmitted through the dichroic mirror 4 and then focused on the phase mask 7, the phase mask 7 diffracts the excitation light into various levels of diffraction with different energies. The light has different grating periods, and the sample can be detected with different grating periods for many times, so as to obtain heterodyne signals under different grating periods.

The shading device 8 is used to shield the diffracted light of all levels that is not used for exciting the acoustic wave after the 532 nm laser is diffracted by the phase mask 7 .

Specifically, the first achromatic lens 9 and the second achromatic lens 11 are used to focus the excitation light diffracted by the phase mask 7 and used to excite the surface acoustic wave of the sample to the surface of the sample, so that the two beams of excitation light are in the space of the sample surface and The time overlaps to form interference fringes, which generate surface ultrasound.

In a further embodiment, the light-emitting emission module comprises a 532nm pulsed laser 1, an attenuator 2 and a cylindrical mirror 3, the 532nm pulsed laser 1, the attenuator 2 and the cylindrical mirror 3 are located on the same optical axis, and the cylindrical mirror 3 will The excitation light is focused onto the phase mask 7 .

Specifically, a 532 nm pulsed laser 1 was used to generate excitation light.

Attenuator 2 is used to adjust the energy of the laser light emitted by the 532nm pulsed laser to excite the acoustic signal.

The cylindrical mirror 3 is used to focus the excitation light on the phase mask 7, and the height in the vertical direction is the same as that of the laser light exit hole.

In a further embodiment, the detection light emission module includes an 830 nm continuous laser 5 and a spherical mirror 6, and the 830 nm continuous laser 5 and the spherical mirror 6 are located on the same optical axis.

Specifically, an 830 nm continuous laser 5 is used to generate detection light.

The spherical mirror 6 is used to focus the detection light onto the phase mask 7 .

Specifically, when the detection light reflected by the dichroic mirror 4 is focused on the phase mask 7, the phase mask 7 diffracts the detection light into various levels of diffracted light with different energies and different grating periods. The detection of different grating periods is used to obtain heterodyne signals under different grating periods.

Specifically, the phase adjuster 10 is an electro-optic phase modulator based on the electro-optic effect. A voltage is applied to the input electrode to make the light generate a phase difference controlled by the voltage, but the polarization direction is not affected. By adjusting the voltage, the laser beam can be adjusted To the required state, the phase of the incident light can be adjusted accurately and dynamically. After the phase adjustment, the detection light and the reference light reach the phase matching condition, and the heterodyne phase is optimized, and finally the ultrasonic micro-vibration can be detected.

In a further embodiment, the signal receiving module includes a focusing spherical lens 13 and a photoelectric signal receiver 14:

The focusing spherical lens 13 is used to couple the probe light and the reference light carrying the sample information excited by the excitation light into the photodetector 14 to obtain a heterodyne signal.

Specifically, the moving sample stage is a three-dimensional motorized translation stage 12, which is used to move the sample to the focused focal spot of the excitation light.

In a further embodiment, the detection light reflected by the dichroic mirror 4 and the excitation light transmitted by the dichroic mirror 4 converge at the same point on the phase mask 7, and the detection light and the excitation light form a fixed angle to ensure the phase regulator. 10 Let only the probe light pass without blocking the excitation light.

Specifically, the angle between the detection light and the excitation light is 7°.

In a further embodiment, the heterodyne signal received by the signal receiving module is:

Among them, _q is the wavelength of the excited acoustic wave, _IS is the light intensity of the probe light after reflection by the sample, IR is the light intensity of the reference light after diffraction by the sample, λ is the wavelength of the excitation light, and u(t) is the microvibration correlation of the sample surface. function.

In a further embodiment, the sample surface micro-vibration correlation function is specifically:

u(t)=U ₀ sin(qx)

In the formula, U ₀ is the amplitude of the ultrasonic displacement, and q is the wavelength of the excited acoustic wave;

The reference light intensity after diffraction by the sample is specifically:

In the formula, I ₀ is the light intensity of the reference light when it is not diffracted by the sample.

Based on the principle of photoacoustic excitation and detection, the invention combines the photoacoustic nondestructive detection technology with the heterodyne detection technology, and further optimizes the traditional heterodyne detection system, so that the detected photoacoustic signal is stronger, and the included The relevant information is more accurate, so the thermal diffusivity of the nano-films analyzed is more accurate. And when optimizing the optical path, the position, angle, etc. of the related components are adjusted to avoid blocking the light by the related components. Finally, the present invention can achieve non-destructive, accurate and fast measurement of nano-thin film materials without contacting the sample

A method for non-contact detection of thermal diffusivity of nano-film, specifically:

Build a laser ultrasound system;

In some embodiments, the specific steps of constructing the laser ultrasound system are:

Step 1. Install the 532nm pulsed laser, attenuator, cylindrical mirror, dichroic mirror, phase mask, shutter, first achromatic lens 9, and second achromatic lens 11 on the base plate in sequence. Among them, the 532nm pulse laser light exit hole, attenuator, cylindrical mirror and phase mask are all located on the same horizontal optical path, and the shutter and achromatic lens are located on the same optical path.

The center height of each element perpendicular to the direction of the fixed bottom plate is the same, which is 68.5mm.

The reason why the cylindrical lens is chosen instead of the spherical lens is that under the same focal spot width, the energy density collected by the cylindrical lens is larger than that of the spherical lens, and the amplitude of the excited acoustic waves is larger. Moreover, the time-domain waveform of the acoustic wave excited by the laser and the directionality of the acoustic wave are related to the spatial distribution of the focused spot. When the light spot is focused by a cylindrical lens, the excited surface acoustic wave is bipolar, and compared with the point light spot focused by a spherical lens, the surface acoustic wave excited by the linear light plate focused by the cylindrical lens has stronger directivity.

Step 2. Set the phase mask period.

The phase mask is first placed on the smallest grating period. When the period is the smallest, the diffraction angle is the largest, ensuring that the size of the optical element meets the maximum requirements. For example, if the minimum period is 4 μm, and the selected probe light and reference light are the 0th order and -2nd order diffracted light of the 830nm laser, respectively, according to the calculation, the maximum diffraction angle is 11.97°. And due to changing the grating period, the diffraction angle of the probe light/reference light changes. The period becomes larger, and the two beams of the probe light/reference light "converge" toward the center of the system.

Step 3. Adjust the angle of the dichroic mirror.

The horizontal direction of the dichroic mirror is fixed at 48.5°, which makes the angle between the excitation light and the detection light 7° to ensure the spatial separation of the excitation light and the detection light/reference light.

The vertical direction of the dichroic mirror is fixed at 11.97°. This angle is obtained according to the period of the phase mask being 4 μm, and the selected probe light and reference light are the 0-order and -2-order diffracted light of the 830 nm laser, respectively.

Step 4. Use a shutter to block out unnecessary excitation light and retain useful excitation light.

The ±1st order light of the 532nm laser was selected as the excitation light. The ±1-order excitation light after passing through the shading plate is focused on the sample by an achromatic lens, and the two beams overlap in space and time to form interference fringes, which generate slight pulse heating in the spatial geometry of the optical interference pattern. Thermal expansion occurs, which emits counter-propagating surface acoustic waves whose wave vectors are determined by the crossing angle and wavelength of the excitation light. Therefore, by converting different grating periods, the angle between the two excitation beams can be converted, and then the wave vector of the emitted acoustic wave can be converted.

Step 5. Install the 830nm CW laser and the spherical mirror on the base plate in sequence. Among them, the 830nm continuous laser light exit hole and the spherical mirror are located on the same horizontal optical path. The phase mask is passed through the achromatic lens focus.

By adjusting the height and angle of the exit hole of the 830nm laser, as well as the height and position of the spherical mirror, it is ensured that the 830nm laser and the 532nm laser overlap on the grating.

Fixing the phase adjuster in the optical path of the probe light can accurately and dynamically adjust the phase of the incident light. The selected probe light and reference light are the 0-order and -2-order diffracted light of the 830 nm laser, respectively, and the phase adjuster is used for the phase adjustment of the 830 nm 0-order diffracted light. The phase-adjusted 0-order diffracted light and -2-order diffracted light reach the phase matching condition, which can optimize the heterodyne phase, and finally detect the ultrasonic micro-vibration of the nanometer level. Compared with the ordinary system, the rotating plate is used for adjustment. The phase method, using the phase adjuster is more accurate and more convenient. Taking the grating period of 4 μm, the selected detection light is the 0th-order diffracted light of the 830 nm laser, and the focal length of the achromatic lens is 85 mm, as an example, adjust the central vertical height of the phase adjuster to 48 mm.

Due to the limitation of the size of the phase adjuster, if the excitation light and the detection light are in the same plane, the phase adjuster will block the excitation light. Therefore, ensure that the horizontal optical path where the light exit hole of the 532nm pulsed laser, attenuator, cylindrical mirror, phase mask is located, and the shutter, achromatic lens, and phase adjuster are located at 7°. This angle ensures that the excitation light and the probe light/reference light are located. The separation in space ensures that the phase adjuster does not block the light.

Step 5. Install the three-dimensional electric translation stage. The horizontal direction of the three-position electric translation stage is 45° with the optical path of the achromatic lens.

Step 6. Install the focusing spherical lens and the photoelectric signal receiver in the signal receiving system in sequence. The focusing spherical lens and the photoelectric signal receiver are in the same horizontal optical path, and form 90° with the optical path of the achromatic lens.

Step 7. Turn on the 532nm pulsed light laser, the 830nm continuous laser, the three-dimensional motorized translation stage, and the photoelectric signal detector to measure the heterodyne signals under different grating periods.

Obtain the heterodyne signal after reflection and diffraction by the sample;

The thermal expansion rate of the sample is obtained by solving the heterodyne signal.

In a further example, the specific method for obtaining the thermal expansion rate of the sample by calculating the heterodyne signal is as follows:

Substitute the expression of the ultrasonic displacement amplitude U ₀ and the expression of the acoustic wavelength into the heterodyne signal to obtain the thermal diffusivity D of the film, where the ultrasonic displacement amplitude U ₀ is expressed as:

where d is the linear thermal expansion coefficient, E ₀ is the average energy density of the excitation light, ρ is the material density, c is the specific heat, and D is the thermal diffusivity;

The expression of the excited acoustic wavelength q is:

where θ is the diffraction angle obtained after the excitation light is diffracted by the phase mask, λ is the wavelength of the excitation light, and Λ is the grating constant of the thermal grating generated by the excitation light excited the sample;

In a further example, the obtained thermal diffusivity is:

where τ is the relaxation time at the excitation acoustic wavelength, and Λ is the relaxation time at the excitation acoustic wavelength.

The detection principle of the present invention is:

The 532-pulse laser 1 emits excitation light, and after passing through the attenuator 2, the energy of the excitation light is adjusted to reach the cylindrical lens 3, and the excitation light is focused and focused on the phase mask 7. Before reaching 7, the excitation light has to pass through the dichroic mirror 4, and the dichroic mirror has no special effect on the excitation light, but only allows it to pass through. After the excitation light reaches 7, it is divided into many beams of diffracted laser light, and only two beams of excitation light are retained after passing through the shutter 8. After passing through the first achromatic lens 9 and the second achromatic lens 11, the two beams of excitation light are focused at the sample 12 to form interference fringes, and finally generate a signal.

The 830 continuous laser 5 emits detection light, and after passing through the spherical mirror 6, it is focused on the phase mask 7. Before reaching 7 , the detection light is reflected by the dichroic mirror 4 , so that the reflected detection light is just focused on 7 . After the detection light passes through 7, it is divided into many beams of diffracted laser light, and passes through the shutter 8, and only two beams of detection light remain. The beam that has passed through the phase adjuster 10 is called probe light, and the beam that has not passed through the phase adjuster 10 is called reference light. After the probe light passes through the phase adjuster 10, a phase difference is formed between the probe light and the reference light. Finally, the two beams of light are focused on the sample after passing through the first achromatic lens 9 and the second achromatic lens 11, and coincide with the light spot formed by the excitation light. Finally, the probe light and the reference light carry the sample information excited by the excitation light, and reach the detector 14 after being focused by the focusing spherical lens 13 .

The 532nm pulsed laser 1 in the photoacoustic excitation module emits a pulsed laser, which reaches the surface of the sample after a series of modulations. Due to the interference of light, a striped light source is formed on the surface of the sample, thereby exciting the thermal grating of the sample, and based on the thermoelastic effect, it can be Without contacting the sample, the surface acoustic wave is excited in the sample, and the continuous laser emitted by the signal detection module is modulated by a series to coincide with the excitation spot on the sample surface, and the acoustic information carried by the detection light on the sample surface is received by the signal receiving module. The resulting signal should be:

为探测光与参考光初相位之差，通过外差系统中相位调节器10的动态精确调节，使

又由于u(t)＜＜λ，所以可得最终探测器接收到的信号为：where _IS is the light intensity of the probe light after reflection by the sample, IR is the light intensity of the reference light after diffraction by the sample, λ is the wavelength of the excitation light, u( _t ) is the microvibration correlation function on the surface of the sample,

In order to detect the difference between the initial phase of the light and the reference light, through the dynamic and precise adjustment of the phase adjuster 10 in the heterodyne system, the

And because u(t)<<λ, the signal received by the final detector can be obtained as:

所以得到的外差信号为：Since the excitation light forms an interference fringe light source on the sample surface, the microvibration correlation function u(t) on the sample surface can be simplified into a sinusoidal form: u(t)=U ₀ sin(qx), and because

So the obtained heterodyne signal is:

Among them, I ₀ is the light intensity of the reference light without sample diffraction, q is the wavelength of the excited acoustic wave, and U ₀ is the amplitude of the ultrasonic displacement. Since I _S is much larger than I ₀ , the final heterodyne signal I is obtained. (t) is much larger than the signal obtained by the direct parametric method, which helps to detect weaker vibrations on the film surface,

Ultrasonic displacement amplitude U ₀ , its expression is:

where d is the coefficient of linear thermal expansion, E ₀ is the average energy density of the excitation light, ρ is the material density, c is the specific heat, and D is the thermal diffusivity, so the relationship between the detected heterodyne signal I(t) and U ₀ is: :

Among them, the wavelength q of the excited acoustic wave is expressed as:

Among them, θ is the diffraction angle obtained after the excitation light is diffracted by the phase mask 7, λ is the wavelength of the excitation light, and Λ is the grating constant of the thermal grating generated by the excitation light excited the sample.

According to the relationship between the heterodyne signal I(t) and _U0 , the thermal diffusivity is obtained:

By measuring the relaxation time at a certain excitation acoustic wave wavelength, the thermal diffusivity D of the film can be obtained, and by adjusting the grating period of the phase mask 7, the wavelength of the excited acoustic wave can be changed, that is, the vibration caused by the acoustic wave can be changed The grating constant of the grating is verified by measuring the relaxation time at different acoustic wavelengths to verify the correctness of the calculated thermal diffusivity of the thin film.

Figure 3 is a schematic diagram of the simulation of the focal spot of excitation light and probe light/reference light focusing at the final sample. By adjusting the positions of the two achromatic lenses, it is ensured that the two beams of excitation light and the two beams of detection light overlap at the same position of the sample and meet the requirements of spot size and intensity.

FIG. 4 is a schematic diagram showing the reflection and diffraction of the probe light and the excitation light respectively after passing through the film excited by the excitation light. After the probe light and reference light are reflected and diffracted by the micro-vibrating sample respectively, they pass through the focusing spherical mirror in the signal receiving module, and finally reach the photoelectric signal receiver. Finally, what the receiver receives is the heterodyne signal coupled with the probe light and the reference light. Through the dynamic adjustment of the phase regulator in the heterodyne system, the optimal signal is obtained, and the thermal diffusivity of the nanofilm sample is finally demodulated.

The invention can excite the sample with different wavelengths of sound waves conveniently and quickly in the same system, realize the acquisition of multiple groups of data, and make the experimental results more accurate. In addition, the heterodyne detection method used in the excitation and detection module is optimized, and the detection is more convenient and accurate under the condition that each optical element does not block the light path.

Claims (10)

1. a laser ultrasonic system for non-contact detection of nano-film thermal diffusivity, is characterized in that, comprises excitation light emission module, detection light emission module, dichroic mirror (4), phase mask (7), shutter (8), a first achromatic lens (9), a phase adjuster (10), a second achromatic lens (11), a moving sample stage and a signal receiving module, and are set as: The excitation light emitted by the excitation light emission module is transmitted through the dichroic mirror (4) and then focused on the phase mask (7), and the phase mask (7) divides the excitation light into two beams, and the two excitation lights pass through the shutter ( 8), the first achromatic lens (9) and the second achromatic lens (11) focus on the sample surface arranged on the moving sample stage; The detection light emitted by the detection light emission module is reflected by the dichroic mirror (4) and then focused on the phase mask (7), and the phase mask (7) divides the detection light into two beams, one of which is used as detection light and is shielded in turn. The device (8), the first achromatic lens (9), the phase adjuster (10), and the second achromatic lens (11) are focused on the sample surface set on the moving sample stage, and the other beam is used as a reference light and is shielded in turn. The device (8), the first achromatic lens (9), and the second achromatic lens (11) focus on the sample surface arranged on the moving sample stage; The two beams of excitation light, probe light and reference light overlap at the same position of the sample; The signal receiving module is used to receive the heterodyne signal reflected and diffracted by the sample. 2. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to claim 1, wherein the light-emitting emission module comprises a 532nm pulsed laser (1), an attenuator (2) and a cylindrical mirror (3) ), the 532nm pulsed laser (1), the attenuation plate (2) and the cylindrical mirror (3) are located on the same optical axis, and the cylindrical mirror (3) focuses the excitation light on the phase mask (7). 3. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to claim 1, wherein the detection light emission module comprises an 830nm continuous laser (5), a spherical mirror (6), and the 830nm The continuous laser (5) and the spherical mirror (6) are located on the same optical axis. 4. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to claim 1, wherein the signal receiving module comprises a focusing spherical mirror (13) and a photoelectric signal receiver (14): The focusing spherical lens (13) is used for coupling probe light and reference light carrying sample information excited by excitation light into a photodetector (14) to obtain a heterodyne signal. 5. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to claim 1, characterized in that the detection light reflected by the dichroic mirror (4) and the light transmitted by the dichroic mirror (4) The excitation light is converged at the same point of the phase mask (7), and the detection light and the excitation light form a fixed angle, ensuring that the phase adjuster (10) only allows the detection light to pass without blocking the excitation light. 6. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-films according to claim 1, wherein the heterodyne signal received by the signal receiving module is:

Among them, _q is the wavelength of the excited acoustic wave, _IS is the light intensity of the probe light after reflection by the sample, IR is the light intensity of the reference light after diffraction by the sample, λ is the wavelength of the excitation light, and u(t) is the microvibration correlation of the sample surface. function. 7. The laser ultrasonic system for non-contact detection of nano-film thermal diffusivity according to claim 6, wherein the sample surface micro-vibration correlation function is specifically: u(t)=U ₀ sin(qx) In the formula, U ₀ is the amplitude of the ultrasonic displacement, and q is the wavelength of the excited acoustic wave; The reference light intensity after diffraction by the sample is specifically:

In the formula, I ₀ is the light intensity of the reference light when it is not diffracted by the sample. 8. a method for non-contact detection of nano-film thermal diffusivity, is characterized in that, is specially: Build the laser ultrasound system according to any one of claims 1 to 7; Obtain the heterodyne signal after reflection and diffraction by the sample; The thermal diffusivity of the sample is obtained by solving the heterodyne signal. 9. The method for non-contact detection of thermal diffusivity of nano-films according to claim 8, characterized in that, the specific method for obtaining the thermal diffusivity of samples by calculating the heterodyne signal is: Substitute the expression of the ultrasonic displacement amplitude U ₀ and the expression of the acoustic wavelength into the heterodyne signal to obtain the thermal diffusivity D of the film, where the ultrasonic displacement amplitude U ₀ is expressed as:

where d is the linear thermal expansion coefficient, E ₀ is the average energy density of the excitation light, ρ is the material density, c is the specific heat, and D is the thermal diffusivity; The expression of the excited acoustic wavelength q is:

Wherein θ is the diffraction angle obtained after the excitation light is diffracted by the phase mask (7), λ is the wavelength of the excitation light, and Λ is the grating constant of the thermal grating generated by the excitation light to excite the sample. 10. The method for non-contact detection of nano-film thermal diffusivity according to claim 8, wherein the obtained thermal diffusivity is:

where τ is the relaxation time at the excitation acoustic wavelength, and Λ is the relaxation time at the excitation acoustic wavelength.

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